Thermochemistry of monocharged anion substitutions in ionic solids

Modification of materials to achieve specific changes in their physical and chemical properties often involves the substitution of ions. While this process is commonly discussed in structural terms, our recent publication focussed on exploring the thermochemical consequences, including enthalpy, entropy, heat capacity, and formula unit volume, associated with substituting monocharged cations for sodium ions as a reference set. In the current study, we extend our analysis to investigate the consequences of substituting monocharged anions, specifically the halides F⁻, Br⁻, and I⁻, as well as H⁻, OH⁻, and NO₃⁻, for chloride anions. This exploration is conducted through least-squares regression analysis of data obtained from 431 chloride ion-exchanged materials. In the case of cation substitutions, the regression trendlines for different substitutions appear to be roughly parallel to each other but vertically displaced. For anion substitutions, however, the trendlines for enthalpy and formula unit volume exhibit a fan-like spread from their data origin. We delve into the reasons behind this observed difference. A detailed analysis of a few outliers is undertaken to identify potential reasons for the discrepancies. These findings contribute to a better understanding of the implications and variations in ion substitutions, shedding light on the intricacies of material modification processes.