230
J. Phys. Chem. 1993,97, 230-232
Observation of a Crystalline Phase Predicted for Transition-Metal Hexafluorides Lawrence S. Bartell,' James W. Hovick, Theodore S. Dibble, and Paul J. Lemon Department of Chemistry, University of Michigan, Ann Arbor, Michigan 481 09 Received: July 14, 1992; In Final Form: October 9, 1992
Experimental observations of clusters TeF6 together with molecular dynamics simulations have uncovered some kinetic aspects of phase transitions that can be expected to apply to related systems. It was predicted, accordingly, that under conditions of fast cooling transition-metal hexafluorides could be induced to crystallize into a monoclinic form which had never been observed for the compounds. Electron diffraction studies of large clusters of MoF6 and WF6, generated by condensation of the vapor in supersonic expansions through a Laval nozzle, confirmed the prediction. A rationale for this behavior is advanced.
Introduction Theoctahedral hexahalides (AX6) exist at atmospheric pressure in a variety of solid-state forms, including body-centered cubic, rhombohedral, trigonal, monoclinic, and orthorh~mbic.l-~ However, the subset of rather rigid transition-metal hexafluorides (A = Mo-Rh and W-Pt) exhibits much less diversity.2.3 Just below their melting points the substances freeze into a plastically crystalline bcc structure. As they are cooled further, they are all reported to transform from bcc to their lowest temperature phase, a denser, well-ordered orthorhombic structure. The bcc phase and the substantial disorder characterizing it are quite well understood in terms of "orientational frustration".' Also intuitively reasonable is the compact orthorhombic packing of the octahedral molecules, whose intramolecular F-F distances are similar to the intermolecular F-F contact distances. In this phase fluorines are distributed in a pseudohexagonal closest packed array. Therefore, the solid-state behavior of these substances would not have attracted our attention if questions had not been raised by investigations of crystalline clusters Of TeFs.S,9In these studies, clusters were generated by condensation from the vapor in supersonic flow. When clusters were nucleated and grown under conditions adjusted to make the flow very cold, they developed into microcrystals with the same orthorhombic structure observed in bulk TeF6 and in the transition-metal hexafluorides. When, on the other hand, they were formed under warmer conditions of flow, their electron diffraction patterns corresponded unmistakably to the monoclinic phase also possessed by the homologous sulfur and selenium hexafluorides.1° The only aspect of this behavior which initially seemed unusual was the fact that the warmer phase, monoclinic, had a lower symmetry than the colder phase, contrary to the general rule. When a careful investigation of bulk TeF67by neutron diffraction was unable to detect the monoclinic phase, however, some explanation was needed. If clusters as large as lo4 molecules readily adopt the monoclinic form, why does it not exist in the bulk? Prior experience had shown that large clusters exhibit the same structures as bulk phases. In order to gain some insight into the problem, several series of molecular dynamics (MD) simulations of small clusters of TeF6 (with 128-250 molecules) were carried out, as described elsewhere." The behavior of the small clusters in the simulations paralleled that observed for large clusters. When warm bcc clusters were cooled, they spontaneously transformed to monoclinic. Cold orthorhombic clusters were stable and remained orthorhombic when heated until they were warm enough to transform into the bcc form. Results implied that, below the bcc range of stability, the orthorhombic structure is more stable than the monoclinic. Therefore, the metastable monoclinic structure must form by virtue of the kinetics, not the thermodynamics, of the transition.
In conventional crystallographic studies of bulk hexafluorides, times are typically many orders of magnitude longer than in the supersonic experiments and computer runs and, hence, are more conducive to the attainment of equilibrium. These considerations suggested that the observation of the monoclinic phase in hexafluorides of the chalcogens but not in the transition metals did not hinge upon whether the central atom was metallic or nonmetallic. Molecules of tellurium hexafluoride are so nearly identical to those of the transition-metal hexafluorides in size, shape, and flexibility' that it is unnatural to expect its crystallography to be markedly different. Therefore, it seemed worthwhile to explore the possibility of finding the monoclinic phase in the transition-metal hexafluorides. To this end, an investigation of tungsten and molybdenum hexafluorides was undertaken.
Experimental Section Gaseous molybdenum hexafluoride (supplied by Noah Chemicals, stated purity 99.9%, and Ozark-Mohoning, >99%) and tungsten hexafluoride (Air Products, 99.9%) were mixed in various proportions with the carrier gas neon (Cryogenic Gas, 99.99%) or, in some cases, helium or argon (both from Air Products, 99.999%) for introduction into an electron diffractometer described elsewhere.12 Each gas mixture was injected through a miniature glass Laval nozzle into the evacuated diffraction chamber where it was probed by a 40-keV electron beam of -40 nA. Dimensions of each of the four nozzles used in the series of runs are tabulated in the supplementary material. Those of the principal nozzle are listed in Table I. Diffraction patterns were recorded on Kodak medium projector slide plates after being filtered by an rl rotating sector. Experiments were carried out in pulsed mode to eliminate background gas." Representative experimental conditions investigated and corresponding results are given in Table I. The complete set of runs is documented in the supplementary material. Results
Experiments confirmed that clusters of MoF6 and wF6 behave almost identically to those of TeFs9as illustrated in Figures 1 and 2. Results for different flow conditions are tabulated in Table I. If pressures are reasonably low and/or molefractionsof subject gas are rather low, orthorhombicclusters aregrown in the resultant cold flow, as verified by the diffraction patterns shown in Figure 1. Patterns are diffuse because of the small size of the clusters (- 100 A). Diffraction patterns of much higher resolution have been published for macroscopic crystals of all three substance^,^-^^^ removing any doubt about the corresponding phase. If expansion conditions are changed by increasing the pressure and/or the mole fraction so as to condense clusters in a warmer flow, the
0022-3654/58/2091-0230%04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 231
Crystalline Phase of Transition-Metal Hexafluorides
TABLE I: Selected Results of Supersonic Expansions with Neon Carrier Cas through Class Nozzle 31' subject MOF6
subject mole fraction
Ptotb
0.30 0.20
2.1 1.7
m
5
0
10
2.0 2.4 2.8 3.4
m m m m
IO
1.4 1.7
0
0.12
solid phaseC
0.12
10
IO 10 10
0
o+m m m m
10
IO 10
10 10
0
3.1 4.1 5.2 6.2 8.3 4.I 5.1 6.2 8.3
0.03
IO 10 IO 10
0
2.1
0.06
10 10 10 10
0 0 0
3.6 4.1 2.1 2.8 3.1 4.1 5.2 2.1 3.1 4.1
0.03 0.20
10
o+m m m m
2.1 2.4 2.8 2.1 3.1
0.06
DnJk (mm)d
IO
o+m m
10
0 0
10 10
0
10
0
10
o+m m
IO 10 10 10
0 0 0
10
IO
0
Nozzle c..aracteristics: entrance i.-. 0.19 mm, exit i.d. 0.60mm, overall length 9.27 mm. Stagnation pressure, in bar. e m = monoclinic; o = orthorhombic. d This distance corresponds to the end of the nozzle holder to the edge of the opening of the skimmer entrance.
0
1
2
3
4
s (k1) Figure 2. Leveled electron diffraction patterns of 100-A clusters of monoclinic TeF6, MoF6, and WF6 produced by condensation of vapor in supersonic flow. This phase was generated when conditions during nucleation were comparatively warm. The fully grown clusters cooled in the subsequent flow. At low temperatures the monoclinic form is more stable than the well-known bcc phase but appears to be metastable with respect to the orthorhombic phase.
SF6, published high-resolution neutron diffraction patterns make the identification ~nequivocal.5+~ The close similarity between clusters of the transition-metal hexafluorides and those of TeF6 that is evident in Figures 1 and 2 extendsalso to the flow conditions required to control the phase generated. As in the case of TeF6, the molecular weight of the carrier gas had little effect on the cluster phase observed. This lack of sensitivity to carrier is very different from that found for the transition from bcc to monoclinic in SeF6 where argon is appreciably more effective and helium less effective than neon in promoting the tran~ition.~ Nevertheless, if the transition from bcc to monoclinic could have been studied for TeF6 and for the transition-metal hexafluorides in nozzle flow (see below), it is likely that the same sensitivity to carrier gas would have been seen.
Discussion
0
1
2
3
4
s(A-1) Figure 1. Leveled electron diffraction patterns of 100-A clusters of orthorhombic TeF6, MoF6, and WF6 produced by condensation of vapor in supersonic flow. This stable low-temperature form for bulk crystals was generated when conditions during nucleation were comparatively cold.
monoclinic form is produced, as corroborated by the electron intensities plotted in Figure 2. For none of the three compounds represented have high-resolution diffraction patterns of the monoclinic phase been obtained. The diffraction intensities for all three substances are so similar to those of SeF6 and SF6, however, that there can be no doubt about the assignment. For
Results support the hypothesis that, for both TeF6 and transition-metal hexafluorides, the clusters first formed under conditions of warm flow are bcc and that these transform quickly, upon cooling, to the closely related monoclinic phase. If flow conditions are too cold to condense the bcc form from the vapor, orthorhombic clusters are obtained. Unfortunately, under current experimental conditions clusters are formed and grown to full size deep inside the nozzle where their initial structure cannot be probed. Nevertheless, it is possible to reconstruct a reasonable account of cluster temperatures inside the nozzle by a computer modeling of the processes occurring in the nonequilibrium supersonic flow. A numerical integration of the rate equations for condensation and evaporation in a supersonic expansion with a carrier gasI4J5 indicates that initial cluster temperatures are high enough for the bcc phase to be stable under flow conditions observed to lead to monoclinic clusters. The bcc phase, as explicitly demonstrated experimentally for the case of SeF616 and as demonstrated in computer simulations for TeF6,1i transforms rapidly into the monoclinic phase when the bcc phase is substantially supercooled. As shown by Raynerd et aL4 and by Pawley and Dove,17 such a transition is facile because it only involves a reorientation of one-third of the quasispherical molecules by 60°, followed by a minor translational relaxation. A transition
232 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 to orthorhombic requires a major reorganization and, consequently, cannot compete kinetically with the transition from bcc to monoclinic. Although the rate of the phase transition from bcc to monoclinic is now known experimentally for SeF6'6 and theoretically for TeF611JS under certain conditions, no information exists for the rate of the transformation from bcc (or monoclinic) to orthorhombic. That we have never seen such a transition in our experiments means only that the process takes place over a time longer than tenths of a millisecond under our conditions. Because transition rates for reconstructive transformationscan beexpected to decline rapidly when the degree of supercooling is increased, it is quite possible that the monoclinic phase can be observed in bulk transition-metal hexafluorides in carefully controlled experiments. We are currently engaged in a program of M D simulations and packing calculations to gain some insight into the relative stabilities of the various solid-state structures for hexafluorides.lE.l9 These simple molecules are arguably the most symmetrical polyatomic molecules conceivable. This very simplicity leads to a greater variety of potentially favorable packing arrangements than if the molecules had little symmetry. Many different periodic structures differing only slightly in energy appear to be possible. Although only a few of them have been seen to date among the hexafluorides, it is likely that more will be found when conditions of crystallization arevaried over the ranges that new experimental techniques make it possible to attain.
Acknowledgment. This research was supported by a grant from the National Science Foundation. Supplementary Material Available: Tables containing experimental conditions and cluster phases observed in expansions of
Bartell et al. the vapors through various nozzles and dimensions of the nozzles (6 pages). Ordering information is given on any current masthead page.
References and Notes ( I ) Taylor, J. C.; Wilson, P. W.; Kelly, J. W. Acra Crysrallogr. 1973, 829,7. Levy, J. H.; Taylor, J. C.; Wilson, P. W. Acra Crysrallogr. 1974,831, 398. Levy, J. H.; Sanger, P. L.; Wilson, P. W. Acra Crysrallogr. 1974,831, 1065. Taylor, J. C.; Wilson, P. W. J . SolidSrare Chem. 1975,14,378. Levy, J. H.; Taylor, J. C.; Wilson, P. W .J . SolidSrareChem. 1975, 15, 360. Levy, J. H.; Taylor, J. C.; Wilson, P. W. J . Less Common Mer. 1976, 45, 155. Dolline. G.: Powell. B. M.: Sears. V. F. Mol. Phvs. 1979. 37. 1859. Dolling. G.; Poiell, B. M. Mol. Crysr. Liq. Crysr. 1979,'52, 27. Dolling, G.; Powefi, B. M. Can. J. Chem. 1988.66, 897. (2) Taylor, J. C. Coord. Chem. Rev. 1976, 20, 197. (3) Siegel, S.;Northrup, D. A. fnorg. Chem. 1966, 5, 2187. (4) Ravnerd. G.; Tatlock. G. J.; Venables, J. A. Acra Crystalloar. - 1980, 838,' 1896. ( 5 ) Powell. B. M.: Dove. M. T.: Pawlev. G. S.:Bartell. L. S.Mol. Phvs. 1987,62, 1127.'Dove,M. T.iPoweli, B. M.;Pawley, G. S.;Bartell, L. S.Mol. Phys. 1988,65, 353. (6) Cockroft, J. K.; Fitch, A. N. 2.Krisrallogr. 1988, 184, 123. (7) Bartell, L. S.; Powell, B. M. Mol. Phys. 1992, 75, 689. ( 8 ) Valente. E. J.: Bartell. L. S. J . Chem. Phvs. 1983. 79, 2683. (9) Bartell, L. S.; Valente, E.J.; Caillat, J. C.% Phys. Chem. 1987, 91, 2498. (10) Note that the chalcogen hexafluoride phases whose structures are now recognized as monoclinic were first indexed in refs 5 and 9 in terms of
triclinic lattices. (11) Bartell, L. S.;Xu, S.J . Phys. Chem. 1991, 95, 8939. (12) Bartell. L. S.: Heenan. R. K.:Naaashima. M. J. Chem. Phvs. 1983. 78,'236. Bartell, L. S. In Physical Merhds of Chemistry; Weiss&rger, A.; Rossiter, B. W., Eds.; Wiley-International: New York, 1972, Vol. I, Part IIId. Bartell, L. S.;French, R. J. Reo. Sci. Instrum. 1989, 60, 1223. Bartell, L. S. J . Phys. Chem. 1990, 94, 5102. Bartell, L. S.; Machonkin, R. A. J . Phys. Chem. 1990, 94, 6468. Dibble, T. S.; Bartell, L. S. J. Phys. Chem. 1992, 96, 8603. Pawley, G. S.;Dove, M. T. Chem. Phys. Lett. 1983, 99, 45. Xu, S.; Bartell, L. S. Unpublished research. Kinney, K.; Bartell, L. S. Unpublished research.
