High-Pressure Phase Transformations in $\mathrm{TiPO_{4}}$ : A Route to Pentacoordinated Phosphorus

Titanium(III) phosphate, TiPO$_4$ , is a typical example of an oxyphosphorus compound containing covalent P−O bonds. Single‐crystal X‐ray diffraction studies of TiPO$_4$ 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. TiPO4‐V is the first inorganic phosphorus‐containing compound that exhibits fivefold coordination with oxygen. Up to the highest studied pressure of 56 GPa, TiPO$_4$‐V coexists with TiPO$_4$‐IV, which is less dense and might be kinetically stabilized. Above a pressure of about 6 GPa, TiPO$_4$‐II is found to be an incommensurately modulated phase whereas a lock‐in transition at about 7 GPa leads to TiPO$_4$‐III with a fourfold superstructure compared to the structure of TiPO$_4$‐I at ambient conditions. TiPO$_4$‐II and TiPO$_4$‐III are similar to the corresponding low‐temperature incommensurate and commensurate magnetic phases and reflect the strong pressure dependence of the spin‐Peierls interactions