Reevaluation of Raman spectra for KH2PO4 high-temperature phases

The common tetragonal form of KH2PO4 undergoes two high-temperature transformations that have been studied extensively by vibrational spectroscopy. But, because these changes are strikingly heterogeneous, the previously reported spectra, averages over polycrystalline and single crystal domains, have remained ill-defined. We recently identified the crystalline domains by polarization microscopy, physically isolated them, and determined their structures by X-ray diffraction (Subramony, J. A.; Lovell, S.; Kahr, B. Chem. Mater. 1998, 10, 2053-2057). Here, using micro-Raman spectroscopy, we have deconvoluted the signals of the polycrystalline and crystalline domains, showing that the inescapable pyro- and metaphosphate dehydration products are localized in the polycrystalline material. Those debating whether the transformations are genuine structural phase transformations or are merely dehydration reactions can all be vindicated in certain parts of any one sample.