Christodoulos, Fotios (1987) Crystal fields for Er[sup]3+ in gold and zero-field splittings for Gd[sup]3+ in lanthanum ethylsulphate. PhD thesis, University of Warwick.

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Abstract

In the first and dominant part of the thesis, a recently developed theory for Crystal Fields (CFs) in metals has been used, in its first order, to investigate the microscopic origin of CF splittings for the dilute Erᶟ⁺:Au alloy. To first order there are three contributions to the CF splitting of the Rare Earth (RE) 4f -electron ground state. One is due to penetration effects with ligand ions, another due to a 5d semi -localised state about the RE site, the so called virtual bound state (vbs), and the third arises from the interaction of the 4f(RE) electrons with a discrete band state, denoted by lco>, which is split off from the s-like majority conduction band of Gold. The coupling between the 5d-vbs and the conduction band has been assumed weak. We find, within the limitations of our calculations, that the model used is quite capable of explaining the CF splitting of the ground level for Erᶟ⁺:Au. In particular we have found that the penetration effects, in the limit of no screening, considerably enhance the Crystal Field Parameters (CFPs) over their Point Charge Model (PCM) values, which consist of a fourth order CFP of the wrong sign, three times smaller than the experimental value and a sixth order CFP of the right sign but five times smaller than the observed. Contributions to CFPs directly from neighbours may be described in terms of a Coulomb interaction using different effective total charges (which we call pseudo-Point Charges) on the ligands for the fourth and sixth order CFPs. The sixth order CFP is much more affected by such effects. When screening is introduced, in an approximate way, we find that it tends to increase (over the unscreened PCM values) the magnitude of the CFPs by as much as 20% principally by screening out the sizable contribution, of opposite sign, from the next nearest neighbours. We also find that the probability of having a reversal in sign for the fourth order CFP, with respect to PCM values, is small since in the regime of the screening constant where we might expect this effect the energy of the 5d-vbs is negative corresponding to a bound state. We also estimate the CFPs of Erᶟ⁺:Ag, within the same model. It becomes clear that the model fails to explain the observed CFPs, at least within the approximations made in our first order investigation, unless a basic assumption, concerning the nature of the conduction band, is modified.

In the second part of the thesis we study the Zero Field Splitting (ZFS) of Gdᶟ⁺:-doped lanthanum ethylsulphate. On-site excitations of the Gdᶟ⁺: in this particular salt have recently been shown to produce a Spin Correlated Crystal Field (SCCF) which results in a ZFS of the same sign as those from experiment but, approximately, two times too large. Inter-site one-electron excitations have been invoked to produce additional contributions to the SCCF and so produce a ZFS which compensates the ZFS of the on-site mechanisms to finally produce ZFS in good agreement with experiment. The resultant ZFS is thus a delicate balance between a number of on-site mechanisms i.e. from the Gdᶟ⁺ ion itself, and a contribution from the host in which it is embedded.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Lanthanum, Crystal field theory, Gold, Field theory (Physics)
Official Date:	August 1987
Dates:	Date Event August 1987 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Dixon, J. M
Sponsors:	Alexander S. Onassis Public Benefit Foundation ; Vice-Chancellor Fund ; University of Warwick. Department of Physics ; MacMillan Memorial Fund ; Institute of Physics (Great Britain) ; European Physical Society
Extent:	iv, 178 leaves : illustrations
Language:	eng

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