Conventional 14/8 high-pressure assemblies (Fig. ) were used. Microcrystalline powder of wurtzite ZnO (Alfa Aesar, 99.99%, -325 mesh) was placed in a capsule of pyrolytic hexagonal boron nitride hBN or MgO (sample volume ~ 3 mm 3 ), which served as a primary sample environment. The capsule was surrounded by a LaCrO 3 (or graphite) cylindrical resistive heater and placed in an octahedron of Cr 2 O 3 -doped MgO. Eight truncated tungsten carbide cubes (32 mm, 8-mm truncations, see Fig. ) served as the second stage anvils to compress the octahedron in three directions. Pressure and temperature were estimated using the "cell pressure vs. hydraulic oil pressure" and "cell temperature vs. supplied power" calibration curves, respectively. To construct the calibration curves, the direct p-T measurements were made using thermocouples (WRe5% -WRe26%, type C) and electrical resistivity changes in pressure standards (Bi, 2.55 and 7.7 GPa; PbSe, 4.2 GPa; PbTe, 5.2 GPa) at room temperature. In all cases, pressure and temperature uncertainties were estimated to be less than 0.5 GPa and 25 K, respectively. The high-pressure assembly was compressed to 7.7 GPa for 6 hours at room temperature, and then heated to 2000 K for 10 min. The sample was held at this temperature for one minute and quenched by turning off the power. Pressure was then slowly released for 15 hours and the sample was recovered.