Short Notes K16 7

phys. stat. sol. (a) 5, K167 (1971) Subject classification: I. 2; 14.4.2

Department of Chemistry, Indian Institute of Technology, Kanpur

Phase Transitions in ABX3 Type Halides

M. NATARAJANandB. PRAKASH BY

An examination of the recent crystallographic literature, particularly the N. B. S.

monograph (1) on X-ray diffraction patterns, revealed that some of the compounds

of general formula ABC13 where A is Cs o r Rb and B is W or Cd, exhibit inter-

esting phase transitions. While the phase transitions in some of the chlorides have

not been completely described in the literature, there is little or no information on

the phase transitions of the corresponding bromides or iodides. We have presently

investigated the crystallography and phase transitions of the chlorides, ABC13, as

well as the corresponding bromides and iodides. In addition, we have studied the

phase transition in CsCuCl which has not been reported in the literature. We have

employed differential thermal analysis (DTA) , X-ray crystallography, and dielectric

constant measurements to study the phase transitions. These studies may be of

value in examining the possible occurrence of ferro- or antiferroelectricity in some

of the phases of ABX3 type compounds.

The results of our studies on the phase transition of ABX3 halides a re summarized

in Table 1 alongwith the tolerance factors. Of these, the I I S I I I transition of CsPbC13

has been described in (2, 3). CsPbBr3 shows a DTA peak at 130 OC corresponding to

the transition from the distorted perovskite to a cubic perovskite (a = 5.874 %) as-

sociated with negligible volume change and thermal hysteresis indicating that the

transition is of higher order (4). Recent NQR studies of Volkov et al. (6), however,

suggest this transition to be of first order; these workers report another highet

order transition around 106 C. Accordingly, our dielectric constant measurements

also indicate two anomalies at 110 and 139 OC (Fig. 1). We feel that these two tran-

sitions are likely to be similar to those described in the case of CsPbC13 (2). CsPb13

shows two dielectric anomalies at 125 and 340 OC (Fig. l), but the DTA curve shows

one reversible transition at 340 C; this behaviour is similar to that in CsPbC13 and

CsPbBr3. The transition temperature of CsPbI reported by Volkov et al. (6) appears

3

0

0

3

K168

Table 1

physica status solidi (a) 4

Phase transition

halides

CsPbCl '1 3

CsPbBr3

CsPb13

RbCdC13 9

RbCdBr3 g)

RbCdI, h)

CSCUCl3

tolerance

factor

0.826

0.813

0.806

0.835

0.828

0.820

-

DTA results

Tt (OC)

no peak

47

no peak

130

no peak

340

28

137

190

120

130

no peak

140 (130)

AH (cal mol-l) -

20 + 10 -

- 250+ 25 -

1000 + 100

350+ 50

2300+ 250

200+ 25

1060+ 100

2250+ 250

- - - - - - -

450 + 50 -

a) I, 11, and III stand for orthorhombic, tetragonal, and cubic structures respectively.

b) 11, a = 5.5901 andc = 5.477 %; 111, a = 5.605%.

c) see (7).

d) We have started with orange form of the solid (5).

8) The material transforms from brownish yellow monoclinic form (a = b = 6.150 51

f ) I, a = 8.959 % ; b = 14.976 %, and c = 4.035 x; 11, a = 10.304 %: c = 10.399 % : c = 6.230 %, fi = 88O, 15').

111, a = 10.328 S\;N rhombohedral.

Short Notes

in ABX3 type halides

dielectric anomaly

at T (OC)

increase in die- lectric constant around 40 OC

47

110

139

125

340

28

140

200

105

115

150

remarks regarding the a) transformation

second order transformation ac- companied by appearance of super structure

II*III change from antiferroelectric phase to paraelectric phase

distorted perovskite z=5 perovskite (cubic)

-

monoclinic to distorted perovskite

IV-I

I -11 (not readily reversible)

11 #HI

1-11 (not readily reversible)

I1 -III (not readily reversible)

- hexagonal to cubic i)

K169

Table 1

g)I, a = 8 . 9 5 0 % , b = 1 5 . 3 8 0 % , a n d c = 3 . 9 5 4 % .

h) I, a = 10.400 i, c = 10.820 1. i) Hexagonal; a = 7.217 8, c = 18.180 x; cubic, a = 6.221 8 .

K170 physica status solidi (a) 4

u 7600

1200

BOO

400

80 160 T°K) - TIC) -

Fig. 1. Dielectric anomalies in ABX type compounds: a) CsPbBr (sold line), CsPbI (dashed line), b) RbCdCl , c) Rb8dBr (dashed line) and &Cd13 (solid tine)

3

to be in error , Neither CsPbBr3

nor CsPbI showed dielectric

hysteresis loops in the 20 to

350 OC range.

3

The DTA curves of RbCdC13

clearly show the 1-W and II-111

transitions (at 137 and 190 C, re-

spectively) in accordance with the

N.B.S. report (1). We have

determined the cell constant of

the cubic perovskite (III) to be

0

10.328 x at 225 OC. In addition to these two transitions, we have also found another

transition at s 28 C. Considering the similarity between the transitions of RbCdC13

and BaTi03 it is possible that the 28 OC transition is from a rhombohedral phase to

an orthorhombic phase. We find dielectric anomalies associated with all the three

transitions of RbCdCl (Fig. l), but there was no clear evidence of dielectric hyster-

esis in any of these phases.

0

3

While the phase III of RbCdCl readily reverts back to 11, I1 transforms to I 3 slowly depending upon the temperature of quenching. If the quenching temperature

is s 35 OC, it takes nearly 4 h for I1 to come back to I. However, if samples heated

to temperatures above 140 C are quenched to 0 o r -190 C, I1 transforms back to

I almost immediately. Further there was evidence for the presence of small pro-

portions of I1 in the X-ray patterns of samples quenched for 4 to 5 h at room temper-

ature. RbCdCl prepared by fusion, when suddenly quenched to room temperature

exhibits the tetragonal structure (n) and the DTA curve shows only II+III trans-

formation. Samples of RbCdC13 obtained by fusion method come back to I only on long

standing at room temperature; quenching at low temperatures, however, gives rise

to I. These studies clearly bring out the metastable nature of structure II. The

0 0

3

Short Notes K171

enthalpies of the I+II and I I d I I I transformations (estimated from DTA peak areas)

are ss 2300 and x 200 cal mol-l, respectively. The AH value of I+II will be con-

siderably smaller unless care is taken to see that the starting material is pure I.

The preparation and characterization of RbCdBr and RbCdI are not reported

3

3 3 in the literature. Our preliminary investigations show that RbCdBr is orthorhombic

and is probably distorted. DTA shows an endothermic peak at at 120 OC with a

A H of ~ ~ 1 1 0 0 cal m01-l. The transEormation is not readily reversible and is likely

to be similar to the I+II transition in RbCdC13. The transformation is also in-

dicated by the dielectric anomaly at = 105 "C. The large transition enthalpy suggests

that transition is likely to be of first order.

RbCdI is found to possess distorted rutile structure isostructural with CsF%Cl3. 3 DTA shows an endothermic peak at x 130 "C with a large enthalpy of transition

(a 2250 cal mol ) suggesting a first order transition. The transformation is not

readily reversible. Two dielectric anomalies a re observed at a 115 C and also

at 4 150 OC. No satisfactory and reproducible ferroelectric hysteresis loops were

observed either in RbCdBrg or in RbCd13.

the literature (1). The crystals of CsCuC13 formed as dark red hexagonal prism

terminated by bipyramids. On grinding to powder the color of the sample changed

to dark orange yellow. 'Our DTA investigations showed an endothermic transition at

o 140 "C with an enthalpy of about 450 cal mol-l. The transformation is reversible

and is accompanied by an appreciable thermal hysteresis of about 10 deg. The fairly

large A H value and the presence of hysteresis suggest that the transformation is a

first order one. The high temperature structure was ascertained by X-ray investi-

gations and found to be face-centered cubic with a lattice constant, a = 6.221 1 (at 190 OC).

-1

0

CsCuCl is hexagonal and no transformation has been reported for this solid in 3

The authors are grateful to Prof. C. N. R. Rao for suggesting the problem and

valuable guidance throughout the investigation; the authors are also thankful to the

U. S. National Bureau of Standards for a research grant (G-51) under their Special

International Programmes.

K172 physica status solidi (a) 4

References

(1) lWandard X-Ray Diffraction Patterns", U. S. National Bureau of Standards,

Monograph No. 25, Section 5, Ed. H. E. SWANSON, H. F. MCMURDIE,

M. C. MORRIS, and E. H. EVANS, August 1967.

(2) M. NATARAJAN, S. RAMDAS, and C. N. R. RAO, Phys. Letters (Netherlands)

- 29A, 528 (1969).

(3) C.K. IWLLER, Mat. -Fys. Medd. Dan. Vid. Selsk. E, No. 2 (1959).

(4) C. N. R. RAO and K. J. RAO, in "Progress in Solid State Chemistry", Ed.

H. REISS, Vol. 4, Pergamon Press, Oxford 1967.

(5) C.K. IWLLER, Nature 182, 1436 (1958).

(6) A. F. VOLKOV, YU. N. VENETSEV, and G.K. SEMIN, phys. stat. sol. 35,

K167 (1969).

(7) T. SAKUDO, H. UNOKI, Y. FUJII, J. KOBAYASHI, and M. YAMADA, Phys. Letters (Netherlands) =A, 542 (1969).

(Received January 22, 1971)