# Hidden Charge‐Order in the Mixed‐Valent K0.75Li2Cr6O12 High‐Pressure Oxide

## TL;DR

A new mixed-valent chromium oxide is synthesized and studied, revealing hidden charge order and a unique magnetic structure stabilized by potassium disorder and structural compression.

## Contribution

The paper identifies a hidden charge order in K₀.₇5Li2Cr₆O12, crucial for stabilizing its antiferromagnetic state through potassium positional disorder and c-axis compression.

## Key findings

- K₀.₇5Li2Cr₆O12 exhibits a commensurate helical magnetic structure with a propagation vector k = [⅓ ⅓ ¼].
- Density functional theory shows c-axis compression stabilizes antiferromagnetic interactions via direct exchange.
- The material undergoes a paramagnetic-to-antiferromagnetic transition at 75 K with a 45° spin rotation in zig-zag ladders.

## 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.

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.