Tricritical point in ferroelastic ammonium titanyl fluoride: NMR study

Ionic mobility and phase transitions in ammonium titanyl pentafluoride (NH4)3TiOF5 were studied using the 19F and 1H NMR data. The high-temperature phase (I) is characterized by spherically symmetric (isotropic) reorientation of [TiOF5]3− anions and by uniaxial reorientation of these anions in the ferroelastic phase II. A previously unknown second-order phase transition to the low-temperature modification (NH4)3TiOF5(III) was found at 205 K. The transition is accompanied by hindering of uniaxial rotations of [TiOF5]3– anions and by noticeable change of 19F magnetic shielding tensor associated with the influence of pseudo-Jahn–Teller effect. A pressure-induced tricritical point with coordinates pTCR≈2 kbar and TTCR≈170 K is estimated on the base of 19F NMR chemical shift data, and previously studied p–T diagram of (NH4)3TiOF5.