Structural stability of BaMF4 (M = Mg, Zn and Mn) at high pressures

Piezoelectric fluorides of the composition BaMF(4) (M = Mg, Zn, Mn) have been studied in situ at high pressures in diamond anvil cells with single-crystal x-ray diffraction and Raman spectroscopy. All three compounds crystallize in the acentric space group Cmc 2(1) at ambient pressure. BaMgF(4) undergoes a reversible second order phase transition to the paraelectric phase (space group Cmcm) at pressures between 5 and 6 GPa. BaZnF(4) undergoes a reversible first order phase transition to a monoclinic phase (space group P 11n). Both high- and low-pressure polymorphs coexist in the pressure range 5-7 GPa. BaMnF(4) maintains the Cmc 2(1) structure up to pressures of 4 GPa. Above this pressure the diffraction signal decreases rapidly and at 6 GPa no diffraction signal could be detected in our experiment. The compound does not recover its crystallinity on decompression. A comparison of the effects of external and chemical pressure is presented.