Structural phase transition in various methylammonium hexahalometallates(IV) as studied by the NQR of halogens

Structural phase transition in various methylammonium hexahalometallates(IV) as studied by the NQR of halogens

JOURNAL OF MAGNETIC RESONANCE 33, 331-344 (1979) Structural Phase Transition in Various Methylammonium Hexahalometallates(IV) as Studied by the NQR o...

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Structural Phase Transition in Various Methylammonium Hexahalometallates(IV) as Studied by the NQR of Halogens YOSHIO K U M E , RYUICHI IKEDA, AND D A I Y U N A K A M U R A

Department of Chemistry, Nagoya University, Chikusa, Nagoya 464, Japan Received March 14, 1978; revised May 15, 1978 The temperature dependence of halogen NQR frequencies was determined for various methylammonium hexahalometallates(IV): (MA)2MC16; M = Se, Pd, Sn, Pt, and Pb, (MA)2MBr6; M = Se, Pd, Sn, and Pt, and (MA)2PtI6. A single resonance line for each isotope in the chlorine and the bromine complexes and only one vl line of iodine-127 for the iodine complex were observed at various temperatures studied, indicating that all halogen atoms are crystallographically equivalent in crystals. At liquid nitrogen temperature, both vl and v2 lines of the iodine complex were observed. X-ray powder patterns taken at room temperature revealed that all of these complexes form crystals isomorphous with each other belonging to the space group R3m. The crystal structure describable as a rhombohedrally distorted K2PtC16 type is consistent with the results of the present NQR measurements. All the complexes studied show an anomaly in the temperature variation of NQR frequencies below dry-ice temperature. The anomaly was attributed to a structural phase transition from the results of DTA. The transition temperatures determined are concentrated in a rather narrow range of temperature, and no remarkable size effect due to complex anions can be observed. A possible structure of these compounds in the low-temperature phase is deduced, and the mechanism Joy which the phase transition takes place is discussed.


In a previous note (1), we have reported the occurrence of structural phase transitions in the crystals of methylammonium hexachloroplatinate(IV), -stannate(IV), and -selenate(IV) at low temperatures. These compounds are known to form rhombohedral crystals at room temperature (1, 2), the structure of which can be described as a rhombohedrally distorted potassium hexachloroplatinate(IV) type (3). The dumbell-like cations situated along the threefold axis of the unit cell form a layer which separates from one another the layer of octahedral complex anions. Each layer lies normal to the threefold axis. In this structure, all chlorine atoms are crystallographically equivalent in agreement with the results of 35C1 N Q R experiments on the complexes above the respective transition points, Ttr. Below Ttr, these complexes still exhibit a single resonance line. Moreover, the resonance frequency changes continuously with increasing or decreasing temperature through Ttr with receiving no appreciable change in the intensity of resonance signals, although the temperature coefficient of resonance frequencies, dv/dT, is discontinuous at Ttr. This suggests that a slight modification of structure in these crystals takes place due to a phase transition. 331

0022-2364/79/020331-14502.00/0 Copyright© 1979by AcademicPress,Inc. All rightsof reproductionin anyformreserved. Printedin GreatBritain



From the study of broad-line proton magnetic resonance (PMR) and proton relaxation in (MA)2PtC16 and (MA)2SnC16 crystals (MA = CH3NH3) subsequently carried out (4), it has been concluded that the cations perform rapid reorientation even far below Ttr, and that the motion of the cations makes only a minor contribution to the occurrence of the phase transition. Furthermore, the nuclear relaxation of chlorine-35 in both complexes has been known to show a distinct minimum at Ttr (5), indicating that the motion of octahedral anions plays an important role in the mechanism of the phase transition. Consequently, interactions among octahedral complex anions in a layer can be expected to be responsible for the appearance of the phase transition. The present investigation on the temperature dependence of halogen NQR frequencies in various isomorphous methylammonium hexahalometallates(IV), (MA)2MX6, has been carried out in an attempt primarily to examine the adequacy of the'expectation. EXPERIMENTAL A modified Dean type superregenerative spectrometer (6) was employed for the determination of chlorine NQR frequencies over a fairly wide range of temperature above 4.2 K, while two superregenerative spectrometers equipped with Lecher lines (6) were used for the detection of bromine and iodine NQR signals at various temperatures above 77 K. Temperatures between liquid helium and liquid nitrogen temperatures were obtained by allowing the sample in a cryostat to warm from liquid helium temperature (7). Experiments above 77 K were performed with samples placed in a Dewar vessel filled with petroleum ether which was cooled with dry-ice or liquid nitrogen. Temperatures between 4.2 and 77 K were determined by use of a gold(0.03% iron) vs chromel thermocouple which was calibrated by measuring the 35C1 NOR frequency of potassium chlorate (8). A chromel vs alumel thermocouple calibrated at standard temperatures was used for the determination of temperature above 77 K. The observed temperatures were estimated to be accurate within +1 K above 77 K and within +2 K between 4.2 and 77 K. X-ray powder patterns were taken by means of a model D-3F diffractometer from Rigaku Denki Company equipped with a copper anticathode. A model SG-7 wide angle goniometer employed was calibrated with a standard sample. The experiment of differential thermal analysis (DTA) was performed by use of an apparatus constructed by us. Powdered alumina was used as a reference and the difference of temperatures was detected by using two sets of copper vs constantan thermocouples inserted in a sample and the reference tubes. Methylammonium hexachlorometallates(IV), (MA)EMCI6 (M = Se, Pd, Sn, Pt, and Pb), were synthesized according to the method described in literature (9). The preparation of partially deuterated compounds of the platinum complex has already been described elsewhere (4). (MA)ESnBr6 was prepared by adding methylammonium bromide to a solution of tin tetrabromide dissolved in concentrated hydrobromic acid. (MA)2SeBr6 was synthesized in a similar manner from a solution of selenium dioxide dissolved in concentrated hydrobromic acid by adding methylammonium bromide. Metallic powder of platinum and palladium were oxidized by bromine in a concentrated hydrobromic acid solution. (MA)EPtBr6 and (MA)2PdBr6



were crystallized from the resulting solutions by adding methylammonium bromide. (MA)2PtI6 was prepared by dissolving hexachloroplatinic acid in a concentrated solution of a large excess of methylammonium iodide. Black crystals formed were purified by recrystallization from a hydroiodic acid solution. To identify the samples prepared, the quantitative determination of halogens was carried out by the conventional methods. Anal Calcd for (MA)2SeCI6: C1, 59.8. Found: C1, 57.7. Calcd for (MA)2PdC16: C1, 55.51 Found: C1, 54.8. Calcd for (MA)2SnC16: C1, 53.8. Found: CI, 53.4. Calcd for (MA)2PtC16: C1, 45.1. Found: C1, 44.9. Calcd for (MA)2PbC16: C1, 43.9. Found: C1, 39.0. Calcd for (MA)2SeBr6: Br, 77.0. Found: Br, 77.4. Calcd for (MA)2PdBr6: Br, 73.8. Found: 72.9. Calcd for (MA)2SnBr6: Br, 72.4. Found: Br, 72.3. Calcd for (MA)2PtBr6: Br, 64.9. Found: Br: 65.3. Calcd for (MA)2PtI6: I, 74.6. Found: I, 73.9. Although (MA)2SeC16 was stable in a sealed ampoule, partial decomposition rapidly took place emitting the odor of hydrogen chloride when the ampoule was cut. This may be the reason why the analytical results shows a rather poor agreement with the calculated value of chlorine content. (MA)2PbC16 yielded a considerably low content of chlorine. The X-ray powder pattern of this complex showed additional diffraction lines indicating the existence of yellow lead oxide (10) as an impurity.


NOR and DTA A single resonance line was observed for each chlorine isotope in (MA)2PtC16 at various temperatures investigated. A slight anomaly in the temperature dependence of resonance frequencies was found at 125 K as shown in Fig. 1, suggesting the existence of phase transition. The characteristic feature of the anomaly is following. The resonance line continuously changes its frequency with temperature even in the immediate vicinity of 125 K, but with a discontinuity in its temperature coefficient, dv/dT. It has been known that weak hydrogen bonds such as C1...H-O in NaAuC14.2H20 and Br...H-N in anilinium bromide result in an anomalous behavior of the temperature dependence of NQR frequencies (11-I 6), Hence, we determined the temperature variation of chlorine NQR frequencies on the partially deuterated complexes, (CH3ND3)2PtC16 and (CD3NH3)2PtCI6. These compounds show a slight anomaly in the temperature dependence at 127.5 K for the former and 126 K for the latter, and the data obtained near those temperatures are shown also in Fig. 1. Both compounds yielded approximately the same frequency as the fully protonated complex at various temperatures studied. Moreover, all the three platinum complexes show the anomalies at about the same temperatures. These facts indicate that conceivable hydrogen bonds between methylammonium ions and chlorine atoms in crystals are too weak to play a major role in the appearance of the anomaly. To confirm that the NQR anomaly is attributable to a phase transition, we carried out the experiments of DTA for (MA)2PtC16. When the sample was warmed and cooled, DTA curves exhibited endothermic and exothermic peaks, respectively, at about 125 K, clearly indicating the presence of phase transition. The DTA curves are not symmetrical but are considerably distorted having a long tail in the





. • 8


(CH3HH3)2PtCI 6


26.4 125K u


26.46} • Oe





• CH3NH3 • CD3NH3 , CH3ND3

• .= ~,, I; 4,










°~o • %• •




12o I


15o I





... I






Temp./K FIG. 1. Temperature dependence of 350 NQR frequencies in methylammonium hexachloroplatinate(IV) and its partially deuterated analogs.

low-temperature side as indicated in Fig. 2. No hysteresis was observed in either NQR or DTA experiments, suggesting that the phase transition belongs to the second-order or higher-order phase transition (17). Anomalies bearing a strong resemblance to those found in the NQR temperature dependence of the platinum complexes were observed for (MA)2SeC16, (MA)2PdC16, (MA)2SnC16, and (MA)EPbC16 as shown in Fig. 3. These complexes yielded a single resonance line for each chlorine isotope in the whole temperature range studied• Resonance signals could be detected even in the immediate vicinity of the respective anomaly without any appreciable alteration in both intensity and linewidth. The DTA patterns of these complexes also have a characteristic peak resembling that obtained for (MA)EPtCI6, at approximately the same temperature as the NQR anomaly appears• Consequently, it is evident that these complexes undergo a phase transition similar to that of (MA)EPtC16 at low temperatures• For (MA)ESnC16 and (MA)EPtC16, the measurements of temperature dependence were extended-down to liquid helium temperature. On lowering the temperature, the resonance line of the former gradually became broad and weak below about 145 K, and finally faded out in the noise level below about 40 K. For the latter complex, the




(GH3NH3)2PtGI6 B

r,M z

115K 120K

.~_.....~~H3)2PtBr6 133K



FIG. 2. DTA curvesof the three platinum complexeson heating. On cooling,exothermicpeaks havinga long tail again on the low-temperature side can be obtained for the respective complexes.

resonance signal gradually broadened with decreasing temperature, but the broadening was not so remarkable that the signal could be observed even at liquid helium temperature. As reported in a previous paper (5), the spin-lattice relaxation time of 35C1 in (MA)zSnC16 increases steeply with decreasing temperature below Ttr and takes the value of about 300 msec at 100 K, which is longer than that of (MA)zPtC16 and K2PtC16 (18) at the same temperature. Therefore, the line broadening observed for both complexes might be interpreted in terms of the saturation effect of chlorine nuclei owing to the use of a superregenerative spectrometer operated at a high-power level. The broadening of resonance signals was detected for the remaining chlorine complexes below the respective transition temperatures. Figure 4 shows the temperature variation of bromine and iodine N Q R frequencies observed for (MA)2SeBr6, (MA)2PdBr6, (MA)2SnBr6, (MA)2PtBr6, and (MA)2PtI6. All of the bromine complexes gave rise to a single resonance line for each isotope at all temperatures studied. The two resonance lines, ul and u2, corresponding to the lower and the higher transition frequencies, respectively, for nuclei having a nuclear spin equal to 5/2, were observed for iodine-127 in (MA)2PtI6 at liquid nitrogen temperature. However, only one ul line could be detected above liquid nitrogen temperature as shown in Fig. 4. This is partly because of the z,2 signal barely detected even at liquid nitrogen temperature, and partly because of the low sensitivity of the







~/MHz e e°ee~ •

O0 0


- 27.2

(MA)2PdCI 6 -35C[

• 21.2'


Ibe e

e• •




- 27.1 • •e 0


(M A)2SeCI6 --35Cl

• . °.,.,





17.7 ~-


"" e•

- 27.0 ~•0


• ...'-21.0




I 16.0"

(MA)2PbCI6 _35CI moe~ • e,~ ,m • e• • e • • eooeee•


" " * " 17.5




. (MA)2SnCI6--35CI

%% o ..,,. ~o

• 0•




00 • • • • e o e ~


- 15.8











FIG. 3. Temperature dependence of 35C1 NQR frequencies in some methylammonium hexachlorometallates(IV). spectrometer employed in a higher-frequency range. All the bromine complexes yield an anomaly bearing a strong resemblance to that of the corresponding chlorine complexes~ Resonance signals having nearly constant intensity could be observed for each complex at various temperatures studied in a narrow temperature range involving the temperature of the anomaly. The same is not true for (MA)2PtI6. When the temperature of the sample was lowered, the us signal could be observed continuously down to 131.5 K, immediately below which temperature the signal







209.0 V/MHz


. •°eOoeqb

4JmoogD• too • ~ °° qJo•

• •o


o ee

Ooo • 108K

°ooe o


°o• °°°o • ~ 2070

(MA)2 PdBr6-81Br

~ I °°•°° 0•o 0000

• e



- 208.0

.... (MA)2Ptl6 -1271

o• o


o• ° 173.5




(MA)2PtBr6- 81Br e o

e°°• 06oo

°F172. 0 • o


ieD ,0•• 0°0•0• co•




~o,•• 0~ °°0


(MA)2SeBr 6- 81Br


o• 131.5


• o



• o


~tlntomDoeo • • • • -.. ,_.,z.n.ro_79 ~MA~,,~ n,,




• oo °

• o

o o

' 146.0

•....i 131"0


I 100


I 200




FIG. 4. Temperature dependence of bromine and iodine NQR frequencies in some methylammonium

hexabromometallates(IV) and hexaiodoplatinate(IV), respectively.

abruptly disappeared. With lowering further the temperature, a signal again appeared at 131 K, the intensity of which was almost equal to that of the signal observed above 131.5 K. With increasing temperature, the reverse process could be followed exactly with no detectable hysteresis. The DTA curves recorded for this compound strongly resemble those of the other platinum complexes as shown in Fig. 2. Accordingly, the phase transition found in (MA)2PtI6 can be considered as the




77 K

190 K

300 K

(MA)2SnC16 (MA)2PbCI6 (MA)2SeC16 (MA)2PdCI6 (MA)2PtC16 (MA-CD3)/PtC16 (MA-ND3)2PtC16

15.932 -4-0.006 17.665 + 0.008 21.161 ± 0.004 27.247 ± 0.002 26.4804- 0.002 26.483 4-0.002 26.485 ± 0.002

15.857 -4-0.004 17.560 + 0.005 21.076 ± 0.004 27.122 ± 0.002 26.3914- 0.002 26.395 4-0.002 26.400 4-0.002

15.815 -4-0.004 17.505 + 0.005 20.990 ± 0.004 27.052 ± 0.002 26.279 4-0.002 26.282 4-0.002 26.288 ± 0.002

(MA)zSn79Br6 (MA)/Se81Br6 (MA)/PdSlBr6 (MA)zptSaBr6

131.754- 0.05 147.25 4-0.05 174.60 ± 0.05 172.79 + 0.05

131.114- 0.05 146.654- 0.05 174.02 4-0.05 172.214- 0.05

130.79 4-0.05 146.04 4-0.05 173.33 + 0.05 171.544- 0.05

(MA)2PtI6 ~'1 u2

209.22 ± 0.08 416.2±0.5

207.814- 0.08 --

206.92 ± 0.008 --

s e c o n d - o r d e r o n e in spite of t h e d i s a p p e a r a n c e of r e s o n a n c e f r e q u e n c i e s in a v e r y n a r r o w r a n g e of t e m p e r a t u r e b e l o w Ttr. F o r the b r o m i n e c o m p l e x e s , it is c o n f i r m e d b y the m e a s u r e m e n t s of D T A t h a t the N Q R a n o m a l i e s d e t e c t e d a r e a t t r i b u t a b l e to p h a s e transitions. It is i n t e r e s t i n g to n o t i c e that the N Q R f r e q u e n c y of h a l o g e n s in (MA)2SnC16, (MA)2SnBr6, (MA)2PbC16, a n d (MA)2PtI6 has a p o s i t i v e t e m p e r a t u r e coefficient in an e x t r e m e l y n a r r o w t e m p e r a t u r e r a n g e i m m e d i a t e l y a b o v e the r e s p e c t i v e t r a n s i t i o n points. T h e N Q R f r e q u e n c i e s d e t e r m i n e d at v a r i o u s t e m p e r a t u r e s for all t h e c o m p l e x e s d e s c r i b e d a b o v e a r e listed in T a b l e 1. T h e 35C1 N Q R f r e q u e n c i e s of (MA)2SnC16 a n d (MA)zPbC16 d e t e r m i n e d at 300 K a g r e e well with t h o s e r e p o r t e d b y Brill et al. ( 9 b ,

19). X-Ray Powder Pattern T h e X - r a y crystal analysis of (MA)zSnC16 a n d (MA)2PtC16 was c a r r i e d o u t b y W y c k o f f (2, 3). V e r y r e c e n t l y , K i r i y a m a et al. (5) h a v e p e r f o r m e d a r e i n v e s t i g a t i o n of crystal s t r u c t u r e s o n the f o r m e r a n d r e f i n e d the s t r u c t u r e p r o p o s e d b y Wyckoff. A c c o r d i n g to t h e i r results, t h e s e c o m p o u n d s f o r m r h o m b o h e d r a l crystals b e l o n g i n g to t h e space g r o u p R3m (D~d). Since no X - r a y s t r u c t u r a l analysis has b e e n r e p o r t e d o n the r e m a i n i n g c o m p l e x e s as yet, X - r a y p o w d e r p a t t e r n s w e r e t a k e n in o r d e r to c o n f i r m the i s o m o r p h i s m of these c o m p o u n d s with (MA)zSnC16 a n d (MA)zPtC16 a n d to d e t e r m i n e t h e lattice p a r a m e t e r s . A l l the p a t t e r n s r e c o r d e d c o u l d b e well i n t e r p r e t e d as resulting f r o m a r h o m b o h e d r a l l y d i s t o r t e d KzPtCI6 s t r u c t u r e with the s p a c e g r o u p R3rn. T h e r e f o r e , it is e s t a b l i s h e d that t h e c o m p l e x e s s t u d i e d in the





X = C1


X= I

Compound M

a (•)

a (degrees)

Ttr (K)

Sn Pb Pd Pt Se Sn Pd Pt Se Pt

8.42 8.39 8.34 8.31 8.47 8.61 8.58 8.59 8.59 8.92

50.23 51.24 45.55 48.77 50.04 52.22 50.05 50.46 51.43 52.85

156 163 112 125 103 149 108 118 111 132

present investigation are isomorphous with each other at room temperature. The lattice parameters of a rhombohedral unit cell along with the transition temperatures located by the N Q R experiments are given in Table 2. Comparisons between calculated and observed X-ray powder diffraction angles and the Miller indices assigned to them are collected in Table 3 for all the complexes studied. DISCUSSION

Variation of Transition Temperatures The R2MX6-type complexes having an octahedral hexahalometallate(IV) anion are known usually to form cubic crystals of the KzPtC16 type, and have been extensively studied frequently by the N Q R of halogens (15, 20-22). According to those, many of such complexes undergo a structural phase transition, only above which temperature the complexes form cubic crystals. For the cubic complexes with a common cation, the transition temperature corresponding to the phase transition increases with increasing the size of anions involving the same metal atom. For instance, K2ReCI6, KzReBr6, and K2ReI6 have the cubic structure above 111,269, and some temperature higher than 473 K, respectively (23, 24). However, no such correlation between the transition temperature and the size of anions can be discernible for the present complexes. Among the complexes listed in Table 2, the lead and tin complexes, the central metal atoms of which belong to the IVb family of the periodic table, yield transition temperatures nearly equal to each other regardless of the kind of halogens. The remaining selenium, palladium, and platinum complexes show practically the same transition temperature, and considerably lower transition temperatures than those of the lead and tin complexes regardless also of the kind of halogens. The typical example can be seen in the palladium complexes; i.e., the Ttr of (MA)2PdC16 is lower than that of (MA)2PdBr6 only by 4 K, and both transition temperatures are lower than those of the tin complexes by about 40 K. Although the size of complex anions definitely becomes bigger in the order of [PtC16] 2-, [PtBr6] 2-, and [PtI6] 2-, the





~=50.04 °

(MA) 2SeBr6;

[email protected]/degree


~=51.43 °


[email protected]/degree


~=45.55 °

[email protected]/degree

h k 1




h k 1




h k 1




1 1 1 2 2 2 2 2 3 3 2 3

11.98 14.83 16.38 21.54 24.09 24.68 29.92 33.10 34.89 36.48 39.28 40.79

11.71 14.68 16.24 21.47 24.03 24.62 29.95 33.14 34.90 36.54 39.42 40.88

vs m w vw w w w w vw w vw w

1 1 1 2 2 2 2 3 3 3 2 3

1 0 0 2 1 0 0 1 1 3 1 1

11.90 14.28 15.86 23.94 24.25 28.79 32.03 34.06 34.28 36.24 37.73 39.74

11.86 14.28 15.85 23.92 24.22 28.79 32.04 34.07 34.26 36.23 37.75 39.74

vs w w m vw m w vw vw w vw vw

1 1 1 2 2 2 1 2 1 2 2 3

1 0 0 1 2 1 0 0 1 0 0 1

12.11 15.25 16.79 22.93 24.36 25.12 25.61 28.42 29.94 30.78 33.96 35.64

12.16 15.32 16.84 21.94 24.35 25.12 25.67 28.44 29.98 30.81 33.96 35.62

vs m w vw vw w vw vw vw vw w vw

3 2 8 4 2 2 4 3 2

43.75 43.81 ) 44.65





3 2 0 4 2 2 4 3 2

42.21 42.85 43.87

42.21 42.87 43.86

vw vw vw

3 3 3 2 1 0 2 1 1

36.88 39.80 40.46

36.79 39.82 40.47

vw vw vw

2 2 0 4 3 1

51.00 51.03 )



3 4 2 3

48.90 48.86 ) 50.48

41.60 43.95 44.85 44.71) 45.46

41.54 43.82

vw vw

v~ vw

3 4 3 4 4

44.74 45.33

vw vw

i 0 1 1 2 2 0 2 2 3 ~ 3

1 0 0 1 2 1 0 0 1 3 1 1

1 0 1 2 2 0 2 2 1 3 1 3

3 1 2 [

0 1) 0 1

48.90 50.52





~=50.05 °


[email protected]/degree h k 1


1 1 1 2 2 1 2 2 3 3 3 2 3 4 3 4

1 0 1 2 2 ~ 0 2 2 1 3 [ 3 3 2 2

1 0 0 2 1 0 0 0 1 1 3 1 1 3 0 2

4 3 2 4 3 1

3 3 2 2 3

1 3 0 2 2








11.83 14.64 16.16 23.78 24.36 24.53 ) 29.52 32.66 34.43 34.85 36.02 38.75 40.18 42.77 43.08 43.22 )

12.05 14.76 16.29 23.96 24.50

vs w w m w

29.52 32.66 34.48 34.85 36.15 38.66 40.18 42.97

w w vw vw m vw w vw



44.05 50.33

44.08 50.63

vw vw

(MA) 2PtBr6;


1 0 1 1 2 2 1 1 [ 0 2 2

o a=8.59A,

5=50.46 °


h k 1




1 i 1 2 2 2 3 2 2 3 3 4 4 2

11.94 14.07 15.68 24.01 28.37 31.67 33.84 36.45 37.15 39.49 41.70 42.63 43.74 48.04

11.92 14.08 15.70 24.08 28.34 31.70 33.87 36.45 37.17 39.54 41.73 42.70 43.81 48.07

vs m m m m m v~ vw vw vw vw vw vw w

1 0 1 2 0 2 2 [ 1 3 2 2 3 2

1 0 0 2 0 0 1 0 1 1 0 2 2 0


[email protected]/degree

o a=8.92A,


a=8.39A ~=51.24

o ~=52.85

[email protected]/degree

h k 1




h k 1




1 1 1 2 2 2 2

1 0 1 1 2 2 1

1 0 0 1 2 1 0

ii. 83 14.50 16.04 21.13 23.78 24.28 27.10

ii. 92 14.54 16.07 21.18 23.86 24.33 27.10

vS m m vw vw w vw

1 1 1 2 2 2 2

1 0 1 2 2 0 2

1 0 0 2 1 0 0

ii. 59 13.44 15.02 23.25 23.29 ) 27.07 30.30

ii. 59 13.43 14.99 23.32

vS s vs m

27.06 30.29

s m

2 2 3 2 2

0 2 2 [ [

0 0 1 0 1

29.24 32.41 34.25 37.71 38.37

29.25 32.41 34.28 37.67 38.34

w w w vw vw

3 3 2 3 3

2 1 ~ 3 1

1 1 1 1 0

32.50~ 32.53 ) 35.41 37.91 38.00 )



3 3 4 4

3 2 2 3

1 0 2 2

39.97 49.74 43.03 43.91

39.97 42.74 43.01 43.93

w vw vw vw

3 2 0 3 0 0 2 2 0

39.85 41.10 45.75





39.85 41.03 45.66

vw vw w

h k 1




1 1 1 2 2 2 2 1 2 3 2 3 2 2 3 2

12.17 14.67 16.28 21.57 24.49 24.85 27.47 28.70 29.58 32.06 32.89 34.94 38.10 38.79 40.78 43.18

12.19 14.66 16.26 21.55 24.52 24.84 27.44 28.67 29.53 32.04 32.83 34.91 38.05 38.76 40.73 43.29

vs vs s w m m w vw vw vw m w vw w vw vw

4 3 3 4 2 2 4 3 2

43.94 43.97 ) 44.98





4 4 3 4 2 1 4 4 4

48 12 48.17 ) 50.19





1 0 1 1 2 2 1 1 0 2 2 2 1 ~ 3 [

1 0 0 1 2 1 0 1 0 2 0 1 0 1 1 ~



bromine complex yields the lowest transition temperature. This clearly indicates that the size of anions is not a dominant factor in determining the temperature of phase transitions for these complexes. These results are rather unexpected. R6ssler and Winter (25) have examined the data of various hexahalometallates(IV) of the K2PtC16 type and found a systematic trend between the transition temperature and the d-electron configuration of the central metal atom. According to them, the transition temperature decreases with increasing the ligand field stabilization energy, LFSE, of the central metal atoms in the respective 4d and 5d series, and the central atom belonging to the 5d series results in higher transition temperature than the corresponding atom in the 4d series. For transition metal ions in a cubic field, they have estimated the LFSE values from the occupation number, neg and nt2g of d-electrons in eg and t2g orbitals, respectively, by use of a formula, 6ne,-4nt2,. This means that d 6 metal ions such as Pt 4+ and Pd 4+ have the lowest LFSE whereasd 0 or d 1 0 ' Ions such as Pb 4 + and Sn 4 + have the highest. • Therefore, the same conclusion as above may be derived for the rhombohedral complexes, although the difference of the present transition temperatures between tin(IV) and palladium(IV) or between lead(IV) and platinum(IV) is very small as compared with the difference greater than 300 K obtained for the K2PtC16 type complexes. The selenium complexes yielding low transition temperatures are rather exceptional in the rhombohedral complexes as well as in the cubic ones. As noticed by R6ssler and Winter (25), this may be due to the existence of a 5s2 lone pair in an outer sphere. Consequently, the variation of the transition temperatures of the rhombohedral complexes may be partly interpreted in terms of the electronic configuration of the central metal atom. "Since all the complexes studied have MA cations in common, one is tempted to conclude that the motion of MA ions acts as a driving force to initiate phase transition. This explanation seems to be reasonable in view of the fact that the transition temperatures concentrate in a rather narrow range of temperature from 103 to 163 K in contrast to the aforementioned cubic complexes. However, the proton relaxation measurements of (MA)ESnC16 and (MA)2PtC16 have revealed that the motion of the cations is not concerned directly in the occurrence of phase transition (4). Furthermore, the 35C1 nuclear relaxation of both complexes shows a distinct minimum at the transition temperature (5), indicating the existence of a soft rotary mode of the octahedral anions as in the case of phase transition observed for some of the cubic complexes such as K2ReC16 (26) and (NH4)2PtBr6 (27). Accordingly, the motion of octahedral anions rather than of MA cations must be related in' the initiation of the phase transition. In the cubic complexes, an octahedral anion is situated at the body center of a cube defined by eight cations. Each metal-ligand bond is directed to the face-center of the cube. It has been known, in many cases, that the cubic cage of cations transforms into a tetragonal one on lowering the temperature through phase transition (25-29). In a low-temperature phase, the octahedron still occupies the body center of a tetragonal lattice, but its orientation changes in such manner that the octahedron rotates about one of its fourfold axes by a small angle (28). To represent the phase transition of these complexes, therefore, a libration of the anion about the principal axis of the octahedron is important (21, 22). On the other hand, the octahedral anions form



layers in the present complexes, neighboring layers of which have a layer of M A cations intervening between them (2, 3). The anions are packed in such a way that one of the four threefold axes of octahedra is perpendicular to the plane of the layer. Therefore, a libration of the octahedra about the threefold axis would be important in the present crystals, and it is conceivable that the octahedra rotate through a small angle about the threefold axis below transition temperature, to reduce repulsive interactions among the complex anions.

Phase Transition and Possible Structure in the Low-Temperature Phase From the preceding discussion on the results of N Q R and D T A measurements, the phase transition found here is considered to be of the second order in the Landau sense, and possibly, to be driven by a soft mode (17, 21, 26). If the presence of a soft mode is assumed in the present complexes, the softening of the libration of anions about the threefold axis of the unit cell is most plausible, as mentioned before, to represent the observed phase transition. Since this librational mode of octahedra in the D5a system is both infrared- and Raman-inactive, we have no direct means to observe it. However, the assumption of the soft mode may be reasonable in view of the fact that (MA)zSnC16, (MA)2PbC16, (MA)ESnBr6, and (MA)EPtI6 show a positive temperature dependence of N Q R frequencies in a narrow range of temperature immediately above Ttr. The explanation of this is as follows. According to Bayer (30) and Kushida (31), the temperature dependence of N Q R frequencies, v(T), can be described, in a simple case, by the equation




Here, vo is the N Q R frequency of resonant nuclei in a rigid lattice, and w and I denote the torsional oscillation frequency effective on the averaging of the field gradient at the resonant nuclei and the associated moment of inertia of molecules in question, respectively. For the high-temperature approximation, we have (15)



v (T) = v (0) - f ~ 7



v0 = v (0) - ~



where v(0) indicates the N Q R frequency in the limit of T - - 0 and f is the force constant of the torsional oscillation. For the present problem, the torsional oscillation can be regarded as the libration of octahedral anions. Equation [2] with constant f shows that v(T) decreases linearly with increasing temperature in a high-temperature region as was observed in the present complexes. When softening of the libration takes place, the force constant should decrease with decreasing temperature down to Ttr. This means that a term having a positive temperature coefficient is added to the normal Bayer term in a limited temperature range above Ttr. Accordingly, a positive temperature dependence can be expected near Ttr only when the term due to the attenuation of force constant prevails over the normal Bayer term. The above four complexes yield a small Idv/dT I value far above Ttr as compared with the others. This may be the reason why only the four complexes show a positive temperature coefficient.



To obtain information on the symmetry of the low-temperature phase of the present complexes, it is helpful to know the symmetry of the soft mode. By applying the method of factor group analysis (32) to the present crystal in the hightemperature phase, one can obtain six rotatory lattice modes, AI,, 2A2~, 2Eg, and E , in the optical branch. The four modes, Alu, A2g, Eg, and E , involve the rotational vibration of MA cations, whereas the remaining A2g and Eg modes are related to the libration of octahedral anions. In the latter two modes, A2g is of particular interest because this corresponds to the libration of complex anions about the threefold axis of the crystal. According to the theory on the soft mode phonon (33-36), softening of the A2g mode at the F point of the Brillouin zone of the rhombohedral lattice (37) leads to a structure belonging to the space group R3. In this structure, all the complex anions rotate about the threefold axis through a small angle in the same sense from the configuration of the parent structure. On the other hand, softening of the mode at the Z point results in a small rotation of complex anions in such manner that complex anions in a layer rotate about the threefold axis in the same sense, whereas those belonging to neighboring layers rotate in the opposite sense with each other as shown in Fig. 5. This causes a doubling of the unit cell, and the space group of the resulting unit cell becomes R3c. In both structures, all halogen atoms are crystallographically equivalent in agreement with the present NQR observation. Softening at the other points of the Brillouin zone leads to a lower symmetry structure incapable of taking in complex anions leaving all chlorine atoms equivalent. It may be difficult, at the present stage, to determine definitely which is more plausible for the structure of the low-temperature phase. However, we may predict from the following reason that the former is more adequate. Since MA cations perform rapid reorientation about their principal axis, interactions of MA cations with halogen atoms such as hydrogen bonding will be weak. Therefore, interlayer repulsive interaction becomes



" MX6


" MA

FIG. 5. Possible structures of the rhombohedral ( M A ) 2 M X 6 in the low-temperature phase projected along the threefold axis of crystals. The three layers of complex anions are indicated for both structures.



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