DIELECTRIC-CONSTANTS AND THE OXIDE ADDITIVITY RULE COMMENTS ON A RECENT INVESTIGATION OF MGAL2O4 SPINEL

The concept of the additivity of molecular polarizabilities has bern shown to provide a powerful means of rationalizing experimental measurements of the dielectric constants of oxides and of recognizing anomalous data. It is pointed out that the deviant behaviour of MgAl2O4 spinel at room temperature is readily accounted for by an imbalance in the anti-ferroelectric arrangement of permanent dipole moments which arise naturally from its crystal structure. At higher temperatures the dipole moment per octahedral site, calculated to be 7.56 X 10(-31) C m, may give rise to a much larger dielectric effect if these dipoles arise through a mechanism similar to that operating in the isomorph MnFe2O4.