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Introduction:

The research area investigated in my working group is located in the field of inorganic solid state chemistry. Equally important are the targeted synthesis of crystalline solids and their characterization.

The aim of the investigations is to contribute to the understanding of the relationship between crystal structure and physical properties (color, magnetism). The description of the chemical bond between oxygen and transition metals is of particular interest.

In the following, some aspects of the research area with their directions of development are presented.

Call to Action

fabric chemistry:

Anhydrous phosphates and phosphides of the transition metals form the substance-chemical core of the work area. The phase relationships and coexistence conditions between phosphides and phosphates are of particular interest.

The synthesis and characterization of Chromium(II) oxo compounds (Cr₂P₂O₇, Cr₃(PO₄)₂ and chromium (II) oxo halides (Cr₂(PO₄)X , X = Cl, Br, I) has evolved from it.

Similar investigations have already been carried out or are planned for a number of other metal/phosphorus/oxygen systems.
In the course of the previous work, the broad applicability of the method of chemical transport for the preparation, purification and crystallization of anhydrous phosphates of the transition metals could be shown. To illustrate the possibilities of chemical transport reactions, α-CrPO₄, RhPO₄, the mixed-valent chromium(II, III) phosphates Cr₇(PO₄)₆ and Cr₆(P₂O₇)₄, and the copper(II) ultraphosphate CuP₄O₁₁ are particularly emphasized.
The list of phosphates that can be obtained in a well-crystallized form by means of chemical transport reactions now ranges from orthophosphates of divalent, trivalent, tetravalent and pentavalent cations to diphosphates (M²⁺, M³⁺, M⁴⁺), metaphosphates M(PO₃)₃ with M³⁺ and M₂P₄O₁₂ with M²⁺ to the already mentioned ultraphosphate.

In total, more than 70 representatives of this class of compounds have already been crystallized as a result of systematic investigations by us. It was found that in particular those phosphates which coexist with phosphorus or phosphides in the respective three-component systems metal/phosphorus/oxygen deliver good transport results.

Investigations on phosphates of the platinum metals as well as the early transition metals are planned for future synthetic work. The thermal lability to be expected, but also the possible catalytic activity, make these investigations both preparatively and technically interesting. Experiments on the crystallization of phosphates with cations in higher oxidation states (Mo⁶⁺, W⁶⁺, Mn³⁺, Mn⁴⁺) should also be carried out more intensively. The use of electrochemical synthesis methods is also planned here. Using hydrothermal syntheses, we were able to crystallize the long-known but structurally uncharacterized mercury diphosphates Hg₄P₂O₇ and Hg₂P₂O₇ .
As part of the DFG Priority Program "Reactivity of Solids", investigations into the oxidation/hydrolysis of phosphides of the transition metals under mild conditions were carried out with the aim of obtaining information about the reaction behavior and the thermal stability of this class of compounds. Further goals of this work are the elucidation of the corrosion behavior of phosphides and the equilibrium relationships between phosphides and phosphates.

Call to Action

physical properties:

All of our efforts to characterize new phosphates are based on X-ray crystallographic studies on single crystals.

Various publications document the experience gained (e.g.: M₃Cr₄(PO₄)₆, CuP₄O₁₁, Ti₅O₄(PO₄)₄).
In addition, the preparative work forms a broad basis for the further investigation of the physical behavior of anhydrous phosphates of the transition metals. The variety of colors that appear is particularly striking.
Titanium(III) orthophosphate TiPO₄ is green, titanium(III) silicophosphate Ti₄P₆Si₂O₂₅ is burgundy and titanium(III)-(tris)metaphosphate Ti(PO₃)₃ is blue. Similar variability is also found in phosphates of other transition metals (e.g.: Cu₅O₂(PO₄)₂ and Cu₄O(PO₄)₂: intense green; Cu₃(PO₄)₂: green; Cu₂P₂O₇: pale green; Cu₂P₄O₁₂: pale blue; CuP₄O₁₁: colorless).
In a study on the electronic structure of Ti³⁺ in TiPO₄, Ti₄P₆Si₂O₂₅ and Ti(PO₃)₃, the special influence of the p-interactions between oxygen and Ti³⁺ on the energy levels of the transition metal ion could be described within the Angular Overlap Model (AOM).
The results of this work encourage the examination of various questions, which are only briefly outlined here. Which structural conditions influence the ligand behavior of oxygen (from phosphate, or other oxoanions) towards transition metal ions? What is the effect of the coordination number of oxygen? The aim of these investigations is the derivation of a set of AOM parameters for oxo compounds of the transition metals, which allow a prediction of the energy level of the d-electrons at the metal if the crystal structure is known. For this purpose, phosphates are a particularly suitable class of substances, since varying the metal oxide: P₄O₁₀ ratio results in a large number of linkage patterns of the metal-oxygen coordination polyhedra. Thus, in the P₄O₁₀-rich ultraphosphates, the coordination number for oxygen is only 2 (M + P or 2 P), bridging of two metal ions by oxygen (of a phosphate group) is not observed. With increasing metal oxide content, condensation of the metal-oxygen polyhedra occurs in the phosphates, and at the same time higher coordination numbers are found for oxygen. The higher degree of condensation of the metal-oxygen polyhedron not only has an effect on increased magnetic interactions, but also becomes "visible" in the electronic spectra of the respective transition metal compounds via the modified ligand behavior of more highly coordinated oxygen.
For corresponding spectroscopic measurements, a micro single crystal UV/VIS/NIR spectrometer is available in our working group, which allows the characterization of single crystals with an edge length down to approx. 0.2 mm at temperatures around 10 K. The spectrometer is from Dr. Elmars Krausz (ANU Canberra, Australia). In addition, we also examine the magnetic behavior of the phosphates using neutron diffraction. After clarifying the magnetic structures of VPO₄, I have regular measuring time at the HMI in Berlin. Current investigations deal with the magnetic order in diphosphates M₂P₂O₇ (M = Cr, Mn, Fe, Co, Ni, Cu). It is planned to continue this cooperation in the future.