COMPARISON OF THE LOW-TEMPERATURE PHASE TRANSITIONS IN K₂CoCl₄ AND K₂ZnCl₄. J.Kusz, J.Warczewski, University of Silesia, Institute of Physics, ul. Uniwersytecka 4, PL-40007 Katowice, Poland, A. Pietraszko, Institute of Low Temperature and Structural Studies of the Polish Academy of Sciences, PL-50950 Wroclaw, Poland

K₂CoCl₄ and K₂ZnCl₄ be1ong to the A₂BX₄ group of the ferroelectric compounds revealing a sequence of phases starting from the high temperature paraelectric phase (Pmcn) through the incommensurate phase with the modulation vector q= (1/3 - d) c* to the commensurate phase with q= 1/3c* and to the low-temperature commensurate phase with q₁= l/3c* and q₂=l/2a* +1/2b*. The theoretical expectations [1] suggest that for the compounds of this group the symmetry of the low-temperature phase should be monoclinic (Clcl).

Our measurements made down to 100 K on the X-Ray 4-circle diffractometer showed that there is both no deviation of the angle [[beta]] from 90deg. and no anomalies of the lattice parameters in the low-temperature phase. The FWHM of the h0l-type reflections increases for K₂Col₄ below the transition temperature (142 K) [2], contrary to K₂ZnCl₄ . This fact can be interpreted as a very small monoclinic deformation of the low-temperature commensurate phase.

Analysis of the reflection intensities (hkl and ikl) for K₂ZnCl₄ at 155 K and 105 K did not show any increase of the R_eq factor. It means, that there is no significant assymetry, that could confirm any monoclinicity of the low-temperature phase in this compound.

References

[1] Dvorak V. and R.Kind, Phys. Status Solidi (b), 107, K109 (1981).

[2] Kusz J., A. Pietraszko and J.Warczewski, Phase Transitions (1996), in press.