High-Pressure Metathesis of the M 1- x PO 3+4 x N 1-4 x ( x ≈ 0.05) and M 0.75 PO 4 ( M = Zr, Hf) Orthophosphates.

We describe the oxonitridophosphates M _{1- x} PO _{3+4 x} N _{1-4 x} ( x ≈ 0.05) and the isotypic oxophosphates M _0.75 PO ₄ ( M = Zr, Hf) obtained by high-pressure metathesis. The structures (ZrSiO ₄ -type, space group I4 ₁ / amd (no. 141), a = 6.5335(7)-6.6178(12), c = 5.7699(7)-5.8409(9) Å, Z = 4) were refined from single-crystal X-ray diffraction data, and the powder samples were examined with quantitative Rietveld refinement. Infrared spectroscopy did not indicate the presence of X-H ( X = O, N) bonds. The optical band gaps, between 3.5 and 4.3 eV, were estimated from UV-vis data using the Kubelka-Munk function under assumption of a direct band gap. Temperature-dependent powder X-ray diffraction showed a phase transformation of the M _0.75 PO ₄ ( M = Zr, Hf) compounds to ambient pressure polymorphs at 780 (Zr) and 900 °C (Hf). The preparation of the nitrogen containing compounds exemplifies the systematic access to the new class of group 4 nitridophosphates granted by high-pressure metathesis. Moreover, we show that high-pressure metathesis can also be used for the preparation of metastable oxophosphates.