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

The oxygen octahedral rotations in Ca2MnO4, the first member of the CaO(CaMnO3)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 ± 0.25 to −21 ± 1.8 ppm/K at the second order I41/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 I41/acd one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical I41/acd and A c a m structural phases.