Static B-11 NMR studies of the short range order in alkali metal modified B2S3 glasses

The B-11 NMR spectra of xRb(2)S + (1 - x)B2S3 glasses in the range 0 less than or equal to x less than or equal to 0.75 and of xCs(2)S + (1 - x)B2S3 glasses in the range 0 less than or equal to x less than or equal to 0.60 are reported. The addition of Rb2S to B2S3 creates on average approximately two and one-half tetrahedral borons for each added sulfur ion, whereas It is found that the addition of Cs2S creates approximately 2 tetrahedral borons for each added sulfur ion. This behavior while more similar to that seen in the alkali borate glasses, contrasts that seen in the Na and K thioborate glasses, where six to eight and three, respectively, tetrahedral borons are formed for every sulfide anion added to the glass. These findings are supported by the IR and B-11 NMR spectra of the di-thioborate polycrystals (c-Rb2S:2B(2)S(3) and c-Cs2S:2B(2)S(3)) whose structures appear to be comprised of two BS4 tetrahedrals and two BS3 trigonals (N-4 similar to 0.5) like that in the alkali di-borate phases for both Rb and Cs. Unlike the B-11 NMR resonances of the sodium thioborate glasses where a single sharp line is observed for the tetrahedral boron site and a single quadrupolar broadened line is observed for all the trigonal sites, a third resonance line is observed at high alkali fractions for the rubidium and cesium thioborate glasses. This new structural feature may arise from asymmetric MBS2 (meta-thioborate groups) or tetrahedral boron groups possessing a non-bridging sulfur.close8

Evidence of complex ionic motion in xLi(2)S+(1-x)B2S3 glassy fast ionic conductors from Li-7 and B-11 NMR and ionic conductivity measurements

Ion dynamics in the highly conductive glassy fast ionic conductor, 0.7Li(2)S + 0.3B(2)S(3), were studied with NMR line width and nuclear spin-lattice relaxation, R(1)(omega,T), of both mobile Li-7 and immobile B-11 nuclei, NMR pulsed field gradient diffusivity, and electrical (ionic) conductivity, sigma(omega,T) over wide ranges of temperature and frequency. Some measurements were also done in 0.65Li(2)S + 0.35B(2)S(3). The quadrupolar split NMR spectrum of the B-11 indicated that about 80% of the boron were in BS3 groups and 20% were in BS4 groups, and their relaxations could be resolved. We fitted R(1Li) with a double-peaked distribution, Z(Li), of activation energies, E(a) for the hops, where the peaks are due to the differing environments near BS3 and BS4. The conductivity, sigma(omega,T), was calculated quantitatively from Z(Li)(E(a)) in our model with random walk and percolation over the lowest barriers. The apparent correlation time tau(B)((3)) derived from spin-lattice relaxation of B-11 in trigonal BS3 groups, R(1B)(BS3), were much shorter than tau(B)((4)) from R(1B)(BS4), which again were shorter than tau(Li) from R(1Li), although all relaxations are due to fluctuating electric field gradients from the hopping Li+ ions. The times, tau(Li), are those of single ion hopping, while times tau(B) are due to the fluctuations of the electric field gradient at the BSx groups which was due to the combined effect of several Li+ ions hopping and to the interaction of the Li+ ion with the BSx groups where x = 3 or 4. The Li-7 NMR line narrowing and diffusivity yielded additional information on the correlation time for Li+ hopping motion which are consistent, within our model, with the sigma(omega,T) and R(1)(omega,T) data.close8

Electronic and magnetic properties of single-crystal YNi2B2C from B-11 and Y-89 NMR and magnetic-susceptibility measurements

The quaternary intermetallic compound superconductor YNi2B2C with transition temperature T-c = 15.5 K has been investigated by B-11 and Y-89 nuclear magnetic resonance (NMR) and by magnetic susceptibility chi measurements both in the normal and the superconducting states. The NMR and relaxation measurements have been performed in a powder sample and single crystals. B-11 (I = 3/2) NMR spectra display patterns typical for an axially symmetric field gradient with quadrupole coupling frequency upsilon(Q) = 698 +/- 1 kHz and Y-89 (I = 1/2) data show spectra typical for a large anisotropic Knight shift, K, with axial symmetry (3K(ax) = 0.042%). In the normal state, the B-11 K increases with decreasing temperature while Y-89 K decreases. The temperature dependences of both the isotropic (K-iso) and anisotropic (3K(ax)) components of the B-11 and Y-89 Knight shifts are presented together with de magnetic susceptibility (chi) measurements obtained from magnetization measurements and are explained by the sharp features of the density of states near the Fermi level in the system. The analysis of the NMR and chi(T) data when combined with the theoretical calculation of the Van Vleck contribution to chi(T) allows the determination of the hyperfine coupling constants for both nuclei investigated and permits the separation of the different contributions to the total measured X(T) The nuclear spin-lattice relaxation rate (NSLR) (T-1(-1)) results for B-11 show an enhancement of (T1T)(-1) when lowering the temperature, consistent with previous results. It is shown that the enhancement of the B-11 NSLR is not due to the effects of antiferromagnetic fluctuations of Ni magnetic moments but simply due to the increase of the s-band spin susceptibility with decreasing temperature as reflected in the temperature dependence of the Knight shift. Contrary to the case of B-11, the Y-89 NSLR displays a (T1T)(-1) which is independent of temperature, indicating that the dominant contribution is from a large temperature-independent orbital Knight shift. In the superconducting state, the B-11 NSLR drops rapidly without a coherence peak and is found to fit BCS behavior with a superconducting gap parameter at T = 0 given by 2 Delta(0) = (3.4 +/- 0.5)k(B)T(c).This article is published as Suh, B. J., F. Borsa, D. R. Torgeson, B. K. Cho, P. C. Canfield, D. C. Johnston, J. Y. Rhee, and B. N. Harmon. "Electronic and magnetic properties of single-crystal Y Ni 2 B 2 C from B 11 and Y 89 NMR and magnetic-susceptibility measurements." Physical Review B 54, no. 21 (1996): 15341. DOI: 10.1103/PhysRevB.54.15341. Copyright 1996 American Physical Society. Posted with permission

Absence of antiferromagnetic correlations in YNi2B2C

The normal-state magnetic and electronic properties of YNi2B2C single crystals have been investigated by detailed B-11 NMR and magnetic-susceptibility measurements. The data are found to be consistent with band theory, with no evidence of Ni local magnetic moments or antiferromagnetic correlations.This article is published as Suh, B. J., F. Borsa, D. R. Torgeson, B. K. Cho, P. C. Canfield, D. C. Johnston, J. Y. Rhee, and B. N. Harmon. "Absence of antiferromagnetic correlations in Y Ni 2 B 2 C." Physical Review B 53, no. 10 (1996): R6022. DOI: 10.1103/PhysRevB.53.R6022 Copyright 1996 American Physical Society. Posted with permission