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

This research investigates the high-pressure behavior of the Ca(Ti0.95Ge0.05)O3 perovskite, a candidate of the locked-tilt perovskite family (orthorhombic compounds characterized by the absence of changes in the octahedral tilt and volume reduction under pressure controlled solely by isotropic compression). The study combines experimental high-pressure synchrotron diffraction data with density functional theory (DFT) calculations, complemented by the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), to understand the structural evolution of the perovskite under pressure. The results show that CTG undergoes nearly isotropic compression with the same compressibility along all three unit-cell axes (i.e., Ka0 = Kb0 = Kc0, giving a normalized cell distortion factor with pressure dnorm(P) = 1). However, a modest increase in octahedral tilting with pressure is revealed by DFT calculations, qualifying CTG as a new type of GdFeO3-type perovskite that exhibits both isotropic compression and nonlocked tilting. This finding complements two existing types: perovskites with anisotropic compression and tilting changes and those with isotropic compression and locked tilting. The multimethod approach provides valuable insights into the structural evolution of locked-tilt perovskites under high pressure and establishes a protocol for the efficient study of complex high-pressure systems. The results have implications for the design of new functional materials with desirable properties.


Tables
Table S1.Unit-cell parameters (a, b, c, and V) of the CTG perovskite (s.g.Pbnm) up to 24 GPa derived from HP diffraction experiments and extracted by a whole powder pattern decomposition (WPPD) through a Pawley profile fit using TOPAS v. 5.0 software.
Table SI3.Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 0.004 GPa.

Figures a b Figure S1 .
Figures

Figure S2 .
Figure S2.Normalized cell distortion factor with pressure, d norm (P), as a function of pressure for CaB 2+ O 3 orthorhombic perovskites as derived from X-ray diffraction experiments.Solid lines are linear fits to the data.Data for the isotypic series CaB 2+ O 3 (B = Ti and Ge) are from 1-4 .

Table SI4 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 0.206 GPa.

Table SI5 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 1.623GPa.

Table SI6 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 2.530 GPa.

Table SI7 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 3.032 GPa.

Table SI8 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 3.942 GPa.

Table SI9 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 5.038 GPa.

Table SI10 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 7.679 GPa.

Table SI11 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 10.055GPa.

Table SI12 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 15.079GPa.

Table SI13 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 20.081GPa.

Table SI14 .
Atomic coordinates obtained from the DFT calculation for the sample CTG at P = 25.141GPa.

Table S15 .
Unit-cell parameters (a, b, c, and V) and selected bond lengths and angles at room conditions for CaTiO 3 orthorhombic perovskites from the literature and those for the CTG sample investigated here, derived from X-ray diffraction experiments.