Article
Reference
Optical, dielectric and DSC studies of "yellow-type" aluminate sodalite Sr
8[Al
12O
24](CrO
4)
2ROSSIGNOL, Jean-François, et al.
Abstract
The ferroelec. orthorhombic phase and a new nonpolar tetragonal phase of yellow Sr8[Al12O24](CrO4)2 single crystals were studied by polarized light microscopy, DSC, permittivity measurements, spontaneous birefringence, and spontaneous polarization. The high perfection of the title crystals relative to the green form was demonstrated. The symmetry and microtwinning of the crystals are discussed.
ROSSIGNOL, Jean-François, et al. Optical, dielectric and DSC studies of "yellow-type"
aluminate sodalite Sr8[Al12O24](CrO4)2. Ferroelectrics, 1988, vol. 79, no. 1, p. 197-200
DOI : 10.1080/00150198808229430
Available at:
http://archive-ouverte.unige.ch/unige:34099
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Ferroelectrics, 1988, Vol. 79, pp. 197-200 . / Reprints available directly from the publisher© 1988 Gordon and Breach Science Publishers S.A, Printed in the United States of America Photocopying pennitted by license only
OPTICAL, DIELECTRIC AND DSC STUDIES OF "YELLOW-TYPE" ALUMI- NATE SODALITE Srg[Alu024](Cr04)2
JEAN-FRAN<.;OIS ROSSIGNOL, JEAN-PIERRE RIVERA, PAUL TISSOT AND HANS SCHMID
Department of Mineral, Analytical and Applied Chemistry, University of Geneva, CH-1211 Geneva 4, Switzerland.
Abstract The ferroelectric orthorhombic phase and a new non-polar tetragonal phase of yellow single crystals of Srg[Alu024](Cr04)2 are studied by polarized light microscopy, differential scanning calorimetry (DSC), measurements of permittivity, spontaneous birefringence and spontaneous polarization.
INTRODUCTION
On green single crystals of the aluminate sodalite Srg[Alu024](Cr04)2 (abbreviated
·~ SACR) an improper ferroelectric phase transition was found at (295K+ - 299Kt)Cl).
Measurements on cerarnics(2) confirmed the ferroelectric phase. The present paper reports some properties of better quality crystals of yellow colour, typical of the pure chromate ion, and those of a new tetragonal phase not observed on the green crystals and ceramics.
EXPERIMENTS
Crystal Growth and Samples.
Yellow single crystals (1-3 mm) of SACR were grown from a Bi203 flux by using SrCr04 (iustead of Cr03)(1,2), SrO and ,B-Al203 directly as solutes. The shiny (llO)c and (112)c facets served as reference for preparing (llO)c and (lOO)c platelets for the measurements.
Domains and poling.
In polarized light two sharp first order transitions, cubic # tetragonal and tetragonal # orthorhombic, have been evidenced. The tetragonal phase is optically
[197]/491
492/[198] J.-F. ROSSIGNOL et al.
negative, has ferroelastic domain walls //(llO)c moving easily under stress and showing no response to electric fields UP. to 40kV.=-1. Aizu species m3mF4/mmm is probable. The orthorhombic phase has tiny intricate domains at zero field, strong birefringence, parallel and 45 degrees extinction on (lOO)c-cuts. With a field of""
40kV.=-1 // [110Jc, ferroelasticjferroelectric single domains were obtained proving
r
Aizu species m3mFmm2(ss).
Differential Scanning Calorimetry.
By using a Mettler FP800, an Al-crucible, 28.26mg of small crystals and a heating rate of 4 deg.min-1, the transition temperatures and enthalpies were obtained (Fig. 1):
T1(orth. •rtetr.) = (12.l"C -1- -14.9"C t); lill1 = 3.9 J.g-1 T2(tetr. <+cub.) = (26.6°C -1--27.1°C t) ;
m
2 = 1.4 J.g-1Spontaneous birefringence
L'lns·
Figure 2 shows
L'lns
of the orthorhombic and tetragonal principal sections, measured on (lOO)c and (llO)c mono-domain platelets upon heating from 4K to 310K, using a Babinet-Soleil compensator. The discontinuous onsets of Lms clearly show the transi- tions to be of first order.1 • • -+
Spontaneous po anzation P s·
On a platelet (110)0 cooled in a field of 30kV=-1 from the cubic to the orthorhombic phase, the remanent polarization has been measured by charge integration upon heating (electrometer Keithley 616). A spontaneous polarization of 25 Ji.C.=-2 (below 260 K) has been measured on a ferroelastic mono-domain platelet (Fig. 3). Reversal of the poling field reversed the sign of the remanent polarization.
Dielectric constant and loss tangent.
The dielectric constant was measured at 300kHz on a poled (llO)c platelet between 200 and 300K upon heating, using a LF Impedance Analyzer (HP 4192A). In the entire temperature range of the mm2 phase the sample remained single domain up to the maximum
€33 (/ jP -r
J (""
24)1._ at 286K, but it became polydomain in the tetragonal phase, simula- ting a 2nd oq!er transition to the cubic phase. At variance with green polydomain crystals(!), the dielectric loss, tgo, was extremely low, lower than the\
OPTICAL, DIELECTRIC AND DSC STUDIES [199]/493
ENDO
1
t.T10 15 20 25 30 35
Temperature (°C)
FIGURE 1
Differential scanning calorimetry.
orthorhombic tetr. cubic
•2.
Of---+-+--
-2.
-4 ~~~--~~----~~-+~
210 2.30 2.50 2.10 2.90 310
Temperature {K)
FIGURE3
Remanent polarization on reversal of the poling field; sample:
(110)c-cut, thickness (0.063 mm).
·~
(100)pc-cul
o.s .4nslT ... )
r----.,
: tetr. •
~ ~00
(110)pc-CU! ' \
----:~.:.__~- :
(110lpc- cull.P5 ~
orthorhombic
.
'o+-.--r-r~~-,~---.-~~rtY~~:;~~"'.''~
0 50 100 150 2.00 2.50 300 350 Temperature {K}
FIGURE2
Spontaneous birefringence vs.
temperature; sample: (001)0 (llO)c and (110)c parallelepiped (thickness: 0.450 mm x 0.470 mm x 0.470 mm).
. ., 30
~25 c c 0
"2.0
~. u 15 .!! .!!
0 10
5 0
2.00
300kHz (1 Vp.p)
~
I-' 0 corthorhombic
250
FIGURE4
0
_g ~ cubic
•
300 Temperature {K}
Dielectric constant vs.
temperature; sample: (llO)c-cut, thickness (0.063 mm).
494/[200] J.-F. ROSSIGNOL et al.
detection limit of the bridge ( <0.0001).
CONCLUSION
The high perfection of yellow SACR crystals relative to the green ones has been demonstrated by i) the detection of a new non-polar tetragonal phase, ii) the great ease of tetragonal ferroelastic wall motion, iii) the achievement of fully poled
' 7
orthorhombic single domains permitting reliable measurements of P sCT),
L'ln;;(T)
and"33(T). In the green crystals (1) of SACR the cr6+ ions seem to be reduced to Cr3+
to some extent, leading to defects that tend to smear the two transitions to an apparent single one(1,2). Recent X-ray structural work on a green single crystal, was unable to decide between space group 1mJm and 143m for the cubic phase(3). The fact that Ps lies along [llO]c unequivocally requires the prototype to be of point group +
m3m, hence of space group Im3m. However, the cubic phase is eventually 143m microtwinned with the prototype remaining hypothetical. Microtwinning might be demonstrated by successful ferroic · switching with simultaneous electric field and mechanical stress as proposed for a similar problem in the case of spinels( 4).
ACKNOWLEDGEMENTS
The authors thank R. Boutellier, E. Burkhardt, R. Cros, 0. Hirth and H. Lartigue for technical help and the "Fonds National Suisse de la Recherche Scientifique" (no 2.081- 0.86) for support.
REFERENCES
1) N. Setter, M.E. Mendoza-Alvarez, W. Depmeier and H. Schmid, Ferroelectrics ~
49-52 (1984).
2) N. Setter, Ferroeclectrics Letters 7., 1-5 (1987).
3) W. Depmeier, H. Schmid, N. Setter and M.L. Werk, Acta Cryst C, in press.
4) H. Schmid and E. Ascher, J. Phys. C: Solid State Phys.l., 2697- 2706 (1974).
