Oxides can experience structural transformations resulting from variations in cation oxidation states or coordination geometry upon thermal treatment. Whether such structural distortions can affect the stability of high‐entropy oxides has not been studied. In this research, a new, high‐entropy, lanthanide sesquioxide, Gd0.4Tb0.4Dy0.4Ho0.4Er0.4O3 solid solution having a single phase, cubic‐bixbyite structure was synthesized, with no phase transformation from room temperature to 1650°C. The phase stability was examined via both in situ and ex situ, high‐temperature, synchrotron, X‐ray powder diffraction. This high‐entropy oxide could inhibit the phase transformations occurring in constituent monocation sesquioxides, Tb2O3 and Gd2O3, via random arrangement of multications.

The previously unknown experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been elucidated up to 3000°C using a quadrupole lamp furnace and conical nozzle levitator system equipped with a CO 2 laser, in conjunction with synchrotron X-ray diffraction. These in-situ techniques allowed the determination of the following: (a) liquidus, solidus, and invariant transformation temperatures as a function of composition from thermal arrest experiments, (b) determination of equilibrium phases through testing of reversibility via in-situ X-ray diffraction, and (c) molar volume measurements as a function of temperature for equilibrium phases. From these, an experimental HfO 2 -Ta 2 O 5 -temperature phase diagram has been constructed which is consistent with the Gibbs Phase Rule.

In situ X-ray diffraction measurements at the Advanced Photon Source show that α-Al 2 O 3 and MgAl 2 O 4 react nearly instantaneously and completely, and nearly completely to form single-phase high-alumina spinel during voltage-to-current type of flash sintering experiments. The initial sample was constituted from powders of α-Al 2 O 3 , MgAl 2 O 4 spinel, and cubic 8 mol% Y 2 O 3 -stabilized ZrO 2 (8YSZ) mixed in equal volume fractions, the spinel to alumina molar ratio being 1:1.5. Specimen temperature was measured by thermal expansion of the platinum standard. These measurements correlated well with a black-body radiation model, using appropriate values for the emissivity of the constituents. Temperatures of 1600-1736°C were reached during the flash, which promoted the formation of alumina-rich spinel. In a second set of experiments, the flash was induced in a current-rate method where the current flowing through the specimen is controlled and increased at a constant rate. In these experiments, we observed the formation of two different compositions of spinel, MgO•3Al 2 O 3 and MgO•1.5Al 2 O 3 , which evolved into a single composition of MgO•2.5Al 2 O 3 as the current continued to increase. In summary, flash sintering is an expedient way to create single-phase, alumina-rich spinel. K E Y W O R D Salumina, composites, field assisted sintering technology, spinels, zirconia: yttria stabilized

We present a new analytical platform that uses a tilted and Peltier cooling stage interfaced with an environmental scanning electron microscope to directly observe and assess the phase state of individual particles as a function of relative humidity.