Clive Johnson

Synthesis, structural, and magnetic characterisation of magnesium-doped lithium ferrite of composition Li0.5Fe2.5O4

Abstract Spinel-related magnesium-doped Li 0.5 Fe 2.5 O 4 has been synthesised by heating magnesium-substituted corundum-related α-Fe 2 O 3 with Li 2 CO 3 at 600°C which is ca. 600°C lower than the temperature normally used to prepare metal-doped Li 0.5 Fe 2.5 O 4 by conventional sintering methods. Rietveld structure refinement of the X-ray powder diffraction data shows that the Mg 2+ ions substitute for Fe 3+ ions on the tetrahedral sites whilst the Li + ions are located on the octahedral sites. The 57 Fe Mossbauer spectrum and magnetic measurements are consistent with this structural model.

Tin-, titanium-, and magnesium-doped α-Cr2O3: Characterisation and rationalisation of the structures

Tin- and titanium-doped α-Cr2O3 have been prepared by the calcination of precipitates. Rietveld structure refinement of the X-ray powder diffraction patterns shows that the dopant metals occupy interstitial sites and partially substitute on octahedral chromium sites in the corundum-related α-Cr2O3 structure. Interatomic potential calculations show that such defects are energetically more favourable than the exclusive substitution of chromium or the occupation by the dopant of interstitial sites in α-Cr2O3 with charge balance being achieved by an appropriate number of cation vacancies. However, the attempted incorporation of magnesium within α-Cr2O3 results in the formation of spinel-related MgCr2O4. The observation is rationalised in terms of the greater thermodynamic stability of the spinel-related structure.

The influence of ball milling and subsequent calcination on the formation of LiFeO2

The influence of ball milling and subsequent calcination of a 1:1 molar mixture of α-Fe2O3 and Li2CO3 on the formation of LiFeO2 has been investigated. Pre-milling was found to lower the temperature of ferrite formation by ca. 200°C and a thermally stable γ-LiFeO2 phase was found to form in the temperature range 500–600°C. Slow cooling of the pre-milled mixture calcined at higher temperatures resulted in the formation of some LiFe5O8.

A comparison of ab initio cluster and periodic calculations of the electric field gradient at sodium in NaNO2

Ab initio HF and DFT calculations, based upon both cluster and periodic modelling approaches, are reported for the efg tensor at sodium in NaNO2. Calculations based on different-sized clusters are compared and it is shown that resonable agreement with experiment can be obtained for a symmetrical cluster that extends beyond the immediate coordination environment of the sodium cation. The accuracy of calculations based on this cluster are, however, very dependent upon basis set. Hybrid DFT methods were not found to give significantly better results than HF methods. The periodic calculations are shown to give very good agreement with experiment for both a mid-sized, as well as a smaller standard 6-21G, basis set. In both cases the presence of d-polarisation functions on Na and O is essential to the accuracy of the calculations. HF methods were found to be superior to DFT methods in the approach used. It is suggested that periodic ab initio HF calculations using the standard 6-21G basis set, with suitable basis set optimisation to take into account the cationic nature of sodium, can provide a routine and consistent method for predicting sodium efg tensor information for ionic sodium compounds. Such a method would be important for assignment purposes in materials in which there are different crystallographic sites for the sodium cation.

An assignment of the 23Na MAS NMR spectrum of Na5P3O10·6H2O using a general ab initio method

A new assignment method based on the periodic ab initio calculation of 23Na quadrupole coupling information using the CRYSTAL95 code is described and applied to the multi-site problem posed by Na5P3O10·6H2O.

Structural and magnetic characterization of Ti-substituted Li0.5Fe2.5O4 prepared using hydrothermal and solid-state routes

Single-phased spinel-related Ti4+-substituted Li0.5Fe2.5O4 has been synthesized by heating a mixture of hydrothermally prepared Ti4+-substituted α-Fe2O3 and Li2CO3 at 850°C (12 h). This temperature is ca. 250-350°C lower than those at which the material is conventionally prepared by the ceramic techniques. X-ray diffraction was used to analyze the evolution of the formation process and, in conjunction with Mossbauer and magnetization measurements, to determine the cation distribution of the resulting ferrite. The results imply that the Ti4+ ions substitute for Fe3+ ones at the octahedral (B) sites whilst the excess Li+ ions, required for balancing the charge in the spinel-related structure, replace Fe3+ ions at the tetrahedral (A) sites.

Synthesis and Magnetic Properties of Magnesium-Doped Li0.5Fe2.5O4 Nanocrystalline Particles

The structure and magnetic properties of nanocrystalline Mg-doped-Li0.5Fe2.5O4 particles synthesised by the ball milling of polycrystalline sample prepared at 600°C have been examined. The magnetisation of the nanoparticles was found to decrease with decreasing crystallite size. The Curie temperatures were found to be lower than those of the corresponding bulk mareials. These properties are discussed in terms of surface magnetism and superparamagnetism.