First Principles Study of Structural, Electronic and Mechanical Properties of Lanthanum Fluoride and Lanthanum Doped Barium Fluoride
Wabululu, Elicah Nafula
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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.