Chapter 107 Rare earth pyrochlores

Chapter 107 Rare earth pyrochlores

Handbook on the Physics and Chemistry of Rare Earths, Vol. 16 edited by K.A. Gschneidner, Jr. and L. Eyring © 1993, Elsevier Science Publishers B. ld ...

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Handbook on the Physics and Chemistry of Rare Earths, Vol. 16 edited by K.A. Gschneidner, Jr. and L. Eyring © 1993, Elsevier Science Publishers B. ld

Chapter 107 RARE EARTH

PYROCHLORES

M.A. S U B R A M A N I A N

E.I. du Pont de Nemours and Company, Wilmington, DE, 19880-0328, USA A.W. S L E I G H T Department of Chemistry, Oregon State University, Corvallis, OR, 97331-4003,

USA

Contents 1. Introduction 2. Pyrochlore structure 2.1. Crystallography 2.2. Description of structure 2.3. Order disorder and nonstoichiometry 3. Pyrochlore composition 3.1. A~+ M2*+O7 3.2. Complex formulations 4. Electrical properties

5. Magnetic and specific heat properties 6. Spectroscopy 6.1. Vibrational spectroscopy 6.2. UV-visible spectroscopy 6.3. Luminescent properties 6.4. Mössbauer spectroscopy Appendix. Bibliography on rare earth R 2 M 2 0 7 pyrochlores Refcrences

225 226 226 227 228 229 229 231 236

238 239 239 241 241 241 242 245

1. Introduction The occurrence of the pyrochlore structure is very common since the vast majority of the elements in the periodic table are found in this structure. The ideal pyrochlore formula is A 2 M 2 X 7 , where A and M are cations and X is an anion such as oxygen or fluorine. However, defect structures such as A2M2X 6 and AM2X 6 are well known. The oxidation states for the A 2 M 2 0 7 formulation are generally A 2 + M S + O 7 or Az3+ M g+ 0 7 . It is the latter formulation that this chapter will mainly address, i.e., those pyrochlores where the A cation is a trivalent rare earth: R32+ M24+ 0 7 . All the rare earths are known to occupy the A sites, and eighteen different tetravalent cations can occupy the M site. In addition, many rare earth pyrochlores with more complex formulations, such as R 3+ M 3+ M 5+ O~, are known. Since there are over a hundred different rare earth compounds with the pyrochlore structure, there exists a wide diversity of properties. Some rare earth pyrochlores are 225

226

M.A. SUBRAMANIAN and A.W. SLEIGHT

electrical insulators; others are low-activation-energy semiconductors. In rare cases, A23÷ M2~ + 0 7 pyrochlores may possess metallic properties. Ferroelectricity is observed for compounds with the pyrochlore structure, but this does not occur in the R23÷ M42÷ O7 phases. In addition to diamagnetism and paramagnetism, ferromagnetism is also observed for some R 2 M 2 0 7 compounds. Superconductivity might be expected to occur in the pyrochlore structure considering the wide variety of compositions and properties found for compounds with this structure. However, superconductivity has not been observed at any temperature for any compound with this structure, regardless of whether or not a rare earth element is present. An interesting aspect of the pyrochlore structure is the unique coordination found for the rare earth at the A cation site. Although this environment is sometimes viewed as a distorted cube, two oxygens are much closer than the remaining six. These art in fact a m o n g the shortest R - O distances erer observed, and such rare earth cations are in the highest electric field gradient ever observed. Despite the wide occurrence of the pyrochlore structure, compounds with this structure type have been reviewed only once before (Subramanian et al. 1983). The appendix presents a bibliography on rare earth pyrochlores with an organization very different from that which directly follows.

2. Pyrochlore structure 2.1.

Crystallography

The space group for the ideal pyrochlore structure is Fd3m. One has a choice of four origins for the pyrochlore structure. Both the A and M cations are at inversion centers, and either of these cation sites can be used as the origin. The generally preferred description has the M cation at the origin, and this decription is given in tabte 1. In addition to the systematic absences of the space group, there are systematic absences based on the positions occupied for the pyrochlore structure. These additional conditions apply to even indices only and are: h, k and 1 must be equal to 4n + 2 or 4n, or h + k + l must be equal to 4n. This causes the 442 reflection, e.g., to be forbidden even though it is allowed for the Fd3m space group. However, the 442 reflection becomes allowed when anisotropic thermal motions are considered. For the ideal A 2 M 2 0 7 pyrochlore structure, all A cations are equivalent and all TABLE 1

Atomic positions in space group Fd3m for R2M207 pyrochlores. Atom R M O O

Position

Coordinates

16d 16c 48f Sb

½, ½, ½ 0, 0, 0 1 ä 1 x, ä, ~,-~,

RARE EARTH PYROCHLORES

227

M cations are equivalent, but there are two types of oxygen. Thus, it is sometimes preferred to write the pyrochlore formula as A2 M2 0 6 O'. The oxygen referred to as O' is in a special position and is sometimes missing from the structure. There is only one positional parameter for the pyrochlore structure, and this is for the more abundant oxygen species. This parameter, x(O), would be 0.3125 for a regular octahedral oxygen environment around M and would be 0.375 for a regular cubic oxygen environment around A. In fact for R 2 M 2 0 7 pyrochlores, x(O) is found to range from 0.319 to 0.343 (Barker et al. 1970, McCauley 1980, Subramanian et al. 1983, Chakoumakos 1984). Several structural refinements for the pyrochlore structure have been published based on single crystal data; however, for R2 M2 0 7 pyrochlores, structures are based on refinements of powder data obtained by either X-ray or neutron diffraction. Since there is onty one positional parameter to refine, the structures are generally highly overdetermined even when powder data are used. The results of the structural refinements on R2 M 2 0 7 pyrochlores are summarized in table 2. 2.2. Description of structure Two different descriptions of the pyrochlore structure are useful. One description views the structure as derived from the fluorite structure (Aleshin and Roy 1962, Longo et al. 1969), and the other views the pyrochlore structure as a network structure (Sleight 1968a). The relationship to the fluorite structure is shown in fig. 1. For x(O) equal to 0.375, we have the fluorite structure except that A and M cations are ordered on the fluorite cation sites and there are ordered oxygen vacancies. The oxygen vacancies are such that the A cation coordination number remains eight, but the M cation coordination number drops to six. In practice, the oxygen positional parameter always decreases considerably, so that the coordination around M approaches octahedral. This reduced value of x(O) causes the coordination around the A cation to deviate very significantly from cubic. The A cation now has two very close distances (~ 2.3 Ä), lying on the three-fold axis; and there are six more distant oxygens (~2.5 ~). The fluorite-related description of the pyrochlore structure is especially useful to understand the order disorder transformation and ranges of stoichiometry that exist, for example, for phases in the R20» ZrO2 systems. The network description of the pyrochlore structure has become more popular, partly because it leads to a natural description of defect pyrochlores such as A2 M2 0 6 and AM 2 06. In this description, the structure is viewed as made up o f t w o networks which interpenetrate (Sleight 1968a). One network, Az O', is the same as the networks found in C u 2 0 . The O' is tetrahedrally coordinated by A, and the A cation has a coordination number of only two. The other network, MO3, is made up of octahedrally coordinated M cations. These octahedra are linked together by corners to form a three-dimensional network with cubic symmetry. Figure 2 shows how these two networks interpenetrate. The strongest interaction between these two networks is between the A 3÷ cations and the O 2- anions from the MO3 network. For R23+ M24+ 0 7 pyrochlores, the network charges are (R20) 4+ and (MO3) 4-. Thus, the structure can be viewed as made up of two covalent networks with ionic bonding

228

M.A. S U B R A M A N I A N and A.W. SLEIGHT

1 -

B

i

I

,I

¸

I~. B,,/ll

~\"x

fB'/~,"~ i / I

"6 ~\x x~\

i/

,,,

~v/ ,-

B (16c)

/"

i / I/

A (lSd)

O

0 (48f) o' (8b)

0

,q

0

i ~ ~ .... ///«/f

O

C)

r'~~ -,_t

Va¢cmcy (SŒ)

Fig. 1. Pyrochlore structure as derived from the ftuorite structure. The cation positions for ¼ of the unit cell are shown. The shifts of (48f) oxygen towards the (8a) vacant site are indicated.

between the two networks. Although R -O bonding is usually considered more ionic than covalent, one must remember that R - O ' distances in the pyrochlore structure are only about 2.3 •. Mössbauer results are also compatible with this description, since they show that the highest electric field gradient ever observed for rare earth cations is found when such cations reside at the A site of the pyrochlore structure (Bauminger et al. 1974, 1976). This unusual environment at the A site also results in very anisotropic thermal parameters for A cations since they cannot readily vibrate against such close oxygens. The coordination of the M cation is frequently described as octahedral, but in fact, the coordination is actually trigonal antiprismatic. 2.3. Order disorder and nonstoichiometry When M cations are large enough to adopt eight-fold coordination to oxygen, order-disorder transformations and significant ranges of stoichiometry are observed for the pyrochlore structure (Perez Y Jorba 1962, Faucher and Caro 1975). In fact, Zr and H f are the only M cations large enough for these related phenomena to be exhibited (Roth 1956, Michel et al. 1974, 1976, Collongues et al. 1961, 1965). Figure 3 shows the R 2 0 3 - Z r O Œphase diagram for R = Gd and Sm (Collongues 1963). Twophase regions, which presumably must exist, are not shown between the pyrochlore and disordered fluorite region. Thus, it is reasonable to assume that the stability field for a single-phase pyrochlore is somewhat smaller than indicated in fig. 3.

RARE EARTH PYROCHLORES

229

rlä~LE 2 Oxygen x(48f) parameters for R2M20 7 pyrochlores (Bo origin). Compound

x(48f)

Rel.

Sm2Ti20 v Eu2 Ti2 0 7 Gd2Ti20 7 Dy 2Ti 20 7 Er2Ti207 Lu 2Ti 2 0 7 Y2Ti2 O7

0.327 0.327 0.322 0.323 0.331 0.330 0.328

Knop et al. (1965, I969)

Er 2 Mn 2 0 7 Y2MnzO7

0.328 0.327

Subramanian et al. (1988)

Sm2Mo20 7 Eu2Mo20 v Tb2Mo20 7 Y2Mo207

0.343 0.322 0.3363 0.3382

Hubert (1975a,b)

Nd2Ru20 7 Lu2Ru20 v Pr 2 Ru 2 0 v Nd2 Ru2 07 Sm2 Ru2 07 Dy 2 Ru a 0 7 Y2Ru207

0.330 0.336 0.325 0.328 0.329 0.332 0.333

Sleight and Bouchard (1972)

Sm2Pt20 7 Eu 2 Pt 2 0 7 ErŒPt 2 O7 Y2 Pt2 O7

0.325 0.329 0.334 0.329

Hoekstra and Siegel (1968)

La2Sn20 7 Sm2Sn207 Eu2Sn20 7 Gd2Sn207 Er 2 Sn 2 0 7 Y2Sn2Ov

0.325 0.333 0.335 0.332 0.335 0.338

Brisse and Knop (1968)

8c2Si20 7 Ins Si2 O7

0.325 0.329

Reid et al. (1977)

Greedan et al. (1991) Reimers et al. (1988)

Kanno (1992)

Pyrochlore microdomains have been observed in samples quenched from a region reported to be single-phase disordered fluorite.

3. Pyrochlore composition 3.1.

A39+M4+ 07

One might expect that any M 4 + cation that can take on octahedral coordination t o o x y g e n c o u l d f o r m a n R~ + M24+ 0 7 c o m p o u n d w i t h a p y r o c h l o r e s t r u c t u r e for

230

M.A. S U B R A M A N I A N and A,W. S L E I G H T

ißJ/r-)x,0,~

ns

Fig. 2. The pyrochlore structure as interpenetrating networks of A 2 0 and M 2 0 6 .

3

M2Zr207 C]

2500 !

/

\

\

I

T°c

2000

i i

i

I

\ p!

: r 0 2 - Sm203 !

', f

1450

ùi

rrO2_Gd203

1000 -' 10

~

!

,

i '~,

i....~~I P i

i = i 20

CI

:l i il I 3 0 33.3

mol. % MzO 3

i i .= 40

50

Fig. 3. R 2 0 3 - Z r O z phase diagram for R = Sm or Gd. P denotes pyrochlore and C 1 disordered fluorite.

any R 3 + cation. However, there are some further constraints as is obvious from the stability field shown in fig. 4. The size ratio R/M must be neither too large nor too small. Table 3 gives a complete listing of all known R23+ M24+ Ov pyrochlores with their room-temperature cell dimensions. Nearly all M 4 ÷ cations which take on octahedral coordination in binary oxides are represented. However, attempts to use N b 4÷ or W 4+ have failed to date. Some of the phases listed in table 3 require high pressure for their synthesis. In some cases, the pressure was required to stabilize the coordination number, e.g., Si 4+. In other cases, the pressure was required to stabilize the oxidation state, e.g., Cr 4+. Some of the obvious gaps in table 3 could presumably be

RARE EARTH PYROCHLORES

231

1.20

1.12

"6

~Pr --Nd

ò

--Sm

o -o

--Gd

o

A

o

Eu 104

2

n

o

Tb --Dy y

n

o

A

o

o

n

oo

o o

o

oo

n

o

o

oo

tx

o

o

oo

zx

o

o

oo

A

o

o

oo

zx

o

o

o

o

A

o o

o

o

o

o n o o

o

o

A zx

o o o

~ o °o o

~er

> o~ v

o

o

--Tray

oo

o

o

8

8

oo o o

o

o~88

o

o o°

0.96

-In

n

n

0.88 Hf

Si

0.80

Mn Ge

A__~ ,~, 0.40

Q52

er

V

I I 0.54

0.56

Ril

[_, I 0.58

060

Ti

Pd

I

I 0.161

~c 2 +

Ru Pf Ir Os

I ~ff 0.62

I 0.63

064

0.65

sn ~r 0.68

0.72

P0 076

080

rB4+(~,) [Vl fold coord.] Fig. 4. Stability field diagram for Az3+ M24+ 0 7 (R = rare earth, Sc or In) pyrochlores. A denotes highpressure synthesis.

eliminated if there was a particular interest in examining the missing compound, e.g., where M is Tc, Rh or Pd. On the other hand, attempts to expand the series where M is V or Cr have failed (Subramanian et al. 1983). 3.2. Complex formulations Rare earths are found in pyrochlores other than the simple R23+ M24+ 07 formulao3+ A3+ M 2 0 7 solid solution where A 3 + might tion. A trivial example would be an x,~ 2 _xrXx be Bi, T1 or In. In fact, such solid solutions where A 3 + is Bi or T1 have been of some interest because Bi~ + M~ + 0 7 and TI~ + Mä + 0 7 pyrochlores are metallic when M is Ru or Ir, whereas analogous R2 M2 0 7 pyrochlores are semiconducting (Sleight and Bouchard 1972). Pyrochlore solid solutions of the type R2 xBixTi207 have been reported by Bamberger et al. (1985). Another group of complex pyrochlores containing rare earth cations might be viewed as solid solutions between R32+ M~ + 0 7 and A~ + M25+ 0 7 (table 4). Such phases appear to have a narrow homogeneity range close to (A2+R3+)(M»+M4+)OT. Thus, they have been referred to as compounds rather than solid solutions. Still other formulations based on rare earths partially occupying the A site are (A+R3+Sb~+)O7 and ( C d 2+ R 3+ )(Fel/2Nb3/2)O7 3+ 5+ (table 4). Very extensive series of rare earth pyrochlores exist based on substitutions at the M site: R2(M3+MS+)O7 and R2(M3/+ W2/3)O7. 6-F In the A3+ (M3+M»+)O7 series, A 3+ may be Bi 3 + as well as a rare earth; M 3 + may be Cr, Fe, Ga or a rare earth; and M »+ may be Nb, Ga, or Sb (table 4). Pyrochlores containing Ce 4+ are also reported (McCauley and Hummel 1980): (CaCe)Ti20 v and (CdCe)Ti207 (table 4). Wakiya et al. (1991) have reported a cubic

232

M.A. S U B R A M A N I A N and A.W. SLEIGHT

~2

õ

ù~ o

d c~

©

# ©

ee~

eq

RARE EARTH PYROCHLORES

233

TABCE4 Complex pyrochlores containing a rare earth cation. Compound

a o (A)

Rel.*

Pr2(CrSb)O 7 Nd2(CrSb)O 7 Sm2(CrSb)O7 Eu2(CrSb)O v Gd2(CrSb)O 7 Tb2(CrSb)O 7 Dy2(CrSb)O 7 Ho2(CrSb)O 7 Er2(CrSb)O v Yb2(CrSb)O 7 Y2(CrSb)O7

10.405 10.370 10.340 10.330 10.300 10.260 10.250 10.220 10.200 10.160 10.215

[1] [1] [1] [1] [i] El] [11 [1] [11 [1] [1]

Pr2(FeSb)O7 Nd2(FeSb)O 7 Sm2(FeSb)O7 Eu2(FeSb)O7 Gdz(FeSb)O v Tb2(FeSb)O 7 Dy2(FeSb)O 7 Ho2(FeSb)O 7 Er2(FeSb)O 7 Yb2(FeSb)O 7 Y2(FeSb)O v

10.405 10.375 10.305 10.295 10.265 10.230 10.210 10.191 10.160 10.135 10.180

[1] [i] [11 [11 [lJ [11 [1] [1] [11 [11 [I 1

Nd2(NdSb)O v Sm2(SmSb)O v Eu2(EuSb)O7 Gd2 (GdSb)O 7 Tb2(TbSb)O v Dy2(DySb)O7 Ho2(HoSb)O 7 Er2(ErSb)O 7 Tm2(TmSb)O 7 Yb2(YbSb)O7 Lu2(LuSb)Ov Y2(YSb)Ov

10.820 10.720 10.668 10.638 10.550 10.522 10.495 10.448 10.410 10.368 10.359 10.487

[2, 3] [2, 3] [2, 3] [2, 3] [2, 3] [2, 3] [2, 31 [2, 3] [2, 3] F2, 3] [2, 3] [2, 3]

Nd2(GaSb)O7 Gd2(GaSb)O 7 Y2(GaSb)O 7

10.37 I0.26 10.18

[1] [1] [l I

R2(M2/s M4/3)O~ Y2(Mn2/3 Mo4/3)O7

Bi2(YNb)O 7 Bi2 (LuNb)O 7

10.95 10.83

[4] 1-4]

Y2(Mn2/3 Nb4/3)O7 Y2(Mn2/3 Ta4/3)O7

La2 (ScNb)O v Nd2(ScNb)Ov Sm2(ScNb)O 7 Ho2(ScNb)O v 81112(ScTa)O 7

10.672 10.534 10.497 I0.332 10.466

[5] [5] [5] [6] [6]

(RCd)(MS + W 6~ )Ov (GdCd)(VW)O7 (TbCd)(VW)O7 (DyCd)(VW)O7 (HoCd)(VW)O 7

R2(MS +Sbs+)O 7

Compound

a 0 (A,)

Ref.*

10.234 10.207 10.191 10.165 10.137 10.118 10.393 10.078 10.164

[71 [71 [7] [7] [7] [7] [71 [7] [7J

s+ W2/3 6+ )07 R2 (M4/3

Gd2(V4/3 W2/3)O 7

Tb2 (V4/3 W2/3)0 7 Dy2(V4/3W2/3)O 7

Ho2(V4/3 Wz/3)O 7 Er2(V4/3 W2/3)O 7 Tm2(V4j3 W2/3)O ~ Yb2(V4/3 W2/3)O 7 Lu2 (V4/3W2/3)0 7 y2(v4/3 W2/3)07 Gd(Mn4/3 W2/3)O7 Dy(Mn~/a W2/3)O 7 Ho(Mn«/3 W2/3)O 7 Er(Mn4/3 W2/3)O7 Tm(Mn4/3 W2/3)O 7 Yb(Mn«/3 W2/3)O 7 Gd(Fe«/s Wz/3)O 7 Dy(Fe4/3 W2/3)O7 H°(Fe'~/3 W2/3)O7 Er(F'e4/3W2/3)O7 Tm(Fe4/3 W2/3)O7 Yb(Fe4/3 W2/3)O7 Y(Fe4/3 W2/3)Ov 2+

10.363 (92.87) 10.291 (92.73) 10.264 (92.67) 10.215 (92.62) 10.165 (92.58) 10.135 (92.53) 10.337 (92.88) 10.279 (92.77) 10.245 (92.69) 10.219 (92.61) 10.175 (92.60) 10.145 (92.50) 10.232 (92.68)

[8, 9] [8, 9] [8, 9] [8,9] [8, 91 [8, 9] [10] [10] [10] [t0] [10] [10] [_10]

$+

10.30 (91.88)

[11] [11] [11]

10.244 10.255 10.218 10.201

[8] [8] [8] [8]

234

M.A. SUBRAMANIAN and A.W. SLEIGHT TABLE4 (cont'd)

Compound

ao (A_)

Rel.*

(ErCd)(VW)O7 (TmCd)(VW)O 7 (YbCd)(VW)O7 (LuCd)(VW)O7 (YCd)(VW)O7 (GdCd)(CrW)O7 (TbCd)(CrW)O7 (DyCd)(CrW)O7 (HoCd)(CrW)O7 (ErCd)(CrW)O7 (TmCd)(CrW)O7 (YbCd)(CrW)O7 (LuCd)(CrW)O 7 (YCd)(CrW)O 7

10.184 10.174 10.162 10.150 10.205 10.194 10.176 10.165 10.152 10.134 10.126 10.115 10.104 10.149

[8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8]

(GdCd)(MnW)O7 (TbCd)(MnW)O7 (DyCd)(MnW)Ov (HoCd)(MnW)O7 (ErCd)(MnW)O 7 (TmCd)(MnW)O7 (YbCd)(MnW)O7 (LuCd)(MnW)O7 (YCd)(MnW)O7 (GdCd)(FeW)O7 (TbCd)(FeW)O v (DyCd)(FeW)O 7 (HoCd)(FeW)O7 (ErCd)(FeW)O 7 (TmCd)(FeW)O 7 (YbCd)(FeW)O7 (LuCd)(FeW)O7 (YCd)(FeW)O7

10.249 10.226 10.217 10.200 10.182 10.174 10.161 10.153 10.203 10.252 10.233 10.222 10.206 10.194 10.184 10.170 10.159 10.205

[8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8] [8]

(RCd)(Felm Nb3/2)O7 (CdLa)(Fel/2 Nb3/z)O 7 (CdNd)(FelnNb3n)O 7 (CdSm)(Fen2 Nb3/2)O 7 (CdGd)(Feln Nban)O ~ (CdDy)(Fel/2 Nb3/2)O 7 (CdHo)(Fe~/2 Nb3/2)O 7 (CdEr)(Fea/zNb3n)O7 (CdTm)(Fe~/2Nb3/z)O 7 (CdYb)(Fea/2Nb3/2)O 7 (CdLu)(FelnNb3/z)O7 (CdY)(Fetn Nb3n)O 7

10.429 10.381 10.354 10.341 10.317 10.303 10.297 10.285 10.278 10.275 10.305

[12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12]

(RA)(M4 + NbS +)O 7 (CaLa)(TiNb)O7 (CdLa)(TiNb)O7

10.374 10.350

[12] [12]

3+

5+

Compound

ao (A)

Re[*

(PbLa)(TiNb)O7 (CdNd)(TiNb)O-/ (CaSm)(TiNb)O7 (CdSm)(TiNb)O7 (PbSm)(TiNb)O7 (CaEr)(TiNb)O7 (CaLa)(ZrNb)O 7 (SrLa)(ZrNb)O7 (BaLa)(ZrNb)O 7 (CdLa)(ZrNb)O7 (PbLa)(ZrNb)O 7 (CaSm)(ZrNb)O7 (SrSm)(ZrNb)O7 (CdSm)(ZrNb)O 7 (PbSm)(ZrNb)O7 (CaEr)(ZrNb)O7 (CaLa)(HfNb)O7 (SrLa)(HfNb)O7 (BaLa)(HfNb)O7 (CdLa)(HfNb)O7 (PbLa)(HfNb)O7 (CaSm)(HfNb)O7 (SrSm)(HfNb)Ov (BaSm)(HfNb)O7 (CdSm)(HfNb)O 7 (PbSm)(HfNb)O7 (CaEr)(HfNb)O7

10.383 10.315 10.318 10.278 10.369 10.219 10.610 10.642 10.710 10.530 10.668 10.518 10.651 t0.431 t0.620 10.371 10.600 10.635 10.700 10.518 10.662 10.593 10.579 10.537 10.438 10.652 10.316

[12] [12] [12] [12] [12] [12] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13]

(RA)(M4+ MS+)07 (CaLa)(SnNb)O 7 (SrLa)(SnNb)O7 (BaLa)(SnNb)O v (CdLa)(SnNb)O 7 (PbLa)(SnNb)O: (CaSm)(SnNb)O7 (SrSm)(SnNb)O7 (CdSm)(SnNb)O7 (PbSm)(SnNb)O7 (CaEr)(SnNb)O•

10.450 10.562 10.600 10.410 10.580 10.541 10.444 10.480 t0.538 10.385

[13] [13] [13] [13] [13] [13] [13] [13] [13] [13]

(CaLa)(TiTa)O7 (CdLa)(TiTa)O7 (PbLa)(TiTa)O7 (CaSm)(TiTa)O7 (CdSm)(TiTa)Ov (PbSm)(TiTa)O 7 (CaEr)(TiTa)O7 (CdNd)(TiTa)O7 (PbCe)(TiTa)O 7

10.365 10.334 10.376 10.295 10.247 10.337 10.222 10.30 10.34

[13] [13] [13] [13] [13] [13] [13] [13] [13]

RARE EARTH PYROCHLORES

235

TABLE4 (cont'd) Compound

a o (~)

Ref.*

Compound

ao (~)

Ref.*

(PbPr)(TiTa)Ov (PbNd)(TiTa)O7 (PbSm)(TiTa)O7 (PbEu)(TiTa)O7 (PbBi)(TiTa)O7 (ZnNd)(TiTa)Ov

10.39 10.39 10.36 10.36 10.39 10.26

[13] [13] [13] [13] [13] [13]

(CaLa)(HfTa)O7 (SrLa)(HfTa)O 7 (BaLa)(HfTa)O 7 (CdLa)(HfTa)O 7 (PbLa)(HfTa)O7 (CaSm)(HfTa)O7 (CdSm)(HfTa)O v (PbSm)(HfTa)O v (CaEr)(HfTa)O 7

10.600 10.631 10.661 10.520 10.660 10.498 10.371 10.570 10.430

[13] [13] [13] [13] [13] [13] [13] [13] [13]

(RM2 +)(pb4+ BiS+)O7 (LaCa)(PbBi)O7 (NdCa)(PbBi)O7 (SmCa)(PbBi)O7 (EuCa)(PbBi)O7 (GdCa)(PbBi)O7 (HoCa)(PbBi)O7 (ErCa)(PbBi)O7

10.889 10.842 10.810 10.794 10.783 10.746 10.720

[14] [14] [14] [14] [14] [14] [14]

(LaSr)(PbBi)O7 (NdSr)(PbBi)O7 (SmSr)(PbBi)Ov (GdSr)(PbBi)O7 (DySr)(PbBi)O7 (Laßa)(PbBi)O7

10.982 10.920 10.891 10.860 10.831 11.119

[14] [14] [14] [14] [14] [14]

(CaLa)(SnTa)O7 (SrLa)(SnTa)O 7 (BaLa)(SnTa)O7 (CdLa)(SnTa)O 7 (PbLa)(SnTa)O7 (CaLa)(SnTa)O 7 (CaSm)(SnTa)O7 (CdSm)(SnTa)Ov (PbSm)(SnTa)O 7 (CaEr)(SnTa)O7 (CaEr)(ZrTa)O 7

10.448 10.540 10.572 10.389 10.564 10.448 10.430 10.332 10.450 10.388 10.440

[13] [13] [13] [13] [13] [13] [13] [13] [13] [13] [13]

(A2+ Ce4 +)Ti207 (CdCe)Tiz O7 (CaCe)Ti2 O7

10.136 10.211

[15] [15]

(RNa+)(Ti4 + NbS +)OöF (CeNa)(TiNb)O6 F (Pra)(TiNb)O6 F (NdNa)(TiNb)O6 F (EuNa)(TiNb)O6 F (GdNa)(TiNb)O6F (YbNa)(TiNb)O6 F (YNa)(TiNb)O6 F

10.374 10.365 10.347 10.309 10.304 10.255 10.262

[16] [16] [16] [16] [16] [16] [16]

10.270 10.254 10.204 10.190 10.140 10.184

[13] [13] [13] [13] [13] [13]

(RCd z + )(Tl 4 + )z O« F (NdCd)(Ti)z O6 F (EuCd)(Ti)2 O6 F (GdCd)(Ti)2 O6 F (YbCd)(Ti)2 O6 F (YCd)(Ti)2 O6 F

10.240 10.198 t0.184 10.119 10.140

[16] [16] [16] [16] [16]

(RMn 2 + )(Mn z + Sb s + )06

(EuMn)(MnSb)O6 (GdMn)(MnSb)O6 (DyMn)(MnSb)O6 (HoMn)(MnSb)O6 (YbMn)(MnSb)O6 (YMn)(MnSb)O6

*References: [1] Montmory and Bertaut (1961); [2] Nath (1970); [3] Faurie et al. (1976); [4] Smolenskii et al. (1974); [5] Alpress and Rossell (1979); [6] Filipev et al. (1982); [7] Subramanian et al. (1979); [8] Subramanian et al. (1983); [9] Bazuev et al. (1983); [10] Basile et al. (1977); [11] Bazuev et al. (1984, 1985, 1987, 1989); [12] Fedorov et al. (1976); [13] Belyaev et al. (1972a, b, 1978), Belyaev and Sharmova (1975); [14] Subramanian (1990); [15] McCauley and Hummel (1980); [16] Grannec et al. (1974).

p y r o c h l o r e of the type C a 2 x C e 2 _ 2 x S n 2 0 7 over the r a n g e 0.35~
236

M.A. SUBRAMANIAN and A.W. SLEIGHT

4. Electrical properties P y r o c h l o r e s of the t y p e R23+ M~ + 0 7 w h e r e M is f r o m g r o u p f o u r (Si, Ge, Sn, Pb, Ti, Zr, a n d H f ) a r e e]ectrica] i n s u l a t o r s , as e x p e c t e d for these d ° o r d t° e l e c t r o n i c c o n f i g u r a t i o n s . W h e n the d shell is p a r t i a l l y filled, s e m i c o n d u c t i n g p r o p e r t i e s are n o r m a l l y o b s e r v e d w i t h g e n e r a l l y l o w a c t i v a t i o n e n e r g i e s (table 5). E x c e p t for t h e l o w - s p i n d 6 s i t u a t i o n , the c o n d u c t i o n m e c h a n i s m is m o s t likely e l e c t r o n h o p p i n g b e t w e e n t h e M sites. F o r Pt4+(5d6), the t2g b a n d is filled a n d the a c t i v a t i o n e n e r g y is p r e s u m a b l y r e l a t e d to e x c i t a t i o n i n t o the e m p t y eg b a n d . T h e fact t h a t the c o n d u c t i o n m e c h a n i s m of the P t ~ ÷ c o m p o u n d s differs f r o m the o t h e r s is s u p p o r t e d b y the TABLE 5 Electrical data. Compound

p (f~ cm)a

Pr 2Ru a O 7(P)C Nd 2 Ru 2O7(c ) EU 2 Ru z O 7(c)

1.0 1.6 1.2

G d 2 R u 2 0 7 (c)

0.2

E~ (eV)b

Rel.*

Sm2Mo2Ov(p)

1 10 28x 10 2 3.5x10 2 7.0 x 10 .2 2.0 x 10-1 30 1 x 10 2 2 x 10 -~1 7 x 10 2 6 x 10 -2

0.2 0.i 1 0.3 0.1 0.12 0.3 0.08 0.08 0.09 0.07 0.17 0.01 0.14 0.3 0.01

[ l] [1] [ 1] [ 1] [1] El] [1] [1] [1] [i] [1] [2] [2] [2] [3]

Eu2Mo207(p)

5 N 10 -2

0.0l

[4]

Gd2Mo2Ov(p) Dy2Mo207(p)

1 x 10 2 3 x 10 2

0.0l 0.02

2 x 10 2

0,05

1010 3.7 x 103 1 x 106 8 x 107 3 x 10v 2 × 107 2 × 108 3 x 106

0.5 0.25 0.37 0.45 0.47 0.48 0.51 0.38

[4] [4] [4] [5] [1] [3] [3] [3] [3] [3] [3]

Yb2Ru207(c ) Y2Ru2OT(p) Nd2Ir2Ov(c ) Sm2Ir2Ov(c) Eu2Ir2Ov(c ) Dy2Ir2OT(C ) Y2Ir207(p) Gd2Os207(p)

Nd2Pt207(c ) Gd2Pt207(c )

Er2Mo2OT(p)

La2Pb207(p) Gd2Ti2OT(p) Dy2Mn2Ov(p) Ho2Mn2OT(p) Er2Mn2OT(p) Tm 2 Mn 207 (p) Lu2Mn207(p) Y2Mn207(p)

~Resistivity at 298 K. bActivation energy determined from the temperature dependence of the electrical resistivity data. Cc means data from single crystal; p means data taken from powder compact. *References: El] Sleight and Gillson (1970); [2] Shaplygin and Lazarev (1973); Lazarev and Shaplygin (1978a, b); [3] Subramanian et al. (1980b); [4] Greedan et al. (1987); [5] Sleight (1969).

RARE EARTH PYROCHLORES

237

properties when A 3+ is not a rare earth. Such compounds are metallic for Ru 4+, Rh 4+, Os 4+ and Ir 4+ but semiconducting for Pt 4+ (Sleight 1968b, Subramanian et al. 1983). Mixing narrow d bands with the broad post-transition metal 6s or 6p bands broadens the t2g band to allow metallic conductivity, but it does not destroy the gap between the t2g and eg bands. More evidence for a hopping model for the cases other than Pt 4 ÷ comes from the sign of the Seebeck coefficient and the Hall coefficient. Both measurements were made on Eu2 Ru2 07 crystals and opposite signs were observed (Sleight and Gillson 1970). This should not be the case for classical semiconductors and is very strong evidence in favor of electron hopping. Mixed oxides of Ru ~+ and Ir 4+ are frequently metallic as are RuO2 and IrO2. The more localized electron behavior observed when these cations are located in the pyrochlore structure is presumably due to structural considerations. The M O M bond angle in pyrochlores is not the most favorable for delocalization of t2g electrons. The most favorable angle would be 180 °, whereas this angle tends to be about 130 ° in the pyrochlore structure. This is a significant deviation from 180 ° and is therefore presumably adequate to cause an activated hopping process in the absence of mixing with post-transition metal s or p bands. Pyrochlores of the type R~ ÷ M~ ÷ 07 are very close to the delocalized-electron limit when M 4+ is 4d 1 s or 5d 1 2. Thus, it is not surprising that there has been some controversy as to whether these compounds are metallic or low-activation-energy semiconductors. Lazarev and Shaplygin (1978a,b) have argued that R 2 M 2 0 7 (M = Os, Ru and Ir) compounds are metallic based on infrared spectra and the high roomtemperature conductivities. However, their arguments are not convincing. Crystals of many of these compounds have been grown hydrothermally, and four-probe electrical resistivity measurements invariably show electrical resistivity decreasing with increasing temperature (Sleight and Gillson 1970). Furthermore, thermoelectric-power measurements always show a value too high for a metallic compound, e.g., +440 for Eu2 Ru2 07, + 320 for Y2 Ru2 0 7 and + 50 for Yz Irz 07 (Sleight and Gillson 1970). Spectroscopic studies (XPS, UPS, and HREELS) have also been used to substantiate localized electrons for R2 Ru2 07 pyrochlores and delocalized electrons for Bi 2 Ru2 0 7 (Cox et al. 1983, 1986). Magnetic susceptibility studies on R2Ru2 0 7 pyrochlores further confirm localized d-electrons (Leonard et al. 1962, Rosset and Ray 1962, Greedan 1991). Ehmann and Kemmler-Sack (1985) conclude from infrared spectra that R2 Ru2 07 pyrochlores are semiconducting. Several workers have attempted to find the metal insulator transition in Bi2 xRxRu207 pyrochlore solid solutions. Spectroscopic studies (Cox et al. 1986) on the Bi2_xGdxRu207 system indicate a transition at about x -- 1.55, and electrical resistivity data on the Bi2 xYxRU207 system suggest a very similar value (Kanno 1992). The transition is not well-defined in either case. The partial substitution of Ag ÷ or Cu ÷ for A in A2Ru2Ov pyrochlores has been reported (Bouchard 1971, Haouzi et al. 1986) to have a significant effect on the electrical properties of these materials. Such a substitution seems reasonable because both C u 2 0 and A g 2 0 have the same structure as the A 2 0 ' network of the pyrochlore structure. No pyrochlore of the type C u 2 M 2 0 7 or AgzMzO7 has ever been reported, but partial substitution of Ag ÷ or Cu ÷ on the A site can occur. For example, it has been reported that substitution of Cu + for Nd 3÷ can occur in the

238

M.A. SUBRAMANIAN and A.W. SLEIGHT

0.56

u

t

7--T

i

--3

0.52

0.48

-

0 O

Eo 0.44

0.40

-

Q 0

0.36 1.21

I 1.22

I I I I 1.23 1.24 1,25 1.26 Pauling Electronegativity of R

I 1.27

1.28

Fig, 5. Activation energy (from electrical resistivity data) versus the Pauling electronegativity value of R for the R2Mn207 pyrochlores.

N d 2 x C u x R u 2 0 7 _ x system up to a value of x of about 0.375. Such a substitution

apparently leads to metallic behavior (Haouzi et al. 1986). Presumably, the proximity of the Cu 4s and 3d bands has a similar effect on the Ru 4d band as do the 6s or 6p bands of Tl, Pb, and Bi. For R 2 M 2 0 7 pyrochlores where the M cation has the electronic configuration d 2 (e.g., Mo4+), d » (e.g., Mn4+), d 4 (e.g., Ru ~+) or d s (e.g., Ir4+), there is a trend toward greater delocalization of the d-electrons as the R cation becomes more electropositive (Subramanian et al. 1980b, Greedan et al. 1987, Subramanian et al. 1988, Sleight and Gillson 1971). This trend is illustrated for the R 2 Mn 2 07 series in fig. 5. An analogous situation exists for AMO» compounds with the perovskite structure. For pyrochlores of the type R 2 M o 2 0 7 , the increased delocalization apparently results in metallic conductivity for the larger rare earth cations. Oxygen ion conductivity has also been studied in various pyrochlores, such as G d 2 Z r 2 0 7 (van Dijk et al. 1983). The structure of this compound takes on a disordered fluorite structure if the compound is quenched from high temperatures. Such a compound may be represented as MOl.7s , indicating a high level of oxygen vacancies relative to the ideal M O 2 fluorite formula. Higher ionic conductivity is observed for the ordered pyrochlore form, where ideally there are no oxygen vacancies. This suggests that the defect concentration in the pyrochlore modification can remain significant. At 1000 K, a conductivity of 68 (~cm) 1 is reported for G d 2 Z r 2 0 7 (van Dijk et al. 1983).

5. Magnetic and specific heat properties Ferromagnetism has been reported for R 2 M 2 0 7 pyrochlores where M is V (Bazuev et al. 1977, Shin-ike et al. 1977, Soderholm and Greedan 1979, Soderholm et al. 1980, 1982), Mn (Fujinika et al. 1979, Subramanian et al. 1988, Reimers et al. 1991) or Mo (Subramanian et al. 1980b, Mandiram and Gopalakrishman 1980, Ranganathan et al. 1983, Sato et al. 1986, Ali et al. 1989, Greedan 1991). However, the magnetic properties are complex, which, at least in part, is due to frustration. The M cations

RARE EARTH PYROCHLORES

239

may be viewed as clustered into tetrahedral units which share corners to form infinite, intersecting chains. Antiferromagnetic M M interactions would be expected to dominate via M - O - M superexchange. However, these interactions are frustrated by the tetrahedral arrangement. Spins at two points of the tetrahedron may coupte in a antiparallel fashion, but it is then impossibte for the remaining two atoms of the tetrahedron to align their spins antiparallel to the first two. This frustration of the antiferromagnetic interaction apparently frequently allows the ferromagnetic interaction, which might be expected to be weaker, to dominate. The only R2V2Ov pyrochlores reported are for R = Tm, Yb and Lu (Shin-ike et al. 1977). These three compounds are ferromagnetic, as indicated in table 6. In addition, magnetic properties have been investigated in the solid solutions Lu2-xYxV2OT, Lu 2 xScxV207, and R2V 2 xMoxO7 (table 6). Pyrochlores of the type A2Mn207 are known for Dy, Ho, Eu, Tm, Yb, Lu, Y, Sc, In and T1 (Subramanian et al. 1988). All exhibit ferromagnetic behavior (table 6). On the other hand, there are indications that these compounds do not possess a spontaneous moment in the absence of an applied field. Pyrochlores of the type R 2 M o 2 0 7 are reported for R = Nd Lu and Y. However, ferromagnetism apparentty only occurs in the case of the three largest rare earth cations, i.e. Nd, Sm and Gd (Greedan 1991). The highest electrical conductivity also occurs for these ferromagnetic pyrochlores, which are possibly best described as metals. It is interesting that R 2 Mo 2 07 pyrochlores are the only known examples of magnetic order based on Mo 4 + oxides. Spin-glass behavior is reported for Y2 M o 2 0 7 (Sato and Greedan 1987). Specific heat and neutron diffraction studies on Y2 Mo2 07 indicate no magnetic order down to 4.2K. Solid solutions of the type La 2 xY:~Mo20 7 have also been prepared, and their magnetic properties have been determined (table 6). Ferromagnetism has also been reported for pyrochlores of the type R 2 Cr 3 + Sb 5 + 0 7 (table 6). The magnetic properties of R2Ru207 pyrochlores were investigated for R = Pf, Nd, Gd, Tb, Dy, Ho and Y. The magnetic contribution of Ru ~ + is less than expected and has been attributed to a high degree of covalency in these compounds. Specific heat measurements on Y2 Ru2 07, Nd2 Ru2 07, Eu2 Ir2 O 7 and Lu2 Ir2 0 7 also indicate no magnetic ordering down to 4.2 K (Blacklock et al. 1980, Blacklock and White 1980). The magnetic properties of R 2 M 2 07 have also been studied where M is a diamagnetic cation such as Ti 4+ (Townsend and Crossley 1968), Zr ~+, Sn 4+ (Mitina et al. 1970) or Ga 3 +-Sb 5 + (Blöte et al. 1969). Magnetic-ordering temperatures have sometimes been detected at very low temperatures: 1.4K for Dy2Ti2OT, 1.3 K for Ho2Ti207, 1.25 K for Er2Ti207, 0.21 K for Yb2Ti2OT, 0.37 K for N d 2 Z r 2 0 7 , 0.91 K for Nd2Sn207 and 1.2 K for Nd2GaSbO7.

6. Speetroseopy 6.1. Vibrational spectroscopy Theoretical analysis of the normal vibrations of the pyrochlore structure predicts 26 normal modes (McCaffrey et al. 1971, McCauley 1973, Vandenborre et al. 1981).

240

M.A. S U B R A M A N I A N and A.W. S L E I G H T TAULE 6 Magnetic data for some R 2 M 2 0 v compounds: M = V, Mn or Mo. Compound

Tc (K)

0c (K)

TmzV207 Yb2V207 Lu2 V2 0 7 Yo.,~oLul,6o V2 0 7 Yo.8oLu1.2oV/Ov Sco.2o LUl.sO Vz O7 Sco.4o Lu~.6o V2 0 7 Sco.6oLul.4oV20 v Sco. so Lu 1.zoV 2 0 v Scl.ooLul.ooV20 v

71.4 73.2 72.5 70 71.5 69 67 56 46 36

+10 +11 + 83.3 + 79 + 90.2 + 93 + 83 +74 + 62 +49

Tb2Mn20 7 Dy2 Mn2 0 7 Ho2 Mn2 0 7 Er2Mn20 v Tm2Mn20 7 Yb2Mn20 v LuΠM n 2 0 7 Y2 M n 2 0 7 Sc2 Mn2 O7

38 40 37 35 30 35 23 20 15

+ 33 + 33 +40 + 56 +41 + 70 + 50

Nd2Mo20 7 Sm2Mo20 7 Gd2Mo20 7 Tb2 Mo2 0 7 Dy2Mo20 7 Ho2 Mo2 0 7 Er 2 Mo z O v Tm2Mo20 v Yb 2 Mo z 0 7 Y2Mo207

96 93 83 7('?)

Pr2 CrSbO7 Nd 2 CrSbO 7 Sm2CrSbO7 Eu z CrSbO v Gd2CrSbOv Tb 2 CrSbO v Dy2 CrSbOv Ho2 CrSbOv Er 2 CrSbO v Tm2 CrSbOù YbzCrSbO v Y2 CrSbOv

/x~t.f

+7 +8 + 12 +4 +12 + 15 + 16 + 10 + 10 +O + 10 + 15

Ret.* [1 4]

1,92 1,96 1,90 1,94 1,96 2,12 2.24 2.34

0.93 0.95 0.98 1.00 0.93 0.77

14.4 14.4 13.3 10.4 7.6 4.9 5.4

+115 +121 + 17 + 10 +5 12 --32 25 --öl

/-G,

[5, 6]

2.1 2.3 1.5

[4, 7]

(21.8) (26.7) (27.0) (22.7) (14.6) (4. t) (1.06) [8]

*References: [1] Soderholm et al. (1982); [2] Shin-ike et al, (1977); [3] Soderholm and Greedan (1979); [4] Greedan (1991); [5] Subramanian et al. (1988); [6] Troyanchuk and Derkachenko (1988); E7] Sato et al. (1986); [8] Bongers and r a n Meurs (1967).

RARE EARTtt PYROCHLORES

24I

Seven of these are IR active, and six are Raman active. Trends in the position of these bands are discussed by Vandenborre et al. (1983) and Subramanian et al. (1983). Raman spectroscopy has been particularly useful in studying order-disorder phenomena in pyrochlores (Michel et al. 1976). 6.2. UV visible spectroscopy lnteratomic electronic transitions can give rise to very complex absorption spectra dependent on the rare earth cation or the transition metal cation present. In addition, charge transfer bands are found at high energies. These charge transfer bands are generally in the UV; however in some cases, such as the R2PbzO7 pyrochlores, the charge transfer band extends well into the visible region. Most of the information available for electronic absorption spectroscopy in pyrochlores is found in publications concerned with luminescent properties. 6.3. Luminescent properties McCauley (1969) studied the luminescence of Eu 3 + in R 2 M 207 pyrochlores where M was Ti, Zr, Hf, Sn or a mixture of Sn and Ti. Brixner (1984) described the luminescence of Eu 3 +, Tb 3 + and Ti 4 + in the pyrochlore La 2 Hf2 07. The absorption and luminescence spectra of Nd 3 + in Y2Ti207 and G d 2 T i 2 0 7 single crystals were given by Antonov et al. (1977). Berdowski and Blasse (1986) have reported on the luminescence and energy-transfer properties of Eu2Ti2 07 and Gd2Ti2OT:Eu. 6.4. Mössbauer spectroscopy Mössbauer studies of Fe in the RŒFeSbO7 series (Knop et al. 1968, Snee et al. 1977, Sundararajan et al. 1983) and of Sn in the R2Sn207 series (Belyaev et al. 1969, Loebenstein et al. 1970, Calage and Pannetier 1977, Snee et al. 1977) give rise to similar conclusions. In both cases, the isomer shift is essentially constant as the rare earth is varied. On the other hand, there are definite trends with the quadrupole splitting as the rare earth cation is varied (fig. 6). Clearly the environment of the M cation is becoming closer to ideal octahedral as the rare earth cation increases in size. This is to be expected since the oxygen positional parameter, x(O), decreases steadily towards its octahedral value of 0.3125 as the rare earth cation size increases. Mössbauer studies have also been carried out for Eu, Gd, and Dy in various R 2 M 2 O 7 pyrochlores (Bauminger et al. 1974, 1976, Cashion et al. 1973, Dunlap et al. 1978a, b, Kmiec et al. 1975, Chien and Sleight 1978). The unusual environment for the rare earth cation in the pyrochlore structure leads to a very pronounced lattice vibrational anisotropy, which can be estimated from the Mössbauer data. Also, the highly anisotropic environment leads to a very large electric field gradient at the rare earth cation site. In fact, R 2 Ti 2 07 pyrochlores have the largest electric field gradient ever observed for a rare earth ion (Bauminger et al. 1974, 1976). The electric field gradient increases regularly with decreasing unit-cell edge in the Eu 2 M 206 0 ' series where M is Pb, Zr, Hf, Sn, Mo and Ti. This can be attributed to a decreasing

242

M.A. S U B R A M A N I A N and A.W. SLEIGHT

o4,

~ LuYbTmEr U 1 - YHoDyTb - 7 7 -~1E -

I---T----I-Sm

Nd Pr

LQ

1.00 0.96

0.4C 0.92

0.36 0.88

"~ 0,32 od d

E E

0.84 "-: o"

0,28

0.80

0.24

0.76

0.20

0.72

• 0.95

_z__ 1.00

, , 1.05 1.10 r [R3 + (2111)] Ä

, 1.15

1.20

Fig. 6. Variation of the quadrupole splitting versus ionic radius of rare earth for R2Sn207 (circles) and R2FeSbOv (squares) pyrochlores.

Eu-O' distance along the three-fold axis. The isomer shifts of 151Eu in Eu2M207 pyrochlores relative to E u 2 0 3 are negative where M is Pb, Zr, Hf, Sn, Mo and Ti, but are positive where M is Pt, Ir and Ru (Chien and Sleight 1978). The question of magnetic order in Y2Ru207 has been addressed by 99Ru studies (Gibb et al. 1973). The eonclusion was that this compound is magnetically ordered at 4.2 K, but other interpretations of the data are possible.

Appendix. Bibliography on rare earth R2 M2 0 7 pyrochlores Reviews

Subramanian et al. (1983), Greedan (1991)

Struetural papers

Jona et al. (1955), Sleight (1968a), Longo et al. (1969), Barker et al. (1970), McCauley (1980), Subramanian et al. (1983, 1988), Chakoumakos (1984), Darriet et al. (1971), Nikiforov et al. (1972), Nyman et al. (1978), Pannetier and Lucas (1970), O'Keefe et al. (1980)

R2M 207 pyroehlores (M = 3d transition element) R2Ti2Ov pyrochlores Synthesis

Crystal growth Electrical properties Magnetic properties Infrared spectra Raman spectra Mössbauer spectra

Knop et al. (1965), Roth (1956), Brixner (1964), Knop et al. (1969), Collongues et aL (1965), Bocquillon et al. (1971), Mizutani et al. (1974), Sheherbakova et al. (1979) Garton and Wanklyn (1968), Becker and Will (1969, 1970), Kato and Utsunomiya (1970) Brixner (1964), Uematsu et al. (1979), Knop et al. (1969) r a n Geuns (1966), Flood (1974), Cashion et al. (1968), Townsend and Crossley (1968) Knop et al. (1969), Klee and Weitz (1969), McCaffrey et al. (1971), Vandenborre et al. (1983) Sheetz and White (1976); Vandenborre et al. (1983) Bauminger et al. (1974), Dunlop et al. (1978a,b), Bauminger et al. (1976), Chien and Sleight (1978)

RARE EARTH PYROCHLORES Luminescent R 2 V 2 0 7 pyrochlores Synthesis

Electrical properties Magnetic properties Ln2Cr207 pyrochlores Synthesis L n 2 M n 2 0 v pyrochlores Synthesis Crystal growth Electrical properties Magnetic properties

243

McCauley (1969), Antonov et al. (1977) Bazuev et al. (1976), Kitayama and Katsura (1976), Bazuev et al. (1977, 1978), Shin-ike et al. (1977, 1979), Greedan (1979), Soderholm and Greedan (1979), Molodkin et al. (1978) Shin-ike et al. (1977) Bazuev et al. (1977), Shin-ike et al. (1977), Soderholm and Greedan (1979), Soderholm et al. (1980, I982), Greedan et al. (1986) Fujinika et al. (1979) Fujinika et al. (1979), Subramanian et al. (1988) Subramanian et al. (1988). Fujinika et al. (1979), Subramanian et al. (1988) Fujinika et al. (1979), Subramanian et al. (1988), Troyanchuk and Derkachenko (1988), Reimers et al. (1991)

R2M207 pyroehlores ( M - 4d transition element) RzZrzO 7 pyrochlores Synthesis Order-disorder Infrared Raman spectra Electrical

Magnetic and specific heat studies R2 M02 07 pyrochlores Synthesis Electrical Magnetic R 2 Tc 2 0 7 pyrochlores Synthesis R 2 R u 2 0 7 pyrochlores Synthesis Crystal growth Electrical properties

Magnetic properties Infrared Mössbauer XPS, UPS and HREELS Specific heat studies

Roth (1956), Klee and Weflz (1969), Collongues (1963), Perez Y Jorba (1962), Michel et al. (1976) Perez Y Jorba (1962), Michel et al. (1976) Klee and Weitz (1969), Gundovin et al. (1975) Vandenborre et al. (1981, 1983), Gundovin et al. (1975), Sheetz and White (1976) Steele et al. (1968), Chappey and Guillou (1975), Fomina and Pal'guev (1977), Shinozaki et al. (1979), ran Dijk et al. (1980, 1983), van Dijk (1981), van Dijk and Burggraaf (1981) Blöte et al. (1969)

McCarthy (1971), Hubert (1974, 1975a, b), Subramanian et al. (1980) Hubert (1974, 1975), Subramanian et al. (1980b), Mandiram and Gopalakrishman (1980) Ranganathan et al. (1983), Sato et al. (1986), Greedan et al. (1987), Ali et al. (1989), Reimers et al. (1988, 1990), Greedan et al. (1991), Greedan (1991) Muller et al. (1964) Bertaut et al. (1959), Lazarev and Shaplygin (1978a, b), Bouchard and Gillson (1971) Sleight and Bouchard (1972) Sleight and Bouchard (1972), Bouchard and Gillson (1971), Lazarev and Shaplygin (1978a, b), Ehmann and Kemmler-Sack (1985), Haouzi et al. (1986), Kanno (1992) Leonard et al. (1962), Rosset and Ray (1962), Greedan (1991) Kochergina et al. (1978), Ehmann and Kemmler-Sack (1985) Chien and Sleight (1978) Cox et al. (1983, 1986) Blacklock et al. (1980), Blacklock and White (1980)

244

M.A. SUBRAMANIAN and A.W. SLEIGHT

R2Rh207 pyrochlores Synthesis

Lazarev and Shaplygin (1978a, b)

R2Pd20 7 pyrochlores Synthesis Electrical properties

Sleight (1968b), Lazarev and Shaplygin (1978a) Sleight (1968b), Lazarev and Shaplygin (1978a, b)

R 2 M 2 0 7 pyrochlores (M - 5d transition element)

R2Hf2O v pyrochlores Synthesis Infrared Luminescent

Klee and Weitz (1969), Spiridinov et al. (1968), Besson et al. (1966) Gundovin et al. (1975) Brixner (1984)

R2Os207 pyrochlores Synthesis Electrical properties

Shaplygin and Lazarev (1973) Lazarev and Shaplygin (1978a, b)

R21r20 v pyrochlores Synthesis Crystal growth Electrical properties Mössbauer Specific heat R2Pt207 pyrochlores Synthesis Crystal growth Electrical properties Infrared

Sleight and Bouchard (1972), Montmory and Bertaut (1961), Lazarev and Shaplygin (1978a, b) Sleight and Bouchard (1972) Sleight and Bouchard (1972), Bouchard and Gillson (1971), Lazarev and Shaplygin (1978a, b) Chien and Sleight (1978) Blacklock et al. (1980), Blacklock and White (1980) Sleight and Bouchard (1972), Sleight (1968), Hoekstra and Gallagher (1968), Hoekstra and Siegel (1968) Ostorero and Makram (1974) Sleight (1968), Lazarev and Shaplygin (1978a, b) Hoekstra and Gallagher (1968)

R 2 M 2 0 7 pyroehlores ( M - Group IVa element)

R2Si20 v pyrochlores Synthesis R2Ge207 pyrochlores Synthesis R2Sn20 7 pyrochlores Synthesis Infrared/Raman Electrical Magnetic properties Specifie heat Mössbauer (Sn) Mössbauer (Ln) R2Pb20 7 pyrochlores Synthesis

Reid and Ringwood (1974), Reid et al. (1977), Bocquillon et al. (1977), Chateau and Loriers (1979) Shannon and Sleight (1968), Bocquillon et al. (1978), Bocquillon and Padiour (1980) Whinfrey et al. (1960), Whinfrey and Tauber (1961) McCaffrey et al. (1971), Brisse and Knop (1968), Vandenborre et al. (1983) Brisse and Knop (1968) van Geuns (1966) Blöte et al. (1969) Belyaev et al. (1969), Loebenstein (1970), Calage and Pannetier (1977) Cashion et al. (1973), Kmiec et al. (1975), Chien and Sleight (t978), Bauminger et al. (1976) Brisse (1967), Sleight (1969)

RARE EARTtt PYROCHLORES

245

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