Density Functional Theory (DFT) Calculations For The Structure Of Active Oxides

Computational materials and materials design combined with computer techniques are important contents in materials science. The structures and properties of rutile-phase RuO2 and IrO2, rutile-type ZrxRu1-xO2, fluorite-type RuxZr1-xO2 solid solution and fluorite-phase RuO2 had been discussed using computational and theoretical methods. (1) The structures of rutile-phase RuO2 and IrO2 were investigated by plane-wave pseudopotential density functional theory(DFT) method; (2) The rutile composites by mixing ruthenium dioxide with ZrO2 and the fluorite composites by mixing zirconium oxide with RuO2 were investigated and an attempt were made to obtain the definite relationship between the bond length and the bulk modulus, and then to find out the critical factor of structure on the bulk modulus; (3) Thermodynamic properties under high pressure at high temperature of fluorite-phase RuO2 were investigated by using structural optimization and thermodynamic theory.Results indicated that the data calculated by GGA and LDA were fitted well with those of the equation of state. The total energy calculated by LDA method was greater than that of GGA while the supercell volume was less. There’s a difference of 50 GPa between the results of LDA and GGA. The data of bulk modulus, supercell volume, lattice parameter and the bond length calculated from LDA were more accurate than those of GGA. If the coefficient of thermal expansion was induced to optimize the LDA method of supercell volume calculation, the result was in good agreement with the experiment.As ZrO2 molar content increases from 0% to 100%, the bulk modulus decreases continually from 300.4 GPa to 217.22 GPa. The bond lengths of O-O, M-O and M-M are increase gradually with the increasing of ZrO2, but that they show different tendencies. After a comparison, it was found that the O-O and M-O bonds are the most important for the structure.It’s observed that the lattice parameters and bond lengths of the fiuorite decreased with the replacement of Zr4+by Ru4+in the structure, owing to the relative smaller size of Ru4+. As RuO2 molar content increased from 0%to 100%, the bulk modulus increased continually from 267 GPa to 353 GPa. Three sub-lattice structural data including lengths of O-O, M-O and M-M bonds were calculated and compared.The bond lengths of O-O, M-O and M-M were decreased gradually with the increasing of RuO2, but that they show different tendencies. After a comparison, it was found that the M-O bond is the most important one for the structure. As a result, The relative bond length was chosen as a reasonable factor to describe the structural and the bulk modulus changes as the increasing of the fraction of RuO2. As RuO2 molar content increased 0 to 12.5%, the band gap of RuxZr1-xO2 was decreased from 3.2 eV into 0.8 eV, and the conduction band was expanded nearly twice in width;As lattice parameter a was A.1592 A, the structure of fluorite-phase RuO2 was the most stable under zero pressure at zero temperature. The relative volume ratio V/Vo decreased with the pressure increasing, and also the temperature increasing. The bulk modulus increased with the pressure increasing, while decreased with the temperature increasing. The bulk modulus value for fluorite phase RuO2 was 353.025 GPa. As the pressure reached 8.9 Gpa, The bulk modulus value can be calculated for 390.5281 Gpa, has only little errors the calculated result, was very close to the experimental value (399 GPa). The thermal expansion coefficient increased with the temperature increasing, while decreased with the pressure increasing. It’s found that the transition phase from rutile structure to fluorite structure occurd at a pressure of 60 GPa according to the Birch-Murnaghan equation of state.