Radiationless transitions in scheelite and fergusonite type crystals have been studied. It is shown that these transitions can be realized by virtual recharge for the ions having the tendency to form a tetravalent state (Pr3+, Tb3+). The probability of this process is greater than that of multi‐phonon relaxation.
Disordered fluorite-related solid solutions ͑AF 2 ͒ 1−x−y ͑LnЈF 3 ͒ x ͑LnЉF 3 ͒ y , where A = Ca, Sr, Ba; LnЈ = Er, Tm, Yb; LnЉ = Lu, Y; and x Ӷ y = 0.001-0.4, were studied by both optical detection and conventional electron paramagnetic resonance ͑EPR͒ techniques. The EPR spectra of paramagnetic rare-earth ions Er 3+ , Tm 3+ , and Yb 3+ in clusters of diamagnetic Y 3+ and Lu 3+ ions were recorded. It appears that the crystalline electric field at the sites of Ln ions in the clusters is of "nearly" tetragonal symmetry and provides for high values of factors g ʈ , approaching the theoretical limits, and small values of factors g Ќ ϳ 0 in the ground states of the paramagnetic Ln ions. It was assumed that all the clusters of Ln ions in the solid solutions appear to be similar in structure to the hexameric clusters, which are the basic structural units of the homologous series of fluoriterelated superstructures ͑AF 2 ͒ 1−y ͑LnF 3 ͒ y with compositions y =5/m, where m is an integer in the range of 13-19. The structure of "symmetric" hexameric clusters in CaF 2 , SrF 2 , and BaF 2 hosts was established by computer simulation. The crystalline electric field and the spectroscopic ground-state parameters for Er 3+ , Tm 3+ , and Yb 3+ ions in the hexameric clusters were calculated and found to be in agreement with the experimental data, being totally different from those known for the "isolated" simple cubic and tetragonal centers in the fluorite crystals.
Physical properties of semiconducting CdF2 crystals doped with In are determined from measurements of the radio-frequency response of a sample with Schottky barriers at frequencies 10 − 10 6 Hz. The dc conductivity, the activation energy of the amphoteric impurity, and the total concentration of the active In ions in CdF2 are found through an equivalent-circuit analysis of the frequency dependencies of the sample complex impedance at temperatures from 20 K to 300 K. Kinetic coefficients determining the thermally induced transitions between the deep and the shallow states of the In impurity and the barrier height between these states are obtained from the time-dependent radio-frequency response after illumination of the material. The results on the low-frequency conductivity in CdF2:In are compared with submillimeter (10
11− 10 12 Hz) measurements and with room-temperature infrared measurements of undoped CdF2. The low-frequency impedance measurements of semiconductor samples with Schottky barriers are shown to be a good tool for investigation of the physical properties of semiconductors.
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