Coulomb driven chain ordering phase transitions in the quasi-1-D electronic material d8-PtI/en/ClO 4

Abstract

The MX material [Pt(en) 2I 2][Pt(en) 2](ClO 4) 4, where en is per-deuterated ethylenediamine, represents a new structural type for the MX family. The material, which crystallizes in the C 2/m monoclinic space group, forms sheets of ordered and disordered 1-D chains and exhibits two phase transitions at 160K and ∼120K. By comparison of the crystallographic, vibrational, and acoustic data the transition at 160K can be attributed to c-axis doubling with ordering of the en groups in the disordered sheet and formation of an acentric, C 2 crystal. The phase transition is driven by hydrogen bonding in the disordered sheet. The structural transition at ∼120K is a discommensurate to commensurate phase transition in the sheets of disordered chains. The phase transition is driven by coloumbic ordering of the chains in the disordered sheet. Resonant ultrasound and thermal data suggest that both transitions are second or higher order and Raman data suggest ordering of the en's of the ordered chain below 120K.