Borates represent a well known class of compounds with a wide range of applications. Especially in the fields of optical materials, RE oxoborates play a major role. Although a huge number of crystal structures of oxoborates is known, the crystal chemistry of thio- and selenoborates is less investigated. Especially RE thio- and selenoborates as materials which might have interesting (RE-dominated) physical properties are nearly unexplored. Thus, we started the exploration of these compounds with the main focus on preparation of crystalline single phase materials suitable for crystal structure determinations and measurements of their physical properties.

Synthesis Equipment

The synthesis of RE thio- and selenoborates from the elements (and/or binary compounds) is fairly problematic because of the high reactivity of the in situ formed boron chalcogenides towards most of the common container materials at elevated temperatures. Fused silica tubes are attacked by boron chalcogenides at temperatures above 400 °C (B/Si exchange). Carbon-coated silica tubes, often used for the synthesis of alkali and alkaline earth chalcogenoborates, are disadvantageous with respect to longer reaction times and temperatures above 900 °C. For such kind of reactions the vessels must be made of either boron nitride or glassy carbon. The chalcogenoborates of the heavier chalcogens are sensitive against oxidation and hydrolysis and, hence, they have to be handled in an inert environment.

The optimization of high temperature synthesis routes (beyond the melting points of the RE metals) led to the development of specially designed crucibles made of sintered boron nitride (without any binder component) which are enclosed in tantalum ampoules. These ampoules are placed in quartz glass reactors under flowing argon and heated in a tube furnace. For the time being, the maximum temperature of this equipment is limited to 1150 °C. The development of closed (pressure-resistant) boron nitride lined reactors for even higher operating temperatures is in progress.

In addition, we perform experiments under external pressure by means of a  multianvil press. Alternatively, we employ the hot isostatic pressing technique within a system originally designed for spark plasma sintering.

Synthesis Routes

In most cases we use the elements and/or binary compounds (mainly rare earth chalcogenides and boron chalcogenides) as starting materials for synthesis of rare earth chalcogenoborates by means of common solid state preparation methods. Alternatively, we develop low-temperature synthesis routes, e.g. metathesis reactions of alkali chalcogenoborates with rare earth halogenides. These indeed appear to be promising routes especially for the preparation of chalcogenoborates of the late rare earth metals with very high melting points under autogenous pressure.

Crystal Structure Determination and Classification

For crystal structure determinations we apply  diffraction methods assisted by  scanning electron-microscopy, energy-dispersive X-ray spectroscopy (EDXS),  X-ray absorption spectroscopy (XAS) and  chemical analysis (ICP-OES). Despite the tendency to form amorphous glassy solids, we try to prepare RE chalcogenoborates in the crystalline state. After optimization of the reaction conditions and techniques we were already able to prepare some RE thioborates as nearly phase-pure crystalline powders, of which the crystal structures were solved from X-ray powder diffraction data (see figure). More informations are presented in the  scientific report. Further developments of special methods, which are required to obtain single crystals large enough for single crystal structure determinations, are in progress.

Chemical and Physical Properties

The reaction products are characterized by means of  thermal analysis (DTA),  optical spectroscopy and measurements of thermal and electrical transport properties.