Lanthanum phosphate calcium aluminate glasses: 27Al and 31P NMR spectroscopy

A new protocol for synthesis of glassy aluminates with substantial content of the refractory compound LaPO4 has been developed, utilizing hydrated monazite and anhydrous metal oxides as starting materials. Ultra high-temperature NMR of samples containing up to 75 mol% LaPO4 shows that monazite is incorporated homogeneously into melts, while analysis by electron microprobe, X-ray diffraction, solid-state NMR and Raman scattering shows that water quench of samples with 50% La–monazite or less results primarily in glass formation. Network structures of the LaPO4-containing glasses remain aluminate-based, as indicated by our samples’ melting points (>1200 °C), well above those of phosphate glasses. Melting temperatures exhibit a minimum in the range 20–30% LaPO4 content. Raman spectra show only isolated phosphate groups, with no POP bonds (orthophosphates). Dissolution observations, electron microprobe composition measurements and a substantially shifted principal 31P NMR line show that La3+ lies within the second coordination shell around P in all of the glasses. P–Al TRAPDOR NMR studies and a second, more negatively shifted line in the 31P spectra indicate that P is also closely coordinated to one or two Al ions.