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 P4₁2₁2 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 WYRIET3. A brief structural comparison of HT KMgWP with its triclinic pseudo-tetragonal 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.