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Title: Computer modelling of optical materials for laser applications : rare earth doping in YLiF4 and BaMgF4
Author: Littleford, Thomas Edward
ISNI:       0000 0004 5351 6000
Awarding Body: Keele University
Current Institution: Keele University
Date of Award: 2014
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In the field of solid-state laser materials, YLiF4 and BaMgF4 are two actively researched systems, with YLiF4 having been shown as a viable laser source. BaMgF4 is less developed. The work presented here provides an atomistic computational chemistry study into the two materials. A introduction to thefield of optical materials and computational chemistry is given before a detailed description of the atomistic methodologies is provided. The work utilises the widely used and studied methods of this field. The interionic interactions are modelled using the pair-wise approximation, a Coulomb interaction for the long-range interaction, which is summed using an Ewald summation, and a Buckingham potential for the short-range interactions. Electronic polarisability of the fluoride ions is included through a core-shell model coupled with a spring. Energy minimisation of the lattice is achieved through geometry optimisation and the resulting structures reproduced the reported structure of YLiF4 and BaMgF4 to within 2%. The intrinsic defect properties of the two materials are calculated through the Mott-Littleton method. For YLiF4, the Frenkel defect energies were found to be lower than the Schottky energies, with the two lowest energy defect formations being a F Frenkel and a Li Frenkel. Thermal expansion coefficients of the lattice were also calculated using Free Energy minimisation techniques. For BaMgF4, the Schottky defect energies are of similar magnitude to the Frenkel defects. Within the Frenkel defects the fluorine and magnesium Frenkel are 2.5 times smaller than the barium Frenkel defect. In comparison to the intrinsic defect energies for YLiF4, the defect energies are greater. To utilise these material in solid-state laser devices, the addition of rare earth dopant ions to the lattice is needed, as it is these ions that provide the electronic structure required for the laser action. The solution energies for the incorporation of the rare earth ions were calculated at both cation sites in both materials. In YLiF4 the dopant ions will dope at the yttrium site. In BaMgF4, the doping site varies across the rare earth group, as does the charge compensation method. For ions La3+ to Nd3+ the preferred site is barium with a magnesium vacancy. The remaining rare earth ions dope at the magnesium site with either a barium or magnesium vacancy. An attempt is made to calculate the doping limit of rare earth ions in each lattice. This is an important value to obtain so that a comparison across the rare earth ions can be made. It also allows different host lattices to be compared. Rare earth dopant solubility is of importance because, while for many devices the doping level is small, ideally the dopants should be homogenous throughout the host lattice and not clustered. In the case of YLiF4 the doping limit was calculated to be between 0.69% for La3+ and 1.51% for Yb3+. BaMgF4 gives smaller maximum doping limit, and in some cases negative values, implying the lattice is less accepting of rare earth ions than YLiF4. The likelihood of transition metal ion defects being incorporated into a Yb:YLiF4 lattice is studied as a result of the work into YLiF4 as a laser cooling device. A new potential set was derived for various transition metal fluoride ions. The results suggest that the 1+ and 3+ transition metal ions are most likely to be incorporated into the lattice with Cu1+ and Ti3+ being the most likely. The surface properties are also modelled using the same model parameters as in the bulk studies. Surface and attachment energies are given for the low index surfaces and these are used to predict surface morphologies. For YLiF4 the equilibrium morphology is dominated by the (112) and (011) surfaces and the growth morphology by the (001) and (120) surfaces. For BaMgF4 the morphologies are dominated by the (010) and (110) surfaces. The segregation of rare earth dopants to the surfaces is calculated by comparing the difference between defect energy at the surface and in the bulk. For a number of surfaces of YLiF4 a driving force for segregation is found. Simulation cells are scaled to consider concentration effects of rare earth dopants at the surfaces. A Perl script is used to automate the creation of every configuration of the dopant ions at various concentrations. The lowest energy configurations are used to predict how the surface energy of each low index surface would change with the presence of dopant ions. This is used to predict the impact on surface morphology. In YLiF4 the (110), (112), (012), (221), (021), (122) and (010) surfaces showed a reduction in surface energy with the presence of some of the rare earth ions. For BaMgF4 all rare earth dopants segregated to the three surfaces studied, with the segregation force to the (010) surface the greatest. There is a correlation between the ionic radius mismatch between the lattice site and the rare earth ion, and the degree of segregation. The content of this thesis is an important contribution to the research of these two materials, which should aid further research, both computational and experimental, into them.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry