Hidden Charge-Order in the Mixed-Valent K_0.75Li₂Cr₆O₁₂ High-Pressure Oxide

The mixed valent high-pressure oxide K₀.₇₅Li₂Cr₆^3.54+O₁₂ is synthesized at 12 GPa and 1373 K. Synchrotron X-ray and powder neutron diffraction (PND) reveal a P6_3/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, K₀.₇₅Li₂Cr₆^3.54+O₁₂ exhibits strong spin-lattice coupling, where a slight change in the structure has a huge impact in the properties.