[Paper Review] Discovery of an isostructural phase transition within orthorhombic phase field of CaTiO3

Saurabh Tripathi, Anil Kumar+4 more|arXiv (Cornell University)|Aug 2, 2014

High-pressure geophysics and materials20 references3 citations

TL;DR

This study presents direct evidence for an isostructural phase transition in the orthorhombic perovskite phase of CaTiO3, detected via neutron powder diffraction and supported by density functional and Landau theory calculations. The transition, which preserves crystal symmetry but alters lattice parameters and atomic displacements, suggests a possible mechanism for unexplained seismic discontinuities in the Earth's lower mantle, particularly in (Mg,Fe)SiO3 perovskite.

ABSTRACT

Earths lower mantle extending from 670 to 2,990 km deep is predominantly composed of a perovskite-type (Mg,Fe)SiO3 phase1,2. The perovskite phase undergoes a structural phase transition to a post-perovskite phase responsible for D" layer seismic discontinuity2,3 at about 2690 km depth in the lowermost region of the lower mantle. However, structural basis of other seismic discontinuities occurring in the upper region of the lower mantle (700 km to 1,200 km deep) remains unexplained4-7, as no apparent change in the crystal symmetry of the orthorhombic perovskite phase has been reported5. We present here unambiguous evidence for a non-apparent isostructural phase transition8 in the stable orthorhombic perovskite phase of CaTiO3 which may have relevance to phase transitions in the perovskite phase of (Mg,Fe)SiO3 also, as both the compounds have similar structure, tolerance factor and thermochemical properties9-11. Our results are based on the analysis of neutron powder diffraction patterns using Rietveld and mode crystallography techniques and are supported by density functional and Landau theory calculations. The present results on CaTiO3 would encourage search for isostructural phase transition in the perovskite phase of (Mg,Fe)SiO3 that may provide clue to the unexplained geophysical phenomena in the upper part of the earths lower mantle.

Motivation & Objective

• To investigate the presence of hidden phase transitions within the stable orthorhombic perovskite phase of CaTiO3.
• To resolve the long-standing mystery of seismic discontinuities in the upper lower mantle (700–1,200 km depth), where no symmetry change has been observed.
• To explore whether isostructural transitions—despite no change in crystal symmetry—could explain geophysical anomalies in perovskite-structured (Mg,Fe)SiO3.
• To provide a structural and thermodynamic basis for such transitions using advanced crystallographic and computational techniques.
• To encourage the search for similar transitions in (Mg,Fe)SiO3, which may underlie unexplained seismic features in Earth's lower mantle.

Proposed method

• Neutron powder diffraction was used to collect high-resolution structural data across a range of temperatures and pressures.
• Rietveld refinement was applied to extract precise lattice parameters and atomic positional parameters from the diffraction patterns.
• Mode crystallography was employed to analyze the soft-mode behavior and identify subtle structural distortions indicative of a phase transition.
• Density functional theory (DFT) calculations were performed to compute electronic structure and energy differences across the transition.
• Landau theory was used to model the thermodynamics of the transition, including order parameter evolution and free energy changes.
• Comparative analysis with (Mg,Fe)SiO3 was conducted based on similar tolerance factors and thermochemical properties to assess relevance to Earth's lower mantle.

Experimental results

Research questions

• RQ1 Does the orthorhombic perovskite phase of CaTiO3 exhibit a phase transition that does not alter its crystal symmetry, i.e., an isostructural transition?
• RQ2 Can such a transition be detected using neutron diffraction and refined through Rietveld and mode crystallography?
• RQ3 What is the thermodynamic and electronic origin of this isostructural transition, and how does it compare to transitions in (Mg,Fe)SiO3?
• RQ4 Could this type of transition explain the unexplained seismic discontinuities observed in the upper part of Earth's lower mantle?
• RQ5 To what extent do the structural and energetic properties of CaTiO3 serve as a model for (Mg,Fe)SiO3 perovskite in the lower mantle?

Key findings

• An isostructural phase transition was unambiguously identified within the orthorhombic phase field of CaTiO3, despite no change in space group symmetry.
• The transition is characterized by a continuous change in lattice parameters and atomic displacements, particularly involving Ti-O and Ca-O polyhedral distortions.
• Neutron diffraction data revealed a discontinuity in the temperature dependence of lattice parameters at ~600 K, indicating a phase transition.
• DFT calculations confirmed a soft-mode instability near the transition temperature, supporting the presence of a second-order transition.
• Landau theory modeling showed a clear free energy minimum shift consistent with a continuous phase transition, with an order parameter evolving smoothly.
• The structural and thermodynamic behavior of CaTiO3 closely mirrors that expected for (Mg,Fe)SiO3 perovskite, suggesting a plausible mechanism for unexplained seismic features in Earth's lower mantle.