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Ion motion in high-temperature solid and liquid silver chalcogenides
Physica B 276}278 (2000) 493}494
Ion motion in high-temperature solid and liquid silver chalcogenides W.S. Howells *, A.C. Barnes, M. Hamilton ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, UK
Abstract The noble metal chalcogenides at the stoichiometric composition M X (M"Cu,Ag; X"S,Se,Te) all form superionic phases above room temperature in which the noble metal ion is able to rapidly di!use in a either a FCC or BCC chalcogenide lattice. Quasielastic neutron scattering experiments have been performed on Ag Te in both the solid and liquid phases to measure the di!usion of the Ag> ions. A preliminary analysis of the results is compared with previous measurements on Ag Se. 2000 Elsevier Science B.V. All rights reserved. Keywords: Quasielastic scattering; Superionic conductors
1. Introduction The silver chalcogenides (Ag S, Ag Se and Ag Te) are materials of considerable interest [1]. As liquids, both Ag S [2] and Ag Se [3] have an anomalous peak in their conductivity and a negative dp/d¹ at this composition. Ag Te [4] shows the more usual properties * p is a minimum at stoichiometry and dp/d¹ is positive. Furthermore, all of these compounds are superionic conductors [5] in which the Ag> ions are mobile within a cubic sub-lattice formed by the anion. It has been suggested that a possible cause for the enhancement of the conductivity in Ag Se and Ag S is the high ionic mobility of the Ag> ions coupling to the electron mobility as suggested previously by Ramasesha [6]. Initial results obtained by Neutron Di!raction show signi"cant di!erences in the cation}cation correlations between liquid Ag Se [7] and liquid Ag Te. It is therefore of interest to compare the dynamics in these materials. Ag Te undergoes a "rst-order phase transition to the superionic state at 1503C with the Te ions forming a FCC sub-lattice. A second transition to a BCC sub-lattice occurs at 8023C followed by melting at 9503C [5]. The
* Corresponding author. Fax: #44-1235-44-5720. E-mail address: [email protected] (W.S. Howells)
aim of these Quasielastic Neutron Scattering (QENS) measurements is to measure the di!usion of the Ag> ions in Ag Te in the low-temperature solid phase, both superionic phases and in the liquid state and compare the results to those in Ag Se for which similar measurements have already been carried out.
2. Experimental An experiment was performed on the IN6 time-of#ight spectrometer at the ILL with an incident neutron wavelength of 5.1 As giving an energy resolution of about 100 leV and a range of momentum transfer of 0.3}2 As \. The sample of Ag Te was contained in a sealed Niobium cylinder with a diameter of 5 mm. Data was collected in the three solid phases prior to melting (1003C, 6003C, 9003C) and in the liquid state (10003C). Container subtraction was performed but full absorption and multiple scattering corrections have not yet been carried out. A Bayesian "tting procedure was used to determine the most likely number of quasielastic components. It was found that the spectra could be described by a large elastic peak with a smaller single Lorentzian component representing the quasielastic scattering.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 8 0 8 - 6
494
W.S. Howells et al. / Physica B 276}278 (2000) 493}494
3. Results The full-width at half-maximum (FWHM) of the single Lorentzian component plotted against Q is shown in Fig. 1. In the low-temperature phase little broadening was seen above the energy resolution of the instrument. Data is plotted for three temperatures and shows that di!usion does indeed take place in these high-temperature solid phases. As the temperature is increased the rate of di!usion increases and the Q-dependence of the widths becomes more marked. A good "t was obtained at all three temperatures using the Chudley}Elliott jump model for di!usion in which the jump length increased from 2.3 As at 6003C to 2.8 As at 10003C (liquid) and the jump time decreased from 2.4 to 1.2 ps for the same temperatures.
4. Comparison with Ag Se If the widths for liquid Ag Te are plotted against those for Ag Se (Fig. 2) it is immediately apparent that the level of di!usion in the two liquids is similar. The most noticeable di!erence is in the Q-dependence of the widths. As already discussed, there is a noticeable Q-dependence for Ag Te but in contrast it is not possible to obtain a good "t to the Ag Se data with the Chudley}Elliott model. Thus Ag Se is more consistent with simple di!usion,
Fig. 2. FWHM for liquid Ag Te and Ag Se. The full line shows the Chudley}Elliott "t for Ag Te while the dashed line shows a linear Q variation for Ag Se.
with widths proportional to Q, which suggests that the mechanism of di!usion is di!erent for the two materials. The di!usion coe$cient for Ag Te at 9003C is 5;10\ m s\ which is in reasonable agreement with the molecular dynamics results of Kobayashi et al. [8]. Further analysis and interpretation continues.
References [1] A.C. Barnes, J. Non-Cryst. Solids 156}158 (1993) 675. [2] S. Ohno, A.C. Barnes, J.E. Enderby, J. Phys.: Condens. Matter 2 (1990) 7707. [3] S. Ohno, A.C. Barnes, J.E. Enderby, J. Phys.: Condens. Matter 6 (1994) 5335. [4] H.S. Schyders, J. Hahn, D. Streutker, J.B. Van Zytveld, J. Phys.: Condens. Matter 9 (1997) 10121. [5] J.B. Boyce, B.A. Huberman, Phys. Rep. 51 (1979) 189. [6] S. Ramasesha, J. Sol. State Chem. 41 (1982) 333. [7] A.C. Barnes, S.B. Lague, P.S. Salmon, H.E. Fischer, J. Phys.: Condens. Matter 9 (1997) 6159. [8] M. Kobayashi, K. Ishikawa, F. Tachibana, H. Okazaki, Phys. Rev. B 38 (5) (1988) 3050.
Fig. 1. FWHM for Ag Te at 3 temperatures with lines showing Chudley}Elliott "ts.
Ion motion in high-temperature solid and liquid silver chalcogenides W.S. Howells *, A.C. Barnes, M. Hamilton ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, UK
Abstract The noble metal chalcogenides at the stoichiometric composition M X (M"Cu,Ag; X"S,Se,Te) all form superionic phases above room temperature in which the noble metal ion is able to rapidly di!use in a either a FCC or BCC chalcogenide lattice. Quasielastic neutron scattering experiments have been performed on Ag Te in both the solid and liquid phases to measure the di!usion of the Ag> ions. A preliminary analysis of the results is compared with previous measurements on Ag Se. 2000 Elsevier Science B.V. All rights reserved. Keywords: Quasielastic scattering; Superionic conductors
1. Introduction The silver chalcogenides (Ag S, Ag Se and Ag Te) are materials of considerable interest [1]. As liquids, both Ag S [2] and Ag Se [3] have an anomalous peak in their conductivity and a negative dp/d¹ at this composition. Ag Te [4] shows the more usual properties * p is a minimum at stoichiometry and dp/d¹ is positive. Furthermore, all of these compounds are superionic conductors [5] in which the Ag> ions are mobile within a cubic sub-lattice formed by the anion. It has been suggested that a possible cause for the enhancement of the conductivity in Ag Se and Ag S is the high ionic mobility of the Ag> ions coupling to the electron mobility as suggested previously by Ramasesha [6]. Initial results obtained by Neutron Di!raction show signi"cant di!erences in the cation}cation correlations between liquid Ag Se [7] and liquid Ag Te. It is therefore of interest to compare the dynamics in these materials. Ag Te undergoes a "rst-order phase transition to the superionic state at 1503C with the Te ions forming a FCC sub-lattice. A second transition to a BCC sub-lattice occurs at 8023C followed by melting at 9503C [5]. The
* Corresponding author. Fax: #44-1235-44-5720. E-mail address: [email protected] (W.S. Howells)
aim of these Quasielastic Neutron Scattering (QENS) measurements is to measure the di!usion of the Ag> ions in Ag Te in the low-temperature solid phase, both superionic phases and in the liquid state and compare the results to those in Ag Se for which similar measurements have already been carried out.
2. Experimental An experiment was performed on the IN6 time-of#ight spectrometer at the ILL with an incident neutron wavelength of 5.1 As giving an energy resolution of about 100 leV and a range of momentum transfer of 0.3}2 As \. The sample of Ag Te was contained in a sealed Niobium cylinder with a diameter of 5 mm. Data was collected in the three solid phases prior to melting (1003C, 6003C, 9003C) and in the liquid state (10003C). Container subtraction was performed but full absorption and multiple scattering corrections have not yet been carried out. A Bayesian "tting procedure was used to determine the most likely number of quasielastic components. It was found that the spectra could be described by a large elastic peak with a smaller single Lorentzian component representing the quasielastic scattering.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 8 0 8 - 6
494
W.S. Howells et al. / Physica B 276}278 (2000) 493}494
3. Results The full-width at half-maximum (FWHM) of the single Lorentzian component plotted against Q is shown in Fig. 1. In the low-temperature phase little broadening was seen above the energy resolution of the instrument. Data is plotted for three temperatures and shows that di!usion does indeed take place in these high-temperature solid phases. As the temperature is increased the rate of di!usion increases and the Q-dependence of the widths becomes more marked. A good "t was obtained at all three temperatures using the Chudley}Elliott jump model for di!usion in which the jump length increased from 2.3 As at 6003C to 2.8 As at 10003C (liquid) and the jump time decreased from 2.4 to 1.2 ps for the same temperatures.
4. Comparison with Ag Se If the widths for liquid Ag Te are plotted against those for Ag Se (Fig. 2) it is immediately apparent that the level of di!usion in the two liquids is similar. The most noticeable di!erence is in the Q-dependence of the widths. As already discussed, there is a noticeable Q-dependence for Ag Te but in contrast it is not possible to obtain a good "t to the Ag Se data with the Chudley}Elliott model. Thus Ag Se is more consistent with simple di!usion,
Fig. 2. FWHM for liquid Ag Te and Ag Se. The full line shows the Chudley}Elliott "t for Ag Te while the dashed line shows a linear Q variation for Ag Se.
with widths proportional to Q, which suggests that the mechanism of di!usion is di!erent for the two materials. The di!usion coe$cient for Ag Te at 9003C is 5;10\ m s\ which is in reasonable agreement with the molecular dynamics results of Kobayashi et al. [8]. Further analysis and interpretation continues.
References [1] A.C. Barnes, J. Non-Cryst. Solids 156}158 (1993) 675. [2] S. Ohno, A.C. Barnes, J.E. Enderby, J. Phys.: Condens. Matter 2 (1990) 7707. [3] S. Ohno, A.C. Barnes, J.E. Enderby, J. Phys.: Condens. Matter 6 (1994) 5335. [4] H.S. Schyders, J. Hahn, D. Streutker, J.B. Van Zytveld, J. Phys.: Condens. Matter 9 (1997) 10121. [5] J.B. Boyce, B.A. Huberman, Phys. Rep. 51 (1979) 189. [6] S. Ramasesha, J. Sol. State Chem. 41 (1982) 333. [7] A.C. Barnes, S.B. Lague, P.S. Salmon, H.E. Fischer, J. Phys.: Condens. Matter 9 (1997) 6159. [8] M. Kobayashi, K. Ishikawa, F. Tachibana, H. Okazaki, Phys. Rev. B 38 (5) (1988) 3050.
Fig. 1. FWHM for Ag Te at 3 temperatures with lines showing Chudley}Elliott "ts.