Phase transition, ferroelasticity and ferroelectricity in Li3InF6

May 1996

ELSEVIER

Materials Letters 27 (1996) 33-39

Phase transition, ferroelasticity and ferroelectricity

in Li 3InF,

M. Halmadkne a, J. Ravez b, J. Grannec b3*, A. La’idoudi-Guehria

a

a tiboratoire de Cristallographie Applique’e, U.S.T.H.B., BP 32, El Alia, 16111, Alger, Algkrie b Institut de Chimie de la Matike Condenste de Bordeaux, UPR CNRS 9048, Avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France Received 29 September

1995; accepted 3 November

1995

Abstract The ternary fluoride Li,InF, exhibits three allotropic forms, (Y, /3 and y. Single crystals of the (Y and y modifications have been obtained by the Bridgman technique. The X-ray diffraction study shows the low-temperature a-form to be

monoclinic and the high-temperature y-form to be orthorhombic. On the basis of optical and dielectric simultaneous ferroelast:ic and probably ferroelectric behaviours are assigned to this compound.

measurements,

Keywords: Fluorides; Stroctural phase transition; Ferroelasticity; Ferroelectricity

1. Introduction

this investigation, been performed.

Various structural types have been attributed to the compounds of general formula A sMF6 (A = monovalent element and M = 3d or IIIB elements) [I]. When A is sodium, these compounds crystallize with the structure of the cryolite Na,AlF, [2]. The homologous lithium compounds exhibit different structural types. For instance several polymorphic forms have been reported for Li, AlF, [3]: the (Yvariety crystallizes with an orthorhombic symmetry [4]. Many other Li,MF, fluorides (M = Ti, V, Cr, Fe, Ga) have been found to present an orthorhombic and/or a monoclinic symmetry 15-81. Concerning indium hexafluoride Li,InF,, evidence of two phase transitions had been :previously given, but attempts to prepare single crystals failed at that time [9]. It seemed therefore worthwhile to characterize the three allotropic forms. In addition, in order to complete

* Corresponding

author.

dielectric

and optical studies have

2. Experimental LiF was a commercial high-purity grade reagent which has been dried before use, at 150°C under vacuum. Indium trifluoride was prepared by preheating a mixture of In,O, and NH,HF, in a platinum crucible and then treated by gaseous HF at 700°C. These fluorides were kept, weighed and mixed in a dry glove box. Li,InF, was prepared by solid state reaction from a stoichiometric mixture of the binary fluorides. The reaction was carried out in a platinum tube, sealed under dry argon over 20 h at 600°C. Several annealings under the same conditions were necessary to get pure samples. The stabilization of the low-temperature form (a) or high-temperature form (y) depended on the cooling rate. Ceramics were prepared by sintering compressed

00167-577X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0167-577X(95)00257-X

34

M. Hamadhe

et al. /Materials

pellets of the powder in platinum sealed tube for 2 h at 610°C. The compactness reached 0.90. Single crystals of the two varieties could be obtained by the Bridgman method: about 5 g of the appropriate mixture of LiF and InF, were heated over 48 h at 600°C in a biconical shaped platinum tube to achieve the reaction. Then the temperature of the tube was raised up to 680°C (i.e. = 50°C above the melting point) for 2 h and slowly lowered to room temperature at a 5”C/h rate. Small crystals of a-Li,InF, were obtained by this procedure. Slow cooling between T = 680°C and 610°C followed by rapid quenching in water, allowed to extract crystals of y-Li,InF,, but of less crystalline quality. Lane, precession and Weissenberg photographs were used to characterize the symmetry of the different forms. The lattice constants were refined from the data of the powder diffraction patterns by a least-squares method. Micro-DTA allowed to determine the transition temperatures. The sample was set in a platinum tube sealed under dry argon. The thermal dependence of the powder pattern was followed with a GuinierSimon camera. Densities were obtained using the hydrostatic method. Dielectric measurements under dry helium or vacuum at different frequencies (102, lo3 and lo4 Hz) were performed on ceramics after deposition of gold electrodes on the circular faces by sputtering. Optical investigation was carried out on a single crystal after polishing using a polarising microscope (Leitz Ortholux II Pal).

3. X-ray diffraction ature

investigation

at room temper-

The powder patterns of Li,InF, show obvious differences with the other lithium hexafluorides. Unlike these previously reported lithium fluorides, Li,InF, does not crystallize in the space groups Pna2, as a-Li, AlF, [4] or C2/c as Li, FeF, [8]. The substitution of the small trivalent cations A13+ (r = 0.53 A> or Fe3+ (r = 0.64 A> by an ion of larger size such as In3+ (r = 0.80 A> [lo] leads to the formation of new structural types. Precession and Weissenberg photographs showed the symmetry of the low-temperature form C(Y) to be

Letters 27 (199b1 33-39

Table 1 X-ray powder pattern of a-Li,InF6 hkl

(300 K)

d,;,,,

d all\

(9

(‘Q

210

4.460

4.46

100

IOi 101

4.378 4.331

4.37 4.33

33 53

1Ii 111 121 121

3.91 I 3.876 3.083 3.066

3.911 3.876 3.083 3.066

52 38 23 26

201 301

2.828 2.789

2.829 2.789

5 4

31i 311 400 230 410

2.689 2.656 2.598 2.528 2.489

2.689 2.656 2.598 2.528 2.489

14 15 5 5 2

13i 131

2.414 2.406

2.414 2.406

10 9

32i 321

2.361 2.347

2.370 2.346

4 6

401 401

2.299 2.27 1

2.298 2.211

6 8

231 420 040

2.243 2.230 2.170

2.243 2.230 2.170

9 21 7

212 212

2.124 2.102

2.124 2.102

16 22

33i 331

2.022 2.008

2.023 2.008

5 7

14i 141

1.944 1.940

1.944 1.940

7 7

5oi 501

1.912 1.897

1.920 1.898

2 2

I/h

monoclinic. The observed reflections Ok0 with k = 2n are consistent with the possible space groups P2,/m or P2,. The refined unit-ceil parameters are the foljowing: a, = 11.515 )0.005 A, b, = 8.684+0.005 A, c, = 4.794 * 0.005 A, p, = 115.44”. However, we have chosen to index all the diffraction lines in a pseudo-orthorhombic cell (Table l), because its crystalline network is closely related to that of the /3-form subsequently described. The refined lattice constants are then the followitg: a2 = 10.399 f 0.005 o A, b, = 8.684 f 0.005 A, c2 = 4.794 f 0.005 A, p2 = 90.85”, with a, sin p, = a2 sin p2, b, = b, and c, = c2.

M. Hamad&e Table 2 X-ray powder pattern of y-Li,InF, hkl

002 101 011 110 112 103 200 013 020 113 004) 210 202 211 121 114 213 123 220 105 204 301 222

et al. /Materials

35

(300 K)

d talc

d c&s

(A)

(A,

4.473 4.410 4.202 3.470 2.741 2.570 2.535 2.527 2.378 2.261

4.41 4.41 4.20 3.470 2.742 2.571 2.535 2.528 2.379 2.262

100 75 65 52 53 15 1 17 1 25

2.237

2.237

24

2.205 2.170 2.094 1.880 1.789 1.746 1.735 1.687 1.677 1.660 1.617

2.206 2.170 2.095 1.880 1.789 1.746 1.735 1.687 1.617 1.660 1.618

10 18 21 13 9 18 14 7 9 3 6

I/b

The experimental density ( p,, = 3.84 g/cm31 is in good agreement with the calculated one ( pcalc = 3.83 g/cm3) for Z =; 4. In the same way, a single crystal investigation of y-Li,InF, led to an orthorhombic symmetry. The observed reflections: h01 with h + I= 2 n and 0 kl with k + I= 2n, are compatible with the space groups Pnnm or Pnn2. The crystallographic data are the following: d, = 5.069 + 0.005 A, b; = 4.759 h 0.005 z& c; = 8.945 f 0.005 A* The experimental density ( p,, = 3.85 g/cm31 is in good agreement with the calculated one ( pcalc = 3.84 g/cm3) for Z ==2. The X-ray powder pattern is given in Table 2.

4. Structural

Letters 27 (1996) 33-39

phase transition study

The compound Li,InF, undergoes two reversible phase transitions. On the micro-DTA diagram, two endothermal peaks are observed with thermal hys-

ado

lob0

*lo

Fig. 1. DTA curve of Li,InFb.

10.0

9.5

2c,*2c.

b tiwf++ 2c

9.01i’l 350

450

550

650

Fig. 2. Thermal dependence of the unit-cell parameters (The (Y* p transition results in a /3 angle of 90”.)

750

‘WO

of Li,InF,.

M. Hamad&e

36

et al./Materials

teresis at r, = 688 f 5 K and T2 = 783 f 5 K on heating. Such a result implies the two transitions to be of the first order. A third peak detected at 900 f 5 K corresponds to the melting point (Fig. 1). The thermal variation of the powder pattern was then followed from 300 to 873 K. Only the first transition occurring at T, = 688 + 15 K is observed; in fact a reaction between the compound and the quartz capillary takes place before reaching the temperature T2. The temperature dependence of the unit-cell parameters is given in Fig. 2. The intermediate p phase was thus characterized and found to be orthorhombic, isostructural with a-Li 3AlF,. The unit-cell parametets determined at 723 K are: a, = 10.482 $- 0.005 A, b, = 8.725 f 0.005 A, c, = 4.904 f 0.005 A. The powder diffraction data for p-Li, InF, are reported in Table 3. The three varieties of Li,InF, are related by the relationships: a,sinp=a,=2ab, bln = b, = c;

Letters 27 (1996) 33-39 Table 3 X-ray powder pattern of /3-Li,InF, hki

210 020 111 121 311 400 230 410 131 321 401 112 231 420 040 212 022 510 312 431 232

at 723 K

d,,,,

d “b,

(A,

(A)

4.493 4.363 3.958 3.112 2.705 2.620 2.543 2.509 2.433 2.383 2.311 2.303 2.258 2.246 2.181 2.152 2.138 2.038 1.956 1.809 1.765

4.49 4.36 3.960 3.115 2.707 2.620 2.542 2.508 2.433 2.385 2.311 2.301 2.257 2.247 2.180 2.154 2.139 2.039 1.957 1.807 1.766

I “b\

s s s m VW w In In w w w VW m m VW w VW

w w VW VW

and

in which m and o notations correspond respectively to the monoclinic and orthorhombic phases.

5. Dielectric and optical investigations

tanS b m -60

-40

I

600

Fig. 3. Thermal variation Hz and (b) 10’ Hz.

7no

ooo

I

9Qo

of E: and tan 6 for Li,InF,

TO

at (a) 10’

The dielectric measurements performed on ceramics and under vacuum at lo* and lo3 Hz showed that the investigated compound exhibits anomalous variations of the permittivity e: on heating. A sharp maximum of the permittivity, associated with a minimum of the dielectric losses tan 6, occurs at a temperature close to 800 + 15 K (Fig. 3); however, only a flattened phenomenon appears at 700 f 15 K. This result is generally characteristic of a ferroelectric behaviour for T < 800 K. The /3 + y transition is of ferroelectric-paraelectric nature and T, is the Curie temperature. A single crystal of cr-Li,InF, (low-temperature form) was selected and polished in order to get a thickness close to 0.2 mm. Optical observation at

M. Hamadhe et al. /Materials Letters 27 (1996) 33-39

Fig. 4. Thermal evolution

‘of the domain structure of a Li,InF,

crystal on heating. (a) 303 K, (b) 783 K,

37

(c)

784 K, (d) 785 K, (e) 788 K.

38

M. HamadGw et al./Materials Letters 27 (1996133-39

M. Hamadhe

et al. /Materials Letters 27 (1996) 33-39

room temperature shows a ferroelastic domain structure (Fig. 4a). The thermal evolution of the domain structure was followed from 300 to 880 K. The domains remain visible up to 785 f 3 K. Just below this temperature they disappear gradually with increasing slowly the temperature (Fig. 4b, 4c and 4d). At 788 f 3 K, the crystal is single domain and appears in extinguishing position (Fig. 4e). This evolution is reversible when temperature is decreasing. However, by 45” rotation of the microscope stage without any change of temperature ( = 790 K), a new domain structure is revealed (Fig. 5a). Such a domain structure persists by heating from about 790 K up to the melting point (T’,,,, = 900 K). This feature implies that the prototype phase is virtual since it has not been reached even at the melting point; result in good agreement with the crystallographic data. By cooling from about 790 K these domains disappear progressively near 785 f 3 K (Fig. 5b, 5c and 5d). In the temperature range 300 K < T < 785 K and with the same orientation, the single crystal is in extinguishing position (Fig. 5e). Here also, this behaviour is reversible in temperature. The two different domain structures, corresponding to two orientations of the domain walls may be both visualised at a temperature close to 785 K only by rotation of the crystal. Such an effect is characteristic of the coexistence of the two phases p and y, implying thus the transition to be of first order.

6. Discussion

The phase transition sequence in Li,InF, can be determined from the point groups of the three phases Q, p, y and from the ferroelastic and ferroelectric properties for T < T,,_,t = 900 K, and for T < 785 K respectively. In fact the ferroelectric ( /3 )-paraelectric (y) transition :tmplies this one to be of polarnon-polar nature: this allows us to choose the nonpolar mmm point group instead of mm2 for the y-phase. As a ferroelastic-paraelastic transition corresponds to a symmetry change, the prototype phase has to be either tetragonal or cubic. In another way,

39

as PLi,InF, and a-Li, AlF, (whose prototype phase is cubic [ 111) are isotypic, the virtual prototype phase of Li,InF, has to be also cubic. The proposed transition sequence for Li,InF, is thus the following: Tl

TZ

2+mm2+mmmTZ’(--

+m3m)

7. Conclusion The two phase transitions of Li,InF, at Tl = 688 f 5 K and T2 = 785 k 5 K have been confirmed by various techniques. The symmetry of the & and y forms has been determined on a single crystal, and the intermediate pform characterized from its hightemperature XRD pattern. The structural, dielectric and optical studies are in agreement with: - ferroelastic and probably ferroelectric properties for T < 785 K, - only ferroelastic properties for 785 K < T < Tmelt’ A structure determination of the two (Y and y phases would bring more information concerning the nature of these transitions.

References 111D. Babel and A. Tressaud,

in: Inorganic solid fluorides, Crystal chemistry of fluorides, Ed. P. Hagenmuller (Academic Press, New York, 198% p.77. [21 E.G. Steward and P. Rooksby, Acta Cryst. 6 (1953) 49. 131G. Garton and B.M. Wanklyn, J. Inorg. Nucl. Chem. 27 (1965) 2466. 141J.H. Bums, A.C. Tennissen and G.D. Brunton, Acta Cryst. B24 (1968) 225. 151A. de Kozak, Compt. Rend. Acad. Sci. (Paris) 268 (1969) 416. [61A. Tressaud, J. Portier, S. Shearer-Turrell. J.L. Dupin and P. Hagenmuller, J. Inorg. Nucl. Chem. 32 (1970) 2179. [71W. Massa and W. Riidorff, 2. Naturforsh. 26b (1971) 1216. [81W. Massa, Z. Krist. 153 (1980) 201. and J. Portier, Bull. Sot. [91J. Grannec, J.C. Champamaud Chim. Fr. 11 (1970) 3862. [lo] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [ll] J.L. Holm and B. Jensen, Acta Chem. Stand. 23 (1969) 1065.

Fig. 5. Thermal evolution of the domain structure after a 45’ rotation of the crystal as compared (c) 786 K, (df 785 K, (e) 303 K.

to Fig. 4 (on cooling), (a) 790 K, (b) 787 K,