PART I

GENERAL AND PHYSICAL C H EM ISTRY

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View Article Online / Journal Homepage / Table of Contents for this issue

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1 Vibrational Spectroscopy of Solids at High Pressures

By D. M . ADAMS and S. J. PAYNE

Department of Chemistry, University of Leicester, Leicester L E l 7RH

1 Introduction

Interest among chemists in use of high pressures is developing rapidly. In the area of applications to vibrational spectroscopy, activity is still relatively slight, although recent developments suggest that we are near the beginning of a logarith- mic growth phase. As in the development of cryogenics a few years ago, use of high-pressure techniques will be geared largely to the rate of commercial develop- ment of equipment, and especially to the degree of success in bringing prices within reach of non-specialist groups. Based upon 1969 prices, Ferraro’ recently estimated the cost of a high-pressure diamond anvil cell and ancilliary equipment for i.r. use down to 50 cm- ’ to be $7000, roughly equivalent to a sterling cost today of €3500. We consider that within a year a comparable figure will be ca. El500 and that that will be halved in a further year if simplified and somewhat less versatile equipment is acceptable. At today’s prices, €900 will enable anyone to study Raman spectra of solids up to ca. 70 kbar using commercially available equipment. For these reasons we consider it appropriate to report on the ‘state of the art’ and to give a fairly complete set of references for those about to enter the field.

2 Equipment

With the exception of highly specialized designs, there are three broad classes of high-pressure cell suitable for i.r. and Raman spectroscopy.

‘Hydrostatic’ Cells.-The solid sample is immersed in a gas or liquid, pressure being applied by compression of the fluid, often via an intensifier stage. Maximum pressure attainable is 12 to 15 kbar using sapphire windows. These restrict the wavelength range available in the i.r. to a lower limit of ca. 2000cm-’, but are compatible with Raman use. Windows of rock salt extrude too rapidly to be of value and the harder fluorite type are little better ;2 typical designs are given in

’ J . R. Ferraro, in ‘Spectroscopy in Inorganic Chemistry’, ed. C. N. R. Rao and J. R. Ferraro, Academic Press, New York, 1971, vol. 2. R. W. Parsons and H. G. Drickamer, J . Opt. SOC. Amer., 1956,46,464.

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4 D. M . Adams and S . J . Payne

refs. 3-5. Such cells can be used to study liquid-state specimens as well as solids, although there is then the double limitation of windows and fluid absorp- tion. The design is perhaps more suited to Raman work : there is no frequency restriction due to window materials, and water can be used as the pressure- transmitting fluid. Mitra and co-workers have recently described such a cell.' This type of cell does not readily lend itself to use at temperatures substantially different from ambient.

The Diamond Anvil Cell PAC).-This design6 allows work up to pressures of at least 70 kbar routinely, and much higher values have been reached,' although the lifetime of the diamonds is likely to be much reduced. A DAC designed for X-ray use has been operated up to 300 kbar.* Typically, the opposed anvil faces are square and of side 30-40 thousandths of an inch (0.7G1.02mm): they must be aligned accurately parallel and remain parallel throughout each pressure cycle. An improved design of DAC has recently been described' and is available commercially : particular attention was given to maintaining face parallelism, and pressure is applied via a small hydraulic ram in place of the calibrated spring used by others.

The main disadvantage of the DAC is that there is a pressure distribution across the anvil faces and a solid sample therefore experiences non-hydrostatic pressure,", '' although this may be minimized by careful attention to technique. The sample at the diamond face centres may thus experience a pressure 1.5 times that at the edges. In general, the pressure calculated in terms of the contact area of the anvil and the known low-pressure side geometry gives the mean pressure across the faces. Alternatively, calibration can be established by observation under a microscope of known phase changes.I2 Even for a transition between two colourless phases (e.g. the rock salt and CsCl polymorphs of alkali-metal halides), the Beck line is readily seen. Another excellent approach is to monitor the visible absorption spectrum of a complex such as nickel dimethylglyoxime, which shifts ca. 80cm-' kbar-' with pressure; it can be used as an internal calibrant.' Certain trigonal-bipyramidal complexes of nickel, [NiLXIY, e.g. [Ni(CN)Sb((CH,),AsMe, ),]BPh, confer some advantages over nickel dimethyl-

E. Fishman and H. G. Drickamer, Analyt. Chem., 1956, 28, 804. H. D . Stromberg and R. N. Schock, Rev. Sci. Instr., 1970,41, 1880. 0. Brafrnan, S . S. Mitra, R . K. Crawford, W. B. Daniels, C. Postmus, and J. R. Ferraro, Solid State Comm., 1969,7, 449. C . E. Weir, E. R. Lippincott, A. Van Valkenberg, and E. N. Bunting, J . Res. Nut. Bur. Stand., Sect. A , 1959, 63, 5 5 .

' E. R. Lippincott, C. E. Weir, A. Van Valkenberg, and E. N. Bunting, Spectrochim. Acta, 1960, 16, 58. W. A. Bassett, T. Takahashi, and P. W. Stook, Rev. Sci. Instr. 1967, 38, 37. D. M. Adams, K. Martin, and S. J . Payne, Appl. Spectroscopy, in the press.

l o E. R. Lippincott and H. C. Duecker, Science, 1964, 144, 1 1 19. L. C. Towle and R. E. Riecker, J . Geophys. Res., 1966,71, 2609. P. W. Bridgman, Proc. Amer. Acad. Arts Sci., 1937,72, 45.

l 3 H. W. Davies, J . Res. Nat. Bur. Stand., Sect. A , 1968, 7 2 , 149.

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Vibrational Spectroscopy of Solids at High Pressures 5

glyoxime, chiefly in giving more symmetrical bands which show little intensity change with pre~sure. '~. l 5

Almost no Raman work using a DAC has been reported. In two studies of the redt+yellow transition in mercuric iodide, in good agreement, both groups used 0" excitation (ie. a laser beam fired straight through the cell towards the entrance lit).'^,'^ Although it has been claimed5." that the DAC is unsuitable for Raman work (because it was assumed that only 0" scattering geometry was usable), it has recently been shown that excellent results can be obtained using 180" excitation.' One diamond anvil was replaced by one of polished tungsten carbide. Use of the normal two-diamond anvils in the 180" mode'' gave poor spectra, although the performance could probably be restored if the rear diamond were silvered. The most severe limitation on Raman work with a DAC is set by fluorescence of the diamonds. Only type I1 stones can be used (as in the i.r.), and these are only compatible with red (632.8 and 647.1 nm) and purple (476.5 nm, argon ion) excitation. However, using a sapphire anvil, green (514.5 nm), blue (488.0 nm), and (to some extent) red (632.8 nm, only) excitation is compatible with its fluorescence emission system,' although pressure is limited to ca. 12 kbar. Nevertheless, a considerable future for the DAC in Raman work seems likely.

Owing to the small open area of the diamond anvils, i.r. transmission is low. However, the DAC can be used directly in interferometer^,^^^^ although it is necessary to use a simple light pipe in front of the cell. With such equipment, high-quality spectra at high pressure can be obtained routinely, with no special precautions. Using the most recent design' it is simple to operate the DAC in vucuo, thereby allowing series of runs to be made at varying pressures without breaking vacuum.

Transmission of the DAC in normal dispersive spectrometers is very low and some method of beam condensation is essential for routine work, although some poor results have been obtained without it. Most workers use a Perkin-Elmer beam condenser consisting of two 90" off-axis ellipsoidal mirrors with 6 : 1 condensation.21 This is an elaborate and expensive item which can probably be replaced with a much simpler system. Nevertheless, it is clear that the entire i.r. range is accessible using the DAC, except for the region of diamond absorption, cu. 2000cm-', in which sapphire anvils must be used. A specialized six-anvil device with quartz windows has been used up to 35 kbar in conjunction with an interferometer . ' l 4 J . R . Ferraro, Inorg. Nuclear Chem. Letters, 1970, 6, 823. l 5 J . R . Ferraro, D. W. Meek, E. C. Siwiec, and A. Quattrochi, J . Amer. Chem. SOC.,

1971,93, 3862. l 6 C. Postmus, V. A. Maroni, J . R. Ferraro, and S. S. Mitra, Inorg. Nuclear Chem.

Letters, 1968, 4, 269. J . W. Brasch, A. J. Melveger, and E. R . Lippincott, Chem. Phys. Letters, 1968, 2, 99. S. S. Mitra, Indian J . Pure Appl. Phys., 1971, 9, 922.

" D. M. Adams and G . Arie, Nouvelle Rev. Optique Appl. , 1971, 2 , 69. *' N. T. McDevitt, R . E. Witkowski, and W. G. Fateley, Abstract, 13th International

Colloquium on Spectroscopy, Ottawa, Canada, June 18-24, 1967. J . R. Ferraro, S. S. Mitra, and C. Postmus, Inorg. Nuclear Chem. Letters, 1966, 2 , 269.

2 2 C. C. Bradley, H. A. Gebbie, A . C. Gilby, V. V. Kechin, and J . H . King, Nature, 1966, 211, 839.

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6 D. M . A d a m and S. J. Payne

The Drickamer Cells.-A general design type, due to Dri~kamer,’~ uses NaCl as the pressure-transmitting medium. Working around this basic principle, designs of cells for use in just about every kind of spectroscopy have been reported. Basically, two opposed tungsten carbide pistons (driven by a simple hydraulic press) compress NaCl which extrudes throughout the cell. With careful design of the window ports, extrusion is so low as to allow NaCl to act as the cell windows. Samples are loaded into a small hole drilled into the central core of NaCl. Other materials (diamond, sapphire, silicon, etc.) may be used to support the NaCl windows if necessary. Two versions of the Drickamer design are currently used, differing in detailed internal dimensions and maximum attainable pressures (0-60 kbar and 0-200 kbar are commonly quoted ranges). Suitably modified, it has been used up to 180 kbar at 500 0C,24 and to 55 kbar at 80 K,25 and even at liquid-helium temperature.

Since NaCl is the medium through which the radiation passes, its use in the i.r. is limited to the region above 400cm-’. Range extension with caesium halides as the fluid is not possible owing to their much greater ease of extrusion. However, the design is very suitable for Raman use since NaCl has no first-order spectrum. Nicol and co-workers have described a version specifically for Raman work with 90” excitation, capable of working in the range 77--500K.26 The Drickamer design confers the advantages over the DAC of extended pressure range and approximately hydrostatic pressure on the sample. However, its use can hardly be described as routine as it demands a certain amount of expertise, as well as ready access to high-grade engineering facilities for heat treatment and precision grinding.

3 Studies of Vibrational Spectra at High Pressures

To some extent, current work in this field seems to be motivated by the belief that use of high pressures is a good thing per se. The much easier experiment of recording spectra at low temperature can give equivalent information in some cases ; cooling to liquid-nitrogen temperatures has roughly the same effect as applying a few kbar pressure. Also, not all problems require the use of very high pressures, and much valuable work can be done in the ‘hydrostatic’ region up to ca. 12 kbar. Nevertheless, a number of clear and valid reasons for interest in high-pressure spectroscopy emerge.

Foremost among these is the desire to study phases not stable under ambient conditions : several examples are discussed below. Intermolecular forces are pressure dependent, offering a substantial field to the physical chemist. In a solid in which there are two types of bond (a crude over-simplification) the weaker can be expected to show the greater pressure dependence ; thus, hydrogen-bonded materials have been prominent choices for study. There is also some hope that

23 R. A. Fitch, T. E. Slykhouse, and H. G. Drickamer, J. Opt. SOC. Amer., 1957,47, 1015. 2 4 A. S. Balchan and H. G. Drickamer, Rev. Sci. Insfr., 1960,31, 51 1 l 5 W. F. Sherman, J. Sci. Instr., 1966, 43, 462. 2 6 M. Nicol, Y. Ebisuzaki, W. D. Ellenson, and A. Karim, Rev. Sci. Instr., 1972, 43,

1368.

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Vibrational Spectroscopy of Solicls at High Pressures 7

symmetry-related information may be obtained from pressure work with powder samples : in terms of a simple model, a symmetric mode of vibration is a volume pulsation and should be much more pressure-sensitive than an antisymmetric mode which is to some extent self-compensating. Some evidence exists in support of this hope, but the picture is still not entirely clear. Molecular Systems.-Application of pressure to a molecular crystal might be expected to have greatest effect on the external or lattice modes, but virtually no such studies have yet been made. At higher frequencies, the internal modes nearly always show blue shifts, and symmetric modes commonly decrease in intensity and broaden concurrently. All of this work is qualitative. Spectra at elevated pressures have been published for naphthalene, biphenyl, p-dichloro- benzene, o-chloro-p-nitrophenol, and p-nitr~phenol.~ A particularly satisfying study of trioxan has been made. Under pressure it was possible to grow a single crystal, so oriented that almost complete extinction between A, and E (in C:,) species was obtained : shifts of from 5 to 10 cm- at 40 kbar were observed. At higher pressures a new phase grew into the initial crystal in a highly oriented manner : it appeared to have higher symmetry and a distinctive spectrum showing fewer lines than the first phase. It is believed that the rings may have become planar, with D,, ~ymrnetry.’~

In the majority of work reported on molecular systems, interest centres on the effect of hydrogen-bonding. In solutions or in the liquid phase, v(0H) shows red shifts with pressure. Fishman and Drickamer2* found that the magnitude of the shift of v(0H) of butanol solutions depended upon the polarizability of the solvent and was also linearly dependent upon the solvent density squared. These red shifts were believed to be due to dispersive interactions with solvent. There are general correlations between : (a) the redness of the spectral shift under pressure and the magnitude of the vapour-liquid red shift ; (b) the redness of the v(A-H) shift and the electronegativity of A. C-H, N-H, and S-H bond-stretching frequencies are relatively insensitive to pressure, possibly indicating a flatter potential well than is commonly belie~ed.’~ v(C-N) of acetonitrile exhibits blue shifts of less than 2 cm- up to 10 kbar pressure in various solvents, whereas the first and second harmonics of v(C=O) in various ketonic systems shift up to ca. 15 cm-’ in the same pressure range. Relative contributions to the shifts of induction, dispersion, orientation, and repulsive forces have been estimated : for the v(C=O) series, induction forces are responsible for ca. 65 % of the shift.,’

It also appears that increasing pressure shortens the hydrogen bond in liquid alcohols, resulting in large red shifts, without affecting the ‘polymer’ equilibrium of hydrogen-bonded species, in contrast to temperature changes which do cause a shift of polymer eq~ilibrium.~’

2 7 J . W. Brasch, A. J. Melveger, E. R. Lippincott, and S. D . Hamman, Appl. Spectroscopy,

2 8 E. Fishman and H. G. Drickamer, J . Chern. Phys., 1956,24,548. 2 9 A. M. Benson and H. G. Drickamer, J . Chem. Phys., 1957,27, 1164. 3 0 R. R. Wiederkher and H. G . Drickamer, J . Chern. Phys., 1958,28, 311. 3 1

1970, 24, 184.

R. J . Jakobsen, Y. Mikawa, and J. W. Brasch, Appl. Spectroscopy, 1970, 24, 333.

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8 D. M . Adams and S. J. Payne

The trends noted above are also evident in hydrogen-bonded solids, uiz. red shifts of v(0H) and v(NH) when hydrogen-bonded, but with non-hydrogen-bonded modes such as v(CH) suffering small blue shifts. Typically, studies have been made of poly(viny1 alcohol), nylon 66,32 and of acids such as ~ x a l i c , ~ ~ succinic, and benzoic7 using DACs. In succinic acid, for example, the pressure-sensitivity of the i.r. spectrum can be understood in terms of dimers :

/ O ... H - 0 \

R-C C-R

v(C=O) shifts to lower frequencies with increasing pressure, owing to strengthened hydrogen-bonding, whilst the out-of-plane deformation of the hydrogen-bonded hydroxy-group moves from 930 cm- to nearly 970 cm- at 50 kbar.7

With application of pressure, most organic liquids freeze: this has been turned to considerable advantage using the DAC.33 Samples are retained between the diamond faces by a gasket and application of pressure usually causes growth of a polycrystalline mass, but judicious cycling of pressure often results in formation of one single crystal. Successive experiments may yield single crystals of different orientation. Polarized light can be used for analysis since diamond is isotropic, but similar work with sapphire would require careful choice of axis when cutting the anvils. Spectra of crystalline benzene and n-hexane have been reported but not assigned in

In a subtle application of this technique, Brasch was able to obtain single crystals of various rotamers of a number of halogenated ethanes and thus assign the complex liquid-phase spectra with some certainty.34 To induce crystallization of 1,1,2,2-tetrachloroethane it was necessary to plunge the DAC into liquid nitrogen, an act of some bravery. Crystals of this compound obtained by cooling are of the gauche-rotamer, but solidification at high pressure yields the trans- form, a valuable differentiation. However, over a series of halogenated ethanes no consistent pattern of rotamer isolation emerged. Spectra of crystalline CH2Cl, and CH2Br, have also been ~ tud ied .~

Three polymorphs of acetonitrile and [2H3]acetonitrile have been obtained as single crystals by application of pressure, and their polarized i.r. and far4.r. spectra recorded ; possible crystal structures were deduced.36 Single crystals of ethanol and of deuteriates were grown similarly ; from dichroic measurements in the mid-i.r., a crystal structure was proposed which was analogous to that of the y-form of long-chain alkyl alcohol^.^

32 J. Reynolds and S. S. Sternstein, J . Chem. Phys., 1964,41,47. 3 3 J . W. Brasch, Spectrochim. Acta, 1965, 21, 1183. 3 4 J. W. Brasch, J . Chem. Phys., 1965, 43, 3473. 35 C. W. Brown, R. J . Obremski, J. R . Allkins, and E. R. Lippincott, J . Chem. Phys.,

36 R. J . Jakobsen and Y. Mikawa, Appl. Optics, 1970,9, 17. ’’ Y . Mikawa, J . W. Brasch, and R. J . Jakobsen, Spectrochim. Acta, 1971, 27A, 529.

1969,51, 1376.

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Vibrational Spectroscopy o j Solids at High Pressures 9

Inorganic Molecular Crystals.-Upon crystallization in a DAC, bromine shows Raman lines at 302(w) and 294(s) cm- ', corresponding to an A, + B Z g (in D2J correlation doublet. The liquid has a single line at 31 1 cm- '. Within experimental error, the Raman spectrum of liquid CS, in the v1 region (655.5, 646.5cm-') shows no change upon crystallization. Since the two components of the doublet show the same derived polarizability tensor dependence, they must have the same symmetry and are believed to be due to 33S and 34S isotopic species.38 In the far-i.r., solid CS, shows absorption at CQ. 90 cm-' at 12 kbar pressure. There is an anomolously large increase in total absorption associated with the liquid-to- solid phase change, probably owing to the different intermolecular interactions possible for this quadrupolar molecule, in contrast to chlorobenzene which is only dipolar. The latter shows two solid phases at ca. 5 and 20 kbar respectively, both having three far4.r. absorptions between 30 and 100 cm-1.22

The 368 cm- ' i.r. band of solid mercuric chloride, v(HgCl,),,,,, is lowered by 6 cm- ' at 60 kbar pressure. The equivalent mode in HgCl,, dioxan is lowered by 18 cm- ', whilst internal modes of the organic ligand at 854,614, and 290 cm- ' are raised concurrently by ca. 5 cm-'. The anomolous behaviour of v(Hg-C1) in these materials has been rationalized in terms of varying contributions of d-levels to the bonding.39 The 6(HOH) mode of water in hydrates such as CoC1,,6H20, Na2Cr,0,,2H,O, and Na,CrO,,lOH,O does not shift at pres- sures up to 45 kbar, but the integrated intensity of the band appears to vary.?

Sufficient co-ordination complexes of transition metals have been studied under pressure to indicate the main lines of behaviour. As with organic materials, it is the symmetric modes which are the most pressure sensitive: they may not shift very much in frequency but a decrease in intensity is almost always apparent and they often broaden as well. Typical examples are PtCl,(norbornadiene) and ZnCl,(terpyridyl) :40 up to 24 kbar v(Pt-Cl),,,, and v(Pt-Cl),,, both shift up by 3 cm-', but v(Pt-Cl),,, decreases in intensity very considerably relative to v(Pt-Cl),,,,. The five-co-ordinate zinc complex behaves similarly and this pattern is repeated for a variety of other mononuclear complexes. Bromides appear to behave like the chlorides, but as the assignments are sometimes less clear-cut the conclusion is not inviolable.

For trans-[MX,L,] complexes, (M = Pd or Pt; X = C1 or Br; L = Me,S), only V(MX,)~,~, is i.r.-active and this is virtually insensitive to pres~ure .~ ' In cis-[PtBr,L,], which has v(Pt-Br) at 225 and 240 cm-', well removed from v(Pt-S), there is again no shift but the lower band [presumed to be v(Pt-Br),,,] loses intensity relative to the higher one. A mixed bag of effects was found in the internal mode region of the ligand. Only blue shifts were found, in agreement with work on organic solids; v(CH,),,, and v(CH~)~,,, both suffered shifts of ca. 45 cm- ' up to 50 kbar, but the symmetric mode lost intensity considerably

3 8 A. J. Melveger, J . W. Brasch, and E. R . Lippincott, A p p f . Optics, 1970,9, 1 I . 3 9 Y. Mikawa, R. J . Jakobsen, and J . W. Brasch, J . Chem. Phys., 1966, 45, 4528. 40 C. Postmus, K. Nakamoto, and J . R. Ferraro, Inorg. Chem., 1967,6,2194. 4 1 J. R. Allkins, R. J. Obremski, C. W. Brown, and E. R. Lippincott, Inorg. Chem.,

1968, 8, 1450.

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10 D. M. Adamsands. J. Payne

and there was some evidence of re-orientation of the methyl groups, discontinu- ously, at ca. 20 kbar. Methyl rocking modes were slightly sensitive (more so in cis- than in trans-isomers, presumably reflecting different efficiencies of crystal packing), but 6(CH3) modes were invariant.

Monomeric CoCl,(py), is reversibly converted under pressure at 130 "C (but not at room temperature) to its octahedral polymeric isomer.42 v(Co-C1) modes in the monomer behave as detailed above for other MX,L, compounds, but v(Co-CI) bridge modes exhibit appreciable blue shifts, uiz. a band at 174 cm- shifts to 220 cm-' at 36 kbar whilst another at 186 cm-' shifts a similar amount but disappears under a neighbouring absorption. Concurrently, v(Co-N) shifts up by 17 cm-'. These effects are also shown by a series of compounds CoX,L, related to CoCl,(py), , 4 ° v 42 and ZnX,(dithi~dipyridine).~~ Use of pressure to distinguish totally symmetric v(M -N) modes therefore seems as reasonable as for the v(M-halogen) case.

In a study of pyrazine and some of its complexes, the following general behaviour was 0bserved.4~ (i) Bands in the ligand which show (blue) shifts are generally far less sensitive in the complex. (ii) All ligand bands broaden and decrease in inten- sity as they shift. (iii) In the ligand alone, there is some doubling of bands at higher pressures. The authors consider various origins for the splitting; one which they do not mention, and which seems reasonable, is that as the molecules are brought closer together the correlation field causes factor-group splitting. This could be tested by determination of the symmetry species of the bands con- cerned. 2,2'-Bipyridyl, when compressed at 55 kbar, shows some sensitive bands, but others do not shift. Detailed explanation of the behaviour of both this ligand and pyrazine founders on lack of really proven assignments ; nevertheless, com- paring the two, it seems that the most sensitive modes are those associated with volume pulsation.44 I t was noted that the ligands show spectra under pEssure very similar to those of the complexes in respect of band frequencies and, especially, of band splitting. The origins of band splitting in the two situations is unlikely to be the same. 9

Table 1 their shiftslcm- ' at 35 kbar

I.r.-active skeletal vibrationslcm - of metal sandwich compounds, and

Ring tilt v(M -ring) Ambient Pressure

Ambient Shijt Pressure Shiji

Fe(Cp), 49 1 3 46 1 RNCP), 447 29 38 1 MWP), 432 5 409 Cr(n-C6 H6)2 489 2 453 a True shift is higher, but band disappears at high pressures.

13 9"

11" 22

42 C. Postmus, J . R. Ferraro, A. Quattrochi, K. Shobatake, and K. Nakamoto, fnorg.

4 3 J. R. Ferraro, B. Murray, A. Quattrochi, and C. A. Luchetti, Specrrochim. Acru, 1972,

44 R. Bayer and J. R . Ferraro, fnorg. Chem., 1969,8, 1654.

Chem., 1969,8, 1851.

28A, 817.

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Vibrational Spectroscopy of Solih at High Pressures 11

Two brief reports of pressure studies on organometallic compounds have appeared. The v(M-ring) and ring 'tilt' modes of some sandwich compounds vary with pressure, as shown in Table l.,' There is no obvious reason why either band should be particularly sensitive, but the general trend seems to be that v(M-ring),,,, suffers the most ; ruthenocene is an apparent exception, but there is reason for believing that the assignment given is wrong.46 In the only Raman work reported using 180" ex~itation,~ it was found that v(W-CO), alg in W(CO), , is substantially pressure sensitive, but that the elg v(C0) mode is almost unaffected at the same pressure. However, the alg v(C0) mode is more sensitive, shifting some 12 cm- ' at 25 kbar.

From the fragmentary data reported, it is clear that much remains to be done in this area, but that there is real promise that useful information (as opposed to numerical data) will be forthcoming. The ability to get symmetry-related informa- tion from powder spectra, if borne out by wider investigation, is especially attractive and important.

Complex Ionic Crystals.-Although the majority of work in this class is on oxyanionic materials, a good introduction is given by a study of complex halides, as these spectra have been assigned in considerable detail. K,PtCl, has three i.r.-active lattice modes : the A,, mode corresponds to translation along the unique tetragonal axis, whereas the two E, modes represent translations in the (ab) plane. For both K,PtCl, and K,PdCl, the higher E , mode is the most highly pressure-sensitive in the entire spectrum, whilst the A2, mode hardly shifts.,' Concurrently, the internal modes of the [MX,I2- ions shift by amounts intermediate between the A,, and E, lattice vibrations. These observations have not been rationalized but it may be relevant that Drickamer and co-workers showed, by X-ray diffraction, that compression of tetragonal SnO, and MnO, caused contraction of the a-axis but expansion along c.

K,PtCl, and K,PdCl, are cubic and show a single i.r.-active lattice mode which is rather more pressure-sensitive than either of the two anion internal modes, v(Pt-Cl) and 6(PtC1).47 Thus, for K2PtC16 : v(Pt-Cl), 345 (0.62); G(Pt-Cl), 185 (0.60); v(lattice), 87 (0.76) cm- ', where the figures in parentheses are the pressure dependence in cm-' kbar-'. From this isolated result the genera! conclusion seems to have been drawn (by various groups of authors) that lattice modes are more sensitive than internal modes, and that this is a principle of wide application. At best this is an oversimplification, as is clear from K,PtCl,. The Reporters have recently found examples of cubic crystals where the reverse is true.,'

4 5 K. Nakamoto, C. Udovich, J . R. Ferraro, and A. Quattrochi, Appl. Spectroscopy,

46 D. M. Adams and W. S. Fernando, J.C.S. Dalton, 1972,2507. 4 7 J . R. Ferraro, J . Chem. Phys., 1970,53, 117. 48 H. G. Drickamer, R. W. Lynch. R. L. Clendenen, and E. A. Perez-Albuerne, Ado.

49 D. M. Adams and S. J . Payne, unpublished work.

1970, 24, 606.

Solid State Phys., 1966, 19, 135.

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12 D. M. Adams and S. J. Payne

A later and more complete i.r. study5' showed that the principal changes with pressure are: (i) appearance of the normally i.r.-inactive v 1 mode of COS- at 1087 cm-' ; (ii) a discontinuous shift of v2 from 882 to 865 cm-' ; (iii) splitting of v4, originally at 710 cm- ', to 715,690 cm- '. There are some similarities with the spectrum of the aragonite modification of CaCO,, but a more probable explana- tion of the changes is based upon presence ofan amount of CaC0,(1r).~' Bridgman showed in 193952 that there are discontinuities in the isothermal compressibility of CaCO, at 14 and 18 kbar, naming the new phases CaCO,(rr) and CaCO,(m). Their structures are not established with certainty. Raman spectra of both new phases at room temperat~re,~' and of CaCO,(rn) at 77 K,53 have recently been reported ; a cell of the Drickamer type was used. It is definite that neither of the phases (11) nor (HI) is aragonite. Presence of two lines in the v1 region in CaCO,(rrr) implies that the primitive cell contains at least two formula units, but little more can be said at present. Very preliminary results with MgCO,, aragonite, and other related materials showed no features of note.6

Raman spectra of CaMoO, and CaWO, have been investigated up to 40 kbar, also in a Drickamer cell, and a previously unknown high-pressure phase dis- covered for each; they may have structures similar to that of NiW04.54 The vibrational frequencies have dv/dP values between 0.1 and 1.0 cm-' kbar-'. The pressure dependence of the internal modes had been predicted by on the basis of a model involving Davydov splittings, but the majority of the shifts observed were of the wrong sign. the reason for this failure is not known.

The paraelectric crystals of KH2P0, and RbH,PO, have been studied by i.r. absorption at pressures up to 60 kbar in a DAC.56 In both compounds the protons are dynamically disordered between the two possible sites of the 0-He. . O bonds which interconnect the PO:- groups. On cooling below the ferroelectric Curie point there is a phase transition with ordering of the protons, known to be accompanied by substantial changes in i.r. spectrum. RbD,PO, does not show a ferroelectric transition and is also presumed to be ordered. Up to 30 kbar little variation of spectrum is found, apart from steady shifts of lattice modes, but at 60 kbar there is a dramatic change to a new phase in which the protons are ordered and the PO:- tetrahedra distorted. These changes are similar to, but not identical with, those found on cooling below the Curie point. RbD,PO,, which is not ferroelectric at any temperature, shows no high-pressure phase.

NH,C1 has an exceptionally large compressibility : the Raman spectrum at room temperature exhibits substantial shifts, with a sharp change in the frequency us. pressure plot near 10 kbar where an order-disorder transition takes place.57

Calcite, CaCO, , was one of the first compounds used in a diamond anvil

R. N. Schock and S. Katz, Amer. Mineralogist, 1968, 53, 1910. M. Y . Fong and M. Nicol, J . Chem. Phys., 1971,54, 579.

M. Nicol and W. D. Ellenson, J . Chem. Phys., 1972,56,677.

5 1

5 2 P. W. Bridgman, Amer. J . Sci., 1939, 237, 7. 53

5 4 M . Nicol and J. F. Durana, J . Chem. Phys., 1971,54, 1436. 5 5 J . F. Scott, J . Chem. Phys., 1968,49, 98. 5 6 R. Blinc, J . R. Ferraro, and C. Postmus, J . Chem. Phys., 1969, 51, 732. " Y. Ebisuzaki and M . Nicol, Chem. Phys. Letters, 1969,3, 480.

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Vibrational Spectroscopy of Solids at High Pressures 13

The v5 lattice mode (relative movement of NH4+ and C1- sub-lattices) has a pressure dependence of 2.65 cm-' kbar-' in the disordered (low pressure) phase, one of the largest known, and v6 (librational) behaves similarly. With the excep- tion of v2 ( E bend), all the internal modes show negative pressure dependencies varying between -0.1 and - 1.2cm-' kbar-', although v,,v(N-H), shows no break at the transition. These frequency decreases are associated with increasingly effective hydrogen-bonding in the ordered high-pressure phase. In this context we note a Raman study of NH,F by Wong and Whal le~ .~ ' Although the experi- ment was carried out at room pressure in a conventional cryostat, the phase studied was a high-pressure one recovered in metastable form by a technique reminiscent of that used in pre-stressing concrete girders.

Less complete, and often fragmentary, pressure data are known for several other materials. Fong and Nicol" have reported Raman spectra of KNO3(11r) at 3.2 kbar, the spectrum being similar to that previously obtained by heating alone ; and a new phase, at 8.0 kbar, KNO,(IV), which has a much more complex spectrum not yet assigned. In the far-i.r. region, NaNO, shows a broad lattice mode at 212 cm-' which is replaced at 35 kbar by a doublet, 279 and 224 cm- '.*' The internal modes of NO, - in NaNO, are not very sensitive to pressure, suggest- ing that interaction between atomic displacements on the anions is primarily dipolar in type. Thus: vzu 836 (2.4), v , ~ 727 (0), and v J U 1351 (28) cm-', where the figures in parentheses are shifts at 72kbar.59 In NaNO, the 825cm-' 6(N02) mode shifts discontinuously to 855 cm- ' consequent upon a phase transition at 14.5 kbar, and is a useful ~a l ib ran t .~ v4 of the anion in Na2S0, shifts up by 3 cm-' at 35 kbar, but a lattice mode at 183 cm-' rises by no less than 52 cm- 1.21 For both [Ph4As] [GeCl,] and [Ph,As] [SnCl,], v(MCl,),,,, drops in intensity relative to V(MC~,)~~,,,,, but does not shift ~ignificantly.~' v3(T2) of Mn04- in KMnO, does not split but ~4(T2), 6(OMn0), gives three bands under pressure. ' Lattice Compounds.4onsiderable interest has been shown in the past four years in the pressure dependence of vibrational modes in lattice compounds, because such experiments yield data essential for testing theories of lattice dynamics. The temperature dependence of a solid cannot be properly understood without a knowledge of its pressure dependence, since the volume change (at constant pressure) complicates the changes in anharmonic vibrational coupling and the two may or may not act in the same sense. For full comparison with theory it is necessary to have also the pressure dependence of the high- and low-frequency dielectric constants, and of the unit cell volume.

Compounds adopting the rock-salt structure have a single i.r.-active TO mode and no first-order Raman spectrum. The LO mode can often be seen directly as a shoulder on the TO band if the material is sufficiently ionic to have oTo and oLo well separated. 1.r. studies have been made of the pressure dependence of

" P. T. T. Wong and E. Whalley, Rev. Sci. Instr., 1972,43, 935. 5 9 R. Eckhardt, D. Eggers, and L. J . Slutsky, Spectrochim. Acta, 1970, 26A, 2033.

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14 D. M . Adam and S. J. Payne

the TO and LO frequencies of LiF,60 NaF,60 KCl," KBr,2'.6' and CsBr.62*63 In all cases studied increase of pressure caused blue shifts. However, the shifts vary from compound to compound and from structure to structure since the compressibility of the material is involved. For KCl and KBr the phase change to the CsCl structure was observed and, for KCl, coincided with a decrease in TO frequency of ca. 12 cm-' at 25 kbar. This decrease should be (6/8)* sz 0.87 (i.e. the ratio of the co-ordination numbers): observed values were 0.92 for KC1 and 0.88 for KBr.61 This work was done in a DAC and because of the pressure gradients the two phases tended to co-exist over quite wide applied pressure ranges. It should be remarked that the far-i.r. bands observed are extremely broad (see figure 3 of reference 21) and that the centres cannot be measured with great accuracy. The TO frequencies, oT0, of the alkali-metal halides (both structure types) follow a simple relation :64

where p = reduced mass per unit cell, a = cubic lattice constant, and is the isothermal compressibility. The pressure dependence of a phonon frequency, v i , is usually quoted in terms of the Gruneisen parameter,

-d In vi 1 ( dvi) d In V pi d P Yi=-=- -

yi may also be computed on the basis of various models. For LiF, in particular, comparison of temperature and pressure dependencies of oTo showed that there is a substantial anharmonic contribution to the frequency shift, increasing with temperature.60 In contrast, the temperature dependence of oT0 for KBr can be completely accounted for by its volume dependence alone.6' The same is true of the mixed crystals KCl,-,Br,, which show a single mode which varies smoothly with concentration between the values for the two end yTO >, yLo and the ratio yTO/yLO increases with the ionicity of the compound as measured by Szigetti's effective charge per valence electron, (q*/Ze) ; an explana- tion has been given in terms of lattice dynamics.66

TlI, which has a special room-temperature structure in which the inert pairs on thallium are stereochemically active, is transformed to the CsCl lattice (no first-order Raman spectrum) under 5 kbar pressure with accompanying loss of the five-line Raman spectrum (22.5,29, 39.5,62, and 73.5 cm- '). The Gruneisen parameter thereby derived is ca. 3. A 'hydrostatic' cell was used.5

6o S. S. Mitra, C. Postmus, and J. R . Ferraro, Phys. Rev. Letters, 1967, 18,455. 6 1 C. Postmus, J. R. Ferraro, and S. S . Mitra, Phys. Rev., 1968,174,983. 6 2 R. P. Lowndes, Phys. Rev. (B) , 1970,1,2754. 6 3 C. Postmus, J. R. Ferraro, and S. S. Mitra, Inorg. Nuclear Chem. Letters, 1968, 4, 55. 6 4 S. S. Mitra and R. Marshall, J . Chem. Phys., 1964,41, 3158. 6 5 J. R. Ferraro, C. Postmus, S. S. Mitra, and C. J. Hoskins, Appl. Optics, 1970, 9, 5 . 66 S . S. Mitra, 0. Brafman, W. B. Daniels, and R. K. Crawford, Phys. Rev., 1969, 186,

942.

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Vibrational Spectroscopy of Solids at High Pressures 15

There is considerable specialist interest in use of pressure as another parameter in study of impurity ions in alkali-metal halide host lattices. We note these very briefly. In 1957 it was shown that CN- in alkali-metal halides exhibits two v(CN) frequencies. Both show blue shifts with pressure with Av - 65 cm- ' for the lower band and ca. 35 cm-' for the higher, at 50 kbar.67 Very detailed studies of CN- in twelve alkali-metal halide host lattices have since shown that in the NaCl lattice type the anion is preferentially oriented (001), but is (1 10) in the CsCl type. The shifts were accounted for quantitatively in terms of a simple perturbation equation.68 Further detail was added to the picture in a study of alkali-metal halides doped with NCO-, in which the pressure dependence of vinternal f veXternal bands of the 'impurity' was investigated. The important points to emerge were the preference for one orientation of the impurity ions, and the prominence of combinations in which veXternal is a torsional mode.69 Of wider interest is the effect of pressure on CsBr doped with NH,+. By using pressures of up to 50 kbar at 100 K, two energy levels of the same symmetry were 'scanned' through their region of maximum Fermi resonance. The levels involved were members of the strongly resonating group v3, (v, + v4), 2v,, ca. 3100 cm-'.70

Compression of the rutile phase of TiO, above 26 kbar at room temperature yields TiO2(1r). Its Raman spectrum has been obtained and differs from that of rutile principally in the low-frequency region. This implies that the primary co-ordination is not affected but that long-range order varies; the changes are consistent with Ti0,(11) having the a-PbO, structure. The Raman frequencies of rutile show blue shifts with dv/dP ca. 0.4 cm- ' kbar- ', with the exception of the B , , mode near 145 cm-' which has negative pressure dependence, dv/dP = -0.3 f 0.1 cm-' kbar-'. A possible explanation depends upon the suggestion that frequency lowering is a result of broadening of the potential well consequent upon rotating TiO, groups that lie in planes normal to the c-axis, thereby shortening the already small separation between nearest-neighbour oxygens. ' Pressure dependence of the Raman spectra of CaF, , SrF,, BaF, (fluorite struc- ture),'' and Mg,Si, Mg,Ge, Mg,Sn (anti-fluorite structure)72 has also been studied.

Solids with simple tetrahedral structures have also attracted much attention. Those with the (cubic) zinc blende structure have well separated LO and TO mode frequencies, owing to the partially ionic nature of materials; both have been studied under pressure by Raman technique for ZnS,66. 7 3 ZnSe,66 ZnTe,66

as have the sole (triply degenerate) Raman-active optical phonons of diamond (at 1332 cm-1),66 silicon, and g e r r n a n i ~ m . ~ ~ The i.r. spectrum of zinc blende has been studied in a DAC and found to have a Gruneisen parameter y - 1.80, rather lower than for alkali-metal halides.60 The uniaxial

6 7 T. E. Slykhouse and H. G. Drickamer, J . Chem. Phys., 1957,27, 1226. G. R. Field and W. F. Sherman, J . Chem. Phys., 1967,47, 2378.

6 9 M. A. Cundill and W. F. Sherman, Phys. Rev., 1968,168, 1007. ' O W. F. Sherman and P. P. Smulovitch, J . Chem. Phys., 1970,52,5187. 7 '

72 C. J . Buchenauer, F. Cerdeira, and M. Cardona, presented at the Second International

and

M . Nicol and M. Y. Fong, J . Chem. Phys., 1971,54,3167.

Conference on Light Scattering of Solids, Paris, July, 1971.

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16 D. M . Adams and S. J . Payne

wurtzite lattice has A + E l + 2E, Raman-active modes : their pressure depen- dence has been investigated for Zn0,66 CdS,66 G ~ A s , ~ , , A1Sb,72 and GaSb.72 The fundamentals of the cubic crystals all show modest blue shifts, but for the wurtzite set, one low-frequency branch of an E, mode decreases with pressure as it goes over to the transverse acoustic branch. An analysis of data for wurtzite shows that the linear volume dependence of 0 2 ( L O b 2 ( T O ) is due mainly to the similar dependence of the (Szigetti) effective charge.', Crystals of the series ZnS, -xSex and CdS, -xSex show a two-mode dependence, i.e. there are mode frequencies close to those for the two end-members (cJ: KCl, -xBrx which shows one mode dependence). Whether or not one- or two-mode behaviour will occur depends upon the relative masses of the atoms. For the above mixed crystals, both modes showed blue shifts with increasing pressure, the higher mode being the more sensitive.74

a-Quartz, the stable phase at ambient conditions, shows a set of Raman lines (357, 395,407, 1073, 1088, 1240, and 1250 cm- ') which are independent of pres- sure (up to ca. 40 kbar), and another group (128, 207, 265, 464, 697, 795, and 807 cm-') for which significant blue shifts (0.5-1.8 cm-' kbar-') are found. Lines of both A and E symmetry occur in both sets. Significantly, the pressure- sensitive lines are also the only ones which are temperature sensitive, the sign being the same. These observations have been interpreted in outline in terms of the atom motions associated with the displacive phase transition to P-quartz. They lead to a correct prediction of the pressure dependence of the 0r-P transi- tion temperature." Under uniaxial compression at liquid-helium temperature the 128 cm-' (E species) line of a-quarts splits. The factor group symmetry is reduced from D , to C , by the compression used, thereby removing the degeneracy and accounting for the o b ~ e r v a t i o n . ~ ~

Mercuric iodide exists under ambient conditions as a red modification with a layer structure, but is readily converted either by heat or pressure (ca. 13 kbar) to the yellow form which consists basically of linear HgI, molecules. Two concordant studies by 0" Raman technique, using a DAC, show disappearance of lines at 119, 29, and 17 cm-' of the red form and growth of lines at 138 and 40cm-' due to the yellow phase. A slight blue shift of the red-phase lines is noted prior to onset of the transition.16, l 7 An i.r. absorption of HgO at 60 cm-' is apparently insensitive to pressure.63

4 Summary From the modest effort expended to date on the vibrational spectroscopy of solids, it is clear that the field holds exciting prospects. An extensive range of experiments await those concerned with lattice dynamics and intermolecular

7 3 Y. Ebisuzaki and M. Nicol, J . Phys. and Chem. Solids, 1972, 33, 763. 7 4 J. R. Ferraro, S. S. Mitra, C. Postmus, C. Hoskins, and E. C. Siwiec, Appl. Spectroscopy,

7s J . F. Asell and M. Nicol, J . Chem. Phys., 1968, 49, 5395. 7 6 V. J. Tekippe and A. K. Ramdas, Phys. Letters ( A ) , 1971, 35, 143.

1970, 24, 187.

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Vibrational Spectroscopy of Solids at High Pressures 17

forces, whilst chemists can be depended upon to develop the qualitative study of band shifts, especially in inorganic materials, with enthusiasm. The kinetics of the process will be largely dependent upon the interest shown commercially in developing simple and reliable equipment at reasonable prices.

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