The (Mg,Fe)SiO₃ crystallizes into a postperovskite (pPv) phase in polycrystalline form at high pressure-temperature conditions and becomes amorphous upon release of pressure. Structural refinement of the pPv phase at extreme conditions is challenging because of uncertainties in diffraction intensity caused by texturing of the sample and spotty diffraction patterns due to crystal growth under high temperature. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, recent experiments by Carnegie Scientists Li Zhang, Yue Meng (HPCAT), Wenge Yang (HPSync), and Ho-kwang Mao, along with CDAC Partner Wendy Mao (Stanford) and colleagues from the University of Chicago have obtained crystallographic orientations of over 100 crystallites at high pressure in a coarse-grained polycrystalline pPv sample. A few selected pPv crystallites have been selected and tracked using conventional single-crystal structural analysis and refinement methods. The established method opens a door for in situ studying of crystal structures of submicron crystallites in a multiphase polycrystalline sample in a diamond anvil cell. The results show clear similarity of structural models for single-crystal Fe-bearing pPv (∼10 mol% Fe) and Fe-free pPv from previous theoretical calculations, and suggest that the Fe content in the mantle has a negligible effect on the crystal structure of the pPv phase [L. Zhang, et al., Proc. Natl. Acad. Sci., 110, 6292-6295 (2012)].