U. Peuchert et al., AB-INITIO STRUCTURE SOLUTION AND RIETVELD REFINEMENT OF THE HIGH-TEMPERATURE K2MGWO2(PO4)(2) PHASE (T = 773 K) FROM X-RAY-POWDER DIFFRACTION DATA, Journal of applied crystallography, 31, 1998, pp. 10-15
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.