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Phase transitions in triamantane
0038-1098/78/0908-1017 $02.00/0
Solid State Communications, Vol. 27, pp. 1017-1019. © Pergamon Press Ltd. 1978. Printed in Great Britain.
PHASE TRANSITIONS IN TRIAMANTANE R. Cernik, E.H.M. Evans, R. Hine and J.P.G. Richards Physics Department, University College, Cardiff, Wales
(Received 27May 1978 by R.A. Cowley) The triple-cage triamantane, C~aH24, in addition to the previously-reported transition to a plastic pre-melting phase at 150°C, is now found to undergo a gradual transition from an ordered phase at 0°C to a disordered phase at 30°C involving an increasing proportion of molecules re-orienting about one axis. The mechanism for this transition is established and a disorder parameter defined and estimated at three temperatures from X-ray measurements. WE REPORT an interesting and unusual phase transition in triamantane (Fig. 1), ClaI-I24, the third member of the adamantane series. In addition to the previously reported transition to a pre-melting plastic phase at 155°C [ 1] we have found a gradual transition from a perfectly ordered to a disordered phase. This transition, which commences at above 0°C and becomes complete at 30°C, involves an increasing proportion of molecules reorienting about one axis to face in opposite directions. An early limited crystallographic stUdy [2] gives the space group and cell dimensions, and confirms the stereochemistry - no further details are reported and no peculiarities are mentioned. We confirm the space group as being Fddd for the ordered structure; our lattice parameters are a = 18.16 .8,,b = 22.04 A,c = 12.92 Awith Z = 16. But in addition to the usual space group absences for Fddd, we have found the following effects:
Fig. 1. Triamantane skeleton. The hydrogens are omitted.
direct consequence of the rather special orientation of the molecular mirror planes; this readily emerges from a trivial manipulation of the appropriate structure factor (a) spectra with h + k + l = 4n + 2 are completely formula [3] and isnot reproduced here. absent at all temperatures; The explanation for (b) is as follows. Since the (b) spectra with all indices odd decrease rapidly phase above 30°C corresponds to space group Immm, it with temperature above 0°C and vanish completely at is necessary in the present circumstances that each 30°C. This is clearly demonstrated in Fig. 2 which shows grouping of atoms should have pointgroup symmetry [001 ] oscillation photographs at 20 and 30°C arranged mmm. This is brought about, statistically, if half the side by side. molecules are reflected through the mirror planes m' The immediate deduction from (a) and (b) is that the (Fig. 4) perpendicular to [010]. In other words, half phase above 30°C has a space group whose diffraction the molecules turn in the (001) plane and face in pattern conforms to Immm with each of the Fddd latopposite directions; there are no close intermolecular tice parameters halved and Z = 2. The full explanations distances in this plane so the restraints to such reof (a) and (b) emerge from a consideration of the crystal " orientations will be weak. If, at some temperature T structure which we have determined from packing conbelow 30°C, a proportion r of the molecules has become siderations. The molecules, which have point group re-oriented in this manner, it can be shown that symmetry mm2, are centred on 2~ 2 2~ sites in the extremely regular manner shown in Figs. 3 and 4. The r = ~ {1 -- (F~,/F~,)} molecular two-fold axes are along crystal diads parallel where, for a reflexion with odd indices, F~ is the to b, and the molecular mirror planes are parallel to observed structure factor at T, F~ is the calculated (100) and (001), though these mirror planes play no structure factor, based on an ordered structure, appropart in the space group symmetry. The absence at all priate to temperature T. Preliminary approximate values temperatures of the h + k + I = 4n + 2 spectra is a 1017
1018
PHASE TRANSITIONS IN TRIAMANTANE
Vol. 27, No. 10
Fig. 2. [001] Oscillation X-ray photographs at 20°C (left) and 30°C (fight).
m ~a mI
I-
m~o
I "°
a
~ -
I -~\\
omm
m
Fig. 3. View of molecule along [010]. Carbons are shown by large circles and hydrogens by small circles. This projection is unaffected by disorder. for r, at three temperatures, based on the 331 reflexion, are
T/° C
r
15 (+- 1) 20 (-+ 1) 30 (-+ 1)
~ 0.1 ~ 0.2 0.5
t, Fig. 4. View of molecule along [001 ]. Hydrogens are omitted. The full line represents the molecule in an ordered orientation, and the broken line the corresponding disordered orientation; the two are related by reflection in m'. Thus r is a disorder parameter which is varying rapidly over the range 15-30°C. It is proposed to measure r carefully throughout the whole range of temperatures over which it varies, but in view of the rapid variation, it will be necessary to refine the temperature control
Vol. 27, No. 10
PHASE TRANSITIONS IN TRIAMANTANE
1019
system on our diffractometer. This will not alter the finally disappearing into the background at 30°C. Details broad picture presented here. It is also proposed to calof this will be published shortly. culate the potential energy function for molecular It is interesting to note that the reflexions with rotations about [001 ] and to discuss the variation of r h + k + I = 4n are independent of r; thus part of the in terms of this. Full structural details will be published diffraction pattern is disorder dependent, and part not. elsewhere in due course. This model is entirely supported Our powder results suggest that the plastic phase (above 155°C) is tetragonal. by a recent laser-Raman spectroscopy experiment car. ried out by Dr. T.E. Jenkins in this department which shows a trio of weU-defined peaks at 0°C merging into a Acknowledgement - We are indebted to Professor M.A. McKervey of University College, Cork, for samples single broad peak with increasing temperature and of triamantane.
REFERENCES 1.
BURNS W., McKERVEY M.A. & ROONEY J.J.,J. Chem. Soc., Chem. Commun. 995 (1975).
2.
CARRELL H.L. & DONOHUE J., Tetrahedron Lett. 40, 3503 (1969).
3.
International Tables for X-ray Crystallography, Vol. 1, pp. 412-413. The Kynoch Press, Birmingham, England (1952).
Solid State Communications, Vol. 27, pp. 1017-1019. © Pergamon Press Ltd. 1978. Printed in Great Britain.
PHASE TRANSITIONS IN TRIAMANTANE R. Cernik, E.H.M. Evans, R. Hine and J.P.G. Richards Physics Department, University College, Cardiff, Wales
(Received 27May 1978 by R.A. Cowley) The triple-cage triamantane, C~aH24, in addition to the previously-reported transition to a plastic pre-melting phase at 150°C, is now found to undergo a gradual transition from an ordered phase at 0°C to a disordered phase at 30°C involving an increasing proportion of molecules re-orienting about one axis. The mechanism for this transition is established and a disorder parameter defined and estimated at three temperatures from X-ray measurements. WE REPORT an interesting and unusual phase transition in triamantane (Fig. 1), ClaI-I24, the third member of the adamantane series. In addition to the previously reported transition to a pre-melting plastic phase at 155°C [ 1] we have found a gradual transition from a perfectly ordered to a disordered phase. This transition, which commences at above 0°C and becomes complete at 30°C, involves an increasing proportion of molecules reorienting about one axis to face in opposite directions. An early limited crystallographic stUdy [2] gives the space group and cell dimensions, and confirms the stereochemistry - no further details are reported and no peculiarities are mentioned. We confirm the space group as being Fddd for the ordered structure; our lattice parameters are a = 18.16 .8,,b = 22.04 A,c = 12.92 Awith Z = 16. But in addition to the usual space group absences for Fddd, we have found the following effects:
Fig. 1. Triamantane skeleton. The hydrogens are omitted.
direct consequence of the rather special orientation of the molecular mirror planes; this readily emerges from a trivial manipulation of the appropriate structure factor (a) spectra with h + k + l = 4n + 2 are completely formula [3] and isnot reproduced here. absent at all temperatures; The explanation for (b) is as follows. Since the (b) spectra with all indices odd decrease rapidly phase above 30°C corresponds to space group Immm, it with temperature above 0°C and vanish completely at is necessary in the present circumstances that each 30°C. This is clearly demonstrated in Fig. 2 which shows grouping of atoms should have pointgroup symmetry [001 ] oscillation photographs at 20 and 30°C arranged mmm. This is brought about, statistically, if half the side by side. molecules are reflected through the mirror planes m' The immediate deduction from (a) and (b) is that the (Fig. 4) perpendicular to [010]. In other words, half phase above 30°C has a space group whose diffraction the molecules turn in the (001) plane and face in pattern conforms to Immm with each of the Fddd latopposite directions; there are no close intermolecular tice parameters halved and Z = 2. The full explanations distances in this plane so the restraints to such reof (a) and (b) emerge from a consideration of the crystal " orientations will be weak. If, at some temperature T structure which we have determined from packing conbelow 30°C, a proportion r of the molecules has become siderations. The molecules, which have point group re-oriented in this manner, it can be shown that symmetry mm2, are centred on 2~ 2 2~ sites in the extremely regular manner shown in Figs. 3 and 4. The r = ~ {1 -- (F~,/F~,)} molecular two-fold axes are along crystal diads parallel where, for a reflexion with odd indices, F~ is the to b, and the molecular mirror planes are parallel to observed structure factor at T, F~ is the calculated (100) and (001), though these mirror planes play no structure factor, based on an ordered structure, appropart in the space group symmetry. The absence at all priate to temperature T. Preliminary approximate values temperatures of the h + k + I = 4n + 2 spectra is a 1017
1018
PHASE TRANSITIONS IN TRIAMANTANE
Vol. 27, No. 10
Fig. 2. [001] Oscillation X-ray photographs at 20°C (left) and 30°C (fight).
m ~a mI
I-
m~o
I "°
a
~ -
I -~\\
omm
m
Fig. 3. View of molecule along [010]. Carbons are shown by large circles and hydrogens by small circles. This projection is unaffected by disorder. for r, at three temperatures, based on the 331 reflexion, are
T/° C
r
15 (+- 1) 20 (-+ 1) 30 (-+ 1)
~ 0.1 ~ 0.2 0.5
t, Fig. 4. View of molecule along [001 ]. Hydrogens are omitted. The full line represents the molecule in an ordered orientation, and the broken line the corresponding disordered orientation; the two are related by reflection in m'. Thus r is a disorder parameter which is varying rapidly over the range 15-30°C. It is proposed to measure r carefully throughout the whole range of temperatures over which it varies, but in view of the rapid variation, it will be necessary to refine the temperature control
Vol. 27, No. 10
PHASE TRANSITIONS IN TRIAMANTANE
1019
system on our diffractometer. This will not alter the finally disappearing into the background at 30°C. Details broad picture presented here. It is also proposed to calof this will be published shortly. culate the potential energy function for molecular It is interesting to note that the reflexions with rotations about [001 ] and to discuss the variation of r h + k + I = 4n are independent of r; thus part of the in terms of this. Full structural details will be published diffraction pattern is disorder dependent, and part not. elsewhere in due course. This model is entirely supported Our powder results suggest that the plastic phase (above 155°C) is tetragonal. by a recent laser-Raman spectroscopy experiment car. ried out by Dr. T.E. Jenkins in this department which shows a trio of weU-defined peaks at 0°C merging into a Acknowledgement - We are indebted to Professor M.A. McKervey of University College, Cork, for samples single broad peak with increasing temperature and of triamantane.
REFERENCES 1.
BURNS W., McKERVEY M.A. & ROONEY J.J.,J. Chem. Soc., Chem. Commun. 995 (1975).
2.
CARRELL H.L. & DONOHUE J., Tetrahedron Lett. 40, 3503 (1969).
3.
International Tables for X-ray Crystallography, Vol. 1, pp. 412-413. The Kynoch Press, Birmingham, England (1952).