Iono-covalent indexes

We must not be misled by the examples of MgO, BaTi03 or diamond most ceramics do not crystallize in a cubic group arrd this implies that the many physical properties that are described by a second order terrsor are not isotropic [NYE 87]. Thermal expansion, optical index, electric and thermal conductivities, permittivity and permeability are at first view anisotropic, which can misinform some metallmgists, because the most cormnon metals (irort, alttminum, copper) are cubic. An effect of the anisotropy of thermal expansion is to create residual stress at the grain boundaries of the polycrystals. Beyond the properties described by a second order tensor, low synunetries combine with the properties of iono-covalent bonds to make dislocations rare and relatively immobile, which explains the lack of ductility and the impossibility of plastic deformation. 

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