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Register a new userThe fate of compound with AgF$_2$ plus AgO stoichiometry. A theoretical study
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Metal oxyfluorides constitute a broad group of chemical compounds with rich spectrum of crystal structures and properties. Here we predict, based on evolutionary algorithm approach, the crystal structure and selected properties of Ag$_2$OF$_2$. This system may be considered as the 1 to 1 adduct of AgF$_2$ (i.e. an antiferromagnetic charge transfer positive U insulator) and AgO (i.e. a disproportionated negative U insulator). We analyze oxidation states of silver in each structure, possible magnetic interactions, as well as energetic stability. Prospect is outlined for synthesis of polytypes of interest using diverse synthetic approaches.
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We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)$_2cdot$(H$_2$O)] by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group $Pcca$. It features distorted VO$_6$ octahedra connected via HCOO linker (formate anions) forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish quasi-two-dimensional nature of the compound with nearest neighbour interaction $J_1/k_{rm B} simeq 11.7$~K and next-nearest-neighbour interaction $J_2/k_{rm B} simeq 0.02$~K. It undergoes a Neel antiferromagnetic ordering at $T_{rm N} simeq 1.1$~K. The ratio $theta_{rm CW}/T_{rm N} simeq 10.9$ reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of $T_{rm N}$ with magnetic field, consistent with the structural data.
We identify the driving mechanism of the gigantic Seebeck coefficient in FeSb$_2$ as the phonon-drag effect associated with an in-gap density of states that we demonstrate to derive from excess iron. We accurately model electronic and thermoelectric transport coefficients and explain the so far ill-understood correlation of maxima and inflection points in different response functions. Our scenario has far-reaching consequences for attempts to harvest the spectacular powerfactor of FeSb$_2$.
Two charge density wave transition can be detected in LaAu$_x$Sb$_2$ at ~ 110 and ~ 90 K by careful electrical transport measurements. Whereas control of the Au site occupancy in LaAu$_x$Sb$_2$ (for 0.9 < x < 1.0) can suppress each of these transitions by ~ 80 K, the application of hydrostatic pressure can completely suppress the lower transition by ~ 10 kbar and the upper transition by ~ 17 kbar. Clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero.
We have investigated the effect of Ti doping on the transport properties coupled with the magnetic ones in Sm$_{0.55}$Sr$_{0.45}$Mn$_{1-eta}$Ti$_{eta}$O$_3$ ($0 leq eta leq 0.04$). The parent compound, Sm$_{0.55}$Sr$_{0.45}$MnO$_3$, exhibits a first-order paramagnetic-insulator to ferromagnetic-metal transition just below $T_{rm c}$ = 128 K. With substitution of Ti at Mn sites ($B$-site), $T_{rm c}$ decreases approximately linearly at the rate of 22 K$%^{-1}$ while the width of thermal hysteresis in magnetization and resistivity increases almost in an exponential fashion. The most spectacular effect has been observed for the composition $eta$=0.03, where a magnetic field of only 1 T yields a huge magnetoresistance, $1.2 times 10^7$ $%$ at $T_capprox$ 63 K. With increasing magnetic field, the transition shifts towards higher temperature, and the first-order nature of the transition gets weakened and eventually becomes crossover above a critical field ($H_{cr}$) which increases with Ti doping. For Ti doping above 0.03, the system remains insulting without any ferromagnetic ordering down to 2 K. The Monte-Carlo calculations based on a two-band double exchange model show that the decrease of $T_{rm c}$ with Ti doping is associated with the increase of the lattice distortions around the doped Ti ions.
The magnetic structure of honeycomb iridate Na$_2$IrO$_3$ is of paramount importance to its exotic properties. The magnetic order is established experimentally to be zigzag antiferromagnetic. However, the previous assignment of ordered moment to the $bm{a}$-axis is tentative. We examine the magnetic structure of Na$_{2}$IrO$_{3}$ using first-principles methods. Our calculations reveal that total energy is minimized when the zigzag antiferromagnetic order is magnetized along $bm{g}approxbm{a}+bm{c}$. Such a magnetic configuration is explained by adding anisotropic interactions to the nearest-neighbor Kitaev-Heisenberg model. Spin-wave spectrum is also calculated, where the calculated spin gap of $10.4$ meV can in principle be measured by future inelastic neutron scattering experiments. Finally we emphasize that our proposal is consistent with all known experimental evidence, including the most relevant resonant x-ray magnetic scattering measurements [X. Liu emph{et al.} {Phys. Rev. B} textbf{83}, 220403(R) (2011)].
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