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 C12A7 (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.