Observation of Sb₂S₃-type post-post-perovskite in NaFeF₃. Implications for ABX₃ and A₂X₃ systems at ultrahigh pressure

Abstract

We describe the structures and transformations that lead to the crystallization of a post-post-perovskite of Sb2S3 type in a GdFeO3-type fluoroperovskite system at high-pressure conditions, through use of large-volume powder and single-crystal x-ray diffraction techniques. The results of this analysis gives unique access to the relative crystallographic orientations of all the polymorphs encountered (GdFeO3 type, CaIrO3 type and Sb2S3 type). We use this information to extend this description to include other calculated and observed forms that are competitive in ABX3 and A2X3 stoichiometries (e.g. α-Gd2S3 and Be3N2 types) and provide substantial information on inter-relationships between these structures. Such information is critical to the interpretation of transition mechanisms, predicting transition sequences and to the expression of directional properties in those transformed structures. The transformation from CaIrO3 type to Sb2S3 type is group-subgroup, from Cmcm with fc2a, to Pnma c5, with no observable volume change, but considerable change to the morphology of the lattice. There is a concomitant increase in coordination and average bond length compared to the post-perovskite form of NaFeF3.

Phase transitions beyond post-perovskite in NaMgF 3 to 160 GPa

“…Our EOS results suggest the transition from pPv to Sb 2 S 3 -type structure will have a small (∼0.5%) volume change (at 55 GPa), which is less than the corresponding decrease in volume (∼4%) at ∼28 GPa during the Pv-to-pPv transition (25). The small volume change is consistent with observations of the same transition in NaFeF 3 at 22 GPa, where no detectable volume change was reported (16). However, it should be noted that these pressure-volume measurements were performed in absence of a pressure-transmitting medium and/or without laser annealing.…”

“…In addition to NaMgF 3 , pPv and ppPv phases have been synthesized in related compositions with transition elements (e.g., Fe, Ni, and Co) substituting for Mg in the B site (16,17). These compounds show varied high-pressure behavior.…”

The phase diagrams of KCaF3 and NaMgF3 by ab initio simulations

“…Lundegaard et al 2003), as has now been found to occur by experiment in NaFeF 3 (Crichton et al 2016) and in NaCoF 3 (Wood et al 2017). At still higher pressure, Umemoto and Wentzcovitch (2006) found that a further transition occurred to a hexagonal structure with space group P 6 3 / mmc , which does not seem to correspond to any known structure type; this hexagonal structure was considered by Crichton et al (2016) to take the Be 3 N 2 structure (identical to that of InFeO 3 or InMnO 3 ), but although these structures are isopointal (as their space group and occupied Wyckoff positions are the same; Allmann and Hinek 2007), they cannot be considered isoconfigurational as their axial ratios differ by an amount sufficient to cause changes in the primary coordination of the atoms. It should be noted, however, that Umemoto and Wentzcovitch (2006) also concluded that both of these new high-pressure phases would be metastable with respect to dissociation of NaMgF 3 into CsCl-structured NaF and cotunnite-structured MgF 2 (a similar sequence of transitions was predicted for CaSnO 3 by Tsuchiya and Tsuchiya 2006).…”

“…Umemoto and Wentzcovitch (2006) proposed that there would first be a transition from the CaIrO 3 phase to a lower-symmetry, orthorhombic structure, of the Sb 2 S 3 (stibnite) structure type (e.g. Lundegaard et al 2003), as has now been found to occur by experiment in NaFeF 3 (Crichton et al 2016) and in NaCoF 3 (Wood et al 2017). At still higher pressure, Umemoto and Wentzcovitch (2006) found that a further transition occurred to a hexagonal structure with space group P 6 3 / mmc , which does not seem to correspond to any known structure type; this hexagonal structure was considered by Crichton et al (2016) to take the Be 3 N 2 structure (identical to that of InFeO 3 or InMnO 3 ), but although these structures are isopointal (as their space group and occupied Wyckoff positions are the same; Allmann and Hinek 2007), they cannot be considered isoconfigurational as their axial ratios differ by an amount sufficient to cause changes in the primary coordination of the atoms.…”

“…In contrast, Xu et al (2015) proposed a different high-pressure transition in NaMgF 3 , from the CaIrO 3 phase to a structure, with space group Pnma , that they termed “ppPv”, for “post-post-Perovskite”. Xu et al (2015) stated that this structure “has never been previously reported in any material”, but it has since been pointed out by Crichton et al (2016) that this “ppPv” phase actually corresponds to a structure of the La 2 S 3 type2 (Besançon et al 1969). Xu et al (2015) considered that the simulated diffraction pattern of their La 2 S 3 -type structure for NaMgF 3 was consistent with the “N phase” reported to form at high pressure by Martin et al 2006a, b, but they did not investigate its stability relative to the Sb 2 S 3 -type structure, nor did they examine its stability with respect to dissociation into NaF and MgF 2 .…”