D J Newman

The superposition model of crystal fields

The superposition model was originally developed to separate the geometrical and physical information in crystal field parameters. Its success in the analysis of lanthanide spectra has been paralleled by the success of the related angular overlap model in the analysis of d-electron spectra. The basic ideas, method of application and reliability of the superposition model are discussed and its relationship with the angular overlap model is clarified. Developments described are the application of the superposition model to the ground (L=0) multiplet splittings of d5 and f7 ions, orbit-lattice interactions, transition intensities and correlation crystal fields. Special attention is paid to work which has been claimed to support or disprove the postulates of the model.

Theory of lanthanide crystal fields

Abstract Recent progress in understanding the origin of the lanthanide crystal field is summarized. The basic assumption of the crystal field parametrization is shown to be that the crystalline environment can be represented as a one-electron potential, and the consequences of removing this assumption are traced. It is further shown that overlap and covalency make the dominant contributions to the observed field, the electrostatic contributions only being inportant for the potential components with low angular dependence (i.e. the n=2 parameters). In some circumstances it is found that the observed parameters can usefully be analysed into superposed contributions from the neighbouring ions in the crystal. The importance of crystal field concepts in related problems is emphasized as well as the stimulus crystal field theory gives to the development of formalisms for dealing with non-orthogonal basis states.

Interpretation of S-state ion E.P.R. spectra

Abstract Although a considerable body of data exists on the parametrization of the ground-state splittings of S-state ions in crystals, relatively little progress has been made in obtaining a quantitative understanding of the mechanisms which determine these parameters. In the course of summarizing our present understanding, we emphasize the need for making planned experiments explicitly aimed at testing theoretical models, such as those proposed in this article. The variable frequency E.P.R. technique is described in some detail, as it has proved to be particularly useful in this respect.

Crystal Field in Rare‐Earth Trichlorides. I. Overlap and Exchange Effects in PrCl3

Overlap and exchange contributions to the crystal field of PrCl3 have been determined using an LCAO—MO model. The contributions to the parameters A60〈r6〉, A66〈r6〉 are found to be 8.5×greater than the magnitudes predicted by the simple point‐charge electrostatic model. However, the calculated values of these parameters are still 21% less than the experimental values and several reasons for this discrepancy are suggested.

Interpretation of Crystal‐Field Parameters in the Rare‐Earth‐Substituted Garnets

Crystal‐field parameters of Yb3+, Eu3+, Tm3+, Sm3+, Dy3+, and Er3+ ions in various garnet lattices have been analyzed using a development of the superposition model proposed by Bradbury and Newman. This effects, in principle, the separation of the crystal‐field contributions of the different coordinated O2− ions and determines power laws and the angular distortion near substituted ions. A realistic separation of the contributions to the n = 4 parameters is obtained even when no allowance is made for local distortion. However, owing to their greater angular sensitivity, the contributions to the n = 6 parameters can only be separated if local angular distortions of up to 9° are allowed for. Results derived for single‐ligand parameters and power laws are in good accord with theoretical results obtained for other rare‐earth systems.

Crystal Field in Rare‐Earth Trichlorides. IV. Parameter Variations

Parameter variations are determined for changes in the ligand coordination angles and distances in Pr3+:LaCl3. Parameters for Sm3+ and Er3+ substituted in LaCl3 are also calculated. These results are used in conjunction with the superposition approximation to discuss the variation of parameters down the rare‐earth series in the trichlorides and to predict values of the dynamic orbit–lattice parameters in PrCl3.

Spin‐correlated crystal field parameters for lanthanide ions substituted into LaCl3

Fits to spectral energy levels for Ho3+:LaCl3 and Gd3+:LaCl3 establish, for the first time, empirical values of the lanthanide spin‐correlated crystal field parameters. The fitted rank 6 parameters are consistent with the superposition model analysis and provide the solution to a long standing problem in the fitted values of the crystal field parameters for trivalent lanthanide ions in LaCl3.

A new interpretation of the ground state splitting in Gd3

The individual ligand contributions to the dominant (quadrupole) component of the Gd3+ ground state splitting for nine compounds with zircon structure are determined. The results of this analysis show that the theoretical problems previously encountered in explaining the ground state splitting were due to an incorrect assumption about the relative sign of the crystal field and spin-Hamiltonian quadrupole contributions. It is argued that three basic mechanisms make contributions of a similar magnitude to the observed splitting.

Parametrization of rare-earth ion transition intensities

A new parametrization is proposed which enables the intensities of transitions between crystal field split levels to be related to parameters representing the components of a vector field. Unlike the conventional parameters which are related to the odd parity components of the crystal field coupling the 4f to the nd and ng excited states, the new parameters are defined in a way which is explicitly independent of the physical processes involved. Use of these parameters enables the authors to explain a paradox in the interpretation of the odd parity crystal field.

Superposition model analysis of Fe3+ and Mn2+ spin-Hamiltonian parameters

Recent work has shown it possible to derive single-ligand contributions from the observed spin-Hamiltonian parameters for the 8S7/2 ground state of f7 lanthanide ions using the superposition model. Here the authors apply this model to the 6S5/2 ground state of Mn2+ and Fe3+ and show it to give a consistent description of experimentally determined parameters in several systems. The significance of this result is understanding the splitting of the 6S5/2 ground state is discussed.