Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB₄O₇

γ-NiB₄O₇ was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6₅22 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm³, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB₄O₇ was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB₄O₇.