Crystal-field Mechanism Research Articles

The effect of γ-doses on the optical band gap of AgInSe 2 films

Some optical properties of AgInSe 2 thin films are studied as a function of γ-doses. In order to determine the optical energy gap E g, the absorbance spectra of the films of different γ-doses are analyzed. The compound shows an allowed direct transition near the fundamental absorption edge E g1, in addition to transition originating from the crystal field split level E g2. The direct energy gap E g1 increases from 1.21 eV for an unirradiated film to 1.31 eV for an irradiated film (0.7 Mrad) and then decreases to 1.17 eV for higher doses (3.0 Mrad). E g2 exhibits an opposite trend of E g1, i.e. with the first dose (0.7 Mrad) it decreases from 1.76 eV for the unirradiated film to 1.66 eV and then increases to 1.75 eV as the γ-dose increases to 3.0 Mrad. The calculated band tails in the first region ( E e1) and in the second region ( E e2) revealed that the crystal field reduces the crystallinity of the composition. The influence of γ-irradiation on the optical gap was explained in terms of diminution of disorder, defects in the structural bonding and crystal field mechanism. From the absorbance spectra, the relationship between the extinction coefficient ( k) and refractive index ( n) at different γ-doses were derived.

EPR spectroscopy of MRI-related Gd(III) complexes: simultaneous analysis of multiple frequency and temperature spectra, including static and transient crystal field effects.

For the first time, a very general theoretical method is proposed to interpret the full electron paramagnetic resonance (EPR) spectra at multiple temperatures and frequencies in the important case of S-state metal ions complexed in liquid solution. This method is illustrated by a careful analysis of the measured spectra of two Gd3+ (S = 7/2) complexes. It is shown that the electronic relaxation mechanisms at the origin of the EPR line shape arise from the combined effects of the modulation of the static crystal field by the random Brownian rotation of the complex and of the transient zero-field splitting. A detailed study of the static crystal field mechanism shows that, contrarily to the usual global models involving only second-order terms, the fourth and sixth order terms can play a non-negligible role. The obtained parameters are well interpreted in the framework of the physics of the various underlying relaxation processes. A better understanding of these mechanisms is highly valuable since they partly control the efficiency of paramagnetic metal ions in contrast agents for medical magnetic resonance imaging (MRI).

Spectra, energy levels and up-conversion effect of erbium ions in LiNbO 3 crystals

The Optical properties and spectral data of erbium-doped Lithium Niobate crystals, including absorption spectrum, emission spectrum, luminescence lifetime and up-conversion effects are reported in this paper. The energy level scheme, Stark splitting under crystal field and up-conversion mechanism are discussed in detail. The possible applications of this material in waveguide optics and opto-electronics are also prospected.

A crystal field mechanism for the noncubic magnetic anisotropy in garnet: Oxygen vacancy ordering

A general, crystal field mechanism based on oxygen vacancy ordering is suggested to be a source of the growth induced noncubic magnetic anisotropy observed in iron garnet. It is estimated that the reported anisotropy energies of 10 4 erg/cm 3 can be accounted for by an ordered oxygen vacancy concentration of 10 −3.