Equations of State and Crystal Structures of KCaPO 4 , KSrPO 4 , and K 2 Ce(PO 4 ) 2 under High Pressure: Discovery of a New Polymorph of KCaPO 4 .

We have studied by means of angle-dispersive powder synchrotron X-ray diffraction the structural behavior of KCaPO 4 , SrKPO 4 , and K 2 Ce(PO 4 ) 2 under high pressure up to 26, 25, and 22 GPa, respectively. For KCaPO 4 , we have also accurately determined the crystal structure under ambient conditions, which differs from the structure previously reported. Arguments supporting our structural determination will be discussed. We have found that KCaPO 4 undergoes a reversible phase transition. The onset of the transition is at 5.6 GPa. It involves a symmetry decrease. The low-pressure phase is described by space group P 3̅ m 1 and the high-pressure phase by space group Pnma . For KSrPO 4 and K 2 Ce(PO 4 ) 2 , no evidence of phase transitions has been found up to the highest pressure covered by the experiments. For the three compounds, the linear compressibility for the different crystallographic axes and the pressure-volume equation of states are reported and compared with those of other phosphates. The three studied compounds are among the most compressible phosphates. The results of the study improve the knowledge about the high-pressure behavior of complex phosphates.