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 <i>P</i>6₅22 (No. 179), <i>a</i> = 425.6(2), <i>c</i> = 3490.5(2) pm, <i>V</i> = 0.5475(2) nm³, <i>Z</i> = 6, and Flack parameter <i>x</i> = −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-NiB₄O<sub>7