Multicomponent rare earth oxide (REO) nanocrystalline powders containing up to seven equiatomic rare earth elements were successfully synthesized in a single-phase CaF₂-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 (Ce⁴⁺), 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. GRAPHICAL ABSTRACT 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 (Fmm) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO₂ 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 Ce⁴⁺ 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.

High entropy oxides (HEOs) are single-phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show the reversible control of optical properties in a rare-earth (RE) based HEO-(Ce_0.2La_0.2Pr_0.2Sm_0.2Y_0.2)O_2−δ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic band structure underpinning the observed optical phenomena. Heat treatment of the HEO under a vacuum atmosphere followed by reheat treatment in air results in a reversible change in the bandgap energy, from 1.9 eV to 2.5 eV. The finding is consistent with the reversible changes in the oxidation state and related f-orbital occupancy of Pr. However, no pertinent changes in the phase composition or crystal structure are observed upon the vacuum heat treatment. Furthermore, annealing of this HEO under a H₂ atmosphere, followed by reheat treatment in air, results in even larger but still a reversible change in the bandgap energy from 1.9 eV to 3.2 eV. This is accompanied by a disorder–order type crystal structure transition and changes in the O 2p–RE 5d hybridization evidenced from x-ray absorption near-edge spectra (XANES). The O K and RE M_4,5/L₃ XANES indicate that the presence of Ce and Pr (in 3+/4+ states) leads to the formation of intermediate 4f energy levels between the O 2p and the RE 5d gap in HEO. It is concluded that heat treatment under reducing/oxidizing atmospheres affects these intermediate levels, thus offering the possibility to tune the bandgap energy in HEOs.

Multicomponent rutile (P4₂/mnm) structured fluorides, containing 4 to 7 transition metals (Co, Cu, Mg, Ni, Zn, Mn, and Fe) in equiatomic ratios, were synthesized using a simple mechanochemical approach. The high entropy fluorides were characterized using different techniques, all of which indicate that the high entropy fluorides tend to crystallize into a homogeneously mixed solid solution and single-phase structure. These high entropy fluorides represent an additional class of high entropy ceramics, which have recently attracted attention especially due to the development of high entropy oxides. With the introduction of these novel high entropy fluorides, similar interest could be generated due to the variety of different applications for fluoride materials and the improvements the high entropy concept might bring. Here we present an in-depth characterization study and the potential application of high entropy fluorides as a catalyst for the oxygen evolution reaction, in which the high entropy fluorides do show increased performance compared to a state-of-the-art catalyst for the oxygen evolution reaction, IrO₂, despite eliminating noble metal constituents.

A nanocrystalline high entropy oxide with near-equimolar composition consisting of 5 transition metal cations and one alkali cation (Co,Cu,Mg,Na,Ni,Zn)O was synthesised by a reverse co-precipitation (RCP) process and characterised by standard methods of X-ray diffraction (for crystallite size and phases), electron microscopy (for particle morphology and size distribution) and Fourier transform infra-red spectroscopy (for bond identification and bond lengths). Charge compensation in the lattice by the formation of Co³⁺ and/or Ni³⁺ (in order to offset the +1 oxidation state of Na) and the creation of oxygen vacancies was quantified from X-ray photoelectron spectroscopy and Raman spectroscopy and further studied using vibrating sample magnetometry (VSM). The influence of different transition metals in being able to accommodate the larger and aliovalent sodium ion in a single phase-pure rocksalt lattice was investigated and the criteria for element selection in such multicomponent systems for single-phase formation examined. Presence of multivalency/non-stoichiometry to accommodate a different-sized cation and maintaining electroneutrality were identified as the critical criteria for single-phase formation in multicomponent systems and further confirmed through synthesis of various lower combination systems (by systematic removal of one transition metal cation) and by addition of bivalent Ca as well as cations of higher valencies. These criteria would aid in designing the compositions of high entropy oxides with aliovalent substitutions.