Vegard's law: a fundamental relation or an approximation?

Vegard' s law has been used extensively in mineralogy, metallurgy and materials science for the past six decades. According to the law, unit cell parameters should vary linearly with composition for a continuous substitutional solid solution in which atoms or ions that substitute for each other are randomly distributed. Although the law was postulated on empirical evidence, several cases of both positive and negative deviations from this law have been documented. Its theoretical foundations have not been critically explored. Presented in this communication is an analysis of the law within the framework of solution thermodynamics. It is shown that the deviation from Vegard's law is expected even for thermodynamically ideal solutions when there is a significant difference in lattice parameters of the pure components. The law should be reclassified as an approximation valid for specific conditions. The approximation is valid for ideal solutions when the lattice parameters of the pure components differ by less than 5 %. For solid solutions with positive deviations from ideality, there will always be positive deviations from Vegard's law. For solid solutions with moderately negative deviations from ideality, positive deviation from linearity of lattice parameters caused by size mismatch can be compensated for by the attractive interaction between the components, resulting in compliance with Vegard's law.