LOW-TEMPERATURE REENTRANT PHASE-TRANSITION IN THE PEROVSKITE TYPE LAYER COMPOUND (C3H7NH3)2PBCL4

Low-frequency Raman spectra (5-250 cm-1) of a polycrystalline sample of bis(n-propylammonium) lead tetrachloride were recorded at different temperatures between 4 and 300 K. A new first-order reentrant phase transition was observed near 35 K. The phase diagram is: [GRAPHICS] Both phases II and IV present the same degree of disorder and are believed to be isomorphic with a monoclinic cell symmetry. Spectral evolution analysis and analogy with a manganese homologous compound suggest an incommensurate character for phase III between 35 K and 91 K. The pseudo-phase mode characteristic of the incommensurability was identified at about 10 cm-1.