Physical properties and microstructures of La $$_{1-{x}}$$ 1 - x Pr $$_{{x}}$$ x PO $$_4$$ 4 monazite-ceramics

Abstract

Synthetic La $$_{1-{x}}$$ Pr $$_{{x}}$$ PO $$_4$$ monazite-type powders and ceramics with 0 $$\le$$ x $$\le$$ 1 were analysed by scanning electron microscopy, high-temperature powder X-ray diffraction, dilatometry, and plane wave ultrasound spectroscopy. Ceramics were synthesised in a two-step sintering process at 1273 and 1573 K. Final densities were up to 99.3% of the theoretical densities. Each sample shows a homogeneous distribution of grain sizes, which increase with increasing sintering temperature. Grain sizes also depend on composition, with intermediate compositions yielding the largest grains. In-situ high-temperature powder X-ray diffraction shows that the volumetric thermal expansion coefficients of the monazite powders decrease with increasing Pr content. This behavior is not observed in dilatometry measurements of the bulk samples (ceramics) because their thermal expansion mainly depends on their density. Elastic properties show the same dependence on the density.