CaAlSiN3:Eu2+ fine powder phosphors were successfully synthesized from the fully oxidic system CaO/Al2O3/SiO2 using the gas-reduction–nitridation (GRN) method. The GRN reaction was conducted at low temperatures of ∼1370 °C to obtain nitrided precursor powders that could be converted into nearly single-phase CaAlSiN3 by a moderate heat treatment at 1600 °C for 4 h in a N2 atmosphere of 0.92 MPa. Highly efficient CaAlSiN3:Eu2+ red-emitting phosphors were obtained using a fluoride activator, with external quantum efficiencies of up to 72%.

We developed a new quaternary wurtzitic nitride system by formation of the solid solution between ZnGeN2 and GaN. Near stoichiometric and monophasic powder samples in the composition Zn1–xGe1–xGa2xN2 (x ≤ 0.50) were obtained by the reduction–nitridation synthesis conducted at 900 °C. The results of crystal structure refinement clearly revealed that the cation ordering in the structure of ZnGeN2 (Pna21) tends to disappear by introducing Ga into the lattice, and the structure transforms to a simple wurtzite phase (P63mc) with the composition of x ≥ 0.33. The observed structural evolution was further confirmed by the results of 71Ga solid-state nuclear magnetic resonance (NMR) spectroscopy, showing an unsplit single peak observed for x ≥ 0.33. The dissolution of GaN into ZnGeN2 also resulted in a marked narrowing of the band gap, from the ultraviolet region of 3.42 eV to the visible-light range of 3.02–3.05 eV, depending scarcely on the value of x. The results of photocatalytic test reactions for water splitting showed that the synthesized Zn1–xGe1–xGa2xN2 solid solution possessed the H2 evolution rate of 2.8–3.6 μmol/h and the relatively high O2 evolution rate of 100.4–126.6 μmol/h, as well as the capability for overall water splitting under the visible-light irradiation of λ > 400 nm.

We demonstrated, for the first time, formation of the Na-α′-GeGaON (NamGe12–(m+n)Gam+nOnN16–n) solid solution, an analogue of the well-established α′-SiAlON system. We successfully synthesized single-phase powder samples by reduction–nitridation of Na2CO3–GeO2–Ga2O3, in the solubility range of m ≈ 0.8–1.7 with n ≈ 0.2–0.3. The Rietveld refinement of powder X-ray diffraction data for Na-α′-GeGaON was conducted on the basis of the space group P31c of α′-SiAlON, and the refinement converged with RB = 1.78% and RF = 1.02% for the composition of Na1.26(1)Ge10.50Ga1.50O0.24(1)N15.76(1), indicating reliably the isomorphism between the SiAlON and GeGaON systems. The results of 23Na solid-state nuclear magnetic resonance (NMR) spectroscopy clearly showed a single peak at the chemical shift of ∼16 ppm, further proving the accommodation of Na in the α′-GeGaON matrix with the expected coordination environment. The synthesized Na-α′-GeGaON exhibited stable photocatalytic activity for the evolution of H2 from water under ultraviolet irradiation, which was comparable to that attained by β-Ge3N4.

A new facile synthetic route to produce a nitride-based red-emitting phosphor has been established. The (Sr,Ca)2Si5N8:Eu2+-based multicomponent phosphor was successfully synthesized from the stable SrCO3–CaCO3–Eu2O3–Si3N4 system by simple one-step heating at 1600 °C for 4 h in an unpressurized N2 atmosphere. The synthesized (Sr,Ca)2Si5N8:Eu2+-based red broadband emitting phosphor exhibited the peak wavelength as long as 661 nm with a practically high external quantum efficiency of 60% under the excitation at 450 nm, while the coexisting secondary phase was inactive under the blue-light excitation, showing no detrimental effects on the photoluminescent properties. The enhanced red emission compared to the unmodified Sr2Si5N8:Eu2+ phosphor enables further improvement of the color rendering properties of the white light-emitting diodes for solid-state lighting applications.