J. Chem. SOC.,Faraday Trans. 2, 1981,77, 1233-1243 Vibrational Spectroscopy at High Pressures Part 3 1.-Raman and Infrared Spectra of Rubidium and Caesium Nitrates BY DAVIDM. ADAMS"AND TIONG-KIE TAN Department of Chemistry, University of Leicester, Leicester LE 17RH Received 31st October, 1980 Mid-infrared and Raman spectra have been obtained for RbN0, and CsNO, at pressures up to at least 40 kbar in each case and up to 125 kbar for the i.r. spectra of CsNO,. The data are shown to be consistent with the recently proposed structure for RbN0, V based upon the space group Pmmn with z = 2. The clearest evidence comes from the vland v4 regions of the nitrate ion and the libratory lattice mode region. The CsNO, I11 to IV phase transition recently claimed by Kalliomaki and Meisalo on the basis of X-ray measurements has been confirmed but is shown to be extremely sluhgish with phase III/IV intergrowths extending over the range 72-109 kbar.The s.t.p. phase of RbN03 (phase IV) transforms at high pressure to RbN0, V. CsN0, behaves similarly, CsN0, I1 slowly yielding CsN0, I11 above ca. 26 kbar.' (Throughout this paper 1kbar = 0.1 GPa = lo8N mP2.) RbN0, IV and CsN0, I1 are isostructural, as are RbN03 V and CsNO, III.2Very recently, a possible phase transition above 88 kbar has been claimed in CsNO, 111, leading to CsN0, IV which apparently adopts the same orthorhombic unit cell as CsN0, 111 (Pmmn =D::, z = 2), although the cell constants differ.2 There are no other structural data for these materials3 and none of these three high-pressure phases have been investigated by vibrational spectroscopy.We have characterised them by i.r. and Raman spectroscopy and have obtained independent evidence for existence of CsN03 IV. EXPERIMENTAL Samples were compressed in a diamond anvil cell (d.a.c.), using a gasket between the anvils. The gaskets were of inconel. For Raman work the initial thickness and the central hole diameter were 0.2 and 0.5 mm, respectively; corresponding dimensions for the mid-i.r. experiments were 0.05 and 0.3 mm. For Raman work the gasket hole was filled with 4 : 1 methanol: ethanol. The solid sample was then loaded into this liquid and the cell assembled. Pressure was thereby transmitted by the fluid to the solid which consequently experienced a hydrostatic stress.However, since RbN03 and CsNO, are intense i.r. absorbers, it was necessary to dilute them with RbCl or CsCl (as appropriate) for i.r. work: no pressure-transmitting fluid was then used and, although the mixture was contained within a gasket hole, it must be presumed that a small shear stress is also suffered by the sample. This is most clearly shown by the different phase-transition pressures revealed by Raman and i.r. results. Pressures were estimated by the ruby R-line Raman spectra were obtained using a Coderg T800 spectrometer and the collection optics described previously.6 Mid-i.r. spectra were obtained with a Perkin-Elmer 580B spec-trophotometer, fitted with a small-area thermocouple giving a signal-to-noise ratio ca. 1.5 times that of the standard detector, and a refracting beam conden~er.~ 1233 1234 I.R.AND RAMAN SPECTRA OF RbN03 AND CsN03 RESULTS AND DISCUSSION AMBIENT PHASE SPECTRA The isostructural RbN03 IV and CsN03 I1 have the space group P31(C:) or P32(C,3)with t = 9:' a detailed structural analysis has yet to be published. In both space groups there are three independent sets of C1(general) sites, each occupied by three nitrate groups: the same selection rules therefore apply whichever is the true group. By '80-isotopic dilution, Brooker has shown that the vl(NOS) region of the Raman spectra of both compounds is consistent with this formulation,' as is the i.r. polarisation behaviour. lo,l1 The two s.t.p. solids have almost identical Raman spectra and their i.r.spectra are very similar. Two qualitative mid-i.r. studies of RbN03 IV and CsN03 I1 are in general agreernent.12?13 Single-crystal mid-i.r. transmission data have been collected for both RbN03 IV lo and CsN03 II.lo>" Likewise, there are qualitative Raman data9.12,14 as well as reports of single-crystal studies for RbN03 IV and CsN03 I1 "," and a fragmentary account of the lattice mode region of both solids.16 These studies show that the numbers of bands observed in the region of the nitrate internal modes for the s.t.p. phases are many fewer than are predicted by the selection rules of table 1. Most features are accounted for at the level of site group TABLE 1.-VIBRATIONAL SELECTION RULES FOR AMBIENT AND HIGH-PRESSURE PHASES OF RbNO, AND CsNO, RbN03 IV, CsNO, I1 RbN03 V, CsNO, 111 A h r If \ x3 x2C3(crystal) +--Cl(site) D3h NO, u+u(xz) Czu(Site) D2h (Crystal) 3" (A+E) a Takes account of the three independent sets of 3C1sites in the unit cell.selection rules. Our mid-i.r. spectra at s.t.p. and at slightly elevated pressures, fig. 1 and 2, are essentially identical to those of Fernandes et al.l3 and the Raman spectra, fig. 3, closely parallel those of others. PHASE TRANSITIONS The occurrence of a phase transition in RbN03 is seen qualitatively in the i.r. spectra, fig. 1,by disappearance of one of the v1+ v4 components and by a change in the v4 region, but more readily in the plot of mode frequencies, fig. 4. Concurrently, a v3component appears in the Raman spectra, the v4 region is simplified and there are major changes in the external (lattice) mode region, fig.3 and 5. U.M. AUAM3 ANU 1-lL. IAlY 1235 ~ I I RbN03 A f IIIIIIlj.1~ 1 1800 1600 1400 1200 1000 800 v/cm-' FIG. 1.-Mid-i.r. spectra of RbNO, at various pressures. The pressure of the transition in RbN03 is seen to be near 12 kbar in the Raman experiments, in good agreement with the recent X-ray work.* In both the X-ray and Raman studies the same technique was used to apply pressure to the sample, namely immersion in 4 : 1 methanol :ethanol, which is known to remain fluid to at least 50 kbar. In contrast, the IV to V transition was delayed until nearer 20 kbar in the i.r.experiments, in which pressure was transmitted directly to a RbCl+ RbN03 mixture. The I1to I11transition in CsN03 is revealed by much the same kind of change as for RbN03, although the behaviour of the v4region is more complex. The transition is undoubtedly very sluggish: at 3 1kbar two pairs of v4 bands were plainly seen in the i.r. spectra, indicating two co-existing phases, but the sharp and intense v1 Raman 1236 I.R. AND RAMAN SPECTRA OF RbN03 AND CsN03 125 kbar / 51 kbar=' u 1100 900 700 v/crn-l FIG.2.-Mid-i.r. spectra of CsNO, at various pressures. band acquired a new component as low as 18kbar, fig. 6. These values are to be compared with the transition pressures of Kalliomaki and Meisalo2 (26kbar) and of Bridgmanl (ca.27 kbar, after extrapolation of higher temperature data).With further increase of pressure beyond ca. 80 kbar the Raman spectra became very poor in quality, especially in the region of the lattice modes, but high quality mid-i.r. data could still be collected. The mid-i.r. spectrum was progressively simplified as the pressure was raised, being reduced above 90 kbar to a single band in each region except u2. Moreover, vl and v1+ u4 lost intensity steadily and were only just detectable at 125 kbar. The collapse of the v4 region to a single band and the changes of slope in fig, 7 argue for a change of phase. Moreover u2 shows a weak shoulder on the low-frequency side at 72 kbar: this increases in intensity relative to D. M. ADAMS AND T-K.TAN RbN03 12 kbarT I x U'3 E: U E .I 6 kbar 1 1 , I 11 ,,I,, 1450 1300 1050 700 250 150 SO Avlcrn-l FIG. 3.-Raman spectra of RbN03 at various pressures. the main (higher) band as further pressure is applied, until by 125 kbar it is the only absorption remaining in that region. The behaviour of vl in the Raman spectra, fig. 8, also shows a discontinuous change in the band shapes between 57 and 74 kbar. We conclude that there is a sluggish phase transition (I11 to IV) in CsN03,beginning at 72 kbar, or even lower, but with evidence of intergrowth up to 109 kbar. Our results therefore support the claim by Kalliomaki and Meisalo that a further very sluggish phase transition occurs near 88 kbar. ANALYSIS OF THE SPECTRA OF RbN03 V, CsN03 I11 AND IV The structure of the s.t.p. phases of the two salts is greatly simplified at the IV/V (Rb) and II/III (Cs) transitions.The new phases have a bimolecular primitive unit 1238 I.R. AND RAMAN SPECTRA OF RbN03 AND CsN0, RbNO3 18003 0 10 20 30 40 ! P/kbar FIG.4.-Pressure dependences of mid-i.r. modes of RbNO,. cell. We now consider how well the selection rules, table 1, account for the observations. v1 should appear as a single band in the Raman spectra but not coincident with the single i.r. band. This is seen to be the case for RbN03. For CsN03, the new v1 appears, as expected, at lower frequency than in the parent phase: the two phases (I1 and 111) then coexist until near the limit of our observations (77 kbar), the new band increasing at the expense of the higher-frequency component.At 50kbar the Raman component of ul of CsN03111 is 2.5 cm-I below its i.r. equivalent, in accord with the selection rules which require non-coincidence. The rules for u2are analogous to those for u1but, unfortunately, v2is vanishingly weak in the Raman spectra. The i.r. spectrum of RbN03V shows a single v2band, as does CsN03I11 (until formation of CsN03IV sets in), thus conforming to the rules. The v3region, whilst potentially informative, is not so on the basis of the present data. In the Raman experiments much of it is obscured by the intense first-order line ofdiamond (1332 cm-I) whilst the i.r. bands of such an intensely absorbing mode are bound to be severely distorted by reflectance effects.u4 of RbN03V shows a single band in both Raman and i.r. spectra, these bands being coincident within limits of error. CsNO, I11 has the two i.r. components required by full factor-group analysis, there is only one Raman equivalent and it is D. M. ADAMS AND T-K. TAN 1239 I I I yo 220 -dO/ORbN03 T I I / No r0 -200 /O1420 0 A0 -180 / 1400 1080 I E \ 4 106C I loot 0 inn IIWW 1 10 1 20 1 30 40 Plkbar 0 10 20 30 40 Plkbar FIG.5.-Pressure-dependences of Raman modes of RbN03. coincident with one of the i.r. bands. With increase of pressure the higher and weaker of the i.r. v4components loses intensity rapidly: it appears that in RbN03 V this band was vanishingly weak throughout.We note also that the vl+v4 combina-tions, present in the i.r. spectra of both RbN03 V and CsN03 111, exactly mirror the behaviour of the v4 region both as to the numbers of bands and their pressure shifts. In summary, the nitrate internal mode region of both RbN03 V and CsN03 I11 is consistent with the proposed space group. The same selection rules apply to CsN03 IV; with the exception of a missing second v4 component in both i.r. and Raman spectra, they are obeyed. The lattice-mode region of the Raman spectra of both salts under ambient conditions shows many fewer bands than the selection rules require (i.e.33, plus any 1.0. components). Reports of this are confused in detail and require definitive re-investigation using single-crystal methods.However, there is agreement that there are 3A +2E type bands, although Badr et aZ.16report that 1240 I.R. AND RAMAN SPECTRA OF RbN03 AND CsN03 r I I I I CsNO, 180 60 40 20I/'-I 1 1 0 20 40 60 80 Plkbar FIG.6.-Pressure-dependences of Raman modes of CsN03. several more components can be seen at 135 IS. For RbN03 V and CsN03 I1 lattice-mode selection rules are: translatory 2Ag+2B2, +2B3, li bra t ory B1, +B2g +B3g. It is usually the case in nitrates and carbonates that the libratory lattice modes are at higher frequency than those of translatory type.17 Note that RbN03 V shows two bands below 80 cm-' and three above 180cm-' at 16 kbar and above, fig. 3. The three higher bands exactly fit the libratory mode selection rules and constitute the clearest evidence in support of the proposed structure.Only two of the expected six translatory modes were found; however, the spectra were rather weak and it is by no means unlikely that further modes are actually present. D. M. ADAMS AND T-K. TAN 1241 I 1 I 1 I I 1800 -2 CsNO, /" / o, -0-0-0 .xov,t vq /o /6 /O 1775 -,O'" 00 o/o/"1080 v'/O // ./O $-1060 84f84$ V7-0 v2,o-o-s=o" -00-0-0 ---O o---o-o-o 830" -_/--/ o!%----0 -0-0 -0-0 o-o~o-o.n~oo-o-oI 0 1 I I I I I 7001 20 60 60 80 100 120 P/kbar FIG. 7.-Pressure-dependences of mid-i.r. modes of CsN03. The lattice-mode spectra of CsN03 were of much poorer quality than those of RbN03: the same general behaviour was observed, but frequencies could not be measured reliably.PRESSURE SHIFTS From Bridgman's compressibility data1' and the slopes of the least-squares lines on the v,/P plots, the mode Griineisen constants, 'yi, of table 2 were computed from 1242 I.R. AND RAMAN SPECTRA OF RbN03 AND CsN03 77 kbar ,/ \ 1100 1050 u/cm-' FIG. %-Raman spectrum of CsNO, in the v1 region. D. M. ADAMS AND T-K. TAN 1243 TABLE2.-MODE GRUNEISENCONSTANTS FOR RbNO, AND CSNO, RbNO, CsNO, A \ r iphase IIV” Vb.e phase II’ A IIId” 3.1 3.1 3.0 1.1 2.4 1.5 0.04 1.6 0.06 2.7 2.7 0.09 2.6 0.07 2.2 0.08 0.13 0.09 0.07 0.13 0.09 0.15 0.05 0.03 0.16 0.045 0.01 0.05 0.08 0.07 0 0.065 0.09 0.03 0.01 0 0.07 0.09 0.05 0.07 0.06 0.11 xT= (a)4.5, (b) 4.0, (c) 5.2 or (d)3.8 X lop3kbar-’.ui andXT both taken at (e) 16 and (f) 26 kbar, respectively. where xT is the isothermal compressibility. Values for the lattice modes are comparable with those for other simple salts and show no clear difference for the high-pressure phases as compared with those stable at s.t.p. We thank the S.R.C. for support. C. W. F. T. Pistorius, Prog. Solid State Chem., 1978, 11, 1. M. S. Kalliomaki and V. P. J. Meisalo, Acta Crystallogr., Sect. B, 1979, 35, 2829. C. N. R. Rao, B. Prakqsh and M. Natarajan, Natf. Bur. Stand. (U.S.),Monogr., 1975, 53. G. J. Piermarini, S. Block and J. D.Barnett, J. Appl. Phys., 1973, 44, 5377. D. M. Adams, R. Appleby and S. K. Sharma, J. Phys. E, 1976,9, 1140. D. M. Adams, S. K. Sharma and R. Appleby, Appl. Opt., 1977,16, 2572. D. M. Adams and S. K. Sharma, Appl. Opt., 1979,18, 594. T. P. Delacy and C. H. L. Kennard, Aust. J. Chem., 1971, 24, 165. M. H. Brooker, J. Chem. Phys., 1973,59,5828. S. V. Karpov and A. A. Shultin, Phys. Status Solidi, 1970, 39, 33. 11 A. J. Melveger, R. K. Khanna and E. R. Lippincott, J. Chem. Phys., 1970,52, 2747. l2 M. H. Brooker and D. E. Irish, Can.J. Chem., 1970,48, 1183. l3 J. R. Fernandes, S. Ganguly and C. N. R. Rao, Spectrochim. Acta, Part A, 1979,35, 1013. l4 J. P. Devlin and D. W. James, Chem. Phys. Lett. 1970, 7, 237. l5 D. W. James and J. P. Devlin, J.Chem. Phys., 1972, 56, 4688. l6 Ya. A. Badr, S. V. Karpov and A. A. Shultin, Sou. Phys.-Solid State, 1975, 16, 1515. 17 See, for example, G. R. Wilkinson, in Molecular Dynamics and Structure of Solids, ed. R. S. Carter and J. J. Rush, Natl. Bur. Stand. (U.S.),Spec. Publ.. 1969, 301, 77. P. W. Bridgman, Proc. Am. Acad. Arts Sci., 1945, 76, 1 and 9. (PAPER 0/1659)