K₂[HCr₂AsO₁₀]: redetermination of phase II and the predicted structure of phase I

Abstract

Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K(2)[HCr(2)AsO(10)], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch-Pouchot, Durif & Guitel [Acta Cryst. (1978), B34, 3725-3727], led to an independent redetermination of the structure using two separate crystals. The resulting improved accuracy allows the inference that the H atom is located in the hydrogen bonds of length 2.555 (5) angstroms which form between the terminal O atoms of shared AsO(3)OH tetrahedra in adjacent HCr(2)AsO(10)(2-) ions. The largest atomic displacement of 0.586 angstroms between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius approximately 0.60 A about the 3(1) or 3(2) axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates.