A linear empirical correlation was established between Raman stretching wavenumbers of phosphorus-oxygen bonds and their bond lengths in inorganic crystalline phosphates. Although established on samples of inorganic crystalline phosphates, the correlation can be applied to glassy and amorphous phosphate materials (GAMs). Their unpolarized vibrational spectra are often similar because they are determined largely by short-range order. The correlation was used to predict P -O bond length in the a-form of Li 3 PO 4 , which is stable over only a small range of temperatures below the melting-point. It can also be used to estimate the length of P -O single bonds, terminal P -O and O -P -O chain bonds and terminal double bonds in many technologically important amorphous materials containing phosphate groups. This correlation is expected to offer invaluable insight into the structures of phosphate species for which diffraction or other spectroscopic techniques provide incomplete structural information. That would enhance the value of Raman spectroscopy as a complementary technique in structural studies of phosphates.

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Although three forms of lithium phosphate are known, a complete structural description of the highest temperature polymorph has not yet been given. In the present work, the phase transitions of lithium phosphate were investigated at high temperatures using Raman microscopy and x-ray powder diffraction. Both transitions were observed by following the temperature dependence of the totally symmetrical Raman stretching vibration of PO 4 3− . Currently available structural information on the a form, resulting in P -O bond lengths of 1.787-1.899Å, as determined by valence bond calculations, are disputed here. A correlation between Raman wavenumber and bond length in inorganic phosphates estimates the P -O bond length in a-Li 3 PO 4 to be around 1.57(1)Å, which is in closer agreement with values for other orthophosphates of between 1.50 and 1.58Å.

Ba(3−x)Srx(PO4)2 orthophosphates (0≤x≤3) have been prepared by solid state reaction. The final temperature was 1000°C. The X-ray diffraction analyses show the existence of a continuous solid solution. Ba(3−x)Srx(PO4)2 orthophosphates (0≤x≤3) crystallize in the hexagonal system with the space group R3m. Their structure is based on a three-dimensional framework constructed of infinite layers of Ba1/Sr1O12 linked and parallel to infinite layers of Ba2/Sr2O10 polyhedra and PO4 tetrahedra.

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