Negative linear thermal expansion of oblique-angle (monoclinic and triclinic) crystals as a common case

The traditional principle of the mid-twentieth century that 'stronger chemical bonds imply a weaker thermal expansion' appeared to be valid for cubic crystals but failed to explain many cases of real high anisotropic thermal expansion of non-cubic crystals. In particular, the discovery, a quarter of a century ago, of the frequent occurrence of negative linear thermal expansion in monoclinic and triclinic crystals was unexpected. For this reason, it is necessary to consider an additional principle of thermal expansion (symmetrical rearrangement of a crystal structure with temperature). According to both principles, the value of thermal expansion is dictated by the strength of chemical bonds, and the anisotropy of thermal expansion is determined mainly by the tendency of crystal symmetry to increase upon beating, etc. The last 25 [GRAPHICS] years have proved to be a test and development period of thermal expansion theory. The principles mentioned above are considered in the light of their present-day advancements. This allows us to achieve a better understanding of real thermal expansion. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim