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Description
Naturally layered perovskites have been studied in the designing of novel functional materials, ranging from superconductors to multiferroics, including the recently discovered hybrid improper ferroelectrics, to materials that exhibit negative thermal expansion [1,2]. Particularly, the Ruddlesden-Popper family with general formula CaO(CaMnO3)n has gained considerable interest, where the combined effect of the perovskite layered structure and the condensation of oxygen octahedral rotations modes, underlie both hybrid improper ferroelectricity and uniaxial negative thermal expansion (NTE) properties.
The oxygen octahedral rotations in Ca2MnO4, the first member of the CaO(CaMnO3)n Ruddlesden-Popper family, are here probed through a set of complementary techniques, including temperature-dependent neutron and x-ray diffraction, combined with local probe studies and ab initio calculations. Long range order based techniques, such as x-ray or neutron diffraction may present difficulties in correlating, with precision, the evolution of octahedral rotations with the thermal change of the Ca2MnO4 expansion properties, either due to a lower sensitivity to oxygen atomic positions, as in x-ray diffraction, or due to the actual condensation of the octahedral rotation modes that propagate with a short structural coherence length within the crystal lattice. Time differential perturbed angular correlation (TDPAC) experiments combined with neutron and x-ray diffraction measurements, and density functional theory simulations provide a unique tool to characterize the Ca2MnO4 structural transitions at the atomic scale [2]. We show that the detailed measurement of the electric field gradient at the Ca-sites, and local symmetry analysis, allow to accurately probe the MnO6 octahedral rotations that underlie the Ca2MnO4 structural transitions and its uniaxial NTE properties.
Here we demonstrate the enhancement of the uniaxial NTE 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 recently proposed corkscrew atomic mechanism. We also establish that the aristotype I4/mmm symmetry is attained around 1050 K, a much lower temperature than previously predicted. At lower temperatures, within the 10–1000 K temperature range, our first-principles calculations and detailed analysis of the Ca local environment reveal that the reported Aba2 structural phase, coexisting with the I41/acd one, cannot correctly describe this compound. On the other hand, our data allow for the coexistence of the locally identical I41/acd and Acam structural phases.
