Decoupling lattice and magnetic instabilities in frustrated CuMnO
arXiv:2104.06281 · doi:10.1021/acs.inorgchem.1c00435
Abstract
The MnO delafossites (=Na, Cu), are model frustrated antiferromagnets, with triangular layers of Mn~spins. At low temperatures (=65 K), a transition is found in CuMnO, which breaks frustration and establishes magnetic order. In contrast to this clean transition, =Na only shows short-range distortions at . Here we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO. We show that, even in stoichiometric samples, non-zero anisotropic Cu displacements co-exist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures acts to decouple these degrees of freedom. This manifests as an isostuctural phase transition at 10 GPa, with a reversible collapse of the -axis. This is shown to be the high pressure analog of the -axis negative thermal expansion seen at ambient pressure. DFT simulations confirm that dynamical instabilities of the Cu cations and edge-shared MnO layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted re-emergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO are quantitatively different to non-magnetic Cu delafossites, and raise questions about the role of intrinsic inhomogeniety in frustrated antiferromagnets.
arXiv commentSubmitted version, A.S.A.P. article in Inorganic Chemistry
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