The effect of pressure on ftir spectra of ammonium halides

Abstract

FTIR spectra of NH₄Cl, NH₄Br and NH₄I have been measured in the range of 600 - 4800 cm^?1 at pressures up to 30 GPa at 135 K. The spectra show significant changes at the phase transition IV - V, particularly the splitting of v₄ in phase V is clearly demonstrated.     

The effect of pressure on the Raman spectrum of NH4Cl

The Raman spectra of NH₄C1 are reported over a very wide pressure range at room temperature and some features of the well-known disorder-order transition as well as the spectra of the ordered phase at high pressures are discussed. The mode Grüneisen parameter has been determined to be equal to 2.1 ± 0.03 for ν₅(TO) in this phase showing that the volume-dependent anharmonicity is relatively large. Above 110kbar, significant spectral changes take place, a large number of lattice modes appear and some internal modes also reflect changes. Since these features closely resemble the ones observed in the newly discovered V of NH₄I, it is concluded that phase V also exists in NH₄Cl. The structure of phase V as well as the mechanism of the IV–V transition are still largely unknown but it is shown that the IV–V transition pressures in the ammonium halides vary linearly with the anionic radii.     

The structures of the ammonium halides

A comparison with the alkali halides suggests that all the ammonium halides should occur in the NaCl centre-of-mass structure. Experimentally, at room temperature and atmospheric pressure, only NH₄I crystallizes in this structure, while NH₄F is found in the ZnO structure, and NH₄C1 and NH₄Br occur in the CsCl structure. We show that a distributed charge on the NH₄⁺ ion can explain these structures. Taking charges of + 0.2e on each of the five atoms in NH₄⁺, as suggested by other studies, we have recomputed the Madelung energy in the cases of interest. A full ionic theory including electrostatic, van der Waals and repulsive interactions then explains the centre-of-mass structures of all the four ammonium halides. The thermal and pressure transitions are also explained reasonably well. The calculated phase diagram of NH₄F compares well with experiment. Barring the poorly understood NH₄F(II) phase, which is beyond the scope of this work, the other features are in qualitative agreement. In particular, the theory correctly predicts a pressure transition at room temperature from the ZnO structure directly to the CsCl structure without an intermediate NaCl phase. A feature of our approach is that we do not need to invoke hydrogen bonding in NH₄F.     

Vibrational spectra of the ammonia halides NH4I and NH4F at high pressures

The vibrational spectra of ammonium iodide NH₄I at pressures up to 4.1 GPa and ammonium fluoride NH₄F at pressures up to 4.7 GPa were investigated by inelastic incoherent neutron scattering. The pressure dependences of the transverse optical translational and librational modes were obtained. The behavior of the rotational potential barrier for the ammonium ion as a function of the lattice parameter for disordered and ordered cubic phases of ammonium halides with CsCl type structure were calculated. The results obtained confirm that the transition from an orientationally disordered cubic phase into an ordered cubic phase in ammonium halides occurs at close critical values of the positional parameter of hydrogen (deuterium).     

Raman study of NH4NO3 and ND4NO3—250–420K

Raman spectra of NH₄NO₃, and ND₄NO₃, were studied from 250 to 420K. The results show that there are four phases separated by first order transitions. No evidence of the previously reported phase II' was observed.The present results combined with the results of other experiments present the following picture of the state of order of the molecules.In phase I, the highest temperature phase, the NH₄⁺ groups are in a free rotation and the nitrate groups are likely in random reorientation among 12-equivalent positions. In phase II, the NH₄⁺ groups are likely in rapid random reorientation under the local force field of S₄ symmetry. The nitrate groups are in hindered rotation but are disordered with one of the O-N bonds directed in one sense or the other along the c-axis. In phase III, the absence of the librational mode indicates that the NH₄⁺ groups are in nearly free rotation but the rotational motion is restricted by the local force field of C₃ symmetry. The nitrate groups are probably ordered as suggested by the well polarized character of the modes associated with the nitrate groups. In phase IV, the nitrate groups are ordered with their molecular planes perpendicular to the b-axis. The NH₄⁺ groups are in orientational disorder but may undergo bindered rotations. An optical mode was observed to couple to an anomalous mode which is believed to be a zone edge acoustical mode.     

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH₄Cl at pressures of up to 2.6 GPa and of NH₄Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.     

Effects of pressure on the two polymorphs of [Co(NH3)5NO2]I2: The anisotropy of lattice distortion and a phase transition

Abstract

The effect of pressure on the two polymorphs of [CO(NH₃)₅NO₂]I₂ (phase I-orthorhombic, S.G. Pnma; phase II-monoclinic, S.G. C2/m) was studied by X-ray powder diffraction in a diamond anvil cell (DAC). In the presence of the ethanol-methanol-water mixture used as a pressure-transmitting liquid polymorph I was shown to undergo a phase transition at pressures between 0.45 GPa and 0.65 GPa. The diffraction pattern of the high-pressure phase (phase III) could be indexed as tetragonal with lattice parameters similar to those, which were previously reported for polymorph II in a 'pseudotetragonal setting'. The lattice distortions of phases II and III were studied at pressures up to 3.2 GPa and 3.7 GPa, correspondingly, and were shown to be very similar. Phases II and III were supposed to be very closely related. If poly(chlortrifluorethylen)-oil was used as a pressure-transmitting medium, no phase transitions were observed in phase I of [CO(NH₃)₅NO₂I₂ at least up to 1.8 GPa (the point when poly(chlortrifluorethylen)-oil becomes solid), and the anisotropy of lattice distortion could be measured.     

Structural study of ND4I at high pressures and low temperatures

Abstract

Structure, positional, and thermal parameters of ND₄I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND₄I(II) into a recently discovered high pressure phase ND₄I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.     

Raman scattering study of the low temperature phase of ammonium nitrate

Raman spectra of NH₄NO₃ and ND₄NO₃ have been recorded from room temperature down to 11K. A sluggish transition from phase IV to phase V was clearly observed but no evidence was found for another low temperature phase. Large splitting of the ν₃ mode indicates that the interionic interaction between NH⁺₄ and NO^-₃ is rather strong in both phases. A possible mechanism for the transition involving coupled modes is discussed.     

Ultraviolet reflectivity of NH4Cl and NH4I in ordered and disordered phases

Near-normal incidence reflectivity spectra below 10 eV of NH₄Cl in phases II and IV and of NH₄I in phases II and III are reported. An interpretation is given by comparing these spectra with those of alkali halides. A temperature shift of the first exciton peak of NH₄I due to ordering of the crystal was found as was the case with NH₄Br. The absence of this effect in NH₄Cl strongly implies the relation to the transition II–III and the tetragonal distortion of the lattice.     

Temperature dependence of the Raman bandwidths and intensities of a lattice mode near the tricritical and second order phase transitions in NH4Cl

We calculate here the temperature dependence of the damping constant using the expressions derived from the Matsushita's theory and the temperature dependence of the order parameter from the molecular field theory for the tricritical (1.5?kbar) and second order (2.8?kbar) phase transitions in NH₄Cl. Our calculations are performed for the ν ₅ (174?cm^?1) Raman mode of NH₄Cl for the pressures studied. Predictions for the damping constant are in good agreement with our observed Raman bandwidths of this lattice mode for both pressures. The Raman intensities calculated from the molecular field theory by means of the order parameter are also in good agreement with our observed Raman intensities of the ν ₅ (174?cm^?1) mode for both tricritical (1.5?kbar) and second order (2.8?kbar) phase transitions in NH₄Cl. In this study our observed Raman intensities of this lattice mode are analysed using a power-law formula with the critical exponent β for the order parameter at those two pressures considered in NH₄Cl. From our analysis, the value of β?=?0.5 is obtained as the mean field value.     

High pressure phase transformations in ammonium halides emerged in conductivity

Comparative studies of high pressure effect on the resistance of ammonium halides NH₄ X (X = F, Cl, Br, I) at 77–400 K are presented. Conditions and characteristic times of various high-pressure phases formation are determined as a function of the time of pressure treatment duration and temperature. Conductivity and magnetoresistance of NH₄I at pressures above 10 GPa are studied for the first time.     

Mode-Grüneisen parameters in disordered molecular crystals: Raman spectra of NH4Br and NH4I under hydrostatic pressure

The Raman spectrum of NH₄I has been measured under hydrostatic pressure. The high pressure phase of NH₄I was identified with the disordered CsCl-type phase by comparing the spectrum with that of NH₄ Br. The mode-Grünesen parameters of NH₄Br and NH₄I were determined from the frequency shift of phonon bands under hydrostatic pressure. These values were compared with the thermodynamic value and that obtained from the Slater relation. Contributions to the thermodynamic Grüneisen constant are discussed from free energy terms of NH⁺₄-orientation and -interactions with lattice modes.     

Splitting of libration mode frequencies in the vicinity of orientation phase transition in NH4Br

Abstract

Vibrational spectra of NH₄Br at high pressures up to 4·5GPa have been studied by means of incoherent inelastic neutron scattering using sapphire anvil high pressure cell technique. Libration mode splitting was investigated in the vicinity of the orientation phase transition (P_tr = 2·7 GPa) and this effect disappears if pressure is less or higher than P_tr. This effect is explained in terms of two-well asymmetric potential.     

Elastic properties study of single crystal NH3 up to 26 GPa

Single crystal Brillouin and Raman scattering measurements on NH₃ in a diamond anvil cell have been performed under pressures up to 26 GPa at room temperature. The pressure dependencies of acoustic velocity, adiabatic elastic constants, and bulk moduli of ammonia from liquid to solid III and solid IV phase have been determined. All the nine elastic constants in orthorhombic structure phase IV were presented for the first time, each elastic constant grows monotonously with pressure and a crossover of the off‐diagonal moduli C₁₂ and C₁₃ was observed at around 12 GPa because of their different pressure derivative values. We also performed ab initio simulations to calculate the bulk elastic moduli for orthorhombic ammonia, the calculated bulk moduli agree well with experimental results. In Raman spectra the very weak bending modes ν₂ and ν₄ for orthorhombic ammonia are both observed at room temperature, a transition point near 12 GPa is also found from the pressure evolution of the Raman bands. Copyright © 2011 John Wiley & Sons, Ltd.     

Modeling of high-pressure 12-μm NH3 lasers

Experimental measurements of small-signal gain in an optically-pumped NH₃ amplifier are carried out at pressures ranging from 40 Torr to 760 Torr, and the results are used to validate a rate-equation model describing the amplifier dynamics. The gain measurements show that dilute mixtures of <0.5% NH₃ in N₂ are reqired to minimize the problems of gas heating due to pump absorption. The model is used to extrapolate the results to gas pressures of several atmospheres, and to demonstrate the potential for highpressure operation of optically-pumped NH₃ lasers. For a pump intensity of 100 MW/cm², calculations indicate that operation of an NH₃–N₂ laser is feasible up to a pressure of 10 atm, which would provide a maximum continuous tuning range of 4 cm^–1. High-resolution spectroscopy reveals that gain on a few NH₃ transitions is eliminated at high pressures due to the presence of overlapping absorptions in other NH₃ bands.     

Ultrasonic frequencies correlated with the volume changes of the q[110] mode for the first-order phase transition in NH4CI

We correlate here by means of γ-Grüneisen relations the volume changes with the ultrasonic frequencies of the q[110] mode of NH₄Cl for the first-order phase transition in this crystal. The ultrasonic frequencies were calculated by a method, which we have developed, using the length-change data from the literature at pressures of 0, 0.2, 0.4 and 0.6 kbar in the first-order phase region of NH4Cl.

Our calculated frequencies are in very good agreement with the observed data from the literature for the q[11 0] mode at these pressures. This shows that by correlating the volume changes with the ultrasonic frequencies, the observed behaviour of NH₄Cl can be explained adequately near the first-order phase transition in this crystalline system.     

Raman spectroscopic study of the phase transitions in the Cs2NH4WO3F3 oxyfluoride

The Raman spectra of Cs₂NH₄WO₃F₃ elpasolite crystals are studied in the temperature range 93–373 K at pressures of up to 6.3 GPa. No indication of a phase transition is revealed from the Raman spectra as the temperature decreases to 93 K. An analysis of the Raman spectra measured under pressure demonstrates that the Cs₂NH₄WO₃F₃ elpasolite crystals undergo a phase transition at a pressure of 2.58 GPa. Judging from the behavior of the pressure dependences of the vibrational frequencies, the revealed phase transition is associated with the lowering of the symmetry of the WO₃F₃ octahedra.     

The influence of deuteration on the phase transitions in (NH4)3 Me 3+F6 cryolites (Me 3+=Sc and Ga)

The heat capacity of partially deuterated crystals with a cryolite structure, namely, (NH₄)₃ScF₆ and (NH₄)₃GaF₆, is measured in the temperature range from 80 to 370 K. The p-T phase diagrams of these compounds are investigated at pressures up to p=0.6 GPa. It is revealed that the deuteration does not affect the sequences of phase transitions observed in the proton-containing ammonium cryolites studied earlier. The isotope effect most clearly manifests itself in significant changes in the thermodynamic parameters of the $I12/m1 - P\bar 1$ low-temperature transformation in scandium cryolite.     

Vibrational study of H3OUO2PO4 · 3 H2O (HUP) and related compounds. phase transitions and conductivity mechanisms: part I,KUO2PO4 · 3 H2O (KUP)

Infrared and Raman spectra of polycrystalline KUO₂PO₄ · 3 H₂O (KUP) and its isotopic derivatives KUO₂P¹⁸O₄ · 3 H₂O and KUO₂PO₄ · 3 D₂O have been investigated in the 4000-10-cm^?1 range at different temperatures. An assignment of the bands in terms of UO₂, PO₄ and H₂O vibrations has been proposed. Combined differential scanning calorimetry and spectroscopic data show two diffuse phase transitions near 130 and 230 K. Comparison of the vibrational spectra of phase I at 300 K and phase IV at 100 K indicates that ordering of the water molecules with subsequent ordering of PO₄ tetrahedra on a site with lower symmetry appears to be the main mechanism responsible for the phase transformation. All the six O-H distances of water molecules in phase IV are found to be crystallographically nonequivalent. Conducting ion frequencies and the corresponding force constants have been determined for the analogous compounds MUP with M = K⁺, Na⁺, Ag⁺, NH⁺₄, Tl⁺ and H₃O⁺ and compared with other properties of these ionic conductors. Conductivity mechanisms in these materials are discussed.