  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.
