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.
Thermal characteristics of alexandrite fluorescence decay at high temperatures, induced by a visible laser diode emission
Resulting from an experimental study to develop a fiber‐optic temperature sensor based on the fluorescence decay of Cr3+‐doped crystalline alexandrite, a simple configurational coordinate model is proposed to describe the kinetics of alexandrite fluorescence over the temperature region from 20 to 700 °C, to link experimental and theoretical work. This model fits well the experimental fluorescence lifetime data over the above region and successfully explains the observed phenomenon that a sharp reduction in the fluorescence lifetime occurs beyond ∼400 °C. The model was also used to study the temperature dependence of the intensity of the fluorescence emission induced by light from a visible laser diode, and as a result it is suggested that a phonon‐assisted absorption is involved in a specific absorption process, where the energy of a single incident photon is below that of the primary absorption band 4T2.
Optical Absorption of CoO and MnO above and below the Néel Temperature
The optical absorption of single-crystal CoO and MnO both above and below the N\'eel temperature is reported and interpreted. The absorption spectrum in each case is similar to that found in dilute paramagnetic salts of the same ions. Tanabe and Sugano's strong-field theory was used to interpret the data. In CoO the tetragonal distortion of the cubic lattice present in the antiferromagnetic phase was experimentally observed. An estimate of the strength of the tetragonal field is given. It is also found that the temperature dependence of the absorption coefficient in CoO is in accord with a second order electric dipole-phonon transition mechanism.