Synthesis, Stability, and Crystal Structure of CaTiO-Type ZnCrO: High-Pressure Crystal Chemistry of Postspinel Phases.

High-pressure phases of spinel-type O, called postspinel phases, have been of great interest in materials science for the appearance of novel physical properties and geoscience for understanding the Earth's deep interior. The CaFeO- and CaTiO-structured phases are representative postspinel phases. To design a postspinel phase, it is useful to construct a prediction diagram using crystallographic parameters such as ionic radius. Especially, the appearance of the CaTiO-type phase is rare in high-pressure-temperature space, and therefore, its further exploration is desired to develop its material group. Our previous study constructed a structural prediction diagram of O postspinel phases based on ionic radii. Based on the prediction diagram, we here investigated the system ZnCrO at high-pressure-temperature conditions to explore the stability field of the CaTiO-type phase. We found that the spinel-type phase transformed to the CaTiO-structured phase at 23-24 GPa and 1000-1400 °C. Rietveld structure analysis of the recovered high-pressure phase yielded lattice parameters for the CaTiO-type phase of = 2.8634(1) Å, = 9.4990(4) Å, = 9.7568(3) Å, and = 4 (space group: ). Thus, our prediction diagram with ionic radii successfully predicted the structure types of an O postspinel phase and is therefore useful to design a novel postspinel phase.