Observation of 'hyperfine multistage transition' at the 320 K phase transition in CsPbCl3

Precise thermal and X-ray diffraction measurements were carried out for the phase transition in CsPbCl3 using 'mK-stabilized cells' with the temperature stability less than a few mK. It was found that the transition between the perovskite-type phase and the adjacent low temperature phase at 320 K occurs with two thermal anomalies on heating and three anomalies on cooling in the temperature range of 0.1 K. Corresponding multistage change in the interplanar spacing and superlattice reflection intensity was recognized by X-ray diffraction. The results show the inadequateness of commonly accepted explanation of the condensation of M-3-phonon mode of PbCl6 for the 320 K transition. It is needed to introduce the idea that some big density fluctuation is released from the cubic phase through the 'hyperfine multistage transition'. (C) 1999 Elsevier Science B.V. All rights reserved.