Ab Initio Structure Determination and Dehydration Dynamics of YK(C2O4)2·4H2O Studied by X-ray Powder Diffraction

Abstract

The crystal structure of YK(C2O4)2·4H2O has been solved ab initio from powder diffraction data collected with conventional monochromatic X-rays. The symmetry is tetragonal, space group I41/a (no. 88), cell dimensions a = 11.4612(8) A, c = 8.9040(8) A, Z = 4. The structure is built from chains of oxalate groups bridging 8-fold-coordinated yttrium atoms. The structure has an open framework exhibiting tunnels with square cross-sections, filled with zeolitic water molecules, and ellipsoidal cross-sections in which the potassium atoms are located. The hydrogen atoms have been localized, and their role on the precision of the bond lengths within the oxalate group has been demonstrated. The dehydration process is reversible and occurs with a pronounced anisotropic elastic property of the structure framework. The enthalpy, ΔrH, and entropy, ΔrS, of the dehydration reaction have been determined as a function of the number n of zeolitic water molecules. ΔrH decreases from 70.2 to 54.2 kJ mol-1 during the dehydratio...