Heats In The Process From Lanthanide Complexes Based On Dithiocarbamates To Lanthanide Semisulfides Via Thermolysis

The lanthanide sulfides exhibit optical, magnetic and thermophysical properties that make them attractive as material components in thermoelectric, solar photovoltaic, and sensor/detector devices. In this dissertation, a general and practical route to prepare lanthanide sulfides was explored via thermolysis of compound of lanthanide and dithiocarbamate compared with traditional synthesis methods. Heats in the preparation processes were detailedly measured in view of energy, which would elucidate the thermodynamic support for the guiding ideology that the reaction energy is much lowed by changing reaction route in compound thermal decomposition method. The main research contents and conclusions in this dissertation were summarized as follows:1. 39 lanthanide complexes involved of the bond Ln-S in three series, Et₂NH₂[Ln(S2CNEt2)4], Ln(Et₂dtc)₃phen and Ln(PDC)₃phen (Ln = La, Pr, Nd, Sm^Lu;Et₂dtc = N, N-diethyldithiocarbamate;APDC = ammonium pyrrolidinedithiocarbamate), have been prepared based on the basic ligand dithiocarbamate and the ancillary ligand 1,10-phenanthroline in absolute ethanol, and characterized by IR, UV-Vis, FS, ¹H NMR and single-crystal X-ray diffraction. The experimental results revealed that sulfur atoms in dithiocarbamate coordinated to lanthanides in a didentate manner and an introduction of an ancillary ligand phen to the complexes played an important role in the stabilities of the complexes. The features of facile syntheses and stable to water and air on the complexes provided a basis for the preparation of lanthanide sulfides via compound thermal-decomposition route.2. Based on the rational thermochemical circles that are designed according to the synthetic routes of the title complexes, heats in the synthetic processes have been determined by microcalorimetry. The enthalpies of reactions in absolute ethanol are of negative values that benefit the proceeding of the reactions and thus provide a thermodynamic reason for the occurrence of the reactions, while the enthalpies of reactions in solid are of positive values that not favorable for the occurrence of the reactions, which represent the preparation ideology that changing the route of reaction aims at lowing the energy of reaction. The thermokinetic investigation for the preparation reaction s of the complexes indicated that the activation energies of the formation of the reactions in ethanol are close to 63 kJ·mol^-1 at which the reaction occursinstantaneously at room temperature, which provide a thermokinetic support for the instantaneous reactions of formation of the complexes.3. The energies of combustion for the complexes have been determined by a rotating-bomb calorimeter. The standard enthalpies of combustion and the standard enthalpies of formation of the complexes have been calculated. The standard enthalpies of formation of the complexes are of much negative values, showing that the complexes are of quite stability. The specific capacities of the complexes have been measured according to two reference substances. The plots of the related thermochemical data against the lanthanide atom numbers exhibit triplet effect, which is consistent with that of the shifts of the characteristic group in the complexes against the lanthanide atom numbers, indicating that the bonds of Ln-S and Ln-N fit into the ionic bonding model with covalent property.4. The thermal decomposition behaviors of the complexes have been performed by TG-DTG. The complexes Et2NH2[Ln(S2CNEt2)4] (Ln = La, Pr, Nd), Ln(Et2dtc)3phen (Ln = La, Pr, Nd, Sm) and Ln(PDC)3phen (Ln = La, Pr^Nd, Sm^Er) were decomposed into lanthanide chalcogenides in one step at the low temperature close to 400 °C, indicating that the complexes are potential single-source precursors to lanthanide chalcogenides materials. The non-isothermal kinetic investigations for the complexes Ln(Et2dtc)3phen (Ln = La, Pr, Nd) and Ln(PDC)3phen (Ln = La, Pr^Nd, Sm^Er) have been carried out, and the apparent activation energy, the pre-exponential constant and the mechanism functions of thermal-decomposition processes have been obtained.5. Thermolysis of the complexes Ln(PDC)3phen (Ln = La, Pr, Nd, Sm, Eu) in a flow of dry nitrogen at 450 °C followed by annealing at 900 °C lead to well crystallized lanthanide sulfides La2S3 (Ln = La, Pr, Nd, Sm) and EuS supported by X-ray power diffraction data, and lanthanide sulfides particles are within 50 nm in size.