Commercial lithium-ion battery cells were cycled to various depths of discharge at various rates while the relative capacities were periodically measured. After 1000 cycles, lithium cobalt oxide (LiCoO 2) cathode material was extracted from the most severely aged cell. Nanoindentation was performed on individual LiCoO 2 particles. Fractures in these particles exhibited anisotropic behavior, which was confirmed by electron microscopy and diffraction examination indicating both intra-and inter-granular fracture occurred along {001} planes. Computation of the charge density structure for LiCoO 2 indicated that the Li-O bonds along the {001} planes require the lowest energy for cleavage, supporting the experimental findings. Atom probe tomography (APT) analysis indicated the nanoscale composition distributions within specimens from both fresh and cycled material. Among the cycled particles, nanoscale inhomogeneities in the Li content were observed. For APT specimens containing grain boundaries, accumulation of Li (up to 80 at%) on one side of the boundary was observed. Correlation of the electrochemical, mechanical, and compositional results indicates a combination of these mechanical and chemical mechanisms contributed to the measured capacity fade.

Xenotime DyPO4 and GdxDy(1−x)PO4 (x = 0.4, 0.5, 0.6) (tetragonal I41amd zircon structure) have been studied at ambient temperature under high pressures inside a diamond anvil cell with in situ Raman spectroscopy. The typical Raman‐active modes of the xenotime structure were observed at low pressures and the appearance of new Raman peaks at higher pressures indicated a phase transformation to a lower symmetry structure—likely monoclinic. Raman mode softening was observed, resulting in a line crossing at approximately 7‐8 GPa for each material and preceding the phase transformation. The onset of phase transformation for DyPO4 occurred at a pressure of 15.3 GPa. DyPO4 underwent a reversible phase transformation and returned to the xenotime phase after decompression. The transformation pressures of the solid solutions (GdxDy(1−x)PO4) were in the range 10‐12 GPa. The GdxDy(1−x)PO4 solid solutions yielded partially reversible phase transformations, retaining some of the high‐pressure phase spectrum while reforming xenotime peaks during decompression. The substitution of Gd into DyPO4 decreased the transformation pressure relative to pure DyPO4. The ability to modify the phase transformation pressures of xenotime rare‐earth orthophosphates by chemical variations of solid solutions may provide additional methods to improve the performance of ceramic matrix composites.

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Low elastic modulus and hardness, as well as anomalous indentation behavior, have been observed during indentation of xenotime rare-earth orthophosphate ceramics (REPO 4 s) with compositions near the monazite/xenotime phase boundary. Pressureinduced phase transformation has been identified as a potential cause for both observations. This study comprehensively characterizes the mechanical properties and indentation behavior of four elemental REPO 4 materials (EuPO 4 , GdPO 4 , TbPO 4 , and DyPO 4) that span the monazite/xenotime phase boundary using ex situ nanoindentation for a range of loading rates and indentation depths. In situ nanoindentation within a SEM was used to correlate discrete load-depth behavior to the development of surface features. Anomalous, elbow-type behavior was not restricted to xenotimes, but occurred in all four materials; thus we concluded that the presence of an elbow in the indentation data was not a unique identifier of phase transformation in rare-earth orthophosphates. Furthermore, it was shown that the elastic modulus of each of these compositions

A series of nanoindentation tests on both single and polycrystalline specimens of a monazite rare-earth orthophosphate, GdPO4, revealed frequent observation of anomalous unloading behavior with a large degree of recovery, where previously this behavior had only been observed in xenotime-structure rare-earth orthophosphates. An indentation site in the polycrystalline sample was examined using TEM to identify the deformation mechanism responsible for recovery. The presence of a twin along the (100) orientation, along with a series of stacking faults contained within the deformation site, provide evidence that the mechanism of recovery in GdPO4 is the collapse of deformation twins during unloading.