New Aspects of Borate Chemistry under High-Pressure
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New Aspects of Borate Chemistry under High-Pressure H. Huppertz Institute for General, Inorganic, and Theoretical Chemistry, Leopold-Franzens-University Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
ABSTRACT At the moment, over 1100 borate crystal structures are listed in the Inorganic Crystal Structure Database ICSD. High-pressure investigations in this field are rare and were mainly performed from a geological point of view. Starting in 1999, systematic high-pressure / hightemperature experiments up to maximum pressures of 16 GPa have been carried out, using the multianvil technique to explore the synthetic possibilities in the field of borates under extreme conditions. This approach led to a variety of interesting new materials, which are unattainable via conventional solid state syntheses. The communication will give a short survey of the most important results in the last years.
INTRODUCTION Our research interest aims mainly at the structural behaviour and the synthesis of new inorganic materials under extreme conditions of pressure and temperature with a special view on borates. In these compounds, boron coordinates to oxygen in form of planar or non planar BO3 groups as well as BO4 tetrahedra. Due to the fact that nearly no synthetic work had been done under high-pressure / high-temperature conditions, we entered a new fruitful research area in the last years. We obtained several new compounds, which are presented in the following. The first part deals with the synthesis of new polymorphs, the second part with compounds exhibiting new compositions. All portrayed compounds were synthesized in a Walker-type multianvil press in combination with a 1000-ton press. The starting materials for the reactions were boron oxide and the corresponding metal oxides. A precise description of the general synthetic details can be found in reference [1]. SYNTHESES OF NEW HIGH-PRESSURE POLYMORPHS Investigating the high-pressure / high-temperature synthesis of new rare-earth and transition metal borates, several new high-pressure polymorphs of known compositions, e.g. β– MB4O7 (M = Ca [2], Zn [3], Hg [4]), χ–REBO3 (RE = Dy, Er) [5], and ν–DyBO3 [6] were discovered. Furthermore, in the field of rare-earth meta-oxoborates we were able to add two new modifications γ–RE(BO2)3 (RE = La–Nd) [7, 8] and δ–RE(BO2)3 [9, 10] (RE = La, Ce) to the already existing modifications α– and β–RE(BO2)3 [11, 12]. Figure 1 and 2 show the crystal structures of the γ– and δ–modifications.
Figure 1. Structure of γ–RE(BO2)3
Figure 2. Structure of δ–RE(BO2)3
These investigations led to fundamental insights into the structural behaviour of oxoborates under high-pressure conditions. Especially the coordination of the boron and the rareearth atoms roused our interest. Meanwhile, we found out that oxoborates, synthesized under conditions exceeding 7 GPa, exhibit boron exclusively in the tetrahedral coordination. In the field of nonlinear optical (NLO) materials, used to generate new lasers with frequencies that cannot be obtained direct
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