Anomalous elastic properties and phase transitions of (NH4)2PF7

Abstract

Abstract Single crystals of tetragonal (NH4)2PF7 with dimensions of up to 20 × 20 × 15 mm have been grown from aqueous solutions of NH4PF6 after partial decomposition. Two phase transitions of character close to secondorder occur at 231 K and 178 K. Elastic constants and their temperature and pressure derivatives have been evaluated from ultrasonic resonance frequencies of thin plates. The shear stiffness exhibits a positive temperature derivative T 66 which increases as the crystal approaches the transition at 231 K. Below 231 K all elastic modes, in which c 66 is essentially involved, are strongly attenuated. Under the polarizing microscope both transitions are recognizable by the formation of characteristic domain patterns. From DSC runs we found transition enthalpies of 0.08 J g −1 at 231 K and 1.8 J g−1 at 178 K. Slight anomalies are observed also in the thermal expansion curves. The anomalous behaviour of (NH4)2PF7 is comparable to that of K2SnCl6. Apparently in both species the librational motions of the octahedral anions contribute substantially to the interactions driving the transitions.