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The preparation and properties of an antimony sulphide glass
Mat. Res. Bull., Vol. 16, p p . 1569-1577, 1981. Printed in the USA. 0025-5408/81/121569-09502.00/0 Copyright (c) 1981 P e r g a m o n P r e s s Ltd.
THE PREPARATION AND PROPERTIES OF AN ANTIMONY SULPHIDE GLASS
J.R. Gannon*
R.J.D. Tilley**
and
A.C. Wright**
* Optical Waveguide Department Standard Telecommunication Laboratories Ltd London Road Harlow CMI7 9NA Essex England ** School of Materials Science University of Bradford Bradford BD7 IDP West Yorkshire England
(Received October 13, 1981; Communicated b y C. H. L. Goodman)
ABSTRACT A glass has been prepared in the Sb2S3-PbS system at compositions very close to 80 mol % Sb2S 3 : 20 mol % PbS. It has been studied by powder x-ray diffraction, optical and high resolution transmission microscopy and infra-red spectroscopy, and its decomposition characterised by differential scanning calorimetry. Hardness values are also reported. This glass displays a wide range of transparency from 4.5 to 16.5 ~m and may be of interest for a number of infrared applications. Structural studies indicate that it is formed of Sb-S and Pb-s polyhedra similar to those found in the crystalline Pb-Sb-S sulphides, and recrystallisation of the glass, observed by electron microscopy shows that the rearrangement of the polyhedra in transforming from the glass state to the crystalline state is -feasible. The formation of the glass and its limited composition range are discussed in terms of the strain involved in linking the Pb-S and Sb-S polyhedra together and this leads to a number of suggestions as to how the existence region over which the glass forms could be expanded.
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Introduction In recent years there has been a considerable interest shown in non-oxide glasses. This is because they can offer important advantages over their oxide counterparts in areas of application associated with optical (1,2,) acousto-optical (3) or electrical (4) properties. Of this group, sulphide glasses are of some importance because they have the potential for extended infrared transmission, in many cases beyond 12 ~m which is of advantage when currently available laser outputs are considered. In particular a great deal of effort has recently been devoted to finding a stable glass which has high transparency over the 8 - 13 ~m atmospheric window for use, in fibre form, as a waveguiding medium. Typical applications of such fibre waveguides are for passive systems where a detector is linked to associated tracking optics, for power transfer in optical circuits, for laser machining and communication data links (5). Moreover there is an interest in certain chalcide glasses which are able to show ovonic behaviour, i.e. threshold or memory switching (6). During an investigation of the phases occurring in the PbS-Sb2S 3 system we have found a glass in the Sb2S3-rich region. Although there have been a number of investigations of this system (see for example 7,8,9 and references therein) a glass has not been reported previously. Indeed, despite the fact that the PbS rich part of the phase diagram is complex no phases are reported to form over much of the Sb2S 3 rich region, and the phase of closest composition to Sb2S 3 is zinckenite, with a reported composition of approximately 56 mol % S b 2 S 3 : 4 4 mol % PbS. The only feature of interest in the phase diagram between zinckenite and Sb2S 3 is an eutectic occurring at approximately 79 mol % Sb2S 3 and with an eutectic temperature of 996K. Because of the interest in sulphide glasses mentioned above we are reporting our findings in this note. Experimental Various compositions in the PbS-Sb2S 3 system were prepared from elemental lead, antimony and sulphur, of 'Specpure' grade, supplied by Johnson Matthey Ltd. The supplier's analytical data indicated that the total metallic impurity content of each chemical was below 10 ppm. Small chips of lead were pared from the supplied rod with a fresh scalpel, after surface tarnish had been removed. The antimony was supplied in the form of large lumps, which were fractured in a hardened steel percussion mortar. The sulphur was supplied as a powder and was used without further treatment. The samples were prepared by sealing the elements, in their correct proportions, in evacuated quartz ampoules. Usually quartz tube was used for this but sometimes flat bottomed ampoules, of approximately 20 mm diameter, were employed so as to produce a flat sample geometry for property measurement. The total weight of each sample was approximately 3 grams. The materials were melted at temperatures ranging from 823K to 1273K and allowed to remain at that temperature for 30 minutes to allow for complete melting after which they were quenched into a bath of iced brine. After preparation, powder x-ray diffraction patterns were
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recorded using a Guinier-H~gg focussing camera and strictly monochromatic Cuk~ 1 radiation. The samples were also examined electron optically using a JEMIOOB electron microscope fitted with a goniometer stage and operating at iOOkV. For this latter purpose, specimens were prepared by crushing fragments of glass in an agate mortar under n-butanol and allowing a drop of the resultant suspension to dry on a copper support grid which had been coated with a holey carbon film.(10) The chemical composition of the glass was evaluated using a JEOL JXA5OA microprobe fitted with a Link Systems 860 energy dispersive x-ray analyser, operating at 15kV. Critical transition temperatures were determined by means of a DuPont 900 thermal analyser equipped with a differential scanning calorimeter cell. The cell was purged with nitrogen at 0.4 £/min and the heating rate applied to an approximately 50 mg bulk sample of glass was 20°C/minute. The infra-red transmission spectrum was recorded using a Grubb-Parsons GS8 spectrophotometer and hardness data was obtained using a Vickers microhardness indenter with a 5Og load. Results composition range and conditions of formation Material in the glassy state was only found to occur, in any quantity, for starting compositions of 80 mol % Sb2S3, 20 mol % PbS. Moreover, glassy material was only formed in ampoules which had been quenched rapidly from the melt and no glass was formed in samples which had been cooled in air. The formation of glass only took place where the melt had been in contact with the capsule wall, the remainder of the charge being crystalline. This usually gave rise to a thin curved shell of glass although thicker pieces were formed at the end of the capsule. Under no circumstances was a sample consisting purely of glass produced. An electron microprobe analysis showed that, within the limits of experimental error, the composition of the glass was much the same as that of the starting material. Thus glass formation in this system appears to be difficult and the glass formation region restricted to the position of the eutectic. Structural results The material was initially shown to be a glass since x-ray powder diffraction films showed no diffraction lines. Material examined electron optically at high resolution showed the granular appearance expected from a glassy material and in addition gave rise to electron diffraction patterns which consisted solely of a few diffuse rings as shown in figure i. Observations at low magnification also revealed that the samples often contained voids having an approximate diameter of 1 ~m. Examination of thick fragments of glass under high beam intensity revealed that the glass could be crystallised in situ. An example of a partially recrystallised fragment is shown in figure 2(a) and typically recrystallised fragments in figures 2(b) and 2(c). The predominant mode of crystal growth from the glass was seen to be one of the formation of successive layers of crystal growth normal to the observed lattice fringes rather than growth by extension of the layers. That is, the glass appears to crystallise normal to the front A in figure 2(a) rather than parallel to it. The electron diffraction patterns of such recrystallised fragments consisted of ring patterns, as shown in figure 3.
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FIG. 1
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FIG. 2(a)
High resolution electron micrograph of a fragment of the PbS-Sb2S 3 glass, revealing that it does not appear to contain crystalline regions of any appreciable size. The electron diffraction pattern from the material is inset.
High resolution electron micrograph of a fragment of glass that has partially recrystallised in the electron beam. Direction of crystal growth is normal to the front A
FIG 2(b) and 2(c) Typical micrographs of glass that has fully crystallised in the electron beam. Only three crystallites are visible in (b) but many small crystalline regions are visible in (c).
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,;0 5;0 5;0 3~0 3;0 ,~0 ,;0 ~0 Temperature (oC)
FIG. 3 Electron diffraction pattern of crystallised glass phase. The expected pattern from polycrystalline Sb2S 3 is inset and reveals a close correspondence between the two.
FIG. 4 Differential scanning calorimeter trace for the PbS-Sb2S 3 glass.
The disposition and intensities of the rings correspond reasonably well with that expected from polycrystalline Sb2S3, as can be seen from the inset on Figure 3. In addition we can note that the broad maxima in the diffraction patterns from the glass corresponded well with the major concentration of rings in the polycrystalline phase revealing a close correspondence between the two structures. The most reasonable interpretation of this diffraction evidence is to suppose that the glass recrystallises to a disordered Sb2S 3 structure. Indeed the structural features illustrated in Figure 2(b) and 2(c) are observed in crystalline samples of the same composition which have been prepared in this system, also x-ray powder patterns show only diffuse Sb2S 3 reflections and we are satisfied as to the identity of the two materials. Although we have not attempted a thorough examination of the diffuse ring patterns produced by the glass, its overall resemblance to the diffraction pattern of the recrystallised phase together with examination of related studies on PbS-As2S 3 glasses,(ll) suggests that the glass consists of sb-s polyhedra similar to those found in the crystalline state, but with a greater degree of disorder. Such an interpretation agrees well with the crystallisation observed in the electron microscope, as it is clear from the reaction that only slight changes are needed to convert the glass to the crystalline state. Large degrees of strain and the presence of strain fields can usually be imaged, directly or indirectly in an electron microscope, and were not seen, and neither was any cracking or breaking of the crystal, as would be expected if significant strain or volume changes had occurred.
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Properties On first heating, the glass showed a DSC curve consisting of an endothermic glass transition, a multiplicity of exothermic crystallisation peaks and the onset of an endothermic melting peak, as shown in figure 4. Thereafter, cycling at 20°C/minute resulted in an apparent stablisation of the highest temperature crystalline phase, and reproducible melting endotherms. There was no measurable weight loss after two such heating cycles. By taking the extrapolated onset of baseline shifts as being the critical temperatures, values of the glass transition and melting temperatures were recorded as 205°C and 445°C respectively, with a reproducibility of ± 3°C. The onset of crystallisation exotherms were recorded in the same way as being 227°C, 256°C, 301°C and 353°C. These results reveal that the glass is not very stable, and so fully support the observations made concerning the difficulty of preparing the glass and the ease with which it recrystallised during electron microscope observation. In terms of the DSC results, the glass stability is generally reflected in the magnitude of the temperature interval between melting and crystallisation critical points. The closer these two values are to each other, the more stable the glass is, and the less critical is the quench rate. In our material we can note that the first crystallisation exotherm almost overlaps the glass transition peak, resulting in a ready recrystallisation of the glass if cooling is other than extremely rapid. The DSC result also shows at least four "crystallisation" exotherms. The disordered nature of the recrystallised glass, as shown in figures 2(b) and 2(c), suggest that significant improvement in order would be expected to take place as the initially recrystallised glass is annealed, and it is these transformations that are likely to cause the additional exotherms noted. In terms of ultimate stability, a two phase mixture of zinckenite and Sb2S 3 is to be expected, and the ordering process may involve the exsolution of zinckenite. A study of this crystallisation and ordering reaction is underway and will be reported in a future communication. The results obtained to date, however, confirm the reaction to be complex and in agreement with the DSC data given above. The microhardness of the glass, averaged over five readings, was 161 kg.mm -2. This is a rather low value, indicating that the glass is rather soft. It is, though, a typical figure for this type of material. The infra-red transmission spectrum, although lacking in resolution both due to the sample geometry and the high level of free carrier absorption, took the form of an essentially featureless curve having a gradual increase in transmission from 4.5 to 14 ~m, indicative of a fairly high level of scatter in the sample. From the point of maximum transparency at 14 ~m, the spectrum showed an exponential fall in transmission with ultimate cut-off at 16.5 ~m. No absorption bands were detected in this study; however, work is now in hand to explore the spectral characteristics more fully, particularly at low temperatures where free carrier effects would be suppressed.
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Discussion There are a number of possible ways available to explain the existence of a glass in this system, however, for the purposes of this communication a fairly simple consideration of the system strain energy will be presented. A more rigorous approach would be to extend these ideas by employing the concept of equalisation of electronegativities, and this aspect will be pursued in a future communication. Our results, have shown that glass formation is possible in the PbS-Sb2S 3 system, but that we are near to the limits of glass forming ability. This is in accord with the overall trends in the periodic group which contains Sb, ie. Group V, containing the elements P,As,Sb and Bi. Phosphorus is a well known glass forming element, and large numbers of phosphorus containing glasses are known. However, this small ion is somewhat different in behaviour to the other more metallic members of this group, and we can set it to one side having noted this glass forming tendency and consider the closely related trio of As,Sb and Bi. Glasses form fairly readily with As2S 3 and in particular they are easily prepared in the PbS-As2S 3 system (ii). In the case of Sb2S 3, glass formation is far more difficult and, as we have seen, glass formation is only found in the PbS-Sb2S 3 system in a very limited region. The Bi2S3-PbS system exhibits no glass-like phases to the best of our knowledge. There are a number of reasons why this trend should appear. The most trivial of these is related to the ionic size of the atoms As,Sb and Bi. As ionic size increases, the outer electron shells of the Group V atoms overlap more with their neighbours and the compounds become more metallic in nature. While metallic glasses are now known, in general metals are difficulty to solidify in a non-crystalline state. The same considerations apply in the case of As2S3, Sb2S 3 and Bi2S 3 . At a more fundamental level it is necessary to consider the crystal chemistry of the solids and the probable structure of the melt in these PbS-M2S 3 systems. The As2S 3 , Sb2S 3 and Bi2S 3 sulphides come into the group labelled sulphosalts by mineralogists. There have been a number of attempts to classify the sulphosalts based on structural considerations. For our purposes, although it is not the more recent, the classification of Takeuchi and Sandanage (12) is of most use. These authors draw attention to the fact that the As,Sb and Bi atoms form pyramidal MS 3 groups and that the geometry of these groups varies with the atom M involved. Clearly these groups are smallest for AsS 3 and largest for BiS 3. In the PbS-M2S 3 system, these pyramids must link with PbS 6 octahedra, along edges or faces. The octahedron edge in PbS is about 0.42 nm, considerably longer than the edges of the MS 3 pyramids and some appreciable strain results in linking the two units together. The strain is greatest for the smallest MS 3 group, ASS3, and least for the largest _MS3 group, BiS 3. In the melt it is reasonable to assume that both PbS octahedra and MS 3 groups are present. While other coordination polyhedra are possible, of course, they are less reasonable. In the pure sulphides of As,Sb and Bi the _MS3 groups link together in chains, and strain is minimal. In the presence of the PbS 6 octahedra, these have to be accommodated into the chain structures somehow, and this is clearly more readily done when the strain is least. In terms of classical glass theory, the PbS 6 groups
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act as glass forming groups but their purpose is not to prevent networks of chains forming, but to prevent crystallisation by distorting the coordination of the regular _MS3 groups and introducing a degree of strain into the lattice. Thus the system is closely related to the strained mixed cluster theory of glass formation proposed by Goodman (13). (A particular consequence of the SMC model is that the glass composition will be slightly displaced from the eutectic towards the polymorphic end member and in this system it is indeed found that the glass had a composition which was shifted 1 mol % from the eutectic towards sb2S3.) In the As2S3-PbS system, with greatest strain, glass formation takes place over a wide range of compositions. In the Sb2S3-PbS system, the situation is finely balanced. Any crystallisation at all prevents glass formation, and it is only by rapid quenching close to the eutectic composition that the melt can effectively be cooled to a glassy solid. In the Bi2S3-PbS system the strain appears to be too small to cause a glass to form, although ultra rapid quenching may just achieve this state, provided liquidus-solidus regions of the phase diagram were chosen with care. A fairly recent refinement of the structure of zinckenite (14) suggests that it is composed of pyramidal units and trigonal prisms. By introducing a degree of rotation, it is of course possible to distort a trigonal prism to an octahedron and it is, therefore, possible to extend the above arguments to cover a situation where the glass is considered in terms of zinckenite-related and MS 3 units. These ideas are substantiated by the electron microscope observation of the recrystallisation of the PbS-Sb2S 3 glass. The process is easy and involves little rearrangement of the glass, indicating that the glass is composed of the same metal-sulphur polyhedra as in the crystalline solid. The product of the recrystallisation also bears close resemblence to slow cooled crystalline phases produced in this system. In terms of this model it is relatively simple to predict how to extend the glass formation region in this system. Effectively we need to increase the degree of misfit, and hence the degree of strain, between the octahedral and pyramidal groups. To do this we have to increase the size of the ootahedral group, and so nominally replace the Pb by larger ions which still preserve octahedral coordination. It is also possible to replace the sulphur, but, although this is less likely to have as great an effect as that due to the difference in nominal radii between cationic species, it would be of interest to seek an analogous glass in the PbSe-Sb2Se 3 system. In a similar way we can attempt to decrease the size of the MS 3 groups. This can be done by replacing Sb by a smaller cationic species, or by replacing sulphur by a smaller anionic species. Such doping experiments are of interest, not least to check the validity of these ideas, and results will be reported in the future. In conclusion i t is possible to say that although the glass phase which forms in the PbS-Sb2S 3 system, forms only over a restricted temperature and composition region it seems to be of some interest. In particular, the preparation difficulties encountered in this study may be overcome in applications such as CVD sputtered films. Further investigations, particularly of electrical properties on this and doped materials are planned for the future.
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Acknowledgements J.R. Gannon wishes to thank the Directors of STL Ltd for permission to publish this paper. A.C. Wright wishes to thank the Science Research Council for financial support. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14.
J.A. Savage and S. Nielson, Infra-red Physics ~, 195, (1965) J.R. Gannon, J. Non-Cryst. Solids 42, 239, (1980) J.T. Krau~e, C.R. Kurkjian, D.A. Pinnow and E.A. Sigety, AppI. Phys. Letters 17, 367, (1970) S.R. Ovshinsky, Phys. Rev. Letters 21, 1450, (1968) J.R. Gannon, in Proc. SPIE Tech Sym, Los Angeles, Feb (1981) J.A. Savage, J. Mater. Sci. 6, 964, (1971) P . L . Garvin, N. Jb. Miner-Abh 118, 235, (1973) J.R. Craig, L.L.Y. Chang and W.R. Lees, Canad. Mineralogist 12, 199, (1973) B. Salanci, N. Jb Miner-Abh 135, 315, (1979) J.R. Gannon and R.J.D. Tilley, J. Microsc. 106, 59, (1976) J.I. Petz, R.F. Kruh and G.C. Amstutz, J. Chem. Phys. 34, 526, (1961) Y.U. Takeuchi and R. Sadanaga, Zeit. Kristallogr. 130, 346, (1969) C.H.L. Goodman, in P.H. Gaskell (Ed) Structure of non-crystalline materials p. 197 Taylor and Francis, London (1977) J.C. Portheine and W. Nowaki, Zeit. Kristallogr. 141, 79, (1975)
THE PREPARATION AND PROPERTIES OF AN ANTIMONY SULPHIDE GLASS
J.R. Gannon*
R.J.D. Tilley**
and
A.C. Wright**
* Optical Waveguide Department Standard Telecommunication Laboratories Ltd London Road Harlow CMI7 9NA Essex England ** School of Materials Science University of Bradford Bradford BD7 IDP West Yorkshire England
(Received October 13, 1981; Communicated b y C. H. L. Goodman)
ABSTRACT A glass has been prepared in the Sb2S3-PbS system at compositions very close to 80 mol % Sb2S 3 : 20 mol % PbS. It has been studied by powder x-ray diffraction, optical and high resolution transmission microscopy and infra-red spectroscopy, and its decomposition characterised by differential scanning calorimetry. Hardness values are also reported. This glass displays a wide range of transparency from 4.5 to 16.5 ~m and may be of interest for a number of infrared applications. Structural studies indicate that it is formed of Sb-S and Pb-s polyhedra similar to those found in the crystalline Pb-Sb-S sulphides, and recrystallisation of the glass, observed by electron microscopy shows that the rearrangement of the polyhedra in transforming from the glass state to the crystalline state is -feasible. The formation of the glass and its limited composition range are discussed in terms of the strain involved in linking the Pb-S and Sb-S polyhedra together and this leads to a number of suggestions as to how the existence region over which the glass forms could be expanded.
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Introduction In recent years there has been a considerable interest shown in non-oxide glasses. This is because they can offer important advantages over their oxide counterparts in areas of application associated with optical (1,2,) acousto-optical (3) or electrical (4) properties. Of this group, sulphide glasses are of some importance because they have the potential for extended infrared transmission, in many cases beyond 12 ~m which is of advantage when currently available laser outputs are considered. In particular a great deal of effort has recently been devoted to finding a stable glass which has high transparency over the 8 - 13 ~m atmospheric window for use, in fibre form, as a waveguiding medium. Typical applications of such fibre waveguides are for passive systems where a detector is linked to associated tracking optics, for power transfer in optical circuits, for laser machining and communication data links (5). Moreover there is an interest in certain chalcide glasses which are able to show ovonic behaviour, i.e. threshold or memory switching (6). During an investigation of the phases occurring in the PbS-Sb2S 3 system we have found a glass in the Sb2S3-rich region. Although there have been a number of investigations of this system (see for example 7,8,9 and references therein) a glass has not been reported previously. Indeed, despite the fact that the PbS rich part of the phase diagram is complex no phases are reported to form over much of the Sb2S 3 rich region, and the phase of closest composition to Sb2S 3 is zinckenite, with a reported composition of approximately 56 mol % S b 2 S 3 : 4 4 mol % PbS. The only feature of interest in the phase diagram between zinckenite and Sb2S 3 is an eutectic occurring at approximately 79 mol % Sb2S 3 and with an eutectic temperature of 996K. Because of the interest in sulphide glasses mentioned above we are reporting our findings in this note. Experimental Various compositions in the PbS-Sb2S 3 system were prepared from elemental lead, antimony and sulphur, of 'Specpure' grade, supplied by Johnson Matthey Ltd. The supplier's analytical data indicated that the total metallic impurity content of each chemical was below 10 ppm. Small chips of lead were pared from the supplied rod with a fresh scalpel, after surface tarnish had been removed. The antimony was supplied in the form of large lumps, which were fractured in a hardened steel percussion mortar. The sulphur was supplied as a powder and was used without further treatment. The samples were prepared by sealing the elements, in their correct proportions, in evacuated quartz ampoules. Usually quartz tube was used for this but sometimes flat bottomed ampoules, of approximately 20 mm diameter, were employed so as to produce a flat sample geometry for property measurement. The total weight of each sample was approximately 3 grams. The materials were melted at temperatures ranging from 823K to 1273K and allowed to remain at that temperature for 30 minutes to allow for complete melting after which they were quenched into a bath of iced brine. After preparation, powder x-ray diffraction patterns were
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recorded using a Guinier-H~gg focussing camera and strictly monochromatic Cuk~ 1 radiation. The samples were also examined electron optically using a JEMIOOB electron microscope fitted with a goniometer stage and operating at iOOkV. For this latter purpose, specimens were prepared by crushing fragments of glass in an agate mortar under n-butanol and allowing a drop of the resultant suspension to dry on a copper support grid which had been coated with a holey carbon film.(10) The chemical composition of the glass was evaluated using a JEOL JXA5OA microprobe fitted with a Link Systems 860 energy dispersive x-ray analyser, operating at 15kV. Critical transition temperatures were determined by means of a DuPont 900 thermal analyser equipped with a differential scanning calorimeter cell. The cell was purged with nitrogen at 0.4 £/min and the heating rate applied to an approximately 50 mg bulk sample of glass was 20°C/minute. The infra-red transmission spectrum was recorded using a Grubb-Parsons GS8 spectrophotometer and hardness data was obtained using a Vickers microhardness indenter with a 5Og load. Results composition range and conditions of formation Material in the glassy state was only found to occur, in any quantity, for starting compositions of 80 mol % Sb2S3, 20 mol % PbS. Moreover, glassy material was only formed in ampoules which had been quenched rapidly from the melt and no glass was formed in samples which had been cooled in air. The formation of glass only took place where the melt had been in contact with the capsule wall, the remainder of the charge being crystalline. This usually gave rise to a thin curved shell of glass although thicker pieces were formed at the end of the capsule. Under no circumstances was a sample consisting purely of glass produced. An electron microprobe analysis showed that, within the limits of experimental error, the composition of the glass was much the same as that of the starting material. Thus glass formation in this system appears to be difficult and the glass formation region restricted to the position of the eutectic. Structural results The material was initially shown to be a glass since x-ray powder diffraction films showed no diffraction lines. Material examined electron optically at high resolution showed the granular appearance expected from a glassy material and in addition gave rise to electron diffraction patterns which consisted solely of a few diffuse rings as shown in figure i. Observations at low magnification also revealed that the samples often contained voids having an approximate diameter of 1 ~m. Examination of thick fragments of glass under high beam intensity revealed that the glass could be crystallised in situ. An example of a partially recrystallised fragment is shown in figure 2(a) and typically recrystallised fragments in figures 2(b) and 2(c). The predominant mode of crystal growth from the glass was seen to be one of the formation of successive layers of crystal growth normal to the observed lattice fringes rather than growth by extension of the layers. That is, the glass appears to crystallise normal to the front A in figure 2(a) rather than parallel to it. The electron diffraction patterns of such recrystallised fragments consisted of ring patterns, as shown in figure 3.
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FIG. 1
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FIG. 2(a)
High resolution electron micrograph of a fragment of the PbS-Sb2S 3 glass, revealing that it does not appear to contain crystalline regions of any appreciable size. The electron diffraction pattern from the material is inset.
High resolution electron micrograph of a fragment of glass that has partially recrystallised in the electron beam. Direction of crystal growth is normal to the front A
FIG 2(b) and 2(c) Typical micrographs of glass that has fully crystallised in the electron beam. Only three crystallites are visible in (b) but many small crystalline regions are visible in (c).
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FIG. 3 Electron diffraction pattern of crystallised glass phase. The expected pattern from polycrystalline Sb2S 3 is inset and reveals a close correspondence between the two.
FIG. 4 Differential scanning calorimeter trace for the PbS-Sb2S 3 glass.
The disposition and intensities of the rings correspond reasonably well with that expected from polycrystalline Sb2S3, as can be seen from the inset on Figure 3. In addition we can note that the broad maxima in the diffraction patterns from the glass corresponded well with the major concentration of rings in the polycrystalline phase revealing a close correspondence between the two structures. The most reasonable interpretation of this diffraction evidence is to suppose that the glass recrystallises to a disordered Sb2S 3 structure. Indeed the structural features illustrated in Figure 2(b) and 2(c) are observed in crystalline samples of the same composition which have been prepared in this system, also x-ray powder patterns show only diffuse Sb2S 3 reflections and we are satisfied as to the identity of the two materials. Although we have not attempted a thorough examination of the diffuse ring patterns produced by the glass, its overall resemblance to the diffraction pattern of the recrystallised phase together with examination of related studies on PbS-As2S 3 glasses,(ll) suggests that the glass consists of sb-s polyhedra similar to those found in the crystalline state, but with a greater degree of disorder. Such an interpretation agrees well with the crystallisation observed in the electron microscope, as it is clear from the reaction that only slight changes are needed to convert the glass to the crystalline state. Large degrees of strain and the presence of strain fields can usually be imaged, directly or indirectly in an electron microscope, and were not seen, and neither was any cracking or breaking of the crystal, as would be expected if significant strain or volume changes had occurred.
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Properties On first heating, the glass showed a DSC curve consisting of an endothermic glass transition, a multiplicity of exothermic crystallisation peaks and the onset of an endothermic melting peak, as shown in figure 4. Thereafter, cycling at 20°C/minute resulted in an apparent stablisation of the highest temperature crystalline phase, and reproducible melting endotherms. There was no measurable weight loss after two such heating cycles. By taking the extrapolated onset of baseline shifts as being the critical temperatures, values of the glass transition and melting temperatures were recorded as 205°C and 445°C respectively, with a reproducibility of ± 3°C. The onset of crystallisation exotherms were recorded in the same way as being 227°C, 256°C, 301°C and 353°C. These results reveal that the glass is not very stable, and so fully support the observations made concerning the difficulty of preparing the glass and the ease with which it recrystallised during electron microscope observation. In terms of the DSC results, the glass stability is generally reflected in the magnitude of the temperature interval between melting and crystallisation critical points. The closer these two values are to each other, the more stable the glass is, and the less critical is the quench rate. In our material we can note that the first crystallisation exotherm almost overlaps the glass transition peak, resulting in a ready recrystallisation of the glass if cooling is other than extremely rapid. The DSC result also shows at least four "crystallisation" exotherms. The disordered nature of the recrystallised glass, as shown in figures 2(b) and 2(c), suggest that significant improvement in order would be expected to take place as the initially recrystallised glass is annealed, and it is these transformations that are likely to cause the additional exotherms noted. In terms of ultimate stability, a two phase mixture of zinckenite and Sb2S 3 is to be expected, and the ordering process may involve the exsolution of zinckenite. A study of this crystallisation and ordering reaction is underway and will be reported in a future communication. The results obtained to date, however, confirm the reaction to be complex and in agreement with the DSC data given above. The microhardness of the glass, averaged over five readings, was 161 kg.mm -2. This is a rather low value, indicating that the glass is rather soft. It is, though, a typical figure for this type of material. The infra-red transmission spectrum, although lacking in resolution both due to the sample geometry and the high level of free carrier absorption, took the form of an essentially featureless curve having a gradual increase in transmission from 4.5 to 14 ~m, indicative of a fairly high level of scatter in the sample. From the point of maximum transparency at 14 ~m, the spectrum showed an exponential fall in transmission with ultimate cut-off at 16.5 ~m. No absorption bands were detected in this study; however, work is now in hand to explore the spectral characteristics more fully, particularly at low temperatures where free carrier effects would be suppressed.
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Discussion There are a number of possible ways available to explain the existence of a glass in this system, however, for the purposes of this communication a fairly simple consideration of the system strain energy will be presented. A more rigorous approach would be to extend these ideas by employing the concept of equalisation of electronegativities, and this aspect will be pursued in a future communication. Our results, have shown that glass formation is possible in the PbS-Sb2S 3 system, but that we are near to the limits of glass forming ability. This is in accord with the overall trends in the periodic group which contains Sb, ie. Group V, containing the elements P,As,Sb and Bi. Phosphorus is a well known glass forming element, and large numbers of phosphorus containing glasses are known. However, this small ion is somewhat different in behaviour to the other more metallic members of this group, and we can set it to one side having noted this glass forming tendency and consider the closely related trio of As,Sb and Bi. Glasses form fairly readily with As2S 3 and in particular they are easily prepared in the PbS-As2S 3 system (ii). In the case of Sb2S 3, glass formation is far more difficult and, as we have seen, glass formation is only found in the PbS-Sb2S 3 system in a very limited region. The Bi2S3-PbS system exhibits no glass-like phases to the best of our knowledge. There are a number of reasons why this trend should appear. The most trivial of these is related to the ionic size of the atoms As,Sb and Bi. As ionic size increases, the outer electron shells of the Group V atoms overlap more with their neighbours and the compounds become more metallic in nature. While metallic glasses are now known, in general metals are difficulty to solidify in a non-crystalline state. The same considerations apply in the case of As2S3, Sb2S 3 and Bi2S 3 . At a more fundamental level it is necessary to consider the crystal chemistry of the solids and the probable structure of the melt in these PbS-M2S 3 systems. The As2S 3 , Sb2S 3 and Bi2S 3 sulphides come into the group labelled sulphosalts by mineralogists. There have been a number of attempts to classify the sulphosalts based on structural considerations. For our purposes, although it is not the more recent, the classification of Takeuchi and Sandanage (12) is of most use. These authors draw attention to the fact that the As,Sb and Bi atoms form pyramidal MS 3 groups and that the geometry of these groups varies with the atom M involved. Clearly these groups are smallest for AsS 3 and largest for BiS 3. In the PbS-M2S 3 system, these pyramids must link with PbS 6 octahedra, along edges or faces. The octahedron edge in PbS is about 0.42 nm, considerably longer than the edges of the MS 3 pyramids and some appreciable strain results in linking the two units together. The strain is greatest for the smallest MS 3 group, ASS3, and least for the largest _MS3 group, BiS 3. In the melt it is reasonable to assume that both PbS octahedra and MS 3 groups are present. While other coordination polyhedra are possible, of course, they are less reasonable. In the pure sulphides of As,Sb and Bi the _MS3 groups link together in chains, and strain is minimal. In the presence of the PbS 6 octahedra, these have to be accommodated into the chain structures somehow, and this is clearly more readily done when the strain is least. In terms of classical glass theory, the PbS 6 groups
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act as glass forming groups but their purpose is not to prevent networks of chains forming, but to prevent crystallisation by distorting the coordination of the regular _MS3 groups and introducing a degree of strain into the lattice. Thus the system is closely related to the strained mixed cluster theory of glass formation proposed by Goodman (13). (A particular consequence of the SMC model is that the glass composition will be slightly displaced from the eutectic towards the polymorphic end member and in this system it is indeed found that the glass had a composition which was shifted 1 mol % from the eutectic towards sb2S3.) In the As2S3-PbS system, with greatest strain, glass formation takes place over a wide range of compositions. In the Sb2S3-PbS system, the situation is finely balanced. Any crystallisation at all prevents glass formation, and it is only by rapid quenching close to the eutectic composition that the melt can effectively be cooled to a glassy solid. In the Bi2S3-PbS system the strain appears to be too small to cause a glass to form, although ultra rapid quenching may just achieve this state, provided liquidus-solidus regions of the phase diagram were chosen with care. A fairly recent refinement of the structure of zinckenite (14) suggests that it is composed of pyramidal units and trigonal prisms. By introducing a degree of rotation, it is of course possible to distort a trigonal prism to an octahedron and it is, therefore, possible to extend the above arguments to cover a situation where the glass is considered in terms of zinckenite-related and MS 3 units. These ideas are substantiated by the electron microscope observation of the recrystallisation of the PbS-Sb2S 3 glass. The process is easy and involves little rearrangement of the glass, indicating that the glass is composed of the same metal-sulphur polyhedra as in the crystalline solid. The product of the recrystallisation also bears close resemblence to slow cooled crystalline phases produced in this system. In terms of this model it is relatively simple to predict how to extend the glass formation region in this system. Effectively we need to increase the degree of misfit, and hence the degree of strain, between the octahedral and pyramidal groups. To do this we have to increase the size of the ootahedral group, and so nominally replace the Pb by larger ions which still preserve octahedral coordination. It is also possible to replace the sulphur, but, although this is less likely to have as great an effect as that due to the difference in nominal radii between cationic species, it would be of interest to seek an analogous glass in the PbSe-Sb2Se 3 system. In a similar way we can attempt to decrease the size of the MS 3 groups. This can be done by replacing Sb by a smaller cationic species, or by replacing sulphur by a smaller anionic species. Such doping experiments are of interest, not least to check the validity of these ideas, and results will be reported in the future. In conclusion i t is possible to say that although the glass phase which forms in the PbS-Sb2S 3 system, forms only over a restricted temperature and composition region it seems to be of some interest. In particular, the preparation difficulties encountered in this study may be overcome in applications such as CVD sputtered films. Further investigations, particularly of electrical properties on this and doped materials are planned for the future.
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Acknowledgements J.R. Gannon wishes to thank the Directors of STL Ltd for permission to publish this paper. A.C. Wright wishes to thank the Science Research Council for financial support. References i. 2. 3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14.
J.A. Savage and S. Nielson, Infra-red Physics ~, 195, (1965) J.R. Gannon, J. Non-Cryst. Solids 42, 239, (1980) J.T. Krau~e, C.R. Kurkjian, D.A. Pinnow and E.A. Sigety, AppI. Phys. Letters 17, 367, (1970) S.R. Ovshinsky, Phys. Rev. Letters 21, 1450, (1968) J.R. Gannon, in Proc. SPIE Tech Sym, Los Angeles, Feb (1981) J.A. Savage, J. Mater. Sci. 6, 964, (1971) P . L . Garvin, N. Jb. Miner-Abh 118, 235, (1973) J.R. Craig, L.L.Y. Chang and W.R. Lees, Canad. Mineralogist 12, 199, (1973) B. Salanci, N. Jb Miner-Abh 135, 315, (1979) J.R. Gannon and R.J.D. Tilley, J. Microsc. 106, 59, (1976) J.I. Petz, R.F. Kruh and G.C. Amstutz, J. Chem. Phys. 34, 526, (1961) Y.U. Takeuchi and R. Sadanaga, Zeit. Kristallogr. 130, 346, (1969) C.H.L. Goodman, in P.H. Gaskell (Ed) Structure of non-crystalline materials p. 197 Taylor and Francis, London (1977) J.C. Portheine and W. Nowaki, Zeit. Kristallogr. 141, 79, (1975)