Z.J. Wu

A Ti-V-based bcc phase alloy for use as metal hydride electrode with high discharge capacity.

The electrochemical characteristics of single bcc phase Ti-30V-15Cr-15Mn alloy were investigated. It was demonstrated that the single bcc phase alloy has high electrochemical discharge performance at high temperature. Its discharge capacity is closely related with temperature and discharge current. The first discharge capacities of 580-814 mAh g(-1) of the alloy powder were obtained at discharge current of 45-10 mA g(-1) in 6 M KOH solution at 353 K. Although the electrochemical cycle life of the alloy is unsatisfactory at present, it opens up prospects for developing a new hydrogen storage alloy with high hydrogen capacity for use as high performance metal hydride electrodes in rechargeable Ni-MH battery.

The bond ionicity in (R = Pr, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm)

(R = Pr, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm) has been studied using complex chemical bond theory. The results indicated that with the decreasing of R radius, the ionicities for all considered types of bond decrease. This is in good agreement with the experimental fact that decreases with the decreasing of R radius. with no Ba-site Pr in this calculation is also predicted to be a superconductor. This supports the conclusion obtained by Blackstead et al. The ionicity for each bond obeys the following order: Ba - O > R - O > Cu(2) - O(1) > Cu(2) - O(2,3) > Cu(1) - O(4) Cu(1) - O(1).

Density functional study of W2 and W3 clusters

Abstract Equilibrium geometries, electron affinities and dissociation energies of W 2 , W 2 − , W 3 and W 3 − clusters and harmonic vibrational frequencies of W 2 and W 2 − clusters were studied by density functional methods B3LYP, B3PW91, BLYP, BHLYP, B3P86 and MPW1PW91. The results indicate that singlet state is the ground state for W 2 cluster, and doublet for W 2 − cluster. For W 3 and W 3 − clusters, triplet with D 3h symmetry and doublet with C 2v symmetry are the ground state, respectively. Comparing among different methods, the results indicate that the calculated quantities, such as electron affinity and dissociation energy, are sensitive to the methods used and cluster size.

Geometries and electronic properties of AunPdm (n=1–4, m=−1, 0, 1) clusters

Abstract Possible conformers for Au n Pd m ( n =1–4, m =−1, 0, 1) clusters have been presented and studied by use of density functional theory. The results indicate that for n =2, linear conformer with C ∞v symmetry is the most stable for anion species, while for cation and neutral species, conformer with C 2v symmetry is the most stable. For n =3, 4, conformers with C 2v symmetry (kite-shape) are energetically favored. The calculated electron affinities (EAs) and vertical detachment energies (VDEs) are in good agreement with experiments for n =1–4. It is also interesting to note that for even n ( n =2, 4), the most stable conformers do not give the best agreement between calculated and experimental EA and VDE values, while for odd n ( n =3), the lowest energy conformer also gives the best agreement. The ionization potentials (IPs) of Au n Pd clusters are calculated as well.

Potential energy surface of aluminum and tungsten dimers

Abstract Potential energy surface of AlW and W 2 clusters has been studied by density functional theory. The results indicate that at a given spin multiplicity, usually more than one local minimum are found on the potential energy surface depending on the initial geometry. For AlW neutral cluster, global minimum is found at spin multiplicity 6, while for cation and anion species, spin multiplicities at 7 and 3 gives the global minimum, respectively. For W 2 cluster, global minimum is found at the lowest spin state, i.e., spin multiplicities 1, 2 and 2 for neutral, cation and anion species, respectively.

Valences of Cu and Tl and chemical bond parameters in TlxBa2CuyO6+δ, Tl2Ba2CaCu2O8, and Tl2Ba2Ca2Cu3O10

Abstract The valences of Cu and Tl and chemical bond parameters, that is, bond covalency and macroscopic linear susceptibility in Tl x Ba 2 Cu y O 6+ δ , Tl 2 Ba 2 CaCu 2 O 8 , and Tl 2 Ba 2 Ca 2 Cu 3 O 10 have been calculated using a semi-empirical method. For Tl x Ba 2 Cu y O 6+ δ , the results indicate that the average valences of Tl and Cu are between +2.0 and +3.0, indicating that part of Cu and Tl is in a state of +3.0. With the increase of oxygen content and Tl/Cu ( x / y ) ratios, the valence of Tl increases, while for the valence of Cu this trend is only observed for fixed Tl/Cu ratio samples. The valence of Tl changes more rapidly than that of Cu. For samples where both oxygen content and Tl/Cu ratio change, there exists an optimal valence of Cu and Tl for the highest T c . On the other hand, for fixed Tl/Cu ratio samples, the smaller the valences of Cu and Tl, the higher the T c . The bond covalencies of Ba–O, Cu–O (in CuO 2 layer), and Tl–O (the bond of Tl with the inserted oxygen between double TlO layers) are sensitive only to the Tl/Cu ratios, while those of the remaining Tl–O bonds and Cu–O (outside CuO 2 layer) are influenced by both. The macroscopic linear susceptibility tends to decrease with the increase of oxygen content and Tl/Cu ratios. Finally, Tl 2 Ba 2 CaCu 2 O 8 and Tl 2 Ba 2 Ca 2 Cu 3 O 10 have also been investigated.

Calculation of chemical bond parameters in La1-xCaxCrO3 (0.0 <= x <= 0.3)

The chemical bond parameters, that is, bond covalency, bond susceptibility, and macroscopic linear susceptibility of La1-xCaxCrO3 (x = 0.0, 0.1, 0.2, 0.3) has been calculated using a semiempirical method. This method is the generalization of the dielectric description theory proposed by Phillips, Van Vechten, Levine, and Tanaka (PVLT). In the calculation of bond valence, two schemes were adopted. One is the bond valence sums (BVS) scheme, and the other is the equal-valence scheme. Both schemes suggest that for the title compounds bond covalency and bond susceptibility are mainly influenced by bond valence and are insensitive to the Ca doping level or structural change. Larger bond valences usually result in higher bond covalency and bond susceptibility. The macroscopic linear susceptibility increases (only slightly for BVS scheme) with the increasing Ca doping level

Dependence of superconducting temperature on chemical bond parameters in YBa2Cu3O6 + δ (δ = 0 − 1)

Abstract The chemical bond parameters, that is ionicities and average energy gaps, for all types of chemical bonds in YBa 2 Cu 3 O 6 + δ have been investigated with variation of oxygen content δ ( δ = 0.0, 0.35, 0.45, 0.58, 0.64, 0.73, 0.78, 0.81, 0.95, 1.00). The theory used is the complex crystal chemical bond theory, which is the development of P-V-L theory. The two plateaus near 90 K and 60 K in superconducting transition temperatures, and the disappearance of superconductivity with the change of oxygen content, were reasonably explained by chemical bond parameters. The results also showed that the CuO chains play a vital role in the transition from non-superconductors to superconductors, and the highest transition temperature occurred when the plane-chain reached a coupling state.

Theoretical investigation of LaCn+ (n=2-8) clusters

LaCn+ (n = 2-8) have been studied using Hartree-Fock (HF) and B3LYP density functional method. The results indicated that at both levels, isomers with C-2v, C-s symmetry for n = 2, and edge insertion isomer for n = 4, 6, 8, as well as edge binding isomer for n = 3, 5, 7 were found as ground states. This is in good agreement with experimental results. The exceptional case is for n = 6 at B3LYP level, in which edge insertion and edge binding isomers were computed to be near isoenergetic

Investigation of Pr valence and relationship between bond covalency and Tc in Y1−xPrxBa2Cu3O7 (x=0–1)

The valence of Pr and relationship between bond covalency and T-c in Y1-xPrxBa2Cu3O7 (x = 0-1) have been studied using complex chemical bond theory. The results indicate that the depression of superconductivity in Y1-xPrxBa2Cu3O7 can be reasonably explained by bond covalency difference for the bonds between CuO2 plane and CuO chain. T-c decreases with the decreasing of bond covalency difference and reaches zero when bond covalency difference is zero (or bond covalency in CuO2 exceeds that in CuO chain) at Pr concentration 0.55 and valence +3.30. These are in good agreement with the experiments and meanwhile suggest that the valence of Pr is + 3.30 in Y1-xPrxBa2Cu3O7. The results also indicate that for Pr valence less than +3.15, superconductivity always exists for whatever Pr concentration, whereas for Pr with a valence of +4.0, superconductivity disappears as soon as Pr concentration exceeds 0.19. This supports with the viewpoint that higher valence Pr will contribute more electrons to CuO2 plane, filling the mobile holes responsible for conduction. For PrBa2Cu3O7 with no Ba-site Pr, our calculation suggests that it will be a superconductor if the average valence of Pr is less than +3.15