Comprehensive Study of Electronic, Optical, and Thermophysical Properties of Metavanadates CaV₂O₆ and MgV₂O₆

Calcium metavanadate (CaV₂O₆) and magnesium metavanadate (MgV₂O₆) have received considerable attention due to their great potential for many practical applications; however, a fundamental understanding of their intrinsic physical properties is still not well established. Here, we present a comprehensive experimental and theoretical study of the optical, electronic, and lattice dynamic properties of CaV₂O₆ and MgV₂O₆. We find that both compounds are semiconductors with indirect band gaps and exhibit visible light-emitting properties, which are thus expected to be the ideal candidates for phosphors or optoelectronic devices. Through density functional theory (DFT) calculations, we further show that CaV₂O₆ exhibits negative thermal expansion (NTE) below 80 K due to the flexible or intense coupled rocking of the CaO₆ octahedra. Moreover, using the dual-phonon model, we disclose the hierarchical thermal transport features of these two compounds, in which diffusive channels make a significant contribution to the thermal conductivity κ, resulting in the weak temperature dependence of κ deviating from the typical κ ∼T^-1 given by the phonon gas model. With the consideration of normal and diffusive phonons, we predict the ultralow and large anisotropic thermal conductivities of CaV₂O₆ and MgV₂O₆. This work provides a fundamental understanding of the optical, electronic, and thermal properties of metal-ion-intercalated layered vanadium oxides, which may promote the functional applications of metavanadates.