Abstract
Ionic mobility and phase transitions in ammonium titanyl pentafluoride (NH 4) 3TiOF 5 were studied using the 19F and 1H NMR data. The high-temperature phase (I) is characterized by spherically symmetric (isotropic) reorientation of [TiOF 5] 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 (NH 4) 3TiOF 5(III) was found at 205 K. The transition is accompanied by hindering of uniaxial rotations of [TiOF 5] 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 p TCR ≈2 kbar and T TCR ≈170 K is estimated on the base of 19F NMR chemical shift data, and previously studied p– T diagram of (NH 4) 3TiOF 5.
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