尖晶石結構LiTi2-xMxO4 (M = V and Cr)之電性、磁性與熱性質之研究

Abstract

The normal spinel LiTi2-xVxO4 and LiTi2-yCryO4 samples were synthesized by solid state reaction. According to X-ray absorption spectra of V L-edge and Cr L-edge, both cations show trivalent state and occupy octahedral sites. Comparing to the non-magnetic Al substitution for Ti at octahedral site and magnetic Mn substitution for Li at tetrahedral site, the substitution of the magnetic V or Cr for Ti at octahedral site plays an important effect on abruptly superconductivity suppression. Because the transition temperature shows a linear suppression of superconductivity with diluted magnetic impurities, the strong 3d(V) or 3d(Cr) hybridization with 2p(O) is proposed to explain the pair breaking effect rather than weak hybridization of 3d(Mn)-2p(O). From the ionic radii point of view, one can expect that the more extended 3d(V)-2p(O) hybridized orbital should result in a larger superconductivity suppression in the LiTi2-xVxO4 samples than the LiTi2-yCryO4 samples, even though V3+ (s = 1/2) has smaller spin number than Cr3+ (s = 3/2). The superconductivity properties of the LiTi2-xVxO4 and LiTi2-yCryO4 samples have been also evaluated from their magnetic field dependence of specific heat properties. By subtracting the specific heat from the conducting electrons and phonons, the field dependences of residual specific heat, from the magnetic contribution, of low substituted samples (x, y ≤ 0.025) are Kondo-like. For the non-substituted LiTi2O4 sample, the minimum content of magnetic defect is estimated to be 651ppm from the magnetic field shifting the magnetic entropy below 0.4 K to higher temperature. With further increasing doping level (x ≤ 0.4, y ≤ 1), the spin-glass-like behavior is observed in dc and ac susceptibility measurement for both LiTi2-xVxO4 and LiTi2-yCryO4 samples. These samples tend to become insulating and their magnetoresistance changes from positive to negative with increasing doping level at low temperature. This resistive property is consistent with spin-dependent transport property of spin-glass. The magnetic specific heat also shows a corresponding peak slightly larger than the freezing temperature determined from the ac magnetic susceptibility measurement. With increasing the concentration of V, the freezing temperature increases and shows an x1.74 dependence on the substitution level. The minimum doping level of V in the emergence of spin-glass-like, by extrapolation, is 0.037 and the dominant Kondo-like effect in LiTi1.975V0.025O4 is thereby confirmed consistent with the feature of magnetic field dependence of magnetic specific heat.