Investigations on the defect structures for Mn2+ in CdSe nanocrystals and bulk materials and the criterion of occupation for Mn2+ in CdX (X = S, Se, Te) nanocrystals.

The spin Hamiltonian parameters and defect structures are theoretically studied for the substitutional Mn2+ at the core of CdSe nanocrystals and in the bulk materials from the perturbation calculations of spin Hamiltonian parameters for trigonal tetrahedral 3d5 clusters. Both the crystal-field and charge transfer contributions are taken into account in the calculations from the cluster approach. The impurity-ligand bond angles are found to be about 1.84° larger and 0.10° smaller in the CdSe:Mn2+ nanocrystals and bulk materials, respectively, than those (≈109.37°) of the host Cd2+ sites. The quantitative criterion of occupation (at the core or surface) for Mn2+ in CdX (X = S, Se, Te) nanocrystals is presented for the first time based on the inequations of hyperfine structure constants (HSCs). This criterion is well supported by the experimental HSCs data of Mn2+ in CdX nanocrystals. The previous assignments of signals SI as Mn2+ at the core of CdS nanocrystals are renewed as Mn2+ at the surface based on the above criterion. The present studies would be helpful to achieve convenient determination of occupation for Mn2+ impurities in CdX semiconductor nanocrystals by means of spectral (e.g., HSCs) analysis.

Theoretical investigations on the defect structures and g factors for V4+ in 30PbO-5Bi2 O3 -(65-x)SiO2 glasses at different concentrations.

For 3d1 (V4+ ) impurity in 30PbO-5Bi2 O3 -(65-x)SiO2 glass systems with different concentrations x of V2O5, the defect structures and gyromagnetic factors are theoretically investigated by using the perturbation formulas of g factors for a tetragonally compressed octahedral 3d1 group. The concentration dependences of d-d transition band and g factors are suitably explained from the Fourier type concentration functions of the cubic crystal field parameter Dq, covalency factor N and relative tetragonal compression ratio ρ. The above concentration dependences of these quantities are suitably illustrated by the modifications of the local crystal field strength and electron cloud distribution with increasing x. The concentration variations of the electrical conductivity and dielectric relaxation are further analyzed from the stability of the systems in view of two competitive factors (increasing network polymerization and bulk stability at low concentrations and decreasing former SiO2 and stability at high concentrations).

Single noble metals (Pd, Pt and Ir) anchored Janus MoSSe monolayers: Efficient oxygen reduction/evolution reaction bifunctional electrocatalysts and harmful gas detectors.

The binding properties of single noble metal atoms (Pd, Pt and Ir) anchored Janus MoSSe monolayers (MLs), the catalytic activity of Pd- and Pt-MoSSe in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as well as the adsorption behaviors of Ir-MoSSe for harmful NO, CO and NH3 molecules are systematically studied from the first-principles calculations. Current results reflect the ascending order (Pd-MoSSe < Pt-MoSSe < Ir-MoSSe) of stability and binding strength as well as the tunable electronic properties of Janus MoSSe ML by anchoring single Pd, Pt and Ir atoms. Pd- and Pt-MoSSe exhibit excellent bifunctional catalytic performance, especially the former having lower overpotentials 0.43 and 0.50 V for ORR and OER, which are better than the well-known Pt (111) (0.45 V) and IrO2 (0.56 V) electrocatalysts, respectively. The adsorption nature for NO, CO and NH3 molecules changes from physisorption (on pristine MoSSe) to chemisorption (on Ir-MoSSe), especially for NO and CO molecules due to their ultra-low adsorption energies (-3.72 and -2.91 eV, respectively). Thus, Pd- and Pt-MoSSe (particularly the former) may act as promising highly-efficient ORR/OER bifunctional electrocatalysts, and Ir-MoSSe may serve as a potential sensitive harmful gas detector for NO and CO molecules.

Density functional theory calculations of copper-doped rutile crystals: Local structural, electronic, optical, and electron paramagnetic resonance properties.

The local structural, electronic, optical, and electron paramagnetic resonance (EPR) properties are uniformly studied for Cu2+ -doped rutile (TiO2 ) crystals by using the density functional theory (DFT) calculations. The local cation-oxygen bond lengths and planar bond angle, band gap, Mulliken charge and overlapping population, density of state (DOS), and UV-Vis absorption spectra are calculated for pure and copper-doped rutile. The smaller overlapping population of Cu-O bonds in the doped system than Ti-O bonds in pure rutile reflects weaker orbital admixtures or covalency of the former. Compared with pure rutile, Cu2+ doping leads to significant redshift of the UV-Vis absorption band and the narrow impurity band in visible and near-infrared regions arising from the Cu2+ d-d transitions and narrowing of the band gap by about 0.636 eV, possibly suggesting enhancement of visible light activity. The Cu dopant induces a spin magnetic moment of 0.74 µB for the doped rutile. The calculated UV-Vis absorption spectra and spin Hamiltonian parameters for copper-doped rutile show reasonable agreement with the experimental data and some improvement related to the previous perturbation formula calculations. Present systematic studies would be helpful to understand the mechanisms of the enhancement in the optical and magnetic properties of this material with transition-metal (especially Cu2+ ) dopants.

Theoretical studies of the concentration dependences of d-d transition band, g factors, and local distortion for V4+ in Li2 O-PbO-B2 O3 -P2 O5 glasses.

In this work, the g factors, d-d transition band, local distortion, and their concentration dependences for impurity V4+ in 20Li2 O-20PbO-45B2 O3 -(15 - x)P2 O5 :V2 O5 (0 ≤ x ≤ 2.5 mol%) glasses are theoretically investigated by using perturbation formulas of g factors for a tetragonally compressed octahedral 3d1 cluster. In the light of the cubic polynomial concentration functions for cubic field parameter Dq , covalency factor N, and relative tetragonal compression ratio ρ, the calculated concentration dependences of d-d transition band and g factors for V4+ show good agreement with the experimental data. With increasing x, N (≈0.7682-0.8165) displays the monotonously increasing trend, whereas ρ (≈6.5-4.2%) and Dq (≈1504.9-1481.1 cm-1 ) exhibit the decreasing tendencies. The above concentration dependences can be ascribed to the modifications of the V4+ -O2- bonding and orbital admixtures around the impurity V4+ due to the effects of V2 O5 doping on the stability of the glass network, the strength of local crystal fields, and the electron cloud distribution.

Exploring promising gas sensing and highly active catalysts for CO oxidation: transition-metal (Fe, Co and Ni) adsorbed Janus MoSSe monolayers.

From first-principles calculations, the transition-metal (TM) atom (Fe, Co and Ni) adsorbed Janus MoSSe monolayer, toxic gas molecules (CO, NH3 and H2S) adsorbed on the Ni-MoSSe monolayer and CO catalytic oxidation on the Fe-MoSSe monolayer are systematically investigated. An increasing order (Fe-MoSSe < Co-MoSSe < Ni-MoSSe) is found for the stability and band gap of the TM atom adsorbed Janus MoSSe monolayer. These toxic gas molecules are found to be weakly physisorbed and strongly chemisorbed on the pristine and Ni-MoSSe monolayers, respectively. The electronic structure and gas molecular adsorption properties of the Janus MoSSe monolayer can be modulated by adsorbing different TM atoms and gas molecules. Particularly, the CO catalytic oxidation can be realized on the Fe-MoSSe monolayer in light of the more preferable Eley-Rideal (ER) mechanism with the two-step route (CO + O2 → OOCO → CO2 + Oads, CO + Oads → CO2) with highly exothermic processes in each step. The adsorption of TM atoms which may greatly enhance gas sensing performance and catalytic performance of CO oxidation based on the Janus MoSSe monolayer is further discussed.

Studies on the local structure and spin Hamiltonian parameters for V4+ in Na2 O-PbO-Bi2 O3 -SiO2 glass ceramics.

The local structure, d-d transition band, and spin Hamiltonian parameters (SHPs) are theoretically studied for the V4+ probe in Na2 O-PbO-Bi2 O3 -SiO2 (NPBS) glass ceramics containing V2 O5 dopant with various concentration x (0 ≤ x ≤ 5 mol%) by using the perturbation formulas of the SHPs for tetragonally compressed octahedral 3d1 clusters. The first decreasing (or increasing) and then increasing (or decreasing) d-d transition band (= 10 Dq ) and hyperfine structure constants A// and A⊥ (or g factors g// and g⊥ ) with x can be suitably simulated with the similarly varying Fourier type concentration functions of cubic field parameter Dq , covalence factor N, core polarization constant κ, and reduction factor H (or relative tetragonal compression ratio ρ), with the minima (or maxima) at the middle concentration x = 3 mol%, respectively. The above concentration variations of SHPs and the related quantities may originate from the modifications of local crystal field strength, tetragonal compression, and electron cloud distribution near the impurity V4+ with x, corresponding to the highest [V4+ ]/[V5+ ] ratio at 3 mol%. Present studies would be helpful to explore novel sodium lead bismuth silicate glass ceramics by modifying the concentration of V2 O5 dopant.

The freezing Rènyi quantum discord.

As a universal quantum character of quantum correlation, the freezing phenomenon is researched by geometry and quantum discord methods, respectively. In this paper, the properties of Rènyi discord is studied for two independent Dimer System coupled to two correlated Fermi-spin environments under the non-Markovian condition. We further demonstrate that the freezing behaviors still exist for Rènyi discord and study the effects of different parameters on this behaviors.

Studies on the local structures for the trigonal Ni3+ centers in cathode materials LiAly Co1-y O2 (y = 0, 0.1, 0.5, and 0.8).

The local angular distortions Δθ are theoretically studied for the various Ni3+ centers in LiAly Co1-y O2 at different Al concentrations (y = 0, 0.1, 0.5, and 0.8) based on the perturbation calculations of electron paramagnetic resonance g factors for a trigonally distorted octahedral 3d7 cluster with low spin (S = 1/2). Due to the Jahn-Teller effect, the [NiO6 ]9- clusters are found to experience the local angular distortions (Δθ ≈ 5°-9°) along the C3 axis. The variation trend of Δθ with y is in accordance with that of anisotropy (Δg = g||  - g⊥ ). As the substitutions can weaken bond strengths between transition metal and oxygen and the structural stability plays an important role in cathode performances, detailed investigations on the structural properties of the cathode materials LiAly Co1-y O2 can be practically helpful to understand the performances of these materials. The oxy-redox properties of LiAly Co1-y O2 systems are comprehensible in the framework of Ni3+ /Ni4+ couples, and the trigonally compressed octahedral [NiO6 ]9- clusters are applicable to the clarification of the electrochemical properties of lithium nickel oxide batteries. It appears that LiAl0.8 Co0.2 O2 with the largest Al concentration may correspond to the smallest distortion among the mixing systems.

DFT calculations of the defect structures, electronic structures, and EPR parameters for three Rh2+ centers in AgCl.

The local structures for various Rh2+ centers in AgCl are theoretically studied using density functional theory (DFT) with periodic CP2K program. Through geometry optimizing, the stable ground states with minimal energies and electronic structures are obtained for the tetragonally elongated (TE ), orthorhombically elongated (OE ), and tetragonally compressed (TC ) centers, and the corresponding g and hyperfine coupling tensors are calculated in ORCA level. The calculations reveal obvious Jahn-Teller elongation distortions of about 0.109 and 0.110 Å along [001] axis for TE and OE centers without and with 1 next nearest neighbor (nnn) cation vacancy VAg in [100] axis, respectively. Whereas TC center with 1 nnn VAg along [001] axis exhibits moderate axial compression of about 0.066 Å due to the Jahn-Teller effect. For OE and TC centers with 1 nnn VAg , the ligand intervening in the central Rh2+ and the VAg is found to displace away from the VAg by about 0.028 and 0.024 Å, respectively. The present results are discussed and compared with those of the previous calculations based on the perturbation formulas by using the improved ligand field theory.

Investigations of the local distortions and EPR parameters for Cu2+ in xNa2 O-(30-x)K2 O-70B2 O3 (5 ≤ x ≤ 25 mol%) glasses.

The local distortions and electron paramagnetic resonance parameters for Cu2+ in the mixed alkali borate glasses xNa2 O-(30-x)K2 O-70B2 O3 (5 ≤ x ≤ 25 mol%) are theoretically studied with distinct modifier Na2 O compositions x. Owing to the Jahn-Teller effect, the octahedral [CuO6 ]10- clusters show significant tetragonal elongation ratios p ~19% along the C4 axis. With the increase of composition x, the cubic field parameter Dq and the orbital reduction factor k exhibit linearly and quasi-linearly decreasing tendencies, respectively, whereas the relative tetragonal elongation ratio p has quasi-linearly increasing rule with some fluctuations, leading to the minima of g factors at x = 10 mol%. The composition dependences of the optical spectra and the electron paramagnetic resonance parameters are suitably reproduced by the linear or quasi-linear relationships of the relevant quantities (i.e., Dq, k, and p) with x. The above composition dependences are analyzed from mixed alkali effect, which brings forward the modifications of the local crystal-fields and the electronic cloud distribution around Cu2+ with the variation of the composition of Na2 O.

Theoretical studies of the copper gyromagnetic factors for three novel Cu2+ coordination polymers with bi-triazole ligand.

The copper electron paramagnetic resonance gyromagnetic factors are theoretically studied for three novel Cu2+ coordination polymers [Cu(XL)(NO3 )2 ]n (1), {[Cu(XL)(4,4'-bpy)(NO3 )2 ]•CH3 CN}n (1a) and {[Cu(XL)3 ](NO3 )2 •3.5H2 O}n (2) with bi-triazole ligand (XL) = N,N'-bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxdiimide bi(1,2,4-triazole) from the high-order perturbation calculations of the g factors for a rhombically elongated octahedral 3d9 group. The order (1 ≤ 1a < 2) of gz can be illustrated by the dominant second-order perturbation term roughly proportional to the square of the covalency factor N. gx (and gy ) relies on the combination of the contributions from N, cubic field parameter Dq , and axial elongation of the copper sites and exhibits the sequence (1 ≤ 2 < 1a). As regards the axiality (gx  ≈ gy ) of g factors, this is because the perpendicular rhombic contribution from the deviations of the bond lengths and bond angles for the planar ligands with respect to an ideal octahedron and that from the discrepancies between the crystal fields of the planar ligands O2- and N3- largely cancel each other. The present theoretical studies on the copper electron paramagnetic resonance g factors would be helpful to understand the structures and properties of some promising coordination polymers containing copper with the novel bi-triazole ligand XL.

Investigations on the electron paramagnetic resonance parameters for Mn2+ in the ABF3 fluoroperovskites.

The electron paramagnetic resonance (EPR) parameters (g factor, the hyperfine structure constant A and the superhyperfine parameters A' and B') for Mn(2+) in the fluoroperovskites ABF(3) (A=K and Cs; B=Zn, Mg, Cd and Ca) are theoretically investigated from the perturbation formulas of these parameters for a 3d(5) ion under ideal octahedra. In the above treatments, not only the crystal-field mechanism but also the charge transfer mechanism is considered uniformly on the basis of the cluster approach. The theoretical EPR parameters are in good agreement with the experimental data. The charge transfer contribution to the g-shift Δg (≈g-g(s), where g(s)≈2.0023 is the spin-only value) is opposite (positive) in sign and comparable in magnitude to the crystal-field one. Nevertheless, the charge transfer contribution to the hyperfine structure constant shows the same sign and about 10% that of the crystal-field one. So, the conventional argument that the charge transfer contributions to the zero-field splittings are negligible for 3d(5) ions under low symmetrically distorted fluorine octahedra is proved no longer valid for the Δg analysis of ABF(3):Mn(2+) in view of the dominant second-order charge transfer perturbation terms. The unpaired spin densities of the fluorine 2s, 2p σ and 2p π orbitals are determined from the quantitative dependences upon the related molecular orbital coefficients, rather than obtained by fitting the observed superhyperfine parameters in the previous works.

Studies on the defect structure for Cu2+ in CdSe nanocrystals.

The defect structure for Cu(2+) in CdSe nanocrystals is theoretically studied by analyzing the spin Hamiltonian parameters of this impurity center. This center is ascribed to Cu(2+) occupying the octahedral interstitial site, rather than the tetrahedral substitutional Cd(2+) site proposed by previous work. The Cu(2+) center exhibits slight tetragonal elongation distortion (characterized by the elongation parameter rho approximately 0.03) due to the Jahn-Teller effect. The theoretical spin Hamiltonian parameters and optical transition show good agreement with the experimental data. The above unusual defect structure (occupation and symmetry) for Cu(2+) in CdSe nanocrystals is discussed, as compared with the conventional trigonally distorted tetrahedral Cu(2+) centers in bulk II-VI semiconductors.

Investigations of the local structures and spin Hamiltonian parameters for the trigonal Rh4+ and Ir4+ centers in rhombohedral BaTiO3.

The local structures and spin Hamiltonian parameters (g factors and the hyperfine structure constants) of the Rh(4+)(4d(5)) and Ir(4+)(5d(5)) centers in rhombohedral BaTiO(3) are theoretically investigated from the formulas of these parameters for a nd(5) (n=4 and 5) ion with low spin (S=1/2) in a trigonally distorted octahedron. From the calculations, the impurity ions are found not to occupy exactly the host Ti(4+) site in BaTiO(3) but to suffer a slight inward shift ( approximately 0.13A) towards the center of the oxygen octahedron along the C(3) axis, yielding much smaller trigonal distortion as compared with that of the host Ti(4+) site. The theoretical spin Hamiltonian parameters based on the above impurity axial shifts are in good agreement with the observed values.

Investigations on the local angular distortions and the spin-Hamiltonian parameters for Ni+ in sulfides.

The local angular distortions and the spin-Hamiltonian parameters (the g factors and the hyperfine parameters) for Ni(+) in ABS(2) (A=Cu, Ag; B=Al, Ga) ternary sulfides are theoretically investigated from the perturbation formulas of these parameters for 3d(9) ions in a tetragonally distorted tetrahedron. In view of the strong covalency of such systems, the ligand orbital and spin-orbit coupling contributions are taken into account using the cluster approach. The local impurity-ligand bond angles in the Ni(+) centers are found to be about 1.4-4.5 degrees smaller than those of the host monovalent A sites in the pure crystals, due to size mismatching substitution. As a result, the ligand tetrahedra exhibit slight elongation in CuBS(2):Ni(+) and slight compression in AgGaS(2):Ni(+). The calculated spin-Hamiltonian parameters, optical transitions and the relative intensity ratios show reasonable agreement with the experimental data.

Studies on the spin Hamiltonian parameters of vitamin B12r.

The spin Hamiltonian parameters g factors g(i) (i=x, y, z) and the hyperfine structure constants A(i) of vitamin B(12r) have been theoretically studied from the perturbation formulas of these parameters for a Co(2+)(3d(7)) ion with low spin (S=1/2) in rhombically distorted octahedra. The related crystal-field parameters are determined from the point-charge-dipole model and the local structure around Co(2+) in vitamin B(12r). The theoretical spin Hamiltonian parameters are in good agreement with the experimental data.

Studies of the spin Hamiltonian parameters and local structure for ZnO:Cu2+.

The spin Hamiltonian parameters (the g factors and the hyperfine structure constants) and local structure for ZnO:Cu2+ are theoretically studied from the perturbation formulas of these parameters for a 3d9 ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are taken into account from the cluster approach due to the significant covalency of the [CuO4](6-) cluster. According to the investigations, the impurity Cu2+ is suggested not to locate on the ideal Zn2+ site in ZnO but to undergo a slight outward displacement (approximately 0.01 angstroms) away from the ligand triangle along C3 axis. The calculated spin Hamiltonian parameters are in good agreement with the observed values. The validity of the above impurity displacement is also discussed.

Investigations on the defect structures and the g factors for the two orthorhombic Ti3+ centers in CaYAlO4.

The defect structures and the g factors for the two orthorhombic Ti(3+) centers (A and B) in CaYAlO(4) are theoretically investigated from the perturbation formulas of the g factors g(xi), g(eta) and g(xi) for a 3d(1) ion in orthorhombically distorted octahedra. The centers may be attributed to Ti(3+) locating on the Al(3+) site associated with one nearest-neighbouring oxygen vacancy (V(O)) along the z (or c) axis and additional next-nearest-neighbouring Ti(4+) replacing the host Al(3+) (Ti(Al), i.e., center A) or Al(3+) vacancy (V(Al), i.e., center B) perpendicular to the c axis, respectively. Due to the electrostatic interactions arising from the local charge mismatch, the central Ti(3+) undergoes a large displacement DeltaZ away from the V(O) in the c axis, while the ligand O(2-) intervening in the Ti(3+) and Ti(Al) (or V(Al)) suffers a small outward (or inward) shift DeltaX in the corresponding c plane related to the center of oxygen octahedron. The theoretical g factors for both centers based on the above displacements are in reasonable agreement with the observed values.

Theoretical investigation of the EPR parameters and local structure for trigonal Yb3+ ion in Bi4Ge3O12 crystal.

Bi4Ge3O12 single crystals are of great interest for science research and engineering applications. In this paper, the electron paramagnetic resonance (EPR) g factors g||, gperpendicular of Yb3+ and hyperfine structure constants A||, Aperpendicular of 171Yb3+ and 173Yb3+ isotopes in Bi4Ge3O12 crystal are calculated from the perturbation formulas of these parameters. The crystal-field parameters are obtained from the superposition model and the crystal structure data. The EPR parameters for trigonal Yb3+ centers in Bi4Ge3O12 are reasonably explained by involving the defect structures of impurity Yb3+ centers. Based on the calculations, Yb3+ ion is found not to occupy exactly the host Bi3+ site, but to shift away from the center of oxygen octahedron by a distance DeltaZ approximately 0.317 A along C3 axis. The results are discussed.