  Angle-dispersive x-ray diffraction (ADXRD) and x-ray absorption near edge
structure (XANES) measurements have been performed in the AWO4 tungstates CaWO4
and SrWO4 under high pressure up to approximately 20 GPa. Similar phase
transitions and phase transition pressures have been observed for both
tungstates using the two techniques in the studied pressure range. Both
materials are found to undergo a pressure-induced scheelite-to-fergusonite
phase transition under sufficiently hydrostatic conditions. Our results are
compared to those found previously in the literature and supported by ab initio
total energy calculations. From the total energy calculations we have also
predicted a second phase transition from the fergusonite structure to a new
structure identified as Cmca. Finally, a linear relationship between the charge
density in the AO8 polyhedra of ABO4 scheelite-related structures and the bulk
modulus is discussed and used to predict the bulk modulus of other materials,
like zircon.
