Extreme pressure-temperature conditions are a powerful tool for creating nanostructured materials. Such materials are not only interesting for the fundamental understanding of microscopic phenomena due to nanostructuring (mechanical and transport properties, etc.), but also have great potential for advanced applications as wide bandgap semiconductors. In traditional experimental approaches, the recovery of high-pressure phases (diamond, stishovite, rs-ZnO, etc.) is linked to the time-pressuretemperature (t-p-T) synthesis protocol (e.g. rate of decompression/cooling from p-T synthesis conditions, decompression temperature, etc.) and the initial state of the precursors (e.g. grain size, impurities). For example, nanocrystalline stishovite (high-pressure polymorph of SiO 2 ) and rs-ZnO can be quenched down to ambient conditions starting from periodic mesoporous silica and nanopowders of wurtzite ZnO, respectively. The nanostructured rs-ZnO can be synthesized by using w-ZnO nanoparticles prepared either by solution chemistry or by deep grinding in a ball mill. Although this approach is quite successful, it has a number of limitations. The most crucial limitation is the purity of the starting nanopowders, which have high specific surface area and contain a large number of chemically bonded surface groups with possible stabilization effects (solution method) or mechanically introduced impurities (ball mill material in the case of deep grinding). This problem has been solved in the case of nanocrystalline diamond, boron nitride, and stishovite, i.e., in the cases where the phases can be easily recovered at ambient conditions, one only need to optimize the t-p-T conditions of direct solid-state phase transformations in order to ensure nanostructuring of a high-pressure phase. However, in the case of zinc oxide, this approach has not worked so far, and synthesis of pure rs-ZnO remains a major challenge, as does understanding of the properties of rs-ZnO-based materials (nanocomposites, solid solutions with transition metals, etc.). Zinc oxide is one of the most studied wide bandgap ionic semiconductors. At ambient conditions, ZnO has a wurtzite structure (P6 3 mc) that transforms into rock-salt one (Fm3m) at pressures above 5 GPa (the transition pressure p tr is strongly dependent on temperature, grain size, etc., while equilibrium transition pressure p eq is only weakly dependent on T). Upon pressure release, assynthesized rs-ZnO usually reverts back to wurtzite phase, with significant hysteresis at temperatures below 1000 K. Occasionally, rs-ZnO can be recovered in the nanocrystalline state either in a mixture with w-ZnO or even in a pure form -in some special cases. Recently, the recovery of the rs-ZnO has been intensively studied. It was found that macroscopic (> 100 mm 3 ) nanocrystalline (grain size of 28-80 nm) samples of single-phase rs-ZnO can be recovered at ambient conditions from 7.7 GPa and 770-820 K when w-ZnO nanopowders with average grain size below 45 nm and narrow particle size distribution are used as precursors. The recovery of rs-ZnO synthesized from ballmilled w-ZnO (grain size ~ 12 nm) at 15 GPa and 550 K in a diamond anvil cell (DAC) has also been reported. However, the attempts to release pressure down to 0.1 GPa in a DAC indicated that rs-ZnO could be retained as a single phase for only ~10 min. It was assumed that only well-crystallized rs-ZnO could be recovered, however, this hypothesis was later not confirmed. Besides the nano-size of a precursor, the presence of isostructural compounds such as MgO and NaCl 17 may also favor the recovery of synthesized rs-ZnO. At ambient pressure, thus recovered rs-ZnO/NaCl composites containing ~50 nm rs-ZnO particles are stable up to 360 K. All this indicates a strong and unpredictable impact of kinetic factors related to the reversible transition of w-ZnO to rs-ZnO, which have been neglected so far in the analysis of the high-pressure phase recovery.