An X-ray diffraction investigation of chromium-substituted lithium ferrite

Abstract

The loss of order in the lithium spinel system Fe(Li0.sFel.5_~/sCrx/8)O 4 with 0 < x < 2 is examined as a function of the Cr 3÷ ion concentration through an analysis of the X-ray superlattice line integrated intensities and line breadths. In these materials it is found that the growth of the antiphase domains is inhibited by the action of Cr 3÷ ions at the domain boundaries and it is shown that the observed variation of the long-range order parameter may be explained on the basis of a cation distribution on the octahedral sites which adheres to the so-called tetrahedral invariance condition. Furthermore, a correlation exists between the probability of an unstable Cr 3÷ ion configuration and the formation of domain boundaries. Some discussion is presented on the variation of the domain thickness distribution with Cr 3÷ ion content. IntroductionLithium spinels of the type M3+(Li0.sMla.+)O4, M corresponding to Fe, AI or Ga, possess an ordered structure on the octahedrally coordinated sites which is described by the cubic space group P4332 or its enantiomorph P4t32 (Braun, 1952). The stability of the ordered arrangement arises principally because each Li + ion has only M 3+ ions as nearest-neighbour octahedral ions. This configuration is referred to as tetrahedral charge invariance (Anderson, 1956) as it is equivalent to each tetrahedral group of octahedral ions comprising one Li ÷ ion and three M 3÷ ions. For lithium ferrite this form of short-range order is rigidly adhered to in the disordered structure above 1008 K (Brunel & de Bergevin, 1966) and in the partially ordered structure immediately below this temperature (Cheary & Grimes, 1978).When transition-metal ions with a large octahedral site stabilisation energy are substituted on the octahedral sites, the structure gradually reverts to the statistically face-centred form (space group Fd3m) 0567-7394/79/040665-08501.00 normally associated with spinel compounds (Gorter, 1954;Blasse, 1964;Rogers, Germann & Arnott, 1965). This is indicated by the disappearance of the superlattice lines in the X-ray diffraction pattern. For Cr 3+ or Rh 3+ ions the critical concentration is approximately four substituted ions per unit cell, but with Mn 3+ ions it is only two ions per unit cell. In these materials the site stabilization energy of the ions hinders the ordering process to the extent that at the critical concentration it is the dominant factor in determining the distribution of the transition-metal ions. On the basis of site stabilization energy alone the Mn 3÷ ion should be least effective in removing the order; however, being a Jahn-Teller ion it also creates localized distortions in the structure (Rogers, Germann & Arnott, 1965). The latter are made manifest through anomalies in the compositional dependence of the lattice parameter which arise, it is believed, from the formation of Mn 3÷ ion clusters. It is worth remarking that the squareness of the magnetic hysteresis loop in the system Fe(Lio. 5 Fe~. 5 _ x/sMnx:8)O4 is optimized for substitutions x between...