| dc.description.abstract | Ternary and quaternary rare earth chalcogenides have gained prominence in recent years, especially because of their wide array of crystal structures and interesting physical properties attributed to the presence of f-electrons 1-3 Among all rare earth chalcogenides, rare earth sulfides of the Ln2S3 family has been explored (Rare earth, Ln) in the industry of pigment, thermoelectric and ceramics, however, the study of their f-electron chemistry and their photoactivity have not been investigated. In this vein, the work described in this dissertation investigates the structures and role of f- electrons in optical properties of the beta and gamma polymorphs of Ln2S3. This research is a starting point to enlarge the library of chalcogenides available for optical and electronic applications, especially for applications related to energy conversion where ternary and quaternary chalcogenides have gained prominence in the last years.
Chapter 1 surveys the prior literature of chalcogenides, describing synthesis, crystal structure, applications and significance of rare earth sulfides.
In Chapter 2, a greener solid-state synthesis method, not requiring the flow of harmful gases, was applied for the synthesis of the red-orange compound CaCe2S4. This compound was first synthesized as an inorganic pigment, However, in this chapter the photoelectrochemical behavior is studied for the first time. X-ray diffraction showed that this compound adopts the cubic Th3P4 structure (γ-polymorph) with a = 8.614(3) Å. The optical band gap was determined to be ~ 2.1 eV and photoelectrochemical studies showed that CaCe2S4 is an n-type semiconductor.
Chapter 3 presents the study of the role of f-electrons over optical properties in the solution solution, Ca(La1-xCex)2S4 (0 ≤ x ≤ 1). These quaternary compounds showed progressive variations in color ranging from grey in CaLa2S4 to orange-red in CaCe2S4. Neutron scattering and synchrotron X-ray revealed that samples maintain the cubic Th3P4 structure type described in previous chapter. Optical characterization and DFT calculations showed a shrinking of the energy band gap from the UV to visible range with the progressive addition of cerium into the host CaLa2S4 structure.
Finally, in Chapter 4 the synthesis and optical characterization of lanthanide oxysulfides with the composition (La1-xCex)10OS14 (0<x<1) are described. Powder and single crystal samples were obtained by heating the binary Ln2S3 (Ln= La, Ce) compounds. The optical band gaps of (La1-xCex)10OS14 are in the range of 1.7 to 2.5 Ev indicating wide band gap semiconductors. Photoluminescence in all samples show red line emission and a red-shift of the lowest energy photoluminescence band as cerium was added into the structure.
References
1. Gschneidner, K. A., Preparation and processing of rare earth chalcogenides. J Mater Eng Perform 1998, 7 (5), 656-660.
2. Flahaut, J., Handbook on the physics and chemistry of rare earths. In Chapter 31 Sulfides, Selenides and Tellurides, Elsevier 1979; Vol. 4, pp 1-88.
3. Hulliger, F.; Vogt, O., Magnetic investigations of new ternary rare-earth chalcogenides. Phys Lett. 1966, 21 (2), 138-140. | |
