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

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Rocha-Rodrigues, Pedro ; Santos, Samuel S. M. ; Oliveira, Goncalo N. P. ; Leal, Tiago ; Miranda, Ivan P. ; dos Santos, Antonio M. ; Correia, Joao G. ; Assali, Lucy V. C. ; Petrilli, Helena M. ; Araujo, Joao P. ; Lopes, Armandina M. L.

• Document type: Journal article
• Source: PHYSICAL REVIEW B; v. 102, n. 10, p. 9-pg., 2020-09-28.

Abstract

The oxygen octahedral rotations in Ca2MnO4, the first member of the CaO(CaMnO3)(pi) 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 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. , (AU)

• FAPESP's process: 18/07760-4 - Functional lattice instabilities in naturally layered perovskites
• Grantee: Helena Maria Petrilli
• Support Opportunities: Regular Research Grants