Research in the Locmelis Group

MOLECULAR CHEMISTRY AND CHEMICAL TRANSPORT REACTIONS

The Locmelis group is engaged in the controlled synthesis of layer materials such as transition metal chalcogenides, metal thio and selenophosphates by means of solid state synthesis and chemical transport. Furthermore, research is focused on the preparation of their solid solutions and the characterization of their electrical properties.

CHEMICAL TRANSPORT REACTIONS

Schematic representation of a transport ampoule

Chemical transport reactions have been established in inorganic solid state chemistry for decades as a very useful method for the preparation of crystalline solids. Representatives of a large number of chemical groups are accessible in this way: metals and semimetals, intermetallic compounds, oxides and chalcogenides, oxide halides, pnictides, halides, salts such as sulfates, phosphates and more. T
he precipitate reacts reversibly in the source at a certain temperature T with the transport medium and forms only gaseous reaction products.

It is also called dissolution of the precipitate in the gas phase. The equilibrium location of these reaction(s) is determined by the temperature, the free reaction enthalpy ΔRG0T (or the equilibrium constant k) and the experimental conditions and is in principle precisely calculable. A very effective material transfer from T2 to T1 is achieved when the equilibrium constant is close to one. In this way, solid solutions are also available. Especially for this group of subcompounds, deposition from the gas phase has proven to be advantageous, as it provides crystals of the target compound without significant concentration gradients, provided that some basic principles are observed. For pure solid-state reactions, this goal can only be achieved with very long reaction times. Moreover, it is only possible to obtain larger crystals in exceptional cases.

Our central questions: Which experimental parameters like pressure in the vial, means of transport, duration of the experiment, temperature direction and gradient have to be adjusted to influence the size of the crystals and the amount of product? How are impurities and unwanted by-products avoided? What are the causes of an unwanted non-stoichiometric product?

SEMICONDUCTORS WITH LAYERED STRUCTURE

The scheme of the ZrTe5 crystal structure along projections in a , b and c direction. The red rectangles mark the lower cell of ZrTe5. TeI and TeII represent the Te atom at the trigonal-prismatic position of ZrTe3 and at the position between the chains, respectively. Lit.: Y.-Y. lv et al., Journal of Crystal Growth 2017, 457, 250-254.

Layer phases are very attractive compounds because they have many physical and chemical properties that are not usually found in three-dimensional materials. Remarkable are for example the occurrence of charge density waves, the anisotropic two-dimensional magnetic behaviour and the strong anisotropy of conductivity. These specific properties are the result of spatial and mainly covalent bonds in the layers that make up the structure.

How do the electrical conductivities and optical band gaps differ between semiconductors with many and few layers? Which composition of a solid solution can be used to adjust a given optical band gap or electrical conductivity? Which semiconductor is suitable for the intercalation of a metal in its VAN-DER-WAALS gaps without volume change?

The M-X bonds within the layer are predominantly covalent, whereas the sandwich layers are coupled by weak VAN-DER-WAALS forces, allowing the crystal to be easily cleaved along the surface of the layer using the Scotch tape method.

Transition metal chalcogenides represent an important and well-known range of coating materials. The two-dimensional compounds can undergo intercalation reactions involving host-guest redox processes. This type of attractive chemical properties is not only due to the weak VAN-DER-WAALS forces connecting the layers, but also and mainly to the presence of oxidation centers in the structure.

Metal thiophosphates and metal selenophosphates represent a large family of layered compounds whose properties are extremely diverse. From the point of view of stability and in contrast to layered transition metal chalcogenides, which occur mainly with the transition elements of the 4th, 5th and 6th groups, the thiophosphates and selenophosphates exist with most of the elements of groups 1 and 2, the transition metals of the fourth period and some heavier ones like Pd, Ag and Cd. The synthesis of the metal thiophosphates and selenophosphates from the elements is very simple and leads to phases with very good stoichiometry.

Fixed solutions play an important role in recent research and technology. In semiconductor technology, both pure compounds and substitution in the host lattice are used to create certain electrical properties. In addition, the optical properties of solids can be changed by substitution.

COOPERATIONS

HEAD OF THE GROUP

Dr. Sonja Locmelis
Research Staff
Address
Callinstraße 3-9
30167 Hannover
Building
Room
027
Address
Callinstraße 3-9
30167 Hannover
Building
Room
027