Synthesis and rietveld refinement of new phosphate and arsenate analogues of paracelsian

Two new arsenates with the general formula A(2+)(ZnAsO4)(2) (A = Sr, Ba) were synthesized by solid-state reaction. Furthermore, Ba(ZnPO4)(2) was obtained under hydrothermal conditions. X-ray diffractograms of these compounds were indexed with a monoclinic unit-cell, and their structures were refined by the Rietveld method using reported atomic coordinates for paracelsian Ba(AlSiO4)(2). Comparison of these structures with those of aluminosilicate and gallogermanate isomorphs illustrates a strong deviation to the pseudo-orthorhombic symmetry that may be explained by the very different charges and ionic radii of the tetrahedrally coordinated cations. Distortions of the tetrahedral sites as well as of the network of tetrahedra are found. The paracelsian form of Ba(ZnPO4)(2) may only be obtained under hydrothermal conditions and at a high temperature (T = 500 degrees C, P = 2 kbar); it undergoes an irreversible transition at T = 800 degrees C and ambient pressure to a hexagonal form with space group .