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).     

Farbzentren in Ammoniumhalogeniden

Color centers produced by X-rays in ammonium halides at various temperatures between 20°K and room temperature have been investigated by means of paramagnetic resonance and by optical methods. Two kinds of paramagnetic defects were found to be predominant, the self-trapped hole (V _K-center) and another electron deficiency center involving a NH₃ ⁺-radical. The electronic structure of theV _K-center is the same as in the alkali halides, except that the orientation of the molecular axis is along [100] instead of [110]. The kinetics of the thermally activated motion of theV _K-centers and of their recombination with electrons has been studied. The electronic structure of the second center was derived from the hyperfine spectrum of the paramagnetic resonance. The rotation of the NH₃ ⁺ ion and its connection with the order-disorder transition in NH₄Cl has been studied.     

Ammonium dynamics in the disorder a-phase of K1- x (NH4) x Y (Y=Cl,Br, I). a neutron scattering study

Temperature and concentration effects on the lattice parameters and the amplitude weighted phonon density of states (AWPDS) in the mixed salts of ammonium-potassium halides were investigated by neutron powder diffraction (NPD) and incoherent inelastic scattering (IINS). In the disorder α-phase (NaCl type), ammonium ions perform a fast stochastic reorientation at phonon frequencies rate, down to ca. 80 K. The IINS spectra at 10 K displayed the four distinct ammonium excitations, two (resonant) modes below and two (localised) above the cut-off frequency of the AWPDS of potassium halides. The high frequency localised modes correspond to the translational and librational vibrations of NH₄ ions. These modes are typical for ordered phases of ammonium halides. Ammonium concentration effects on the localised and resonant modes were studied for the K_{1? x} (NH₄) _x I mixed salts and the harmonic excitations of ammonium in the hypothetical low temperature α-phase of NH₄I were approximated to ca. 30, 95, 155 and 250 cm^?1. In the real low temperature ordered γ-phase of NH₄I, translational ammonium vibrations were observed at ca. 140-160 cm^?1 and librational at ca. 300 cm^?1.     

An EPR study of a triplet state center in (NH4)2SO4

Heating ammonium sulphate to near its dissociation temperature and then quenching to room temperature produces a stable triplet state center that is identified as NH₂⁺. The symmetry of the EPR spectra indicates that the center is located in two non equivalent sites with each of these sites having slightly different EPR parameters and two measurably differents orientations. Thus the center is apparently occupying the positions of the NH₄⁺ groups in the perfect lattice. A study is made of this center both at room temperature and also well below the ferroelectric phase transition of (NH₄)₂SO₄. It is found that above and below the transition temperature (?50°C) there is very little temperature dependence of the EPR spectra but at the phase transition there is an abrupt change in the spectra. The number of detectable centers doubles below the phase transition indicating that the inversion symmetry of the NH₂⁺ sites is eliminated at the transition temperature.     

Temperature dependence of third order elastic constants of ammonium halides

The third order elastic constants of ammonium halides have been evaluated for the first time using Lundqvist three body potential incorporating the effect of thermal contributions. Theoretical investigations have been carried out above λ-temperature where both NH₄Cl and NH₄Br have cubic CsCl structure. Analysis has been extended to mixed NH₄Cl_1−x Br _x . The repulsive potential which is assumed significant up to first neighbours is taken of the Born-Mayer type. Results have been compared with the recently measured values wherever available.     

Energy - dispersive X - ray diffraction of the ammonium - halides under pressure

Abstract

The structure of the ammonium halides NH₄X (X = Cl, Br, I) has been studied under pressure up to 40 GPa by energy dispersive X-ray diffraction using synchrotron radiation. Equations of state and a discussion on the possible structure of phase V will be presented.     

N.M.R. study of internal motion in hexahydrated acid fluorides of cobalt and nickel

A linear relationship between the viscosity B-coefficient of the Jones-Dole equation for aqueous solutions of certain alkali metal salts and the enthalpy of hydration of the gaseous monatomic constituent ions has been established. The assumption that a similar rectilinear law applies to ammonium halides appears justified and the enthalpies of solution of NH₄ ⁺(g)+X^-(g) have been estimated and used to obtain magnitudes for the lattice energies of NH₄X(c) [X=F, Cl, Br, I]. In conjunction with experimental thermochemical data, the latter yield consistent results for the proton affinity of ammonia ΔH ₁ ^?=860·5±2·0 kJ mol^-1 (298·15 K). The lattice energies of the salts are, (in kJ mol^-1) 834 (NH₄F), 708 (NH₄Cl), 682 (NH₄Br) and 637(NH₄I).     

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.     

Temperature dependence of EPR spectrum of Mn2+ doped in ammonium iodide single crystals

EPR of Mn²⁺ doped in ammonium iodide single crystal has been studied at X-band in the temperature range 573–577 K. The observed temperature dependence of line widths and spin Hamiltonian parameter b₂⁰ below room temperature is related to the structural transformations in the crystal. The coexistence of high temperature phase (NaCl) and low temperature phase (CsCl) is attributed to the large thermal hysteresis in line widths and b₂⁰. The dissociation of ion vacancy pairs occurs near 500 K and is reflected in the reversible change of an anisotropic EPR spectrum in an isotropic sextet near this temperature. The ion vacancy pair models for NaCl and CsCl phases are discussed along with the effects of thermal processing of the samples. Heating the crystals above 500 K leads to expulsion of Mn²⁺ impurity from the crystal.     

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.     

Formation of simple 11C-labelled molecules by the nuclear reaction 14N(p,∝) 11C in nitrogen containing solids

Simple ¹¹C-labelled molecules which are formed by hot processes following the ¹⁴N(p, a) ¹¹C nuclear reactions with 13 MeV protons in cyclotron targets are of interest as precursors for the synthesis of ¹¹C-radiopharma-ceuticals. Solid targets show the advantage of greater stability against radiolysis and heat and a greater variability of chemical reactions. Frozen ammonia NH₃, ammonium halides NH₄X (X = F, C1, Br, I), and complex salts such as Co(NH₃)eCl₃ have been studied as matrices. The interaction of recoil carbon atoms with the lattice constituents leads to a very differentiated spectrum of ¹¹C containing compounds. Analysis of the system is performed by gas chromatography or high performance liquid chromatography. In contrast to the situation in alkali halides, the carbon atoms react to organic compounds such as ¹¹C(NH)2-cyanamide, ¹¹CH₃NH₃+-methylammonium, ₁₁CH(NH₂)₂ ⁺-formamidinium and ¹¹C(NH₂)₃ ⁺-guanidinium with radiochemical yields of 40%, 80%, 30% and 65%, respectively. Even at 5 K, carbon stabilizes via H-abstraction to CNn-radicals and insertion into N-H bonds. The amount of ammonium ions interacting with the carbon depends strongly on the lattice arrangement. This could be shown by experiments in different lattice structures, using low temperature cryostats. The above mentioned products can be transferred to ¹¹C labelled compounds such as pyrimidine-derivatives and barbiturates suited for nuclear medical applications. Moreover, the behaviour of carbon impurities in ionic crystals may serve as a model for the formation of organic compounds in inorganic matter.     

Experimental assembling of gas cells using beta/beta″-alumina type NH+4-gallates electrolytes

Polycrystalline disks of NH⁺₄-gallate with a β″-alumina structure (NH⁺₄-β″-gallate) containing a small amount of β-phase were prepared from Rb⁺-β″-gallate disks fabricated by hot pressing. The small difference in the lattice parameter between NH⁺₄-β″-gallate and Rb⁺-β″-gallate was responsible for the non cracking ion exchange of disks in molten ammonium nitrate. Electromotive forces of a hydrogen concentration cell using the NH⁺₄-β/β″-gallate electrolyte agreed rather well with a simple thermodynamical potential equation.     

Neutron diffraction study of (NH4I)0.73(KI)0.27

Powder and single crystal neutron diffraction experiments were performed on (NH₄I)_0.73(KI)_0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH₄ sites of the four NH₄ molecules per unit cell. The NH₄ group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH₄ molecules are 0.18 Å off center and exhibit C_3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.     

Neutron diffraction study of (NH4I)0.73(KI)0.27

Powder and single crystal neutron diffraction experiments were performed on (NH₄I)_0.73(KI)_0.27 and provide experimental evidence for the occurence of a new phase below 63K. This ε phase has trigonal symmetry (space group R3m) and was, so far, not observed in the ammonium halides. It can be characterized by an almost undistorted NaCl lattice, with two in-equivalent NH₄ sites of the four NH₄ molecules per unit cell. The NH₄ group at the trigonal axis (the former cubic <111> axis) is almos perfectly tetrahedral. The other three NH₄ molecules are 0.18 Å off center and exhibit C_3v symmetry with the polar axis along the remaining three body diagonals. Hence, this ε phase reveals a complex dipolar order with a residual moment along the trigonal axis.     

Fascinating interaction of the ammonium cation with [2.2.2]paracyclophane: experimental and theoretical study

By means of electrospray ionisation mass spectrometry, it was evidenced experimentally that the ammonium cation (NH₄⁺) reacts with the electroneutral [2.2.2]paracyclophane ligand (C₂₄H₂₄) to form the cationic complex [NH₄(C₂₄H₂₄)]⁺. Moreover, applying quantum chemical calculations, the most probable conformation of the proven [NH₄(C₂₄H₂₄)]⁺ complex was solved. In the complex [NH₄(C₂₄H₂₄)]⁺ having a symmetry very close to C₃, the ‘central’ cation NH₄⁺ is coordinated by three strong bifurcated intramolecular hydrogen bonds to the corresponding six carbon atoms from the three benzene rings of [2.2.2]paracyclophane via cation–π interaction. Finally, the interaction energy, E(int), of the considered complex [NH₄(C₂₄H₂₄)]⁺ was evaluated as ?625.8 kJ/mol, confirming the formation of this fascinating complex species as well. It means that the [2.2.2]paracyclophane ligand can be considered as an effective receptor for the ammonium cation in the gas phase.     

An ab initio LCAO-MO-SCF study of reaction paths for proton transfer in ammonium aqueous solution

The possible mechanisms for proton transfer in ammonium aqueous solutions are discussed through ab initio LCAO-MO-SCF calculations for the following hydrogen-bonded complexes : [NH₄ ⁺ … NH₃] ; [NH₄ ⁺ … OH₂] ; [NH₄ ⁺ … OH₂ … OH₂] ; [NH₄ ⁺ … OH₂ … NH₃] and [H₂O … NH₄ ⁺ … OH₂ … OH₂]. The energy curve along the reaction coordinate is drawn for the first three systems. A double well potential curve is obtained for the two symmetrical systems with a very low barrier to proton transfer : 2·9 kcal/mole for the system [NH₄ ⁺ … NH₃] and 4·3 kcal/mole for the system [NH₄ ⁺ … H₂O … NH₃]. For both systems the exchange mechanism involves three successive steps : association, transfer and dissociation. Solvation may affect the energetics of the first and third steps. For the unsymmetrical system NH₄ ⁺ + H₂O, the energy would increase continuously during the steps of proton transfer and dissociation. Hence the process of proton transfer between an ammonium ion and a water molecule may take place in solution only if assisted either by solvation or by a concerted push-pull mechanism involving a third molecule [NH₄ ⁺ … OH₂ … NH₃]. Theoretical results for the systems [NH₄ ⁺ … OH₂ … OH₂] and [NH₃ … H₃O⁺ … H₂O] show, indeed, that solvation should make the proton transfer easier. In any case the proton transfer is found to occur through a contraction of the associated species formed in the first step.     

NH<Subscript>3</Subscript>-assisted growth approach for ZnO films by atmospheric pressure metal-organic chemical vapor deposition

In this paper, we investigated the two effects of a little H₂ and NH₃ gas on the properties of ZnO films grown by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) using deionized water (H₂O) and diethylzinc (DEZn) as the O and Zn sources, respectively. Experimental results showed that compared to the effect of a little H₂, it could more effectively improve the surface morphology, crystalline structure, and optical quality of ZnO epilayers by adding a little NH₃ gas into the growth ambient. Furthermore, by adding a little NH₃ gas into the growth ambient, the hydrogen-related D⁰X₁ (I₄):3.365 eV peak disappeared in 10 K photoluminescence spectrum of ZnO films, which indicated the elimination of the unintended hydrogen impurity. The result of the Huang–Rhys factor S (0.113) showed that it was effective for reducing the probability of exciton–phonon scattering of ZnO films by adding a little NH₃ gas into the growth ambient. The electron mobility of ZnO films were also significantly improved by this method with the mobility of 100 cm²V^-1s^-1 measured by Hall measurement at room temperature. PACS 71.55.Gs; 81.15.Gh; 78.55.-m; 78.55.Et; 68.55.-a     

Phase transitions in PbTe under quasi-hydrostatic pressure up to 50 GPa

PbTe has been investigated using synchrotron X-ray diffraction (XRD) in a diamond anvil cell under quasi-hydrostatic pressures up to 50 GPa. Upon compression to 6.6 GPa, the initial NaCl phase transforms to an intermediate phase, which is confirmed to be an orthorhombic structure with a space group Pnma. At 18.4 GPa, the intermediate Pnma phase undergoes a phase transition to the CsCl structure. The systemic analysis of the crystal structures between the NaCl and intermediate phases indicates that the structure of the Pnma phase could be derived from the distortion of the NaCl structure. The bulk modulus of the CsCl phase is B₀=52(2) GPa with V₀=60.8(4) ų and B′₀=4.0 (fixed), slightly larger than the NaCl phase (B₀=44(1) GPa) and the intermediate phase (B₀=49(3) GPa).     

Semi-quantitative theory of the structures of simple ionic crystals

A simple theory is developed which shows that the regions of stability of the CsCl, NaCl and ZnS structures can be demarcated in a two-dimensional plot of the radius ratio versus the strength of the van der Waals interaction. There is good agreement with experiment. The effect of pressure on these structures is explained qualitatively. The increased occurrence of the ZnS structure and the decreased stability of the CsCl structure in the A²⁺ B²⁻ crystals compared to the A⁺B⁻ crystals is also explained. Finally it is shown that the radius ratio and the polarizabilities of the ions are the important factors that determine the structures of AB₂ crystals.