Density functional calculation of the heats of formation of rare-earth orthophosphates

TL;DR

This study uses electronic structure calculations to estimate the heats of formation of rare-earth orthophosphates, revealing a systematic electronic origin for discrepancies with experimental data and insights into ionic radius effects.

Contribution

It provides a detailed computational analysis of heats of formation for rare-earth orthophosphates, highlighting electronic contributions and ionic radius effects not previously quantified.

Findings

Calculated heats of formation are about 40 kJ/mol less exothermic than experimental values.

Discrepancies are mainly due to electronic effects, not zero-point or thermal corrections.

Heats of formation decrease with decreasing ionic radius due to charge localization and lattice energy changes.

Abstract

Electronic structure calculations are carried out to estimate the heats of formation of rare-earth orthophosphates from their oxides. The calculated heats of formation are systematically about 40 kJ/mol less exothermic than the measured values. Based on estimated corrections for zero-point energies and H(298.15)-H(0), the discrepancy is almost entirely electronic in origin. The decreasingly exothermic {\Delta}Hfox with decreasing ionic radius (i.e. LaPO4 more exothermic than ScPO4) results from the higher charge localization on the oxide anion (O2-) relative to the phosphate anion (PO43-). The lattice energy, of course, becomes more negative with decreasing ionic radius for both oxide and phosphate phases, but does so more rapidly for the oxide, making the reaction less exothermic as ionic radius becomes smaller. This effect should carry over to {\Delta}Hfox other oxyacids, such as silicates and sulfates.