Perovskite to Postperovskite Transition in NaFeF₃

Abstract

The GdFeO3-type perovskite NaFeF3 transforms to CaIrO3-type postperovskite at pressures as low as 9 GPa at room temperature. The details of such a transition were investigated by in situ synchrotron powder diffraction in a multianvil press. Fit of the p-V data showed that the perovskite phase is more compressible than related chemistries with a strongly anisotropic response of the lattice metrics to increasing pressure. The reduction in volume is accommodated by a rapid increase of the octahedral tilting angle, which reaches a critical value of 26° at the transition boundary. The postperovskite form, which is fully recoverable at ambient conditions, shows a regular geometry of the edge-sharing octahedra and its structural properties are comparable to those found in CaIrO3-type MgSiO3 at high pressure and temperature. Theoretical studies using density functional theory at the GGA + U level were also performed and describe a scenario where both perovskite and postperovskite phases can be considered Mott-Hubbard insulators with collinear magnetic G- and C-type antiferromagnetic structures, respectively. Magnetic measurements are in line with the theoretical predictions with both forms showing the typical behavior of canted antiferromagnets.

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

“…The lower-pressure large-volume data are discussed in greater detail in Bernal et al (2014). It is sufficient to illustrate that further pv diffraction patterns (Fig.…”

“…2c.). Consequently, the degree of anisotropy increases with pressure, as reflected by the value of the tilting angle that quickly reaches the approximate limiting value (Tateno et al, 2010) of  = 26.9º at 9.5 GPa (see also Table 1, from single-crystal refinement, and the powder diffraction results of Bernal et al, 2014).…”

“…As compression results in significant bond-length shortening, it is not unexpected that the more rapid b-axis compression will tend to saturate, leading to a consequent reduction in out-of-plane anisotropy with increasing pressure. The ppv form is recoverable in bulk, Bernal et al (2014), though the single-crystal samples do back-transform to pv.…”

“…Powders of NaF, Fe and FeF 3 were ground finely and mixed in a 3 : 2:1 ratio according to the synthesis method of Benner and Hoppe (1990) and were of the same batch as used by Bernal et al (2014). The so-obtained mixtures were then pelletized and loaded into an Fe tube which was used as a reaction vessel.…”

The phase diagrams of KCaF3 and NaMgF3 by ab initio simulations

“…A major disadvantage of the ABF 3 fluorides, where A = Na and B = a first-row transition element, however, is that in the majority of these compounds the d -electrons of the B cation will have unpaired spins leading to magnetic ordering at low-temperatures. Bernal et al ( 2014 ) have reported recently that PV- and CaIrO 3 -type NaFeF 3 show antiferromagnetic ordering at ~90 and 48 K, respectively (with weak ferromagnetism observed in PV-NaFeF 3 ); similarly, PV- and CaIrO 3 -type NaNiF 3 order antiferromagnetically at 156 and 22 K, respectively (Shirako et al 2012b ) and the corresponding phases of NaCoF 3 have antiferromagnetic transitions at 74 K (Friedman et al 1970 ) and 24 K (A S Wills, pers. comm.).…”