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Neutron diffraction study of solid SiD4
Solid State Communications, Vol. 18, pp. 1353—1355, 1976.
Pergamon Press.
Printed in Great Britain
NEUTRON DIFFRACTION STUDY OF SOLID SiD4 E. Legrand Materials Science, S.C.K./C.E.N., B-2400 Mol, Belgium and W. Press Institut für Festkorperforschung der Kernforschungsanlage, Jülich, Germany (Received 5 January 1976 by S. Amelinckx) Neutron diffraction measurements on polycrystalline SiD4 confirm the existence of three crystalline phases. From these measurements, it is clear that the symmetry of solid silane is much lower than that of solid methane. THE RECENT interest manifesting itself in many detailed studies of the molecular motions of solid methane, has led to similar studies on silane, e.g. references 1, 2 and 3. Little is known about the structure of solid silane, however. Interpretations usually have been based on the assumption that silane is isostructural with methane. In particular, the high temperature transition known to occur at 63.75 Kwith in Sill4 and at 67K inorder 4 has been associated the orientational SiD4 disorder transition occurring in methane. Recently, a second phase transition in solid SiD has been about 5 In order to obtain better4information detected. the various phases, we have performed neutron dif-
In Fig. 1, three diffraction patterns taken at 86, 49.5 and 26 K are shown. Form the diffraction pattern at 86 K, it is clear that the high temperature phase I of solid silanehas a lower symmetry than the “corresponding” phase of solid methane: too many reflections are present in the spectrum to allow an indexing in terms of a f.c.c. unit cell. The diffraction spectrum of phase I is indexed on measurements the basis of a tetragonal from X-ray on SiH unit cell, as found 4 by Sears andwith 7 An indexing of the powder pattern Morrison. higher symmetry cells molecules been attempted, but was notcontaining successful.less The unit cell has parameters of SiH 4 at 77 K, a = 12.5 A and c = 14.2 A, are fraction measurements on solid SiD4 at a number of adjusted with the linear expansion coefficient given by temperatures. these authors to the values a = 12.63 A and c = 14.34 A SiD4 gas from Merck, Sharp and Dohme (quoted at 85.5 K. With this unit cell the most important difisotopic purity > 99%) was condensed in a spherical fraction lines can be indexed in a satisfactory way as is aluminium sample holder with 1 cm diameter. The D2 shown on the lowest spectrum in Fig. 1. However, from content of the gas was first removed from the gas, by the extinctions in the X-ray measurements it was concondensing it and pumping on it several times. To obtain cluded that silane has a body-centered unit cell, which satisfactory powder samples, one has to avoid the means that only reflections with h + k + 1 = 2n are growth of large crystallites, which was accomplished as allowed. In the neutron diffraction experiment, where far as possible by cooling the sample rapidly. In addition, also the deuterium atoms contribute significantly more the sample was rotated continuously around a vertical to the diffractedintensities, there is no evidence for axis throughout the measurements in order to decrease this extinction rule. Wespectrum did not try to determine a unit 6 cell for the diffraction taken at 49.5 K of the influence ofatpreferred orientation in the Measurements the BR2 reactor in Mol weresample. performed silane in phase II, since the number of well-defined difwith a neutron wavelength X = 1.263 A at temperatures fraction lines in this measurement is too limited to fit T= 86, 49.5, 26 and 6.9 K. The relative intensity of the them with a large unit cell. diffraction lines obtained for different samples at the The upper diffraction spectrum in Fig. 1 is obtained same temperature were approximately the same, which at 26 K and hence at a temperature below the 38 K indicates the absence of preferred orientation. Only in transition found by Wilde and Srinivasan.5 This pattern one case did some obvious “fibre” texture remain. The is once again different from the two other ones, which data were recorded in two-theta scans in steps of 10 proves the existence of a second phase transition in solid minutes of arc. Due to the small sample size, counting silane. A diffraction pattern taken at 6.9 K had the same rates generally were low, features as at 26 K, showing that the structure remains the same down to this temperature. 1353
1354
NEUTRON DIFFRACTION STUDY OF SOLID SiD4 9c23 tor~g ~r~q[e
2100
Vol. 18, Nos. 9/10
20
27° 11)
600
/
H Phase S
T26K
I ~
1500
CI)
--
~—
2(X)
•
4/)
30 T
K
900 1000 2135
31K
(205
323<.
•)55
/235
600
C
Phase U
T~495K
200
II 26° 28° 26° 28°26° 28° 26° 28° 26° 28°26° 28°
Scalter~ng ar
-
900
300
-
_________
900
J~haseiT~855s.
300
—
-
~ 150
20° 250 30° Scaller~~gar~q)e 20 ( degrees
35°
Fig. 1. Neutron diffraction patterns of SiD4 at 86, 49.5 and 26K (X = 1.263 A). In order to determine the transition temperature we followed the intensity change of the maximum of the peak at a scattering angle 20 = 27°10’ as a function of temperature. This is plotted in Fig. 2(a). We may note that the intensity starts to change at about 38 K. In Fig. 2(b) the scattered intensity in the interval 26°~ 20 ~ 28 is shown at various temperatures. It should be remarked that down to about 38 K there is just one peak; the width of the intensity distribution is determined by the instrumental resolution. At lower temperatures the peak gradually splits into a doublet which reflects the breaking of symmetry at Til/LU 38 K. The transition seems to occur in a continuous way, but certainly better statistics are required for a final conclusion, As can be seen from the diffraction spectra in Fig. 1, more well resolved peaks appear at angles 25°( 20 ~ 35°in the diffraction spectra of phases 11 and III than in that of phase I. We therefore extended
20 ( degrees
Fig. 2. Determination of the transition temperature between phase 11 and phase III of solid silane. (a) Change of the intensity diffracted under a scattering angle of 27°10’ as a function of temperature. (b) Change of the shape of the diffraction peaks between 26°and 28°as a function of temperature. the measurements in phase Ito 20 = 55°.In this part of and the rapid decrease with scattering angle of the corthe spectrum only ~i few weak peaks are visible. The absence of reflections at higher scattering angles could possibly be interpreted as due to orientational disorder responding molecular form factors. The presence of orientational disorder, however, is inconsistent with the low symmetry cell of phase I, since plastic phases in general have close-packed structures. As it happens, similar measurements in SiD4 III also show only a few additional peaks. The absence of strong reflections at higher scattering angles in both phases may therefore indicate large thermal motions at all temperatures. These conclusions are not firmly based, however, because neither the centre-of-mass structure nor the orientational structure is known. At the moment, there remains the problem of assigning unit cells in all three phases of solid silane. Better resolution of the diffraction patterns together with improved counting statistics are required. For a complete structure determination single crystal data seem to be necessary. Nevertheless, it is clear from the present measurements that solid silane is not isostructural with any phase of solid methane. All phases seem to have rather low symmetry and contain several molecules in each unit cell, as inferred from a recent i.r. analysis for phases II and III.~It is not surprising, therefore, that inelastic neutron scattering experiments performed in
Vol. 18, Nos. 9/10
NEUTRON DIFFRACTION STUDY OF SOLID SiD4
the high temperature phases 3of SiH4 and CH4 show completely different spectra. One may speculate why the analogy between methane and silane does not hold. In the methane case the intermolecular forces may be grouped into those between the molecular centres (attractive part: van-der-Waals forces), and orientation dependent ones
1355
(octupole—octupole interaction). Thisbecause concept break down in solid silane, probably theseems silaneto molecules are much larger and also less rigid. Acknowledgements Thanks are due to D.E. Cox for his careful reading of the manuscript. The authors wish to thank Messrs. Fredel, Van Roy and Baudeweyns for their technical assistance during the measurements. —
REFERENCES 1. 2.
FOURNIER R.P., SAVOIE R., THE NGUYEN DINI-1, BELZILE R. & CABANE A., Can. J. Chem. 50,35 (1972). JANSSENS G., VAN HECKE P. & VAN GERVEN L., Proc. XVHJth Congrès Ampere, Nottingham (Edited by ANDREW E.) North-Holland, Amsterdam.
3.
VORDERWISCH P. & HAUTECLER S. (private communication).
4.
KLEIN M.L., MORRISON J.A., & WEIR R.D., Discussions of the Faraday Society 48, 93 (1969).
5.
WILDE R.E. & SRINIVASAN T.K.K.,J. Phys. Chem. Solids 36, 119 (1975).
6.
PRESS W., J. Chem. Phys. 56, 2597 (1972).
7.
SEARS W.M. & MORRISON J.A., J. Chem. Phys. 62, 2736 (1975).
Pergamon Press.
Printed in Great Britain
NEUTRON DIFFRACTION STUDY OF SOLID SiD4 E. Legrand Materials Science, S.C.K./C.E.N., B-2400 Mol, Belgium and W. Press Institut für Festkorperforschung der Kernforschungsanlage, Jülich, Germany (Received 5 January 1976 by S. Amelinckx) Neutron diffraction measurements on polycrystalline SiD4 confirm the existence of three crystalline phases. From these measurements, it is clear that the symmetry of solid silane is much lower than that of solid methane. THE RECENT interest manifesting itself in many detailed studies of the molecular motions of solid methane, has led to similar studies on silane, e.g. references 1, 2 and 3. Little is known about the structure of solid silane, however. Interpretations usually have been based on the assumption that silane is isostructural with methane. In particular, the high temperature transition known to occur at 63.75 Kwith in Sill4 and at 67K inorder 4 has been associated the orientational SiD4 disorder transition occurring in methane. Recently, a second phase transition in solid SiD has been about 5 In order to obtain better4information detected. the various phases, we have performed neutron dif-
In Fig. 1, three diffraction patterns taken at 86, 49.5 and 26 K are shown. Form the diffraction pattern at 86 K, it is clear that the high temperature phase I of solid silanehas a lower symmetry than the “corresponding” phase of solid methane: too many reflections are present in the spectrum to allow an indexing in terms of a f.c.c. unit cell. The diffraction spectrum of phase I is indexed on measurements the basis of a tetragonal from X-ray on SiH unit cell, as found 4 by Sears andwith 7 An indexing of the powder pattern Morrison. higher symmetry cells molecules been attempted, but was notcontaining successful.less The unit cell has parameters of SiH 4 at 77 K, a = 12.5 A and c = 14.2 A, are fraction measurements on solid SiD4 at a number of adjusted with the linear expansion coefficient given by temperatures. these authors to the values a = 12.63 A and c = 14.34 A SiD4 gas from Merck, Sharp and Dohme (quoted at 85.5 K. With this unit cell the most important difisotopic purity > 99%) was condensed in a spherical fraction lines can be indexed in a satisfactory way as is aluminium sample holder with 1 cm diameter. The D2 shown on the lowest spectrum in Fig. 1. However, from content of the gas was first removed from the gas, by the extinctions in the X-ray measurements it was concondensing it and pumping on it several times. To obtain cluded that silane has a body-centered unit cell, which satisfactory powder samples, one has to avoid the means that only reflections with h + k + 1 = 2n are growth of large crystallites, which was accomplished as allowed. In the neutron diffraction experiment, where far as possible by cooling the sample rapidly. In addition, also the deuterium atoms contribute significantly more the sample was rotated continuously around a vertical to the diffractedintensities, there is no evidence for axis throughout the measurements in order to decrease this extinction rule. Wespectrum did not try to determine a unit 6 cell for the diffraction taken at 49.5 K of the influence ofatpreferred orientation in the Measurements the BR2 reactor in Mol weresample. performed silane in phase II, since the number of well-defined difwith a neutron wavelength X = 1.263 A at temperatures fraction lines in this measurement is too limited to fit T= 86, 49.5, 26 and 6.9 K. The relative intensity of the them with a large unit cell. diffraction lines obtained for different samples at the The upper diffraction spectrum in Fig. 1 is obtained same temperature were approximately the same, which at 26 K and hence at a temperature below the 38 K indicates the absence of preferred orientation. Only in transition found by Wilde and Srinivasan.5 This pattern one case did some obvious “fibre” texture remain. The is once again different from the two other ones, which data were recorded in two-theta scans in steps of 10 proves the existence of a second phase transition in solid minutes of arc. Due to the small sample size, counting silane. A diffraction pattern taken at 6.9 K had the same rates generally were low, features as at 26 K, showing that the structure remains the same down to this temperature. 1353
1354
NEUTRON DIFFRACTION STUDY OF SOLID SiD4 9c23 tor~g ~r~q[e
2100
Vol. 18, Nos. 9/10
20
27° 11)
600
/
H Phase S
T26K
I ~
1500
CI)
--
~—
2(X)
•
4/)
30 T
K
900 1000 2135
31K
(205
323<.
•)55
/235
600
C
Phase U
T~495K
200
II 26° 28° 26° 28°26° 28° 26° 28° 26° 28°26° 28°
Scalter~ng ar
-
900
300
-
_________
900
J~haseiT~855s.
300
—
-
~ 150
20° 250 30° Scaller~~gar~q)e 20 ( degrees
35°
Fig. 1. Neutron diffraction patterns of SiD4 at 86, 49.5 and 26K (X = 1.263 A). In order to determine the transition temperature we followed the intensity change of the maximum of the peak at a scattering angle 20 = 27°10’ as a function of temperature. This is plotted in Fig. 2(a). We may note that the intensity starts to change at about 38 K. In Fig. 2(b) the scattered intensity in the interval 26°~ 20 ~ 28 is shown at various temperatures. It should be remarked that down to about 38 K there is just one peak; the width of the intensity distribution is determined by the instrumental resolution. At lower temperatures the peak gradually splits into a doublet which reflects the breaking of symmetry at Til/LU 38 K. The transition seems to occur in a continuous way, but certainly better statistics are required for a final conclusion, As can be seen from the diffraction spectra in Fig. 1, more well resolved peaks appear at angles 25°( 20 ~ 35°in the diffraction spectra of phases 11 and III than in that of phase I. We therefore extended
20 ( degrees
Fig. 2. Determination of the transition temperature between phase 11 and phase III of solid silane. (a) Change of the intensity diffracted under a scattering angle of 27°10’ as a function of temperature. (b) Change of the shape of the diffraction peaks between 26°and 28°as a function of temperature. the measurements in phase Ito 20 = 55°.In this part of and the rapid decrease with scattering angle of the corthe spectrum only ~i few weak peaks are visible. The absence of reflections at higher scattering angles could possibly be interpreted as due to orientational disorder responding molecular form factors. The presence of orientational disorder, however, is inconsistent with the low symmetry cell of phase I, since plastic phases in general have close-packed structures. As it happens, similar measurements in SiD4 III also show only a few additional peaks. The absence of strong reflections at higher scattering angles in both phases may therefore indicate large thermal motions at all temperatures. These conclusions are not firmly based, however, because neither the centre-of-mass structure nor the orientational structure is known. At the moment, there remains the problem of assigning unit cells in all three phases of solid silane. Better resolution of the diffraction patterns together with improved counting statistics are required. For a complete structure determination single crystal data seem to be necessary. Nevertheless, it is clear from the present measurements that solid silane is not isostructural with any phase of solid methane. All phases seem to have rather low symmetry and contain several molecules in each unit cell, as inferred from a recent i.r. analysis for phases II and III.~It is not surprising, therefore, that inelastic neutron scattering experiments performed in
Vol. 18, Nos. 9/10
NEUTRON DIFFRACTION STUDY OF SOLID SiD4
the high temperature phases 3of SiH4 and CH4 show completely different spectra. One may speculate why the analogy between methane and silane does not hold. In the methane case the intermolecular forces may be grouped into those between the molecular centres (attractive part: van-der-Waals forces), and orientation dependent ones
1355
(octupole—octupole interaction). Thisbecause concept break down in solid silane, probably theseems silaneto molecules are much larger and also less rigid. Acknowledgements Thanks are due to D.E. Cox for his careful reading of the manuscript. The authors wish to thank Messrs. Fredel, Van Roy and Baudeweyns for their technical assistance during the measurements. —
REFERENCES 1. 2.
FOURNIER R.P., SAVOIE R., THE NGUYEN DINI-1, BELZILE R. & CABANE A., Can. J. Chem. 50,35 (1972). JANSSENS G., VAN HECKE P. & VAN GERVEN L., Proc. XVHJth Congrès Ampere, Nottingham (Edited by ANDREW E.) North-Holland, Amsterdam.
3.
VORDERWISCH P. & HAUTECLER S. (private communication).
4.
KLEIN M.L., MORRISON J.A., & WEIR R.D., Discussions of the Faraday Society 48, 93 (1969).
5.
WILDE R.E. & SRINIVASAN T.K.K.,J. Phys. Chem. Solids 36, 119 (1975).
6.
PRESS W., J. Chem. Phys. 56, 2597 (1972).
7.
SEARS W.M. & MORRISON J.A., J. Chem. Phys. 62, 2736 (1975).