J. Phys. Chem. B 2001, 105, 2951-2956
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Transition from Ionic to Metallic Glasses by Rapid Quenching of Bi-BiCl3 and Bi-BiCl3-KCl Melts S. Engelberg, U. Beck, and W. Freyland* Institute of Physical Chemistry, UniVersity of Karlsruhe, D-76128 Karlsruhe, Germany ReceiVed: September 28, 2000; In Final Form: February 5, 2001
Melts of Bi-BiCl3-KCl and Bi-BiCl3 mixtures have been rapidly quenched into the glassy state at cooling rates of ∼106 K/s. In this way glasses with electronic properties intermediate between the ionic and metallic state have been obtained for the first time. At low Bi-additions the optical characteristics of these glasses are very similar to those of the corresponding melts. Measurements of the electronic conductivity of Bi-BiCl3 glasses indicate that the transition from nonmetallic to metallic behavior occurs at high Bi-doping of xBi > 0.6 whereby the concentration dependence of the conductivity in the glassy and liquid state within experimental errors is the same. Changes of the microscopic structure of these glasses have been determined by smallangle X-ray scattering (SAXS) and by extended X-ray absorption fine structure (EXAFS) experiments. In the nonmetallic salt-rich glasses the SAXS-results exhibit Guinier-type scattering behavior consistent with Bi-clusters of spherical radii between 10 and 13 Å. In the nonmetal-metal transition region the SAXS data follow a power law behavior indicating a coagulation of Bi-clusters and a percolative structure. From the EXAFS results an intra-cluster Bi-Bi distance of 3.11 ( 0.02 Å is derived which remains constant for the Bix (BiCl3)1-x glasses studied (0.2 e x e 0.8).
Introduction In the limiting case of ionic or metallic bonding a number of glassy or amorphous materials are known and their structural and electronic properties are well characterized. This is not the case for glasses with bonding characteristics intermediate between the ionic and metallic state. In principle, they should be obtainable from the corresponding metal-molten salt solutions by rapid quenching. However, little is known about the glass-forming ability of these melts, their critical nucleation rate, or critical cooling rate. On the other hand, only a few examples of metal-molten salt systems have been explored in sufficient detail which exhibit a continuous metal-nonmetal transition with varying composition at temperatures and pressures which are accessible experimentally. Among the metal-molten salt systems whose thermodynamic, structural and electronic properties have been investigated quite intensively so far, are the alkali metal-alkali halide and bismuth-bismuth halide melts (for reviews see refs 1,2). The first exhibit a metal-nonmetal transition in the salt rich melts, e.g., at a metal mole fraction of x ∼ 0.2 for K-KCl.1 The transition is characterized by a strong increase of the electronic mobility,3 the onset of Pauli behavior of the magnetic susceptibility,4 and an electronic conductivity of about 102 Ω-1 cm-1 which is comparable in magnitude with the Mott minimum metallic conductivity.5 In the nonmetallic melts the electronic transport is determined by a dynamic equilibrium of localized and mobile electrons,1,3,4 whereby electron localization in the form of liquid F-centers and bipolaronic states prevails.6,7 In Bi-BiX3 (X ) I, Br) melts, metallic conductivities are reached only at high metal additions, x > 0.6.8 In the salt-rich melts Bi-ion clusters of different oxidation states have been postulated.9 Optical absorption and EMF measurements in Bi-BiX3 * Author to whom correspondence should be addressed.
melts indicate that at low metal additions Bi+ is the dominating species.10,11 The phase diagram of Bix (BiCl3)1-x exhibits a eutectic near xE ) 0.27 and TE ) 200 °C and a wide liquidliquid miscibility gap with a consolute point at xc ) 0.52 and Tc ) 780 °C.12 To transform these melts into the glassy state, extreme cooling rates have to be considered. For pure alkali halides, condensation from the gas phase on a low-temperature substrate and with a corresponding cooling rate of the order of 1014 K/s is not sufficient to achieve the amorphous phase.13 A slightly lower critical cooling rate of ∼1013 K/s is estimated for liquid BiCl3 on the basis of a non stationary heterogeneous nucleation model as described by Gutzov et al.14 On the other hand it has been reported that liquid mixtures of BiCl3 and KCl which form a deep eutectic near 70 mol % BiCl3, can be quenched into the glassy state with glass temperatures up to 318 K.15 The necessary cooling rates are below 106 K/s, a value typically achieved with the splat cooling technique.16 In this study we present the first results on Bix (BiCl3)1-x glasses which have been obtained by rapid quenching (106107 K/s) of the melts over a wide composition range 0.2 e x e 0.8 encompassing the metal-nonmetal transition region. At low Bi-addition, glasses of Bi-BiCl3-KCl have been investigated. The amorphous state of the quenched splats has been characterized by X-ray diffraction. The main objectives of this study are the following. First, the transformation into the glassy state had to be demonstrated. Of main interest is the variation of the electronic properties of the glasses in comparison to the respective melts. This has been followed by measurements of the electrical conductivity in the metal-nonmetal transition region and by absorption spectroscopy in the nonmetallic glasses. Another problem of interest concerns the size and structure of Bi-clusters in the glassy state and their transformation or aggregation approaching metallic
10.1021/jp003545o CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001
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behavior. For this aim, the glassy Bi-BiCl3 splats have been investigated by small-angle X-ray scattering (SAXS) and by extended X-ray absorption fine structure spectroscopy (EXAFS) at various compositions 0.2 e x e 0.8. Experimental Section Rapid Quenching and Characterization of Glasses. A splat cooling apparatus has been constructed following the anvilpiston principle first applied for quenching glassy metals.16 In short, the metal-salt sample was heated inside a quartz capillary with a small opening at both ends employing an induction heating coil. On melting the sample, liquid droplets leave the capillary. They cross a laser beam which triggers the acceleration of anvil and piston by magnetic coils in such a way that the droplets are quenched when they reach the center between the polished copper plates of the anvil and piston, respectively. Typically splats of 50 µm thickness and about 1 cm diameter are obtained. Following the calculations of Kroeger et al.17 for Newtonian cooling we estimate a cooling rate of ∼106 K/s in our experiments.18 The quenching procedure has been performed inside a stainless steel chamber under pure argon atmosphere with impurities of O2 and H2O of about 5 ppm. Gloves were connected to this chamber which allowed in-situ preparation and sealing of the splats inside the respective measurement cells (for further details see ref 18). A major problem in quenching Bix (BiCl3)1-x melts is due to the high vapor pressure of BiCl3sthe boiling temperature is 713 K and the critical point is 1178 K and 100 bar.19 Furthermore, for x g 0.45 and 590 K the two-phase region occurs. As a consequence of this the concentration of the splat deviated from the nominal concentration of the prepared sample. Therefore the composition of the splats had to be analyzed separately, either by atomic absorption spectrometry and by ion chromatography or by gravimetric analysis.18,20 The error in this concentration determination is estimated to be (7%. The uncertainty of the concentration homogeneity of a splat determined at different sections is found to be less than 1 mol % taken for the example of a splat with x ) 0.67. The amorphous structure of the splats was controlled by X-ray diffraction with a Θ-2Θ diffractometer in reflection geometry. For this the splats were sealed by a 12 µm thin Al-foil and the X-ray amorphicity was tested for 15 e 2Θ e 80°. At room temperature no crystallization of Bix (BiCl3)1-x splats was observed over a period of one week. Electrical Conductivity and Optical Absorption. The electrical conductivity has been determined using the microwave resonance absorption method. For this, tablets of about 1 mm diameter (d) and 1 mm length have been compressed from the splats and sealed in thin-walled quartz capillaries of about 4 cm length. For a given microwave frequency ν0 (X-band) the change ∆Q of the quality factor and of the frequency ν0 has been measured moving the sample in and out of the cavity. Since in the conductivity range of interest here (σ > 10-1 Ω-1 cm-1) the skin depth is smaller than the sample dimensions, the conductivity σ has been evaluated according to:21
σ ) ν30
(
)
R‚9π‚d 2 3 4π µ0 20 2 3 N ‚2 ‚∆Q
(1)
Here R ) 1.6 Vs/Vc, with Vs ) sample volume, Vc ) cavity volume, N ) de-electrization factor, and the other symbols have their usual meaning. N has been determined separately from the frequency shift ∆ν0. A calibration was performed with a
Figure 1. Absorption spectra of Bi-BiCl3-KCl glasses at different Bi-additions in mole percent taken at 298 K; the composition of the pure salt is 60BiCl3-40KCl.
sample of pure crystalline BiCl3 yielding an estimate of the errors in the conductivity data of (30%.18 The optical absorption of Bi-doped BiCl3-KCl glasses (x (Bi) e 0.05) was measured in the UV-vis-IR region (300 nm e λ e 2000 nm) with a double beam Perkin-Elmer spectrometer. The splats have been sealed between two glass plates with a fixed aperture of 2 mm diameter. From the measured extinction and the known sample thickness, the absorption coefficients have been determined according to the Lambert-Beer law. SAXS and EXAFS Measurements. Small-angle X-ray scattering experiments have been performed with the JUSIFAspectrometer at the synchrotron source of HASYLAB. Details of this instrument including data calibration and correction are described in ref 22. Scattering experiments were performed for momentum transfer q in the range 10-2 to 0.8 Å-1. Samples were sealed between two 50 µm thick mica plates in order to reduce the window background scattering at low q values. Focusing on the scattering of Bi-clusters of the glassy splats, scattering experiments have been performed at different energies around the Bi-LIII-edge of 13 427 eV. The difference spectra obtained by this contrast variation give direct information of the scattering by the Bi-aggregates and eliminate contributions, e.g., by window scattering or structural inhomogeneities of the sample.23 EXAFS measurements on amorphous Bi-BiCl3 splats have been recorded in transmission at the ×1.1 spectrometer at the synchrotron source of HASYLAB. Sample dimensions and sealing with capton foils were similar to the SAXS measurements. Spectra were taken at the Bi-LIII edge, and for reference a Bi-foil was used in each experiment. The background correction of the EXAFS spectra was performed with autobk as described by Newville et al.24 and for the evaluation of the spectra the compiler program EXCURV 92 has been used (see also ref 25). Results and Discussion Bi-BiCl3-KCl Glasses. Glasses of Bi-BiCl3-KCl have been obtained by rapid quenching with compositions near 60 mol % BiCl3 and for metal additions up to 9 mol % Bi. They have been characterized both by UV-vis spectroscopy and SAXS and EXAFS measurements. The latter will be discussed in the context of the Bix (BiCl3)1-x glasses below. In Figure 1 spectra of splats (about 50 µm thick) with different Bi-content are compared which exhibit the following features. At 2.4 eV
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or 516 nm a broad absorption band occurs whose intensity increases slightly at low x, but whose position within experimental errors remains constant with x. It is responsible for the intensive deep red color of the splats. A monotonic background absorption or scattering is visible which is energy dependent and strongly increases with Bi addition. Above 3 eV all spectra merge the fundamental edge of the charge-transfer band of BiCl3, which is slightly blue shifted with reference to pure liquid BiCl3.26 The absorption band at 2.4 eV for the Bi-BiCl3-KCl glasses has characteristics comparable with those of the excitation spectra of Bi-BiCl3 melts26 or Bi-BiCl3 solutions in eutectic molten salt mixtures.27 This applies to both the excitation energies and the magnitude of the oscillator strengths of ∼10-2 at low Bi-additions(x ) 0.002). At this concentration the dominating cation species in the melts is Bi+, 26,27 and the electronic excitation is ascribed to electric dipole transitions, 3P f 3P .28 So we conclude that in the glasses with low 0 2 Bi-addition the electronic structure of the corresponding melt is conserved. On the basis of electrochemical studies the existence of a low concentration of colloidal Bi in Bi-BiCl3 melts has been considered.11 If Bi-colloids persist on quenching to the glassy state they will contribute to a background extinction due to Mie scattering which continuously increases with energy (see also refs 29,30). Therefore, we have approximated the background absorption in the spectra of Figure 1 by the Mie theory according to
K(λ) )
(
2(λ) 18πF‚V(〈〉)3/2 λ (1(λ) + 2〈〉)2 + 2(λ)2
)
(2)
Here F is the concentration of colloids, V denotes their volume, 〈〉 is the dielectric constant of the matrix, and 1(λ) and 2(λ) are the real and imaginary part of the complex dielectric constant of Bi at the wavelength λ. Fitting the background absorption with eq 2 we have taken 1 and 2 for bulk Bi from the literature,31 〈〉 was determined from the refractive index n ) 1.916 of a 60BiCl3-40KCl glass15 and the diameter of the Bi-colloids was assumed to be 10 nm. Furthermore, an additive wavelength-independent term A had to be included to correct the extinction measurements, e.g., for reflection losses. The influence of the Urbach tail on the dispersion of the background scattering was accounted for. With these assumptions and variation of the two fitting parameters, F and A, a correct description of the background scattering has been obtained (see the example in the upper part of Figure 2). From these fits a lower limit of the concentration of Bi-colloids of 3 × 1020 m-3 has been estimated which with the assumed particle volume of 5 × 10-25 m-3 corresponds to 0.04 mol % Bi. Thus only a small fraction of the added Bi should form Bi-colloids, the major part yielding the lower valent Bi+-species. This qualitatively agrees with observation made in Bi-BiCl3 melts.11 In disordered materials the fundamental absorption edge is characterized by an exponential dependence on the photon energy, p, i.e., the absorption constant K (pω) is given by the Urbach rule:
K(pω) ) K0 exp
(
)
γ(pω - E0) kT
(3)
Here E0 is a measure of the valence-conduction band separation and γ describes the tailing of the band edges. In general, disorder-induced Gaussian fluctuations of the intermolecular
Figure 2. Upper part: Fitting of the background absorption of a 2.7Bi57BiCl3-40.3KCl spectrum by the Mie scattering of Bi-colloids (- - -); Lower part: Fundamental absorption edge of a (BiCl3)0.6 (KCl)0.4 glass; experimental curve (o) and Urbach fit (-).
potentials lead to exponential tailing of the valence and conduction band density of states which explains the exponential Urbach behavior of K(pω).32 For the Bi-BiCl3-KCl glasses we find a quantitative agreement of the UV-part of the spectra with the prediction of the Urbach rule. This is shown for the example of a 60BiCl3-40KCl glass in the lower part of Figure 2. From a fit of the spectra according to eq 3 the following Urbach parameters are obtained: E0 ) 3.50 ( 0.01 eV, γ/kT ) 20 (eV)-1 at 300 K, K0 ) 1.7 × 104 cm-1. Whereas γ/kT lies in the range typical of most glasses,33 K0 is about an order of magnitude lower. On the other hand, the K0 value of these glasses is comparable in magnitude with that of the corresponding Bi-BiCl3 and BiI3 melts (see also ref 18). Bi-BiCl3 Glasses. Results of the electrical conductivity σ of Bix (BiCl3)1-x glasses (0.49 e x e 0.77) are presented in Figure 3. Also included in this figure are literature data of liquid K-KBr34 and liquid Bi-BiX3 mixtures.8 Comparing the conductivity behavior of the liquid systems it is obvious that the transition to metallic behavior occurs at relatively high metal concentrations in the BiX3 melts. Taking into account the uncertainty of the x- and σ-determination in the measurements of the Bi-BiCl3 glasses, a very similar concentration dependence of the electrical conductivity of the Bi-BiX3 systems is found, in the liquid and glassy state. On the basis of Mott’s minimum metallic conductivity criterion of ∼102 Ω-1 cm-1 (see ref 5), a transition from nonmetallic to metallic states is expected for Bi mole fractions near x ∼ 0.6. Given this similarity of the electronic properties of the liquid and glassy Bi-BiCl3 systems the change of the microscopic structure across the nonmetal-metal transition is of particular interest. Figure 4 shows a selection of Guinier plots (ln I vs q2) of nonmetallic Bi-BiCl3 glasses (x < 0.4) and of a Bi0.09 (BiCl3)0.55(KCl)0.36 glass for comparison. Shown are the difference plots of the SAXS results measured near the Bi-LIII edge (contrast variation). Fitting the small angle scattering results
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Figure 3. Electrical conductivity as a function of metal addition of Bi-BiCl3 glasses at 298 K (this work) in comparison with literature data of various metal-molten salt solutions (see ref 2).
by the Guinier scattering law35 a radius of gyration is obtained which slightly increases from 8 ( 1.4 Å for x ) 0.09 to 9.9 ( 1.1 Å for x ) 0.37 (see also Figure 4). Assuming Bi-aggregates of spherical shape these values correspond to sphere radii Rs between 10 and 13 Å. These results indicate that Bi-clusters or Bi-polycations larger than Bi5+ 9 (see also 9) prevail in nonmetallic Bi-BiCl3 glasses with high Bi-concentrations up to x < 0.4. Comparing these Rs-results from the SAXS measurements with the colloid radius of 5 nm assumed above in the Mie scattering fits it seems that according to eq 2 a higher concentration of Bi-colloids should be considered. However, with the different assumptions and uncertainties which are contained in the Mie scattering analysis above we think that a more quantitative estimate of the concentration is not reasonable. In the transition region from nonmetallic to metallic glasses the information on the change in the microscopic structure which can be derived from the SAXS results is less clear. In the composition range 0.63 e x e 0.7 the SAXS data in general follow a power law, i.e., I(q) R q-P; however, the fractal exponent P varies from 2.97 to 3.63. An exponent of P < 3 corresponds to mass fractals, that of P ) 3 indicates a strongly connected three-dimensional internal surface, and 3 < P < 4 is typical of surface fractal scattering. Figure 5 shows the example of a Bi0.67 (BiCl3)0.33 glass measured at two energies near the Bi-LIII edge exhibiting power law behavior with P ) 2.97. This scattering behavior indicates a strong coagulation of Bi-clusters in the nonmetal-metal transition region consistent with a percolative structure. The variation in the P-values possibly reflects differences of the quenching conditions in independent experiments. EXAFS spectra have been recorded for different Bi-BiCl3KCl and Bi-BiCl3 glasses at various Bi-additions up to x ) 0.81. Typical spectra of an ionic and a metallic glass are shown in Figure 6a, the corresponding Fourier transforms are presented in Figure 6b. In both cases the experimental data are compared with a model calculation of the local structure which is fitted to the EXAFS spectra. The phase shifts of the partial wave approximation of the EXAFS function have not been optimized
Figure 4. Typical Guinier-plots (ln I vs q2) nonmetallic Bi-BiCl3KCl and Bi-BiCl3 glasses together with the corresponding radii of gyration RG; shown are the SAXS results of the intensity differences measured near the Bi-LIII absorption edge at 298 K.
Figure 5. Power law behavior of the SAXS intensity (log I vs log q plot) of a Bi0.67 (BiCl3)0.33 glass in the nonmetal-metal transition region; shown are results at two energies near the Bi-LIII absorption edge.
in this fitting procedure and the corresponding uncertainty resulting for the distances is reflected by the R-parameter in the r-axis of the radial distribution functions of Figure 6b. This parameter can be calibrated for pure BiCl3 or Bi. Comparing the Fourier transforms in Figure 6b it is apparent that besides a nearest neighbor Bi-Cl distance of 2.52 ( 0.01 Å a second distance of 3.11 ( 0.02 Å occurs for the Bi-BiCl3 glass which we ascribe to the intra-cluster Bi-Bi-distance. Within the
Transition from Ionic to Metallic Glasses
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2955 Conclusions In summary, we have shown that Bi-BiCl3-KCl and BiBiCl3 glasses can be obtained by rapid quenching from the melts. The composition of these glasses can be varied over a wide range from ionic to metallic characteristics, which has not been reported before. The electronic properties of these glasses are similar to the corresponding melts, both in the nonmetallic composition range and in the metal-nonmetal transition region. This has been demonstrated by UV-vis spectra and by the concentration dependence of the electronic conductivity at high Bi-additions. The microscopic structure of the nonmetallic glasses is characterized by Bi-clusters of uniform size of about 10 Å radius of gyration and a constant intracluster nearest neighbor distance of 3.11 ( 0.02 Å. Above the metal-nonmetal transition (x > 0.6) a power law behavior of the small angle scattering with a fractal exponent near 3 is observed. This is consistent with the formation of perculative networks of the Bi-clusters. However, for a discussion of the nature of the metal-nonmetal transition in these glasses a more detailed study of their electronic and structural properties in the transition region is needed. Acknowledgment. We thank the group of E. Dormann (Karlsruhe) for support in performing the conductivity measurements and the group of H. Bertagnolli (Stuttgart) for assistance in the EXAFS data analysis. Financial support of this work by a BMBF network program and in part by the Fonds der Chemischen Industrie is acknowledged. References and Notes
Figure 6. (a) EXAFS-spectra, q3χ(q), vs momentum transfer, q, of a (BiCl3)0.58 KCl0.42 and a Bi0.61 (BiCl3)0.39 glass, respectively, measured at 298 K near the Bi-LIII absorption edge. (b) Fourier transform of the EXAFS spectra of Figure 6a; whereas the modulus curve of (BiCl3)0.58 (KCl)0.42 exhibits only one peak corresponding to the Bi-Cl distance, that of Bi0.61 (BiCl3)0.39 shows an additional peak of the Bi-Bi intracluster distance; the abscissa, r + R, denotes the bond distance r plus a parameter R which describes the uncertainty in r due to the partial wave approximation (see text).
experimental error of ( 0.02 Å this distance is constant for the Bix (BiCl3)1-x glasses and has been reproduced in several independent measurements for compositions of 0.2 e x e 0.8.20 It is comparable to the Bi-Bi-distance in crystalline Bi (3.07 Å37) or in crystalline Bi6Cl7 (3.15 Å37), but is clearly smaller than in liquid Bi (3.38 Å38).
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