Shunchong Wang

Stable Oxoborate with Edge‐Sharing BO4 Tetrahedra Synthesized under Ambient Pressure

Analysis of an atom s coordination and the linkage of polyhedra is of vital importance for understanding crystal structures, especially considering our general inability to forecast structure types for new systems of elements. As a general rule, the presence of shared edges or faces of polyhedra in a coordinated structure is common for large cations, but scarcely seen for high-valence low coordinated small cations. This conclusion is the main thrust of the Pauling s third and fourth rules and is especially strict for compounds such as borates, 6] silicates, and phosphates. Until now, edge-sharing of those cation–oxygen (cation = B, Si, P) polyhedra was considered impossible except under extreme conditions. Unlike silicon and carbon, boron has the ability to bind to either three or four oxygen atoms to form a BO3 triangle or a BO4 tetrahedron. Polymerization of those B O blocks can give omnifarious types of anion groups (the BO3 and BO4 groups can occur isolated or linked in the form of rings, chains, layers, or networks) and endow over 1000 borate compounds with amazing structural diversity from triclinic symmetry to cubic symmetry. 11] On the basis of the borate structures discovered, Ross and Edwards in 1967 postulated that B O groups can only link to each other through common corners, not by edge-sharing or face-sharing. This hypothesis reduced the number of possible fundamental building blocks (FBB, the repeat B O block of the structure) greatly, making it possible for subsequent researchers to develop concise theories and clearer nomenclature for the unique borate structural chemistry. The hypothesis is valid except under extreme conditions. In 2002, Huppertz and van der Eltz claimed first violation of this hypothesis as they synthesized Dy4B6O15 under high pressure (HP) (8 GPa, 1000 K). Since then, several more edge-sharing HP borates have been synthesized under high-pressure/high-temperature conditions; thus, the appearance of edge-sharing BO4 is a significant phenomenon for distinguished HP borates. Herein, we present a novel borate KZnB3O6 synthesized under ambient pressure which is built from edge-sharing BO4 tetrahedra and is stable up to its melting point. Our work demonstrates that high pressure is not an indispensable prerequisite for the formation of edge-sharing BO4 polyhedra, and that the original hypothesis should be reexamined. KZnB3O6 was synthesized through solid-state reaction in air with K2CO3, H3BO3, and ZnO powders as the starting materials. The compound thus obtained is airand waterstable. The crystal structure was solved and refined on the basis of single-crystal data, which confirms the title compound to be the first ambient pressure borate with the edgesharing BO4 tetrahedra. Figure 1 shows the structure, in which the metal–borate framework is built up from corner-sharing B6O12 and Zn2O6 blocks, and weakly bonded K ions are

Effect of Ag substitution on the transport property and magnetoresistance of LaMnO3

Abstract The structural, magnetic and electronic properties of the polycrystalline La1−xAgxMnO3 are systematically investigated as a function of Ag-doping level. The result of the Rietveld refinement of X-ray powder diffraction shows that the samples are single rhombohedral ( R 3 C ) structure phase for x R 3 C perovskite phase and a nonmagnetic metal Ag phase. The temperature dependence of resistivity shows that all samples undergo a sharp insulator–metal (I–M) transition accompanying a paramagnetic to ferromagnetic transition with the decrease of temperature. However, for x

Effect of Li substitution on the crystal structure and magnetoresistance of LaMnO3

The crystal structure and magnetoresistance of the polycrystalline La1−xLixMnO3 (x=0.10, 0.15, 0.20, 0.30) are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that the room temperature structural transition from rhombohedral (R3C) to orthorhombic (Pbnm) symmetry occurs at the Li-doped level x⩾0.2. Accompanying the occurrence of the structural transition, the lattice distortion and the bending of the Mn–O–Mn bond increase and the ferromagnetic transition temperature TC decreases. For x=0.10 and 0.15 samples, double metal–insulator (M–I) transitions accompanying a single ferromagnetic transition and a negative magnetoresistance as high as 26% in a magnetic field of 0.8 T are observed. For x=0.20 and 0.30, the samples manifest nonmetallic behavior throughout the measured temperature range. We suggest that the double M–I transitions phenomena of low Li-doped samples originate from the magnetic inhomogeneity due to the formations of the Mn3+ and Mn4+-rich regions induced ...

Visible-light-induced persistent photoconductivity in perovskite manganite films of (La0.3Nd0.7)2/3Ca1/3MnO3

The persistent photoconductivity (PPC) induced by visible light of a He–Ne laser with a wavelength of 632.8 nm in perovskite manganite films of (La0.3Nd0.7)2/3Ca1/3MnO3 at low temperatures, below ∼50 K, was observed. Based on the measurements of transport and magnetic properties, it is suggested that the samples undergo a transition of cluster-glass state below ∼50 K. The PPC phenomenon can be qualitatively explained in terms of a photoinduced ferromagnetic cluster growth model.

Ordered double-perovskite Ca2FeMoO6 compounds with nanometer-scale grains: structure, magnetism, and intergrain tunneling magnetoresistance

Abstract Polycrystalline ordered double-perovskite Ca2FeMoO6 bulk samples with nanometer-scale grain size have been synthesized using a sol–gel method. A large magnetoresistance (MR) of 7.6% at a low magnetic field of 0.5 T and room temperature is obtained. The result of ac susceptibility shows that the sample has two magnetic transitions at TC1=374.6 K and TC2=336.4 K, which may correspond to the orthorhombic structure and the monoclinic structure phase, respectively. Due to the large MR at room temperature and well ferromagnetic conductivity of Ca2FeMoO6, it may be a desirable material for practical applications in future.

Effect of interfacial structures on spin dependent tunneling in epitaxial L10-FePt/MgO/FePt perpendicular magnetic tunnel junctions

Epitaxial FePt(001)/MgO/FePt magnetic tunnel junctions with L10-FePt electrodes showing perpendicular magnetic anisotropy were fabricated by molecular beam epitaxial growth. Tunnel magnetoresistance ratios of 21% and 53% were obtained at 300 K and 10 K, respectively. Our previous work, based on transmission electron microscopy, confirmed a semi-coherent interfacial structure with atomic steps (Kohn et al., APL 102, 062403 (2013)). Here, we show by x-ray photoemission spectroscopy and first-principles calculation that the bottom FePt/MgO interface is either Pt-terminated for regular growth or when an Fe layer is inserted at the interface, it is chemically bonded to O. Both these structures have a dominant role in spin dependent tunneling across the MgO barrier resulting in a decrease of the tunneling magnetoresistance ratio compared with previous predictions.

Effect of MgO/Co interface and Co/MgO interface on the spin dependent transport in perpendicular Co/Pt multilayers

Effect of the metal/oxide interface on spin-dependent transport properties in perpendicular [Co/Pt]3 multilayers was investigated. The saturation Hall resistivity (ρxy) is significantly increased by 45% with 1.4 nm thick CoO layer inserted at the top Co/MgO interface; whereas it is increased only 25% with 1 nm thick CoO layer at the bottom MgO/Co interface. The interfacial structures characterized by X-ray photoelectron spectroscopy show that the MgO/Co interface and Co/MgO interface including chemical states play a dominant role on spin-dependent transport, leading to different anomalous Hall behavior.

Visible-light induced persistent photoconductivity in trilayered films of perovskite manganites

The persistent photoconductivity (PPC) effect has been observed in trilayered films made of perovskite manganites La2/3Ca1/3MnO3/(La0.3Nd0.7)2.3Ca1/3MnO3La2/3Ca1/3MnO3(LNL) induced by He–Ne laser with wavelength of 632.8 nm. According to the result obtained in the thin film of (La0.3Nd0.7)2/3Ca1/3MnO3(LNCMO), which the PPC effect is also observed below ∼50 K, the PPC effect observed in trilayered film LNL should originate from the middle layer LNCMO. Compared with the thin film of LNCMO, the PPC effect of the trilayered film LNL appears at ∼86 K, which is higher than that of LNCMO at ∼50 K. The PPC effect of LNL can be quenched on thermal cycling in the vicinity of 98 K, which is also higher than that of LNCMO at ∼77 K. The difference of PPC behavior between the thin films of LNCMO and LNL can be attributed to the variation of cluster-glass state in the trilayered films of LNL caused by the strong coupling of interlayer between the middle layer LNCMO and the top/bottom layers LCMO.

Observation of stimulated emission from a single Fe-doped AlN triangular fiber at room temperature

Aluminum nitride (AlN) is a well known wide-band gap semiconductor that has been widely used in fabricating various ultraviolet photo-electronic devices. Herein, we demonstrate that a fiber laser can be achieved in Fe-doped AlN fiber where Fe is the active ion and AlN fiber is used as the gain medium. Fe-doped single crystal AlN fibers with a diameter of 20–50 μm and a length of 0.5–1 mm were preparated successfully. Stimulated emission (peak at about 607 nm and FWHM ~0.2 nm) and a long luminescence lifetime (2.5 ms) were observed in the fibers by a 532nm laser excitation at room temperature. The high quality long AlN fibers are also found to be good optical waveguides. This kind of fiber lasers may possess potential advantages over traditional fiber lasers in enhancing power output and extending laser wavelengths from infrared to visible regime.

Temperature dependence of refractive indices for 4H-and 6H-SiC

The refractive indices of 4H- and 6H-SiC single crystals at wavelengths of 404.7, 435.8, 480.0, 546.1, 587.5, 643.8, 706.5, 852.1, 1014.0, 1529.6, and 2325.4 nm are carefully measured from 293 to 493 K by the method of minimum deviation. We find that ordinary (no) and extraordinary (ne) refractive indices for both crystals increase with the elevated temperature. The temperature-dependent Sellmeier equations of refractive indices for 4H- and 6H-SiC are obtained and then thermo-optic coefficients (TOCs) are derived. The TOCs diminish very quickly at visible light region and become less dependent in the infrared light region. Such changing trends of TOCs can be explained by single-oscillator approximation. The results may provide the important reference value for designing the optoelectronic and nonlinear optical devices based on SiC.