- Email: [email protected]
The coordination environment of Ag and Cu in ternary chalcogenide glasses
cQ& .-Elg
EL.SEVIER
JOURNAL
OF
NON-CRYSTALLINESOL Journal of Non-Crystalline
Solids 205-207
(1996)
172-175
The coordination environment of Ag and Cu in ternary chalcogenide glasses Philip S. Salmon * , Jian Liu School of Physics,
University
of East Anglia,
Norwich
NR4 7TJ, UK
Abstract An analysisis madeof the informationprovidedby the methodof isotopicsubstitutionin neutrondiffraction on the metal atomcoordination environment in M-A-X glasses, where M denotesAg or Cu, A a GroupIVB or VB element,andX a chalcogen.It is found that the M-X coordinationnumberis low (N 3-4) but thereis no clearcorrelationbetweenits precise valueandwhetherthe materialis a semiconductor or fast-ionconductor.Whenthe concentrationof M is high it is possible to measurethe M-M partialpair distributionfunction g,-,(r). In thosesystemsfor which resultsareavailableit is found that, irrespective of the electrical conductivity mechanism, short M-M nearest-neighbour distances are present. However, the first peak in g,-,(r) is moreclearly definedin the caseof semiconducting glasses.
1. Introduction The object of this paper is to present neutron diffraction results on the metal atom coordination environment in ternary chalcogenide glassesof the form M-A-X, where M denotes Ag or Cu, A denotesa Group IVB or VB element, and X denotes a chalcogen (S, Se or Te). Motivation for the study of these materials stems from the observation that when a large mole fraction of a network modifier of the type M,X, which is a joint fast-ionic/electronic conductor in its high temperature crystalline phase [1,2], is mixed with a semiconducting network former, such as A,X, or AX, the resultant glasses sometimesbecomefast-ion conducting with M+ ions as the mobile species[3]. It is therefore of interest to investigate the coordination environment of the metal
* Corresponding author. Tel.: -l-44-1603 1603 259 515; e-mail: [email protected]. 0022-3093/96/$15.00 Copyright PII SOO22-3093(96)00225-6
592 580; Fax:
0 1996 Elsevier
+44-
atom to help elucidate the reasonsfor the different electrical conductivity mechanisms. For example, ionic motion may be prevented by a deeptrapping of the metal atom in the potential well of its neighbours or, if translational motion is allowed, the movement may not be long ranged owing to the occurrence of defects which block diffusion pathways. Definitive structural information at the atomic level is a prerequisite for the development of realistic microscopic models for ionic transport in glasses[4]. Significant progress on the structure of M-A-X glassescan be made by applying the method of isotopic substitution in neutron diffraction [5,6]. This follows from the large coherent scattering length contrast of the silver (b(lo7Ag) = 7.555(11) fm, b(“‘Ag) = 4.165(11) fm) and copper (b(63Cu) = 6.43(15) fm, b(65Cu) = 10.61(19) fm) isotopes;from an ability to measurethe reciprocal spacefunctions over a wide range of scattering vector, k, such that information is provided on both the short and intermediate range atomic ordering; and from the relative
Science B.V. All rights reserved.
P.S. Salmon, J. Liu/Journal
of Non-Crystalline
ease of glass formation in many M-A-X systems which allows for the preparation of large reproducible bulk quenched samples. In particular, for many glasses the atomic fraction of M is sufficiently high to allow for measurement of the metal-metal atom pair distribution function, g,-,(r), which leads to unprecedented information on the metal atom coordination environment.
Solids 205-207
(19961 172-175
correlation functions associatedwith F(k) can also be reduced [7] by combining it with a suitably weighted A,(k) to give, in r-space, AG( r) = -&b,b’,[
In a neutron diffraction experiment on a glass, the coherent scattered intensity can be representedby the total structure factor
where c, and b, are, respectively, the atomic fraction and bound coherent scattering length of atomic species 0~.The partial structure factors, S,&k>, are related to the partial pair-distribution functions; g, s( r), through the Fourier transform relation
X
sin( kr)dk,
(2) where no is the atomic number density of the sample. By making diffraction experiments on two MA-X glasseswhich are identical in every respect, except for the isotopic composition of M, two total structure factors F(k) and ‘F(k) are measured,corresponding to bM and bh, respectively. The first order difference function A,(k) = F(k) -‘F(k) then gives, when transformed to real-space, AG,(
r) = AGM-J
r) + c&Abh[ g,-,(
r> - 11, (3)
where A%-,(r)
=2cMcxbxAbM[gM-x(r) + 2c,c,b,Ab,[
- 11 gh?-*( I-) - 11. (4)
p denotes a matrix atom (A or X), Ab& = bL - bt and Ab, = b, - b&. AG,(r) comprisesonly those correlations involving M atoms. The complexity of
g,-,(
r) - 11
+ %4G?4bxEK4-.(r) + cib,i [ g,-,(r) + cibi[
2. Theory
173
gx-dr)
- 11
- 11 - 11.
With this combination, the M-A and M-X correlations are eliminated from F(k) and the M-M correlations are the only ones which are given a negative weighting, i.e., since bA and b, are positive and g,& r) 2 0, any real features in AG( r) corresponding to values < AG(0) must contain a contribution from g,-,(r). If a third total structure factor “F(k) is measured for the same sample, corresponding to a scattering length b&(-f b& # b,), it is then possible [5,6] to extract S,-,(k) and hence gMWM(r) explicitly, provided the scattering length contrast and cM are sufficiently large. Hence the complexity of correlations associated with either AG,(r) or AG(r) can be further reduced via the elimination of g,-,(r).
3. Results and discussion The method of isotopic substitution in neutron diffraction has been successfully applied to glasses in the Ag-As-S [7], Ag-As-Se [5,6,8], Ag-As-Te [9], Ag-Ge-Se [lo] and Cu-As-Se [6] systems.The resultsfor the mean nearest-neighbourdistances, F,,, and mean coordination numbers, Et, of species p around speciesa are summarisedin Table 1 and the available g,-,(r) and AG,-,(r) functions are plotted in Fig. 1. The latter were obtained by the minimum noise reconstruction method of Soper et al [ill which gives, essentially, a smoothedversion of the Fourier transformed k-space function while ensuring that the gap(r) take physical values, i.e., gap(r) = 0 at low-r, 0 I r s rmin, and g+(r) 2 0 for r 2 r,,. The glassesare either semiconducting (S) or fast-ion conducting (FE). The first noteworthy feature is the existence of metal-metal distances, F,-,, which are relatively short in the sensethat they are comparable with the
P.S. Salmon, J. Liu / Journal of Non-CrystaIline Soiids 205-207 119961 172-175
174 Table 1 Summary diffraction
of the metal atom coordination
environment
in M-A-X
glasses as measured
using the method
Glass
Measured
(Ag,S),,,,,(As,S,),.90, &z,%,(As,S,),,, (Ag2Se)o.096(As,Se,)o.sol
- 3.00)
(Ag,Se),,,j(AsSe)o,,j (Ag,Se),.t,,(Ge,.,,,,Seo.s)0.87j (Ag,Te),,,(As,Tej),., (Cu,Se)o.zs(AsSe)~.,,
2.58(l) 2.54(l) 2.69(2) 2.66(3) 2.68(l) 2.80(3) 2.42(2)
-
3.30(10) 3.02(5) 3.03(2) 2.70(4)
2.7(2) 4.2(2) 2.8(4) 1.0(3)
The measurements were made at room temueramre a The first peak in AG,,(r) is ill-defined. ’
(= 20-25°C)
4.2(3) 3.3(2) 3.4-5.0 4.0(2) 3.00) 2.9(2) 2.9(2) 2.0(2) 1.2(2) 0 except
J
0 Fig.
1.
g,-,(r)
5 (broken
curve)
rl‘i
[AGM-,(r)g,-,(r) (fu11 curve) for (a) the fast-ion conductor (Ag,Se),.,,(AsSe),,,, where A = 0.6192 [6]; (b) the semiconductor (Cu,Se),,,,(AsSe),,,j where A = 0.6192 [6]; and (c) the semiconductor (AgzTe)o,j(As2Te3)o,j where A = 0.5672 [9].
Ac,-,(0)1/(2c,cxb,Abh,~) = g,-,(r)
and
10
+A
n
0 1 2 3.5(2)
for the Ag-Ge-Se
of isotopic
functions
AG&),AG(r) AG&),AG(r) AG,,(r),AG(r) g,+&),AGAg&) AG.,&),AG(r) g+&)>“G+&~) gcu-&),AGc&)
substitution
in neutron
Conductivity mechanism
Ref.
s FIC S FE FIC S s
t71 [71
181 161 DO1 PI k31
glass when T = 10 K.
nearest-neighbourdis!ancesin metallic Ag (2.89 A) or metallic Cu (2.56 A). This observation is independent of the nature of the electrical conductivity mechanism and is a characteristic feature of the structural chemistry of several crystalline Ag(1) and Cu(1) compounds [12]. The distancesare, however, longer than the metal-matrix atom distances, TMey, and it is not clear as to whether there exists bonded or non-bonded M-M interactions. Notwithstanding, for the semiconductors (Ag,Te)o.s(AszTe,)o,s and (Cu,Se),,,(AsSe),,,, the first peak in g,-,(r) is well defined in that it returns to the g, -M(r = 0) limit on its high-r side while for the fast-ion conductor (Ag2Se)o,25(AsSe)o,,, a much broader distribution of Ag-Ag nearest-neighbour bond lengths is found. These trends relating to the sharpnessof the first peak in g,-,(r) may therefore be transferable to other semiconducting and fast-ion conducting M-A-X glasses. By comparisonwith the structuresof several crystalline M-A-X and M-X systems,the Ag nearestneighbour matrix atoms are identified with the chalcogen whereasCu is likely to have both A and X nearest-neighbours.Nevertheless,in every case Yhlis much smaller than the sum of ionic radii for M P (0.96 and 1.26 w for Cu+ and Ag*, respectively) and X2- (1.84, 1.91 and 2.11 A for S2-, Se2- and Tes-, respectively) [13] which is indicative of substantial covalent character for the M-X bond [14]. Indeed, in crystalline fast-ion conductors, it is argued [15,16] that the potential barrier to ionic motion between sites is lowered in the case of Ag(1) and
P.S. Salmon,
J. Liu/JoumaI
of Non-Crystalline
Cu(1) owing to their ability to form bonds with substantial covalent character at each intermediate position in their motion. The coordination number IER is low (- 3-4) for all of the compounds in Table 1 but there is no clear correlation between its precise value and the nature of the electrical conductivity mechanism. The appearance of threefold coordinated M atoms is not consistent with the model of Kastner [17] which predicts that Group I metal atoms will be fourfold coordinated by the chalcogen when the covalent component to the bonding is important and electronic d states are not involved. It appears, therefore, that d states are important and the relative stability of threefold versus fourfold coordination for Cu(1) and Ag(1) compounds has recently been investigated by Burdett and Eisenstein [18] using an orbital approach. It is argued that the lower coordination number conformation can be stabilised over the regular tetrahedral arrangement if there is a distortion via a second order Jahn-Teller effect wherein the d orbitals of the occupied outer shell are mixed with the s orbitals of the valence shell.
4. Conclusions The overlap between the first peaks in gMvI--M(r) and AG,-,( r > sh own in Fig. 1 demonstrates the need to separate AG,(r) into these two functions for M-A-X glasses when C~ is large, especially since the existence of small F,-,M values can lead to a misinterpretation of the first peak in AG,(r) [6]. Moreover, the distribution of nearest-neighbour M-M distances appears as an important factor in characterising fast-ion versus semiconducting behaviour in the M-A-X glasses studied to date. For instance, the definition of the first peak in gMeM(r) for the semiconducting compounds of Fig. 1 points to a trapping of the metal atom in the potential well of its nearest neighbours [9]. In the crystalline state the ability of silver, and to a lesser extent copper, to form fast-ion conducting compounds is related, in part, to the low coordination number and flexible stereochemistry of these atoms [15,16]. The results presented in this paper show that similar considera-
Solids 205-207
(1996)
172-175
175
tions hold for the amorphous state. Nonetheless, a full appreciation of the mechanisms for ionic motion will also require information on the coordination environments of the less electropositive A and X species and on the intermediate range order [6,7,9,10].
Acknowledgements It is a pleasure to thank the UK Engineering and Physical Sciences Research Council for financial support and Drs Ian Penfold and Chris 3enmore for their substantial contributions to our research programme on M-A-X materials. J.L. also thanks the support of the University of East Anglia, the CVCP and the Great Britain-China Education Trust.
References [ll K. Shahi, Phys. Status Solidi (a)41 (1977) 11. [2] J.B. Boyce and B.A. Huberman, Phys. Rep. 51 (1979) 189. [3] Z.U. Borisova, Glassy Semiconductors (Plenum, New York, 1981). [4] S.R. Elliott, J. Non-Cryst. Solids 160 (1993) 29. 151 P.S. Salmon and C.J. Benmore in: Recent Developments in the Physics of Fluids, ed. W.S. Howells and A.K. Soper (Hilger, Bristol, 1992) p. F225. [6] C.J. Benmore and P.S. Salmon, Phys. Rev. Lett. 73 (1994) 264. [7] LT. Penfold and P.S. Salmon, Phys. Rev. Lett. 64 (1990) 2164. [8] C.J. Benmore andP.S. Salmon, J. Non-Cryst. Solids 156-158 (1993) 720. 191 J. Liu and P.S. Salmon, in preparation. [lo] R.J. Dejus, S. Susman, K.J. Volin, D.G. Montague and D.L. Price, J. Non-(&t. Solids 143 (1992) 162. [ll] A.K. Soper, C. Andreani and M. Nardone, Phys. Rev. E47 (1993) 2598. [12] A.F. Wells, Structural Inorganic Chemistry (Clarendon, Oxford, 1984). [13] R.C. Weast, ed., CRC Handbook of Chemistry and Physics, 67th Ed. (CRC, Boca Raton, FL, 1986) p, F1.57. [14] J.E. Enderby and A.C. Barnes, Rep.’ Prog. Phys. 53 (1990) 85. [15] R.D. Armstrong, R.S. Bulmer and T. Dickinson, J. Solid Stare Chem. 8 (1973) 219. [16] P. McGeehin and A. Hooper, J. Mater. Sci. 12 (1977) 1. [17] M. Kastner, Philos. Mag. B37 (1978) 127. [18] J.K. Burdett and 0. Eisenstein, Inorg. Chem. 31 (1992) 1758.
EL.SEVIER
JOURNAL
OF
NON-CRYSTALLINESOL Journal of Non-Crystalline
Solids 205-207
(1996)
172-175
The coordination environment of Ag and Cu in ternary chalcogenide glasses Philip S. Salmon * , Jian Liu School of Physics,
University
of East Anglia,
Norwich
NR4 7TJ, UK
Abstract An analysisis madeof the informationprovidedby the methodof isotopicsubstitutionin neutrondiffraction on the metal atomcoordination environment in M-A-X glasses, where M denotesAg or Cu, A a GroupIVB or VB element,andX a chalcogen.It is found that the M-X coordinationnumberis low (N 3-4) but thereis no clearcorrelationbetweenits precise valueandwhetherthe materialis a semiconductor or fast-ionconductor.Whenthe concentrationof M is high it is possible to measurethe M-M partialpair distributionfunction g,-,(r). In thosesystemsfor which resultsareavailableit is found that, irrespective of the electrical conductivity mechanism, short M-M nearest-neighbour distances are present. However, the first peak in g,-,(r) is moreclearly definedin the caseof semiconducting glasses.
1. Introduction The object of this paper is to present neutron diffraction results on the metal atom coordination environment in ternary chalcogenide glassesof the form M-A-X, where M denotes Ag or Cu, A denotesa Group IVB or VB element, and X denotes a chalcogen (S, Se or Te). Motivation for the study of these materials stems from the observation that when a large mole fraction of a network modifier of the type M,X, which is a joint fast-ionic/electronic conductor in its high temperature crystalline phase [1,2], is mixed with a semiconducting network former, such as A,X, or AX, the resultant glasses sometimesbecomefast-ion conducting with M+ ions as the mobile species[3]. It is therefore of interest to investigate the coordination environment of the metal
* Corresponding author. Tel.: -l-44-1603 1603 259 515; e-mail: [email protected]. 0022-3093/96/$15.00 Copyright PII SOO22-3093(96)00225-6
592 580; Fax:
0 1996 Elsevier
+44-
atom to help elucidate the reasonsfor the different electrical conductivity mechanisms. For example, ionic motion may be prevented by a deeptrapping of the metal atom in the potential well of its neighbours or, if translational motion is allowed, the movement may not be long ranged owing to the occurrence of defects which block diffusion pathways. Definitive structural information at the atomic level is a prerequisite for the development of realistic microscopic models for ionic transport in glasses[4]. Significant progress on the structure of M-A-X glassescan be made by applying the method of isotopic substitution in neutron diffraction [5,6]. This follows from the large coherent scattering length contrast of the silver (b(lo7Ag) = 7.555(11) fm, b(“‘Ag) = 4.165(11) fm) and copper (b(63Cu) = 6.43(15) fm, b(65Cu) = 10.61(19) fm) isotopes;from an ability to measurethe reciprocal spacefunctions over a wide range of scattering vector, k, such that information is provided on both the short and intermediate range atomic ordering; and from the relative
Science B.V. All rights reserved.
P.S. Salmon, J. Liu/Journal
of Non-Crystalline
ease of glass formation in many M-A-X systems which allows for the preparation of large reproducible bulk quenched samples. In particular, for many glasses the atomic fraction of M is sufficiently high to allow for measurement of the metal-metal atom pair distribution function, g,-,(r), which leads to unprecedented information on the metal atom coordination environment.
Solids 205-207
(19961 172-175
correlation functions associatedwith F(k) can also be reduced [7] by combining it with a suitably weighted A,(k) to give, in r-space, AG( r) = -&b,b’,[
In a neutron diffraction experiment on a glass, the coherent scattered intensity can be representedby the total structure factor
where c, and b, are, respectively, the atomic fraction and bound coherent scattering length of atomic species 0~.The partial structure factors, S,&k>, are related to the partial pair-distribution functions; g, s( r), through the Fourier transform relation
X
sin( kr)dk,
(2) where no is the atomic number density of the sample. By making diffraction experiments on two MA-X glasseswhich are identical in every respect, except for the isotopic composition of M, two total structure factors F(k) and ‘F(k) are measured,corresponding to bM and bh, respectively. The first order difference function A,(k) = F(k) -‘F(k) then gives, when transformed to real-space, AG,(
r) = AGM-J
r) + c&Abh[ g,-,(
r> - 11, (3)
where A%-,(r)
=2cMcxbxAbM[gM-x(r) + 2c,c,b,Ab,[
- 11 gh?-*( I-) - 11. (4)
p denotes a matrix atom (A or X), Ab& = bL - bt and Ab, = b, - b&. AG,(r) comprisesonly those correlations involving M atoms. The complexity of
g,-,(
r) - 11
+ %4G?4bxEK4-.(r) + cib,i [ g,-,(r) + cibi[
2. Theory
173
gx-dr)
- 11
- 11 - 11.
With this combination, the M-A and M-X correlations are eliminated from F(k) and the M-M correlations are the only ones which are given a negative weighting, i.e., since bA and b, are positive and g,& r) 2 0, any real features in AG( r) corresponding to values < AG(0) must contain a contribution from g,-,(r). If a third total structure factor “F(k) is measured for the same sample, corresponding to a scattering length b&(-f b& # b,), it is then possible [5,6] to extract S,-,(k) and hence gMWM(r) explicitly, provided the scattering length contrast and cM are sufficiently large. Hence the complexity of correlations associated with either AG,(r) or AG(r) can be further reduced via the elimination of g,-,(r).
3. Results and discussion The method of isotopic substitution in neutron diffraction has been successfully applied to glasses in the Ag-As-S [7], Ag-As-Se [5,6,8], Ag-As-Te [9], Ag-Ge-Se [lo] and Cu-As-Se [6] systems.The resultsfor the mean nearest-neighbourdistances, F,,, and mean coordination numbers, Et, of species p around speciesa are summarisedin Table 1 and the available g,-,(r) and AG,-,(r) functions are plotted in Fig. 1. The latter were obtained by the minimum noise reconstruction method of Soper et al [ill which gives, essentially, a smoothedversion of the Fourier transformed k-space function while ensuring that the gap(r) take physical values, i.e., gap(r) = 0 at low-r, 0 I r s rmin, and g+(r) 2 0 for r 2 r,,. The glassesare either semiconducting (S) or fast-ion conducting (FE). The first noteworthy feature is the existence of metal-metal distances, F,-,, which are relatively short in the sensethat they are comparable with the
P.S. Salmon, J. Liu / Journal of Non-CrystaIline Soiids 205-207 119961 172-175
174 Table 1 Summary diffraction
of the metal atom coordination
environment
in M-A-X
glasses as measured
using the method
Glass
Measured
(Ag,S),,,,,(As,S,),.90, &z,%,(As,S,),,, (Ag2Se)o.096(As,Se,)o.sol
- 3.00)
(Ag,Se),,,j(AsSe)o,,j (Ag,Se),.t,,(Ge,.,,,,Seo.s)0.87j (Ag,Te),,,(As,Tej),., (Cu,Se)o.zs(AsSe)~.,,
2.58(l) 2.54(l) 2.69(2) 2.66(3) 2.68(l) 2.80(3) 2.42(2)
-
3.30(10) 3.02(5) 3.03(2) 2.70(4)
2.7(2) 4.2(2) 2.8(4) 1.0(3)
The measurements were made at room temueramre a The first peak in AG,,(r) is ill-defined. ’
(= 20-25°C)
4.2(3) 3.3(2) 3.4-5.0 4.0(2) 3.00) 2.9(2) 2.9(2) 2.0(2) 1.2(2) 0 except
J
0 Fig.
1.
g,-,(r)
5 (broken
curve)
rl‘i
[AGM-,(r)g,-,(r) (fu11 curve) for (a) the fast-ion conductor (Ag,Se),.,,(AsSe),,,, where A = 0.6192 [6]; (b) the semiconductor (Cu,Se),,,,(AsSe),,,j where A = 0.6192 [6]; and (c) the semiconductor (AgzTe)o,j(As2Te3)o,j where A = 0.5672 [9].
Ac,-,(0)1/(2c,cxb,Abh,~) = g,-,(r)
and
10
+A
n
0 1 2 3.5(2)
for the Ag-Ge-Se
of isotopic
functions
AG&),AG(r) AG&),AG(r) AG,,(r),AG(r) g,+&),AGAg&) AG.,&),AG(r) g+&)>“G+&~) gcu-&),AGc&)
substitution
in neutron
Conductivity mechanism
Ref.
s FIC S FE FIC S s
t71 [71
181 161 DO1 PI k31
glass when T = 10 K.
nearest-neighbourdis!ancesin metallic Ag (2.89 A) or metallic Cu (2.56 A). This observation is independent of the nature of the electrical conductivity mechanism and is a characteristic feature of the structural chemistry of several crystalline Ag(1) and Cu(1) compounds [12]. The distancesare, however, longer than the metal-matrix atom distances, TMey, and it is not clear as to whether there exists bonded or non-bonded M-M interactions. Notwithstanding, for the semiconductors (Ag,Te)o.s(AszTe,)o,s and (Cu,Se),,,(AsSe),,,, the first peak in g,-,(r) is well defined in that it returns to the g, -M(r = 0) limit on its high-r side while for the fast-ion conductor (Ag2Se)o,25(AsSe)o,,, a much broader distribution of Ag-Ag nearest-neighbour bond lengths is found. These trends relating to the sharpnessof the first peak in g,-,(r) may therefore be transferable to other semiconducting and fast-ion conducting M-A-X glasses. By comparisonwith the structuresof several crystalline M-A-X and M-X systems,the Ag nearestneighbour matrix atoms are identified with the chalcogen whereasCu is likely to have both A and X nearest-neighbours.Nevertheless,in every case Yhlis much smaller than the sum of ionic radii for M P (0.96 and 1.26 w for Cu+ and Ag*, respectively) and X2- (1.84, 1.91 and 2.11 A for S2-, Se2- and Tes-, respectively) [13] which is indicative of substantial covalent character for the M-X bond [14]. Indeed, in crystalline fast-ion conductors, it is argued [15,16] that the potential barrier to ionic motion between sites is lowered in the case of Ag(1) and
P.S. Salmon,
J. Liu/JoumaI
of Non-Crystalline
Cu(1) owing to their ability to form bonds with substantial covalent character at each intermediate position in their motion. The coordination number IER is low (- 3-4) for all of the compounds in Table 1 but there is no clear correlation between its precise value and the nature of the electrical conductivity mechanism. The appearance of threefold coordinated M atoms is not consistent with the model of Kastner [17] which predicts that Group I metal atoms will be fourfold coordinated by the chalcogen when the covalent component to the bonding is important and electronic d states are not involved. It appears, therefore, that d states are important and the relative stability of threefold versus fourfold coordination for Cu(1) and Ag(1) compounds has recently been investigated by Burdett and Eisenstein [18] using an orbital approach. It is argued that the lower coordination number conformation can be stabilised over the regular tetrahedral arrangement if there is a distortion via a second order Jahn-Teller effect wherein the d orbitals of the occupied outer shell are mixed with the s orbitals of the valence shell.
4. Conclusions The overlap between the first peaks in gMvI--M(r) and AG,-,( r > sh own in Fig. 1 demonstrates the need to separate AG,(r) into these two functions for M-A-X glasses when C~ is large, especially since the existence of small F,-,M values can lead to a misinterpretation of the first peak in AG,(r) [6]. Moreover, the distribution of nearest-neighbour M-M distances appears as an important factor in characterising fast-ion versus semiconducting behaviour in the M-A-X glasses studied to date. For instance, the definition of the first peak in gMeM(r) for the semiconducting compounds of Fig. 1 points to a trapping of the metal atom in the potential well of its nearest neighbours [9]. In the crystalline state the ability of silver, and to a lesser extent copper, to form fast-ion conducting compounds is related, in part, to the low coordination number and flexible stereochemistry of these atoms [15,16]. The results presented in this paper show that similar considera-
Solids 205-207
(1996)
172-175
175
tions hold for the amorphous state. Nonetheless, a full appreciation of the mechanisms for ionic motion will also require information on the coordination environments of the less electropositive A and X species and on the intermediate range order [6,7,9,10].
Acknowledgements It is a pleasure to thank the UK Engineering and Physical Sciences Research Council for financial support and Drs Ian Penfold and Chris 3enmore for their substantial contributions to our research programme on M-A-X materials. J.L. also thanks the support of the University of East Anglia, the CVCP and the Great Britain-China Education Trust.
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