Due to the high-temperature conditions (intense diffusion processes in ZnO start above 1000 K), in our experiments at 7.7 GPa and 2000 K, the as-formed rs-ZnO phase was usually microcrystalline, and hBN-to-cBN transformation was observed when BN capsules were used. However, in some cases, the formation of nanocrystalline rs-ZnO (50-200 nm according to TEM, ~60 nm according to powder Xray diffraction data) was observed in recovered samples during the quenching/decompression stage.