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A semiphenomenological theory for Gd3+ ground state splitting in low-symmetry crystals is proposed. It allows to explain this splitting with an explicit indication of the Gd3+ excited states taken into account and to extract the characteristic parameters of the splitting which are independent of the angular and radial coordinates of the ligands. IIpeAnomeHa nony~eHoMeHonorHYecKaR Teopm pacwennemn OcHoBHoro S-COCTORHMR

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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