Growth, Structures, and Properties of Li₂Zn₂(MoO₄)₃ and Co-doped Li₂Zn₂(MoO₄)₃

On the basis of our previous work on the investigation of the subsolidus phase relation of the system ZnO- Li₂O-MoO₃, we for the first time successfully prepared single crystals of Li₂Zn₂(MoO₄)₃ (I) with dimensions up to 10 × 4 × 3 mm³ and Co-doped Li₂Zn₂(MoO₄)₃ (II) with a size of 15 × 10 × 3 mm³ by using the top-seeded solution growth method. The X-ray diffraction analysis shows that the Li⁺ cation and transition metal ion fill in the octahedral and trigonal prism interstice composed of surrounding MoO₄ tetrahedrons. The UV−vis spectra show that by doping with Co²⁺, there is an obvious red shift of the absorption band at 276 nm with the steep absorption edge changing from 370 nm (I) to 430 nm (II). The band gaps of I and II calculated with CASTEP are ≪ 2.4 eV and ≪ 1.9 eV, respectively, indicating that Co²⁺ play roles in the change of the band gap of the host crystal. The X-ray powder analysis of the system Li_2−2xZn_2+x(MoO₄)₃ suggests the framework is stable over a range of composition for −0.1 ≤ x ≤ 0.3. The magnetic experiments indicate that the Co-doped Li₂Zn₂(MoO₄)₃ is paramagnetic with a rather large μ_eff (5.37 μ_B). The crystal Li₂Zn₂(MoO₄)₃ may have at least two feasible applications: (i) the absorption in the ultraviolet band suggested that they can be used as a light filter; (ii) since the framework of Li_2−2xZn_2+x(MoO₄)₃ is stable over a range of composition for −0.1 ≤ x ≤ 0.3, the refractive index of the crystal may vary with different ratios of Li/Zn, suggesting that the crystals can be used as tunable refractive index materials in the middle- and near-ultraviolet region.