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 temperatu re stability less than a few mK. It was found that the transition between t he perovskite-type phase and the adjacent low temperature phase at 320 K oc curs with two thermal anomalies on heating and three anomalies on cooling i n the temperature range of 0.1 K. Corresponding multistage change in the in terplanar 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 t ransition. It is needed to introduce the idea that some big density fluctua tion is released from the cubic phase through the 'hyperfine multistage tra nsition'. (C) 1999 Elsevier Science B.V. All rights reserved.