  We report detailed temperature-dependent inelastic neutron scattering and
ab-initio lattice dynamics investigation of magnetic perovskites YCrO3 and
LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant
changes with temperature and dominates at low momentum transfer regime.
Ab-inito calculations performed including magnetic interactions show that the
effect of magnetic interaction is very signicant on the low- as well as
high-energy phonon modes. We have also shown that the inelastic neutron
spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic
system, which is consistent with previously reported magnetic structure in the
compound. The ab-initio lattice dynamics calculations in both the compounds
exhibit anisotropic thermal expansion behaviour in the orthorhombic structure
and predict negative thermal expansion along the crystallographic a-axis at low
temperatures. We identify the anharmonic phonon modes responsible for this
anamolous behaviour in LaCrO3 involving low-energy La vibrations and
distortions of the CrO6 octahedra.
