Density functional calculations of the enthalpies of formation of rare-earth orthophosphates

Abstract

Electronic structure calculations are carried out to estimate the heats of formation of rareearth 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 H f ox with decreasing ionic radius (i.e. LaPO 4 more exothermic than ScPO 4 ) results from the higher charge localization on the oxide anion (O 2-) relative to the phosphate anion (PO 4 3-). 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 H f ox other oxyacids, such as silicates and sulfates. a Ni et al., 1995; b antiferromagnetic electronic states; c ferromagnetic electronic state

Defect formation energies in A 2 B 2 O 7 pyrochlores

“…Because the strongly correlated 4f electrons make the straightforward application of DFT problematic, we used two approaches which we had successfully applied to study the monazite-type lanthanide-phosphates [50,51]: the DFT + U method with the Hubbard U parameter derived ab initio using the linear response method of [52] and the f-in core approach in which f electrons are modeled by the pseudopotential. The second approach results in energies and structures that are usually better than when f electrons are computed explicitly, and consistent with the DFT + U method [50,53,51], being at the same time computationally more stable. [54] performed DFT calculations of the O-D transition temperatures for a series of A 2 B 2 O 7 pyrochlores and obtained temperatures that are systematically lower by 250 K than the measured values.…”

Electronic structure and optical properties of TbPO4: Experiment and density functional theory calculations

“…Today, theoretical studies have become effective methods to study the electronic structure, optical properties, crystallographic evolution, morphology, and the reaction mechanism [11][12][13][14][15]. Recently, density functional calculations are used to investigate the enthalpies of formation of rare-earth monophosphates and to make a comparative analysis of their experimental and theoretical vibrational dynamics [16,17]. To our knowledge, there are not any theoretical results on the electronic and optical properties of TbPO 4 , including the dielectric function, absorption coefficient, reflectivity, and energy-loss function.…”

DFT and two-dimensional correlation analysis methods for evaluating the Pu3+–Pu4+ electronic transition of plutonium-doped zircon

“…As one of several crystalline phases in glass ceramic waste, crystalline zircon (ZrSiO 4 ) has been proposed as a durable ceramic for the immobilization and disposal of excess weapons-grade Pu in the US and high-actinide waste in Russia [8][9][10]. However, numerous quantitative scientific techniques [11][12][13][14][15] must be applied to understand the often complex nature of these systems. The damage caused by Pu doping must be studied for a wide variety of reasons, given the complex structural phase and electronic transitions in zircon [16,17].…”