Penternary Wurtzitic Nitrides Li 1- x Zn x Ge 2- x Ga x N 3 : Powder Synthesis, Crystal Structure, and Potentiality as a Solar-Active Photocatalyst.

We developed a new penternary wurtzitic nitride system Li _{1- x} Zn _x Ge _{2- x} Ga _x N ₃ (0 ≤ x ≤ 1) by hybridizing LiGe ₂ N ₃ and ZnGeGaN ₃ . Fairly stoichiometric fine powder samples were synthesized by the reduction-nitridation process at 900 °C. While the end member LiGe ₂ N ₃ possessed a relatively large band gap of 4.16 eV, the band gap of the developed penternary system varied in a broad range of 3.81 to 3.10 eV, showing promising responsivity to the solar spectrum. The crystal structure of LiGe ₂ N ₃ was precisely determined by time-of-flight neutron powder diffraction for the first time, revealing the complete ordering of Li and Ge in the Cmc 2 ₁ structure. The structural evolution from completely ordered LiGe ₂ N ₃ to fully disordered ZnGeGaN ₃ was quantitatively analyzed by Rietveld refinement based on a partially disordered Cmc 2 ₁ model, and the obtained results were also supported by ⁷¹ Ga solid-state NMR spectroscopy. The synthesized Li _{1- x} Zn _x Ge _{2- x} Ga _x N ₃ powder samples exhibited photocatalytic activities for the water reduction and oxidation reactions under solar light irradiation, with the H ₂ evolution rate of 0.3-59.0 μmol/h and the O ₂ evolution rate of 3.1-296.2 μmol/h, depending on the composition. Stable solar hydrogen generation of up to 48 h was demonstrated by the x = 0.80 sample, with the total amount of H ₂ evolved over 1.6 mmol and an external quantum efficiency of 2.1%.