NH4Br Crystals Research Articles

Effect of an electric field on nucleation and growth of crystals

The effect of the electric field strength on nucleation and growth of the crystals of ammonium halides and alkali metal sulfates has been studied. The optimal electric field strength for NH4Cl and NH4Br crystals was found to be 15 kV/cm, and for NH4I, it equaled 10 kV/cm. No effect of the electric field strength on the crystal growth was found for alkali metal sulfates. This difference is analyzed in terms of the crystal growth thermodynamics. In case, when the electric field is small and the Gibbs energy is of a significant value, the influence of the electric field at the crystal growth is negligible. A method to estimate the critical radius of homogeneous nucleation of the crystal is suggested.

IDENTIFICATION OF THE TRANSITION RULE IN A MODIFIED CELLULAR AUTOMATA MODEL: THE CASE OF DENDRITIC NH4Br CRYSTAL GROWTH

A method of identifying the transition rule, encapsulated in a modified cellular automata (CA) model, is demonstrated using experimentally observed data of the evolution of dendritic crystal growth patterns in NH4Br crystals. The influence of the factors, such as experimental set-up and image preprocessing, color and size calibrations, on the method of identification are discussed in detail. A noise reduction parameter and the diffusion velocity of the crystal boundary are also considered. The results show that the proposed method can in principle provide a good representation of the dendritic growth anisotropy.

Characterization of NH 4Br:Cu 2+ crystals: Direct evidence of NH 3 through Raman spectroscopy

At variance with the results found for NH 4Cl : Cu 2+ it is shown here that the same Cu 2+ centre is formed in NH 4Br crystals grown either in basic solutions or in acid solutions with a pH down to 2. In order to definitely clarify the nature of this Cu 2+ species polarized Raman spectra have been recorded from which the Cu 2+ centre can be ascribed to a D 4 h CuBr 4(NH 3) 2− 2 species. Aside from detecting the two A 1 g vibration frequencies of the complex a remarkable feature strongly supporting this model is the clear observation of the internal “umbrella” mode of the NH 3 molecule detected at 1283 cm −1 and 1293 cm −1 for CuBr 4(NH 3) 2− 2 in NH 4Br and for CuCl 4(NH 3) 2− 2 in NH 4Cl respectively. The detection of three Raman peaks of CuBr 4(NH 3) 2− 2 in NH 4Br crystals containing only a Cu 2+ concentration of ∼ 3000 ppm is seen to be favoured by the positron of the first charge transfer transition lying only at 26 000 cm −1. This situation gives rise to preresonance phenomena clearly observed in the present work.

ChemInform Abstract: EPR Study of VO2+ Ions Doped in Single Crystals of NH4I and NH4Br.

AbstractAus den EPR‐Spektren im Temp.‐Bereich 430‐77 K wird auf Flüssigkeit‐artiges Verhalten der Titelkristalle gegenüber VO2+ geschlossen.

EPR study of VO2+ ions doped in single crystals of NH4I and NH4Br

The results of the EPR study of VO2+ doped in NH4I and NH4Br single crystals in the temperature range 430–77 K are reported. The observed spectra suggest a liquid-like behavior of these crystals for VO2+. A single eight line isotropic spectrum was observed in the temperature ranges ∼373 to ∼245 K in NH4I and ∼430 to ∼233 K in NH4Br. No spectrum was observed in the temperature ranges ∼245 to ∼173 K and ∼ 233 to ∼223 K in NH4I and NH4Br crystals, respectively. A spectrum with two sets of eight isotropic lines was observed from ∼173 K for NH4I and ∼223 K for NH4Br to 77 K. The observed mI dependence and temperature variation of linewidths are discussed. The anomalies in temperature dependence of linewidth confirmed the structural phase transitions in the host lattices.

Effect of structural phase transition on the exciton luminescence in NH 4Br crystals

A marked hysteresis associated with the III–IV phase transition of NH 4Br crystals has been found by monitoring the luminescence arising from self-trapped excitons. Based on these results, the influence of a structural phase transition on exciton relaxation is discussed.

Anomalous temperature dependence of third-order elastic constants in NH 4Cl and NH 4Br near the λ point

Second order elastic constants of NH 4Cl and NH 4Br crystals were measured under hydrostatic and uniaxial pressures near the λ-type phase transitions. Third order elastic constants were calculated from the pressure derivatives of the second order elastic constants. Results show that the temperature dependences of the third order elastic constants of NH 4Cl differ from those of NH 4Br dramatically. In addition, it is found that the coefficients 1 54 (C 111 + 6C 112 + 2C 123) and ( 1 24 √3)(C 111 − 3C 112 + 2C 123) of strains in the free energy of NH 4Cl show quite different temperature dependences from those of NH 4Br.

Structural correspondences and mechanisms for the polymorphic transformations NaCl⇄CsCl type and NaCl⇄Rhombohedral

Experimental evidence is presented for a martensitic mechanism of transformation [NaCl]→[CsCl] type in supercooled NH 4Br crystals. Two published structural relationships are shown to be equivalent to the original Shoji-Buerger correspondence. The effect of conditions on the mechanism is discussed. Interface immobilization in transformations is attributed to relaxation of the interface structure.

The proton magnetic resonance lineshapes of tunnelling NH 4+ ions in an NH 4Br single crystal at low temperatures

The proton magnetic resonance spectrum was observed for a single crystal of NH 4Br at liquid helium temperature. The spectra recorded for various orientations of the single crystal could not be fitted to the lineshape calculated for rigid NH 4 + ions in crystals. The calculated spectra, including the effect due to the tunnel splitting of the librational ground state of the NH 4 + ion, agree fairly well with experimental observations and yield a tunnel splitting energy of 2.2 G or 9.37 kHz for the NH 4 + ion in NH 4Br crystals. The simulation method employed for the lineshape fitting can be applied to cases where the estimation of the tunnel splitting of an NH 4 + ion is as small as 1 G or 4.26 kHz.

EPR studies of MoO 3+ in NH 4Br single crystals

Electron paramagnetic resonance studies of molybdenyl doped NH 4BR crystals were made at room temperature. The resonance due to even isotopes of Mo and the hyperfine interaction due to 95,97Mo is observed. The angular variation of the spectrum is studied in the crystallographic (001)plane. The symmetry axes of the magnetic complexes are found to be along <100>, <010> and <001> directions of the crystal. The spectra are analysed using a spin hamiltonian corresponding to tetragonal symmetry. The spin hamiltonial parameters indicate the formation of a stable[MoOCl 5] 2− complex in the lattice. The ligand spin orbit effects on g and A values is discussed. The spin hamiltonian parameters are correlated with the ligand properties like electronegativity, covalent nature, etc.

Isotope effect on the migration of protons in NH 4Cl and NH 4Br crystals

Excess electrical conductivity is induced by plastic deformation in NH 4Cl, NH 4Br, ND 4Cl and ND 4Br crystals. The annealing of the excess conductivity is fitted to a monomolecular decay characteristic of protons and deuterons. The relaxation parameters are examined in the light of existing theories of isotope effect in mass transport. Values of 0.96 and 0.93 are deduced for g, the fraction of kinetic energy possessed by the jumping ion at its saddle point.

Electrical conductivity of NH 4Cl and NH 4Br crystals in phases I and II

The electrical conductivity of single crystals of ammonium chloride, ammonium bromide and of the complete range of their solid solutions is measured from the simple cubic (sc, CsCl type) to the face centred cubic (fcc, NaCl type) phases through the structural transition. The effects of added divalent cationic impurities in NH 4Cl and of purification by recrystallization are studied. The sc → fcc transition of NH 4Br crystals is marked by the onset of a pronounced conductivity peak in the conventional log (σ T) vs. 1/ T plot. NH 4Cl crystals show a marked drop in conductivity at the transition point. The λ-type conductivity peak persists in the solid solutions with large bromide content. The activation energy and preexponential factors of conductivity are markedly less in the higher-temperature phase than in phase II (sc) for undoped NH 4Cl and undoped NH 4Br. The conductivity as well as the activation energy for NH 4Cl are decreased in phase II but increased in phase I as a result of the doping. In the case of NH 4Br the conductivity and activation energy are gradually increased in successive stages of purification. These results in phase II of the two salts are interpreted in terms of an anion vacancy mechanism of conductivity in the pure crystals and extrinsic cation vacany transport in the impure materials. The excess conductivity at the transition is ascribed to proton release from freely rotating ammonium ions. The observed variation of the transition temperature with composition of NH 4Cl 1− x Br x fits well into an ideal two-component solid-solution model.

Über die doppelbrechende Phase der Mischkristalle von NH4Br mit TIBr. NH4Br mit CsBr und NH4Cl mit CsCl

Abstract The range of existence of the birefringent tetragonal phase of mixed crystals of NH4Br or NH4Cl and other isomorph halides with the same anion is being examined. Especially, it will be shown that by adding CsCl even NH4Cl can be caused to change into that phase, i. e. to behave qualitatively like NH4Br.

Vapor-phase growth and electrical conductivity of pure and impurity-doped ammonium halide crystals

Ideal growth parameters for obtaining large transparent crystals of NH4Cl and NH4Br by vapor-phase technique are described. The electrical conductivity of such cystals and crystals grown from solution are reported. In pure NH4Cl crystals two regions with slopes 1.26 and 0.8 eV are obtained in the logσT vs 1/T plot. In impurity-doped NH4Cl crystals a third region with slope 1.04 eV is also observed. In pure NH4Br crystals two regions with slopes 1.06 and 0.66 eV are obtained.

On the dendritic growth of NH 4Br-crystals from aqueous solutions

The dendritic growth of NH 4Br-crystals from aqueous solutions was studied, and the growth velocity as well as the tip radius measured as function of supersaturation and temperature. At low supersaturation ∏ the growth velocity increases as ∏ 2, at higher supersaturation as ∏, while the tip radius decreases as 1/∏. The temperature dependence of the growth velocity in the parabolic region is weak, indicating diffusion as rate determining step, while the stronger temperature dependence in the linear region is an indication for a rate determining reaction at the interface.