Influence of annealing on X-ray diffraction of natural alexandrite
In this work we present results of X-ray diffraction using powder method, on natural alexandrite samples from Minas Gerais State (Brazil), as a function of a sequence of annealing. From these measurements we determine lattice parameters before (a=9.405 Å, b=5.471 Å, c=4.409 Å) and after annealing, and its structure is confirmed as orthorhombic. Measurements done after an annealing of 15 minutes at 700 °C and for 5 hours at 1000 °C indicate the migration of atoms present in the sample through different phases, which were also identified by Microprobe Analysis (WDS). However we have verified that such migration does not modify the structure. X-ray diffraction measurements have been carried out in conjunction with optical absorption in the UV–Vis as a function of annealing.
Analysis of the spin‐Hamiltonian parameters for Cr3+ in mirror and inversion symmetry sites of alexandrite (Al2−xCrxBeO4). Determination of the relative site occupancy by EPR
The EPR spectra of Cr3+ in single crystals of alexandrite (Al2−xCrxBeO4, x<0.01) have been obtained at temperatures between RT and 4.2 K in the frequency range 9.5–9.1 GHz. The orthorhombic structure of Al2BeO4 has two distinct Al3+ sites in the unit cell corresponding to Cs and Ci symmetry. The EPR spectra at 35 GHz of Cr3+ in the Cs site have been previously analyzed by other investigators. This study reports the observation of Cr3+ in the Ci site and the refined spin‐Hamiltonian parameters for both magnetic centers. The absolute sign of the fine structure tensor elements for the Ci site has been determined to be positive. The observed orientations of both the chromium mirror and inversion magnetic axes are consistent with the crystallographic symmetry relationships of pure Al2BeO4. A comparison of the measured Cr3+ EPR intensities for resonances from both sites indicates that for two magnetically dilute crystals, the mirror site is preferentially occupied with a relative site occupancy of 78%±3%.
Thermodynamics on the Carbochlorination of Titanium-bearing Ores
The equilibrium compositions of the multi-component, multi-phase and multi-reaction system of carbochlorination of titanium-bearing ores are calculated with HSC, a computational chemistry software. The thermodynamic equilibrium calculation of the reaction system shows that a complete conversion of titanium-bearing ores is feasible at a temperature near 200oC. At temperatures ranging from 800 to 1600oC, the products of the reaction system are all in gaseous phase. Thermodynamic analysis accords with the industrial practice. The simulated results provide reasonable scientific basis for improving suitable technological conditions of the carbochlorination reduction.