Spectroscopic investigations of cation vacancy defect centers in low-dimensional lattices.

A relatively low concentration of defect centers can cause dramatic changes in material bulk properties. For this reason, studies correlating changes in material properties with defect center creation and modification are of both academic and practical interest. Cation vacancy defect centers in one and two-dimensional lattices have been characterized in this work. The low-dimensional lattices used in this study simplify defect structure investigations. A wide variety of experimental techniques ranging from absorption, photoluminescence and photoexcitation spectroscopy to thermal gravimetric analysis, differential scanning calorimetry and x-ray analysis were used to reach my conclusions. One-Dimensional Lattices: The family of CsMX$\sb3$ crystals (M = Mg or Cd; X = Br or Cl) adopt the CsNiCl$\sb3$ structure consisting of linear anionic (MX$\sb3$) $\sp-$ chains with the Cs$\sp+$ ions positioned between the chains. Trivalent cations, M$\sp{3+}$, can enter these anionic chains and form either a weakly interacting magnetic pair in the case of M$\sp{3+}$-V-M$\sp{3+}$, or a strongly interacting magnetic pair as in M$\sp{3+}$-M$\sp{3+}$-V (V = Vacancy). High resolution laser induced photoluminescence spectra of CsMgX$\sb3$:M$\sp{3+}$ (M = Cr or Mo; X = Cl or Br) and CsCdBr$\sb3$:M$\sp{3+}$ have established the dominant defect structure as M$\sp{3+}$-V-M$\sp{3+}$ and the minority defect as M$\sp{3+}$-M$\sp{3+}$-V. The observed exchange splittings correlated well with those predicted from a spin Hamiltonian analysis using parameters calculated from previous EPR studies. Two-Dimensional Lattices: CdPS$\sb3$ is a broad-band semiconductor whose lamellar structure consists of CdPS$\sb3$ sheets weakly interacting through a van der Waals gap. An array of cations can be placed into the van der Waals gap by cation exchange chemistry. This research has associated the host lattice luminescence with a cation vacancy defect center created during crystal growth or by cation exchange. Changes in the optical properties and band structure of CdPS$\sb3$ have been correlated with structural changes at the cation vacancy defect centers for CdPS$\sb3$:M (M = Na$\sp+$, K$\sp+$, Mg$\sp{++}$ or Ca$\sp{++}$). Crystalline site assignments consistent with D$\sb{\rm 3d}$ symmetry have been made through a Hamiltonian analysis of the multiline rare earth emission obtained from CdPS$\sb3$:Sm$\sp{3+}$ and CdPS$\sb3$:Nd$\sp{3+}$.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103448/1/9319581.pdfDescription of 9319581.pdf : Restricted to UM users only