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

Our prediction that phase II of dipotassium hydrogen chromatoarsenate, K₂HCr₂AsO₁₀], is ferroelectric, based on the analysis of the atomic coordinates by Averbuch‐Pouchot, Durif & Guitel Acta Cryst. (1978), B 34 , 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) Å which form between the terminal O atoms of shared AsO₃OH tetrahedra in adjacent HCr₂AsO₁₀²⁻ ions. The largest atomic displacement of 0.586 Å between phase II and the predicted paraelectric phase I is by these two O atoms. The H atoms form helices of radius ∼0.60 Å about the 3₁ or 3₂ axes. Normal probability analysis reveals systematic error in seven or more of the earlier atomic coordinates.