Coexistence of long-range order and spin fluctuation in geometrically frustrated clinoatacamite Cu2Cl(OH)3

Muon spin rotation experiments are carried out on clinoatacamite, Cu2Cl(OH)3, which is a new geometrically frustrated system featuring a three-dimensional network of corner-sharing tetrahedral 3d Cu2+ spins. A long-range antiferromagnetic order occurs below 18.1 K with a surprisingly small entropy release of about 0.05Rln2/Cu. Below 6.5 K, the static long-range order transforms abruptly into a metastable state with nearly complete depolarization of muon spins which suggests strong fluctuation. The system then enters a state in which partial long-range order and spin fluctuation coexist down to the lowest experimentally attainable temperature of 20 mK. This work presents a novel system for studying geometric frustration.     

Diagonal antiferromagnetic easy axis in lightly hole doped Y1-xCaxBa2Cu3O6

Hole induced changes in the antiferromagnetic structure of a lightly Ca doped Gd:Y(1-x)CaxBa2Cu3O6 copper oxide single crystal with x approximately 0.008 is investigated by Gd3+ electron spin resonance. Holes do not localize to Ca2+ ions above 2.5 K since the charge distribution and spin susceptibility next to the Ca2+ are independent of temperature. Both hole doped and pristine crystals are magnetically twinned with an external magnetic field dependent antiferromagnetic domain structure. Unlike the undoped crystal, where the easy magnetic axis is along [100] at all temperatures, the easy direction in the hole doped crystal is along the [110] diagonal at low temperatures and changes gradually to the [100] direction between 10 and 100 K. The transition is tentatively attributed to a magnetic anisotropy introduced by hole ordering.     

Coulomb- and antiferromagnetic-induced fission in doubly charged cubelike fe-s clusters

We report the observation of symmetric fission in doubly charged Fe-S cluster anions, [Fe(4)S(4)X(4)](2-)-->2[Fe(2)S(2)X(2)](-) (X=Cl,Br), owing to both Coulomb repulsion and antiferromagnetic coupling. Photoelectron spectroscopy shows that both the parent and the fission fragments have similar electronic structures and confirms the inverted energy schemes due to the strong spin polarization of the Fe 3d levels. The current observation provides direct confirmation for the unusual spin couplings in the [Fe(4)S(4)X(4)](2-) clusters, which contain two valent-delocalized and ferromagnetically coupled Fe2S2 subunits.     

Conformational equilibria in dihaloheptasilanes X2Si[SiMe(SiMe3)2]2 with X = F,Cl, Br,I: A Raman spectroscopic and quantum chemical DFT study

Equilibrium geometries, stabilities and vibrational wavenumbers for conformers of the dihaloheptasilanes X₂Si[SiMe(SiMe₃)₂]₂ with X = F, Cl, Br and I were calculated at the density functional B3LYP level employing 6‐311G(d) basis sets and SDD pseudopotentials for Br and I. Two spectroscopically distinct low‐energy conformers were located for all four heptasilanes with energy differences of 5.5, 4.7, 1.9 and 1.2 kJ mol⁻¹ for X = F, Cl, Br and I, respectively. Five more conformers were found for difluoroheptasilane and four for X = Cl, Br and I. They all have relative energies larger than 7.5 and up to 17 kJ mol⁻¹ and are negligibly populated at room temperature. Variable temperature solution Raman spectra (−70 to + 100 °C) in a wavenumber range typical for Si Si stretching vibrations (280‐350 cm⁻¹) confirm these results. For X = Br and I, no temperature effects at all could be observed as a very rapid inter‐conversion between the two low‐energy conformers, which is fast even on the time scale of Raman spectroscopy, occurs. For X = Cl, rapid inter‐conversion also occurs, and a third conformer could be detected at higher temperatures (50–100 °C). For X = F, intensity changes with temperature are consistent with the presence of two low‐energy conformers. Copyright © 2007 John Wiley & Sons, Ltd.     

Studies on Electronic Structure and Magnetic Properties of an Organic Magnet with Metallic Mn^2＋ and Cu^2＋ Ions

The electronic structure and the magnetic properties of the non-pure organic ferromagnetic compound MnCu(pbaOH)(H2O)3 with pbaOH=2-hydroxy-1, 3-propylenebis (oxamato) are studied by using the density-functional theory with local-spin-density approximation. The density of states, total energy, and the spin magnetic moment are calculated. The calculations reveal that the compound MnCu(pbaOH)(H2O)3 has a stable metal-ferromagnetic ground state, and the spin magnetic moment per molecule is 2.208 μa, and the spin magnetic moment is mainly from Mn ionand Cu ion. An antiferromagnetic order is expected and the antiferromagnetic exchange interaction of d-electrons of Cu and Mn passes through the antiferromagnetic interaction between the adjacent O, 0, and N atoms along the path linking the atoms Cu and Mn.     

CX2+CH2O (X=F, Cl, Br)环加成反应的密度泛函理论计算

用密度泛函理论,在B3LYP/6-311 G(d)水平上研究了CX2 CH2O(X=F,Cl,Br)环加成反应一条三过渡态三中间体路径的反应机理,全参数优化了反应势能面各驻点的几何构型,用内禀反应坐标(IRC)和频率分析方法,对过渡态进行了验证.用高级电子相关校正的耦合簇[CCSD(T)/6-311 G(d)]方法对优化构型进行了单点能计算.采用经Wigner校正的Eyring过渡态理论和热力学方法,研究了该反应通道的热力学及动力学性质.从热力学和动力角度综合分析,该途径CF2与GH2O的环加成反应难以发生,而CCl2及CBr2与CH2O反应的适宜温度范围均为400～1000K,如此,反应既具有较大的自发趋势和平衡常数,又具有较快的反应速率.     

Photostimulated luminescence in BaX 2 :Eu 2+ (X=Br,Cl) X-ray storage phosphors

The photostimulated luminescence (PSL) effect in BaX 2 :Eu 2+ (X=Br, Cl) is comparable to that observed in BaFBr:Eu 2+ which is used in commercial X-ray storage phosphor screens. After X-irradiation the PSL stimulation spectra of BaX 2 :Eu 2+ (X=Br, Cl) single crystals are identical to the F centre absorption spectra, i.e. the F centres are the PSL-active electron trap centres. The nature of the hole centres is still unknown. The PSL response time of about 0.70 v s is within experimental error of 0.02 v s identical to the Eu 2+ radiative lifetime, whereas in BaCl 2 :Eu 2+ the PSL response time is 0.60 v s, and thus longer than the Eu 2+ radiative lifetime of 0.47 v s.     

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

In this paper, we present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu2+ ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of (T) were observed in the vicinity of TN=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.     

Quantum magnetism in the paratacamite family: towards an ideal kagomé lattice

We report muon spin rotation measurements on the S=1/2 (Cu2+) paratacamite ZnxCu4-x(OH)6Cl2 family. Despite a Weiss temperature of approximately -300 K, the x=1 compound is found to have no transition to a magnetic frozen state down to 50 mK as theoretically expected for the kagomé Heisenberg antiferromagnet. We find that the limit between a dynamical and a partly frozen ground state occurs around x=0.5. For x=1, we discuss the relevance to a singlet picture.     

Quaternary antiferromagnetic Ba_2BiFeS_5 with isolated FeS_4 tetrahedra

We report the detailed physical properties of quaternary compound Ba_2BiFeS_5 with the key structural ingredient of isolated FeS_4 tetrahedra. Magnetization and heat capacity measurements clearly indicate that Ba_2BiFeS_5 has a paramagnetic to antiferromagnetic transition at about 30 K. The calculated magnetic entropy above ordering temperature is much smaller than theoretical value for high-spin Fe~(3+) ion with S = 5/2, implying the possible short-range antiferromagnetic fluctuation in Ba_2BiFeS_5.     

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet

We present a detailed analysis of the heat capacity of a near-perfect S=1/2 kagome antiferromagnet, zinc paratacamite Zn(x)Cu(4-x)(OH)(6)Cl(2), as a function of stoichiometry x-->1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu2+/Zn2+ exchange between the Cu and the Zn sites, which was measured independently for x=1 using neutron diffraction. The evolution of the heat capacity for x=0.8...1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x>0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.     

Helicoidal magnetic structure of Ru2FeSi

Magnetic susceptibility, X-ray and neutron diffraction measurements have been performed on Ru₂FeSi intermetallic compound, which was found to be antiferromagnetic below 280 K. Neutron diffraction data obtained at 300 K indicate that Ru₂FeSi exhibits a chemically ordered structure with some admixture of L2₁ type of ordering. The magnetic ordering observed at 4.2 and 78 K consists of two components: - a collinear one formed by ferromagnetic (111) planes coupled antiferromagnetically, - an antiferromagnetic cone spiral with propagation vector k = 0.6a^*, parallel to the [001] direction. The total magnetic moment of 3.7μ_B at 4.2 K was found to be localized on iron ions only.     

External magnetic field effects on a distorted kagome antiferromagnet

We report bulk magnetization, and elastic and inelastic neutron scattering measurements under an external magnetic field H on the weakly coupled distorted kagome system, Cu2(OD)3Cl. Our results show that the ordered state below 6.7 K is a canted antiferromagnet and consists of large antiferromagnetic ac components and smaller ferromagnetic b components. By first-principles calculations and linear spin wave analysis, we present a simple spin Hamiltonian with nonuniform nearest neighbor exchange interactions resulting in a system of coupled spin trimers with a single-ion anisotropy that can qualitatively reproduce the spin dynamics of Cu2(OD)3Cl.     

Study on Chemical Bond and Electronic State of New Gold Mixed Valence Complexes Cs2[AuIX2][AuIIIY4] (X,Y=Cl,Br, I) by Means of 197Au Mössbauer Spectroscopy

Hyperfine Interactions - Cs2[AuIX2][AuIIIX4] (X=Cl, Br, I) is known for a perovskite-type gold mixed-valence system. We have synthesized new gold mixed valence complexes, Cs2[AuIX2][AuIIIY4] (X,...     

99Ru Mössbauer spectroscopic study on salts of ruthenocene with halogens

⁹⁹Ru Mössbauer spectra at 5 K have been measured on samples of salts of ruthenocene with halogens, expressed as [Ru(C₅H₅)₂X]Y (X=Cl, Br, Y=PF₆, and X=1, Y=13). The values of both the isomer shift and the quadrupole splitting of these salts with halogens are larger compared to those of ruthenocene. It is concluded that ruthenocene gives salts having direct chemical bonding between Ru and Cl, Br, or I, and that the Ru atom in each salt is in a higher oxidation state than 2+ in ruthenocene.     

Magnetic structure of the intermetallic compound DyAg

Neutron diffraction measurements were performed for an antiferromagnetic compound DyAg with T_N=56.7 K. Magnetic structures were found to be the (π π 0)-type at 12 K and rthe sinusoidally modulated transverse static spin wave propagating along the 110 direction and polarized in the 001 direction at 50.7 K.     

Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2

It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.     

Precise determination of the orientation of the Dzialoshinskii-Moriya vector in kappa-(BEDT-TTF)2Cu[N(CN)(2)]Cl

A novel electron spin-reorientation transition is discovered by 13C NMR in the quasi-two-dimensional organic antiferromagnet kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl. The spin reorientation occurs as an external field is swept through the orientation of the characteristic vector of the Dzialoshinskii-Moriya (DM) interaction, thus providing a precise determination of the orientation of the DM vector. Such a spin reorientation could help to characterize the DM interaction in other antiferromagnetic systems.     

Theoretical studies on the electronic structure and magnetic properties of Cu(2,5-dmpz)Cl2

First-principles calculations have been performed to study the electronic structure, the metallic and magnetic properties of Cu(2,5-dmpz)Cl₂. The calculations are based on the density functional theory (DFT) with the generalized gradient approximation (GGA) and the full-potential-linearized-augmented plane wave (FPLAPW) method. The total energy, magnetic moment, density of states (DOS) and electronic band structure are calculated. The results reveal that the compound has a stable semiconductive antiferromagnetic (AFM) ground state and a semiconductive ferromagnetic (FM) metastable state, which is in good agreement with the experimental results. Based on the spin distribution and the DOS, it is found that the spin magnetic moment is mainly from the Cu²⁺, and with relative small contribution from Cl, N atoms.     

A comparative study and development of an improved method for the reduction of nitroarenes into arylamines by Al/NH4X (X=Cl, Br, I) in methanol under ultrasonic conditions

Nitroarenes were shown by us earlier to undergo reduction when treated with Al/NH4Cl in methanol under sonic conditions to give anilines in high yields [D. Nagaraja, M.A. Pasha, Tetrahedron Lett. 40 (1999) 7855]. Now, a comparative study has been carried out and an efficient and improved procedure for this reduction by using Al/NH4X (X=Cl, Br, I) is reported. A plausible mechanism of the reaction is envisaged.