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Abstract: La(PO3)3–Ca(PO3)2 meta-phosphate glass–ceramic composites were prepared by quenching from the melt, followed by heating at 800°C for 20h, in air. Microstructural characterization showed that exposure to water vapor pressures equal to or in excess of 4.2kPa at as low as 550°C resulted in the disproportionation of the lanthanum metaphosphates, with formation of some LaPO4. This reaction puts important limitations on the conditions under which the lanthanum metaphosphates could be used as electrolytes in electrochemical devices. The conductivities at uniform air/water partial pressures around 3kPa, were independent of the water partial pressures used. Comparison of the total ionic effective transference numbers and the proton effective transference number suggested that electronic transport contributed on the average about 7% to the total conductivity in the examined temperature range. The measurements suggested that oxygen ions also participate. The H/D isotope effect, assessed from the ratio σ H 2 O /σ D 2 O ranged from 1.28 at 400°C to 1.35 at 500°C, evidencing dominant proton conduction. At 550°C, the proton effective transference numbers, t H + , determined from the OCVs of cells with oxidizing differential atmospheres were around 0.87, in good agreement with the value derived from the σ H 2 O /σ D 2 O conductivity ratio at 550°C. However, at lower temperatures such agreement was not found. [Copyright &y& Elsevier]

Abstract: We report a TEM microstructural and compositional study of proton-conducting (La,Ca)P3O9 (Al-free) and (La,Ca,Al)P3O9 (Al-containing) glass-ceramics, prepared by the crystallization of quenched glass melts. Bright-field Transmission Electron Microscopy imaging, annular dark-field Scanning Transmission Electron Microscopy, and energy dispersive x-ray compositional analysis were used to characterize the structure and chemistry of the glass and crystalline phases of the samples that developed following the treatments at various temperatures, heating-rate and dwell-time. TEM chemical characterization revealed that the crystallized domains were mainly Ca-doped LaP3O9, while the remaining phase was a Ca-rich phosphate glass. Crystallization improved conductivity first (up to a conductivity of nearly 10−4.5 S/cm at 550°C in air). However, the conductivity dropped when the samples were treated at higher temperature or for longer dwell-times, due to a continued Ca-departure from the La-rich crystalline into glass phase. Possible proton transfer mechanisms and strategy to improve conductivity in this doped rare earth metaphosphate glass-ceramic are discussed. [Copyright &y& Elsevier]

Abstract: Al2O3 was added to a 2CaO–La2O3–5P2O5 metaphosphate, to replace 10% of the Ca2+ ions by Al3+, forming a phosphate with the nominal composition 1.8CaO–0.1Al2O3–La2O3–5P2O5. The effect of Al2O3 addition and heat treatment on the microstructure and conductivity of the resulting glass–ceramics was investigated by XRD, SEM, TEM, and AC impedance spectroscopy. Upon transformation from glass to glass–ceramic, conductivities increased significantly. The glasses were isochronally transformed at 700 and at 800 °C for 1 h or 5 h, in air, following heating at 3 or 10 °C/min. With Al2O3 addition, after a heat treatment at 700 °C, 100–300 nm nano-domains of LaP3O9 crystallized from the glass matrix. Annealing at 800 °C produced a further order of magnitude conductivity increase for the Al-free glass, but less so for the Al-containing glass. [Copyright &y& Elsevier]

Abstract: A dual phase glass–ceramic composite is produced by heating a CaLa(PO3)5 glass at 800 °C for 20 h. The glass–ceramic consists of intertwined crystalline La(PO3)3 and amorphous Ca(PO3)2 with an overall conductivity of 1.52×10−5 S/cm−1 at 550 °C in humidified air. For the water vapor treated glass, the vibration modes of the incorporated water, as well as of P–OH groups, are detected by infrared spectroscopy in the range 3440–1660 cm−1. The conductivity of the glass–ceramic is higher in humidified air than in dry air, consistent with proton conduction. [Copyright &y& Elsevier]