Synthesis of new fluorido/ fluoride borates

The goal of this project is the exploitation of new ternary and multinary rare-earth fluorido- and fluoride borates, based on the systematic preparation via ambient- and high-pressure / high-temperature syntheses. Emphasis is put on the study of fluoridoborates, which exhibit fluorine atoms bound covalently to boron atoms. At the beginning of our preliminary work, the field of rare-earth fluorido- and fluoride borates was only represented by two well- characterized structure types (four different compounds overall). Therefore, the preparation of additional compounds with yet unknown compositions and crystal structures should be possible under both ambient- and high-pressure conditions. Ambient-pressure syntheses will be performed in conventional high-temperature kilns. Since the system of fluorido- and fluoride borates is inclined to form glasses under ambient pressure, a gradual increase of pressure can evoke pressure-induced crystallization. Knowledge about these crystalline phases might have a large impact on the understanding of the glass phases. For high-pressure syntheses, our well-established multianvil device, based on a Walker-type module and a 1000 ton press, is perfectly suited. With this device, high pressures (up to 25 GPa) and high temperatures (up to 2500 C), with simultaneously quite large sample volumes, can be realized. Applying such extreme conditions, new phases with still unknown compositions, new structures with exceptional structural motifs, exciting physical properties, and access to new chemical systems are expected. Due to their increased density, high-pressure phases can posses material properties (fluorescence, hardness, magnetism, piezoelectricity) of great interest. Furthermore, spectroscopic investigations (UV/Vis, IR, Raman, NLO-properties, solid state-NMR) will play a major role in this context. The preparative works, mentioned above, will be completed by in-situ diffraction experiments in laser-heated diamond anvil cells (DACs) and in-situ multianvil experiments, to identify quickly the stability ranges of potential high-pressure phases. By means of our innovative preparative approach towards this nearly untapped field of solid state research, we expect to obtain a wide range of new materials to be thoroughly examined and characterized concerning their potential material properties.

The goal of this project was the exploitation of new rare earth fluorido and fluoride borates, based on the systematic preparation via ambient- and high-pressure/high-temperature syntheses. The ambient-pressure syntheses were performed in conventional high-temperature furnaces and the high-pressure syntheses in a multi anvil device based on a Walker-type module and a 1000ton press. With this device, extreme pressures up to 25 GPa (250,000 bar) and high temperatures of up to 2500 C could be realized. Next to the thermodynamic parameters chemical composition and temperature, which are used from the beginning of chemical synthesis, we are able to use the additional parameter pressure to open up new fields for materials syntheses.At the beginning of our preliminary work, the field of rare earth fluorido and fluoride borates was represented by only two well-characterized structure types (four different compounds overall). Our systematic investigations in the frame of this project led to the discovery of about 25 new fluoride/fluorido borates and about 20 new compounds from neighboured systems like the borates, borophosphates, oxides, and fluorides. As examples from the field of fluoride borates, CeB2O4F, LaB2O4F, and the series RE4B4O11F2 (RE = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er) were synthesized supplemented by the compounds RE2(BO3)F3 (RE = Tb, Dy, Ho). In side reactions, new borates with the compositions La4B10O21, Ce4B14O27, RE2B8O15 with RE = La, Ce, Pr, and Nd, and AnB4O8 (An = Th, U) were discovered. Also new oxide fluorides like HP-REOF (RE = Pr, Nd, Sm, Eu, Gd) resulted from our investigations. Opening up the synthetic strategy to neighboured systems resulted in a new borate phosphate with the composition U2[BO4][PO4]. One of the most remarkable discoveries was the synthesis of a new rare earth oxide with the composition Tb3O5, which was unknown up to now. These results have a remarkable input to the knowledge about rare earth fluoride/fluorido borates in solid state chemistry and materials science. In addition, the side products, e.g. the new binary oxide Tb3O5, are of significant interest for the community of solid state chemists. So, finally this project has a tremendous impact into the field of basic research in materials synthesis under high-pressure conditions.