Elsevier

Physica C: Superconductivity

Volume 386, 15 April 2003, Pages 588-592
Physica C: Superconductivity

Chemical doping effect on the crystal structure and superconductivity of MgB2

https://doi.org/10.1016/S0921-4534(02)02167-6Get rights and content

Abstract

Substituent alloys of Mg1−xMxB2 with M=Ti, Zr, Mo, Mn, Fe, Ca, Al, Ag, Cu, Y, and Ho and 0⩽x⩽10% have been synthesized by the solid-state reaction. The solid solubility for most of these dopants at Mg site is found to be very low except for M=Al. In most of these alloys, the Vegard relationship holds as the doping level is low. The lattice constants for the fictitious compounds such as AgB2, CuB2, FeB2 have been extrapolated. The superconductivity transition temperature, Tc, shows a systematic changes with the doping level in some of these substituent alloys. Relevant mechanisms of superconductivity suppression have been discussed.

Introduction

The newly discovered superconductor MgB2 [1] has attracted considerable interest from theoretical and experimental points of view since MgB2 has achieved a record high Tc in the conventional superconductors. Theory indicates that MgB2 can be treated as phonon-mediated superconductor with strong coupling [2], [3]. Photoemission spectroscopy [4], tunneling spectroscopy [5], isotope effect measurements [6], [7], and inelastic neutron scattering measurement [8] also support that MgB2 is a typical BCS superconductor with a strong electron–phonon (e–p) interaction. It is found that electron doping is deleterious to superconductivity in MgB2, as demonstrated in Al-doped MgB2 [9]. Superconductivity can also be greatly suppressed by pressure [10], [11]. On the other hand, the chemical doping effect on MgB2 is not very clear although some very preliminary results show a suppression of Tc by Li+ and Al doping [9], [12]. Therefore, it is necessary to provide clearer results of both the hole and electron doping effects in MgB2 to understand why MgB2 has such a high Tc compared to other conventional superconductors, especially other diborides with an isostructure to MgB2.

In this paper, we report the structure and superconductivity of MgB2 doped with various chemical dopants. We find that most of the dopants have a very low solid solubility in the Mg site of MgB2 except for Al. The Vegard relations holds for all these dopants. The lattice constants for fictitious compounds such as AgB2, CuB2, FeB2 have been extrapolated from the Vegard relationship, the corresponding lattice constants are different from what have been predicted by theoretical calculation [13]. Our results also suggest that the suppression of superconductivity by chemical doping originates largely from the chemical pressure effect.

Section snippets

Experimental

Mg1−xMxB2 samples with M=Ti, Zr, Mo, Mn, Fe, Ca, Al, Ag, Cu, Y, and Ho and 0⩽x⩽1.0% have been synthesized by the solid-state reaction using fine powders of Mg, B, Ti, Zr, Mo, Mn, Fe, Al, Ag, Cu, and Y, and granular Ca with high purity (⩾99.9%) as starting materials. Powders of the raw materials with the stoichiometry composition were mixed in argon atmosphere for 1 h, pressured into bar shape, placed on a MgO plate, and first heated at 600 °C for 1 h, and then sintered in a range of temperature

Results and discussion

The doping level dependence of the lattice constants for the Ag-doped MgB2 is shown in Fig. 1(a). At x<0.5%, the lattice constants decrease linearly with increasing doping level. However, the decrease of the lattice constants is getting saturated as the doping level is higher than x>0.5%, showing a sharp turn at x∼0.5%, as shown in the inset of Fig. 1(a). For convenience, hereinafter, the doping level at the turning point is denoted as xL. Similar phenomenon was also observed for the samples

Summary

In summary, we have prepared Mg1−xMxB2 compounds with M=Ti, Zr, Mo, Mn, Fe, Ca, Al, Ag, Cu, Y, and Ho using solid state reaction method and studied the doping effect on structure and superconductivity of MgB2. The solid solubility for most of these dopants at Mg site is found to be very low except for M=Al. In most of these alloys, the Vegard relationship holds in the low doping level region. The lattice constants for the fictitious compounds such as AgB2, CuB2, FeB2 have been extrapolated. The

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