O17 nuclear-magnetic-resonance spectroscopic study of high-Tc superconductors

Abstract

We have obtained solid-state O17 NMR spectra of a number of enriched17 oxides and high-temperature oxide superconductors, including Cu2IO, CuIIO, KCuIIIO2, Bi2O3, Tl2O3, La1.85Sr0.15CuO4, YBa2Cu3O7−x, Bi2Sr2CaCu2O8+x, Tl2Ba2CaCu2O8+x, and Ba0.6K0.4BiO3. Spectra of all of the simple diamagnetic oxides contain relatively sharp resonances in a ‘‘diamagnetic’’ region of ∼-200 to +700 ppm (from H2O, International Union of Pure and Applied Chemistry δ scale). CuIIO exhibits a broad resonance centered at ∼4500 ppm. Spectra of all of the Cu-containing superconductors (at 300 K) contain a relatively broad and highly paramagnetically shifted peak, due to CuO2 planes, at ≊1800 ppm, together with a peak or series of peaks, in a more normal diamagnetic region, ≊100–600 ppm from H2O. YBa2Cu3O7−x also contains a peak at ∼2950 ppm, assigned to the chain [O(4)] sites. The highly shifted (CuO2) features undergo a ≊1000–1300 ppm diamagnetic shift in YBa2Cu3O7−x and Tl2Ba2CaCu2O8+x upon cooling to 77 K (<Tc). These highly shifted features are absent in Ba0.6K0.4BiO3, although the resonance in Ba0.6K0.4BiO3 is ≊500 ppm paramagnetically shifted from that in the parent compound, BaBiO3. O17 NMR spectra of magnetically aligned samples of YBa2Cu3O7−x yield diagonal elements of the electric-field-gradient tensor (χii) for the CuO2 plane oxygens, O(2) and O(3), of ±(4.02,-6.46,2.44) and ±(3.95,-6.51,2.56) MHz. The observed resonance frequencies for both sites (H0∥c) are ≊1900 ppm from H2O. For the CuO2 plane in Bi2Sr2CaCu2O8+x (at 300 K) and in Tl2Ba2CaCu2O8+x (at 77 K) the quadrupole coupling constant (e2qQ/h) is ≊6.4 MHz also. For the column or bridging oxygen [O(1)] in YBa2Cu3O7−x, e2qQ/h∼7.3 MHz and η≊0, at 300 K, the isotropic frequency shift is ∼458 ppm, and the anisotropy of the chemical or Knight shift (Δσ) is ∼657 ppm, with σ?∥c. A similar result is found for the apical oxygen in La1.85Sr0.15CuO4, with σ33=σ?∼694 ppm and σ11=σ22=σ⊥∼474 ppm, determined on a magnetically ordered sample. For the chain site in YBa2Cu3O7−x [O(4)], we find χii=±(3.38,-10.94,7.56) MHz. Calculations suggest for all sites that Vzz is negative and is aligned along the Cu-O-Cu bond axis. Results on oxygen-depleted YBa2Cu3O7−x show considerable broadening of the plane-site features, consistent with the onset of antiferromagnetic ordering, while the column oxygens [O(1)] are much less affected. Temperature-dependence studies of T1 for O(2,3) in YBa2Cu3O7−x indicate Korringa-type behavior above Tc, and the Knight shift predicted is in good accord with that observed. There is no BCS-type relaxation enhancement just below Tc. Overall, our results give a broad basis for the assignment of O17 NMR resonances in most of the classes of high-temperature superconductors currently being investigated (2:1:4, 1:2:3, Bi2Cu2, Tl2Cu2, and Cu-free Bi-containing phases) and show considerable similarities in frequency shifts (and their temperature dependence), T1, e2qQ/h values, and spin-echo decay behavior, for the CuO2 planes in each system, and between the (La(Sr)O,SrO,BaO) and (TlO,BiO) planes as well.

Source-reported materials — not catalogue approval

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