Evidence for a different electronic configuration as a primary effect during compression of orthorhombic perovskites: The case of NdM3+O3(M=Cr

Abstract

SiO 3 perovskite is the most abundant mineral of the Earth's lower mantle, and compounds with the perovskite structure are perhaps the most widely employed ceramics. Hence, they attract both geophysicists and material scientists. Several investigations attempted to predict their structural evolution at high pressure, and recent advancements highlighted that perovskites having ions with the same formal valence at both polyhedral sites (i.e., 3+:3+) define different compressional patterns when transition metal ions (TMI) are involved. In this study, in situ high-pressure synchrotron XRD measurements coupled with ab initio simulations of the electronic population of NdCrO 3 perovskite are compared with the compressional feature of NdGaO 3. Almost identical from a steric point of view (Cr 3+ and Ga 3+ have almost the same ionic radius), the different electronic configuration of octahedrally coordinated ions-which leads to a redistribution of electrons at the 3d orbitals for Cr 3+-allows the crystal field stabilization energy (CFSE) to act as a vehicle of octahedral softening in NdCrO 3 or it turns octahedra into rigid units when CFSE is null as in NdGaO 3. Besides to highlight that different electronic configurations can act as a primary effect during compression of perovskite compounds, our findings have a deep repercussion on the way the compressibility of perovskites have to be modeled.

Structure Evolution of Ge-Doped CaTiO3 (CTG) at High Pressure: Search for the First 2:4 Locked-Tilt Perovskite by Synchrotron X-ray Diffraction and DFT Calculations

“…Conversely, when A and B have the same formal charge (i.e., both cations are 3+; perovskites 3:3), A O 12 is stiffer than B O 6 , and the volume reduction is partially compensated by a decrease in the octahedral tilt . Although true for most of the GdFeO 3 -type perovskites, the above description is incomplete and has recently been revised. ,, In addition to defining dichotomous trends depending on the formal charge of A and B cations, the evolution of orthorhombic perovskites with pressure is influenced by whether transition metal ions (TMIs) are hosted at the octahedral site. , The use of geochemical constraints (i.e., valence, ionic radius, diadochy rules) and the evaluation of the “normalized cell distortion factor with pressure, d norm ( P )″, for several perovskite compounds, ,− has allowed the identification of other possible locked-tilt perovskite formulations, viz., La­(Mn 0.69 Ga 0.31 )­O 3 , Ca­(Ti 0.95 Ge 0.05 )­O 3 , and (Sc 0.86 Y 0.14 )­AlO 3 , respectively.…”

“… 1 , 20 , 21 In addition to defining dichotomous trends depending on the formal charge of A and B cations, the evolution of orthorhombic perovskites with pressure is influenced by whether transition metal ions (TMIs) are hosted at the octahedral site. 20 , 21 The use of geochemical constraints (i.e., valence, ionic radius, diadochy rules) and the evaluation of the “normalized cell distortion factor with pressure, d norm ( P )″, for several perovskite compounds, 1 , 19 − 22 has allowed the identification of other possible locked-tilt perovskite formulations, viz., La(Mn 0.69 Ga 0.31 )O 3 , Ca(Ti 0.95 Ge 0.05 )O 3 , and (Sc 0.86 Y 0.14 )AlO 3 , respectively.…”