Investigations in the systems Sr-As-O-X (X= H, Cl): Preparation and crystal structure refinements of the anhydrous arsenates(V) Sr_3(AsO_4)_2, Sr_2As_2O_7, alpha- and beta-SrAs_2O_6, and of the apatite-type phases Sr_5(AsO_4)_3OH and Sr_5(AsO_4)_3Cl

摘要

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 Sr_3(AsO_4)_2 and Sr_5(AsO_4)_3Cl were grown from a borate and NaCl melt, respectively, whereas single crystals of alpha- and beta-SrAs_2O_6 and of Sr_5-(AsO_4)_3OH were obtained under hydrothermal conditions. Microcrystalline samples of Sr_2As_2O_7 were prepared by solid-state reaction of As_2O_5 with SrCO_3 and, alternatively, by thermolysis of SrHAsO_4. Crystal structure determinations based on powder X-ray diffraction data using the Rietveld method (Sr_2As_2O_7), and from single-crystal X-ray diffraction data (all other phases) revealed isotypism with known structure types (except beta-SrAs_2O_6 which represents a new structure type). Sr_3(AsO_4)_2 crystallizes in the K_2Pb(SO_4)_2 structure type; Sr_2As_2O_7 is isotypic with the high-temperature polymorphs of Ca_2P_2O_7 and Sr_2V_2O_7, but itself shows no polymorphism; Sr_5(AsO_4)_3OH and Sr_5(AsO_4)_3Cl crystallize with the hydroxylapatite and chloroapatite structure, respectively; SrAs_2O_6 is dimorphic. Like all other known M~(II)As_2O_6 metaarsenates (M = Mn, Co, Ni, Sr, Pd, Cd, Hg, Pb), the alpha-polymorph crystallizes in the PbSb_2O_6 structure type, whereas the novel beta-polymorph is the first example of a M~(II)As_2O_6 superstructure with a doubled c-axis. Additional analytical methods using Raman spectroscopy and thermal analyses support the results of the X-ray structure work.