Haidong Zhou

Physical properties of Hastelloy® C-276™ at cryogenic temperatures

In recent years, the Ni–Mo–Cr superalloy Hastelloy® C-276™ has been used as a substrate material for fabricating superconducting tapes such as YBCO and MgB2 coated conductors. With increasing piece length, these coated conductors are within reach of large scale commercial applications. However, data on the physical properties of Hastelloy C-276 at temperatures relevant for these applications are not yet available. In this work, physical properties including magnet succeptibility, specific heat, electrical resistivity, and the Seebeck coefficient are measured from 2to300K and thermal conductivity from 2to200K. Our results show that Hastelloy C-276 exhibits Curie paramagnetism between 4 and 300K with a Curie constant C=0.091K. A spin-glass-like behavior is observed below 3K. The electrical resistivity has a minimum at ∼12K, and shows a linear weak T dependence at higher temperatures. The specific heat Cp between 15 and 40K follows Cp=γT+AT3. Below ∼10K, an upturn in Cp∕T with decreasing T is interpreted by ...

Probing disorder in isometric pyrochlore and related complex oxides

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.

High-Pressure Sequence of Ba3NiSb2O9 Structural Phases: New S=1 Quantum Spin Liquids Based on Ni2+

Two new gapless quantum spin-liquid candidates with S = 1 (Ni(2+)) moments: the 6H-B phase of Ba(3)NiSb(2)O(9) with a Ni(2+)-triangular lattice and the 3C phase with a Ni(2/3)Sb(1/3)-three-dimensional edge-shared tetrahedral lattice were obtained under high pressure. Both compounds show no magnetic order down to 0.35 K despite Curie-Weiss temperatures θ(CW) of -75.5 (6H-B) and -182.5 K (3C), respectively. Below ~25 K, the magnetic susceptibility of the 6H-B phase saturates to a constant value χ(0) = 0.013 emu/mol, which is followed below 7 K by a linear-temperature-dependent magnetic specific heat (C(M)) displaying a giant coefficient γ = 168 mJ/mol K(2). Both observations suggest the development of a Fermi-liquid-like ground state. For the 3C phase, the C(M) perpendicular T(2) behavior indicates a unique S = 1, 3D quantum spin-liquid ground state.

Di-, tri-, and tetranuclear nickel(II) complexes with oximato bridges: magnetism and catecholase-like activity of two tetranuclear complexes possessing rhombic topology.

Oxime-based tridentate Schiff base ligands 3-[2-(diethylamino)ethylimino]butan-2-one oxime (HL(1)) and 3-[3-(dimethylamino)propylimino]butan-2-one oxime (HL(2)) produced the dinuclear complex [Ni2L(1)2](ClO4)2 (1) and trinuclear complex [Ni3(HL(2))3(μ3-O)](ClO4)4·CH3CN (2), respectively, upon reaction with Ni(ClO4)2·6H2O. However, in a slightly alkaline medium, both of the ligands underwent hydrolysis and resulted in tetranuclear complexes [{Ni(deen)(H2O)}2(μ3-OH)2{Ni2(moda)4}](ClO4)2·2CH3CN (3) and [{Ni(dmpn)(CH3CN)2}2(μ3-OH)2{Ni2(moda)4}](ClO4)2·CH3CN (4), where deen = 2-(diethylamino)ethylamine, dmpn = 3-(dimethylamino)-1-propylamine, and modaH = diacetyl monoxime. All four complexes have been structurally characterized. Complex 1 is a centrosymmetric dimer where the square planar nickel(II) atoms are joined solely by the oximato bridges. In complex 2, three square planar nickel atoms form a triangular core through a central oxido (μ3-O) and peripheral oximato bridges. Tetranuclear complexes 3 and 4 consist of four distorted octahedral nickel(II) ions held together in a rhombic chair arrangement by two central μ3-OH and four peripheral oximato bridges. Magnetic susceptibility measurements indicated that dinuclear 1 and trinuclear 2 exhibited diamagnetic behavior, while tetranuclear complexes 3 and 4 were found to have dominant antiferromagnetic intramolecular coupling with concomitant ferromagnetic interactions. Despite its singlet ground state, both 3 and 4 serve as useful examples of Kahns model for competing spin interactions. High-frequency EPR studies were also attempted, but no signal was detected, likely due to the large energy gap between the ground and first excited state. Complexes 3 and 4 exhibited excellent catecholase-like activity in the aerial oxidation of 3,5-di-tert-butylcatechol to the corresponding o-quinone, whereas 1 and 2 did not show such catalytic activity. Kinetic data analyses of this oxidation reaction in acetonitrile revealed that the catalytic activity of 3 (kcat = 278.4 h(-1)) was slightly lower than that of 4 (kcat = 300.0 h(-1)). X-band EPR spectroscopy indicated that the reaction proceeded through the formation of iminoxyl-type radicals.

M3−x(NH4)xCrO8 (M = Na, K, Rb, Cs): A New Family of Cr5+-Based Magnetic Ferroelectrics

Upon consideration of the hydrogen-bonding properties of the NH(4)(+) cation, we synthesized a new class of compounds, M(3-x)(NH(4))(x)CrO(8) (M = Na, K, Rb, Cs). These magnetic compounds with the simple 3d(1) ground state become ferroelectric. X-ray studies confirmed that the phase transition involves a symmetry change from I42m to Cmc2(1) to P1. The transition temperature depends linearly on the composition variable x. The transitions are of the order-disorder type, with N-H···O bonding playing the central role in the mechanism. Extension of this idea to the introduction of ferroelectricity in several other classes of materials is suggested.

Magnetic properties of the geometrically frustrated S= 1 2 antiferromagnets, La2 LiMoO6 and Ba2 YMoO 6, with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S = 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected. La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below 150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below 20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a Weiss temperature, \theta_c, which depends strongly on the temperature range of the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence for long range magnetic order to 2K. On the other hand oscillations develop below 20K in muSR indicating at least short range magnetic correlations. Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below 150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat capacity, neutron powder diffraction and muSR data show no evidence for long range order to 2K but a very broad maximum appears in the heat capacity. The 89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, 1/T1. These results are discussed and compared with those from other isostructural S = 1/2 materials and those based on S = 3/2 and S = 1.

Origin of the phase transition in IrTe2: structural modulation and local bonding instability

We used x-ray and neutron diffraction to determine the low-temperature structure of IrTe

Spin-Glass Behavior in LaFexCo2–xP2 Solid Solutions: Interplay Between Magnetic Properties and Crystal and Electronic Structures

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High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe12O19

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Excess-hole induced high temperature polarized state and its correlation with the multiferroicity in single crystalline DyMnO3

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