J. Phys. Chem. 1992,96, 8173-8176 phase. The tentative explanation that the metastable phase is phase IV has recently been confirmed in an X-ray diffraction study using synchrotron radiation.Is Summary and Conclusions Raman spectra of deuterated and hydrogenated cyclohexane have been obtained over the pressure range 1 bar to 30 kbar at room temperature. Data were also collected in the low-temperature phase I1 and in the metastable phase. As pressure is increased from ambient, the phase sequence in both C&II2and C6DIZ is I I11 IV V. The monoclinic phase 11was not obtained at room temperature up to 30 kbar. The general similarity of spectra in the region 5-300 K and ambient to 30 kbar suggests that the phase diagram topologies of these compounds are identical. A new phase (phase V) has been discovered in both materials but its structure remains unknown. The similarity of spectra in all phases suggests that there is only a small degree of molecular distortion accompanying these first-order transitions. Comparison of the metastable phase spectrum with those of the other phases suggests that it is most similar to the phase IV high-pressure polymorph,
---
Acknowledgment. We acknowledge Mr. H. Vass for his expert assistance in performing the experiments and Dr. N. B. Wilding, Prof. A. Warflinger, and Dr. D. F. R. Gilson for helpful discussions. J.C. acknowledges a departmental scholarship from the University of Edinburgh and an Overseas Research Studentship Award. This work was funded in part by the Science and Engineering Research Council and the Nuffield Foundation. This paper is dedicated to the memory of our coauthor and colleague Mr. A. Cairns-Smith after his untimely death.
8173
References and Notes (1) Kitaigorodskii, A. I. Molecular Crysrals and Molecules; Academic Press, New York, (1973). (2) Cavagnat, D.; Roberts, M. P.; Cavagnat, R. M.; Vahedir-Banisaeid, S.J. Phys. Chem. 1991,95, 1821. (3) Dosseh, G.; Fressigne, C.; Rousseau, B.; Wilding, N. E.;Fuchs, F. H. J . Chim. Phys. 1990,87, 1821. (4) Carpenter, G. B.; Halford, R. S.J . Chem. Phys. 1990, 15, 99. (5) Andrew, E. R.; Eades, R. G. Proc. R.Soc. London 1953, A216,398. (6) Kahn, R.; Fourme, R.; Andr'e, D.; Renaud, M. Acta Crysrallogr.1973, 829, 131. (7) Renaud, M.; Fourme, R. J . Chim. Phys. 1966,63, 27. (8) Ito, M. Spectrochim. Acta 1965, 21, 2063. (9) Burns, G.; D a d , F. H. Solid State Commun. 1984, 51, 773. (10) Wurflinger, A. Eer. Bunsen-Ges. Phys. Chem. 1975, 79, 1195. (11) Andersson, P. J . Phys. Chem. Solids 1978, 39, 65. Amtz, H.; Schneider, G. M. Faraday Discuss. Chem. Soc. 1980,69, 139. (12) Schulte, L.; Wiirflinger, A. J . Chem. Thermodyn. 1987, 19, 363. (13) Haines, J.; Gilson, D. F. R. J . Phys. Chem. 1989, 93, 7920. (14) Haines, J.; Gilson, D. F. R. J . Phys. Chem. 1990, 94, 4712. (15) Obremski, R. J.; Brown, C. W.; Lippincott E. R. J. Chem. Phys. 1968, 49, 185. (16) Brown, C. W.; Obremski, R. J.; Lippincott, E. R. J. Chem. Phys. 1970, 52, 2253. (17) Wilding, N. B.; Hatton, P. D.; Pawley, G. S.Acra Crysfallogr.1991, 847, 797. (18) Wilding, N. B.; Crain, J.; Hatton, P. D.; Bushnell-Wye, G. To be published. (19) Mayer, J.; Chrusciel, J.; Habrylo, S.; Holderna, K.;Natkaniec, I.; Hartmann, M.; Wiirflinger, A.; Urban, S.; Zajac, W. High Pressure Res. 1990, 4, 460. (20) Mayer, J.; Urban, S.;Habrylo, S.;Holderna, K.; Natkaniec, I.; Wiirflinger, A.; Zajac, W. Phys. Srarus Solidi (E) 1991, 166, 381. (21) Shimanouchi, T. Narl. Stand. ReJ Dura Ser. 1967, 58. (22) Millar, F. A.; Golab, H. R. Spectrochim. Acta 1969, 21, 1969. (23) Snyder, R. G.; Schactschnieder,A. Spectrochim. Acra 1%9,21, 169. (24) Anobe, D.; Ceccaldi, D.; Szwarc, H. J. Phys. (Paris) 1984, 45, 731. (25) Saesinska, E.; Sciesinki, I. Mol. crysr. Liq. Cryst. 1979, 51, 9.
High-pressure Phase Transition in LiOD: A Neutron Powder Diffraction and Vibrational Spectroscopic Studyt David M. Adam,* Andrew G. Christy, and Julian Haines Department of Chemistry, University of Leicester, Leicester LEI 7RH, England (Received: February 28, 1992; In Final Form: May 5, 1992)
Lithium deuteroxide, LiOD, exhibits a phase transition beginning about 17 kbar on compression. A significant decrease in the 0-D stretching frequency in the infrared and Raman spectra indicates that the transition involves the formation of hydrogen bonds. Neutron powder diffraction data from the high-pressure phase were indexed on a monoclinic cell with cell parameters a = 6.266 (10)A, b = 3.097(8)A, c = 4.847 (36)A, (3 = 108.91 (26)O,and 2 = 4 at 18 kbar. The structure is likely to be of the TI1 type, distorted by the hydrogen bonding as in the low-temperature phase of NaOD. A model is proposed in which the space group would be P2,la with bifurcated hydrogen-bonded chains running along b.
Introduction Recently, it has been shown that the alkali metal hydroxides and deuteroxides, with the exceptions of LiOH, LiOD, and NaOD, undergo transitions at low temperature to antiferroelectric or ferroelectric phases, which are characterized by the presence of hydrogen-bonded chains.'-" We have shown that LiOH, which under ambient conditions adopts the tetragonal anti-PbO (litharge) structure ( W / n " = D42, Z = 2 ) with a = 3.55 A and c = 4.33 A,I2J3 Figure 1, undergoes a transition beginning at close to 7 kbar in which, similarly, hydrogen-bonded chains are formed.14 The spectroscopic results indicated that the primitive unit cell
doubled while remaining centrosymmetric, but the structure of the new phase is not known. These transitions have been shown to exhibit large isotope effects. Deuteration increases the transition temperature by about 30 K for the potassium, rubidium, and cesium hydroxides, and in the case of sodium hydroxide no transition is observed down to 6 K, while the deuteroxide exhibits a transition at 153 K. A large isotope effect could therefore be expected to occur for LiOD, which was subject to the present study as it is amenable to neutron diffraction, thereby potentially allowing a determination of the high pressure structure.
'A preliminary account of this work appeared as part of the Proceedings of the XXIX EHPRG Meeting in Thessaloniki, Greece, October 21-25.1991, in High Pressure Research.
Experimental Section 7LiOD was prepared from the reaction of 7Li metal (AERE Stable Isotope, 99.99% 'Li) with D 2 0 (Aldrich "Gold Label",
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8174 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Adams et al.
TABLE I: ViblrtloDll lhta for LiOD v(IR)/cm-' (2 kbar) 2712
a Intercept
phase I1 v(Raman)/cm-I du/dp/cm-' ( 1 kbar) kbar-I 0.40 2705 0.28 324 0.93 299 0.37 0.33 283
uoa/cm-' 2711 2704 323 299 282
phase 111 v(Raman)/cm-I du/dp/cm-l (58 kbar) kbar-I -0.04 2600 -0.03
v(IR)/cm-l (53 kbar) 2605
uoa/cm-' 2607 2602
of least-squares line. 2750
I
I
2700
C
t
L
a Figure 1. Structure of LiOH at STP.
99.8% D) with subsequent dehydration for 1 h under vacuum at between 25 and 100 OC. 'LiOD was studied as 6Li and H are strong neutron absorbers and incoherent neutron scatterers, respectively. For the spectrampic measurements, LiOD was loaded into the diamond anvil cell (DAC) under a dry nitrogen atmosphere, along with a ruby chip or 0.21% (w/w) NaN03 in NaBr as a pressure calibranti5J6for the Raman and IR experiments, respectively. Stainless steel gaskets of varying thicknesses with an initial hole diameter of 400 I.cm were used to contain the sample and calibrant. Raman spectra were obtained in backscattering configuration on a Coderg T800 spectrometer using a Spectra Physics Model 164 argon ion laser (488.0-nm line) for excitation. Infrared spectra were acquired on a Bio-Rad Digilab FTS-40 spectrometer. The Raman and infrared spectra were obtained at 24 and 31 OC, respectively. Ambient pressure neutron diffraction data on a sample in a 5 mm diameter vanadium can were collected using the POLARIS time-of-flight diffractometer at the Rutherford Appleton Laboratory. A McWhan pressure cell containing a 5 mm diameter aluminum can, in which approximately 100 mm3of sample and Fluorinert as a pressure transmitting medium were placed, was used for the high pressure runs. Data were collected at 90° scattering angles wing a 3He and a scintillation detector; in addition low-angle data were collected and examined for large dspacing reflections. The high-pressure data accumulation time was approximately 20 h.
ResultS Infrared and Raman Spectroscopy. The phase transition in LiOD was found to begin at close to 17 kbar upon compression, much higher than the 7 kbar observed for LiOH.I4 As was observed for LiOH, a new OX (X = H, D) stretching band appeared at lower frequency in both the infrared and Raman spectra and was found to increase in intensity at the expense of the original band as the pressure was increased, Figure 2. The decrease in frequency at the transition of just over 100 an-',Table I, is close to what is expected considering the relative reduced of OH and OD; AuOD/AVOH = ( k D / p ~ ~ )=' 0.728. / ~ As was the case for LiOH, the OX stretching bands broadened and exhibited slightly negative dv/dp values, indicating that weak hydrogen bonds are formed. The IR and Raman modes remained noncoincident indicating that the high-pressure structure is cen-
r(
[E 0
'=Q. n
E 2650 3 C Q
> 0
z
2600
2550 _ _-.
0
20
40
60
80
pressure/kbar Figure 2. Pressure dependence of the IR (dashed line) and Raman (solid line) bands of LiOD (2750-2550 cm-' region).
trosymmetric. The Raman-active lattice modes were found to dissappear at the transition and those of the new phase were too weak to be detected. A large region of intergrowth was observed both on compregsion and decompression, with small amounts of the high-pressure phase, phase 111, remaining in the DAC when the pressure was released. This indicates that the transition kinetics are slow at ca. 300 K. The reverse transition was found to begin at below 10 kbar. Increasing the pressure on the low-pressure phase, phase 11, with a trace of phase 111, resulted in the transition being 3/4 of the way to completion by 15 kbar. Pressure cycling of a mixture of phases between 14 and 22 kbar led to the formation of pure phase I11 at 20 kbar as indicated by the disappearance of the phase I1 0-D stretch in the Raman spectrum. Neutroa Powder Diffraction. The neutron powder diffraction patterns of ambient and high-pressure LiOD are shown in Figure 3. The ambient data are in agreement with the known antilitharge structure. The high-pressure phase was obtained by increasing
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8175
High-pressure Phase Transition in LiOD 18 kbar
- 90'
scintillation detector
D-spacingla 18 kbar
-
I
90" 'He detector
*
+ VI
5
E
D-spacingli Ambient
IO
2o
3.0 0
D-spacing/ A
Figure 3. Neutron powder patterns for LiOD (additional reflections in pattern obtaipd with the scintillation detector arise from the A20, insert of the pressure cell).
TABLE II: High-Pressure Neutron Powder Diffraction Data for UOD (18 kbu) d d A 2.914 2.573 2.537 1.467 1.336 1.282
dOJA 2.966 2.964 2.567 2.528 1.467 1.337 1.283
100 X Adld 0.27 0.34 0.23 0.36 0.00 0.07 0.08
hkl 201 200 01 1 iii 02 1 412,410 022
the pressure to 21 kbar and cycling the pressure between 8 and 18 kbar, before finally increasing the pressure to 18 kbar. The high-pressure diffraction pattern indicated that the sample had transformed completely. The peaks in the high-pressure pattern bore no obvious relation to those of the ambient phase, indicating that very significant changes occur at the phase transition.
A structure model resembling TlI, to which the low-temperaturc structurca of MOD (M= Na, K,Rb, CS) are all related,u*69 succesfully fitted the neutron diffraction pattern with cell parameters u = 3.13 A, b = 3.10 A, and c = 9.17 A. Peak splitting, however, indicated a slight monoclinic distortion of the ideal Bmmb symmetry, as occurs in the hydrogen-bonded low temperature form of NaODS2A cell comparable to that of NaOD was obtained with cell parameters a = 6.266 (10) A, b = 3.097 (8) A, c = 4.847 (36) A, /3 = 108.91 (26)O, and Z = 4 at 18 kbar, Table 11. The space group would be P2,/a with hydrogen-bonded chains running along b. The quality of the data did not allow the neutron intensities to be fitted; however, attempts at obtaining correct relative intensities of the 01 1 and T 11 reflections required the x coordinate of the oxygen atoms to be adjusted and the OD- ions to be strongly canted as shown in the model structure, Figure 4. In particular, the intensity of T 11 would be very close to zero for a Oo cant angle, with its intensity increasing with the cant angle. The observed 01 1:T 11 intensity ratio was best fitted by a cant angle of approximately 50°, which produces bifurcated hydrogen bonds. In order to proceed any further a better data set would be needed. The proposed model is reasonable; however, the atomic positions are still uncertain. Bifurcation could help to explain the pressure insensitivity of the 0-D stretch, which is much less pressure sensitive than the 0-H stretches of strongly hydrogen bonded systems, such as H20.17 A related system, Mg(OH)2,'8 in which each hydroxyl hydrogen is weakly hydrogen bonded to three hydroxyl oxygens, also exhibits a relatively small negative pressure
Figure 4. Model structure for LiOD I11 (dashed lines show the hydrogen bonds belonging to one chain).
shift for the 0-H stretch of -0.06 cm-'/kbar. This space group is consistent with the vibrational data which, due to the mutual exclusion of the infrared and Raman bands, indicated that the unit cell is centrosymmetric. In addition, deconvolution of the IR spectrum of LiOH in the 0-H stretching regionI4 yielded two components, thereby requiring the unit cell to double. A model was proposed b a d on the vibrational spectra of LiOH in which weak hydrogen bonds are formed by the rotation of the hydroxyl ions, yielding an orthorhombic structure. It can now be seen that the transition is more drastic and involves a rearrangement of the Li-0 lattice in addition to the formation of hydrogen bonds. The volume reduction relative to phase I1 at ambient pressure is large (18%), consistent with the onset of H-bonding and the increase in Li coordination from 4 to 5. The reconstructive nature of the transformation accounts for the slow kinetics. As was previously mentioned, the transition in LiOD began at a higher pressure than that in LiOH: 17 kbar as opposed to 7 kbar, although the true equilibrium transition pressures are probably lower in both oompounds. The isotope effects in the other hydroxides arise from tunneling of protons (deuterons) between two minima of a double potential well. The energy levels are lower in the wells for the deuterated compounds and hence the barrier is wider and the transition occurs at a higher temperature. This increase in transition temperature is typically close to 30 K l o however, for NaOH no transition is observed down to 6 K,while NaOD transforms at 153 K. A phase transition, which could be the same as the low-temperature NaOD transition, occurs in NaOH at 8.4 kbar.Ig If NaOD follows the trend that substitution with a heavier isotope is equivalent to the application of pressure or, in the case of a variable-pressure experiment, a reduction in temperature,m one would expect the transition in NaOD to m r at a lower pressure than in NaOH. This is opposite to what is observed for LiOH(D). The high-pressure LiOH(D) transition is different from the low-temperature NaOD transition in that it involves an increase in lithium coordination in addition to hydrogen bond formation. One possible explanation for the transition pressure in LiOD being higher than LiOH is that the dT/dp line on the phase diagram has a negative slope. If the dT/dp line for the I1 111 transition has a negative slop, one would expect a transition at high temperature. LiOH has been shown to undergo a phase transition at 686 Ka21 It is unlikely, however, that this is the same phase observed at high pressure and ambient temperature as in the case of the rest of the alkali metal hydroxides, the hydrogen bonds break at high temperature. The structure could be similar to ambient NaOH, in which hydrogen bonding is absent. If this were the case, the phase diagram of LiOH(D) will be similar to that of NaOH(D), except that it will be shifted to higher pressure and the transition temperatures
-
J. Phys. Chem. 1992, 96, 8176-8180
8176
will be different. This would be consistent with the phase behavior of the alkali halides for which the hdvier alkali halides behave as their light and analogues under pressure.
Acknowledgment. We thank Dr.S. Hull (RAL)for help with the neutron diffraction experiments. This work was supported by SERC (U.K.). J.H.acknowledges the award of a fellowship by NSERC (Canada). Registry No. LiOD, 12159-20-5.
References pnd Notes (1) Bastow, T. J.; Amm, D.T.; Segel; S. W.; Heyding, R. D. Z . Narurforsch. 1986, 41a, 283. (2) Bastow, T. J.; Elcombe, M. M.; Howard, C. J . Solid Srare Commun. 1986, 57, 339. (3) Bastow, T. J.; Elcombe, M. M.; Howard, C. J. Solid Stare Commun. 1986, 59, 257. (4) Elschner, S.; Bastow, T. J. Solid Srare Commun. 1986, 60, 75. ( 5 ) White, M.A.; Moore, S. A. J. Chem. Phys. 1986, 85, 4629. (6) Bastow, T. J.; Elcombe, M.M.; Howard, C. J. Solid Srare Commun. 1987, 62, 149.
(7) Jacobs, H.; Mach, B.; Lutz, H.-D.; Henning, J. 2.Anorg. AIIg. Chem. 19877
5441
28.
( 8 ) Jacobs, H.; Mach, B.; Harbrecht, B.; Lutz, H.-D.; Henning, J. Z. Anorg. Allg. Chem. 1987, 544, 55.
(9) Mach, B.; Jacobs, H.; Schgfer, W. Z . Anorg. Allg. Chem. 1987,553, 187. (10) Bastow, T. J.; Elcombe, M. M.; Howard, C. J. Ferroelectrics 1988, 79* 269. (1 1) Bastow, T. J.; Segel, S. L.; Jeffrey, K. R. Solid Srare Commun. 1991, 78, 565. (12) Ernst, T. Z. Phys. Chem. 1933, B20, 65. (13) Dachs. H. Z. Krisrallow. 1959. 112. 60. (14j Adams, D. M.; HainesYJ. J. Phys. Chem. 1991, 95, 7064. (15) Barnett, J. D.; Block, S.; Piermarini, G. J. Rev. Sci. Insrrum. 1973, 44, 1. (16) Klug, D. D.; Whalley, E. Rev. Sci. Insrrum. 1983, 54, 1205. (17) Minceva-Sukarova, B.; Sherman, W. F.; Wilkinson, G. R. J. Phys. C Solid Srare Phys. 1984, 17, 5833. (18) Kruger, M. B.; Williams, Q.;Jeanloz, R. J. Chem. Phys. 1989, 91, 5910. (19) Pistorius, C. W. F. T. Z . Phys. Chem. 1969, 65, 51. (20) Hiiller, A. Faraday Discuss. Chem. Soc. 1980, 69, 66.
(21) Khitrov, V. A.; Khitrova, N. N.; Khmel'kov, V. F. Zh. Obshchei Khim. 1953, 23, 1630.
NMR Study of the Effects of Electric Dipole Interactions on the Ordering of Chaln Solutes in the Nematic Phase D. J. Photinoqt C.-D. Peon,? E.T.Samulski,**tand H.ToriumiS Department of Chemistry, Venable and Kenan Laboratories, University of North Carolina, Chapel Hill, North Carolina 27599-3290; Department of Physics, University of Patras, Patras 261 10, Greece; and Department of Chemistry, University of Tokyo, Komaba, Meguro, Tokyo 153, Japan (Received: March I O , 1992)
The existence of residual electric dipole interactions in nematic liquid crystals is demonstrated by comparatively analyzing deuterium NMR spectra of a,w-dibromoalkanesand n-alkanes dissolved in nematic solvents. It is shown that these. interactions generate an additional orientational bias that has sizable and readily measurable effects on the order parameter profiles of the dibromoalkane solute. The residual electric dipole interaction modifies both the local and global ordering of the electric dipole carrying chain solutes. The study of the dependence of these changes in solute ordering on the dipole moment constitution of the solvent offers new insights into the interplay of the different types of intermolecular interactions operative in the liquid crystal's average intermolecular potential.
I. Introduction With very few exceptions, molecules forming thermotropic nematic or smectic liquid crystal phases possess strong electric dipoles. Aside from their obvious role in the formation of polar mesophases (e.g., ferroelectric smectic phases), electric dipoles are often assumed to be a determining factor of thermodynamic stability and/or self-organization in apolar mesophases as well (pair association' giving reentrant phases? biaxial smectics? etc.). While it is generally agreed that the dominant mechanism accounting for the molecular orientational ordering in apolar phases originates from packing considerations (excluded vol~me),"~ here we show experimental NMR data for dibromoalkane solutes in different nematic solvents which clearly imply the existence of residual solute-solvent electric dipole interactions. In apolar mesophases electric dipole interactions are effective only over a short range of intermolecular distances, and therefore their contribution to the ordering depends strongly on the shape of the molecule and the location of the dipole moments on the molecule. An accurate evaluation of the orientational bias experienced by a solute molecule as a result of dipoldipole interactions with the nematic solvent molecules is a major task even for molecules having simple geometry. Nevertheless, crude es-
* Corresponding author.
University of North Carolina. 'University of Patras. 'University of Tokyo.
0022-3654/92/2096-8 176$03.00/0
TABLE I Molecular Primary Structures
50CB
Idealized Shapes & Dipoles
N-c-@@o-(cH,),-~H,
5CB
DBr-C& c10
CHJ-(CH2)g-CHJ
&%w
timates of the anisotropy of this bias for molecular dimensions and dipole moment strength usually encountered in liquid crystals give values as high as 1 kcal/mol and, therefore, is comparable to the typical order of magnitude of the total anisotropy of the nematic potential of mean torque. Accordingly, the dipole-generated bias could play an important role in the alignment Here we initiate an inquiry into this subject by studying the orientational ordering differen- exhibited by related solutes with 0 1992 American Chemical Society
