Crystal Growth, Structures, and Optical Bandgaps of Cuprous Rare-Earth Molybdates

Cuprous oxide semiconductors have been growing in research interest because of their promising optical and catalytic properties for solar energy conversion. While much recent research has focused on binary and ternary cuprous oxides, the more structurally complex quaternary and higher systems remain significantly less explored. Crystal growth in the cuprous rare-earth (RE) molybdate system has been investigated by using high-temperature and arc-melting synthesis techniques. Prior studies show the formation of two closely related compounds in these systems, occurring as either Cu₆RE₄(MoO₄)₉ (Space group: R3c, No. 161; exclusive for RE = La) or CuRE-(MoO₄)₂ (Space group: Pbca, No. 61; for RE = Nd and heavier REs). By contrast, our new synthetic investigations for RE = Ce and La demonstrate the crystallization of both structure types, as red-colored crystals of Cu₆RE₄(MoO₄)₉ (RE = La (1) or Ce (2)) and as yellow-colored crystals of CuRE-(MoO₄)₂ (RE = La (3) or Ce (4)), as obtained by slow cooling from 950 °C and from arc-melting techniques, respectively. Both structures similarly consist of [MoO₄]^2- tetrahedra bridged either by highly distorted CuO₄ tetrahedra and REO₉ tricapped trigonal prisms in 1 and 2 or by T-shaped CuO₃ and REO₈ square antiprisms in 3 and 4. Optical UV-vis diffuse reflectance measurements on powders of CuRE-(MoO₄)₂ for RE = La, Ce, Sm, Eu, and Yb show relatively larger direct bandgaps in the range of 2.36 to 2.47 eV, while the crystals of Cu₆RE₄(MoO₄)₉ for RE = La and Ce yield smaller indirect bandgaps of about 1.89 to 2.05 eV. Electronic structure calculations show band gaps that stem predominantly from electronic transitions between the filled 3d ¹⁰-based orbitals of the Cu-(I) cations and the empty d-based orbitals of the Mo-(VI) cations. Thus, the RE cations are found to have an indirect effect on the optical bandgaps via changes in the local coordination environments of the transition-metal cations. The change in crystal structure, such as from Cu₆Ce₄(MoO₄)₉ to CuCe-(MoO₄)₂, has a notably larger effect of decreasing the bandgap by ∼0.4 to 0.5 eV as compared to only changing the RE cation within the same structure type. In summary, new synthetic investigations of the quaternary cuprous molybdates have elucidated the impact of RE cations on their crystal structures, compositions, and visible-light bandgaps, with the underlying relationships revealed via electronic structure calculations.