Decoupling lattice and magnetic instabilities in frustrated CuMnO${}_2$

Abstract

The $A$MnO${}_2$ delafossites ($A$=Na, Cu), are model frustrated antiferromagnets, with triangular layers of Mn${}^{3+}$~spins. At low temperatures ($T_N$=65 K), a $C2/m\to P\overline{1}$ transition is found in CuMnO${}_2$, which breaks frustration and establishes magnetic order. In contrast to this clean transition, $A$=Na only shows short-range distortions at $T_N$. Here we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO${}_2$. 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 $\sim$10 GPa, with a reversible collapse of the $c$-axis. This is shown to be the high pressure analog of the $c$-axis negative thermal expansion seen at ambient pressure. DFT simulations confirm that dynamical instabilities of the Cu${}^{+}$ cations and edge-shared MnO${}_6$ 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${}_2$ are quantitatively different to non-magnetic Cu delafossites, and raise questions about the role of intrinsic inhomogeniety in frustrated antiferromagnets.

Submitted version, A.S.A.P. article in Inorganic Chemistry