Abstract. This thesis is subdivided into four major subjects. It deals with equilibrium investigations in systems metal / phosphorus / oxygen and chemical vapour transport, crystal structures of selected anhydrous phosphates, investigations on the electronic structure (colour) of phosphates of trivalent transition metals, and, finally with the magnetic behaviour of orthophosphates MPO₄ and diphosphates M₂P₂O₇.
Within the first area (chapter 2; "Kapitel 2") results from equilibrium investigations on the systems M / P / O (M = Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn) are reported. In this context information from literature as well as results from our own work is summarized, concerning the known anhydrous transition metal phosphate phases. The redox behaviour of phosphates or metal oxides towards phosphorus and of P₄O₁₀ towards metals is discussed with respect to the experimentally determined equilibrium relations. Furthermore, several synthetic routes to anhydrous phosphates are described. Particular emphasis is given to synthesis and crystallization of such phosphates containing transition metals in low oxidation states (Ti³⁺, V³⁺, Cr²⁺, Mo³⁺, Fe²⁺). Eventually, a detailed report on chemical vapour transport experiments with transition metal phosphates is presented. More than 60 phosphates have been crystallized and purified by this method, some of them for the first time ever. As transpor agents have been successfully used plain chlorine or iodine in combination with reducing agents (few mg of metal, metal phosphide or phosphorus). In addition to the detailed description of the experimental practice the results of the transport experiments are discussed under thermochemical aspects.

Crystal structures of selected anhydrous phosphates are treated in the second section of the thesis (chapter 3; "Kapitel 3"). The range of structures under discussion spans from phosphates of divalent chromium [Cr₃(PO₄)₂), Cr₂P₂O₇, Cr₇(PO₄)₆, Cr₆(P₂O₇)₄], to RhPO₄ and Rh(PO₃)₃, to the MP₂O₇ structure family and some ultraphosphates MP₄O₁₁ and reaches eventually ?₃Ti₅O₄(PO₄)₄ as well as some other members of the [Beta]-Fe₂O(PO₄) structure family. In a short summary the crystal structure of silicophosphates [M₂Si(P₂O₇)₂, M₄P₆Si₂O₂₅, MP₃SiO₁₁, MP₃Si₂O₁₃, M₃Si₂O(PO₄)₆, M₅O(PO₄)₆] and their crystal chemical relationships are described. At the end of this second section considerations are given regarding the radial and angular distortion of PO₄ tetrahedra within the crystal lattices of anhydrous phosphates.
The third subject (chapter 4; "Kapitel 4") comprises investigations on the colour of anhydrous phosphates of trivalent transition metals. UV/VIS spectra have been measured on powders and/or single crystals of MPO₄ (M = Ti, V, Cr, Fe), M(PO₃)₃ (M = Ti, V, Cr, Mo, Mn, Fe), M₄(P₂O₇)₃ (V, Cr, Fe), and M₄P₆Si₂O₂₅ (M = Ti, V, Cr, Fe). Within the framework of the angular overlap model the astonishing variability of the electronic structure of a particular ion in different phosphate structures can be rationalized. Assumption of a strong anisotropy in the p-bonding behaviour of O^2- appears to be essential for a fit between observed and calculated electronic energy levels. To our understanding this anisotropy is due to variations in the coordination number of oxygen. The employed bonding model as well as the simulation of the spectra using the computer program CAMMAG are described in detail.
As examples for the magnetic properties of phosphates some results obtained on orthophosphates MPO₄ (M = Ti, V, Cr) and diphosphates M₂P₂O₇ (M = Cr, Mn, Fe, Co, Ni, Cu) are presented in the last section (chapter 5; "Kapitel 5"). The magnetic structures of VPO₄ and [Alpha]-Cr₂P₂O₇ have been solved and refined from neutron diffraction data. The results are compared to information in literature on similar compounds. A discussion of the magnetic ordering in VPO₄ and [Alpha]-Cr₂P₂O₇ with respect to the Goodenough-Kanamori rules is given.