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The solid solution $\mathrm{Eu}{({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x})}_{4}$ was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds ${\mathrm{EuGa}}_{4}$ and ${\mathrm{EuAl}}_{4}$, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for $x=0--1$ evidenced by magnetization and temperature-dependent specific heat measurements. ${T}_{\mathrm{N}}$ changes nonmonotonously with $x$, and it reaches a maximum around 20 K for $x=0.50$, where the $a$ lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist only for $x=0.50$ and 1. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the $x=0.50$ structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

We report x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements as well as multiplet calculations for Dy, Ho, Er, and Tm atoms adsorbed on Pt(111), Cu(111), Ag(100), and Ag(111). In the gas phase, all four elements are divalent and we label their $4f$ occupancy as $4{f}^{n}$. Upon surface adsorption, and depending on the substrate, the atoms either remain in that state or become trivalent with $4{f}^{n\ensuremath{-}1}$ configuration. The trivalent state is realized when the sum of the atomic correction energies ($4f$\ensuremath{\rightarrow}$5d$ promotion energy ${E}_{fd}\phantom{\rule{0.16em}{0ex}}+$ intershell coupling energy $\ensuremath{\delta}{E}_{c}$) is low and the surface binding energy is large. The latter correlates with a high substrate density of states at the Fermi level. The magnetocrystalline anisotropy of trivalent RE atoms is larger than the one of divalent RE atoms. We ascribe this to the significantly smaller covalent radius of the trivalent state compared to the divalent one for a given RE element. For a given valency of the RE atom, the anisotropy is determined by the overlap between the $spd$ states of the RE and the $d$ states of the surface. For all investigated systems, the magnetization curves recorded at 2.5 K show absence of hysteresis indicating that magnetic relaxation is faster than about 10 s.

X-ray standing-wave measurements, along with first-principles local-density molecular-cluster calculations, have determined the Bi-dimer orientation, location, and bond length for the Si(001)-(1{times}2):Bi surface. The results for Bi directly scale with the covalent radii and with adsorption characteristics of other group-V elements (As and Sb) on Si(001).

In order to quantitatively characterize the effect of motion of the guest atom in type-I clathrates ${X}_{8}$Ga${}_{16}$Ge${}_{30}$ ($X$ $=$ Sr, Ba) on thermal conductivity, electrostatic potential in the crystal was visualized by synchrotron radiation x-ray diffraction. The obtained electrostatic potential clearly exhibited structural aspects of the guest atom, rattling, which is considered as a cause of suppression of thermal conductivity \ensuremath{\kappa}. The parameterized degree of rattling ``rattling factor,'' ${f}_{\mathrm{R}}$, being defined as a volume ratio of the electrostatic interaction region of the guest atom to the spherical volume estimated by the covalent radius of the guest atom, was found to well classify \ensuremath{\kappa}. The \ensuremath{\kappa} monotonously decreases with increasing the ${f}_{\mathrm{R}}$. Consequently, ${f}_{\mathrm{R}}$ was judged as a better criterion to explore high-performance thermoelectric system of the clathrates.

The previous systems of molecular single-, double-, and triple-bond self-consistent, additive covalent radii $R$(AB) $=$ $r$(A) $+$ $r$(B) are now completed with a fit for crystals, where both atoms are tetrahedrally coordinated. A self-consistent primary fit is carried out using elements such as Si and binary compounds such as GaAs or ZnS. Further checks involve also ternary compounds such as chalcopyrites, CuInS${}_{2}$, and the quaternary compounds Cu${}_{2}$ZnSnS${}_{4}$ and Cu${}_{2}$ZnSnSe${}_{4}$, the latter two with both kesterite and stannite structures. The input bond lengths $R$ are taken from experimental or theoretical data. For the 30 elements in the primary fit, the mean-square deviation of the predicted $R$(AB) values is only 0.67 pm for the 48 systems used. The results are compared to earlier radii by Pauling and Huggins [Z. Kristallogr. A 87, 205 (1934)] or by Van Vechten and Phillips [Phys. Rev. B 2, 2160 (1970)], and to the recent molecular covalent radii.

The 2\ifmmode\times\else\texttimes\fi{}2 structure formed on a Ga-adsorbed Si(100) surface is determined using tensor low-energy electron diffraction. I-V curves of the parallel dimer model are in excellent agreement with those of the experiment, indicating that the actual surface has parallel dimer structure. Specific displacements of the topmost two surface layers (the protrusion of the Ga dimer toward the vacuum, the increase of the bond length of the Ga dimer, the stretching of the Si dimer, and the movement of the Si dimer toward the Ga dimer) with the elongation of the Si dimer back bond are observed in the optimized geometry. The Ga-Si bond angle measured from the Si(100) surface plane is recovered with these displacements from that of the ideal geometry where each bond length is assumed to be the sum of Pauling covalent radii. Subsurface layers are also deformed to keep the bond lengths near their bulk values.

We report a quantum-mechanical calculation of impurity effects on adhesion between two materials. Adhesive properties of the Mo(001)/${\mathrm{MoSi}}_{2}$(001) heterophase interface with and without monolayers of C, O, B, S, and Nb impurities are calculated using a first-principles local-density-functional approach. Adhesive energies, peak interfacial strengths, and bonding characteristics are found to be strongly dependent upon impurity-atom type. The interfacial spacings increase in proportion to impurity covalent radii. All of the impurities were found to decrease the Mo/${\mathrm{MoSi}}_{2}$ adhesive energy, with S lowering it by approximately a factor of 2. The substitutional impurities S and Nb decrease the peak interfacial strength, while the interstitial impurities C, O, and B increase it. Our results are discussed in terms of experimental results on impurity effects in adhesion and embrittlement in other metallic systems.

X-ray standing-wave measurements on Cs and Rb adlayers on the Si(111)7\ifmmode\times\else\texttimes\fi{}7 surface are presented. Low [\ensuremath{\approxeq}0.06 monolayer (ML)] and medium (\ensuremath{\approxeq}0.15 ML) coverages have been analyzed. The (111) and the (1\ifmmode\bar\else\textasciimacron\fi{}11) reflections have been used. Since the experimental results clearly indicate that multisite occupancy takes place, the threefold hollow (${\mathit{H}}_{3}$), the threefold filled (${\mathit{T}}_{4}$), and all the dangling-bond sites have been considered as possible adsorption sites. Distinction has been made between the faulted and the unfaulted half of the 7\ifmmode\times\else\texttimes\fi{}7 cell. The Si--alkali-metal bond length has been assumed to be the sum of the covalent radii of the two elements. Our results indicate that both Rb and Cs adsorb preferentially on the ${\mathit{H}}_{3}$ and the ${\mathit{T}}_{4}$ sites. The remaining atoms adsorb most at the atop sites for Rb and at the adatom sites (i.e., on top of the silicon adatom) for Cs.

X-ray reflectivity measurements of the liquid Bi surface are presented and analyzed together with previous liquid Sn results. Published measurements on liquid Ga, In, and K all exhibit a single strong maximum at a wave-vector transfer of the order of the reciprocal of an atomic-diameter, due to surface-induced layering. In contrast, both Sn and Bi exhibit---in addition---a weak broad peak at much smaller wave-vector transfers. This feature is an unambiguous signature of an enhanced electron density in the near-surface region. Possible ways of modeling this enhancement are presented. Once the different surface-roughening effects of thermal capillary waves are accounted for, the surface structure factors of Sn and Bi are remarkably similar. The principal difference between the two is that the depth of the layering below the surface is more than $\ensuremath{\sim}40%$ larger for Bi than for Sn. This is considerably larger than the ratio of their covalent radii which is only $\ensuremath{\sim}10%$. No theoretical explanation can be offered at this time for the surface structure difference between Sn and Bi and other elemental liquid metals studied to date: Ga, In, and K.

We present a structural investigation of both clean and alkali-metal- (Na,Cs) covered Si(100)2\ifmmode\times\else\texttimes\fi{}1 surfaces using photoemission extended x-ray-absorption fine structure. We find that the Na-Si bond length is the same for both low (0.4 monolayer) and 1-monolayer coverages at 2.80\ifmmode\pm\else\textpm\fi{}0.1 \AA{}, close to the sum of covalent radii, supporting the model of covalent bonding between Na and Si, even below 0.5 monolayer. Furthermore, in contrast to III-V semiconductors, Na and Cs do not significantly modify the structure of the Si(100)2\ifmmode\times\else\texttimes\fi{}1 surface. We also comment on the dramatic changes in the Si-Si nearest neighbors during the initial stages of oxygen chemisorption on a sodium-covered silicon surface.

~Received 2 July 2002; revised manuscript received 19 September 2002; published 15 January 2003! First-principle studies on the total energy, electronic structure, and bonding nature of RNiIn (R5La, Ce, and Nd!, and their saturated hydrides ( R3Ni3In3H45RNiInH1.333) are performed using a full-potential linear muffin-tin orbital approach. This series of phases crystallizes in a ZrNiAl-type structural frame-work. When hydrogen is introduced in the RNiIn matrix, anisotropic lattice expansion is observed along @001# and lattice contraction along @100#. In order to establish the equilibrium structural parameters for these compounds we have performed force minimization as well as volume and c/a optimization. The optimized atomic positions, cell volume, and c/a ratio are in very good agreement with recent experimental findings. From the electronic structure and charge density, charge difference, and electron localization function analyses the microscopic origin of the anisotropic change in lattice parameters on hydrogenation of RNiIn has been identified. The hydrides concerned, with their theoretically calculated interatomic H-H distances of ;1.57 A, violate the ‘‘2-A rule’’ for H-H separation in metal hydrides. The shortest internuclear Ni-H separation is almost equal to the sum of the covalent radii. H is bonded to Ni in an H-Ni-H dumbbell-shaped linear array, with a character of NiH2 subunits. Density of states, valence charge density, charge transfer plot, and electron localization function analyses clearly indicate significant ionic bonding between Ni and H and weak metallic bonding between H-H. The paired and localized electron distribution at the H site is polarized toward La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. The calculations show that all these materials have a metallic character.

We report spectral features of localized vibration of hydrogen bound to group-III acceptors in Si. To avoid the Fano effect on the line broadening, we homogeneously doped specimens with group-III acceptors and hydrogen with low concentrations; consequently, the line shapes were well fitted by Lorentzian functions, indicating that the shapes were affected by two dynamical effects, i.e., energy relaxation and dephasing. While the spectra of B-H, Al-H and Ga-H pairs were found to show only one sideband in the lower-frequency region of the main peak, sidebands appeared in both the lower-and higher-frequency regions in the case of In-H. This appearance of the sidebands in the case of In-H pairs was unexpected from a previous model [M. Stavola et al., Phys. Rev. B 37, 8313 (1988)]. By analyzing the linewidth of the main peak at around 6 K, we estimated the energy relaxation times T 1 , which were found to be much shorter than those previously observed for the complexes of the point defect and H in Si. Moreover, we also found a chemical trend: As the covalent radius of the group-III acceptor increased, the T 1 became shorter. Temperature dependences of the linewidth and peak shift of the main peak were analyzed based on dephasing models. The analysis showed that the experimental results cannot be explained by the Persson-Ryberg model in the weak-coupling limit of the anharmonic coupling between the local vibrational mode and hath ones, suggesting that the anharmonic coupling is intermediate or strong.

Harrison's bonding theory, the valence force field (VFF), and an elastic continuum are combined in a study of the substitution energies ${\ensuremath{\Delta}}_{s}$ and local (first-shell) bond lengths ${d}_{1}$ of isoelectronic impurities in semiconductors. Explicit expressions for ${\ensuremath{\Delta}}_{s}$ and ${d}_{1}$ are derived, which enable us to absorb measured elastic constants into the calculation and to study the chemical effects arising from differences in the covalent radii and polarities. Several models based on VFF alone are also derived for comparison. The full theory and at least five VFF models are found toalculated self-consistently in the Hartree approximation. The degenerate valence-band structure and the matching of the wave function at the interface are taken into account. Without magnetic field we calculate the subband dispersion parallel to the interface. The subbands are found to be strongly nonparabolic and spin split. The calculated classical cyclotron effective masses do not agree very well with those found in cyclotron resonance experiments. We have therefore included the magnetic field in the calculation. The B dependence of the Landau levels is found to be strongly nonlinear. The calculated transition energies are partly in very good agreement with experiment. The dependence of the results on areal hole density, doping concentrations, valence-band discontinuity, etc., is also investigated.

The relaxations of hydrogenated Si and Ge (111) surfaces are determined using ab initio self-consistent calculations in a slab configuration. The Si-H and Ge-H bonds are found to be considerably larger than the sum of covalent radii. The substrate relaxations are small and their physical origin can be explained in terms of electronic charge transfer which eliminates the surface dipole moment. The calculated frequencies of the hydrogen vibrational modes are in excellent agreement with experiment. A surface-atom vibrational mode is compared to similar modes in the amorphous hydrogenated materials. The comparison predicts that internal surfaces (microvoids) in the amorphous network are locally much softer than the corresponding crystalline surface configuration.

It is shown that the Wigner-Seitz radii of metal lattices can be reasonably well approximated starting from the electronic densities of the corresponding free atoms at the radial distances where their electrostatic potentials equal their chemical potentials. These radial distances have earlier been shown to be quite similar to the covalent radii of the atoms. Thus there appears to be a significant similarity between the electronic densities in the bonding regions of metal lattices and of the free atoms.