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Crystallographic and magnetic structure of ternary carbides of the type Nd2Fe17Cx
Journal of the Less-Common
Metals, 155 (1989)
15 - 21
15
CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF TERNARY CARBIDES OF THE TYPE Nd,Fe,,C, R. B. HELMHOLDT
Netherlands Energy Research Foundation,
ECN, 1755 ZG Petten (‘The ~ether~unds)
K. H. J. BUSCHOW
Philips Research Laboratories, (Received
February
5600 JA Eindhoven
(The Netherlands)
13,1989)
Summary Neutron diffraction measurements were made at 300 K and 4.2 K on a polycrystalline sample of the composition NdzFel,CO.s. The crystal structure of this ternary carbide is very similar to that of the parent compound Nd~Fe~~(Th*Zn~,-typed R&r), the carbon atoms partially filling up the empty Q(e) site. At 4.2 K, the compound Nd,Fe,&, adopts a magnetic structure in which the easy magnetization direction is perpendicular to the c-axis.
1. Introduction During an investigation into the conditions under which formation of the tetragonal compound can take place in NdzFe14C it was found that above about 870 “C, Nd,Fe,& decomposes into other ternary carbides, the main component being a phase of the rhombohedral Th2Zni7 structure type [ 11. An interesting property of this phase is that it can contain fairly large amounts of carbon. In the series NdzFel&, the lattice constants were found to increase with X, from which it was concluded that the carbon atoms occupy an interstitial position. The Q(e) position was considered to be the most likely site for the carbon atoms [ 11. However, there are also reports advocating the occupation of the 3(a) site located between the two iron atoms that form the so-called dumb-bell pair [2, 31. In the carbon-free compound Nd,Fe,, these dumb-bell atoms have a relatively short interatomic separation, which was generally held responsible for an anti-ferromagnetic coupling b&ween these iron atoms leading to an abnormally low Curie temperature. Occupation of the 3(a) position would then increase the interatomic separation between these iron atoms, which would explain the considerable increase of the Curie temperature with carbon content in Nd,Fe,,C, . Previous investigations of the magnetic properties of various Nd-Fe-B compounds had shown that there is no correlation between the 0 Elsevier
Sequoia/P~inted
in The Netherlands
16
occurrence of short Fe-Fe distances in these materials and the magnitude of their Curie temperatures [4]. For this reason, the increase of T, with ;Yin Nd,Fer&, need not require the filling up of the 3(a) position. F~hermore~ occupation of the 3(a) site by carbon would lead to rather short Fe-C distances and hence would be rather unlikely. In order to clarify this situation we have re-investigated the structure of these ternary carbides by means of neutron diffraction”
2, Experimental details The compound NdzFel,C& was prepared by arc melting and subsequent vacuum annealing at 900 “C for four weeks, After annealing, the sample was quenched in water. Neutron diffraction measurements were made at 300 K and 4.2 K. The diffraction patterns obtained at both temperatures were analysed by means of Rietveld’s profile refinement technique [5]. The maximum absorption correction was 5%, @R-= 0.442. For the scattering lengths we used the values 0.769 X 10-i’ cm for neod~ium~ 0.954 X 10-l” cm for iron and 0.6643 X 10-i* cm for carbon. The magnetic form factors for neodymium and iron were taken from the papers published by Watson and Freeman 161 and Blume et al. [ 71. The quality of the fit between the various experimental and the calculated diffractograms was assessed by the expression
where yi(obs) and yi(calc) are the observed and calculated values of the ith measuring point, wi being its statistical weight. The quantity v represents the number of points minus the number of parameters.
3. Results and discussion The neutron diffraction diagram obtained at 300 K is shown in the top part of Fig. 1. Attempts to refine nuclear as well as magnetic parameters at this ~m~erature were not successful, although the Curie temperature is 420 K. For this reason we focussed our attention exclusively on the nuclear parameters, i.e. we considered Nd,Fe,,Co,., as a paramagnet. The results of the refinement procedure are given in column A of Table 1. The atomic position parameters are very close to those obtained earlier by X-ray diffraction [ 1 J . A schematic representation of the structure is shown in Fig. 2. The most important result is the location of the carbon atoms, which are seen from the table to occupy partially the 9(e) site. Because of the low X-ray scattering factor for carbon it has not been possible to determine the location of carbon in Nd,Fe,,C, by means of a refinement procedure of the
17 z
50000
; 2 ‘J r z5
..,.....
OBSERVED
PROFILE
__c
CALCULATED
PROFILE
40000
z
30000
i
20000
1 ODOC
0
25OOG
1875E
I250(
)-
6251
Fig. 1. Neutron diffraction diagram of NdZFel&e.4 obtained at 300 K (top part) and 4.2 K (battom part). The solid line through the data points represents a fit which is discussed in the main text. Diffraction lines due to the sample holder and the cryostat were not included in the fitting procedure,
X-ray data. The 9(e) site was proposed mainly on the basis of interstit~~ hole sizes and it is gratifying that this site occupancy has now been confirmed by neutron diffraction. Less obvious and less easily explained is the fact that the refinement of the neutron data led to an occupation number for carbon at the 9(e) site which is slightly lower than expected on the basis of the carbon
18
o Fe
0 Rare earth
Fig. 2. Schematic representation of the crystal structure of NdzFe1,C3. The carbon atoms occupy the 9(e) position. In the compound NdzFel,C 0.4 these 9(e) sites are only partially occupied,
concentration applied in preparing the carbide, i.e. it would correspond to a formula composition Nd,Fe,,Co,4 rather than to Nd,Fe,,Co,s. A compound of the composition Nd,Fe&Z 1 (equivalent to Nd,Fe,,JJo,7) was investigated earlier by means of neutron diffraction by Luo Sheng et al. [2]. Our analysis agrees with theirs with respect to the metal atoms. In both investigations, the basic metal atom structure is found to be the rhombohedral Th,Zn,, structure type. However, Luo Sheng et al. proposed different sites for the carbon atoms which, according to these authors, occupy the 3(a) position. For this reason, we investigated whether their structural model would also lead to a satisfactory fit of our neutron data, which have a considerably higher resolution than their data. The results of our refinement procedure, obtained by restrict~g the carbon atoms to the 3(a) position, are included in Table 1. If the occupation number is considered as an adjustable parameter we find that the refinement leads to zero occupation of carbon at 3(a). Keeping the position (3(a)) fixed and using the same carbon concentration as considered in column A leads (column C) to a fit of considerably lower quality than those obtained with the parameters in column A of Table 1. Note that the reliability factor R and the quantity x2 have increased by a factor of about three. Another argument against occupation of the 3(a) site by the carbon atoms is the extremely short C-Fe distance. According to the data of Luo Sheng et al. we find d(C-Fe,) = 0.106 nm. By using the parameters of the fourth column of Table 1, the distance d(C-Fe,) is found to be even smaller (0.085 nm). On the basis of these small values it is unlikely that the carbon atoms occupy the 3(a) position. A similar criticism regarding occupation of the 3(a) site by carbon was made already by Stadelmaier et al. [8]. These authors propose the 18(h)
19
TABLE 1 Parameters derived from fitting the neutron diffracto~am obtained for the compound NdzFer7Coe.s at 300 K. Column A current results; columns B and C results for carbon at 3(a); column D results for carbon at 18(h) with the coordinates as given in ref. 8
a = b
(A)
c f.4
@B)(A21
A
B
C
D
8.6245 (3) 12.4724 (6) 0.72 (5)
8.6231 (4) 12.4707 (7) 0.44 (6)
8.6247 (5) 12.4724 (9) 0.08 (7)
8.6232 (5) 12.4705 (8) 0.42 (6)
z (Nd in 6(c)) x (Fe in 18(f)) x (Fe in 18(h)) Y z (Fe in 6(c)) z (C in 9(e)) x (C in 3(a)) x (C in 18(h)) z (C in 18(h)) n (C) R nucl(‘70) X2
-
0.0100 0.2871 0.5023 0.4977 0.1556 0.2382 0.5
(4) (2) (1) (1) (2) (2)
-
0.13 (2) 3.5 11.3
0.0095 0.2864 0.5025 0.4975 0.1558 0.2385 0.0
0.00 (1) 6.0 15.9
(5) (2) (1) (1) (2) (3)
-
0.0133 0.2872 0.5024 0.4976 0.1556 0.2345
(6) (3) (2) (2) (3) (4)
0.0103 0.2867 0.5025 0.4975 0.1554 0.2381
(5) (2) (2) (2) (2) (3)
0.0
0.38 9.2 27.5
0.1150 0.05 0.06 7.0 19.1
interstitial position as more likely than the 3(a) position. The results of our refinement procedure obtained when restricting the carbon atoms to the 18(h) site and using the same carbon concentration as considered in column A of Table 1 are included in column D of the same table. The quality of the fit is seen to be much better than the fit corresponding to column C, but it is still inferior to the fit of column A. The neutron diffraction diagram obtained at 4.2 K is shown in the bottom part of Fig. 1. Satisfactory fits to the experimental data were obtained by using the same n(C) value as found from the 300 K data and restricting the easy magnetization direction to be perpendicular to the c-axis. The corresponding parameters are given in column A of Table 2. Included in the table (column B) are the corresponding parameters when the easy magnetization direction is restricted to be parallel to the c-axis. Although the R factors and x2 are in favour of the former direction, it may be seen that the difference between the two sets of reliability factors is not very large. For this reason also we include intermediate easy magnetization directions in the fitting procedure. However, they did not lead to R factors significantly different from those found for the magnetization perpendicular to c (column A). 4. Concluding remarks From the analysis of our neutron data it can be derived that the crystal structure of ternary carbides of the type Nd,Fel& is essentially that of the
20 TABLE 2 Parameters derived from fitting the neutron diffractogram obtained for the compound NdzFer,Co.s at 4.2 K for the magnetic moments perpendicular (A) and parallel (B) to c respectively
a = b (8) c (8) (B) (A’)
A
B
8.6185 (4) 12.4869 (6) -0.25 (6)
8.6189 (4) 12.4871 (7) -0.20 (6)
z (Nd in 6 (c)) x (Fe in 18(f)) x (Fe in 18(h)) Y z z (Fe in 6(c)) x (C in 9(e)) n (C) & (Nd in 6(c))
0.0098 (4) 0.2883 (2) 0.5020 (1) 0.4980 (1) 0.1558 (3) 0.2379 (2) 0.5 0.13 4.07 (9)
E: (Fe in 9(d))
2.18 (15)
Et (Fe in 18(f))
2.45 (13)
Ez (Fe in 18(h)) & k (Fe in 6(c)) I-& R nucl(%) Rw (%) X2
2.16 (12)
0.0116 0.2879 0.5021 0.4979 0.1556 0.2378 0.5 0.13
(5) (2) (1) (1) (3) (3)
3.59 (11)
1.28 (11) 1.51 (8) 1.65 (10) 2.89 (13) 3.6 5.0 5.5
1.75 (14) 4.9 7.3 6.8
corresponding parent compound Nd,Fe,, (Th,Zni7 type) in which the carbon atoms partially fill up the empty 9(e) site. In this structure, the carbon atoms have not only iron atoms as neighbours but also neodymium atoms, the corresponding strongly negative value of the enthalpy of the R-C bond [9] contributing much to the stability of these ternary carbides. Bringing carbon atoms into close contact with the rare-earth atoms in Nd,Fe,,C, does not lead to drastic changes in the rare-earth sublattice anisotropy (at least not for x = 0.4) since the easy magnetization direction in NdzFe17Co,d at 4.2 K is still the same as that of the parent compound. The main effect of the introduction of carbon atoms is to increase the lattice constants and simultaneously to increase greatly the Curie temperature .
References 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 142 (1988) 349. 2 S. Luo, Z.-K. Liu, G.-W. Zhang, X.-D. Pei, W. Jiang and W.-N. Ho, IEEE Trans. Magn., 23 (1987) 3095.
21 3 S. Luo, G.-W. Zhang, Z.-K. Liu, X. D. Pei and W. Jiang, J. Magn. Magn. Mater., 70 (1987) 311. 4 K. H, J. ~usehow,~~SSymp. Proc., 96 (1987) 1. 5 H. M. Rietveld, J. Appl. Crystallogr., 2 (1969) 65. 6 R. E. Watson and A. J. Freeman, Acta Crystallogr., 14 (1961) 27. 7 M. Blume, A. J. Freeman and R. E. Watson, J. Chem. Phys., 36 (1962) 1248. 8 H. H. Stadelmaier, E.-Th. Henig, G. Schneider and R. Grieb, Mater. Lett., 7 (1988) 155. 9 A. R. Miedema, A. K. Niessen, F. R. de Boer, R. Boom and W. C. M. Mattens, Cohesion in petals, North-Holland, Amsterdam, 1988.
Metals, 155 (1989)
15 - 21
15
CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF TERNARY CARBIDES OF THE TYPE Nd,Fe,,C, R. B. HELMHOLDT
Netherlands Energy Research Foundation,
ECN, 1755 ZG Petten (‘The ~ether~unds)
K. H. J. BUSCHOW
Philips Research Laboratories, (Received
February
5600 JA Eindhoven
(The Netherlands)
13,1989)
Summary Neutron diffraction measurements were made at 300 K and 4.2 K on a polycrystalline sample of the composition NdzFel,CO.s. The crystal structure of this ternary carbide is very similar to that of the parent compound Nd~Fe~~(Th*Zn~,-typed R&r), the carbon atoms partially filling up the empty Q(e) site. At 4.2 K, the compound Nd,Fe,&, adopts a magnetic structure in which the easy magnetization direction is perpendicular to the c-axis.
1. Introduction During an investigation into the conditions under which formation of the tetragonal compound can take place in NdzFe14C it was found that above about 870 “C, Nd,Fe,& decomposes into other ternary carbides, the main component being a phase of the rhombohedral Th2Zni7 structure type [ 11. An interesting property of this phase is that it can contain fairly large amounts of carbon. In the series NdzFel&, the lattice constants were found to increase with X, from which it was concluded that the carbon atoms occupy an interstitial position. The Q(e) position was considered to be the most likely site for the carbon atoms [ 11. However, there are also reports advocating the occupation of the 3(a) site located between the two iron atoms that form the so-called dumb-bell pair [2, 31. In the carbon-free compound Nd,Fe,, these dumb-bell atoms have a relatively short interatomic separation, which was generally held responsible for an anti-ferromagnetic coupling b&ween these iron atoms leading to an abnormally low Curie temperature. Occupation of the 3(a) position would then increase the interatomic separation between these iron atoms, which would explain the considerable increase of the Curie temperature with carbon content in Nd,Fe,,C, . Previous investigations of the magnetic properties of various Nd-Fe-B compounds had shown that there is no correlation between the 0 Elsevier
Sequoia/P~inted
in The Netherlands
16
occurrence of short Fe-Fe distances in these materials and the magnitude of their Curie temperatures [4]. For this reason, the increase of T, with ;Yin Nd,Fer&, need not require the filling up of the 3(a) position. F~hermore~ occupation of the 3(a) site by carbon would lead to rather short Fe-C distances and hence would be rather unlikely. In order to clarify this situation we have re-investigated the structure of these ternary carbides by means of neutron diffraction”
2, Experimental details The compound NdzFel,C& was prepared by arc melting and subsequent vacuum annealing at 900 “C for four weeks, After annealing, the sample was quenched in water. Neutron diffraction measurements were made at 300 K and 4.2 K. The diffraction patterns obtained at both temperatures were analysed by means of Rietveld’s profile refinement technique [5]. The maximum absorption correction was 5%, @R-= 0.442. For the scattering lengths we used the values 0.769 X 10-i’ cm for neod~ium~ 0.954 X 10-l” cm for iron and 0.6643 X 10-i* cm for carbon. The magnetic form factors for neodymium and iron were taken from the papers published by Watson and Freeman 161 and Blume et al. [ 71. The quality of the fit between the various experimental and the calculated diffractograms was assessed by the expression
where yi(obs) and yi(calc) are the observed and calculated values of the ith measuring point, wi being its statistical weight. The quantity v represents the number of points minus the number of parameters.
3. Results and discussion The neutron diffraction diagram obtained at 300 K is shown in the top part of Fig. 1. Attempts to refine nuclear as well as magnetic parameters at this ~m~erature were not successful, although the Curie temperature is 420 K. For this reason we focussed our attention exclusively on the nuclear parameters, i.e. we considered Nd,Fe,,Co,., as a paramagnet. The results of the refinement procedure are given in column A of Table 1. The atomic position parameters are very close to those obtained earlier by X-ray diffraction [ 1 J . A schematic representation of the structure is shown in Fig. 2. The most important result is the location of the carbon atoms, which are seen from the table to occupy partially the 9(e) site. Because of the low X-ray scattering factor for carbon it has not been possible to determine the location of carbon in Nd,Fe,,C, by means of a refinement procedure of the
17 z
50000
; 2 ‘J r z5
..,.....
OBSERVED
PROFILE
__c
CALCULATED
PROFILE
40000
z
30000
i
20000
1 ODOC
0
25OOG
1875E
I250(
)-
6251
Fig. 1. Neutron diffraction diagram of NdZFel&e.4 obtained at 300 K (top part) and 4.2 K (battom part). The solid line through the data points represents a fit which is discussed in the main text. Diffraction lines due to the sample holder and the cryostat were not included in the fitting procedure,
X-ray data. The 9(e) site was proposed mainly on the basis of interstit~~ hole sizes and it is gratifying that this site occupancy has now been confirmed by neutron diffraction. Less obvious and less easily explained is the fact that the refinement of the neutron data led to an occupation number for carbon at the 9(e) site which is slightly lower than expected on the basis of the carbon
18
o Fe
0 Rare earth
Fig. 2. Schematic representation of the crystal structure of NdzFe1,C3. The carbon atoms occupy the 9(e) position. In the compound NdzFel,C 0.4 these 9(e) sites are only partially occupied,
concentration applied in preparing the carbide, i.e. it would correspond to a formula composition Nd,Fe,,Co,4 rather than to Nd,Fe,,Co,s. A compound of the composition Nd,Fe&Z 1 (equivalent to Nd,Fe,,JJo,7) was investigated earlier by means of neutron diffraction by Luo Sheng et al. [2]. Our analysis agrees with theirs with respect to the metal atoms. In both investigations, the basic metal atom structure is found to be the rhombohedral Th,Zn,, structure type. However, Luo Sheng et al. proposed different sites for the carbon atoms which, according to these authors, occupy the 3(a) position. For this reason, we investigated whether their structural model would also lead to a satisfactory fit of our neutron data, which have a considerably higher resolution than their data. The results of our refinement procedure, obtained by restrict~g the carbon atoms to the 3(a) position, are included in Table 1. If the occupation number is considered as an adjustable parameter we find that the refinement leads to zero occupation of carbon at 3(a). Keeping the position (3(a)) fixed and using the same carbon concentration as considered in column A leads (column C) to a fit of considerably lower quality than those obtained with the parameters in column A of Table 1. Note that the reliability factor R and the quantity x2 have increased by a factor of about three. Another argument against occupation of the 3(a) site by the carbon atoms is the extremely short C-Fe distance. According to the data of Luo Sheng et al. we find d(C-Fe,) = 0.106 nm. By using the parameters of the fourth column of Table 1, the distance d(C-Fe,) is found to be even smaller (0.085 nm). On the basis of these small values it is unlikely that the carbon atoms occupy the 3(a) position. A similar criticism regarding occupation of the 3(a) site by carbon was made already by Stadelmaier et al. [8]. These authors propose the 18(h)
19
TABLE 1 Parameters derived from fitting the neutron diffracto~am obtained for the compound NdzFer7Coe.s at 300 K. Column A current results; columns B and C results for carbon at 3(a); column D results for carbon at 18(h) with the coordinates as given in ref. 8
a = b
(A)
c f.4
@B)(A21
A
B
C
D
8.6245 (3) 12.4724 (6) 0.72 (5)
8.6231 (4) 12.4707 (7) 0.44 (6)
8.6247 (5) 12.4724 (9) 0.08 (7)
8.6232 (5) 12.4705 (8) 0.42 (6)
z (Nd in 6(c)) x (Fe in 18(f)) x (Fe in 18(h)) Y z (Fe in 6(c)) z (C in 9(e)) x (C in 3(a)) x (C in 18(h)) z (C in 18(h)) n (C) R nucl(‘70) X2
-
0.0100 0.2871 0.5023 0.4977 0.1556 0.2382 0.5
(4) (2) (1) (1) (2) (2)
-
0.13 (2) 3.5 11.3
0.0095 0.2864 0.5025 0.4975 0.1558 0.2385 0.0
0.00 (1) 6.0 15.9
(5) (2) (1) (1) (2) (3)
-
0.0133 0.2872 0.5024 0.4976 0.1556 0.2345
(6) (3) (2) (2) (3) (4)
0.0103 0.2867 0.5025 0.4975 0.1554 0.2381
(5) (2) (2) (2) (2) (3)
0.0
0.38 9.2 27.5
0.1150 0.05 0.06 7.0 19.1
interstitial position as more likely than the 3(a) position. The results of our refinement procedure obtained when restricting the carbon atoms to the 18(h) site and using the same carbon concentration as considered in column A of Table 1 are included in column D of the same table. The quality of the fit is seen to be much better than the fit corresponding to column C, but it is still inferior to the fit of column A. The neutron diffraction diagram obtained at 4.2 K is shown in the bottom part of Fig. 1. Satisfactory fits to the experimental data were obtained by using the same n(C) value as found from the 300 K data and restricting the easy magnetization direction to be perpendicular to the c-axis. The corresponding parameters are given in column A of Table 2. Included in the table (column B) are the corresponding parameters when the easy magnetization direction is restricted to be parallel to the c-axis. Although the R factors and x2 are in favour of the former direction, it may be seen that the difference between the two sets of reliability factors is not very large. For this reason also we include intermediate easy magnetization directions in the fitting procedure. However, they did not lead to R factors significantly different from those found for the magnetization perpendicular to c (column A). 4. Concluding remarks From the analysis of our neutron data it can be derived that the crystal structure of ternary carbides of the type Nd,Fel& is essentially that of the
20 TABLE 2 Parameters derived from fitting the neutron diffractogram obtained for the compound NdzFer,Co.s at 4.2 K for the magnetic moments perpendicular (A) and parallel (B) to c respectively
a = b (8) c (8) (B) (A’)
A
B
8.6185 (4) 12.4869 (6) -0.25 (6)
8.6189 (4) 12.4871 (7) -0.20 (6)
z (Nd in 6 (c)) x (Fe in 18(f)) x (Fe in 18(h)) Y z z (Fe in 6(c)) x (C in 9(e)) n (C) & (Nd in 6(c))
0.0098 (4) 0.2883 (2) 0.5020 (1) 0.4980 (1) 0.1558 (3) 0.2379 (2) 0.5 0.13 4.07 (9)
E: (Fe in 9(d))
2.18 (15)
Et (Fe in 18(f))
2.45 (13)
Ez (Fe in 18(h)) & k (Fe in 6(c)) I-& R nucl(%) Rw (%) X2
2.16 (12)
0.0116 0.2879 0.5021 0.4979 0.1556 0.2378 0.5 0.13
(5) (2) (1) (1) (3) (3)
3.59 (11)
1.28 (11) 1.51 (8) 1.65 (10) 2.89 (13) 3.6 5.0 5.5
1.75 (14) 4.9 7.3 6.8
corresponding parent compound Nd,Fe,, (Th,Zni7 type) in which the carbon atoms partially fill up the empty 9(e) site. In this structure, the carbon atoms have not only iron atoms as neighbours but also neodymium atoms, the corresponding strongly negative value of the enthalpy of the R-C bond [9] contributing much to the stability of these ternary carbides. Bringing carbon atoms into close contact with the rare-earth atoms in Nd,Fe,,C, does not lead to drastic changes in the rare-earth sublattice anisotropy (at least not for x = 0.4) since the easy magnetization direction in NdzFe17Co,d at 4.2 K is still the same as that of the parent compound. The main effect of the introduction of carbon atoms is to increase the lattice constants and simultaneously to increase greatly the Curie temperature .
References 1 D. B. de Mooij and K. H. J. Buschow, J. Less-Common Met., 142 (1988) 349. 2 S. Luo, Z.-K. Liu, G.-W. Zhang, X.-D. Pei, W. Jiang and W.-N. Ho, IEEE Trans. Magn., 23 (1987) 3095.
21 3 S. Luo, G.-W. Zhang, Z.-K. Liu, X. D. Pei and W. Jiang, J. Magn. Magn. Mater., 70 (1987) 311. 4 K. H, J. ~usehow,~~SSymp. Proc., 96 (1987) 1. 5 H. M. Rietveld, J. Appl. Crystallogr., 2 (1969) 65. 6 R. E. Watson and A. J. Freeman, Acta Crystallogr., 14 (1961) 27. 7 M. Blume, A. J. Freeman and R. E. Watson, J. Chem. Phys., 36 (1962) 1248. 8 H. H. Stadelmaier, E.-Th. Henig, G. Schneider and R. Grieb, Mater. Lett., 7 (1988) 155. 9 A. R. Miedema, A. K. Niessen, F. R. de Boer, R. Boom and W. C. M. Mattens, Cohesion in petals, North-Holland, Amsterdam, 1988.