Ab initio structure solution and rietveld refinement of the high-temperature K2MgWO2(PO4)2 phase (T = 773 K) from X-ray powder diffraction data

Large crystals of the new orthophosphate K2MgWO2(PO4)(2) [dipotassium magnesium tungsten dioxide bis(monophosphate), 'KMgWP'] were grown from a stoichiometric melt at about 1068 K. They were found to crystallize tetragonally in point symmetry group 422. On cooling to room temperature, they undergo at least three reversible, ferroic, solid-state phase transitions. Two additional anomalies were found within the stability field of KMgWP's tetragonal high-temperature phase (HT KMgWP), which is stable above T = 537 K. Its structure was solved and refined in space group P4(1)2(1)2 at T = 773 K from X-ray powder diffraction data by the Rietveld method (R-wp = 3.59%, R-Bragg = 3.39%) using the program WYR1ET3. A brief structural comparison of HT KMgWP with its triclinic pseudotetragonal room-temperature modification reveals that the potassium ions, occupying interstices within the rigid network of [MgWO2(PO4)(2)](2-), are affected most by the phase transitions.