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Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF 2-type (Fm-3 m) structure. The addition of more than six elements resulted in the formation of a secondary phase. Annealing at 1000°C for 1 h led to the formation of a single-phase (Ia-3) even in the 7-component system. In the absence of cerium (Ce 4+), secondary phases were observed irrespective of the number of cations or the extent of thermal treatment indicating that cerium cations played a crucial role in stabilizing the multicomponent REOs into a phase pure structure. IMPACT STATEMENT Multicomponent equiatomic rare earth oxides pioneer a new group of materials that crystallize into a single-phase structure with the dominant role of a single element instead of entropy.

New multicomponent equiatomic rare earth oxides (ME-REOs) containing 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiatomic proportions are synthesized using nebulized spray pyrolysis. All the systems crystallized as a phase pure fluorite type (Fm3[combining macron]m) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO is observed in all ME-REOs. X-ray photoelectron spectroscopy performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidation state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidation state, respectively. The presence of Ce contributes substantially to the observed stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concentration rapidly increases and then saturates with the number of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the composition or thermal treatments. Considering the high level of oxygen vacancies present and the observed low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.

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The detailed characterization of garnet-type Li-ion conducting Li 7 La 3 Zr 2 O 12 (LLZO) solid electrolyte thin films grown by novel CO 2 -laser assisted chemical vapor deposition (LA-CVD) is reported. A deposition process parameter study reveals that an optimal combination of deposition temperature and oxygen partial pressure is essential to obtain high quality tetragonal LLZO thin films. The polycrystalline tetragonal LLZO films grown on platinum have a dense and homogeneous microstructure and are free of cracks. A total lithium ion conductivity of 4.2 · 10 −6 S · cm −1 at room temperature, with an activation energy of 0.50 eV, is achieved. This is the highest total lithium ion conductivity value reported for tetragonal LLZO thin films so far, being about one order of magnitude higher than previously reported values for tetragonal LLZO thin films prepared by sputtering and pulsed laser deposition. The results of this study suggest that the tetragonal LLZO thin films grown by LA-CVD are applicable for the use in all-solid-state thin film lithium ion batteries. Over the last decades a progressive miniaturization of electronic components took place. As a result there is an increasing demand for micro-sized power sources that drives the current research on thin film batteries. Possible applications range from sensors and radiofrequency identification tags to implantable medical devices. For these applications an all-solid-state thin film Li-ion battery is desirable due to considerable advantages such as excellent safety properties and easy integration in microelectronics. Furthermore, the energy density of such an all-solid-state cell can be increased compared to a cell with conventional liquid electrolyte, if the chosen solid electrolyte material is stable toward high voltage cathode materials as well as toward a lithium metal anode. The garnet-type solid electrolyte Li 7 La 3 Zr 2 O 12 (LLZO), first reported in 2007 by Murugan et al., 1 is a material that combines those properties with reasonably high lithium ion conductivity of up to 2.3 · 10 −5 S · cm −1 in its tetragonal 2 and up to 1.2 · 10 −3S · cm −1 in its cubic 3 modification. However, these high conductivity values have so far only been achieved for bulk ceramics, but not for thin films. Apparently, over the last years researchers have focused on the optimization of the lithium ion conductivity in bulk ceramics, e.g. by stabilization of the cubic LLZO phase via doping, 4,5 while only a few publications have addressed LLZO thin film deposition at all. 6-12The advantage of thin films is that their lithium ion conductivity for the use in an all-solid-state battery does not need to be as high as compared to bulk solid electrolytes, taking into account the electrolyte resistance R:where A is the contact area, l is the thickness and σ is the Li-ion conductivity of the solid electrolyte, respectively. For example, in their recent study Liu et al. 13 report on a functioning bulk all-solid-state battery using cubic LLZO as solid electrolyte with ∼ 1 mm thickn...

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