X-ray study on the phase transition of Cu-Cl boracite

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X-ray study on the phase transition of Cu-Cl boracite

UESU, Y., et al.

Abstract

The temperature and electric field dependences were measured of the lattice strains in Cu-Cl boracite. The spontaneous pure shear is 8 times larger than that of Fe-I boracite, whereas spontaneous dilation strains are .apprx.1/2 those of Fe-I boracite. The electrostrictive constants show anomalous temperature variations near the transition tempeature. The results were explained by the theory of improper ferroelectrics.

UESU, Y.,

. X-ray study on the phase transition of Cu-Cl boracite.

, 1978, vol. 20, no. 3-4, 3-4 (Proc. Fourth Int. Meet. Ferroelectr., Leningrad, USSR, September 1977.

Part 2), p. 167-168

DOI : 10.1080/00150197808237199

Available at:

http://archive-ouverte.unige.ch/unige:33388

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

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ferroelectrics © Gordon and Breach Science Publishers L t d . . 1978 1978, Vol. 20. pp. 167-168 Printed in Great Britain

X - R A Y S T U D Y ON T H E P H A S E T R A N S I T I O N O F C u - C l B O R A C I T E

Y. U E S U t . J . K O B A Y A S H I t and I. A N J O H t

i Department of Physics, t Department of Applied Physics, Waseda University, Nishiokubo, Shinjuku-ku, Tokyo, Japan

and H. SCHMID

Battelle Geneva Research Centre, CH-1227, Carouge-Geneva, Switzerland

(Received September 18.1977)

Temperature and electric field dependences of lattice strains of C u - C l boracite were accurately measured. The spontaneous pure shear x, 2 of C u — C I boracite is 8 times as large as that of F e - I boracite. while spontaneous dilatation strains are about half of those of F e - I boracite. Electrosirictive constants Qj^ and Q^, show anomalous temperature dependences near the transition temperature. These peculiarities have been successfully explained by our theory of improper ferroelectrics.

I N T R O D U C T I O N

Boracite crystals are regarded as typical improper ferroelectrics.'-^ They undergo a sequence of phase transitions.^ Especially the ferroelectric phase transition from cubic T-^ to orthorhombic is of interest, because of peculiar physical properties.'* We are interested in relating the strength of coupling between an order parameter and polarization to peculiar physical properties of boracites. For this reason, we studied lattice strains of Cu—CI boracite under an electric field.

EXPERIMENTAL RESULTS

We used a c-plate specimen with the dimension of 1.5 X 0.5 X 0.037 mm^. The specimen was held in a vacuum of 10"^ torr at 2 0 0 ° C during 120 hours and cooled gradually. This treatment diminished a broad spread of X-ray spectra.

Lattice strains of Cu—CI boracite were measured by using high precision x-ray strainmeter with the accuracy of 10~^ combined with the two- dimensional reciprocal lattice method. Figure I shows the temperature dependence of lattice constants of Cu—CI boracite along the

orthorhombic A. B and C axes. From these results, we obtained deformation strain D = -v, — .v,. and dilatation strains 5 = .v, -1- .v,. and 5 , = .v,. The pure shear .v,, = D/2 of C u - C l boracite is found to be 8 times as large as that of F e - I boracite.' while dilatation strains S, and S, are about half of those of Fe—I boracite. It is interesting to note that Cu—CI boracite is similar to KH^PO^* as far as lattice strains are concerned.

The electric field dependences of D and S were measured in the vicinity of TQ. D increases with increasing ( + ) electric field both in paraelectric and ferroelectric phases, with steeper slopes in the vicinity of TQ. On the other hand, the electric field dependence of strain Sy is quite unusual as shown in Figure 2(a); the spontaneous 5; is positive showing the dilation of the lattice, while the induced 5, is negative at temperatures remote from T^. However, the induced S, changes its sign and becomes positive immediately above Tp. Electrostictive constant 0, is shown in Figure 2(b) as a function of temperature.

has a strong dependence upon temperature: it changes sign at 98.8°C and has a pronounced peak at T^. The same anomaly has been also observed in the strain 53. These peculiar behaviours of electromechanical properties of Cu—CI boracite have been explained by our theory of improper ferroelectrics as djscussed below section.

168 Y . VESVel al.

5 - - -

jrthorho"^:3 z c u b c

' G T; p e r c* i j r e ("CJ

F-ICiL'RF I Temperature dependence of lattice constants of Cu C! boracite along the orthorhombic .4. B and C axes.

DISCUSSIONS

According to this theory.'' strain 5 is described by polarization P and an order parameter 6 as.

These expressions provides for ad hoc strains, that is. latent lattice strains.- which come from the mechanical coupling with 9. From these equations, we derive the following expressions for electro- sirictive constants.

For spontaneous strain.

Qh=Q,i+ico/f)'R„: (2)

For induced strains.

^3, - 03, + (f//J)'Rj(T- r«)-(in para). (3)

=Qyi-(ojllp)R„•/{T - Tf) (in ferro). (4)

Experimental results shown in Figure 2a and b are explained if the constant R^^ is positive and Q,,-

F I G U R F 2 i . T ) Fleciric field dependence of the strain 5 , = .v, - . V , of Cu CI boracite. th^ Temperature dependence of the electrosirictive constant Q",, of C u - C l boracite. (cl and (d) Schemaiical representation of 5 , and Q'' predicted from the theory of improper ferroelectrics.

negative. The electric field dependence of 5 and temperature dependence of Q'' predicted from the theory are schematically depicted in Figures 2c and d. respectively, which clearly manifest that the peculiar electromechanical properties of C u - C l boracite can be explained by the theory. Com- parison with the result of Fe—I boracite indicates that the strength of the coupling/between P and 9 for Cu—CI boracite is stronger than that for F e - I boracite.

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