Ca$_2$MnO$_4$ structural path: Following the negative thermal expansion at the local scale

The oxygen octahedral rotations in Ca$_2$MnO$_4$, the first member of the $\textrm{Ca O ( CaMnO}_3)_n$ 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 $-1.26 \pm 0.25$ to $-21 \pm 1.8$ ppm/K at the second order $I4_1/acd$ 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 $I4_1/acd$ one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical $I4_1/acd$ and $Acam$ structural phases.