Lawler Keith V, Smith Dean, Evans Shaun R, Dos Santos Antonio M, Molaison Jamie J, Bos Jan-Willem G, Mutka Hannu, Henry Paul F, Argyriou Dimitri N, Salamat Ashkan, Kimber Simon A J
Department of Chemistry and Biochemistry, University of Nevada Las Vegas, Las Vegas, Nevada 89154, United States.
Department of Physics and Astronomy and HiPSEC, University of Nevada Las Vegas, Las Vegas, Nevada 89154, United States.
Inorg Chem. 2021 Apr 19;60(8):6004-6015. doi: 10.1021/acs.inorgchem.1c00435. Epub 2021 Mar 31.
The AMnO delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn spins. At low temperatures ( = 65 K), a 2/ → 1̅ transition is found in CuMnO, which breaks frustration and establishes magnetic order. In contrast to this clean transition, A = 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, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act 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 analogue of the -axis negative thermal expansion seen at ambient pressure. Density functional theory (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 reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.
AMnO铜铁矿（A = Na、Cu）是典型的受挫反铁磁体，具有由锰自旋构成的三角形层。在低温（= 65 K）下，在CuMnO中发现了2/→1̅转变，该转变打破了受挫状态并建立了磁有序。与这种清晰的转变形成对比的是，A = Na仅在 时表现出短程畸变。在此，我们报告了对CuMnO进行的系统晶体学、光谱学和理论研究。我们表明，即使在化学计量比的样品中，非零的各向异性铜位移也与磁有序共存。利用X射线/中子衍射和拉曼散射，我们表明高压作用使这些自由度解耦。这表现为在约10 GPa时的等结构相变，伴随着 轴的可逆塌缩。这被证明是在常压下观察到的 轴负热膨胀的高压类似物。密度泛函理论（DFT）模拟证实，在常压下铜阳离子和边缘共享的MnO层的动力学不稳定性相互交织。然而，高压选择性地激活了前者，最终预计在最高压力下磁性会再次出现。我们的结果表明，CuMnO的晶格动力学和局部结构在数量上与非磁性铜铁矿不同，并引发了关于固有不均匀性在受挫反铁磁体中的作用的问题。
