Spin-phonon coupling and high-pressure phase transitions ofRMnO3(R=Caand Pr): An inelastic neutron scattering and first-principles study

Abstract

We report inelastic neutron scattering measurements over 7--1251 K in $\mathrm{CaMn}{\mathrm{O}}_{3}$ covering various phase transitions, and over 6--150 K in $\mathrm{PrMn}{\mathrm{O}}_{3}$ covering the magnetic transition. The excitations around 20 meV in $\mathrm{CaMn}{\mathrm{O}}_{3}$ and at 17 meV in $\mathrm{PrMn}{\mathrm{O}}_{3}$ at low temperatures are found to be associated with magnetic origin. We observe coherent magnetic neutron scattering in localized regions in reciprocal space and show it to arise from long-range correlated magnetic spin-waves below the magnetic transition temperature (${T}_{\mathrm{N}}$) and short-range stochastic spin-spin fluctuations above ${T}_{\mathrm{N}}$. In spite of the similarity of the structure of the two compounds, the neutron inelastic spectrum of $\mathrm{PrMn}{\mathrm{O}}_{3}$ exhibits broad features at 150 K unlike well-defined peaks in the spectrum of $\mathrm{CaMn}{\mathrm{O}}_{3}$. This might result from the difference in the nature of interactions in the two compounds (magnetic and Jahn-Teller distortion). Ab initio phonon calculations have been used to interpret the observed phonon spectra. The ab initio calculations at high pressures show that the variations of Mn-O distances are isotropic for $\mathrm{CaMn}{\mathrm{O}}_{3}$ and highly anisotropic for $\mathrm{PrMn}{\mathrm{O}}_{3}$. The calculation in $\mathrm{PrMn}{\mathrm{O}}_{3}$ shows the suppression of Jahn-Teller distortion and simultaneous insulator-to-metal transition. It appears that this transition may not be associated with the occurrence of the tetragonal phase above 20 GPa as reported in the literature, since the tetragonal phase is found to be dynamically unstable, although it is found to be energetically favored over the orthorhombic phase above 20 GPa. $\mathrm{CaMn}{\mathrm{O}}_{3}$ does not show any phase transition up to 60 GPa.