Phase transitions and thermal expansion in langbeinite type compounds

"

°~ Solid State Communications,

Vol.66,No.4,

pp.375-378,

1988.

0038-1098/88 $3.00 + .00 Pergamon Press plc

~a~_~ Printed in Great Britain.

PHASE TRANSITIONS

AND THERMAL EXPANSION

IN LANGBEINITE

TYPE COMPOUNDS*

Mojtaba Kahrizi and M.O. Steinitz Department of Physics St. Francis Xavier University Antigonish, Nova Scotia Canada B2G IC0 (Received 26 January 1988 by M. F. Collins)

We report thermal expansion measurements using capacitance dilatometric methods on several polar and non-polar compounds of the langbeinite family, from liquid helium to room temperature. In K2Mn2(SO4) 3 and T 1 2 C d 2 ( S O 4 ) 3 new thermal e x p a n s i o n anomalies indicative of phase transitions were detected. Similarly, two phase t r a n s i t i o n s were detected for the first time at 227.8 K and 330.8 K respectively in TI2Mg2(SO4) 3. In (NH4)2Mg2(SO4) 3 two phase transitions were discovered at 220 K and 241 K with d e c r e a s i n g temperature, a l t h o u g h with increasing temperature only one phase transition could be detected.

I.

Introduction

T I 2 M n 2 ( S O 4 ) 3, (TMnS); T12Mg2(S04)3, (TMgS) and (NH4)2Mg2(SO4)3, (AMgS). All of these, with the exception of KMnS, are f e r r o e l e c t r i c at low temperature.

Crystals of the langbeinite family, with the general formula (M+)2(M2+)2(S04)3, (M + and M 2÷ represent monovalent and divalent ions respectively), have been the subject of many investigations. Many of these compounds have shown several structural phase transitions below room temperature. Among these some, such as (NH4)2Cd2(SO4) 3, T12Cd2(SO4) 3, and Rb2Cd2(SO4) 3, are f e r r o e l e c t r i c s below the h i g h e s t phase transition temperature, and others, such as K2Mn2(SO4)3, and K 2 C o 2 ( S O 4 ) 3 , undergo nonferroelectric phase transitions. However, very few of these transitions have been investigated in detail and the n a t u r e and type of these transitions have remained unexplained. X-ray studies have shown that these crystals have a cubic structure with space group P213 at room temperature I and that the lattice parameter is roughly I0 A, with four formula units per cell. Ferroelectric behavior in these crystals was first discovered by Jona and Pepinsky 2 using dielectric measurements on (NH4)2Cd2(SO~) 3. D v o r a k 3 has t h e o r e t i c a l l y enumerated all possible phase transitions in these c r y s t a l s . According to his work these crystals, with space group P213 at room temperature, can transform to low temperature phases with the possible space groups P21, R3, PI and P21212 I. The occurrence of ferroelectricity and the various phase transitions reported in the crystals of this family in the literature are summarized in table I. In this paper we report thermal expansion measurements from liquid helium temperature to room temperature on R d 2 C d 2 ( S O 4 ) 3 , (RCdS) ; (NH4)2Cd2(SO4)3, (ACdS); TI2Cd2(SO4) 3, (TCdS); K2Mn2(S04) 3, (KMnS) ; (NH4) 2Mn2(SO4) 3 , (AMnS) ;

2.

Experiment

and Results

A highly sensitive capacitance dilatometer 4, (potentially sensitive to length changes as small as 10 -10 m) was used to measure the thermal expansion of the crystals. The t e m p e r a t u r e (varied at a rate of about 0.1 K/min) was measured using a calibrated silicon diode. All the data were collected using a Commodore B128 microcomputer. The measurements reported are relative to the thermal expansion of the brass capacitance-cell material. Single crystals were grown by s l o w evaporation of an aqueous solution c o n t a i n i n g (M+)2S04 and (M2+)SO4 (in a 1:2 molar ratio) at a temperature above 358 K. ACdS, TCdS and RCdS have similar growth habits5 and grow along <111> axes, while KMnS~ TMnS a n d A M n S have similar growth habits U different from those of the f i r s t - m e n t i o n e d compounds, but with easily r e c o g n i z e d <111> planes. All measurements on these compounds were carried out along <111>-axes of the crystals. The preparation of single crystals of TMgS and AMgS from aqueous solutions is impossible. Therefore, polycrystalline samples of TMgS and AMgS were prepared from aqueous solutions at 393 K.7 Attempts to prepare several other langbeinite crystals, such as K 2 N i 2 ( S 0 4 ) 3 , (PNiS), (NH4)2Ni2(SO4)3, (ANiS), K2Ca2(SO4) 3, (PCaS) and T12Ca2(SO4)3, (TCaS), from aqueous solutions failed. Among the above compounds, it has been r e p o r t e d that PNiS and ANiS can be prepared by solidifying the molten salts at 973 K and 593 K respectively7, but we did not attempt this. For the m e a s u r e m e n t s presented in this paper, samples with s~zes roughly 2x2x2 mm3 with parallel faces p e r p e n d i c u l a r to the axis of

* This work was supported in part by the Natural Sciences and E n g i n e e r i n g Research Council of Canada and the St. Francis X a v i e r U n i v e r s i t y Council for Research. 375

376

THERMAL EXPANSION IN LANGBEINITE TYPE COMPOUNDS

measurement were chosen. Measurements on each sample were r e p e a t e d several times, and the results were reproducible. The anomalies always appeared at the same temperature for the same direction of temperature change, and size of the anomalies varied by less than 10% over several measurements. A f t e r r e p e a t e d w a r m i n g and cooling, the samples absorbed moisture and the results began to d e v i a t e more. Thermal hysteresis was examined for all transitions. 2.1

Rb2Cd2(S04)3:

I

I

I

I

I

I

I

l

>,

E 3x~0-

(Fig.l)

As the t e m p e r a t u r e was i n c r e a s e d , the crystal expanded monotonically with the following exceptions: a) Around 60 K some interruptions appeared, followed by a relatively sharp anomaly at 64.5 K. The sharp change in the t h e r m a l expansion curve and the 2 d e g r e e t h e r m a l hysteresis indicate that the phase transition at this temperature is of first order, b) At 97 K the thermal expansion of the sample showed the beginning of a further change, which proceeded through two steps up to 104.5 K. A thermal hysteresis of 4 K was d e t e c t e d for this transition upon cooling and warming the samples, revealing that this transition is of first order. c) At 129 K an apparently second order phase t r a n s i t i o n a p p e a r e d in the thermal expansion curve. No thermal hysteresis was observed for this transition. Further details are included in Table I. 2.2

I

Vol. 66, No. 4

(NH4)2Cd2(S04)3:

I

I

i

i

I

I

I

I

I

90 91 92 93 94 95 96 9? 98 99 100 Temperature ( Kelvins ) F i g u r e 2: Thermal expansion <100> axis versus temperature, brass dilatometer cell. '

I

'

I

'

I

of ACdS along a relative to the

'

I

'

I

'

I

>L_

(Fig. 2)

A single phase transition was observed for the case of ACdS at 93 K. The thermal expansion coefficient changes sharply at this temperature. A thermal h y s t e r e s i s of about I K e l v i n was detected, indicating that the transition has some first order nature. I

2.3

TI2Cd2(S04)3:

At least three phase transitions were observed, at 95.2 K (with 2.5 K thermal hysteresis), 121 K

' ........

I

I

I

I

85

(Fig. 3)

I

I

I

I

I

t

93

I

t

I

I

I

J

101 109 117 Temperature ( Ketvins )

I

125

i

I

133

Figure 3: Thermal expansion of TCdS along a <111> axis as a function of temperature, relative to the brass dilatometer cell.

I

f//"

/ J

,/

>

.//

~ 2x10-" E 3x10-

/

/ / /

/

rr

50

I

I

60

?0

I

I

I

I

I

I

I

80 90 100 110 120 130 140 150 Temperoture ( Kelvins )

Figure I: Thermal expansion of RCdS along a <111> axis as a function of temperature, relative to the brass dilatometer cell.

I

175

180

I

I

185 190 TemperQture ( Kelvins )

i

195

200

Figure 4: Thermal expansion of KMnS along a <111> axis as a function of temperature, relative to the brass dilatometer cell.

377

THERMAL EXPANSION IN LANGBEINITE TYPE COMPOUNDS

Vol. 66, No. 4

Table I : Phase transition temperature, phase transition type, space group and ferroelectricity of various langbeinltes. ((F) indicates a first order, (S) a second order phase transition).

Crystal

Transition Temp (K) This work (type) Literature 93

ACdS

RCdS

TCdS

(F)

(S)

12910,11,5

97

(F)

10310,11, 5

64.5

(F)

6811

127.5 (F)

12812

121

(F)

12012

107

(F)

---

95.5

(F)

921 2

--

KMnS

190.5 (F) 178

Ferroelectric

892 , 929

129

AMnS

Reported Space Group Below Transition

P21

Yes

P21

Yes

PI

No

P212121

No

P21

Yes

Pl

Yes

PI

Yes

P21 21 21

No

P21 21 21

No

1938

(F) ?

TMnS AMgS

241

(F)

220

(F)

2218,7

230.8 (S)?

TMgS

227.8

(S)?

(2 K hysteresis) and 127.5 K (2.5 K hysteresis), in good agreement with p r e v i o u s reports. A further transition at 107 K, with I K thermal hysteresis was a l s o detected. The phase transitions at 121 K and 127.5 K appear to proceed in two steps. The length change at the 107 K transition is very small compared to that at the other transitions, which may be the reason it was not detected by other techniques. 2.4

K2Mn2(S04)3:

(Fig. 4)

Among the compounds investigated in this work, only KMnS is non-ferroelectrlc. KMnS is the only l a n g b e i n i t e type c o m p o u n d with the divalent Mn ion which has shown phase transitions at low temperatures (see s e c t i o n II.6). Two first order phase transitions were detected at 178 K and 190.5 K in this compound. Thermal hystereses of 4 K for the 190.5 K transition and

3.5 K for the 178 K transition were detected. Only one phase transition, at 193 K, had previously been reported in this compound. 8 No information is available about the phase below the 178 K transition. 2.5 T12Mg2(S04) 3 and (NH4)2Mg2(S04)3 : (Figs 5 and

6) In TMgS (Fig. 5) two phase transitions were detected at 227.8 K and 230.8 K. It is difficult to determine the type of the transitions from the shape of the anomalies in the thermal expansion curves. No hysteresis greater than 0.1 K was seen when the runs were repeated for cooling and warming, which may indicate that these phase transitions are of second order. This is the first time that phase t r a n s i t i o n s have been reported in TMgS. In A M g S (Fig. 6), u p o n lowering the

378

THERMAL EXPANSION I

I

I

I

I

I

I

I

IN LANGBEINITE

I

-TT

~1 5x10_5~-

/

I/I 12

m .>

I

220

I I I I I I I I 224 228 232 236 240 Temperature (Ketvins)

Figure 5: Average (polycrystalline) thermal expansion of TMgS as a function of temperature, relative to the brass dilatometer cell.

I I~ I I

I

I

I

I

I

Vol. 66, No. 4

temperature, phase transitions were observed at 247 K and 220 K. Both phase transLtions have a very large thermal h y s t e r e s i s . The phase transition seen at 220 K on cooling had a thermal hysteresis of 49.5 degree (upon warming it was detected at 269.5 K). The phase transition at 241 K had even larger hysteresis (we could not detect it when the temperature was raised to 300 K). The measurements were repeated several times and were checked on several samples. The results are quite reproducible. The relative dilatation of the crystal at 241 K is almost 10 times larger than that which occurs at 220 K. The large thermal hysteresis in both transitions, and also the sharp a n o m a l i e s at the phase t r a n s i t i o n temperatures, indicate that both phase transitions are first order. Although a phase transition in AMgS at 221 K was reported by Hikita et al.7, the ferroelectrlc properties of TMgS and AMgS were not determined. Therefore, the nature of the transitions in both these compounds are not known and more investigation will be r e q u i r e d in order to clarify them. 2.6

~J

TYPE COMPOUNDS

T12Mn2(S04)3

and (NH4)2Mn2(S04)3:

No phase transitions were d e t e c t e d for these compounds in the temperature range from 77 to 300 K. The dielectric properties of these materials were investigated by Hikita et al. 7, and they have not found any phase transitions in these crystals.

m

~Q.4 x 10 Summary

m

200

I

I I I I I I I I 220 240 260 280 300 Temperature ( Kelvins )

Figure 6: Average (polycrystalline) thermal expansion of AMgS as a function of temperature, relative to the brass dilatometer cell for a) cooling and b) warming the sample.

The thermal expansion of crystals of several langbeinite compounds was examined. New phase transitions were d e t e c t e d in TCdS and KMnS. Phase transitions were found in TMgS and AMgS, previously unexamined members of this series of compounds. In the case of AMgS, an unusually large h y s t e r e s i s was o b s e r v e d in both phase transitions. It was found that AMgS has the closest phase t r a n s i t i o n to room t e m p e r a t u r e among the langbeinite crystals.

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