Structural Phase Transitions in K3Na(SeO4)2 Crystals

T. KRAJEWSKI et al.: Structural Phase Transitions in K3Na(Se0,), Crystals

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phys. stat. sol. (a) 135, 557 (1993) Subject classification: 64.70; 65.40; 77.20; S1 1.1 Institute of Physics, A . Mickiewicz University, Poznah I )

Structural Phase Transitions in K,Na(SeO,), Crystals BY T. KRAJEWSKI, P. PISKUNOWICZ, a n d B. MROZ Thermal, dielectric, and elastic properties of K,Na(SeO,), crystals grown from aqueous solution are studied versus temperature. Two improper structural phase transitions are observed, on cooling, in the temperature range from 300 to 400 K: the nonferroic continuous phase transition with no change of 3m point group at 346 K, and the ferroelastic discontinuous phase transition from the trigonal point group 3m to the monoclinic 2/m one at 334 K. The first transition is manifested as anomalous temperature behaviour of heat capacity and thermal expansion of the crystal. The second one is detected by direct observations of domain structure in the polarized light below 334 K and by softening of elastic moduli in selected crystallographic directions. Die thermischen, dielektrischen und elastischen Eigenschaften von K,Na(SeO,),-Kristallen, die aus waDriger Losung geziichtet wurden, werden als Funktion der Temperatur untersucht. Im Temperaturbereich von 300 bis 400 K werden beim Abkiihlen der Kristalle zwei uneigentliche Phasenumwandlungen festgestellt : eine kontinuierliche, nichtferroische Umwandlung in der Punktsymmetriegruppe ?m bei To = 346 K und eine nichtkontinuierliche, ferroelastische Umwandlung aus der trigonalen Punktgruppe Jm in die monoklinische 2/m bei T, = 334 K. Die erste wird als eine anomale Temperaturabhangigkeit der Warmekapazitiit und der thermischen Ausdehnungskoeffizienten beobachtet. Die zweite wird direkt durch Beobachtung der Domanenstruktur in polarisiertem Licht bei T < 334 K und als eine Anderung der elastischen Koeffizienten in ausgewahlten kristallographischen Richtungen festgestellt.

1. Introduction Experimental studies of crystals containing BX, tetrahedra as structural elements relatively frequently prove the presence of order-disorder phase transitions which appear a s a consequence of spatial ordering of these elements in the crystal lattice [l to lo]. The spatial ordering of CrO, tetrahedra in the K,Na(CrO,), crystal leads, a t T, = 239 K, to the ferroelastic phase transition from the trigonal point group 3m t o the monoclinic 2/m one. This was recently observed by Krajewski et al. [ll] and confirmed then by Madariaga a n d B r p e w s k i [I21 and Mroz et al. [13]. One can expect that the K,Na(SeO,), crystal will demonstrate a similar temperature behaviour. Thus, attempts were undertaken t o grow such a crystal and t o measure some of its physical properties as a function of temperature. The paper presented contains the preliminary results obtained from the thermal, dielectric, and elastic studies of K,Na(SeO,),.

2. Experimental Results Single crystals of K,Na(SeO,), were grown isothermally a t 300 K from saturated aqueous solution by steady-state and/or dynamic methods. T h e initial reagents used for the synthesis were chemically pure sodium and potassium hydroxides and selenate acid taken in I)

Grunwaldzka 6, PL-60-780 Poznah, Poland.

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T. KRAJEWSKI, P. PISKUNOWICZ, and B. MROZ

stoichiometric ratio. The product of synthesis was purified by threefold recrystallization from distilled water. The colourless and transparent crystals were obtained in the form of hexagonal prisms of density Q = 3.15 g cm-3 and with edge length varying from 10 to 20 mm. Their chemical composition was confirmed by atomic spectroscopy (Na', K+). The heat capacity C,(T) of K,Na(SeO,), crystals was measured by the DTA method in the temperature range from 100 to 1300K. Two different rates of temperature change, 5 and 10 K/min, were used. Three different phase transitions were observed: the melting point at T, = 1170K, the high temperature phase transition in the solid state at TI = 744K, and the low temperature continuous phase transition at To = 346K. The temperature dependence of C, for K,Na(SeO,), around To is shown in Fig. 1. The C,(T) values of K,Na(SeO,), were calculated using the method proposed by Kaisersberger et al. [14]. The obtained results were used for the determination of the entropy S and the enthalpy H changes at this transition. The increase in the entropy AH and enthalpy AS for K,Na(SeO,), at To was found as (1.7 A 0.2)J rno1-I K - ' and (550 60) J/mol, respectively. Thus, the entropy of the transition is ASIR = 0.20. The thermal expansion of K,Na(SeO,), was measured in the temperature range from 300 to 400 K using a push-rod type dilatometer [15] of a sensitivity to Measurements were carried out both on cooling and heating the crystal rods oriented along the three main crystallographic directions. Thermal expansion coefficients cli (i = 1,2, 3) of the crystal rods as function of temperature are given in Fig. 2a. Above Tolinear changes of ui versus T were observed. From the curves presented in Fig. 2a the jumps in Aui at Towere determined K-', and Au3 K-' , Au, = -(3.4 0.5)~ as follows: Aal = - (9.4 k 1.0) x = + (32.2 3.2) x K-'. Hence, the jump in the volume expansion coefficient K-I. Fig. 2b shows the temperature A/) = Aai was found to be (19.4 k 4.7) x

+

i

dependence of the volume expansion coefficient

p. The effect of hydrostatic pressure

460 300

320

340

360

380

Temperature ( K )

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Fig. 1 . Temperature dependence of heat capacity C, for K,Na(SeO& in the 300 to 400 K region

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Structural Phase Transitions in K,Na(SeOJz Crystals

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on the phase transition temperature To may be determined from the relation: dTo/dp = (Ag To)/(AC,Q)= - (3.4 f 1.5) x K/Pa. This value will be verified in further experimental studies of K3Na(Se04)2crystals. The dielectric properties of K,Na(SeO,), were studied in the temperature range from 300 to 400 K using an automatic C-bridge at 1 kHz. Fig. 3a and b show temperature dependence of dielectric permittivities cI1, E ~ E~~~ and , dielectric losses, tg 6, measured in the directions [ 1001, [OlO], and [OOl]. The only indication of the phase transition at To = 346 K is the change in slope of the c l l ( T ) and E , ~ ( T functions ) observed below To. The dielectric permittivity c33 exhibits a linear temperature dependence in the entire temperature region studied. As it is evident from Fig. 3b the coefficients tg 6 increase continuously with increasing temperature. Above To = 346 K, K3Na(Se04)2belongs to the trigonal point group 3m for which the elastic stiffness tensor contains six independent components: c I 1= cZ2,c33, c4, = c ~ ~ , c12, cI3, c14, and 2c, = cI1 - cI2. The elastic properties of K,Na(Se04)2 were measured in the 300 to 400 K temperature region using two independent methods: torsion vibration technique [16] and composite bar method [17].

T. KRAJEWSKI, P. PISKUNOWICZ, and B. MROZ

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Fig. 3. Temperature dependence of a) real components E , , , czz, E~~ of the dielectric permittivity tensor and b) dielectric losses tg 6 of K3WSeOJ2

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The first one allowed us to observe the temperature dependence of torsional moduli G, cs,), and G,(c,,, c ~ ~ )The . obtained results are given in Fig. 4. As is evident from Fig. 4 all torsional moduli are affected by the low temperature transition. The highest anomalies were found for samples cut along the [loo] and [OlO] directions. The minima of Gi were detected at T, = 334 K, which is by about 12 K lower than the anomalies observed in the course of thermal and dielectric studies. The elastic anomalies point to a discontinuous character of the transition observed at T,. The composite bar method was applied to find the temperature dependence of the longitudinal elastic constants cI1 = c Z 2 and c , ~ . The transducers were X-cut quartz bars with resonant frequencies of 195 and 202 kHz. The samples were prepared in the form of X - , Y-, and Z-cut bars. Fig. 5 presents the temperature dependence of c l l r cZ2,and c , ~ elastic constants. The effect of acoustic phonon softening is clearly seen in case of the cll elastic constant, whereas c,,(T) shows only a slight anomaly at T, = 334 K. Again, as in the case of torsional moduli related with the elastic shear constants, the minima of c1 and c3, were detected at T, = 334 K. (i = 1,2,3) related with the elastic shear constants, namely G , ( C ~c,,), ~ , G,(c,,,

561

Structural Phase Transitions in K,Na(SeO,), Crystals

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380 Temperature ( K )

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Fig. 4. Temperature dependence of torsional moduli G i(i = 1,2,3) of K3Na(SeOJ2

Below T, = 334 K a ferroelastic domain structure was observed in the plane (001) using a polarizing microscope.This domain structure is shown in Fig. 6. The orientation of domain walls is described by the following equations: and Y = f 1 / 5 X .

Y=O

In the (100) and (010) planes no domain walls were observed.

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Fig. 5. Temperature dependences of c l l rc 2 2 ,and c , ~elastic moduli of K,Na(SeO,), measured by the composite bar method

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T. KRAJEWSKI, P. PISKUNOWICZ, and B. MROZ

Y I

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Fig. 6. Ferroelastic domain structure of K,Na(SeO,), observed at room temperature along the [OOl] direction

Neither pyroelectric nor piezoelectric phenomena were observed in the K,Na(SeO,), crystal in the whole temperature region studied. 3. Summary and Conclusions

The K,Na(SeO,), crystal belongs to the glaserit family whose well-known representatives are K,Na(SO,), and K,Na(CrO,),. Taking into account a similar temperature behaviour of the corresponding sulphates and chromates, resulting from the phase diagrams and structure studies performed by Hilmy [18], Eysel [19, 201, and Goldberg et al. [21], one can suggest for K,Na(SeO,), at TI = 744 K a phase transition from the high temperature phase 6/mmm to the phase containing the glaserit structure with the point group 3m. Thus, at a sufficiently low temperature a discontinuous elastic transition 3rn 2/m may be expected. This suggestion follows also from the results of group-theoretical considerations performed for order-disorder transitions by Boccara [22] and Janovec et al. [23]. Such a sequence was confirmed in the case of K,Na(CrO,), crystals [ll]. The thermal and dielectric behaviour of K,Na(SeO,), at To = 346 K points to the occurrence of a continuous phase transition at this temperature. The relatively small softening of the elastic moduli cij observed at T, = 334 K may be connected with a

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Structural Phase Transitions in K,Na(SeO,), Crystals

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discontinuous elastic transition 3m -, 2/m in accordance with the results obtained by Boccara [22] and Janovec et al. [23]. Preliminary structural data resulting from X-ray studies performed recently by Fabry et al. [24] for K,Na(SeO,), crystals at two different temperatures point to the symmetry change of this crystal from the space group P3ml at 390 K down to C2/c at 291 K accompanied by the fourfold multiplication of the unit cell in the low temperature phase. Such a result, together with the temperature behaviour of the thermal, dielectric, and elastic properties of K,Na(SeO,), presented here, in accordance with the results obtained from the analysis based on the Landau theory performed by Toledano and Toledano for nonferroic [25] and ferroic [26] phase transitions may be explained as follows: 1. At To = 346 K, on cooling, K,Na(SeO,), undergoes a continuous nonferroic phase transition from the P3ml space group to the P k l one accompanied by the twofold multiplication of the unit cell in the c-direction, 2. At T, = 334 K a discontinuous ferroelastic phase transition takes place from the P k l space group to the P2,/a, P2/a, or C2/c one, accompanied by the doubling of the unit cell in the a-direction. The twofold doubling of the unit cell at these transitions may explain the fourfold multiplication of the unit cell in the monoclinic phase with respect to the trigonal one as it results from X-ray studies [24]. As both transitions are accompanied by a modification of the translational symmetry of the crystal, they must be classified as improper ones, since the order parameters of these transitions are not translationally invariant. Acknowledgements We are grateful to Dr. J. Fabry from the Institute of Physics, Czechoslovak Academy of Sciences, and his collaborators for presenting us unpublished data of their X-ray studies of K,Na(SeO,), crystal. References [I] [2] [3] [4] [5]

Y. MAKITA,A. SAWADA, and Y. TAKAGI,J. Phys. SOC.Japan 41, 167 (1976). S. SHIOZAKI, A. SAWADA, Y. ISHIBASHI, and Y. TAKAGI, J. Phys. SOC.Japan 43, 1314 (1977). E. F. DUDNIK,Fiz. tverd. Tela 19, 865 (1977). H. G. UNRUH,Ferroelectrics 25, 507 (1980); 36, 359 (1981). K. S. ALEXANDROV, L. I. ZHEREBTSOVA, 1. M. ISKORNEV, A. I. KRUGLIK,0.V. ROZANOV,and I. N. FLEROV,Fiz. tverd. Tela 22, 3673 (1980). [6] A. PIETRASZKO, P. E. TOMASZEWSKI, and K. LUKASZEWICZ, Phase Transitions 2, 141 (1981). [7] G. PAKULSKI, B. MROZ,and T. KRAJEWSKI, Ferroelectrics 48, 259 (1983). [8] H. SCHULZ,U. ZUCKER,and R. FRECH,Acta cryst. B41, 21 (1985). [9] H. KLAPPER, TH. HAHN,and S. J. CHUNG,Acta cryst. B43, 147 (1987). [lo] A. 1. KRUGLIK, S. V. MELNIKOVA, and V. I. VORONOV, Fiz. tverd. Tela 28, 1215 (1990). [ l l ] T. KRAJEWSKI, B. MROZ,P. PISKUNOWICZ, and T. BRECZEWSKI, Ferroelectrics 106, 225 (1990). [I21 G. MADARIAGA and T. BRQCZEWSKI, Acta cryst. C46, 2019 (1990). and J. A. TUSZYNSKI, Phys. Rev. B43, 641 (1991). [I31 B. MROZ,H. KIEFTE,M. J. CLOUTER, [I41 E. KAISERSBERGER, J. JANOSCHEK, and E. WASSER, Thermochim. Acta 148, 499 (1989). [I51 P. PISKUNOWICZ, T. BRBCZEWSKI, and T. WOLEJKO,Mater. Sci. 15, 69 (1989). [I61 G. PAKULSKI, J. Phys. E 15, 951 (1982). [I71 S. KUDOand T. IKEDA,Japan. J . appl. Phys. 19, L45 (1980). [18] E. M. HILMY,Amer. Mineralogist 38, 118 (1953).

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[19] W. EYSEL,Z. Krist. 132, 426 (1970). [20] W. EYSEL,Amer. Mineralogist 58, 736 (1973). W. EYSEL,and TH.HAHN,Neues Jahrbuch Mineralogie 6,241 (1973). [21] A. GOLDBERG, [22] N. BOCCARA, Ann. Phys. (Paris) 47, 40 (1968). [23] V. JANOVEC, V. DVORAK, and J. PETZELT,Czech. J. Phys. B25, 1362 (1975). [24] J. FABRY,T. BRFCZEWSKI, F. J. ZUNIGA,and V. PETRICEK, Acta cryst., to be published. [25] P. TOLEDANO and J. C. TOLEDANO, Phys. Rev. B 25, 1946 (1982). [26] J. C. TOLEDANO and P. TOLEDANO, Phys. Rev. B 21, 1139 (1980). (Received June 25, 1992)