Nanocrystalline ceramics have great potential for applications in electronics, sensors and energy-related areas due to their remarkable functional properties. Conventionally, in the design of materials for targeted applications, the composition at the apex of a multinary phase diagram is taken as a basis as the properties at the apical composition are, by and large, well established. Modification of the properties and rendering them suitable for the targeted application is then achieved by minor additions/doping with other phases/components. Hence, the centre of the phase diagram, where there are two or more principal components of (near-)equimolar proportions, re-
mains as an unexplored field in this method of design. To explore the properties at the (near-)equimolar regime, multicomponent alloys were developed in metallic systems, with five or more elements substituted in a single Wycoff position at (near-)equimolar proportions. The presence of a multiple number of elements leads to stabilization of a single phase through an increase in the configurational entropy which is the highest when the elements are present in equimolar proportions. Such materials are called high entropy materials (HEMs). Similarly, in the class of ceramics also, only doped, co-doped and binary ceramics have been extensively studied, while the realm of equimolar or near-equimolar, multicomponent ceramics has been largely an unexplored field.
High entropy ceramics have been reported relatively recently in the form of oxides, nitrides, borides and carbides. The first entropy stabilised transition metal based high entropy oxide (HEO) was synthesized by Rost et al. (2015) wherein 5 transition metal cations were used to get a phase-pure rocksalt (Co,Cu,Mg,Ni,Zn)O. Although selection of the cations was based on Hume-Rothery and Pauling’s rules, the applicability of these criteria for single phase formation, effect of aliovalent additions, stability in the nanocrystalline form etc. have been, by and large, not rigorously explored.
In the present work, the transition metal based high entropy oxide (TM-HEO), (Co,Cu,Mg,Ni,Zn)O was synthesised from aqueous inorganic nitrate precursors with the primary intention of studying and analyzing the phase formation, structure and optical band gap. Three bottom up approaches having different residence times were used: flame spray pyrolysis (FSP), nebulized spray pyrolysis (NSP) and reverse coprecipitation (RCP). Among these processes, FSP has the least residence time resulting in ultra-fine nanocrystalline sizes as well as giving rise to the possibility of the formation of non-equilibrium or metastable phases while RCP has the highest residence time resulting in equilibrium product(s). To understand effect of aliovalent cations in the TM-HEO, monovalent, trivalent and quadrivalent cations were, in turn, systematically added to the lattice.
Nanocrystalline phase-pure (Co,Cu,Mg,Ni,Zn)O powders were obtained by all the three processes (FSP, NSP and RCP) as confirmed from X-ray diffraction (XRD). How-ever, the XRD peaks in all three cases showed some asymmetry and minor distortion from the rocksalt structure with the most significant deviation in the FSP powders and least in the RCP powders. The presence of distortion in the rocksalt crystal structure, also confirmed from Raman spectroscopy and correlated with magnetic measurements from SQUID and EPR studies, could be attributed to the additive effects of exchange striction (from the magnetic constituents) and magnetic anisotropy (from the decreased crystallite size), in addition to the Jahn-Teller effect. Addition of some Fe to the TM- HEO revealed that a higher amount of magnetic constituent increased the distortion in the lattice. The present study showed that the presence of asymmetry due to distortion in the crystal structure arising from magnetic effects is possible in a lattice with extreme chemical disorder. Optical band gap of the TM-HEO was studied with a specific view to understand the influence of each cation by synthesizing four component systems, eliminating one of Co, Zn and Cu, in turn, from the (Co,Cu,Mg,Ni,Zn)O system. A further understanding in the variation of the band gap with defect states and crystallite sizes was done by comparing the TM-HEO powders synthesised using the three processes. It was seen that copper played a major role in influencing the band gap primarily because of an increase in the covalent nature of the bond and formation of multiple oxidation states, confirmed through XPS. A comparison of the synthesis processes revealed that the band gap energy also depended on the off-stoichiometry of the product.Effect of phase formation in the TM-HEO with the addition of an aliovalent cation was studied. Incorporation of an equimolar content of sodium (Na+1) in the TM-HEO could be achieved with the product being phase-pure rocksalt with the phase formation being characterized by XRD, particle morphology and size distribution by electron mi-croscopy, and bond identification and bond lengths by FTIR spectroscopy. 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. This was further confirmed through the synthesis of various lower combination systems (by the systematic removal of one transition metal cation) and also by addition of bivalent Ca as well as cations of higher valencies (+3 oxidation state cations - Cr, Mn and Fe) at equimolar proportions. These criteria would aid in designing the compositions of high entropy oxides with aliovalent substitutions.
Addition of a +4 oxidation state cation Ti to the rocksalt (Co,Cu,Mg,Ni,Zn)O system induced spinel phase formation and when the Ti content was as per the stoichiometry of (Co,Cu,Mg,Ni,Zn)2TiO4, pure orthotitanate with single-phase spinel structure formed. The structure was found to be of an inverse spinel type from XRD analysis and further confirmed from the presence of the A1g sub-band in the Raman spectrum. The degree of inversion and stoichiometry was determined from XPS. It was deduced that cations occupying the tetrahedral and octahedral sites needed to be in the ratio of 1:2 for the formation of a phase-pure spinel structure. Any deviation in this ratio due to the removal of cations that occupy the tetrahedral sites (Zn2+, Co2+) resulted in the formation of secondary phases as confirmed from the XRD patterns. Band gap of (Co,Cu,Mg,Ni,Zn)2TiO4 was found to be much lower than that of any of the individual (unary) oxides or titanates.