Fig. 2 XRD spectra and molecular structures of tetragonal and monoclinic crystal phase KDP samples. XRD spectra and molecular structures of tetragonal and monoclinic crystal phase KDP samples. Red trace: XRD spectrum of powder of commonly used tetragonal phase (bulk) KDP crystal (inset: molecular structure and packing arrangement). Blue trace: XRD spectrum of a small sample of single-crystal monoclinic-phase KDP microstructures showing a characteristic XRD pattern substantially different from the usual tetragonal-phase KDP crystal (inset: molecular structure and packing arrangement). Yan Ren et al. Sci Adv 2016;2:e1600404 Copyright © 2016, The Authors