First Principles Study of Structural, Electronic and Mechanical Properties of Lanthanum Fluoride and Lanthanum Doped Barium Fluoride
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Date
2020
Authors
Wabululu, Elicah Nafula
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
Lanthanum Fluoride (LaF3) has several applications for instance, in electrodes
manufacture, fluorescent lamps and fiber optic devices due to its high heat capacity.
Barium fluoride (BaF2) is an alkaline-earth element with a wide band gap with many
optical applications, such as lithography. BaF2 has been established as an excellent
candidate that is useful for hosting optically active centers (OAC).Lanthanide
elements have been used as dopants in BaF2 lattice to achieve the desired optical
properties, which has not been fully explored .With these vast applications of these
compounds, it is necessary to carry out complementary theoretical investigations on
their properties to explore full applications. The structural, electronic, mechanical and
optical properties for LaF3, BaF2 and BaF2:La have been investigated in this current
work using theoretical computational modeling. These properties have been studied
using the generalized gradient approximation (GGA) employing pseudopotentials
and plane wave basis sets as implemented in Density functional theory. Lattice
constants and bulk modulus for LaF3 were calculated and the values obtained were
found to be in good agreement with experimental and other theoretical values. Density
of states and energy band structure for LaF3, BaF2 and BaF2:La have been obtained
along high symmetry points in k-space, respectively. A band gap of 7.79eV was
obtained for LaF3 which is an underestimation in comparison to the 9.0eV of
experiment. Values of elastic constants calculated for LaF3 are comparable with those
from other experimental and theoretical calculations.LaF3 exhibits mechanical
stability from the obtained elastic constants. Doping BaF2 with La reduced the band
gap and introduced new energy bands within the band gap from the charge
compensating fluorine. The elastic constants calculated for BaF2:La show a decrease
in the first two elastic constants with no change in the third component of the elastic
constants compared to the pure BaF2.This has been attributed to the lattice distortion
introduced by the La atom. From the defect formation energy calculated, nearest
neighbor (NN) formation energy was found to be -26.48eV compared to the next
nearest neighbor (NNN) of -27.58e.Itwas observed that the next nearest neighbor is
most favorable in BaF2 lattice. For optical properties, there was a shift in the
absorption coefficient from 5.32 m-1 to 6.25 m-1 of BaF2 when doped with La. The
refractive index of BaF2 is obtained as 1.52 compared to experimental value of 1.45.
The obtained refractive index is in good agreement with experimental values. The
introduction of La atom together with the interstitial fluorine affects electronic,
mechanical and optical properties of BaF2.
Description
A Thesis Submitted In Partial Fulfillment Of The
Requirements for the Award of Degree of
Doctor of Philosophy (Physics) in the School of
Pure and Applied Sciences of Kenyatta University
Keywords
Properties of Lanthanum Fluoride, Properties of Lanthanum Doped Barium Fluoride