Structure and Thermal Expansion of YSZ and La₂Zr₂O₇ above 1500°C from Neutron Diffraction on Levitated Samples

Higherature time-of-flight neutron diffraction experiments were performed on cubic yttria-stabilized zirconia (YSZ, 10 mol% YO_1.5) and lanthanum zirconate (LZ) prepared by laser melting. Three spheroids of each composition were aerodynamically levitated and rotated in argon flow and heated with a CO₂ laser. Unit cell, positional and atomic displacement parameters were obtained by Rietveld analysis. Below ~1650°C the mean thermal expansion coefficient (TEC) for YSZ is higher than for LZ (13 ± 1 vs. 10.3 ± 0.6) × 10^-6/K. From ~1650°C to the onset of melting of LZ at ~2250°C, TEC for YSZ and LZ are similar and within (7 ± 2) × 10^-6/K. LZ retains the pyrochlore structure up to the melting temperature with Zr coordination becoming closer to perfectly octahedral. Congruently melting LZ is La deficient. The occurrence of thermal disordering of oxygen sublattice (Bredig transition) in defect fluorite structure was deduced from the rise in YSZ TEC to ~25 × 10^-6/K at 2350°C-2550°C with oxygen displacement parameters (U_iso) reaching 0.1 Ų, similar to behavior observed in UO₂. Acquisition of powder-like higherature neutron diffraction data from solid-levitated samples is feasible and possible improvements are outlined. This methodology should be applicable to a wide range of materials for higherature applications.