Spectroscopy at very high pressures: Infrared study of the high pressure phase transitions in NaNO2 and KNO2

Infrared spectra of NaNO₂ and KNO₂ in the region of the anion internal modes have been obtained using a diamond anvil cell at pressures up to 65 kbar. At 39°C NaNO₂ undergoes a phase transition at ca. 10.0 kbar, not at 14 kbar as reported from an earlier IR study. A similar transition was found at 6.3 kbar for KNO₂. The symmetric modes lose intensity with increasing pressure, and all modes suffer blue-shifts. For KNO₂ ν(NO)₅ shifts much more than ν(NO)_a.     

Phase transitions in the dimethyl thallium(III) halides, (CH3)2TIX(X = Cl,Br, I)

The dimethyl thallium(III) halides were investigated at low temperature and at high pressure by IR and Raman spectroscopy. These compounds have a tetragonal structure under ambient conditions, in which the methyl groups are disordered. Ordering transitions were observed at low temperature in all three halides. The low-temperature unit cells are monoclinic or triclinic with C_2h or C_i symmetry and Z = 1 and Z = 2 per primitive unit cell for the chloride and for the bromide and iodide, respectively.At high pressure, the iodide and bromide underwent phase transitions at just below 10 kbar to phases similar to those observed at low-temperature. A second transition was observed in both the iodide and bromide at 27 and 45 kbar, respectively, which involved a change in conformation of the (CH₃)₂Tl⁺ ion from linear to bent and a distortion of the (TlX)_nlayers. Significant spectral changes were also observed for the iodide and bromide at close to 65 kbar and 70 kbar, respectively, indicating the presence of another transition involving a change in methyl group orientation.The chloride underwent a transition at about 15 kbar at which the methyl groups become ordered. This phase appears to be related to that observed at low-temperature for the bromide and iodide. Further changes were observed at just below 25 kbar indicating that the (CH₃)₂Tl⁺ ions become bent. There is evidence for another transition above 35 kbar from changes in slope on plots of ν versus p.     

Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

Vibrational spectroscopy at very high pressures—XLVI. A new high pressure phase transition in NaNO2

A Raman study to 100 kbar has confirmed existence of a second-order phase change in NaNO₂ at 8.5 kbar at ambient temperature, and revealed a new transition at 48.0 kbar. This transition appears to be first-order in type and involve cell doubling.     

The conformation and vibrational spectra of isocyanato and isothiocyanatocyclohexane

The IR spectra of isocyanato and isothiocyanatocyclohexane (C₆H₁₁NCX) as liquids and as amorphous and crystalline solids at low temperatures have been recorded in the region 4000-50 cm^?1. High pressure (0–30 kbar) IR spectra of the neat samples were obtained in a diamond anvil cell and various high-pressure solid phases were studied. Raman spectra of the compounds as liquids and as low-temperature solids were obtained.Isocyanatocyclohexane crystallized directly as anisotropic solids containing equatorial molecules at low temperature and axial molecules at high pressure. Isothiocyanatocyclohexane formed a possibly plastic phase between 225 and 260 K where both equatorial and axial conformers are present. A solid high-pressure phase (1–3 kbar) at ambient temperature appeared anisotropic and contained both the e and a conformers. Below 225 K (atmospheric pressure) and above 10 kbar (ambient temperature) anisotropic crystals were formed which both contained equatorial conformers only.Normal coordinate analyses were carried out for the equatorial and axial conformers of the two molecules with different orientations (C_s and C_l symmetries) of the side chain. Force constants were transferred from various halo and pseudohalocyclohexanes. Tentative assignments of the fundamentals belonging to both the e and a-conformers are presented.     

Oxygen phase equilibria near 298 K

Raman spectra of fluid and solid oxygen have been measured at temperatures near 298 K to pressure greater than 180 kbar (18 GPa). At 298 K, fluid oxygen freezes at 59.1±0.5 kbar which is 2 kbar higher than the freezing pressure of n-H₂ at this temperature. Solid—solid phase transitions are observed near 96 and 99 kbar. The phase boundaries near room temperature and the intense visible absorption spectra of the very high pressure phase are described.     

Pressure-tuning Raman and infrared spectra of N-thiocyanato[tris(o-tolyl)]tin(IV), (o-CH3C6H4)3SnNCS

The effect of high external pressures on the Raman and IR spectra of the title compound (I) has been examined at ambient temperature. A pressure-induced phase transition was observed at 13–16 kbar, which is most likely second-order, resulting from slight rotations of the phenyl rings and/or the CH₃ groups under the influence of pressure. No new peaks were observed in the spectra with increasing pressure indicating that no pressure-induced linkage isomerism or SnNCS⋯Sn bridging took place. The average pressure sensitivity (dν/dP) of the Raman-active vibrational modes is lower in the low-pressure region (0.23 cm⁻¹/kbar) than in the high-pressure one (0.47 cm⁻¹/kbar). In general, the IR-active modes are less sensitive to increasing pressure than are the Raman-active modes and the average dν/dP value for the IR-active modes in the low-pressure region is quite similar to that in the high-pressure region, i.e., about 0.23 cm⁻¹/kbar.     

High-pressure FT IR measurements of crystalline methylene chloride up to 120 kbar in the diamond anvil cell

The FT IR spectra of pressure-induced crystalline CH₂Cl₂ at room temperature were measured at hydrostatic pressures up to 120 kbar in the diamond anvil cell. The pressure dependences of the internal modes (ν₃, ν₉, ν₈, and ν₂) are reported and compared with the result of Raman scattering measurements. The discontinuity of the slope (dν/dP) at ≈ 45 kbar for the ν₉ antisymmetric CCl streching mode indicates the pressure-induced second-order phase transition which seems to be triggered by the interaction between the ν₉ mode and the ν₃ symmetric CCl stretching mode.     

Vibrational spectroscopy at high pressures—LVI. A Raman study of some alkali metal halates,and the X-ray compressibility of KClO3

Raman spectra at high pressure have been obtained for KBrO₃, NaClO₃ and NaBrO₃ in a diamond anvil cell. A phase change has been found for KBrO₃ at 19 kbar; at 24 kbar for NaBrO₃; and at 36 kbar for NaClO₃. The bulk compressibility of KClO₃ has been determined by X-ray diffraction using synchrotron radiation and found to be 2.36 × 10⁻² GPa⁻¹, in good agreement with Bridgman's value.     

Vibrational spectroscopy at high pressure: Part 53. Alkali metavanadates and copper metagermanate

Essentially complete Raman and infrared spectra at ambient and 80 K have been obtained for the isostructural metavanadates AVO₃, (A  K, Rb, Cs). An assignment is offered on the basis of line and factor group analyses. A similar study has been made on CuGeO₃. Under pressure up to 100 kbar both KVO₃ and RbVO₃ show a single phase transition, at 56 and 53 kbar. The remarkable insensitivity of the transition pressure to change of cation implies that the major effect of pressure is chain torsion and repacking, driven by the need to reduce the cation coordination number.     

High pressure raman study of isotropic lineshapes of aqueous sodium nitrate solutions

The isotropic lineshape of the v₁ (A₁) stretching mode of the nitrate ion in solutions of sodium nitrate has been studied at 25°C as a function of NaNO₃ concentration ranging from 0.1 to 6M. The pressure dependence has been determined for 1 and 6M solutions at pressures ranging from 1 bar to 3 kbar. The isotropic band becomes more asymmetric with increasing concentration, and its v₁ peak frequency undergoes a blue shift both with increased concentration and increased pressure. At low concentration the vibrational correlation function is well described by the Kubo formula, whereas at higher concentration it becomes more Gaussian. The experimental data indicate that the v₁ vibrational lineshape in aqueous solutions of NaNO₃ is dominated by strong intermolecular interactions which produce inhomogeneous broadening at higher concentration.     

Vibrational spectroscopy at very high pressures: Part 21. Raman and infrared study of mercury(II) iodide [1]

The solid phase behaviour of HgI₂ is reviewed, together with IR and Raman data for the red and the high-temperature yellow forms. New IR transmission, single-crystal reflectance, and Raman data were obtained at temperatures down to 80 K for red HgI₂ but no evidence of a temperature-induced transition to HgI₂ IV was found.The high pressure results showed a possible break in the slope of the pressure versus frequency plot for the 29 cm^?1 Raman mode of red Hgl, at 6 ± 0.5 kbar, close to the reported III/IV phase boundary. Both Raman and far-IR spectra of the high pressure form of yellow HgI₂ differ significantly from those of high temperature yellow HgI₂ : the two forms are regarded as having different structures. These, and other data, reveal a much closer similarity between the phase diagrams of HgBr₂ and HgI₂ than has been suspected hitherto.     

Vibrational spectroscopy of solids under high pressures. Part I. Raman spectra of inorganic hexafluorometallate compounds

Raman spectra of a series of isomorphous hexafluorometallates (TlSbF₆; BaMF₆, M = Si, Ge, Sn, and Ti, space group $\text{R}\text{3}\text{m-D}{}^5{}_{\mathrm{3d}}\text{, Z = 1}$) have been measured under high pressures up to 25 kbar with a new opposed sapphire anvil cell. Pressure calibration has been achieved by means of the R₁ fluorescence band of ruby. The frequency of the only Raman active lattice mode (libration of the octahedral anion) of each compound shows a higher pressure dependence than do the internal vibrations of the anions. The E_g components of the ν₅ modes of the MF₆ octahedra which are coupled to the E_g lattice modes because of their same symmetries and small frequency differences exhibit higher pressure sensitivities than do the uncoupled internal modes. These findings may be used as a further means for symmetry assignment. BaGeF₆ undergoes a reversible phase transition at 9.6 kbar; the split librational modes of the high-pressure phase show the highest pressure dependences of up to about 3 cm^?1 · kbar^?1 found in this investigation.     

The pressure dependence of the low-frequency Raman spectra of crystalline biphenyl and p-terphenyl

Raman spectra as a function of hydrostatic pressure are presented for crystalline biphenyl and p-terphenyl. The observed changes in the low-frequency Raman spectra of crystalline biphenyl indicate that there are probably some changes in the crystal or molecular structure with increasing pressure. The Raman spectra of p-terphenyl have no evident anomalies at pressures up to 33 kbar.     

Spectres infrarouges et raman du complexe solide MgBr22Et2O: Changement de phase et isomérie de rotation des molécules d'éther coordonnées

Infrared and Raman spectra of solid magnesium bromide dietherate MgBr₂2Et₂O have been studied at different temperatures between 300 and 90 °K in the 4000 to 30 cm^? range. Infrared spectra of this compound at various pressures up to 10 kbar have also been investigated. The MgBr₂2Et₂O crystal has two phases and there is a reversible transition between them. This phase transition concerns primarily the conformational change of (C₂Hg₅)₂O molecules and much less the arrangement of the ligands around the central magnesium atom. At room temperature or low pressure the GG conformation appears to be predominant while at low temperature or high pressure there is only TG (or TT) conformation of ether molecules. The enthalpy difference and transition temperature between the phases have been determined calorimetrically. An assignment of the spectra of both phases is given. The analysis of intramolecular vibrations gives information about the relative contribution of conformational change and coordination effect to the frequency shifts of (C₂H₅)₂O vibrations. The assignment of magnesium-ligand vibrations on the other hand allows us to estimate the Mg-Br and Mg-O force constants.     

High-pressure Raman study of liquid and molecular crystal at room temperature. Methylene chloride

Raman spectra of liquid and crystalline CH₂Cl₂ were measured at hydrostatic pressures up to 85 kbar and at room temperature. The pressure dependences of the internal modes (ν₄, ν₃, ν₉, ν₂ and ν₁) and the two external (lattice) modes are reported; the ν₄ symmetric CCl₂ bending mode is split into two major peaks at the liquid—solid phase transition point (at 11.3 kbar), and the discontinuities of the slopes for their peak frequencies against pressure suggest a second-order phase transition at ≈ 45 kbar. The pressure data are used to test the applicability of the vibrational scaling law proposed by Zallen for a molecular crystal.     

High-pressure synthesis and structural behavior of sodium orthonitrate Na3NO4

Sodium orthonitrate (Na₃NO₄) is an unusual phase containing the first example of isolated tetrahedrally bonded NO₄³⁻ groups. This compound was obtained originally by heating together mixtures of Na₂O and NaNO₃ for periods extending up to >14 days in evacuated chambers. Considering the negative volume change between reactants and products, it was inferred that a high-pressure synthesis route might favor the formation of the Na₃NO₄ compound. We found that the recovered sample is likely to be a high-pressure polymorph, containing NO₄³⁻ groups as evidenced by Raman spectroscopy. The high-pressure behavior of Na₃NO₄ was studied using Raman spectroscopy and synchrotron X-ray diffraction in a diamond anvil cell above 60 GPa. We found no evidence for major structural transformations, even following laser heating experiments carried out at high pressure, although broadening of the Raman peaks could indicate the onset of disordering at higher pressure.     

Structure and properties of titanium-zinc borophosphate glasses

The structure of 50ZnO-10B₂O₃-40P₂O₅+xTiO₂ glasses prepared by slow cooling (for x=0-24 mol% TiO₂) or by quenching (for x=28-64 mol% TiO₂) was studied by Raman, ³¹P and ¹¹B MAS NMR spectroscopy. TiO₂ incorporates into the glass structure, which results in a decrease in the molar volume and an increase in the glass transition temperature. According to Raman spectra Ti atoms form distorted TiO₆ octahedra and their incorporation into the structural network results in the depolymerization of phosphate chains. ¹¹B NMR spectra of the glasses with increasing TiO₂ content show gradual changes from one symmetrical signal of B(OP)₄ units to a broad complex signal which was separated into five signals ascribed to four different types of BO₄ units and one BO₃ unit. With increasing TiO₂ content a part of boron atoms changes from tetrahedral BO₄ to trigonal BO₃ units, which is ascribed to the formation of TiO₆ units in the glass structure.     

Effect of high external pressures on the vibrational spectra of Zeise’s complexes, K[Pt(η-C2H4)Cl3] and [Pt(η-C2H4)Cl2]2

The pressure dependences (dν/dP) of the main IR and Raman bands of Zeise’s complexes, K[Pt(η²-C₂H₄)Cl₃] and [Pt(η²-C₂H₄)Cl₂]₂, have been determined for the first time for selected pressures up to ∼33 kbar with the aid of diamond-anvil cells. Neither complex undergoes a pressure-induced structural change throughout the pressure range investigated. The dν/dP values range from −0.13 to 0.79 cm⁻¹ kbar⁻¹. The negative values have proved particularly informative in identifying the location of the CC stretching modes of the Pt-ethylene groups, a topic of considerable disagreement in the literature.     

Vibrational spectroscopy at high pressures—57 [1]. Phosphonitrilic chloride trimer and tetramer

Raman and IR spectra at high pressures are reported for (PNCl₂)₃ and (PNCl₂)₄. The trimer exhibits a second order phase transition near 22 kbar to a structure of probable monoclinic symmetry, whilst the tetramer shows evidence of a similar change near 10 kbar.