Ammonium Chloride Crystals Research Articles

Irregular Fractal-Like Crystal Growth of Ammonium Chloride

Irregular dendritic crystals of ammonium chloride (NH 4 Cl) are grown two-dimensionally between two parallel glasses, one of which has rough surface. The pattern clearly reminds us of those of Witten-Sander diffusion-limited aggregation (DLA) model. The radius of gyration is measured to give the fractal dimension d f =1.67±0.002, which is in good agreement with two-dimensional DLA. Our result implies that any random perturbations strong enough to overcome crystalline anisotropy always give rise to random DLA-like crystals, instead of otherwise regular dendritic crystals.

Hyper Rayleigh and hyper Raman scattering from ammonium chloride and ammonium bromide

AbstractThe hyper Rayleigh and hyper Raman (0–1600 cm−1 wavenumber shift) spectra of oriented single crystals of ammonium chloride and ammonium bromide are reported. It is demonstrated that hyper Rayleigh scattering is a very sensitive measure of the presence of a centre of symmetry within a crystal structure and this property is used to investigate the residual ordering within an ammonium chloride crystal at 55 K above the order‐disorder phase transition. The hyper Raman wavenumber shifts associated with the transverse (TO) and longitudinal (LO) components of the optic mode, the librational mode which is infrared and Raman inactive, and the H–N–H internal mode were observed for both crystals. A basis for the interpretation of the relative hyper Raman intensities of the LO and TO modes is provided by an extension of the Poulet treatment of the relative intensities in the Raman spectrum.

Third Order Elastic Constants of Ammonium Chloride

Sound velocities along several directions in ammonium chloride crystals have been measured under hydrostatic and uniaxial pressures by means of an ultrasonic method. Second order elastic constants and their pressure derivatives were evaluated in the vicinity of the phase transition point in disordered phase, and third order elastic constants were calculated from the data. It was found that the absolute values of the third order elastic constants C 111 , C 112 and C 123 considerably increased with decreasing temperature near the phase transition point.

Thermal depolarization in ammonium chloride crystals, I

The thermal depolarization of crystals of NH 4Cl polarized above the phase transition temperature (242.5K) by an applied electric field was studied in 13 crystals prepared under a variety of conditions in order to examine the effects of doping with divalent anions or cations and of acid or alkaline conditions during crystal growth. All the features found in crystals grown from solution also occur in a crystal grown from the vapor. We may conclude, therefore, that the depolarization effects observed are properties of crystalline NH 4Cl and are not due to water or urea occluded during crystal growth. Four main depolarizations were observed: one of these is ascribed to space charge and the other three are attributed to dipole relaxation processes. The activation energies for these are 0.58, 0.73, and 0.85 eV, respectively. The various doping experiments lead to the conclusion that one of these is favored by acid conditions and the presence of cation vacancies and the other two by alkaline conditions and the presence of anion vacancies. Specific models for the dipolar defects are formulated and discussed.

Thermal depolarization in ammonium chloride crystals. II. Effect of hydrochloric acid and ammonia

Thermal depolarization in ammonium chloride crystals. II. Effect of hydrochloric acid and ammonia

Electric Field- and Self-Induced Polarization in Ammonium Chloride Crystals

Crystals of NH4Cl grown from solution or from the vapour display large displacement currents on cooling or warming through the phase transition. The internal field associated with the phase transition is sufficiently large to induce the ordering of dipoles and the displacement of charge carriers. Dipole relaxation processes and space charge effects in the high-temperature (disordered) phase of NH4Cl are studied following polarization in an external electric field above the transition temperature. By variations in the polarization time and by partial depolarization in successive steps, three dipole relaxation processes can be identified, along with space charge release. One of the dipole processes is enhanced by treatment of the crystal with HCl gas. Aus der Losung oder Dampfphase gewachsene NH4Cl-Kristalle zeigen bei Abkuhlung oder Erwarmung uber den Phasenubergang grose Verschiebungsstrome. Das mit dem Phasenubergang verbundene innere Feld ist genugend gros, um einen Ordnungsprozes der Dipole und eine Verschiebung der Ladungstrager zu induzieren. Dipol-Relaxationsprozesse und Raumladungseffekte in der (fehlgeordneten) Hochtemperatur-Phase von NH4Cl werden nach einer Polarisation in einem auseren elektrischen Feld oberhalb der Ubergangstemperatur untersucht. Durch Anderung der Polarisationszeit und durch partielle Depolarisation in sukzessiven Schritten lassen sich drei Dipol-Relaxationsprozesse zusammen mit einer Raumladungsfreisetzung identifizieren. Einer der Dipolprozesse wird durch die Behandlung des Kristalls mit HCl-Gas verstarkt.

Raman Scattering from Nonfundamental States in Ammonium Chloride Crystals at Low Temperatures

AbstractThe Raman spectra of the δ‐phase of ammonium chloride crystals at low temperatures are carefully investigated. Particular attention is focused on the Raman spectra in the region of the internal vibrations of the ammonium group. At liquid helium temperatures intense sharp lines of definite symmetry type are observed. These lines are identified as two particle bound states (biphonons) in agreement with theoretical predictions.

The thermodynamics of mixed crystals of ammonium chloride and ammonium bromide. II. An analysis of the heat capacity of ammonium chloride, ammonium bromide, and an approximately equimolar solid solution of these salts

The heat capacity C p of ammonium chloride, ammonium bromide and the mixed crystal NH 4 Br 0.55 Cl 0.45 has been analysed, for temperatures in the range 0-500 K, on the assumption that can be expressed as the sum of the terms ( C p — C v ), C (lat.), C (lib.), C (int.) and C (extra); C (lat.), C (lib.) and C (int.) are respectively, the contributions from the lattice vibrations (phonons), and the librations (torsional oscillations) and internal vibrations of the ammonium ions. C (extra) is the contribution from order-disorder changes or any other ‘abnormal’ sources. The analysis requires a knowledge of certain thermodynamic and spectroscopic properties of the crystals, and current information on these has been surveyed. The ‘normal ’ or ‘baseline’ heat capacity C´ p for the three solids has been estimated; C´ p is the molar heat capacity to be expected from the progressive excitation of the lattice vibrations, torsional oscillations and internal vibration of the cations, and from the expansion of the lattice. By comparing C´ p and the observed heat capacity C p , C (extra) has been evaluated, making it possible to examine the development with rising temperature of the configurational or otherwise ‘extra’ entropy. The main results emerging from this are the following, ( a ) For all three solids, the entropy gain at the λ-transition (or at the two lower transitions, taken together, in ammonium bromide) is not about R ln 2, as has often been stated, but is roughly 50 % larger. The excess over R ln 2 is to be attributed to changes in the phonon spectrum and to a reduction in the frequency of the torsional oscillations of the ammonium ions, ( b ) The heat capacity of the β-phase of all three solids is abnormally large. Possible reasons for this are briefly discussed, but a definitive explanation is lacking. ( c ) The overall entropy gain for all three solids when the high-temperature face-centred a-phase has been formed is considerably greater than R In 6, the configurational contribution to be expected from the structural evidence that each ammonium ion in this phase has access to six distinguishable orientations. The balance is believed to be due to enhanced freedom of torsional movement of the ammonium ions, which must be a complex motion. However, from its observable thermodynamic consequences, this motion is still far from true free rotation.

The thermodynamics of mixed crystals of ammonium chloride and ammonium bromide. I. The heat capacity from 8 K to 476 K of an approximately equimolar mixture

The heat capacity of a mixed crystal of ammonium chloride and ammonium bromide, of formula NH4Br,0.55Cl0.45, has been measured for temperatures in the range 8-300 K in one calorimeter, and 270K-476K in a second calorimeter. The mixed crystal undergoes a >.-type transition atT= 271.2K, and an essentially isothermal transition, to a facecentred cubic crystal, at 382.8 K. At this second transition, considerable superheating of the low-temperature form and supercooling of the hightemperature form can occur. Thermodynamic parameters for the mixed crystal are given at rounded temperatures.

Nucleation and growth rates of ammonium chloride in organic media: Development of an on‐line measurement technique

AbstractNucleation and growth rates of small ammonium chloride crystals precipitated from organic mother liquors were measured with an electronic particle counter using an on‐line measurement and data reduction technique. Current values for nucleation and growth rates could be obtained within a period of 3 min, thus permitting rapid scanning of variables affecting crystal size distribution. Population average sizes of 1 to 3μ were typically measured in these organic liquors. The average size of particles increased with increase in ammonium chloride solubility. Addition of surfactants (sodium dodecylbenzene sulfonate, sodium oleate) either had no effect on nucleation or inhibited growth and nucleation equally, thus did not change particle size. The average size of particles was slightly increased with addition of magnesium chloride.

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.

Growth of Ammonium Chloride Dendrites from Vapor Phase

Large symmetrical and transparent dendritic crystals of ammonium chloride have been grown below transformation temperature by vapor phase technique. A regular change in the morphology of crystals starting from needle type crystals to tree like branching dendrites have been observed. Observations indicate that non-faceted needle type crystals grow by interface diffusion and volume diffusion field is responsible for the dendritic crystals, as proposed by Mullin & Sekerka and Cahn theoretically. In addition to diffusion fields and supersaturation, the growth temperature has been found to be an important factor in determining the basic morphology of crystals. The observed facets on the tips of dendritic arms and on the main arm of crystals may be explained by considering the decrease in supersaturation in later stage of growth. Characteristic features of dendritic crystals have also been discussed on the basis of existing theories.

On the growth of single crystals of ammonium chloride from vapor-phase techniques

Kinetics of growth of ammonium chloride single crystals have been studied employing vapor-phase techniques. The crystals grown by the Piper-Polich technique were found polycrystalline in nature. Single crystals of ammonium chloride 1–2 mm wide and 5–7 mm long have been grown below its transformation temperature employing sublimation techniques. A possible growth mechanism has been suggested.

Optical absorption spectrum of Cu 2+ doped in NH 4Cl single crystal

Single crystals of copper-doped ammonium chloride were grown at room temperature. Spectra in the near infrared at room temperature and in the visible at room and liquid-air temperatures were recorded on a Unicam SP-700 spectrophotometer and Hilger medium quartz spectrograph, respectively. The observed absorption bands have been attributed to an ion of Cu 2+ in tetragonal symmetry with 2B 1 as the ground state. Taking into consideration spin-orbit coupling associated with a tetragonal field, a successful interpretation of all the observed bands has been made. The crystal parameters derived are Dq = −1360 cm −1; Ds = −2000 cm −1; Dt = −140 cm −1 and λ = −830 cm −1.

Growth of ammonium chloride single crystals from the vapor phase

Abstract Large single crystals of ammonium chloride have been grown in vacuum, from the vapor phase in self-sealing glass ampoules. The conditions for reproducible growth of these crystals have been established. A possible growth mechanism has also been suggested.

ESR of copper complexes in NH 4Cl, ND 4Cl and CsCl

A detailed investigation of the ESR spectra of two copper centres in ammonium chloride crystals and one copper centrum in cesium chloride crystals is presented. Hyperfine interaction with nuclei of the ligands is observed for copper centres in ND 4Cl and proves helpful for the clarification of the structure of these centres. An analysis of the parameters in the spin-Hamiltonian is presented. Details are also given of the preparation and doping of single crystals of NH 4Cl, ND 4Cl and CsCl.

Raman spectrum of ammonium chloride at high pressures

Raman spectra of single crystals of ammonium chloride have been studued to 40 kbar at room temperature. Effects of pressure on the frequencies of the Reststrahlen, librational, and internal modes are described, and the origins of these shifts are discussed.

Paramagnetic resonance of a hydrazine-like defect in irradiated ammonium chloride crystals

A radiation induced paramagnetic defect center in ammonium chloride has been characterized by hyperfine interaction with two nitrogen nuclei and four hydrogen nuclei. The nuclei were identified by observing the changes of the EPR spectra caused by isotopic substitutions. Previously this EPR spectrum had been thought to be due to an electron trapped at a negative ion vacancy ( F-center).

Group-Theoretical Analysis of Lattice Vibrations in Molecular Crystals

A practical method of analysing the external vibrations of molecular crystals according to the representation theory of space group is presented. The procedure, which is a direct extension of the method given for the ordinary crystal by one of the authors, is applicable to any complicated molecular crystal. The result should be useful to both neutron and Raman spectroscopy. Detail on hexamine, adamantane, and ammonium chloride crystals has been worked out, and the result for hexamine is compared with actual calculation by Cochran and Pawley.

Microscopic determination of near solid state changes in aged propellants.

Study of solid-propellant thin sections by petrographic microscopy has led to a thorough optical characterization of the phase composition. In aged propellants, phases with different optical properties demonstrate the presence of reaction compounds with lower energy and altered mechanical properties. In six motors containing composite polyurethane propellant aged between 3.5 and 5.6 years, the reaction compounds have localized into a series of structures termed phenoblasts. These phenoblasts exhibit a structural genesis in size and complexity accompanied by characteristic color changes. The older phenoblasts are composed of a large central aluminum particle and peripheral ammonium chloride crystals, all enclosed in a high modulus binder. The ammonium chloride is shown to form in the solid state from ammonium perchlorate. A concentration gradient of phenoblasts expands with age and is related to grain design. Near the bore surface of the oldest motors, the phenoblast concentration decreases inward in opposition to an increasing binder swelling gradient. Two independent reaction mechanisms are thereby indicated which apparently interacted to yield a subsurface binder degradation zone. Comparison of phenoblasts in motors of different ages gives an approximate rate and model of chemical aging for these propellants.