Structural studies on the phase transition of rubidium formate

The thermal phase transition of rubidium formate, observed at 363 K, was studied by means of thermal analysis and X-ray diffraction method, The transition was found to be a reversible first-order transition. In the phase transition, the orthorhombic phase (Pca2(1)) with lattice constants a=9.229(1), b=4.630(2) and c=7.408(2) Angstrom, was transformed to the monoclinic phase (P2(1)/m) with one of a=4.6552(4), b=4.6528(7), c=7.5170(6)K and gamma=97.610(6)degrees. The values of the enthalpy, Delta H-tr, and entropy change, Delta S-tr,, for the transition were 0.42+/-0.02 kJ mol(-1) and 1.16+/-0.05 J K-1 mol(-1), respectively. Although the relative positions of rubidium atoms hardly changed in the phases before, and after, the transition, the orientation of formate ions differed in both the phases. The values of Delta H-tr, and Delta S-tr seem to reflect a slight change of structure in the orientation of formate ions. (C) 1998 Elsevier Science B.V.