This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution 4.0 International License. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung 4.0 Lizenz. Superposition Model Analysis of the Spin Hamiltonian Parameters of Two Gd 3+ Doped Thorium Dichalcogenides G. Amoretti*, C. Fava, and V. Varacca* Istituto di Fisica dell'Universita, Parma, Italy Z. Naturforsch. 37a. 5 3 6 - 5 4 5 (1982); received January 8, 1982 The spin Hamiltonian parameters of Gd 3+ in a single crystal of Thorium disulfide (TI1S2) are analysed by means of the superposition model, in its most general form of a two exponents power law. In fact, the approximated one exponent power law is not suitable for this compound, where the ion-ligand distances show a quite large spread. The results of the analysis are then applied to the case of Gd 3+ doped Thorium oxysulfide (ThOS), where the fitting of the experimental data is possible only assuming some distortion of the ligand cage. 1. Introduction The superposition model (SPM) has been suc- cesfully applied to the analysis of the spin Hamil- tonian parameters describing the ground state splitting of S-state ions in many host matrices [1—7]. The main assumption underlying the SPM is that the total crystal field acting on the ion can be built up from individual contributions from each of the ions of the crystal. This essentially means that the crystal field interaction is considered as ruled up by overlap and covalence mechanisms and therefore it is sufficient to take into account con- tributions from the nearest neighbours only. This is also the principal limit of the SPM: in fact, in strongly ionic compounds, or when the dominant interaction is almost purely electrostatic, better results seem obtainable by the polarizable point charge model, at least for / 7 ions [8, 9]. However, in the quoted limit, the application of the SPM has given valuable results for the second degree and recently also for the fourth degree parameters [6], The contribution to the parameters of each ion- ligand pair is given, apart from a geometrical factor depending on the angular coordinates of the ligand, by an "intrinsic parameter", the value of wdiich depends on the ion-ligand distance. This dependence is usually explicited in terms of a simple power law, that is hn(Ri) = bn(R0) (Ro/Ri)'" [1], where Ri is the * Gruppo Nazionale di Struttura della Materia del C.N.R., Parma, Italy. Reprint request to G. Amoretti, Istituto di Fisica, Via M.D'Azeglio 85, 43100 Parma, Italy. given distance and Ro a fixed reference distance. In effect, this approximation is valid for small variations of Ri with respect to R Q only. This derives from the fact that such a power law r can be considered as an approximation of the sum of two opposing terms, as pointed out by Newman and Urban [1], in agreement with the low values of normally found for the second degree parameters, in the case of / 7 ions. As far as we know, the SPM has never been successfully applied to compounds showing a large spread of ion-ligand distances. An attempt on PbCl2 [10] has not given satisfactory results, owing prob- ably also to the prevalent ionic character of the Pb-Cl bond. Therefore, we consider it necessary to analyze the most general form of the SPM as reported in Ref. [1], to show that the model can be applied also in the case of a relatively wide range of ion-ligand distances. In this w r ork we report the application of the SPM to a Gd 3+ doped single crystal of Thorium disulfide (TI1S2), which has the same structure as PbCl2, but appears more suitable for such applica- tion, owing to the more covalent character of the bonding with sulphur ligands. Using a power law with two exponents, we analyze the possible solu- tions for the parameters, obtained by numerical procedures, and apply the results to a second compound, namely Thorium oxysulfide (ThOS). One of the advantages of the SPM is the possibility to calculate the spin Hamiltonian parameters for a complex, using the intrinsic parameters obtained from other compounds. In this way, as we will show, one can also point out local distortions at the im- purity site. 0340-4811 / 82 / 0500-550 $ 01.30/0. - Please order a reprint rather than making your own copy.