Melt and glass structure in the Al 2O 3–CaO–LaPO 4 system studied by 27Al and 31P NMR, and by Raman scattering

Abstract

NMR chemical shift and Raman scattering measurements have been performed on molten and drop-quenched samples of the La-monazite containing ternary system Al 2O 3–CaO–LaPO 4. In Al 2O 3–LaPO 4 melts without CaO modifier, the observed chemical shift of 27Al increases linearly towards ∼72 ppm with increasing LaPO 4. For C 12A 7 (Ca 12Al 14O 33) melts, increasing LaPO 4 leads initially to little change in the strongly tetrahedral 27Al chemical shift (∼85 ppm), but between 25 and 50% LaPO 4 the shift sharply decreases, indicating strong Al bonding changes. Liquid CA: xLaPO 4 samples exhibit only moderate shift variations with LaPO 4 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 Al 2O 3–LaPO 4 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 LaCa 3(PO 4) 3.