LONG-RANGE ORDER IN THE HIGH-TEMPERATURE PHASE OF K2ZNCL4

X-ray-diffraction investigations of K2ZnCl4 reveal an ordered ZnCl4 co nfiguration that has a long-range-order character of the stable phase, through the observation of Bragg peaks located on the superstructure positions of the low-temperature phase, over the whole temperature ran ge [110 K, 600 K], i.e., in the disordered phases (Tau > 144 K). The s trong broadening of these superlattice reflections along a above 144 K reveals a loss of the translational periodicity along a and thus poin ts out a strong one-dimensional distortion of the ordered state in the disordered phases. The phase transition into the ordered phase at 144 K is observed via an increasing of the low-temperature superstructure peaks at the high-Q values. At these Q points an additional broad dif fuse scattering was detected 50 K above the phase transition. The exis tence of the broad intensity can be interpreted from consideration of two different phenomena that could have the same origin: the disordere d ZnCl4 configuration generated by the relative sizes of the K cation and the rigid ZnCl4 anion group. The behavior of the diffuse scatterin g was first interpreted in terms of local ZnCl4 ordering, which could be a precursor of the phase transition at 144 K. A correlation between the ZnCl4 ordering and the successive modulated-phase transitions is given from structural considerations. On the other hand, the diffuse s cattering also could he the result of the defect structure that is res ponsible for the lattice distortion along the a direction.