Nearest-neighbor Ions Research Articles

Discrete gap breathers in chains with strong hydrogen bonding

We consider a diatomic chain of heavy ions coupled by hydrogen bonds which are sufficiently strong compared with other interactions in the system. In this case, each proton in the hydrogen bond is subject to a single-minimum potential resulting from its interaction with nearest-neighbor heavy ions through the Morse potential that contains soft anharmonicity. This diatomic chain of nonlinearly coupled masses admits discrete breather solutions in the gap of the phonon spectrum. Simple analytical arguments accompanying explicit solutions that demonstrate the existence of the gap breather with only one type of symmetry, namely the odd-parity pattern centered at a hydrogen-bonded proton, are present. These arguments are supported by the numerically exact procedure using the anticontinuous limit. Some other multi-breather solutions in the gap are also obtained exactly from the anticontinuous limit.

Actinyl Ions in Cs2UO2Cl4

The electronic energy levels of the uranyl ion (UO22+) and the neptunyl ion (NpO22+) in the crystalline environment of Cs2UO2Cl4 are studied theoretically and compared with the spectroscopic work of Denning and co-workers. A layered-cluster computational method is used. The valence electrons of the actinyl ion and the nearest-neighbor chloride ions are treated explicitly, the closest cesium ions are replaced by all-electron core potentials, and all ions further away are replaced by point charges. The cluster is approximately spherical overall and contains 1873 ions. For the electrons treated explicitly, we use relativistic quantum chemical theory, including relativistic effective core potentials, corresponding spin−orbit operators, and spin−orbit, graphical unitary group configuration interaction. The effects of the crystalline environment on bond distances, vibrational frequencies, excitation energies, energy splittings, and wave function character are examined. Shifts are generally more accurate than absolute values, and the electron correlation treatment is generally the limiting factor in the accuracy.

Tungsten Atoms and Clusters Adsorbed on the MgO(001) Surface: A Density Functional Study

Adsorption of tungsten clusters Wn (n = 1−4) on the ideal MgO(001) surface has been studied computationally using a scalar relativistic density functional method and a gradient-corrected exchange-correlation functional. Structure and energetic features of the adsorption complexes Wn/MgO(001) have been analyzed. The oxide surface was represented by cluster models embedded in a large array of point charges (PCs). To reduce the artificial polarization of oxygen anions in the immediate vicinity of positive PCs, the cations at the cluster boundaries were treated as Mg2+ ions at either all-electron or pseudopotential (PP) level. Compared to the all-electron + PC embedding, the significantly more economic PP + PC approach is demonstrated to impose cluster model boundary conditions appropriate to the ionic oxide MgO. The cluster size dependence of the adsorption properties is found weak. Like other transition metal clusters considered previously, tungsten species favor adsorption sites in the proximity of oxygen centers of MgO(001). Rather small calculated adsorption-induced deformations of the tungsten clusters manifest notably stronger W−W bonds compared to W−O bonds between metal and substrate. The tetrahedron shape of W4, most stable in the gas phase, is calculated to be energetically preferred also in the adsorbed state, in particular over a square-planar adsorbate. This finding is at variance with a model of two-dimensional W4 clusters on MgO(001) derived from a recent high-resolution electron microscopy investigation (Tanaka, N., et al. Surf. Rev. Lett. 1998, 5, 723). The configuration of W3 with two W atoms located close to two nearest-neighbor oxygen ions is favored over that where two W atoms are close to next-nearest-neighbor substrate anions. In both cases, the adsorbed W3 cluster tilts considerably from an upright orientation.

Ab initiostudy ofMF2(M=Mn,Fe,Co,Ni)rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the groundstate structural, electronic, and magnetic properties of the rutile-type compounds MF2 ( M5Mn, Fe, Co, and Ni!. All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the @001#, @111#, and @100 #~ or @010#! directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed. Several transition-metal ~TM! oxides and fluorides crystallize in the rutile-type structure, which is the simplest and most common MX 2 structure, where each M atom is octahedrally coordinated by the X ligands. Slight distortions of the MF6 structural units are present due to electrostatic and/or Jahn-Teller effects. The physical properties of the TM compounds with rutile-type structure vary considerably: from ionic or covalent insulators to metals, from diamagnetic to strong ferromagnetic or antiferromagnetic semimetallic or insulating systems. This feature is a challenge to modern theoretical methods. In this context a series of compounds whose physical properties exhibits a smooth variation from one member to the other provide a suitable test of theory. This is precisely the case of the MF2 ( M5Mn, Fe, Co, and Ni! family of ionic compounds in which the interplay between structural parameters, chemical bonding, and magnetic coupling may be expected to display such a smooth variation. Two different ab initio periodic approaches are commonly used in solid-state physics to study TM compounds: the Hartree-Fock, 1,2 and the density-functional ~in its local or gradient corrected variants 3 ! schemes. In the latter both the exchange and correlation parts of the electron-electron interaction are taken into account in an approximate way. In the former the electron exchange part is treated exactly while electron correlation is neglected. When the spin-unrestricted form of the Hartree-Fock approach ~UHF! is used in the study of insulating magnetic systems, the correct sign and a reasonably good description of the magnitude of the magnetic coupling constants usually results. The ab initio UHF

Phase separation in inhomogeneous mixed-valence systems: Application to manganites

Manganites are inhomogeneous mixed-valence systems in which the Mn ions coexist in two magnetic configurations, Mn4+ and Mn3+. We consider a simple alloy-analogy model with a charge interaction between nearest neighbor Mn ions. In addition there is an antiferromagnetic (AF) (superexchange) interaction between nearest neighbor ions of equal valence, while ions of different valence are coupled ferromagnetically (FM) (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of AF and charge ordered FM domains. In a magnetic field the FM domains increase at the expense of the AF ones.

EPR studies of Cu2+ in K1−xLixTaO3

Electron paramagnetic resonance (EPR) of copper centers in KTaO3:Li:Cu single crystals have been studied. Superhyperfine (SHF) structure of an exchange pair Cu2+-Cu2+ center, associated with the eight surrounding nearest-neighbor potassium ions was found. The EPR signal behaviour with the temperature may characterize the dielectric properties of crystals at the microwave frequency.

Ab initio study of hydrogen dissociation at a surface divacancy on the (001) MgO surface

An embedded-cluster Hartree–Fock approximation is adopted for simulating the heterolytic dissociation of hydrogen at a “tub” divacancy of MgO corresponding to the removal of two nearest neighbor ions at the (001) surface. Two stable dissociated configurations are identified, where the hydroxyl is formed either at a surface four-coordinated oxygen (Tα) or at the five-coordinated oxygen at the bottom of the tub (Tβ). In both configurations, the other atom of the molecule forms a hydride-vacancy complex (HV) near the site of the missing oxygen. The results are discussed with reference to a previous investigation concerning the isolated anion vacancy, where the dissociated configuration (Fα) was similar to Tα. The dissociation process in the three cases is shown to critically depend on the electrostatic field at the defect and on steric hindrance constraints. Tα and Tβ are stable with respect to the undissociated molecule by a few kcal/mol, but only the latter can be reached with low activation energy; Fα is thermodynamically unstable. The dissociation of the HV complex in the presence of ultraviolet radiation is next studied, resulting in the removal of a neutral H atom, while a lone electron remains trapped at the vacancy. This paramagnetic state is characterized, and its features compared with those obtained from recent experiments concerning the Fs(H) color center. On the whole, only the Tβ model is in fair agreement with experimental evidence concerning that center, although definite discrepancies are left.

Nature of Ce-Ce Interaction in CeB6and Its Consequences

The interaction between nearest-neighbor Ce ions mediated by conduction electrons is carefully analyzed for CeB 6 by fully taking into account the symmetry connecting two Ce ions as well as the nature of Γ 8 wave functions. It is shown that the interaction Hamiltonian resulting from the analysis has a simple form containing a very few parameters because of constraints due to the symmetry. It is concluded quite generally from the interaction Hamiltonian that when the AF quadrupole-quadrupole interaction of Γ 5 -type is strong, the AF octupole-octupole interaction of T x y z type is equally strong. Thus the present analysis gives a strong support to the importance of T x y z octupoles in CeB 6 .

Phase separation in inhomogeneous mixed-valence systems

A simple alloy-analogy model for inhomogeneous mixed-valence systems is considered in the context of manganites. It is assumed that the Mn ions coexist in two magnetic configurations, ${\mathrm{Mn}}^{4+}$ and ${\mathrm{Mn}}^{3+},$ and that they interact via a nearest-neighbor charge interaction. In addition, two nearest-neighbor ions with equal valence interact antiferromagnetically (superexchange), while ions of different valence are coupled ferromagnetically (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of antiferromagnetic and charge ordered ferromagnetic domains.

Electron paramagnetic resonance of a cation antisite defect in ZnGeP2

Electron paramagnetic resonance (EPR) has been used to characterize a new donor center in ZnGeP2 crystals. An equally spaced triplet of lines with a 1:2:1 intensity ratio is observed at g=2.0026 when a crystal is exposed to 633 nm light (below band gap) while at temperatures below 50 K. These EPR lines are approximately 70 G wide and they exhibit only a slight angular dependence (their separation changes from 189 to 179 G as the magnetic field is rotated from the c axis to the a axis). The proposed model for the new donor is a germanium ion on a zinc site. This antisite center is paramagnetic (S=1/2) in the singly ionized state (i.e., GeZn+) and the triplet nature of its EPR spectrum is attributed to nearly equal strong hyperfine interactions with two of the nearest-neighbor phosphorus ions.

High-pressure X-ray diffraction study on the structure of NaCl melt using synchrotron radiation

Molten NaCl was analyzed by high-pressure X-ray diffraction experiments using synchrotron radiation up to 5 GPa and 1600 °C along the melting curve. The interference function Qi(Q) and the correlation function g(r) were derived from the diffraction data. The first-neighbor distance r 1 is about 2.7 Å and the second-neighbor distance appears around 1.4r 1 -1.5r 1 . The coordination number, CN, of the nearest neighbor ions increases with pressure from 3.5 at 0.1 MPa to 4.5 at 5 GPa. This is the evidence that the NaCl melt has a B1-like structure with large vacancies over this pressure range and becomes densified by an increase in CN as a result of second neighbor compaction.

An ab initio approach to bulk and surface properties of many-body energies and adsorptivity in MgO crystal

A finite MgO crystal, in which Coulomb potential in the bulk region closely approximates the Madelung potential in the host crystal, is constructed. A method was then suggested for generating the crystal surface and matrix Hartree–Fock calculations were carried out for isolated clusters and for clusters embedded in the bulk and the surface of the crystal. Overlap effects of nearest neighbor ions are tested. The convergence properties of many-body energies were examined as well as the effective pair potentials for some symmetrical distortions in the central region. The adsorptivity of Mg 2+ and O 2− ions as well as the nucleation mechanism of (Mg 2−O 2−) 2 tetramer on crystal surface are discussed in relation to crystal growth and screw dislocation.

Low temperature photoluminescence properties of p- and n-type Al x Ga 1- x As with x>0.42

The photoluminescence (PL) spectra of n- and p-type Al x Ga1−x As (x>0.42) grown by metalorganic vapor phase epitaxy (MOVPE) and liquid phase epitaxy show typically a broad PL band (BB) centered about 300meV below the near band-gap PL lines. In the MOVPE grown samples the BB is composed out of four lines. The BB intensity increases with the doping level and dominates the spectrum at concentrations > 1017 cm−3. The temperature dependence of the BB intensity shows two distinct maxima at ≈ 19K and ≈ 80K. Hydrogenation of MOVPE grown samples at 170°C reveal an effective passivation of the shallow acceptors and the centers associated with the BB line. On the contrary hydrogenation at temperatures >250°C leads to an increase in the BB intensity which is related with the evaporation of As from the surface and the generation of VAs in the layers at these temperatures. Our results suggest that the BB is related with isoelectronic centers formed when VAs and CAs or SiAs associate to form next nearest neighbor ion pairs. The PL is then due to a recombination of excitons in analogy to the case of (Zn, O) isoelectronic complexes in GaP.

Electronic states and magnetic properties of edge-sharing Cu-O chains

The electronic states and magnetic properties for the copper oxides containing edge-sharing Cu-O chains such as Li$_2$CuO$_2$, La$_6$Ca$_8$Cu$_{24}$O$_{41}$ and CuGeO$_3$ are systematically studied. The optical conductivity $\sigma(\omega)$ and the temperature dependence of the magnetic susceptibility $\chi(T)$ for single crystalline samples Li$_2$CuO$_2$ are measured as a reference system and analyzed by using the exact diagonalization method for small Cu-O clusters. It is shown that the spectral distribution of $\sigma(\omega)$ is different between edge-sharing and corner-sharing Cu-O-Cu bonds. The charge transfer gap in edge-sharing chains is larger than that of high-$T_{c}$ cuprates. The exchange interaction between nearest-neighbor copper ions in edge-sharing chains $J_1$ depends sensitively on the Cu-O-Cu bond angles. In addition to $J_1$, the exchange interaction between next-nearest-neighbor copper ions $J_2$ has sufficient contribution to the magnetic properties. We calculate $J_1$ and $J_2$ for all the copper oxides containing edge-sharing Cu-O chains and discuss the magnetic properties.

Magnetic excitations from the singlet ground state in the S=1/2 quasi-one-dimensional system Sr14-xYxCu24O41.

Inelastic neutron scattering measurements have been performed on the S=1/2 quasi-one-dimensional system ${\mathrm{Sr}}_{14\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Cu}}_{24}$${\mathrm{O}}_{41}$, which has both simple chains and two-leg ladders of copper ions. Strong magnetic inelastic peaks, which originate from the simple chains, have been observed at 9--14 meV in ${\mathrm{Sr}}_{14}$${\mathrm{Cu}}_{24}$${\mathrm{O}}_{41}$. These spin gaps have been confirmed to originate from a dimerized state of the chain. The dimers are formed between spins which are separated by 2 and 4 times the distance between the nearest-neighbor copper ions in the simple chain. In addition, a weaker magnetic inelastic peak around 11 meV, which originates from a dimerized state in the ladder, has also been observed. These dimers are formed between the nearest-neighbor copper ions which are connected by the interladder coupling. We have also studied the effect of ${\mathrm{Y}}^{3+}$ substitution for ${\mathrm{Sr}}^{2+}$ site on the dimerized states. It was found that the yttrium substitution suppresses the gap energies drastically. Possible origins of the dimerized ground state and the excitations are discussed. \textcopyright{} 1996 The American Physical Society.

Ce3+, Fe3+-Induced Optical Absorption in Ce, Fe:YAG Prepared by Coprecipitation

To investigate optical transitions induced by Ce3+ and Fe3+ in garnet, Ce, Fe-substituted Y3Al5O12 (Ce, Fe:YAG) ceramics have been successfully prepared by urea coprecipitation and subsequent heat treatment at temperatures as low as 820–880° C. Absorption spectra of Y2.9Ce0.1Al5-x Fex O12 (x=0–1.0), estimated by diffuse reflection measurements, showed that, as x increases, the absorption peak at h ν=2.7 eV due to the 4f–5d transition in Ce3+ remains unchanged, while a new broad absorption peak occurs at around 1.4 eV. These results indicate that the transition at 1.4 eV observed in Ce-substituted iron garnet, which strongly enhances the magneto-optical effect, is not the 4f–5d transition in Ce3+ shifted by the influence of the next nearest neighbor ions.

Continuous-wave and pulsed EPR studies of Cr2+ defects in CaF2.

${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{CrF}}_{3}$ doped ${\mathrm{CaF}}_{2}$ single crystals have been studied by cw-EPR and electron spin echo envelope modulation (ESEEM) techniques. In both cases ${\mathrm{Cr}}^{2+}$ ions enter the fluorite structure at substitutionally distorted cation sites. For ${\mathrm{Cr}}_{2}$${\mathrm{O}}_{3}$ doped crystals, X and Q band EPR measurements at temperatures between 4 and 200 K allowed us to determine the ion symmetry and spin Hamiltonian parameters, such as the g factors and the zero-field splitting tensors, as well as the $^{53}\mathrm{Cr}^{2+}$ hyperfine tensor. Electron spin echoes (ESE) were detected on that system at temperatures below 20 K. The analysis of the ESEEM and cw-EPR superhyperfine (shf) structure lead us to propose a model composed of two nearest-neighbor fluoride ions placed in the (110) plane containing the ${\mathit{C}}_{2}$ axis with the ones opposite substituted by an ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion. For ${\mathrm{CrF}}_{3}$ doped crystals the distortion is also orthorhombic but the shf interaction is with four fluoride ions presumably in the (110) plane perpendicular to the z defect axis. No ESE was detected for this system. A linewidth narrowing effect was observed. We assign this distortion to a dynamical Jahn-Teller effect corresponding to ${\mathit{T}}_{2\mathit{g}}$\ensuremath{\bigotimes}(${\mathit{t}}_{\mathit{g}}$+${\mathit{e}}_{\mathit{g}}$) coupling stabilized by lattice stresses. \textcopyright{} 1996 The American Physical Society.

Pressure-induced structure change of molten KCl

Abstract Energy-dispersive X-ray diffraction experiments of molten KCl under high pressure have been carried out by using synchrotron radiation. The diffraction profiles of molten KCl were acquired just above the melting temperature of KCl up to 4 GPa. The reduced structure factor S(Q)'s for molten KCl do not show any change in their primary features, except for a gradual increase in the first peak intensity with increasing pressure. This implies that molten KCl does not show a first-order phase transition, such as the B1-B2 transition, found in solid KCl, but that the local structure in molten KCl must be changed by compression. According to a molecular-dynamics simulation, this change of S(Q) can be explained by a continuous increase in the coordination number of the nearest-neighbor ions in molten KCl with pressure.

Embedded molecular cluster modeling of Tl 0(1), In 0(1) and Ga 0(1) centers in potassium chloride

Ab initio, embedded-cluster RHF-SCF-LCAO molecular-orbital calculations were performed on both ground and excited states of the Tl 0(1) center in potassium chloride. Effective core potentials and valence orbitals were employed on the thallium atom and its nearest-neighbor chlorine ions. The remaining ions in the cluster were represented either by bare effective core potentials or by point ions. Spin-orbit effects were calculated perturbatively in the intermediate-coupling regime. External interactions of the molecular cluster were represented by pair potentials, and simultaneous relaxation of the cluster and surrounding lattice was accomplished with a modified lattice-statics program. Optical absorption energies for transitions to the first two excited states, the optical emission energy and a ground-state vibrational frequency were determined successfully. A simplified model based on a single-center expansion and fixed point-ion lattice, validated for the Tl 0(1) center, was also employed to determine the optical absorption energies of the analogous In 0(1) and Ga 0(1) centers in potassium chloride and to explain the non-radiative de-excitation of these centers.

Determination of the Piezospectroscopic Coefficients for Chromium‐Doped Sapphire

The coefficients of the piezospectroscopic tensor for a 0.05 wt% ruby are evaluated by direct measurement of the frequency shift for uniaxial compression along the three principal crystallographic directions and for loading in the three principal shear directions. Although the numerical value of the trace of the tensor is the same äs previously reported, the piezospectroscopic coefficients along the c and m directions are slightly different. Contrary to expectation from the rotational symmetry of sapphire about the c axis, the coefflcient along the a direction was found to be different from that along the m direction. It also varies nonlinearly with strain in the a direction. The difference is attributed to the applied stress breaking the rotational symmetry about the c axis and is interpreted in terms of the relative motion of the Cr3+ ion with respect to the nearest‐neighbor oxygen ions. An additional finding is that there is a signiflcant difference in the piezospectroscopic tensor for the Rl and R2 lines. This will facilitate a more complete determination of the stress tensor from fluorescence measurements than has hitherto been possible.