P-O and Al-O Bonding in Alumina-Calcia-Monazite Melts Studied by Raman Scattering and Ultra High-temperature NMR*

Raman scattering and NMR of $^{27}$Al have been used to investigate the structure of molten samples of ceramics in the Al$_{2}$O$_{3}$-CaO-LaPO$_{4}$ system. Raman spectra of quenched samples indicate the presence of PO$_{4}$ structures similar to those of metaphosphate glasses, involving Q$_{2}$ tetrahedral$_{ }$units. NMR of $^{27}$Al in melts shows strong 4-fold coordination, but also 5- and 6-fold Al-O bonding and diffusivities far more rapid than those expected for networked AlO$_{4}$ tetrahedra. Evidence that Al cross links tetrahedral chains, in addition to forming these tetrahedra, will be discussed.