Synthesis of New Mixed Metal Chalcogenides: Crystal structure, Characterization and Properties Investigation

Abstract

Metal chalcogenides are one of the most important class of compounds in the field of Inorganic Chemistry. A wide variety of chalco-anion building blocks provides excellent opportunities to synthesize new compounds with unique structure and properties, essential drives in maximizing technological impact.

In this dissertation, the exploratory synthesis of new mixed-metal chalcogenide compounds is carried out. The novel phases were characterized using a wide spectrum of techniques, and their properties were investigated.

The project started by investigating the synthesis of zeolite-like chalcogenides using a solid-state reaction. As a result, the thioaluminogermanate Na(AlS2)(GeS2)4 was synthesized with successful insertion of Al3+ cations into the chalcogenogermanate framework. This effectively extended the structural chemistry for this family of materials and approximated them to the aluminosilicate zeolites. The crystal structure of Na(AlS2)(GeS2)4 displayed a [(AlS2)(GeS2)4]1- 3D polyanionic framework, in which Al and Ge atoms share atomic positions and Na cations occupy the channels in-between. At room temperature and in a solvent medium, this compound exhibits a unique cation-exchange property with monovalent Ag+ and Cu+ ions, resulting in the formation of the isostructural compounds Ag(AlS2)(GeS2)4 and Cu(AlS2)(GeS2)4. The replacement of Na+ in the parent compound with Ag+ or Cu+ results in enhanced properties such as higher stability in air and narrower bandgap energies. The completeness of the ion-exchange reactions was confirmed using various analytical tools including single crystal XRD, EDX, and 23Na NMR.

Following this initial success, a systematic study was carried out to synthesize unknown phases of transition and main group mixed-metal chalcogenides. As a result, the first example of an alkali/transition metal thioaluminate compound K2Cu3AlS4 was synthesized. For this, a solid-state reaction with K2S acting as a self-flux was used. The crystal structure of K2Cu3AlS4 consists of [Cu3AlS4]2- polyanionic anti-PbO type layers, in which Al and Cu atoms share the atomic positions, separated by K+ cations. The coordination environments of the Al and K cations were confirmed by solid-state 27Al and 39K NMR spectroscopies. The optical property and thermal stability of this new quaternary compound were also studied.

The mixed-metal chalcogenides class is not restricted only to purely inorganic components; it can also be extended to inorganic-organic hybrid materials. In an attempt to synthesize main group chalcogenides mixed with transition metal complexes, the new compound [Ni(en)3]GeS2(OH)2•H2O was obtained. In the complex cation [Ni(en)3]2+, the ethylenediamine (en) ligands are bidentate to the Ni2+ through the N atoms resulting in a distorted octahedral geometry which is charge balanced by the rarely observed [GeS2(OH)2]2- tetrahedral anion. In agreement with single crystal data, the solid-state 1H NMR spectrum exhibits four signals corresponding to the -CH2 and NH2 protons of the (en) in addition to the H2O and -OH protons. This compound exhibits a paramagnetic response, studied by EPR spectroscopy and ZFC/FC magnetization measurements. The optical properties including UV-Vis absorption and photoluminescence emission were also measured.

Knowing that it was possible to synthesize various types of mixed-metal chalcogenides, the focus was shifted to the production of those with interesting functional properties. In this way, Na2BiSbQ4 (Q = S, Se, Te) compounds were synthesized by reacting Bi and Sb in the corresponding Na2Q flux. The three phases obtained are isostructural and crystallize with NaCl-type structure. The unique feature of these structures is the existence of only one crystallographic metal site in the unit cell (where Bi, Sb and Na share the same atomic position). These mix of position sites provide the desirable lattice complexity with a totally random distribution of Na, Bi and Sb atoms. As expected, extremely low thermal conductivities at room temperature have been observed for the studied phases. The optical properties, solid-state 27Na NMR spectra, chemical and thermal stabilities are discussed.