DFT Calculation Combined With Thermodynamic Analysis For RuO₂-CeO₂ Composite Oxide

The nature of phase separation of(Ru_1-x,Ce_x)O₂ solid solution and its structure are calculated by a combination of ab initio density functional theory and thermodynamic calculations.The structure parameters,ground energies and formation energies of rutile and fluorite(Ru_1-x,Ce_x)O₂ solid solutions are investigated by optimizting the crystal structures using plane-wave pseudopotential method within density functional theory?DFT?.The electronic structure is analyzed based on the structural optimization.Thermodynamic stability is researched and the miscibility gap curves are plotted based on thermodynamic model.Then the phase diagram of RuO₂-CeO₂ is depicted.Ru_0.6Ce_0.4O₂ samples are prepared by sol-gel method.XRD and in situ TEM characterizations provide proofs of quasi-spinodal decomposition.Results indicate that:1.The calculated result is reliable by compared the calculated lattice constants and unit cell volume of rutile and fluorite RuO₂?CeO₂?with other ab initio results and the JCPDS cards.The band of R-RuO₂ reveals its metallic properties.F-RuO₂ is an indirect band gap semiconductor with the band gap of 0.88 eV.R-CeO₂ is a direct band gap semiconductor with the band gap of 2.129 eV.F-CeO₂ is an indirect band gap semiconductor with the band gap of 2.62 eV.2.For rutile and fluorite(Ru_1-x,Ce_x)O₂,lattice constants a,b,c increase monotonously with the fraction of CeO₂.The unit cell volume V0 increases with the fraction of CeO₂ for rutile and fluorite(Ru_1-x,Ce_x)O₂,slightly deviates from the Vegard-relation.The rutile(Ru_1-x,Ce_x)O₂ has bigger deviation than fluorite(Ru_1-x,Ce_x)O₂.3.Total energy E0 decreases monotonously with the fraction of CeO₂ for both rutile and fluorite(Ru_1-x,Ce_x)O₂.The truth that total energy E0 is larger than the total energy of mechanical mixture of RuO₂ and CeO₂ indicates that spinodal decomposition may occur in rutile and fluorite(Ru_1-x,Ce_x)O₂ system.4.The critical temperature is overestimated when ? is T-independent.The calculated SN and BN curves are more reasonable by using linearly T-dependent,exponential T-dependent and average models.5.The BN curves for rutile and fluorite(Ru_1-x,Ce_x)O₂ solid solution calculated by the three models indicate that the solid solubility of RuO₂ in CeO₂ is larger than that of CeO₂ in RuO₂ at any temperature.6.The phase diagram indicates that the solubility between ruthenium oxide and cerium oxide is very low at temperatures below 1173 K.Solid solution phases,if existed under normal experimental conditions,are metastable and quasi-spinodal decomposition will occur.This phenomenon is proved by the experiment.7.The calculation of IrO₂-CeO₂ in the same method shows that the solid solubility of IrO₂ and CeO₂ is very low,which is consistent with experiment.It shows the study method in this paper is applicable to other systems.The solid solubility of IrO₂ in CeO₂ is smaller than that of CeO₂ in IrO₂.The solid solubility of CeO₂ in RuO₂ is smaller than in IrO₂.The solid solubility of RuO₂ in CeO₂ is smaller than that of IrO₂ in CeO₂.