Surface vibration correlation with d-electron (hole) per atom ratio (d-bandfilling) in CO chemisorption on supported transition metal particles: Dominance of hund's rule and d-electron degeneracy in real supported catalysts

The findings of Guerra and Schulman (ref.1) of a correlation between force constant of metal-CO chemisorbed species (admolocules) with the number of d-holes per atom (1-number of d-electrons per atom, or d-bandfilling) of the transition metal particle (supported) surface is treated using the ten-fold degenerate Hubbard (ref.2) (TDHM) and Anderson (TDAM) (ref.3) models developed for magnetic and Mott metal-insulator transition problems in transition metals, alloys and compounds. Their experimental finding that via infrared spectroscopy on the CO adsorbed species (admolecule) bond, the infrared surface vibrational frequency, CO bond order and CO bond force constant were nearly linear versus the number of d-holes per transition metal surface atom is easily explained via the TDHM and TDAM via their inherent d-bandfilling dependent (alloy specific) Coulomb and exchange enhanced coupling strength between d-electrons or d-holes (ref.4). Even the Fulde (ref.5) correction to the number of d-electrons (holes) per $\text{surface}$ atom as a function of transition metal particle size does not change the excellent agreement, but merely renormalizes the measured functional properties versus d-hole per atom ratio for a suite of equally fine sized transition metal particles. So it seems that in admolecule chemisorption (at least for CO) the degenerate electronic properties of the surface dominate the vibrational and bonding properties of the admolecular species rather completely.     

Magnetic susceptibility under pressure and anisotropy of electrical conductivity in quasi-one-dimensional (perylene)2(AsF6)0.75(PF6)0.35 × 0.85CH2Cl2

The magnetic susceptibility and electrical conductivity of the quasi-one-dimensional organic metal (Perylene)₂(AsF₆)_0.75(PF₆)_0.35 × 0.85CH₂Cl₂ was studied in the temperature range 3–300 K. The measured susceptibility can be separated into its defect χ_d, core χ_c and enhanced paramagnetic spin susceptibility χ_p components. χ_p is found to decrease upon lowering the temperature or applying pressure, in analogy with results on charge transfer compounds such as TTF-TCNQ. The conductivity ratio $\text{σ}{}_6\text{σ}{}_{\mathrm{\perp }}$ also decreases with falling temperature. The present results are discussed within the context of a model which takes into account band-narrowing (electron localization) due to strong electron-phonon coupling.     

The Meissner effect of polysulfur nitride, (SN)x

The magnetic susceptibility of (SN)_x was measured by the a.c. method. The Meissner effect occurs at 250 ± 3 mK. The real part of the susceptibility nearly saturates at 100 mK and reaches 93 ± 7% of ?$\text{1}\text{4}\text{π}$ at 30 mK for weak field perpendicular to the polymer axis, while for the parallel direction it remains at 65 ± 10% of ?$\text{1}\text{4}\text{π}$. The susceptibility depends strongly on the a.c. magnetic field. There is a peak in the imaginary part of the susceptibility at the transition region to the superconducting state.     

Superconductivity and magnetic susceptibility of TiU alloys

The superconducting transition temperatures of $\text{TiU}$ alloys have been measured. They peak at a temperature of 3.8 K at 20 a/o U. Similar behavior is seen for $\text{Ti}$Cr, $\text{Ti}$Mo, and $\text{Ti}$W indicating that U in Ti behaves like a hexavalent transition metal and is not magnetic. Magnetic susceptibility measurements support this viewpoint.     

Electrical conductivity of (Bi,Pb)2MO4 (M = Pd,Pt) linear chain compounds

Partial oxidation of Pd in Bi₂PdO₄ is achieved by substitution of Pb²⁺ for Bi³⁺ up to Bi₁₉₁Pb₀₀₉PdO₄, partial oxidation is necessary to stabilize the isostructural Pt compound, Bi_1?xPb_xPtO₄ within the range 0.33 ? x ? 0.52. In both cases, the tetragonal cell c parameter, therefore metal-metal distance $\text{(d}{}_{\mathrm{<mtext>M?M</mtext>}}\text{=}\text{c}\text{2}\text{)}$, decreases linearly with increasing mean oxidation degree (MOD) of transition metal atom For the insulator B1₂CuO₄, no substitution occurs Powder electrical conductivity measurements of the partially oxidized compounds show that these materials are semiconductors Platinum compounds exhibit relatively high conductivities $\text{(σ?10 (Ω}\text{cm}\text{)}{}^{\mathrm{?1}}\text{)}$ and low activation energies (?0 02 eV) with small variations with x Palladium compounds exhibit lower conductivities which linearly increases with MOD These electronic properties are comparable with those of the most one-dimensional Pt or Pd chain conductors.     

Magnetoelastic effects in dilute magnetic alloys

Ultrasonic studies of $\text{Cu}$Mn, $\text{Au}$Mn and $\text{Au}$Fe alloys where the concentrations of the transition metal ranges between 1 and 10% show that the longitudinal sound velocity has a marked field dependence at temperatures below the peak in the magnetic susceptibility and a negligible field dependence above it. This suggests that there may be an ordered magnetic state in these alloys.     

The interaction of oxygen with Bi(0001): Kinetic,electronic, and structural features

The interaction of oxygen with clean Bi(0001) was studied for adsorption between 118 and 296 K using LEED, Auger, electron energy loss, and work function measurements. Oxygen adsorption kinetics show an activated process with a dissociative sticking probability (<10^?4) which smoothly decreases over two orders of magnitude up to saturation coverage. Changes in surface electronic properties indicate that an oxide-like bond is formed from the onset of adsorption. There is no evidence for a stable chemisorbed phase. LEED shows simultaneous growth of epitaxial BiO(0001) and a $\text{3}$ overlayer. At 296 K the adsorption terminates after about three equivalent monolayers of BiO(0001). Periodic trends extended from the transition metal series suggest that local and atomic characteristic rather than long-range electronic properties determine the low reactivity of this surface toward O₂.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

Itinerant electron theory of transition metal compounds of mixed valency: La1−xSrxMnO3

We use a simplified model to calculate the electronic band structure of compounds of the type of La_1?xSr_xMnO₃. The model includes two nonequivalent sites for the transition metal ions as well as strong Coulomb and exchange interactions between d electrons. Using the resulting band structure we discuss the correlation between magnetic order and conductivity in these compounds.     

The magnetic properties of RuF5

The structural unit of the pentafluorides MF₅ (M = Ru, Rh, Os, Ir, Pt) consists of M₄F₂₀ tetramers with the transition elements occupying the corners of the rhombus. In the case of RuF₅ the magnetic susceptibility vs. temperature curve shows a broad maximum at T ? 40 K. Neutron diffraction experiments below and above this temperature show that this maximum does not correspond to a magnetic three-dimensional ordering. Theoretical calculations based on isolated tetra-nuclear clusters of identical $\text{S =}\text{3}\text{2}$ spins have been applied. The intracluster exchange interactions are antiferromagnetic. The fitting of the susceptibility data leads to an exchange constant of $\text{J}\text{k}\text{= ?8.3 K}$.     

Magnetism and structural chemistry of ternary silicides: (RE,Th, U)Pt2Si2 (RE = rare earth)

Ternary silicides (RE, U, Th)Pt₂Si₂ have been prepared from the elements. All the compounds (RE= Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and U, Th) were found to be isotypic and crystallize with the primitive tetragonal CePt₂Si₂-type structure closely related to the CaBe₂Ge₂-type. The magnetic properties of these alloys were studied in the temperature range 1.5 K < T < 1100 K and in fields up to 1.3 T revealing a typical Van Vleck paramagnetism of free RE³⁺-ions for temperatures T > 200 K. A nonmagnetic ground state is reflected from the magnetic susceptibility data of CePt₂Si₂, which are interpreted in terms of interconfiguration fluctuations (ICF). The magnetic results of SmPt₂Si₂ (μ_eff = 0.7 BM) compare well with the ideal Van Vleck behavior of Sm₃₊ ions with a $\text{J =}\text{5}\text{2}$ ground state and a low-lying excited first level $\text{J =}\text{7}\text{2}$. At temperatures below 40 K antiferromagnetic ordering is found for (Gd, Tb, U)Pt₂Si₂; whereas in case of (Dy, Ho, Er, Tm)Pt₂Si₂ the onset of ferromagnetism is indicated below 4 K. None of the samples exhibited a superconducting transition above 1.8 K.     

On the magnetic properties of Y(FexCo1−x)2 compounds

The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe_xCo_1?x)₂ compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 μβ if there are least two Fe atoms as nearest neighbours. The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie-Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic interactions on the transition metal moments is discussed.     

One-dimensional Ising chain with generalized molecular field

The behavior of a spin-$\text{1}\text{2}$ one-dimensional nearest-neighbor Ising chain coupled to a generalized internal molecular field is considered. The internal field is shown to be equivalent to long-range spin-spin interactions. An assumed expansion of the internal field in powers of the magnetization results in the possibility of first- or second-order phase transitions and softening in the spontaneous magnetization and specific heat near the transition temperature. The internal energy and magnetic susceptibility are also discussed.     

Constant coupling approximation for antiferromagnets with mixed ferro- and antiferromagnetic interactions

The constant-coupling approximation is extended to an antiferromagnetic spin-$\text{1}\text{2}$ system with two distinct anisotropic exchange interactions. The thermodynamic properties such as the transition temperature, magnetization, susceptibility and specific heat are discussed for three special cases: (i) ferro- and antiferromagnetic Ising interactions, (ii) isotropic ferro- and antiferromagnetic Heisenberg interactions; and (iii) isotropic ferromagnetic Heisenberg interactions and antiferromagnetic Ising interactions, allowing in each case for two different nearest-neighbour coupling constants. Numerical calculations have been performed for a layer structure with z = 6 intraplanar and z' = 6 interplanar nearest neighbours and the results are compared with those obtained in other approximations. Applying the theory to FeCl₂, the exchange constants are evaluated. It is shown that the calculated magnitudes of the interactions strongly depend upon the exchange-interaction model assumed.     

The cooperative Jahn-Teller effect in uranium (4+) hydroxy-sulphate

The magnetic susceptibility measurements of orthorhombic U(OH)₂SO₄ within the temperature range 4.2–300 K have revealed a magnetic anomaly at T_D = 21 K associated with crystallographic transition induced by the cooperative Jahn-Teller effect. Above 21 K the magnetic susceptibility of the uranium (4+) ion corresponds to the electronic ground doublet ${}_{\mathrm{¡}}\text{M}{}_{\mathrm{J}}\text{= ± 2〉}$ confirming thus the antiprismatic symmetry (D_4d) of the crystal field at the uranium site. Below T_D the system of two singlets ($\text{1}\text{√2}$)|2〉 ± ($\text{1}\text{√2}$)|$\text{2}$〉 separated by δ(T) is the ground state of the uranium ion.     

Electric and magnetic properties of the synthetic metal-copper subrodanide Cu(SCN)13

The temperature dependence of the resistance and magnetic susceptibility of the copper subrodanide Cu(SCN)_{$\text{1}\text{3}$} samples were measured at the magnetic field up to 7.3 kG and the temperature range 4.2–400 K. The resistance of the pressed pellet samples linearly increases with the temperature increasing and at 100–150 K the ?(T) line slope smoothly changes. The magnetic susceptibility at zero field linearly decreases with decreasing of the temperature, showing unusual exponential field dependence, which is rather strong at the room temperature and disappears at T → 0. It was concluded that a copper subrodanide is a synthetic metal.     

Concentration dependence of magnetic susceptibility for the system of dilute magnetic impurities

The concentration dependence of the magnetic susceptibility for the system of randomly distributed dilute magnetic impurities in a d-dimensional metal or on a metal surface is studied at zero-temperature under the mean-random-field approximation. It is shown that, if the amplitude of the RKKY-type interaction between the magnetic impurities behaves like $\text{1}\text{r}{}^{\mathrm{n}}$ at a large distance r, the susceptibility is proportional to $\text{c}{}^{\mathrm{<mtext>1?(</mtext><mtext>n</mtext><mtext>d</mtext><mtext>)</mtext>}}$, where c is the concentration of the magnetic impurities.     

On the magnetic properties of Y(FexCo1-x)2 compounds

The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe_xCo_1-x)₂ compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 μ_B if there are at least two Fe atoms as nearest neighbours. The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie-Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic interactions on the transition metal moments is discussed.     

Sodium nuclear magnetic resonance and dynamic second order quadrupole effects in the superionic conductor nazirpsio

The sodium nuclear magnetic resonance interaction in nazirpsio, Na_1+xZr₂P_3-xSi_xO₁₂ (x = 1.9) is dominated by dynamic second order quadrupole interactions. This has been determined by observing the temperature and frequency dependence of the dipolar-quadrupolar linewidth of the $\text{m=+}\text{1}\text{2}\text{?m=-}\text{1}\text{2}$ transition. This behaviour has been anticipated by Werbelow. The motional narrowing of the quadrupole interaction appears to be thermally activated process and agrees with a model of three dimensional diffusion of the sodium ions. The measured activation energies are consistent with those derived from conductivity measurements.     

Soliton lattice to metal: A first order phase transition

$\text{In-situ}$ measurements of the electron spin resonance of [Na⁺_y (CH)^?y]_x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.