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 fro m a stoichiometric melt at about 1068 K. They were found to crystalliz e tetragonally in point symmetry group 422. On cooling to room tempera ture, they undergo at least three reversible, ferroic, solid-state pha se transitions. Two additional anomalies were found within the stabili ty field of KMgWP's tetragonal high-temperature phase (HT KMgWP), whic h is stable above T = 537 K. Its structure was solved and refined in s pace 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 progr am WYR1ET3. A brief structural comparison of HT KMgWP with its triclin ic pseudotetragonal room-temperature modification reveals that the pot assium ions, occupying interstices within the rigid network of [MgWO2( PO4)(2)](2-), are affected most by the phase transitions.