The thermal expansion of alkali borate glasses and the boric oxide anomaly

A brief review is presented of the major theories for the so-called "boric-oxide anomaly" in alkali borate glasses, and a comparison is effected with recent published information on the structure of B₂O₃ and alkali borate glasses. These indicate serious shortcomings in current theories. An experimental study is reported of the mean linear thermal expansion coefficients between 25°C and -196°C of a series of glasses, prepared at composition intervals of 2 mole% alkali oxide, in all five alkali borate systems. The thermal expansion versus composition relations are characterized by broad, flat minima, with no single composition being uniquely identifiable with the minimum. It is concluded that the compositional variation of the thermal expansion coefficient of alkali borate glasses is not truly anomalous, but rather seems to vary with structural variations in a generally predictable manner. It is suggested that the term "anomaly" be reserved for the more serious problem of explaining the change in boron coordination with composition.