Bao-Hua Teng

Magnetocaloric effect in GdCoxAl2−x system for (0.15 ≤ x ≤ 1) compositions

Structure and magnetic properties of the GdAl2-GdCo2 system including magnetocaloric effect have been investigated for various compositions. The as-cast GdCox Al 2-x alloys prepared by arc-melting crystallize in the dual-phase structure of cubic MgCu2-type and hexagonal MgZn2-type phases. All the as-cast dual-phase alloys exhibit only one magnetic phase transition near 77 K even though two ferrimagnetic phases coexist. The magnetic transition was broadened by the continuous transitions of the component phases, which resulted in enhancement of relative cooling power at the expense of the magnetic entropy changes.

Enhancement of magnetocaloric effect in the Gd2Al phase by Co alloying

To understand the effect of Co doping on the magnetic entropy changes in Gd2Al phase, a series of Gd2AlCox alloys with 0 ≤ x ≤ 0.6 were synthesized by arc-melting and the crystal structure was analyzed by XRD. The magnetic properties were investigated, and the entropy changes were calculated for a magnetic field change of 50 kOe. All the as-cast alloys doped with Co exhibited greater magnetic entropy changes than the original binary Gd2Al phase. The main reasons attributed to this are the increase of ferromagnetic interaction indicated by the disappearance of cusp and sharp drop in magnetization and the reduction of the critical field required to trigger the field-induced transition below 50 K in Gd2Al phase after Co alloying.

Theoretical investigations on the g factors and local structures for Cu2+ in the ZnX (X= O, S and Se) nanocrystals

ABSTRACT The g factors and local structures for Cu2+ in the ZnX (X = O, S and Se) nanocrystals at room temperature are theoretically investigated by the perturbation calculations for a tetragonally distorted tetrahedral 3d9 cluster in a consistent way, and the isotropic g factor is predicted for the ZnS:Cu2+ nanocrystals at room temperature. The bond angles θ between the four equivalent Cu2+−X2− bonds and the C4 axis are found to be about 1.26°, 1.24° and 1.07°, respectively, larger in the ZnO, ZnS and ZnSe nanocrystals than that (θ0 ≈ 54.74°) for an ideal tetrahedron, inducing tetragonally compressed tetrahedra. The declining tendency (ZnO > ZnS > ZnSe) of the tetragonal angular distortion Δθ (= θ − θ0) can be ascribed to the decreasing strength of the dynamic Jahn–Teller effect via the vibration interactions of the [CuX4]6− groups due to the weakening Cu2+−X2− bonding. The isotropic g factors are attributable to the appropriate Δθ due to the dynamic Jahn–Teller effect and the internal stress. The slightly increasing (ZnO < ZnS < ZnSe) g factors can be illustrated by the declining cubic field parameter Dq, angular distortion Δθ and covalency factor N of the systems. GRAPHICAL ABSTRACT

Analysis on the local structures for 3d1 impurities (Ti3+ and V4+) in KTiPO4

Making use of the perturbation formulae for 3d1 ions (Ti3+ and V4+) under orthorhombically compressed octahedra, the spin Hamiltonian parameters (g factors: gx, gy, gz and hyperfine structure constants: Ax, Ay, Az) and local structures of the 3d1 impurity centres C1, C2, and C3 in KTiOPO4 crystals are theoretically analyzed in a consistent way. The remarkable local distortions (i.e., the relative axial compression ratios 11.2%, 7.0%, and 5.5% along Z axis and the relative planar bond length variation ratios 15.9%, 7.0%, and 6.0%) are obtained for the [Ti2O6]9− cluster on Ti2 site and [VO6]8− clusters on Ti1 and Ti2 sites, respectively, in view of the Jahn–Teller effect. The above local orthorhombic distortion parameters in the impurity centres are found to be more significant than the host Ti1 and Ti2 sites in pure KTiOPO4. The sequences (C1 > C2 > C3) of the local orthorhombic distortion parameters ρ and τ are in accordance with those of the axial and perpendicular anisotropies Δg and δg of g factors, respectively.

Investigations on the local structures of Cu2+ at various BaO concentrations in 59B2O3–10K2O–(30−x)ZnO–xBaO–1CuO glasses

Abstract The local structures and electron paramagnetic resonance (EPR) parameters for Cu2+ in 59B2O3–10K2O–(30−x)ZnO–xBaO–1CuO (BKZBC) glasses are theoretically investigated with distinct modifier BaO concentrations x (= 0, 6, 12, 18, 24 and 30 mol %). The clusters are found to undergo the relative tetragonal elongations of about 13.5 and 5.0% at zero and higher BaO concentrations. The concentration dependences of the measured d–d transition bands, g factors and A// are suitably reproduced from the Fourier type functions or sign functions of the relevant quantities with x by using only six adjustable parameters. The features of the EPR parameters and the local structures of Cu2+ are analysed in a consistent way by considering the differences in the local ligand field strength and electronic cloud admixtures around Cu2+ under addition of Ba2+ with the highest ionicity and polarisability. The present theoretical studies would be helpful to the researches on the structures, optical and EPR properties for the similar potassium barium zinc borate glasses containing copper with variation concentration of modifier BaO.

The magnetic field generated by a rotating charged polygon

The magnetic field along the symmetry axis of a regular polygon carrying a uniform electric charge on its edges is calculated systematically when the polygon is rotated about this axis of symmetry. A group of circular current-carrying coils arranged concentrically about the axis of the polygon has been designed to simulate the magnetic field characteristics of the rotating charged polygon. The magnetic field of the simulated coils is measured using the PASCO magnetic field sensor. The results show that the theoretical calculation agrees well with the experimental results.

The DFT Calculations of Structures and EPR Parameters for the Dinuclear Paddle-Wheel Copper(II) Complex {Cu2(μ2-O2CCH3)4}(OCNH2CH3) as Powder or Single Crystal

Abstract Density functional theory (DFT) calculations of the structures and the Cu2+ g factors (gx, gy and gz ) and hyperfine coupling tensor A (Ax , Ay and Az ) were performed for the paddle-wheel (PW)-type binuclear copper(II) complex {Cu2(μ2-O2CCH3)4}(OCNH2CH3) powder and single crystal. Calculations were carried out with the ORCA software using the functionals BHandHlyp, B3P86 and B3LYP with five different basis sets: 6-311g, 6-311g(d,p), VTZ, def-2 and def2-TZVP. Results were tested by the MPAD analysis to find the most suitable functional and basis sets. The electronic structure and covalency between copper and oxygen were investigated by the electron localisation function and the localised orbital locator as well as the Mayer bond order for the [CuO5] group. The optical spectra were theoretically calculated by the time-dependent DFT module and plotted by the Multiwfn program for the [CuO5] group and reasonably associated with the local structure in the vicinity of the central ion copper. In addition, the interactions between the OCNH2CH3, NH3 and H2O molecules and the uncoordinated PW copper(II) complex were studied, and the corresponding adsorption energies, the frequency shifts with respect to the free molecules and the changes of the Cu–Cu distances were calculated and compared with the relevant systems.

Theoretical Studies of the Spin Hamiltonian Parameters and Local Distortions for Cu2+ in Alkaline Earth Lead Zinc Phosphate Glasses

Abstract The spin Hamiltonian parameters and local structures are theoretically studied for Cu2+-doped alkaline earth lead zinc phosphate (RPPZ, R=Mg, Ca, Sr, and Ba) glasses based on the high-order perturbation calculations for a tetragonally elongated octahedral 3d9 cluster. The relative elongation ratios are found to be ρ≈3.2%, 4.4%, 4.6%, and 3.3% for R=Mg, Ca, Sr, and Ba, respectively, because of the Jahn-Teller effect. The whole decreasing crystal-field strength Dq and orbital reduction factor k from Mg to Sr are ascribed to the weakening electrostatic coulombic interactions and the increasing probability of productivity of nonbridge oxygen (and hence increasing Cu2+–O2− electron cloud admixtures) under PbO addition, respectively, with increasing alkali earth ionic radius. The anomalies (the largest Dq and the next highest k among the systems) for R=Ba are attributed to the cross linkage of this large cation in the network. The overall increasing order (Mg≤Ba<Ca<Sr) of ρ is largely due to the decreasing crystal-field strength Dq and hence the decreasing force constant of the Cu2+–O2− bonds. The present studies would be helpful to understand local structures and the influences on the optical properties of RPPZ glasses containing copper dopants.

Hysteresis in the phase transition of chocolate

We designed an experiment to reproduce the hysteresis phenomenon of chocolate appearing in the heating and cooling process, and then established a model to relate the solidification degree to the order parameter. Based on the Landau–Devonshire theory, our model gave a description of the hysteresis phenomenon in chocolate, which lays the foundations for the study of the phase transition behavior of chocolate.