Rare earth rhodium borides with the perovskite structure

Rare earth rhodium borides with the perovskite structure

Journal of the Less-Common Metals, 97(1984) RARE EARTH STRUCTURE RHODIUM BORIDES 223 223-229 WITH THE PEROVSKITE H. TAKE1 and T. SHISHIDO The ...

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Journal of the Less-Common Metals, 97(1984)

RARE EARTH STRUCTURE

RHODIUM

BORIDES

223

223-229

WITH

THE PEROVSKITE

H. TAKE1 and T. SHISHIDO The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980(Japan) (Received June l&1983)

Summary Single-crystal perovskite borides of LnRh,B (Ln = Sm, Gd, Er or Yb) were prepared by the flux method. X-ray analysis revealed that the crystal structures were of the cubic perovskite type with cell dimensions of 4.193 A for SmRh,B, 4.183 A for GdRh,B, 4.147 A for ErRh,B and 4.136 A for YbRh,B. These data are in good agreement with those obtained for arc-melted polycrystalline ingots. The stability of these compounds is considered in terms of a charge transfer mechanism from the lanthanide to the Rh-B bonds to form three non-metal p orbitals or two non-metal p orbitals and two hybrid sp orbitals. The nonstoichiometry of boron and the valence state of the rare earth elements are also discussed.

1. Introduction A large number of carbides, nitrides and oxides with perovskite structures have been reported. However, relatively few studies of perovskite borides have been performed. Schobel and Stadelmaier prepared a perovskite boride with the formula NiJnB,,, [l, 21. In this compound the nickel and indium atoms form a metal framework of the ordered Cu,Au type and the boron atoms are located at the body-centred position. Holleck [3] found that scandium borides with the general formula ScT,B, -x (T - In, Tl, Sn, Pb and Rh; 0 < x d 1) had a perovskite structure. He also reported the existence of boron-stabilized ternary compounds LnRh,B, -x (Ln 5 rare earth element) [4]. Rogl and coworkers reported the formation of perovskite borides in the (Zr, Hf)-(Rh, Ir)-B [5] and YRh,B, -x [6] systems. They found that the perovskite structure was stable for 0 < x < 1 in ZrRh,B, _x and for 0 < x: < 0.78 in HfRh,B, -X. The perovskite phases of LnRh,B (Ln = La, Ce, Pr, Nd, Sm, Eu, Cd, Tb, Dy, Ho, Er, Tm, Yb and Lu) are investigated in this paper. The results of singlecrystal syntheses and X-ray structural analyses are also reported. 0022.5088/84/$3.00

NC> Elsevier Sequoia/Printed

in The Netherlands

224

2. Experimental

procedure

The polycrystalline pellets were prepared as follows. Lanthanides (purity, 99.9%) rhodium (purity, 99.9%) and boron (purity, 99.8%) obtained from commercial sources were arc melted in a titanium-gettered argon atmosphere. Each sample was remelted three times to ensure homogeneity. After melting, the samples were wrapped in tantalum foil and were annealed for 50 h at temperatures between 1400 and 1100 “C in helium gas flowing at a rate of about 200mlmin~‘. Single crystals were grown using the flux method with copper (purity, 99.999%) as the solvent. Stoichiometric quantities of the rare earth, rhodium and boron were mixed with copper in a ratio of about 1: 8 by weight. The mixture was put into a high purity alumina crucible and was heated in an electric furnace under a purified helium gas flow. The heating and cooling procedures were as follows. The samples were heated at a rate of about 400 “C h-i, maintained at a temperature between 1250 and 1350 “C for 10 h and cooled at a rate of 1 “C h- ’ to 1080 “C. The furnace was then cooled rapidly to room temperature. The boride crystals were separated from the copper by treatment with hot HNO,. Powder X-ray diffraction data were obtained using a Toshiba ADX301 diffractometer with nickel-filtered Cu Ka radiation. Silicon powder was used as an internal standard. The line intensities were determined by weighing paper tracings of the diffraction peaks. Single-crystal analyses were performed using a Burger precession camera with zirconium-filtered MO Kcl radiation. The lattice parameters were obtained from the (331) and (420) diffraction peaks and by a computer calculation using the least-squares program U-CELL.11 [7]. The discrepancies in the parameters obtained using the two methods were found to be very small. Microprobe analysis of crystal cross sections was performed using a Horiba EMAX energy dispersion analyser. No promethium samples were prepared.

3. Results and discussion 3.1. Single crystals of LnRh,B (Ln = Sm, Gd, Er and Yb) Single crystals of volume 0.1-3 mm3 were successfully obtained by the flux method. As shown in Fig. 1,the crystals were small black cubes with welldeveloped habits. Precession photographs showed that the crystal structure is simple cubic with lattice parameters of 4.1-4.2 A. The cube surfaces are { lOO} planes. The cocrystallization of LnRh,B, in the form of thin hexagonal platelets was observed in all cases except Ln z Yb. Perovskite crystals for Ln = La, Ce, Pr and Nd could not be obtained using the present technique. Chemical analysis revealed that the atomic ratio Er:Rh:B was 1:3.2:1.1 or 1:2.9:1.0. These values closely correspond to the formula ErRh,B. Electron probe microanalysis of the crystals showed that copper radiation due to solvent contamination was very weak.

225

Table 1 shows X-ray powder diffraction data for pulverized ErRh,B crystals. The diffraction peaks are easily indexed on the basis of a simple cubic cell. The observed peak intensities were compared with the calculated intensities which were obtained using the ideal perovskite structure (eight rhodium atoms at (O,O,O) etc., six rhodium atoms at (*,+,O) etc. and one boron atom at (+,t,+)), ignoring the temperature factor. The reliability factor R = Cl&,, - Iccalc~/~lobs for the lines listed in Table 1 is 0.082. Similar results were obtained for the samarium and gadolinium perovskites. The calculated interatomic distances for the observed d spacing are presented in Table 2. It should be noted that the Rh-B distance is very similar to the close-packed length obtained using Pauling’s single-bond metallic radii. The lattice parameters of the LnRh,B crystals listed in Table 3 decrease with increasing atomic number. The fact that there is a linear dependence of the unit cell parameters on the radii of the Ln3’ ions (Fig. 2) and that the Er-Rh and Er-Er interatomic distances are much larger than the corresponding sums of the atomic radii (Table 2) imply that the rare earth elements in the compounds behave as trivalent rather than metallic ions. A similar inference has previously been made for LnIr,B,, LnRh,B, and LnRu,B, [9, lo]. In the perovskites investigated here each boron atom is surrounded by six rhodium atoms. This means that the coordination number (CN) of boron is 6. This situation is unusual in binary or ternary borides because most of the interstitial borides prefer a trigonal prismatic metal surrounding. As has been pointed out by Aronsson [ll, 121, the existence of TiB, ZrB and HfB with the NaCl structure, for example, is unlikely. Aronsson believed that this type of boride would be a ternary phase containing carbon, oxygen or nitrogen,

(2

a

740

2

730

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720 71 0 700

4 IO

‘095’ Sm Gd

090

’ Er

’ Yb

3 ti c r 3

09:

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IONIC

RADIUS

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i

Fig. 1. ErRh,B crystals. Fig. 2. Lattice parameters of the LnRh,B Yb).

crystals as a function of ionic radii (Ln = Sm, Gd, Er or

226

TABLE 1 Powder diffraction data for pulverized ErRh,B crystals

100 110 111 200 210 211 220 221 300 310 311 222 320 321 400 322 410 330 331 420

4.150 2.9345 2.3970 2.0749 1.8565 1.6940 1.4675

4.1465 2.9320 2.3940 2.0733 1.8544 1.6928 1.4660

4 4 100 49 2 2 41

1.1

1.3822 _a

6

1.1 36.5 11.2 0.5 1.3 6.0

1.0057

0.8

0.97734 0.95127 0.92719

17 21

0.3 21.3 22.8

1.0375

1.3112 1.2502 1.1970 1.1500 1.1082 1.0366

_=

1.2506 1.1977 -a _a

_a

0.95179 0.92746

3.1 5.5 100.0 51.9 2.1 2.4 40.0

44 11

ErRh,B: perovskite-type structure; space group, Pm3m; Z = 1; a,, = 4.1466 A. ’ Only weak spots were observed in the precession photograph. TABLE 2 Interatomic Atoms

Er-Er ErrRh Er-B Rh-Rh Rh-B

distances in ErRh,B Distance

Close-packed length”

(4

(4

4.147 2.932 3.591 2.932 2.073

3.160 2.832 2.380 2.504 2.062

aThese values were calculated from the sum of Pauling’s single-bond metallic radii listed in ref. 8.

The reason for the stability of boron atoms with CN 6 in the LnRh,B compounds may be due in part to the charge transfer concept proposed for transition metal monocarbides and nitrides by Rundle [13, 143. A detailed description of the charge transfer mechanism is given in the review by Toth [15]. Bonding between a metal and a non-metal is accomplished either by three nonmetal p orbitals or by two non-metal p orbitals and two hybrid sp orbitals, and consequently the short Rh-B bond is formed (see Table 2). A maximum of three electrons is required for boron to form these orbitals. In the perovskites

227

TABLE 3 Lattice parameters of single-crystal Compound

Lattice parameter Single crystal

LaRh,B CeRh,B PrRh,B NdRh,B SmRh,B EuRh,B GdRh,B TbRh,B DyRh,B HoRh,B ErRh,B TmRh,B YbRh,B LuRh,B

4.193 4.183

4.147

and polycrystalline

LnRh,B

(A) for the following samples Polycrystalline

ingot __

As melted

Annealed”

4.237 4.147 4.209 4.210 4.192 4.184 4.182 4.172 4.169 4.156 4.150 4.127

4.242 4.213 4.210 4.206’ 4.195 4.184d 4.182 4.175 4.168 4.151 4.154 4.129

4.123

4.120

4.136

a Annealed at 1400 “C. bThe numbers in parentheses denote x in LnRh,B, ‘Annealed at 1300 “C. dAnnealed at 1370 “C.

Holleck’s data for LnRhdBl --x [4] b

4.244 4.096 4.213 4.195 4.192 4.187 4.178 4.165 4.159 4.148 4.142 4.146 4.137 4.132

(0.2) (0.45) (0.25) (0.1) (0.2) (0.1) (0.15) (0.1) (0.1) (0.05) (0.02) (0.02) (0.15) (0.2)

_-X.

investigated here these excess electrons would be transferred from rare earth elements to the Rh-B bonds and would form the octahedral coordination. 3.2. Comparison of single-crystal LnRh,B withpolycrystalline LnRh,B The lattice parameters of the polycrystalline LnRh,B ingot prepared from the stoichiometric melt are shown in Fig. 3. Small changes in the parameters of most of the specimens (except for CeRh,B) were observed on annealing. Table 3 summarizes our results as well as those of Holleck [4] who investigated the nonstoichiometric perovskite borides LnRh,B, _-x. He also found that the stability region of the perovskite ScRh,B, _x ranged from x = 0 (Cu,Au type) to x = 1 (complete perovskite type), and the lattice constant varied from 3.90 A to 4.08 A with increasing x [S]. Similar results have been reported for ZrRh,B, -x and HfRh,B 1 x [5]. The present experiment also showed that the lattice parameter of ErRh,B, -x varied with x and became constant when r reached zero (ErRh,B composition), as shown in Fig. 4. The fact that the lattice parameters of the single crystal agree with those of Holleck’s polycrystalline specimen (Fig. 3 and Table 3) suggests that there is a small boron deficiency in the perovskite crystals investigated here. The marked increase in the lattice parameter of CeRh,B after annealing (see Fig. 3) should be noted. This fact is attributable to valency differences observed in CeRh,B and CeRu,B [9, lo] where the small lattice parameters and

228

-424 6 -

,$

A

t 422

0” !

t

x I

420-

4 16-

cc Lz

414-

z 4 14-

s f 6 u

8 = 2

412-

418-

ai r”

4 IZ4 IO408-

4 IO-

,,E+,1,, , , , , / , , , , , , ] 57 58 59 60

62636465666768697071

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

RARE

EARTH

ELEMENTS

z ; ‘0 J

4064041

, 0

Atomic

,

5

IO

%

of

,

,

15

20

Boron

, 25

(As

cast)

Fig. 3. Lattice parameters of rare earth rhodium boride (LnRh,B) polycrystals as a function of the atomic number: 0, as prepared; 0, annealed at 1400 “C; l, single-crystal data; A, Holleck’s data [4]. Fig. 4. Lattice parameters of ErRh,B, _x as a function ofthe boron content: 0, polycrystal; crystal. The solubility limit occurs at 20 at.% B (ErRh,B composition).

0, single

paramagnetic properties indicate the existence of tetravalent behaviour in rare earth elements. Annealing in a helium atmosphere would be effective in reducing the valence state from tetravalent to trivalent, and consequently the lattice parameter would become larger. Rogl and Nowotny [5] found a marked solubility in the intermetallic compounds ZrRh, and HfRh, (Cu,Au-type structure) up to the compositions ZrRh,B and HfRh3B,,s, where zirconium and hafnium are tetravalent. Furthermore, ScRh,B has been reported by Holleck [S], who argued that scandium was tetravalent in this compound. However, this scandium valence state is believed to be anomalous [16]. The present study indicates that perovskite rhodium borides containing trivalent rare earths are very stable. This evidence and the results obtained by Rogl and Nowotny [5] suggest that in MRh,B perovskites M is trivalent, tetravalent or has an intermediate valence. Such a situation would be achieved by a change in the valency of rhodium and/or in the electron density of the Rh-B bond. A more detailed investigation using magnetic measurements is now in progress. Acknowledgments The authors thank Professor T. Muto of Tohoku University for his encouragement and Mr. K. Yamagishi of the Mitsui Mining and Smelting Co. for performing the electron microprobe analysis measurements. This research was carried out under the terms of Grant 57055005 for nuclear fusion research from the Ministry of Education, Science and Culture.

229

References 1 2 3 4

5 6 7

8 9 10 11 12 13 14 15 16

J. D. Schobel and H. H. Stadelmaier, 2. Metallkd., 55(1964) 378. H. H. Stadelmaier, Developments in the Structural Chemistry of Alloy Phases, Plenum, New York, 1969, p. 141. H. Holleck, J. Less-Common Met., 52 (1977) 167. H. Holleck, Proc. 4th Int. Conf. on Solid Compounds of Transition Elements, Geneva, April 9-13, 1973, in Rep. AED-Conf. 73-128-015, 1973, p. 210 (Zentrastelle fiir Atomkernenergie-Dokumentation, Eggenstein, Leopoldshaffen). P. Rogl and H. Nowotny, J. Less-Common Met., 67(1979) 41. P. Rogl and L. Delong, J. Less-Common Met., 91(1983) 97. Y. Matsui, U-CELL II: a Crystallographic Least-squares Program, Okayama University, 1978. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 3rd edn., 1960, p. 403. H. C. Ku, G. P. Meisner and F. Acker, Solid State Commun., 35 (1980) 91. K. Hiebl, P. Rogl, E. Uhl and M. J. Sienko, Inorg. Chem., 19 (1980) 3316. B. Aronsson, Borides, Silicides and Phosphides, Methuen, New York, 1965, p. 13. B. Aronsson, Ark. Kemi, 16(1960) 379. R. E. Rundle, J. Am. Chem. Sot., 69(1947) 1327. R. E. Rundle, Acta Crystallogr., 1(1948) 180. L. E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971, p. 247. C. T. Horovitz (ed.), Scandium, its Occurrence, Chemistry, Physics, Metallurgy, Biology and Technology, Academic Press, London, 1975, p. 152.