First-Principles Study of Electronic and Magnetic Properties of AA'FeReO_6 and La_2NaRuO_6 Double Perovskite Structures.

【摘要】： Perovskite oxides are perhaps the most studied family of compounds in solid-state chemistry due to their inherent ability to accommodate a wide range of elemental compositions and to display a wealth of structural variations. The transition-metal oxides in the double-perovskite structure can be represented by the A_2BB'O_6 formula (where A = alkaline earth or rare earth and BB' are transition-metals). Most of these alloys have been up to now found to take a rock-salt crystal structure, and ordered one of the alternate perovskite (octahedral) units BO_6 and B'O_6 along three crystallographic axes. There are currently immense research interests in A_2BB'O_6 (BB'=FeRe) double perovskite oxide materials, because of their varied structure, composition and physical characteristics. These materials have attracted intense research activities in many applied and fundamental areas of solid state science and advanced materials research due the exotic properties such as large spin-polarization and Curie temperature (Tc), which is much higher than room temperature, superconductivity, colossal magneto-resistance, half-metallicity, and magneto-dielectricity. The exotic property is characterized by the differentiated conducting response of the spin up and spin down orientations. Recently, among the double perovskites, compounds that contain Ru(V) also have been investigated extensively owing to the wide variety of potential compositions. In this thesis, we have studied the ground state electronic and magnetic properties of AA'FeReO_6 series and La_2NaRuO_6 double perovskites using CASTEP method based on density functional theory. We have systematically varied the cation size in (AA')FeReO_6 double perovskites by substituting the compounds AA' = Ba_2, Ba_(1.5)Sr_(0.5), BaSr, Ba_(0.5)Sr_(1.5), Sr_2, Sr_(1.5)Ca_(0.5), SrCa, Sr_(0.5)Ca_(1.5), Ca_2. By using CASTEP geometric optimization task, we optimized the structures to obtain the configurations that possess the local minimum total energy. This is done by performing an iterative process in which the coordinates of the atoms and possibly the cell parameters are adjusted so that the total energy of the structure is minimized. After geometric optimization, the electronic properties of the systems have been calculated using single point energy calculation within the spin-polarization. We estimated that the half-metallic of these compounds may be caused by the indirect (Re-O-Fe-O-Re) pdd -πcouplings which are simultaneously responsible for their ferrimagnetic character. The inter-atomic distances between the transition atoms (Fe/Re) decrease due to the atomic size reduction, which benefits the d-d' overlapping and therefore increases the hopping amplitude of the kinetically driven magnetic interaction. To our knowledge, there is no complete set of data correlating Tc and the structural parameters obtained by previous studies, but according to the small increase of hybridization term V_(dpd-π)～(d_(Fe-O)×d_(Re-O)~(-4) based on Okimoto et al. relations, we conclude that a slight increase of Tc with doping in the direction of small cation-size series. A significant strong pd-hybridization effect in this systems is evident for the formal high valence Re 5d~(5+) ions, which is supported by a higher covalence bonding seen in Re-O bonds in compare to that of Fe-O ones. Substitutions of small cation size induce a little broadening on energy peaks shown far below E_F, and small splitting between the Fe t_(2g) and Fe e_g levels in up and down spin states, this splitting decreases the majority-spin gap from 1.21 to 0.94eV and from 0.95 to 0.87 eV for the cubic and tetragonal (AA')FeReO_6 samples of the double perovskite studied, respectively. The total magnetic moment observed in these materials mainly comes from Fe and Re species for all compounds. The values of the magnetic moments are closed to 3μ_B/f.u for all compounds. In the case of La_2NaRuO_6 double perovskite, and in order to understand the nature of structure and preferred spin configurations, the ferromagnetic (FM) spin ordering and the type-I anti-ferromagnetic spin ordering are both considered in calculation processes. Finally, we found that the AFM spin ordering structure is more stable than FM magnetic spin ordering structure. Although the optical properties of this material are interesting, to our knowledge, theoretical progresses have not been reported before. In this work, the linear optical properties from the complex dielectric function were also studied using DFT method. Here, the hard norm-conserving pseudo-potential was used to deal with the core states, instead of the ultra-soft pseudo-potential. The calculation results within the FM and AFM spin show that this material is the type of an insulating material, but the AFM state is 0.032 eV lower in energy than that of the FM state. Thus, we predicted that La_2NaRuO_6 is anti-ferromagnetic insulator. The calculated AFM band structure shows a forbidden gap of 0.97 eV. The Ru ions in the cell exhibit the antiparallel spin orderings within the AFM ground state calculation. The calculated total magnetic moment of the FM state is 3μ_B that mainly comes from Ru and oxygen atoms. According to simple relationship between electron energy-loss spectra and dispersive part of the dielectric function in optical properties, the two peaks of energy-loss spectra observed in this study correspond to the roots in dispersive part spectrum, which are agreement to those obtained in the previous reports.