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.

A computer controlled ENDOR spectrometer and special software are used to determine the coordinates of 19F nuclei in four anionic spheres of cubic MeF2:Gd 3 § (Me = Ca, Sr, Ba, Pb) centers. The computer simulation of Gd 3 § cubic impurity center in crystals MeF 2 (Me = Ca, Sr, Ba) has been also performed. The analysis of lattice relaxation near the impurity center carried out on the base of ENDOR experiments data has allowed us to obtain potential of interaction Gd3 § . For the first time not only radial displacements but also angular displacements of F ions of the third sphere have been taken into account and it has been shown that it influences the determination of lattice relaxation around the impurity center essentially. The influence of hydrostatic pressure (up to 30 kbar) on the local structure of the impurity center has also been investigated using computer simulation. The comparison of the experimental and calculated displacements of distant ions gives a reliable test of the validity of theoretical lattice relaxation model and accuracy of calculations of impurity-directed shifts of ligands.

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Adiabatic potential of K2ZnF4:Cu2 impurity system is calculated within the frames of pair potential approximation and shell model. The Jahn-Teller multi-body energy contribution is approximated by the lower branch of [CuF6] cluster adiabatic potential. It is shown that Qe-and Q-displacements ofthe distorted octahedron ofF ions surrounding Cu impurity ion may be assumed as the normal vibrational modes. Thus K2ZnF4:Cu2 vibronic states energy spectrum calculation reduces to the solution ofthe (Exe)-problem with the adiabatic potential, which contains not only the main terms (1/2.Ko'p2 H Vel P) but also relatively small corrections (-1/2.MC.p2cos(2p)+F.p'cosq) treated by perturbation theory. g-tensor components temperature dependences calculated by averaging over vibromc levels are consistent with the experimental data. Simulation of hydrostatic pressure influence on the K2ZnF4:Cu2 properties predicts essential changes in g-tensor components values and temperature dependences.

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