Strong anisotropic thermal expansion in borates

The paper summarizes results of high-temperature single crystal X-ray diffraction studies for six borate structures and high-temperature powder X-ray diffraction data for about 50 borates. The characteristics on thermal expansion of 50 borates of univalent, bivalent and trivalent metals of different dimensionality and anion structure are listed. The basic principles of high-temperature borate crystal chemistry emerge from these studies and the regularities of borate thermal behavior are revealed. On heating the BO3 and BO4 polyhedra and rigid groups consisting of these polyhedra practically do not change their configuration and size but sometimes rotate like hinges exhibiting greatly anisotropic thermal expansion including linear negative expansion. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim