Vacancy Tuning in Li,V-Substituted Lyonsites

ABSTRACT

The lyonsite structure, characterized by the formula A₄M₃O₁₂, is a relatively understudied tunneled crystal structure. This host structure is known to be compositionally flexible, able to incorporate a number of cations in various oxidation states into the A site. In the parent compound Co_3.75V_1.5Mo_1.5O₁₂, it is apparent that a stoichiometric vacancy of 0.25 is unavoidable as a result of Coulombic repulsion. This work focuses on the systematic elimination of vacancies by chemically introducing guest Li ions while maintaining host integrity. A full solid solution was found to exist with the formula □_0.25–1/8xLi_xCo_3.75–7/8xV_1.5–3/4xMo_1.5+3/4xO₁₂ (0 ≤ x ≤ 2), terminating at the known end member Li₂Co₂Mo₃O₁₂. Lattice refinements on PXRD data confirmed the isostructural nature of the whole series, and detailed structural analysis revealed that competition between Li and Co in the same crystallographic site is unequal, with Li exhibiting a stronger site preference for larger interstitial sites. Diffuse reflectance analysis revealed that the optical band gap is directly tunable with x, and supporting structure-property relationships were also explored via magnetometry and dielectric measurements.

KEYWORDS:

• Lyonsite
• crystallography
• magnetism
• diffuse reflectance
• dielectric

Acknowledgments

We acknowledge the support of the National Institute of Standards and Technology, U.S. Department of Commerce, in providing the neutron research facilities used in this work. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Declaration of interest

The authors declare no conflict of interest.

Additional information

Funding

This research was supported by the National Science Foundation under Grant [DMR 1508527].