High-pressure structural behavior of ingersonite, Ca3Mn2+Sb45+O14: An in-situ single-crystal X-ray study

Abstract

An in-situ, high-pressure, single-crystal X-ray diffraction study has been carried out at room temperature up to 7.42 GPa on a crystal of ingersonite, ideally Ca 3 Mn 2+ Sb 5+ 4 O 14 , from the type material. Ingersonite is isostructural with the synthetic weberite-3T polytype and related to the pyrochlore structure-type. Owing to the P range investigated and the quality of data, a second-order Birch- Murnaghan Equation of State (EoS) is the best approximation to describe the ingersonite volume evolution with P. The refined EoS parameters are V 0 = 810.6(1) Å 3 and K 0 = 154.5(2.4) GPa. The behavior of ingersonite with pressure is almost isotropic and the decrease of the unit-cell volume is mainly due to the kinking of the polyhedra rather than their volume decrease. The overall mean distances are quite constant, indicating virtually no compressibility of both the A and B polyhedra in the P range investigated. However, some geometrical changes in the pyrochlore-like AB 3 layer can be observed and compared with those observed in synthetic pyrochlore compounds. The largest change is observed for the z atomic coordinate of the O 2 atom. Using an anion-centered polyhedral description, O 2 is the only O atom that is asymmetrically located in an octahedral interstice, this feature being the most remarkable difference between the structure of ingersonite (i.e., weberite-3T type, space group P3 1 21) and that of zirconolite-3T (pyrochlore structure type, space group P3 1 21), where all the O atoms occupy the tetrahedral interstices of a cubic A 2 B 2 array. With the increase of pressure, the O 2 atom migrates from the A 4 B 2 octahedral cavity toward the adjacent AB 3 tetrahedral cavity, suggesting that a transition from weberite-3T to zirconolite-3T structure type could occur at pressures higher than 11 GPa.