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Aluminate sodalite Sr<sub>8</sub>[Al<sub>12</sub>O<sub>24</sub>](CrO<sub>4</sub>)<sub>2</sub> - a new ferroelectric material

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Aluminate sodalite Sr

8

[Al

12

O

24

](CrO

4

)

2

- a new ferroelectric material

SETTER, Nava, et al.

Abstract

On flux-grown polydomain single-crystals of Sr8[Al12O24](CrO4)2, exploratory domain studies and measurements of birefringence, polarization, dielectric constant and transition enthalpy demonstrate the existence of a 1st-order ferroelectric phase transition at 295-299K.

SETTER, Nava, et al . Aluminate sodalite Sr

8

[Al

12

O

24

](CrO

4

)

2

- a new ferroelectric material.

Ferroelectrics , 1984, vol. 56, no. 1, p. 49-52

DOI : 10.1080/00150198408012717

Available at:

http://archive-ouverte.unige.ch/unige:33202

Disclaimer: layout of this document may differ from the published version.

1 / 1

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Ferroclectrics. 1984, Vol. 56, pp. 49-52 0015.()193/84/5601-0049$18.5010

© 1984 Gordon and Breach. Science Publ~>hcr.. Inc. Prontcd '" the United St•t"-' of America

ALUMlNATE SODALITE Sr8[Al1202~](Cr0~)2 -A NEW FERROELECTRIC HATERIAL

NAVA SETTER*, HARIA EUCENIA HENDOZA-ALVAREZ, WULF DEPHEIER AND HANS SCHHID

Department of Hineral, Analytical and Applied Chemistry, University of Geneva, CH 1211 Geneva 4, Switzerland

* Present address: P.O.B. 2250 Haifa, 31021, Israel Abstract On flux grown polydomain single crystals of Sra[Al12024](Cr04)2 exploratory domain studies and measure- ments of birefringence, polarization, dielectric constant and transition enthalpy demonstrate the existence of a first order ferroelectric phase transition at about 295~ - 299tK.

INTRODUCTION

The general formula of the members of the sodalite family is Me[T12024]X2.Corner-sharing tetrahedra T04 (T = Si4+, Al3+, . . . ) form an open framework, in the cavities of which are contained the

. + 2+ z+ . -

cage cat~ons M (Na , Ca , Sr , ... ) and the cage an~ons X (Cl ,

sot- ,

CrO~-, ... ). Aluminate sodalites contain only Al3+ as T cat-

~ons. The sodalite framework is flexible and adapts itself to the sizes and shapes of the cage ions. Displacive phase transitions have been found to occur in most aluminate socialites containing tetrahedral cage anions 1 '2. The result of the first structure de- termination of a non-cubic sodalite has recently been reported2'3

Symmetry considerations made it conceivable that ferroelectricity might occur in at least some of the aluminate sodalites. Because of its convenient transition temperature, the title compound has been chosen for a first search for ferroelectricity amongst alu- minate sodalites.

EXPERIHENTS

Crystal growth and samples

Light green single crystals of l - 2 mm size and imperfect (110) and (112) facets - the rhombic dodecahedron is usually dominating- have been grown from a Bi203 flux4

. Platelets parallel to (lOO) and (110) have been prepared with a thickness of 40 - 60wm and electroded with a transparent gold layer.

110531/49

I

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N. SETTER. M. E. MENDOi'A·ALVAREZ. W IJEPMF.IER A:-;1) H ~CHMID

Polarized light microscopy of the domain structure.

lising a polarizing microscope in conjunction with an optical helium flow cryostat, observation of (lOO) platelets showed two kinds of domains, one with extinction parallel [100], the other parallel [110], suggesting orthorhombic symmetry. Oomains with ex- tinction parallel [lOO] are separated by (110) walls, whereas tho- se with extinction parallel [110] are linked to the former by walls having traces on (lOO) along [lOO]. Cooling of the sample through T , with 20 kV·cm-1 applied perpendicular to the surface, increasedcthe amount of domains with extinction parallel [100].

This behaviour is consistent with mm2 symmetry with the spontane- ous polarization parallel [110] (Aizu species m3mF2mm(sp)). Beca- use the domains did not change the orientation of the indicat~ix

upon reversal of polarization, the high temperature symmetry 43m can be excluded. Application of the same field below T had no visible effect on the domain pattern. c

Spontaneous birefringence.

On a (100)-cut platelet (thickness 74~m), the spontaneous birefri- ngence ~n has been measured on a domain with extinction parallel [ lOO] from T down to 4K, using a tilting compensator (Fig.l). The discontinuou~ onset of ~n and the thermal hysteresis at T clearly show the transition to be of first order (hysteresis of m~asured domains: T

=

295K on cooling, T

=

299K on heating; hysteresis of

the entirecplatelet: +285- t299R). No further transition appear- ed below T .

c

Spontaneous polarization.

Platelets cut parallel (110) and (lOO) have been cooled in a field of 20 kV·cm-1 through T and the remanent polarization has been measured by charge inte~ration upon heating using an electrometer

(Keithley 616). The charge integration was possible owing to the very high resistivity of the samples. Maximum polarizations of 7 and S~C·cm-2 (at 248K) have been achieved on (110) and (lOO) cuts, respectively (Fig.2). Reversal of the paling field reversed the sign of the remanent polarization (Fig.J). After a second cooling of the poled samples in zero field strong depolarization was ob- served. Therefore, the slope aP /aT is certainly not intrinsic and the apparent second order type ~anishing of the polarization at T can be attributed to smooth depoling effects. Although the sample~

1vere not fully saturated, the ratio P[llO]/P[lOO] = "<Jl/5 "<J/2is also consistent with

P

lying along [110].

s

Dielectric constant and loss tangent.

Figure 4 shows the dielectric constant and loss tangent measured at lOO kHz on a polydomain (lOO) platelet between 240 and JOOK by

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I'ERROELECTRIC Sr,IAI110,.iCCrO.I, 1105W51

means of a LF Lmpedance Analyzer (HP 4192A). for a heating/cooling rate uf l deg/min the maximum of c (E. 'V 90) was found at 292K and 287K, respectively. The temperatu~~xdependence of c below T

. . . c

and 1ts tndependence of temperature above T are typtcal for an improper ferroelectric5, reminiscent of, e.g., Gd2(Mo0")36 ln the range l to 1000 kHz E is nearly independent of frequency. Measure- ment of E. in a bias field of 20 kV·cm-1 decreased c by about 20Z without changing T(c ), an effect probably du~a~o partial

. . . max ~ ( . 4)

pollng. The d~electr~c loss, tgu, was extremely low F1g. : above T lower than the detection limit of the bridge (<0.0001), whereas tf,e maximum of tgo was still lower than 0.0001 for f

=

l kHz and increased with frequency to 0.003 for 100 kHz and to 0.01 for ltffiz.

Calorimetric measurement.

With a differential scanning calorimeter the enthalpy of the transition could be determined to be 5.76 kJ·mol-1

CONCLUSIONS.

Although the different results are of preliminary character - owing to the insufficient size and quality of the crystals - the observ- ed polarization and its reversal upon cooling in an electric field through T clearly demonstrate that Sr8[Al 120 24](Cr0 4)2is ferro- electric.cHerewith it is the first example of ferroelectricity oc- curring in the aluminate socialite family in which many more ferro- electrics and ferroelastics are to be expected. The dielectric anomaly suggests that the compound is an improper ferroelectric and the domain pattern and paling behaviour show that the most pro- bable Aizu species is m3mF2mm(sp). The apparent sluggishness of the phase transition needs further study.

Acknowledgements

The authors thank R. Boutellier, E. Burkhardt and R. Cros for tech- nical assistance, G. Wildermuth (Konstanz) for the DSC measurements and J.-P. Rivera for discussions. The work was supported by the Fonds National Suisse de la Recherche Scientifique.

REFERENCES

1. W.Depmeier, J.Appl.Cryst. ~. 623 (1979) 2. W.Depmeier, Habilitation Thesis, Geneva (1983) 3. W.Depmeier, Acta Cryst.C, in the press

4. to be published

5. V.Dvorak, Ferroelectrics

I•

1 (1974)

6. J.Fousek and C.Koni.1ik, Czech.J.Phys. B22, 995 (1972)

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I' Sl:TTER. M E MENDOZA·At.VARE7.. W OEPMEIER ANO 1-t S\IIMIO

~

• heotong

&

7 (110) -cut

'23 p--_..,.., o coolong

0 ~

u

w ::t.

u z 5

UJ UJ

'-'>2 u (100)-cut

z z

Oi w

l1. z 3

w

~1 ~

!Il ~

UJ a:

0 0 100 TEMPERATURE 200 [K} 300 LOO 200 220 240 260 280 300 TEMPERATURE [K]

FIGURE 1 Spontaneous birefrin- FIGURE 2 Remanent polarizat- gence vs. temperature.

·3

·2 (110)-cut

.... ,

~ 0

u

~_,

~ -2

~ -3

3

·4

·5 . 6

E: +20kV,cm"0

E=-20kv.cm·'

200 220 240 260 200 300 TEMPERATURE (K]

FIGURE 3 Sign reversal of the remanent polarization on revers- al of the poling field.

ion vs. temperature.

100

80

·w

...

z <(

:;:; 60 0 z u u a:

t; t.O

...

..J

a

...

20

FIGURE 4

30 20

Dielectric constant and loss tangent vs. temperature,

(lOO) platelet, 100kHz.

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