Ca2MnO4 structural path: Following the negative thermal expansion at the local scale

Abstract

The oxygen octahedral rotations in ${\mathrm{Ca}}_{2}\mathrm{Mn}{\mathrm{O}}_{4}$, the first member of the $\mathrm{Ca}\mathrm{O}{({\mathrm{CaMnO}}_{3})}_{n}$ Ruddlesden-Popper family, is probed through a set of complementary techniques, including temperature-dependent neutron and x-ray diffraction, combined with local probe studies and ab initio calculations. Here we demonstrate the enhancement of the uniaxial negative thermal expansion coefficient from $\ensuremath{-}1.26\ifmmode\pm\else\textpm\fi{}0.25$ to $\ensuremath{-}21\ifmmode\pm\else\textpm\fi{}1.8$ ppm/K at the second order $I{4}_{1}/acd$ to $I4/mmm$ structural phase transition, providing direct evidence for the corkscrew atomic mechanism. We establish, also, that the predicted $I4/mmm$ high symmetry is attained around 1050 K. At lower temperatures, within the 10--1000 K temperature range, our first-principles calculations and detailed analysis of the Ca local environment reveals that the reported $Aba2$ structural phase, coexisting with the $I{4}_{1}/acd$ one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical $I{4}_{1}/acd$ and $Acam$ structural phases.