K0.75Li2Cr6O12 is one of the few mixed valent chromium oxides. It shows a single angle spiral magnetic structure with a k = [1/3 1/3 ¼] propagation vector. Density functional theory reveals a hidden charge order that is necessary to stabilize the antiferromagnetic (AFM) ground state. This hidden order is induced by the inherent K+ positional disorder due to the loose character in its channel. The strong c compression helps on stabilizing direct-exchange AFM interactions.
Abstract
The mixed valent high-pressure oxide K₀.₇5Li2Cr₆3.54+O12 is synthesized at 12 GPa and 1373 K. Synchrotron X-ray and powder neutron diffraction (PND) reveal a P63/m
average structure. Analogous to hollandites, CrO6 octahedra form a corner-sharing double-chain framework, this creates two types of channels respectively occupied by Li+ and K+. However, electron microscopy and pair distribution function analysis suggest the loss of correlation in and between the K+ partially occupied channels. A magnetostrictive paramagnetic-to-antiferromagnetic transition occurs at T
N = 75 K, where low temperature PND reveals a propagation vector k = [⅓ ⅓ ¼] realising a commensurate helical structure with 45° rotation between Cr3⁺ and Cr⁴⁺ spins in zig-zag ladders, stabilized by competing ferromagnetic and antiferromagnetic exchanges. Density functional theory calculations highlight the critical role of the c-axis compression in determining the magnetic direct exchange interactions thus revealing the ferromagentic-correlated paramagnet → charge-ordered antiferromagnetic transition, reminiscent to the pressure induced FM to AFM transition in the related K2Cr8O16 hollandite. Despite hidden long-range charge order due to K⁺ disorder, K₀.₇5Li2Cr₆3.54+O12 exhibits strong spin-lattice coupling, where a slight change in the structure has a huge impact in the properties.
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