Melt and glass structure in the Al₂O₃ -CaO-LaPO₄ system studied by ²⁷Al and ³¹P NMR, and by Raman scattering

NMR chemical shift and Raman scattering measurements have been performed on molten and drop-quenched samples of the La-monazite containing ternary system Al₂O₃-CaO-LaPO₄. In Al₂O₃-LaPO₄ melts without CaO modifier, the observed chemical shift of ²⁷Al increases linearly towards ∼72 ppm with increasing LaPO₄. For C₁₂ A₇ (Ca₁₂Al₁₄O₃₃) melts, increasing LaPO₄ leads initially to little change in the strongly tetrahedral ²⁷Al chemical shift (∼85 ppm), but between 25 and 50% LaPO₄ the shift sharply decreases, indicating strong Al bonding changes. Liquid CA:xLaPO₄ samples exhibit only moderate shift variations with LaPO₄ content. These dependences of ²⁷Al chemical shifts upon melt composition are reflected in substantial observed differences between molten and quenched samples, and may be related to the strong variations seen in physical properties of higher phosphate glasses. Quenched samples along the Al₂O₃-LaPO₄ join fractionate into distinct phases, but CaO content acts to suppress phase separation and often results in glass formation. Raman, ²⁷Al and ³¹P NMR spectra of glassy samples indicate a structure based upon orthophosphate groups, each associated with one or more Al atoms in P-O-Al linkages. At 75% monazite content X-ray spectra of quenched samples show the presence of the crystalline double phosphate LaCa₃(PO₄)₃.