Metal alloys

Mahan, G. D.

doi:10.1107/97809553602060000635

International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

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International Tables for Crystallography (2006). Vol. D. ch. 1.8, p. 222

Section 1.8.3.2. Metal alloys

G. D. Mahan^a ^*

^a Department of Physics, 104 Davey Laboratory, Pennsylvania State University, University Park, Pennsylvania, USA
Correspondence e-mail: gmahan@psu.edu

1.8.3.2. Metal alloys

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Alloys are solids composed of a mixture of two or more elements that do not form a stoichiometric compound. An example is Cu_xNi_1−x, in which x can have any value. For small values of x, or of (1 − x), the atoms of one element just serve as impurities in the other element. This results in the type of behaviour described above. However, in the range $[0.2 \,\lt\, x \,\lt\, 0.8]$ , a different type of resistivity is found. This was first summarized by Mooij (1973), who found a remarkable range of behaviours. He measured the resistivity of hundreds of alloys and also surveyed the published literature for additional results. He represented the resistivity at T = 300 K by two values: the resistivity itself, ρ(T = 300), and its logarithmic derivative, $[\alpha = {\rm d}\ln(\rho)/{\rm d}T]$ . He produced the graph shown in Fig. 1.8.3.2, where these two values are plotted against each other. Each point is one sample as represented by these two numbers. He found that all of the results fit within a band of numbers, in which larger values of ρ(T = 300) are accompanied by negative values of $[\alpha]$ . Alloys with very high values of resistivity generally have a resistivity $[\rho(T)]$ that decreases with increasing temperature. The region where $[\alpha =0]$ corresponds to a resistivity of $[\rho^*=150]$ µΩ cm, which appears to be a fixed point. As the temperature is increased, the resisitivities of alloys with $[\rho\,\gt\,\rho^*]$ decrease to this value, while the resisitivities of alloys with $[\rho \,\lt\, \rho^*]$ increase to this value.

Figure 1.8.3.2 | top | pdf |

The temperature coefficient of resistance versus resistivity for alloys according to Mooij (1973). Data are shown for bulk alloys ( [+] ), thin films ( $[\bullet]$ ) and amorphous alloys ( $[\times]$ ).

Mooij's observations are obviously important, but the reason for this behaviour is not certain. Several different explanations have been proposed and all are plausible: see Jonson & Girvin (1979), Allen & Chakraborty (1981) or Tsuei (1986).

Recently, another group of alloys have been found that are called bad metals. The ruthenates (see Allen et al., 1996; Klein et al., 1996) have a resistivity $[\rho\,\gt\,\rho^*]$ that increases at high temperatures. Their values are outliers on Mooij's plot.

References

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International Tables for Crystallography (2006). Vol. D. ch. 1.8, p. 222