Breathing Distortion Research Articles

On the competition between ferromagnetic and antiferromagnetic states in Sr 2MnMoO 6

It is argued that the magnetic behavior of Sr 2MnMoO 6 is determined by the existence of two total energy minima corresponding to the metallic ferromagnetic and insulating antiferromagnetic states, which may be nearly degenerate depending on the magnitude of the breathing distortion.

Electronic structure and stability of the ferrimagnetic ordering in double perovskites

Using results of first-principles band structure calculations and a model tight-binding approach, we investigate the local stability of the half-metallic ferrimagnetic (FiM) states in double perovskites ${\mathrm{Sr}}_{2}\mathrm{Fe}M{\mathrm{O}}_{6}$ $(M=\mathrm{Mo},$ Re, and W). In ordered compounds, a generalized double-exchange (DE) mechanism operating in the metallic minority-spin channel, in a hybrid $\mathrm{Fe}\ensuremath{-}M$-derived ${t}_{2g}$ band, always competes with the strong antiferromagnetic superexchange (SE) interactions in the Fe sublattice mediated by virtual electron hoppings into the unoccupied $M(d)$ states. In the local-spin-density approximation (LSDA), the SE mechanism largely prevails and the FiM phase is unstable with respect to a noncollinear spin-spiral alignment. The situation appears to be more generic. So, the onsite Coulomb repulsion between the $\mathrm{Fe}(3d)$ electrons $(\ensuremath{\Delta}U)$ on the top of the LSDA picture suppresses the SE interactions but may also modify some of the DE interactions through the change of the $\mathrm{Fe}\ensuremath{-}M$ hybridization. The total change of the electronic structure, caused by $\ensuremath{\Delta}U$ alone, does not explain the local stability of the FiM state. Therefore, we conclude that the FiM phase cannot be stabilized by purely electronic mechanisms. According to our scenario, that is exactly the situation realized in ${\mathrm{Sr}}_{2}{\mathrm{FeWO}}_{6}.$ An interpretation of the experimental FiM ordering observed in ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6}$ should require an additional mechanism, which destroys the half-metallic character of the electronic structure and suppresses the saturation moment. We consider two possibilities: the alternating breathing distortions of the ${\mathrm{FeO}}_{6}$ and $M{\mathrm{O}}_{6}$ octahedra, and the antisite $(\mathrm{Fe}\ensuremath{-}M$ interchange) disorder. In the former case, the oxygen displacement towards the Fe-sites leads to the partial depopulation of the majority-spin $\mathrm{Fe}{(e}_{g})$ band and thereby activates an effective channel for the ferromagnetic DE interactions, similar to colossal-magnetoresistive manganites. In the latter case, the FiM ordering can be stabilized by SE interactions of a small (less than 10%) amount of Fe impurities with the host atoms. We discuss possible implications of these scenarios to different compounds with emphasis on their magnetic and optical properties.

LOCAL DISTORTIONS IN Ba1-xKxBiO3 OBSERVED IN LOW FREQUENCY PHONON RAMAN SCATTERING

High quality single crystals of Ba 1-x K x BiO 3 ( x =0; 0.15, 0.2, 0.33, 0.42) have been investigated by Raman light scattering in the 5-300 K temperature range. All spectra are interpreted as phonon states that are introduced in Raman scattering by folding back of modes from different points of the zone boundary due to structural distortions. Direct comparison of our spectra with inelastic neutron scattering data provides clear evidence for the existence of local R-type tilt and breathing distortions together with some additional distortion possibly related to the M-point.

Electron-phonon coupling and properties of dopedBaBiO3

We report density-functional calculations based on the local-density approximation (LDA) of the properties of doped barium bismuthates. Using the linear-response approach developed in the framework of the linear muffin-tin-orbital method the phonon spectrum of the ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_{3}$ system is calculated and is compared with the results of the neutron-diffraction measurements. The effect of doping in the calculation is modeled by the virtual crystal and mass approximations. The electron-phonon coupling constant $\ensuremath{\lambda}$ is then evaluated for a grid of phonon wave vectors using the change in the potential due to phonon distortion found self-consistently. A large coupling of the electrons to the bond-stretching oxygen vibrations and especially to the breathinglike vibrations is established. Also, a strongly anharmonic potential well is found for the tiltinglike motions of the oxygen octahedra. This mode is not coupled to the electrons to linear order in the displacements; therefore an anharmonic contribution to $\ensuremath{\lambda}$ is estimated using the frozen-phonon method. Our total (harmonic plus anharmonic) $\ensuremath{\lambda}$ is found to be 0.34. This is too small to explain high-temperature superconductivity in ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_{3}$ within the standard mechanism. Finally, based on standard LDA and LDA+$U$ like calculations, a number of properties of pure ${\mathrm{BaBiO}}_{3}$ such as tilting of the octahedra, breathing distortion, charge disproportionation, and semiconducting energy gap value is evaluated and discussed in connection with the negative-$U$ extended Hubbard model frequently applied to this compound.

Syntheses, Crystal Structures, and Magnetic Properties of Sc19Br28Z4 Compounds with Z = Mn, Fe, Ru, or Os. Structural and Bonding Trends in R16X20Z4-Type Oligomers

The title compounds are obtained from reactions of the appropriate proportions of Sc, ScBr3, and elemental Z in sealed Nb tubing at 800−900 °C. The structures with Z = Mn, Ru, Os were established by single crystal means to be Sc-deficient examples of the Gd4I84+(Gd16I20Mn44-) structure (cubic P4̄3m, Z = 1, a = 10.941(9), 10.9897(5), 11.0032(3) Å) with 3.6(1), 3.0(1), and 2.9(1) scandium atoms in the tetrahedral cation portion, respectively. The cation deficiency with Z = Ru, Os affords 61-electron Sc16Br20Z4 oligomeric units relative to the closed-shell expectation of 60 e-. The dimensions, proportions, and bond orders in the Sc16Br20Z4 oligomers (S4 symmetry) are compared with those in the four yttrium and one gadolinium examples previously known in this or other structure types. The Sc−Mn combination again displays the longest Sc−Z distances. A systematic breathing distortion of R16X20Z4 clusters that increases in the order Y−I, Y−Br, Sc−Br(Ru,Os,Ir) to Sc−Br−Mn and Gd−I−Mn parallels many other comparisons. Extended Hückel MO calculations show these variations depend not only on relative atom sizes but also on appreciable variations in orbital energies and the resulting R−Z mixing in the frontier cluster orbitals (Köckerling and Martin, unpublished). Unusual variations in the magnetic susceptibilities of the Ru, Os examples over the range of 6−300 K are shown to be fit very well by a model based on a Boltzmann distribution of the odd electron between close-lying frontier a1 and t1 orbitals with a temperature-dependent energy separation.

Cooperative Jahn-Teller effect and electron-phonon coupling in La1-xAxMnO3.

A classical model for the lattice distortions of $\lax$ is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d-electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of $LaMnO_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely $\approx 0.6$ eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x-dependence of the transition temperature is argued to occur because added holes produce a "random" field which misaligns the nearby sites.

Model for the insulating behavior of Pb- or K-doped BaBiO3.

A simple model is presented to describe the nonmetallic behavior observed over wide doping ranges of perovskite-type ${\mathrm{BaBi}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{O}}_{3}$ and ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ alloys. The model is based on the idea that the oxygen octahedra endure static distortion waves of a ``breathing'' type, with wave vectors Q(x) nesting to the respective alloy Fermi surfaces as obtained from the energy bands of a virtual-crystal band model. For ${\mathrm{BaBi}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Pb}}_{\mathit{x}}$${\mathrm{O}}_{3}$, Bi-Pb ordering waves are added by placing the Bi atoms onto those octahedral sites that exhibit the largest breathing distortions. Using a simple tight-binding Hamiltonian based on realistic energy-band calculations, the model has been studied on large supercells, testing a multitude of lattice configurations. Experimental consequences are discussed.

Microscopic mechanism of atomic diffusion in Si under pressure.

We have performed the first-principles total-energy calculations on the atomic diffusion of group-V impurities in Si, and have revealed the pressure effect on the activation energy of the diffusion. For the vacancy mechanism, the activation energies for P, As, and Sb decrease with pressure. For the interstitial mechanism, on the other hand, the formation energy of the interstitial impurity shows a general tendency to increase with pressure. Combining the results with the experimental data, we have concluded that there is a dominant contribution of the vacancy mechanism for the As diffusion. The microscopic origin of the pressure dependence is clarified in terms of the local strain around the defects. The negative pressure dependence common to the group-V impurities for the vacancy-mediated diffusion is well explained by the peculiar nature of the isolated vacancy: (a) breathing distortion of the surrounding Si atoms toward the vacancy site, which gives rise to a tensile strain around the vacancy, and (b) the characters of the wave function and the lattice distortion originating from the vacancy, which cause the weak vacancy-impurity interaction. The positive pressure dependence for the interstitial-mediated diffusion is closely related to the atomic structures of the interstitial impurities which cause compressive strain to the surrounding Si-Si bonds.

Structural phase diagram and electron-phonon interaction in Ba1-xKxBiO3.

Using the local-density approximation and the full-potential linear muffin-tin-orbital method we have calculated the total energy of ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ for x=0 and 0.5 as a function of tilting and breathing distortions. For x=0 we find an instability for a combination of both distortions and we correctly reproduce the monoclinic structure. For x=0.5 we correctly find the cubic phase to be stable. The frequencies of the tilting and breathinglike phonons are calculated for x=0 and 0.5. In the cubic structure we calculate the strength ${\ensuremath{\lambda}}_{\mathit{b}}$ of the electron-phonon coupling for the breathing mode as a function of x using a rigid-band approximation. For 0.37x0.5, ${\ensuremath{\lambda}}_{\mathit{b}}$\ensuremath{\approxeq}0.3 and follows the trend of ${\mathit{T}}_{\mathit{c}}$. The superconductivity is conceivably mediated by the electron-phonon coupling.

Structure and dynamics of the DX center in GaAs:Si.

We have carried out local-density functional cluster calculations on the ${\mathrm{Si}}_{\mathrm{Ga}}^{\mathrm{\ensuremath{-}}}$ defect in GaAs. We find that the distortion proposed by Chadi and Chang involving a large Si movement along 〈111〉 breaking an Si-As bond has the same energy as a simple breathing distortion. The calculated local vibratory modes of one Chadi-Chang structure do not agree with those recently assigned to DX from an infrared-absorption experiment. On the other hand, the calculated triplet mode due to the other structure above is in reasonable agreement with these observations. The effective charge of the second type of defect is about three times that of the first. It is proposed that both defects coexist with the Chadi-Chang one being almost infrared inactive.

Structural phase diagram of BaBiO 3 in the potential induced breathing model

Structural stability of BaBiO 3 and of BaKBi 2O 6 is studied using an ionic density- functional-based model. The Experimental structure is reasonably reproduced. It is shown that there is no instability against charge disproprtionation and against breathing distortion separatly, but there is an instability against them both simultaniously. This proves that at least partially the breathing instability is not due to the “nesting” effect. BaKBi 2O 6 is stable in the cubic structure, as in the experiment.

Total-energy gradients and lattice distortions at point defects in semiconductors.

A parameter-free, self-consistent method for calculating the gradient of the total energy (i.e., forces on atoms) of point defects in semiconductors is described. It is shown that under two conditions, (i) the pseudopotential approach and (ii) the inclusion of basis-set derivatives, the Hellmann-Feynman theorem can be applied. The convergence properties of the force calculation are examined, and the method is used to study the breathing distortions of the vacancy in silicon. The results, such as the direction and amplitude of the distortions and the force constants, are compared to other calculations and to available experimental data.

Effects of lattice distortion on the deep Cu center in ZnS

Self-consistent linear combination of atomic orbitals cluster calculations are performed for the substitutional copper impurity in ZnS, showing the electronic changes induced by the impurity. The new bonding forces lead to a relaxation of the neighboring atoms. Total-energy calculations yield a new energy minimum at an outward breathing distortion of 0.21 \AA{}. Consequently the $e$ and ${t}_{2}$ levels are shifted in the gap, and the crystal-field splitting parameter $10Dq=\ensuremath{\epsilon}({t}_{2})\ensuremath{-}\ensuremath{\epsilon}(e)$ is calculated to be 0.68 eV for the relaxed cluster compared to 0.77 eV from experiment.