Abstract
The oxygen octahedral rotations in , the first member of the 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 to ppm/K at the second order to structural phase transition, providing direct evidence for the corkscrew atomic mechanism. We establish, also, that the predicted 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 structural phase, coexisting with the one, cannot describe correctly this compound. On the other hand, our data allow for the coexistence of the locally identical and structural phases.
- Received 12 May 2020
- Revised 29 July 2020
- Accepted 28 August 2020
DOI:https://doi.org/10.1103/PhysRevB.102.104115
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