Electronic properties of liquid mercury alloys

] O U R N A L OF

Journal of Non-Crystalhne Solids 156-158 (1993) 281-284 North-Holland

Electronic properties of liquid mercury alloys Toshio Itami, Takahiro Kusakabe and Yuko Yasuhara Department of Chemtstry, Faculty of Sctence, HokkaMo Unwerstty, Sapporo, HokkaMo 060, Japan

The thermoelectric power, Q, of hqmd R b - H g and the electrical reslstwlty, p, of hquld K - H g and R b - H g alloys were measured. A deep minimum of Q is present at 5 at % Rb At this alkah content, the temperature coeffmlent of Q shows a minimum for hqmd R b - H g , and the temperature coefficmnt of p shows a maximum for hqmd K - H g and R b - H g The resistivity shows a maximum value at 60 at.% alkah for hqmd K - H g and R b - H g alloys Two types of chemmal short-range order (CSRO) seem to be present m hqmd alkah-Hg alloys and features of these CSROs are discussed on the basis of the mformatmn avadable on the electromc properties.

1. Introduction

Recently considerable progress has been made in studies of properties of liquid alkali-polyvalent alloys [1] and many peculiar properties have been found for alkali-Hg alloys [2-6]. One such peculiar property is the minimum in the thermoelectric power in the dilute alkali concentration range [2-4]. This phenomenon has been attributed to higher-order correlation function (HOCF) effects of atomic configurations [24] which are induced by the charge transfer effects from solute atoms to Hg atoms [4]. Another such peculiar property is the 'W shape' phenomenon of the isotherm curve of the magnetic susceptibility, XM [5,6]. An enhancement of XM has been found at the intermediate concentration in the isotherms of liquid Na- and Cs-Hg alloys, although XM, as a whole, shows negative deviations from the linear law ('W shape'), which may be a reflection of chemical short-range order (CSRO) in concentrated liquid alkali-Hg alloys. In order to obtain a better understanding of these peculiar features m liquid Hg alloys, exper-

Correspondence to Dr T Itaml, Department of Chemistry, Faculty of Scmnce, Hokkaldo University, Sapporo, Hokkaldo 060, Japan Tel : +81-11 716 2111, ext 3532. Telefax- +81-11 746 2548

~mental information is reqmred as a detailed function of the concentration over the entire concentration range. In this respect, previous data of p for liquid alkali (Na, K and Cs)-Hg alloys [7,8] are insufficient. In addition, to date, no measurements of p and Q have been reported for liquid Rb-Hg alloys. One purpose of this paper is to study experimentally the thermoelectric power, Q, of liquid Rb-Hg alloys and the electrical resistivity, p, of liquid K-Hg and Rb-Hg alloys. The other purpose is to discuss particular features of the CSRO in hquid alkali-Hg alloys.

2. Experimental

The thermoelectric power, Q, was measured by the so-called 'small AT method' and the dc four-probe method was adopted for measurements of the electrical resistivity, p The possible error in the measurements is +0.5 I~V/K for Q and +0.2 txf~ cm for p [2]. The cell used for measurements was already described in detail m a previous paper [2]. The alkah metal was kept in vacuo in a glass capsule, which was destroyed in vacuo. The concentration of samples was determined by weighing. The purities fo the metals used are: Hg:99.99%; K:99.93%; Rb:99.9%. The temperature range measured was between the liquidus temperature T L and TL + 50-100 K.

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T. ltamt et al /Electrontc properttes of hqutd mercury alloys

282

3. Results

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The concentration dependence of Q for liquid R b - H g alloys, shown in fig. 1, is quite similar to that of liquid K - H g alloys [9], i.e., a deep minimum in Q at a few at.% alkali (K or Rb), and a rather monotonic concentration dependence in the intermediate concentration range. The depth of the minimum in Q for liquid R b - H g is a little deeper than that of liquid K-Hg. As the origin of this minimum in Q, an anomalous behaviour of HOCF has been proposed [2-4]. HOCF effects are important for the temperature derivative of p, ap/ST. The Ziman expression for p contains only pair distribution functions. On the other hand, Op/OT contains the temperature derivatives of pair distribution functions expressed by three- and four-body (higher-order) correlation functions in addition to pair correlation functions [2]. Similar anomalies have been found, for example, for the temperature derivative of Q and the magnetic susceptibility, XM, and the thermal expansion coefficient [2-4,6]. In the present study, similar anomalies are found: at 5 at.% alkali, OQ/OT shows a minimum for liquid R b - H g (fig. 2) and Op/ST shows a maximum for liquid K - H g and R b - - H g (fig. 5). By contrast, p itself shows no anomalous concentration dependence around 5 at.% alkali (figs. 3 and 4). Therefore, the HOCF effects are confirmed for liquid K-Hg and R b - H g alloys. The sharpness of the maximum in Op/OT in its

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isotherms can be estimated from the magnitude of curvature parameter, CP = aZ(Op/aT)/OC2 (C: at.% alkali). With a decrease of the minimum value of Q, Qm, the value of the parameter CP decreases. The data set (Qm/P~W/K, CP/10 -4 Ixl~ c m / K C 2) at 423 K of liquid alkali-Hg alloys [2,4] is (-10.8, -3.4) for liquid Li-Hg, (-10.9, -3.6) for Na-Hg, (-16.4, -20.4) for K-Hg, (-18.3, -41.2) for R b - H g and (-19.4, -46.9) for Cs-Hg. CP is 14.9 × 10 -4 txlq c m / K C 2 for the liquid Sn-Hg system at 432 k. Sn-Hg is a typical system with no minimum in Q. The positive value of CP is by contrast with the negative values for liquid alkali-Hg alloys with minimum in Q.

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Fig. 2. The concentration dependence of the temperature coefficient of the thermoelectric power Q, aQ/aT, for hqmd R b - H g alloys.

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Fig 3. The concentration dependence of the electrical reststwlty, p, of liqmd K - H g alloys at 573 K.

T ltamt et al. / Electromc properttes of hquM mercury alloys 300

of which is nearly the same as that of pure Hg. The Q of Hg is more negative than that of simple liquid metals, due to the strong energy dependence of its pseudopotential.

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4. Discussion I00' 50

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Fig. 4 The concentration dependence of the electrical reslstwlty of hqmd R b - H g alloys at 473 K

The concentration dependence of p for liquid K-Hg alloys shows a maximum value of about 250 tx12 cm at 60 at.% K (fig. 3). The concentration dependence of liquid Rb-Hg is also very similar to that of liquid K-Hg (fig. 4), with maximum value at 60 at.% Rb of about 280 ~l-I cm. These maximum values are still smaller than the maximum metallic p (300-1000 ix12 cm). In fact, in this experiment, weakly positive values of 8p/ST are observed even for liquid alloys corresponding to the maximum of p (60 at.% Rb or K). This metallic nature may explain the small negative value of Q at 60 at.% Rb, the magnitude 0.20 t

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Fig. 5 The concentration dependence of temperature coefficient of resistivity p, Op/aT for hquld K - H g and R b - H g alloys, o, Rb; E3, K

At present we have no microscopic information about the atomic and electronic structures of liquid alkali-Hg alloys. However, in a previous paper [4], the occurrence of a minimum in Q has been found to be related to charge-transfer effects from alkali to Hg atoms in liquid alloys. This has been demonstrated in a modified Miedema diagram in which a minimum in Q is found only in liquid Hg alloys containing elements with electronegativity lower than that of Hg. Therefore, it is possible to consider that, in the dilute alkali concentration range, positively charged alkali atoms may be surrounded by negatively charged Hg atoms within a short distance. Such a solvation mechanism has already been considered by Mott [10]. The minimum in Q may correspond to the establishment of a solvation structure with, for example, first and second neighbours. This point must be confirmed by direct structure analysis. In the higher alkali range another type of order with a longer range may appear. This is supported by the fact that two curves (with the crossing point at 18 at.% K) can be seen in the concentration dependence of p for liquid K-Hg alloys (fig. 3). From this point of view, previous data for the resistivity, p, of liquid Na-Hg [7] also show a crossing of two curves at 30 at.% Na (see also ref. [11]). Recent information on the electronic structure, such as valence band (VB) spectra, is available for liquid polyvalent alloys by photoelectron spectroscopy [12]. A clear splitting of the s and p bands can be observed for liquid T1, Pb and Bi. As for liquid Hg, the essential points are the overlapping between s and p bands, a dip is present at the Fermi level and a 6s-5d hybridization. For the dilute alkali-Hg alloys, the data for the VB spectra for liquid Hg are a little confusing. An extended character might be attributed to

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T. Itamt et al / Electromc properties of hquld mercury alloys

the 6s and 6p electrons of the Hg atoms and the outer s electrons of the alkali atoms; on the other hand, a rather localized character of the electrons might be required to explain the solvation mechanism of positively charged alkali atoms by negatively charged Hg atoms. Unfortunately, no spectroscopic measurements have been performed for liquid alkali-Hg alloys. However, it is plausible to consider that a splitting of the s and p bands may be present in concentrated liquid alloys of Hg with alkali metals, because of large Hg-Hg interatomic distance due to the large atomic size of the inserted alkali atoms. In this respect, it is very interesting to note the W-shape behaviour in the concentration dependence curve of the magnetic susceptibility, XM, for liquid Na-Hg and Cs-Hg alloys [5,6]. The separated s band may be occupied fully by 6s electrons of Hg. If electrons derived from alkali atoms are accommodated in the separated p band, it is possible, as a zeroth approximation, to consider that the electrons from the alkali atoms are simply diluted as a monovalent metal. Here we remember the important work on the magnetic susceptibility for metallic and expanded fluid Cs [13]. XM is enhanced with decreasing density from a typical metallic density (resistivity ~ 30 pA) cm) to the density, dmm , corresponding to the minimum metallic conductivity (300-1000 ixIl cm); below dmm , XM decreases. It may be considered that the addition of Rb and Cs liquid Hg corresponds to the process of approaching dram from the metallic side. This process brings an enhancement of Xr~- The most important factor for this mechanism is the electron-electron correlation effect [13]. As already described, p shows a maximum at 60 at.% alkali for liquid K-Hg and Rb-Hg. Therefore, at 60 at.% alkali the degree of localization is largest. Previous data of liquid Na-Hg [7,11] and Cs-Hg [8] seem to show a maximum of resistivity at 60 at.% Na and Cs, respectively. There is no intermetallic compound at 60 at.% alkali for the K-Hg, Rb-Hg and Cs-Hg systems [14]. However, for Na-Hg, the intermetallic compound Na3Hg 2 is present [14]. In this compound, a nearly square Hg 4 unit is isolated and sur-

rounded by Na [15]. Liquid Na-Hg alloys show a maximum p at the composition Na3Hg2, although the maximum value is only 120 txfl cm [7,11]. The resistity maximum for liquid K-Hg, Rb-Hg and Cs-Hg alloys also may be caused by the formation of this Hg 4 unit. This situation is quite similar to the case of liquid alkali-In and -TI alloys: resistivity maxima for heavier alkaliHg appear at the composition for which the compound exists only in the case of lighter alkali element [1]. It as very important to clarify the electronic structure of Na3Hg 2 and the nature of the chemical bond in Hg 4 as a test of the existence of s-p band separation in liquid alkali-Hg alloys.

5. Conclusions

It is very important to advance the study of atomic and electronic states to clarify the mysterious features of liquid Hg alloys.

References [1] W van der Lugt, Phys Scr T39 (1991) 372 [2] T Itaml, T Wada and M Shlmojl, J Phys F12 (1982) 1959 [3] T. Itaml, N Takahashl and M Shlmojl, J Phys F13 (1983) 1225. [4] T Itaml, S Takahashl and M. Shlmojl, J Phys F14 (1984) 427 [5] T Itaml, T. Sato and M. Shlmojl, J Phys Soc Jpn. 55 (1986) 2823 [6] T Itaml, K Shlmokawa, T Sato and M Shlmojl, J Phys. Soc Jpn 55 (1986) 3545. [7] P Muller, Metallurgle 7 (1910) 755. [8] M Kltajima and M Shlrnojl, Inst. Phys Conf Ser No 30 (1977) 226 [9] T Itaml and M Shlmojl, Phdos Mag. 28 (1973) 85 [10] N.F Mott, Phdos Mag 13 (1966) 989 [11] H A Davies, J.S.L Leach and P H. Draper, Phdos. Mag 23 (1971) 1163 [12] P Oelhafen, G. Indlekofer and H.-J Guntherodt, Z Phys Chem NF 157 (1988) 483. [13] W Fredand, Phys Rev B12 (1979) 5104. [14] M. Hansen, Conshtuhon of Binary Alloys (McGraw-Hill, New York, 1958) p. 827 [15] W Nielsen and N.C Baenzlger. Acta Crystallogr 7 (1954) 277