A correlation between collections of the experimental values of the crystal-field parameters (Pi, b:. A:) characterizing the nuclear quadrupole interaction. the ground-state and excited-state splitting for %dS* in various ionic crystals with the same type of anions has been established. The correlation consistsola tendency towards a decrease in P: and an increasein Ay throughaseriesofthese colleclionsprovidedthatthey are arranged according to descending values of b!. The great variety of the centres involved and the considerable magnitudes of the intervals within which the parameters of the collections obeying the correlation fallsuggest that thiscorrelation isa highly general characteristicofthe collections {Pi. b:. A!} and its explanation may be regarded as an important criterion of validity for microtheory crystal-field models. We have developed a model to interpret the correlation mentionedandsomecasesofdeviation from it. Thephenomenologicalvaluesofthe Sternheimer antiscreening coefficient y. have been determined in different ways. These values agree with each other and with the value of y. for er'* calculated by Ahmad and Newman taking the interelectron correlation into account in addition to the interactions considered by Sternheimer originally.

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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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