DSC and Raman spectroscopy studies of high temperature structural phase transitions in (C 3H 7NH 3) 2PbCl 4

Abstract

The Raman spectra (260-10 cm −1) and differential scanning calorimetry of polycrystalline samples of (C 3H 7NH 3) 2PbCl 4 were investigated in the temperature range 300–450 K. This work shows three first-order phase transitions I → γ → β → α, respectively at 340 K (Δ H 1 = 3.73 kJ mol −1), 371 K (Δ H 2 = 0.22 kJ mol −1) and 406 K (Δ H 3 = 0.25 kJ mol −1). The thermal behaviour and crystal structure analogy with the Mn derivative suggest an incommensurability of the γ phase, embedded between two commensurate phases I and β, both belonging to the same space group. The phase transition I → γ is governed by an order-disorder mechanism involving mainly alkylammonium chains; the other transitions are driven by displacive mechanisms involving deformation of PbCl 6 octahedra. The highest temperature phase α seems to be a plastic phase.