Abrahamsen, A.B.

Andersen, N.H.

Grivel, Jean-Claude

Xu, G.-J.

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Downloaded from orbit.dtu.dk on: Apr 01, 2019Structure and transport properties of double doped Mg1-x(Al0.5Li0.5)xB2 (poster)Xu, G.-J.; Grivel, Jean-Claude; Abrahamsen, Asger Bech; Andersen, Niels HesselPublished in:Superconductivity and magnetism: Materials properties and developments.  Extended abstractsPublication date:2003Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Xu, G-J., Grivel, J-C., Abrahamsen, A. B., & Andersen, N. H. (2003). Structure and transport properties ofdouble doped Mg1-x(Al0.5Li0.5)xB2 (poster). In N. H. Andersen, N. Bay, J-C. Grivel, P. Hedegård, D.McMorrow, S. Mørup, L. T. Kuhn, A. Larsen, B. Lebech, K. Lefmann, P-E. Lindelof, S. Linderoth, ... N. F.Pedersen (Eds.), Superconductivity and magnetism: Materials properties and developments.  Extendedabstracts (pp. 79-80). Roskilde: Risø National Laboratory.STRUCTURE AND TRANSPORT PROPERTIES OF  DOUBLE DOPED Mg1-x(Al0.5Li0.5)xB2 G.-J. Xu, J.-C. Grivel, A. B. Abrahamsen, N. H. Andersen (Department of Materials Research, RisØ National Laboratory, 4000 Roskilde, Denmark)  A series of polycrystalline samples of Mg1-x(Al0.5Li0.5)xB2 (0≤x≤0.6) and Mg1-xAlxB2 (0≤x≤0.4) were synthesized by the conventional solid state reaction method and their structure and superconductivity were investigated by means of x-ray diffraction (XRD), ac susceptibility and resistivity. The XRD patterns of Mg1-x(Al0.5Li0.5)xB2 (0≤x≤0.6) samples show that the predominant phase was of MgB2 type and there was a minor amount of impurity phases for the x≤0.4 samples. A marked increase in intensity of impurity phases can be clearly observed for the x = 0.6 sample. Similar to Al doping, the double doping leads to a decrease in both the lattice parameters a and c as shown in the inset of Fig. 1.  To obtain the superconducting transition temperature, both resistivity and ac susceptibility measurement were made. The Tc obtained from magnetic measurement is approximately consistent with that from ρ-T curves. Fig. 1 illustrates the variation of zero resistivity, Tc0, as a function of substitution level x for Mg1-x(Al0.5Li0.5)xB2 and Mg1-xAlxB2. The Tcs for the double-doped samples are systematically higher than that of the single Al doped samples. The difference of Tc0s between the two systems increases and it reaches about 10 K for the two x = 0.4 samples. High-pressure experimental results indicated that MgB2 remains strictly hexagonal until the high pressure [1]. The large value of critical temperature variation with small modification in the unit-cell volume demonstrates that Mg-B and B-B bonding distances are crucial in the superconductivity, which can be used to understand ourdecrease in lattice parameters tthe decrease in Tc with Al dopi0.0 0.1 0.2 0.3 0.40102030400.0 0.1 0.2 0.3 0.4 0.53.053.103.43.53.6Mg1-xAlxB2Mg1-x(Al0.5Li0.5)xB2T c0x   Mg1-xAlxB2 Mg1-x(Al0.5Li0.5)xB2a,c (A)x Fig. 1. Tc0 dependence of doping level x forMg1-x(Al0.5Li0.5)xB2 and Mg1-xAlxB2. The inset illustrates thelattice parameter a and c as a function of x.  results, i.e., Al doping in Mg1-xAlxB2 leads to a faster han the double doping with the same doping level, thus ng is faster than that with double doping. To clarify the factors that influence the superconducting transition temperature, we calculated the dependence of Tc on the unit-cell volume and plotted Tc(V) in Figure 2. The result from high-pressure experiment was also given in the figure for comparison. At low doping level, the volume dependence of Tc values are close for the three cases and the Tcs show a approximately linear volume dependence. With further decreasing V, a remarkable deviation from linear Tc(V) behavior for the chemical substituted cases can be clearly observed. The structural transition for the Al doped MgB2 system taking place as x  0.1 may be a possible factor causing the deviation. However, the structural transition, as well as the ‘pressure effect’, cannot be used to account for the fact that with same unit-cell volume the double-doped system has lower Tc than Al doped system. Obviously the change of carrier concentration is not the main cause resulting in different Tc(V) between the two systems. In fact, the effect of carrier concentration on Tc is a little different between MgB2 and high Tc cuprates. Hall effect experimental results [2] showed that the charge carriers in MgB2 are holes with a density of 1.7 – 2.8×1023 holes cm-3 at 300 K, which is about two orders of magnitude higher than the charge carrier density for high Tc cuprates. This value corresponds to 2.5 – 4 holes per unit-cell in MgB2. Therefore a little change in carrier concentration may not show significant influence on Tc in MgB2 system, which is in contrast to that in high-Tc cuprates. From the variation of lattice parameters with doping one may find that in order to get same unit-cell volume, the doping level for the double doped system is always higher than the Al doped system, which means that the disorder for the former is higher than that for the latter with same unit-cell volume. The strong disorder or localized structural distortion may be the origin causing the different Tc(V).  TF27 28 2910203040 MgB2Mg1-x(AlLi)xB2Mg1-xAlxB2cV (A3) ig.2 Tc as a function of unit-cell volume forMg1-x(Al0.5Li0.5)xB2 and Mg1-xAlxB2 and MgB2.    References 1. A.F. Goncharov, V.V. Struzhkin, E. Gregoryanz, Phys. Rev. B 64 (2001) 100509.  2. C. Buzea and T. Yamashita, Supercond. Sci. Technol. 14 (2001) R115.  

Structure and transport properties of double doped Mg1-x(Al0.5Li0.5)xB2 (poster)

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Structure and transport properties of double doped Mg<sub>1</sub><sub>-</sub><sub>x</sub>(Al<sub>0</sub><sub>.</sub><sub>5</sub>Li<sub>0</sub><sub>.</sub><sub>5</sub>)<sub>x</sub>B<sub>2</sub> (poster)

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