2011/07/21 by James R. Rustad, Rustad, James R.
Chemistry · Environmental Science · Materials Science · Physics and Astronomy · #Chemical Synthesis and Characterization #Crystal Structures and Properties #FOS: Physical sciences #Luminescence Properties of Advanced Materials #Materials Science (cond-mat.mtrl-sci) #Zeolite Catalysis and Synthesis #cond-mat.mtrl-sci
paper · pdf · doi:10.48550/arxiv.1107.4261
35 pages 6 figures
arxiv created 2011/07/21 · openalex publication_date 2011/07/21 · arxiv updated 2011/07/22 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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 ΔHfox 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 ΔHfox other oxyacids, such as silicates and sulfates.