Investigations in the systems Sr–As–O–X (X = H, Cl): Preparation and crystal structure refinements of the anhydrous arsenates(V) Sr3(AsO4)2, Sr2As2O7, α- and β-SrAs2O6, and of the apatite-type phases Sr5(AsO4)3OH and Sr5(AsO4)3Cl

The previously uninvestigated atomic arrangements of six strontium arsenate(V) phases in the systems Sr-As-O-X (X = H, Cl) have been determined. Single crystals of Sr3(AsO4)2 and Sr5(AsO4)3Cl were grown from a borate and NaCl melt, respectively, whereas single crystals of α- and β-SrAs2O6 and of Sr5-(AsO4)3OH were obtained under hydrothermal conditions. Microcrystalline samples of Sr2As2O7 were prepared by solid-state reaction of As2O5 with SrCO3 and, alternatively, by thermolysis of SrHAsO4. Crystal structure determinations based on powder X-ray diffraction data using the Rietveld method (Sr2As2O7), and from single-crystal X-ray diffraction data (all other phases) revealed isotypism with known structure types (except β-SrAs2O6 which represents a new structure type). Sr3(AsO4)2 crystallizes in the K2Pb(SO4)2 structure type; Sr2As2O7 is isotypic with the high-temperature polymorphs of Ca2P2O7 and Sr2V2O7, but itself shows no polymorphism; Sr5(AsO4)3OH and Sr5(AsO4)3Cl crystallize with the hydroxylapatite and chloroapatite structure, respectively; SrAs2O6 is dimorphic. Like all other known MIIAs2O6 metaarsenates (M = Mn, Co, Ni, Sr, Pd, Cd, Hg, Pb), the α-polymorph crystallizes in the PbSb2O6 structure type, whereas the novel β-polymorph is the first example of a MIIAs2O6 superstructure with a doubled c-axis. Additional analytical methods using Raman spectroscopy and thermal analyses support the results of the X-ray structure work.

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