Ab initio equilibrium constants for H2O–H2O and H2O–CO2

Ab initio 6‐31G** electronic structure calculations have been used to determine the minimum energy geometries and vibrational frequencies of molecular clusters of water and carbon dioxide. Application of statistical thermodynamics leads to theoretical equilibrium constants for gas phase dimerization of water and the formation of an adduct of carbon dioxide with water.The low energy vibrations of the clusters lead to much larger contributions to the vibrational partitioning of the energy than do the fundamental vibrations of the monomeric species. A new ‘‘Harmonic‐Morse’’ formula is derived to estimate anharmonicity from optimized harmonic frequencies and two additional values on the potential surface for each vibration. These ab initiocalculations of equilibrium constants are very close to recent measurements and fall within the range of values obtained by other methods. This no‐parameter treatment gives excellent agreement for the equilibrium of H2O–CO2 near the supercritical fluid range of CO2 and suggests that a ‘‘Theory of Significant Clusters’’ may be extended to a model of supercritical fluids which includes the effects of anharmonicity

Charge Order Superstructure with Integer Iron Valence in Fe2OBO3

Solution-grown single crystals of Fe2OBO3 were characterized by specific heat, Mossbauer spectroscopy, and x-ray diffraction. A peak in the specific heat at 340 K indicates the onset of charge order. Evidence for a doubling of the unit cell at low temperature is presented. Combining structural refinement of diffraction data and Mossbauer spectra, domains with diagonal charge order are established. Bond-valence-sum analysis indicates integer valence states of the Fe ions in the charge ordered phase, suggesting Fe2OBO3 is the clearest example of ionic charge order so far.Comment: 4 pages, 5 figures. Fig. 3 is available in higher resolution from the authors. PRL in prin

Incommensurate Charge Order Phase in Fe2OBO3 due to Geometrical Frustration

The temperature dependence of charge order in Fe2OBO3 was investigated by resistivity and differential scanning calorimetry measurements, Mossbauer spectroscopy, and synchrotron x-ray scattering, revealing an intermediate phase between room temperature and 340 K, characterized by coexisting mobile and immobile carriers, and by incommensurate superstructure modulations with temperature-dependent propagation vector (1/2,0,tau). The incommensurate modulations arise from specific anti-phase boundaries with low energy cost due to geometrical charge frustration.Comment: 4 p., 5 fig.; v2: slightly expanded introduction + minor changes. PRL in prin

Orbitally driven spin-singlet dimerization in SS=1 La4_{4}Ru2_{2}O10_{10}

Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the $S$=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in which the orbital physics drives the formation of spin-singlet dimers in a quasi 2-dimensional $S$=1 system.Comment: 5 pages, 4 figures, and 1 tabl

Giant anharmonicity and non-linear electron-phonon coupling in MgB2_{2}; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $\sigma$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$Comment: In this revised version (to appear in PRL) we also discuss the boron isotope effect. Please visit http://www.ncnr.nist.gov/staff/taner/mgb2 for detail

Strongly linked current flow in polycrystalline forms of the new superconductor MgB2

The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One of the central questions is whether this new superconductor resembles a high-temperature-cuprate superconductor or a low-temperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks[2]. Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive[3,4]. These studies of MgB2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains[3]. Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determined by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered