Role of size, alio-/multi-valency and non-stoichiometry in the synthesis of phase-pure high entropy oxide (Co,Cu,Mg,Na,Ni,Zn)O†
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.