Solid-state nuclear magnetic resonance study of the structure of lanthanum phosphate crystals and glasses
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Lanthanum phosphate glasses were synthesized by melt quenching and characterized by x-ray diffraction, electron microprobe analysis, and solid-state nuclear magnetic resonance spectroscopy. A range of compositions near the metaphosphate composition (75 mol% P2O5) was examined. Comparison of 31P chemical shifts and shielding anisotropies of glasses with those of the crystalline phases of La(PO3)3 (metaphosphate) and LaP5O14 (pentaphosphate) were consistent with the presence of primarily chainlike Q2 phosphate groups.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Phosphate glasses and ceramics have applications in glass–metal joining, laser technology, and biological implants. In general, phosphate glasses suffer from poor chemical durability and extreme hygroscopicity but, with additives, can exhibit high coefficients of thermal expansion, good optical properties, and reasonable chemical durability. An area of special current interest is the study and development of rare-earth-doped phosphate glasses and ceramics that exhibit unusual luminescence and magnetic behavior. For example, La2O3-doped phosphate glasses and glass fibers exhibit improved optical response with enhanced chemical durability and a higher elastic modulus.1 While some reports of structural investigations of rare-earth (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, and Er1–3) containing glasses have been reported, a comprehensive picture of the structure of rare-earth phosphate glasses remains incomplete at present. Lanthanum phosphate glasses serve as good structural models for other rare-earth phosphate glasses and are easily studied by 31P solid-state nuclear magnetic resonance (NMR) spectroscopy, whereas NMR is difficult or impossible to use for glasses containing large concentrations of paramagnetic ions. In favorable cases, 31P NMR allows the speciation of the phosphate network to be determined in terms of the number of bridging oxygens per P atom, denoted Qn where n is the number of bridging oxygens. In this work, we present results of a solidstate NMR study of lanthanum phosphate glasses [varying slightly in P content near the metaphosphate composition La(PO3)3] and the structural interpretations of the chemical shift parameters observed in the NMR spectra.
The La2O3–P2O5 phase diagram for temperatures between 650 and 1550 °C4 indicates a liquid region at high P2O5 content. Liquid formation in the region of 75 to 95 mol% P2O5, accompanied by the deep eutectic at approximately 80 mol% P2O5 suggests glasses can form upon rapid quenching. Lanthanum metaphosphate (75 mol% P2O5) glasses were prepared from powders of P2O5 and LaPO4, which were mixed and melted in a platinum crucible in air (Deltech Model DT-31 Mode C furnace, Denver, CO). The melt was held at 1400 °C for approximately 40 to 60 min and quenched between two brass plates. Due to the volatility of P2O5, an excess of P2O5 was added to each batch to compensate for losses during melting. Glass compositions were determined by electron microprobe analysis (EMA), and the analyzed P2O5 molar percenta
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