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 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 R = Nd and heavier REs). By contrast, our new synthetic investigations for RE = Ce and La demonstrate the crystallization of both structure types, i.e., 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 oC and from arc-melting techniques, respectively. Both structures similarly consist of [MoO4]2- tetrahedra bridged either by highly-distorted CuO4 tetrahedra and CeO9 tricapped trigonal prisms in 1 and 2 or by T-shaped CuO3 and CeO8 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 bandgaps which stem predominantly from electronic transitions between the filled 3d10-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 each of these cations. The change in crystal structure, such as from Cu6Ce4(MoO4)9 to CuCe(MoO4)2, has a dramatically 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 investigation of the quaternary cuprous molybdates has elucidated the impact of RE cations on their crystal structures, compositions, and visible-light bandgaps, with the underlying relationships revealed via electronic structure calculations.