THERMAL-EXPANSION OF CHEMICAL-BONDS

Using the bond-valence model, a relationship is developed between the thermal expansion of a chemical bond, its amplitude of thermal vibrati on and its force constant. An empirical expression found between bond valence and the force constants derived from vibrational spectroscopy allows all of these quantities to be predicted from either the expecte d or the observed bond valence. The thermal expansion predicted by the se relations is in excellent agreement with the average expansion obse rved around cations in inorganic solids, but individual bonds are foun d to expand more or less than this depending on strains and constraint s within the structure. Comparison between the theoretical and observe d amplitudes of thermal vibration gives a quantitative measure of corr elation between the thermal motions of atoms that form the bond. The t heory also shows how the parameters used in calculating bond valences from bond lengths should be corrected for temperature.