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) we re synthesized by solid-state reaction. Furthermore, Ba(ZnPO4)(2) was obtai ned 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 B a(AlSiO4)(2). Comparison of these structures with those of aluminosilicate and gallogermanate isomorphs illustrates a strong deviation to the pseudo-o rthorhombic symmetry that may be explained by the very different charges an d ionic radii of the tetrahedrally coordinated cations. Distortions of the tetrahedral sites as well as of the network of tetrahedra are found. The pa racelsian form of Ba(ZnPO4)(2) may only be obtained under hydrothermal cond itions and at a high temperature (T = 500 degrees C, P = 2 kbar); it underg oes an irreversible transition at T = 800 degrees C and ambient pressure to a hexagonal form with space group <P(3)over bar m1>.