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Earths lower mantle extending from 670 to 2,990 km deep is predominantlycomposed of a perovskite-type (Mg,Fe)SiO3 phase1,2. The perovskite phaseundergoes a structural phase transition to a post-perovskite phase responsiblefor D" layer seismic discontinuity2,3 at about 2690 km depth in the lowermostregion of the lower mantle. However, structural basis of other seismicdiscontinuities occurring in the upper region of the lower mantle (700 km to1,200 km deep) remains unexplained4-7, as no apparent change in the crystalsymmetry of the orthorhombic perovskite phase has been reported5. We presenthere unambiguous evidence for a non-apparent isostructural phase transition8 inthe stable orthorhombic perovskite phase of CaTiO3 which may have relevance tophase transitions in the perovskite phase of (Mg,Fe)SiO3 also, as both thecompounds have similar structure, tolerance factor and thermochemicalproperties9-11. Our results are based on the analysis of neutron powderdiffraction patterns using Rietveld and mode crystallography techniques and aresupported by density functional and Landau theory calculations. The presentresults on CaTiO3 would encourage search for isostructural phase transition inthe perovskite phase of (Mg,Fe)SiO3 that may provide clue to the unexplainedgeophysical phenomena in the upper part of the earths lower mantle.

Discovery of an isostructural phase transition within orthorhombic phase field of CaTiO3