High-pressure transitions in NaZnF3 and NaMnF3 perovskites, and crystal-chemical characteristics of perovskite-postperovskite transitions in ABX3 fluorides and oxides

Phase transitions in NaZnF3 and NaMnF3 were examined up to 24 GPa and 1100 degrees C using a multianvil apparatus. NaZnE3 perovskite transforms to postperovskite above 11-16 GPa at 600-1000 degrees C, and the postperovskite is quenchable at ambient conditions. The NaZnE3 perovskite-postperovskite transition boundary is expressed as P (GPa) = 4.9 + 0.011T (degrees C). At 8-11 GPa and 900-1100 degrees C, NaMnF3 perovskite dissociates into two phases of Na3Mn2F7 and MnF2. The latter phase is suggested to have the structure of orthorhombic-I type ZrO2 or cotunnite. Using available experimental data on the perovskite-postperovskite transitions in thirteen compounds of A(2+)B(4+)O(3) and A(+)B(2+)F(3), several crystal-chemical characteristics of the transition are elucidated as follows. In the transition, the volume change is between -1% and -2%, and the Clapeyron slope of the boundary is 10-17 MPa/degrees C. These support reliability of recently determined Clapeyron slope of 13 MPa/degrees C in MgSiO3 which suggests that the perovskite-postperovskite boundary intersects the temperature profile twice in the D" layer. Postperovskites of ABX(3) whose enthalpies are higher by more than 70 kj/mol relative to the phase stable at I atm are unquenchable, while those by less than 15 kj/mol are quenchable to ambient conditions. Structure refinements indicate that A(+)B(2+)F(3) postperovskites quenched at 1 atm are more similar to that of MgSiO3 postperovskite at high pressure, than those of quenched A(2+)B(4+)O(3) postperovskites. With increasing pressure, octahedral tilt angles of both A(2+)B(4+)O(3) and A(+)B(2+)F(3) perovskites increase, resulting in transition to postperovskite at the angle of about 26 degrees, and fluoride perovskites are more rapidly distorted with pressure than oxide perovskites. Covalent character of B-X bonds of ABX(3) postperovskite is suggested to be favorable for stabilization of the postperovskite structure. All these features suggest that NaNiF3 is a good quenchable, low-pressure analogue compound to MgSiO3 to investigate the perovskite-postperovskite transition. (C) 2013 Elsevier B.V. All rights reserved.