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A suitable heating condition designed for a particular type of ruby is used to enhance the quality to increase its value. Heating Vietnamese rubies in an oxygen atmosphere may improve color and clarity, and thus increase their prices. The color of rubies is due to the presence of trace amounts of Cr³⁺. Fe³⁺ yields a pale yellow color and the charge transfer mechanism between Fe²⁺ and Ti⁴⁺ gives ruby an undesirable bluish color. Reducing this mechanism is possible by heating the ruby in oxygen so that most of the Fe²⁺ is converted into Fe³⁺ ions and thus they appear a more intense red. By using XRD we found that the c/a ratio of the hexagonal structure was smallest after heat treatment at 1300°C. The number of Fe²⁺ ions converted to Fe³⁺ was detected by an electron spin resonance spectrometer and found to increase with temperature. The ruby appeared a most intense red after heating at 1500°C for 12 hours. The color change is due to both the decrease in c/a ratio and the increase in the number of Fe³⁺ ions. ESR experiments on ruby crystals by rotation about their c-axis show that Fe³⁺ ions are a little off axis before heat treatment. After heat treatment at 1300°C they move to new asymmetric equilibrium positions towards larger O^2- triangles. It is not clear, however, if this Fe³⁺ movement is related to the change in color. The asymmetry may arise due to some O^2- vacancies.

In this report, Fe³⁺ impurity ions present in green sapphire (Al₂O₃) were studied experimentally, by heating a light green sapphire in flowing oxygen atmosphere for 12 h from 1200, 1300, 1400, 1500 and 1600°C, respectively. Electron spin resonance (ESR) spectra in X-band (~9.45 GHz) were recorded by mounting the crystal with the c-axis perpendicular (θ = 90°) to the magnetic field direction. The spectra were recorded and simulated by a numerical diagonalization of spin Hamiltonian matrix in the range from 0 to 180 degrees for every 15 degrees of rotation angle (φ). In our case, only the last two sets of peaks strongly depend on the rotation angle (φ), and each exhibits C₃ symmetry due to two magnetically inequivalent Fe³⁺ sites in the corundum structure. For polycrystalline ESR spectra, seven main Fe³⁺ ESR absorption peaks occur at the resonance magnetic fields of 100.20, 310.24, 486.80, 525.00, 550.60, 761.00 and 777.00 mT respectively. Specifically, ESR signals show that the number of paramagnetic Fe³⁺ ions increase roughly linearly with the heat treating temperature, having the ratio ~1.41 at 1600°C.

Cu-doped Cadmium sulfide (CdS) films were prepared by chemical bath deposition method. Various quantities of Cu were used for mixing. Surface characterization, structure, morphology and defect structures of the films were studied. The structure of all Cu-doped CdS films was a unique CdS cubic phase. The grain size of CdS was decreased when doped with Cu. The F-type defect in the undoped CdS is clearly confirmed by an ESR signal arising from the hyperfine interaction of electron spin (S=1/2) trapped in sulfur vacancies and neighboring cadmium nuclear spin (I=1/2). In the Cu-doped CdS films, the ESR peaks shift towards low fields as the copper concentration is increased. This is due to the change in crystal field experience by Cu²⁺ (3d⁹) ions while the Cd²⁺ signal disappears. The Cu²⁺ ions in the Cd_1-xCu_xS itself are ESR silent due to a very short relaxation time. The dark resistance increased with increasing amount of Cu concentrations up to about 0.03 M. This 0.03 M Cu-doped CdS sample also possesses the maximum photosensitivity.

The CdS : Ni films were fabricated on glass substrates by chemical bath deposition method (CBD), where Ni concentrations are 0%, 10%, 20%, 30% and 40%. X-ray diffractometry (XRD), Raman spectroscopy and electron spin resonance (ESR) were employed to study the film structures. The XRD patterns revealed the presence of cubic CdS and trace of NiS. The Raman spectra were observed at 300 and 600 cm^-1, corresponding to the first and second orders of the longitudinal optical phonon modes. Both results confirm that slightly lower order of crystallinity of CdS : Ni was found at the higher concentration of Ni. The ESR spectra showed the presence of F-type defects in CdS : Ni films. The band gaps of the samples were found to increase with the increase of Ni concentration.

We have studied paramagnetic Mn²⁺ ions present in the freshwater snail, Sinotaia ingallsiana (FS), Viviparus which are abundant in Thailand. The FS shells in our study were ground into fine powder. A set of seven samples was each then separately annealed for 2 hours in air atmosphere at 300°C, 400°C, 500°C, 600°C, 700°C, 800°C and 900°C, respectively. Our detailed ESR spectral analyses of FS show that Mn²⁺ ions enter Ca²⁺ sites during a biomineralization process. The hyperfine coupling constant (A) and zero-field splitting (D) in the ESR spectrum of Mn²⁺ in calcite and aragonite were determined. The spreading of the non-central allowed transitions was analyzed and the experimental transitions were attributed. For calculating the hyperfine coupling constant, five methods for calculating the zero-field splitting, based on the analysis of the allowed and forbidden transitions, were provided. The values of the hyperfine coupling constant range from 87.50 to 89.00 G and those of the zero-field splitting range from 110.00 to 116.00 G.

Temperature dependences of the ESR spectrum in Y₂Cu₂O₅ compound are studied for both polycrystalline and single crystal samples. The g-factors along the main axes are found to be g_a=2.099, g_b=2.048, and g_c=2.21 at room temperature, T=293 K. As temperature decreases, the linewidth ΔH increases in agreement with the Huber’s formula ΔH=A(T_N/(T−T_N))^β+B(θ/T+1) with the following parameters: A=1513±10 Oe, B=509±10 Oe, T_N=7±1 K, θ=18±2 K, β=1.3±0.3. In the vicinity of the Neel Temperature, T_N, the ESR spectrum has a very complicated form indicating the existence of clusters with different internal magnetic fields. Below T_N, a single line symmetrical ESR spectrum is recovered.