High-Pressure Phase Transformations in TiPO : A Route to Pentacoordinated Phosphorus.

Titanium(III) phosphate, TiPO  , is a typical example of an oxyphosphorus compound containing covalent P-O bonds. Single-crystal X-ray diffraction studies of TiPO reveal complex and unexpected structural and chemical behavior as a function of pressure at room temperature. A series of phase transitions lead to the high-pressure phase V, which is stable above 46 GPa and features an unusual oxygen coordination of the phosphorus atoms. TiPO -V is the first inorganic phosphorus-containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO -V coexists with TiPO -IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO -II is found to be an incommensurately modulated phase whereas a lock-in transition at about 7 GPa leads to TiPO -III with a fourfold superstructure compared to the structure of TiPO -I at ambient conditions. TiPO -II and TiPO -III are similar to the corresponding low-temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin-Peierls interactions.