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The incommensurate composite structure of (PbS)1.14(NBS2)2, “PbNb2S5”
Micron and Microscopica Acta. Vol. 21. No. 4, pp. 289—290, 1990. Printed in Great Britain.
0739-6260/90 $3.00 +0.00 Pergamon Press plc
The Incommensurate Composite Structure of (PbS)1 14(NbS2)2, ‘tPbNb2S5t’. S. Kuypers and J. Van Landuyt University of Antwerp (RUCA) Groenenborgerlaan 171; 8-2020 Antwerp, Belgium.
The compound with approximate composition PbNbS
3 (1-1-3) has a composite layer structure. It consists of an alternation of two-atom-thick NaC1-type PbS layers, and three-atom-thick NbS2 sandwiches with the Nb atoms in trigonal prisms of sulphur. The substructures of PbS and NbS2 can be described in C-centred orthorhombic unit cells (a1, b1, C1), with i = 1, 2 for PbS and NbS2 respectively ; the corresponding axes are parallel; the c-axis is chosen perpendicular to the layers (Wiegers et a!., 1990). Electron diffraction patterns (EDPs) along the [001] zone axis reveal that the pseudo-square (PbS) and pseudo-hexagonal (NbS2) meshes (a1,b1) coincide along the b-direction (b1 = b2), but along the a-direction they are incompatible (fig.!). The ratio a1/a2, although close to 7/4, is irrational. The complete structure is therefore incommensurate , as is evidenced by satellite reflectionsat non-fractional positions with respect to the basic reflections (Kuypers er a!., 1990). The ratio a1/a2 determines the actual composition of the compound. One finds (PbS)1 14NbS2, rather than PbNbS3. From EDPs along the [100] zone axis a c-repeat period of =1.2 nm is deduced (fig.2). Heavy streaks at k=odd imply stacking disorder and breaking of the C-centring, possibly induced by bending ofthe foil.
_____________________ •j
Fig.! EDP along [001] in (PbS)1 14NbS2. Apart from a square and a hexagonal arrangement of intense reflections due to PbS and NbS2 respectively, satellites at incommensurate positions are observed.
Guemas er a!. (1988) succeeded in growing single crystals with approximate composition PbNb2S5 (1-2-5). This suggests a composite layer structure in which two three-atom-thick sandwiches of NbS2 alternate with one two-atom-thick slab of PbS. The c-parameter will be =1.8 nm (or a multiple). 289
290
S. Kuypers and J. Van Landuyt
Generally, a batch of single crystals grown from a 1-2-5 mixture of constituent elements containi 1-2-5 as well as 1-1-3 crystals. A qualitative EDX analysis allowed to link composition with crysi morphology and made it possible to hand-pick all crystals with assumed composition PbNb
2S5. The [00 EDP of a 1-2-5 compound is very similar to the corresponding pattern of (PbS)1, l4NbS2 (fig.!). The vali for a1/a2 is very nearly the same and hence the actual composition is (PbS)1,14(NbS2)2. For both sublattic the reflections hkO, h + k = 2n+1, are extinct in accordance with C-centring. A [100] EDP obtained at t] folded edge of a (001) foil is reproducedin fig.3. It contains the reflectionsOk! common to both subsysterr The rows 021, 041,... contain sharp reflections connected by weak streaks. The spacing along the
TO\
corresponds with a repeat period of =3.6 nm. The rows 01!, 03!,... contain no sharp reflections but are (oft heavily) streaked, which implies not only stacking disorder but also the violation of C-centring. All ti indicates that although one of the subsystems has an average repeat period of 3.6 nm along c, the oth subsystem contains considerable stacking disorder. Diffuse lines in X-ray diffraction show the disordert system to be the PbS system (Meerschaut et al., 1990). The stacking disorder can be understood if one not that the atom rows along a in the PbS system are located in the grooves formed by the sulphur atoms of ti NbS2 sandwiches. This will still be the case when the PbS layer is displaced over a vector with a compone b/2, but then the shift with respect to the previous PbS layer will not be b/3 (fig. 4) but -b/6. Stacking fau. of that type might also explain the discrepancy, viz, a b/6 shift of successive PbS layers in the P1 substructure and a b/3 shift of the same layers in the “complete” structure, in the X-ray structu determination (Meerschaut eta!., 1990).
Fig. 2 EDP along [100] in (PbS)1 14NbS2. The spot separation along c* corresponds with 1.2 nm. Fig. 3 EDP along [100] in (PbS)1 14(NbS2)2. The spot separation along c* corresponds with =3.6 nm. Note the diffuse intensity in figs. 2 & 3. Fig. 4 Structure model for (PbS)1 14(NbS2)2 viewed along [100]. Successive slabs of PbS are shifted over b/3 (Meerschaut er al.,1990). References Guemas L., Rabu P., Meerschaut A. and Rouxel J., 1988, Mar. Res. Bull. 23:1061-1069. Kuypers S., Van Landuyt J. & Amelinckx S., 1990, J. Solid State Chem. 86: 212-232. Meerschaut A., Guemas L., Auriel C. and Rouxel J., 1990, European J. Inorg. Chem., in the press. Wiegers G.A., Meetsma A., Haange R.J., Van Smaalen S., Dc Beer J.L., Meerschaut A., Rabu P. at Rouxel J., 1990, Ada Cryst. B 46: 324-332.
0739-6260/90 $3.00 +0.00 Pergamon Press plc
The Incommensurate Composite Structure of (PbS)1 14(NbS2)2, ‘tPbNb2S5t’. S. Kuypers and J. Van Landuyt University of Antwerp (RUCA) Groenenborgerlaan 171; 8-2020 Antwerp, Belgium.
The compound with approximate composition PbNbS
3 (1-1-3) has a composite layer structure. It consists of an alternation of two-atom-thick NaC1-type PbS layers, and three-atom-thick NbS2 sandwiches with the Nb atoms in trigonal prisms of sulphur. The substructures of PbS and NbS2 can be described in C-centred orthorhombic unit cells (a1, b1, C1), with i = 1, 2 for PbS and NbS2 respectively ; the corresponding axes are parallel; the c-axis is chosen perpendicular to the layers (Wiegers et a!., 1990). Electron diffraction patterns (EDPs) along the [001] zone axis reveal that the pseudo-square (PbS) and pseudo-hexagonal (NbS2) meshes (a1,b1) coincide along the b-direction (b1 = b2), but along the a-direction they are incompatible (fig.!). The ratio a1/a2, although close to 7/4, is irrational. The complete structure is therefore incommensurate , as is evidenced by satellite reflectionsat non-fractional positions with respect to the basic reflections (Kuypers er a!., 1990). The ratio a1/a2 determines the actual composition of the compound. One finds (PbS)1 14NbS2, rather than PbNbS3. From EDPs along the [100] zone axis a c-repeat period of =1.2 nm is deduced (fig.2). Heavy streaks at k=odd imply stacking disorder and breaking of the C-centring, possibly induced by bending ofthe foil.
_____________________ •j
Fig.! EDP along [001] in (PbS)1 14NbS2. Apart from a square and a hexagonal arrangement of intense reflections due to PbS and NbS2 respectively, satellites at incommensurate positions are observed.
Guemas er a!. (1988) succeeded in growing single crystals with approximate composition PbNb2S5 (1-2-5). This suggests a composite layer structure in which two three-atom-thick sandwiches of NbS2 alternate with one two-atom-thick slab of PbS. The c-parameter will be =1.8 nm (or a multiple). 289
290
S. Kuypers and J. Van Landuyt
Generally, a batch of single crystals grown from a 1-2-5 mixture of constituent elements containi 1-2-5 as well as 1-1-3 crystals. A qualitative EDX analysis allowed to link composition with crysi morphology and made it possible to hand-pick all crystals with assumed composition PbNb
2S5. The [00 EDP of a 1-2-5 compound is very similar to the corresponding pattern of (PbS)1, l4NbS2 (fig.!). The vali for a1/a2 is very nearly the same and hence the actual composition is (PbS)1,14(NbS2)2. For both sublattic the reflections hkO, h + k = 2n+1, are extinct in accordance with C-centring. A [100] EDP obtained at t] folded edge of a (001) foil is reproducedin fig.3. It contains the reflectionsOk! common to both subsysterr The rows 021, 041,... contain sharp reflections connected by weak streaks. The spacing along the
TO\
corresponds with a repeat period of =3.6 nm. The rows 01!, 03!,... contain no sharp reflections but are (oft heavily) streaked, which implies not only stacking disorder but also the violation of C-centring. All ti indicates that although one of the subsystems has an average repeat period of 3.6 nm along c, the oth subsystem contains considerable stacking disorder. Diffuse lines in X-ray diffraction show the disordert system to be the PbS system (Meerschaut et al., 1990). The stacking disorder can be understood if one not that the atom rows along a in the PbS system are located in the grooves formed by the sulphur atoms of ti NbS2 sandwiches. This will still be the case when the PbS layer is displaced over a vector with a compone b/2, but then the shift with respect to the previous PbS layer will not be b/3 (fig. 4) but -b/6. Stacking fau. of that type might also explain the discrepancy, viz, a b/6 shift of successive PbS layers in the P1 substructure and a b/3 shift of the same layers in the “complete” structure, in the X-ray structu determination (Meerschaut eta!., 1990).
Fig. 2 EDP along [100] in (PbS)1 14NbS2. The spot separation along c* corresponds with 1.2 nm. Fig. 3 EDP along [100] in (PbS)1 14(NbS2)2. The spot separation along c* corresponds with =3.6 nm. Note the diffuse intensity in figs. 2 & 3. Fig. 4 Structure model for (PbS)1 14(NbS2)2 viewed along [100]. Successive slabs of PbS are shifted over b/3 (Meerschaut er al.,1990). References Guemas L., Rabu P., Meerschaut A. and Rouxel J., 1988, Mar. Res. Bull. 23:1061-1069. Kuypers S., Van Landuyt J. & Amelinckx S., 1990, J. Solid State Chem. 86: 212-232. Meerschaut A., Guemas L., Auriel C. and Rouxel J., 1990, European J. Inorg. Chem., in the press. Wiegers G.A., Meetsma A., Haange R.J., Van Smaalen S., Dc Beer J.L., Meerschaut A., Rabu P. at Rouxel J., 1990, Ada Cryst. B 46: 324-332.