Interplay of dual-site metal disorder on the thermodynamic stability and electronic structures of Cu( <scp>i</scp> )-containing molybdovanadate and tungstovanadate compounds

Compounds combining Cu(I) and early transition-metal cations have attracted much attention as small-bandgap semiconductors. However, synthetic progress and new discoveries in these systems have been inhibited by their limited stability. Two new Cu(I)-molybdovanadates and one new Cu(I)-tungstovanadate have been synthesized in the Cu(I)-V(V)-M(VI)-O (M = Mo, W) systems, CuVMoO₆ (1), Cu_1.51(1)V_3.57(1)Mo_0.43(1)O₁₁ (2) and Cu_1.59(1)V_3.52(1)W_0.48(1)O₁₁ (3). Their structures were characterized by single crystal X-ray diffraction to contain layers of distorted VO₆/MO₆ octahedra with their mixed occupancy driven by the similar atomic radii of the early transition-metal cations. Interlayer sites are occupied by uncommon octahedrally-coordinated Cu, and in 2 and 3 by ∼25-30% occupancy of distorted tetrahedral sites. Electronic structure calculations show that while 1 is stable <i>versus</i> binary oxides, the CuVWO₆ analogue is not stable and could not be synthesized. The isostructural compounds, 2 and 3, also exhibited greater stability when the CuO₄ occupied sites are directly bridged to the VO₆ octahedra rather than the MoO₆/WO₆ octahedra. A small optical bandgap of ∼0.8 eV was measured for 1, with 2 and 3 being undetectable and calculated to be semi-metals. Thus, these results provide deeper insights into the energetic driving forces underlying the successful synthesis of Cu(I)-containing oxides.