NH4Cl Single Crystals Research Articles

Optical properties of the Tl+ center doped in NH4Cl crystals with the CsCl structure

AbstractWe have investigated the optical properties of the Tl+ center doped in NH4Cl single crystals with the CsCl structure. The A, B and C absorption bands due to the cationic transition in the Tl+ ion are observed at 5.22, 6.22 and 6.59 eV at low temperature, respectively. When the A absorption band is photoexcited, the AX luminescence band appears at 4.35 eV, and the AT luminescence band at 3.37 eV. As the temperature increases, the AT band that is dominant at 13 K decreases around 20 and 60 K. On the other hand, the AT band exhibits an increase that compensates the decrease of the AX luminescence intensity. The blue‐shift of the peak energy and the narrowing of the halfwidth with increasing temperature are observed in the temperature ranges from 18 to 25 K for the AX band, and from 25 to 45 K for the AT band. Such complex temperature behaviors of the AX and AT bands are explained on the basis of the thermally activated non‐radiative transitions among four relaxed excited states introduced semiempirically. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Variable-frequency EPR study of Mn 2+-doped NH 4Cl 0.9I 0.1 single crystal at 9.6, 36, and 249.9 GHz: structural phase transition

Multifrequency electron paramagnetic resonance studies on the Mn 2+ impurity ion in a mixed single crystal NH 4Cl 0.9I 0.1 were carried out at 9.62 (X-band) in the range 120–295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn 2+ ions with their respective magnetic Z-axes oriented along the crystallographic [1 0 0], [0 1 0], [0 0 1] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn 2+ spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of λ-transitions in the mixed NH 4Cl 0.9I 0.1 crystal at 242 and 228 K consistent with those observed in the pure NH 4Cl and NH 4I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn 2+ ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn 2+ ion substitutes for a monovalent NH 4 + ion in the NH 4Cl 0.9I 0.1 crystal. This reorganization is almost the same as that observed in NH 4Cl and NH 4I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.

Hyperfine interaction in 199Hg+ and 201Hg+ (6s1): electron paramagnetic resonance studies of Hg+ in NH4Cl single crystals

The mercurous ion Hg+ is produced by gamma irradiation of Hg2+-doped ammonium chloride single crystals, and electron paramagnetic resonance studies of it have been conducted in the 10-300 K temperature range. It is shown that Hg+ occurs substitutionally at the NH4+ site. For odd isotopes of mercury, 199Hg and 201Hg, hitherto unreported transitions have been observed and are found to be highly sensitive to variations in the hyperfine coupling constant A, and thereby to the temperature dependence of A. The temperature dependence of A (for odd isotopic species) has shown an anomalous increase below the lambda transition of ammonium chloride. The observation of new transitions has enabled the hyperfine coupling constants to be determined accurately and also the hyperfine anomaly for 199Hg+ and 201Hg+ to be re-estimated.

Optical Absorption Spectrum of Cr3+ Ions Doped in NH4Cl Single Crystals

AbstractOptical absorption spectrum of Cr3+ ions doped in NH4Cl single crystals is studied in the temperature range from 300 to 143 K. From the observed spectrum it is concluded that Cr3+ ions enter interstitially into NH4Cl lattice and the site symmetry of the Cr3+ ions are lowered from octahedral to tetragonal. The crystal field and the tetragonal parameters are evaluated.

High-Pressure Brillouin Scattering Cell with Crystal Rotation Axis

A high-pressure optical cell with four windows and a manipulating rod for crystal rotation was designed and constructed from SNCM-8 steel. Brillouin scattering on a cylindrical NH4Cl single crystal was performed as a function of the crystal orientation angle. The angular dependence of the Brillouin shift and linewidth was measured both at 22.5°C, 0.1 MPa and -11.4°C, 0.21 GPa. The spacious sample chamber permitted the installation of optical components, enabling us to perform small-angle scattering experiments using this optical cell.

Correlation of Zero Field Splittings and Site Distortions Interstitial Incorporation of Mn2+ in Urea‐Ammoniumchloride (H2N)2CO · NH4Cl

AbstractIn (H2N)2CO · NH4Cl single crystals doped with Mn2+ three crystallographically inequivalent sets of EPR spectra with relative intensities of 8:3:2 are observed and analyzed. The second order zero field splitting parameters b02 = ‐744.7; ‐712.2; ‐736.5 and b22 = ‐127.1; ‐160.8; ‐111.2 (all in 10−4 cm−1) and the directions of their principal axes can only be explained by incorporation of Mn2+ into interstitial sites midway between two NH4 sites with two water molecules replacing these ammonium ions in close analogy to NH4Cl:Mn.

Cobalt Centres in Ammonium Chloride: Optical Absorption Studies

AbstractThe optical absorption data for Co2+ ions in NH4Cl single crystals grown under different conditions and measured in the temperature range of 300 to 10 K are presented. The observed absorption spectrum is interpreted by considering the absorption within the crystal field levels determined mainly by the ligands of octahedral coordination. The spectrum at low temperatures is considered to be due to two different centres in which Co2+ ion occupies an interstitial position, i.e. the centre of a square of four Cl− ions with either two NH3 ions or H2O ions on either side of the square. In addition to these, the stabilization of a centre in which a Co2+ ion occupies an edge centre position at room temperature is also suggested. The crystal field parameters for the cobalt centres stable at low temperature are evaluated.

EPR studies of Rh and Rh(II) centers stabilized in NH4Cl single crystals

The electron paramagnetic resonance (EPR) studies of gamma irradiated ammonium chloride single crystals doped with rhodium have given evidence to suggest the stabilization of Rh0 and Rh(II) at interstitial sites in a compressed octahedral coordination with D4h symmetry. The EPR spectrum of Rh0 has been found to be highly temperature dependent, the intensity increasing with the lowering of temperature and was observable only below −100 °C. Well resolved hyperfine structure due to 103Rh has been observed for Rh0 center. Its g values suggest that the unpaired electron is in a pure ‖3z2−r2〉 orbital, in contrast to the ‖x2−y2〉 character of the unpaired electron of Rh(II) center. At 77 °K superhyperfine structure due to four equivalent chlorines was observed in the EPR spectrum of Rh(II) center, indicating the overlap of ‖x2−y2〉 orbital with the equatorial ligands.

Quasielastic neutron scattering experiments near the order–disorder phase transition on NH4Cl single crystals

Quasielastic neutron scattering experiments on NH4Cl single crystals near the order–disorder phase transition were interpreted in terms of a model with statistically independent three- and fourfold rotational jumps. By proper choice of the orientation of the scattering vector, the corresponding jump probabilities could be determined separately. At the phase transition, the fourfold jump rate, which introduces rotational disorder, changes stepwise by a factor of ∼3, whereas the threefold jump rate remains nearly unchanged. The results are consistent with measurements applying the ’’fixed-window method’’ (high resolution experiment at zero energy transfer), and with a number of existing NMR T1 experiments. A model of rotations without a preferred rotational axis could be excluded. Results for the order parameter in the vicinity of the phase transition were also obtained. Good agreement was obtained between the activation energy for fourfold jumps from the experiments and the calculations by Hüller et al.

ESR of Cr 3+ in CsCl and NH 4Cl single crystals

An ESR study of Cr 3+-ions doped in CsCl and NH 4Cl single crystals has been carried out at room temperature and, in the case of NH 4Cl, also at temperatures around the λ-transition point. Three different Cr 3+ centres have been identified in CsCl and two in NH 4Cl. The ESR spectra of these centres can be described by an axially symmetric spin hamiltonian, the centre axes are parallel to the crystal axes, except for centre l in NH 4Cl for which an intensity anomaly is found. There is a change in the tilt angle and the zero-field splitting of this centre at the NH 4Cl λ-transition temperature. It is found that the structure of centre III in CsCl is associated with the six coordinated Cr 3+ complex [CrCl 6] 3-; the structures of the other complexes are still not certain.

Electro-optical properties of NH4Cl single crystals near their order-disorder phase transition

Electro-optical properties of NH4Cl single crystals near their order-disorder phase transition

Molybdenyl centers in NH4Cl single crystals

Molybdenyl centers are studied in single crystals of NH4Cl using a set of correlated experimental techniques like EPR, optical absorption, infrared absorption, conductivity, and dielectric loss. EPR studies on NH4Cl single crystals doped with (MoOCl5)2− are made over a wide temperature range. At room temperature the resonance due to even isotopes of Mo and the hyperfine interaction due to the odd isotopes are observed. At 77 K the ligand hyperfine interaction due to 35,37Cl is also observed. The spectra are analyzed using a suitable spin Hamiltonian. An LCAO–MO method is used to interpret all the results. In the optical absorption spectra the bands corresponding to the d–d transitions and charge transfer transitions are observed. Spin–orbit coupling constant is estimated using the EPR and optical results. Molecular orbital parameters are calculated taking into account the spin–orbit effects of the ligands also. The calculated bonding coefficients are correlated with the EPR results and the nature of binding in these complexes is discussed. The spin density on the ligand orbitals is estimated using the results of the ligand hyperfine interactions. The Mo–O stretching frequencies are observed in the infrared absorption spectrum. The evidence for the existence of the bound cation vacancies comes from the conductivity and dielectric loss experiments. The impurity–vacancy binding energy is also estimated.

Growth of NH 4Cl and ND 4Cl single crystals by vapor deposition

A simple vapor deposition process is reported for growing large single crystals of NH 4Cl and ND 4Cl directly in the disordered phase II (125 °C). The ND 4Cl powder used in the preparation of the single crystal was deuterated at 70–80 °C in D 2O. The exchange process was followed by the Raman spectrum of the ammonium ion.

Piezoelectric properties of NH 4Cl near its order-disorder phase transition

The piezoelectric constants of NH 4Cl single domain crystals were measured in the neighborhood of the order-disorder phase transition near 242 K and the the critical exponent β of the order parameter was determined.

Growth of NH 4Cl single crystals from the melt

Single crystals of NH 4Cl having the high temperature NaCl structure were grown under a pressure of about 50 atm with a Bridgman apparatus and then cooled slowly (5°C/h) through the phase transition at 183.1°C into the low temperature CsCl structure. The growth apparatus is described in detail. One obtains single crystals of 2–3 cm 3 at a growth rate of 0.7 mm/h and a temperature gradient of 50°C/cm if the ampoule is equiped with a small platinum wire in its tip in order to favour the formation of one main nucleus.

ENDOR Investigation of Cu++ Centres in NH4Cl I. Experimental Results

AbstractTwo Cu++ centres in NH4Cl single crystals grown from acid (I) and neutral (II) aqueous solution were investigated by ESR and ENDOR at 4.2 °K. From the superhyperfine and quadrupole data obtained for several shells of neighbour nuclei the following centre models are proposed: in both centres Cu++ is at an interstitial site with 2 NH4+ vacancies along [001] at both sides of the Cu++. In centre I the vacancies are filled by H2O, in centre II by NH3. The precise configuration of the H2O and NH3 molecules could be determined.

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.

Order-disorder phenomena. VIII. Acoustic attenuation and dispersion in NH4 Cl

The attenuation coefficient a of longitudinal ultrasonic waves propagating in the [100] direction of an NH4 Cl single crystal has been measured as a function of temperature at 1270, 1500, 1700, and 2656 bar. The elastic constant c11 corresponding to this wave has also been measured at 1500 and 2656 bar. By combining these data with a variety to other ultrasonic and hypersonic information about NH4 Cl it is possible to determine the dynamical behavior near the order-disorder transition in terms of the variation in the relaxation strength and the relaxation time. At a given value of the reduced temperature t=|T−Tλ|/Tλ the acoustic velocity and attenuation in the ordered phase are almost identical at all pressures, but the behavior is different in the disordered phase at 1 atm and at the ``tricritical'' pressure of ∼ 1500 bar. In particular, the relaxation rate 1/τ (in units of 108 sec−1) is 414 t0.60 at 1 atm and is 123 t0.88 at 1500 bar.

Raman scattering study of the disorder-order phase transition in NH4Cl. II

A complete Raman scattering study of an NH4Cl single crystal at both sides of Tλ has been carried out and new results are reported. The study includes measurements and interpretations of the Raman spectral bandwidths, intensities and frequencies of the lattice and internal modes as a function of temperature. By the careful examination of the polarization properties of various scattering spectral components, it is suggested that well-defined Raman selection rules are operative in the orientationally disordered crystal.

Order-Disorder Phenomena. VII. Critical Variations in the Length of NH4Cl Single Crystals at High Pressures

A high-precision capacitance method has been used to investigate variations in the length L of an NH4Cl single crystal in the vicinity of its order—disorder transition line. At low pressures, there is a small first-order discontinuity ΔL superimposed on the lambdalike variation in L. At 255.95°K and 1491.8 bar, L varies continuously but κT and α appear to diverge. At higher pressures, the variation in L at the transition becomes progressively more gradual as the pressure increases. The linear isothermal compressibility in the immediate vicinity of the transition is well represented along an isotherm by κ L = (κ lim ∼L + B φ −y) / (1 + 2 pB φ −y), where φ ≡ | p−p c | / p c and pc is the transition pressure for the given isotherm. The quantity κL has been assigned the constant value 1.57 × 10−6bar−1 for all isotherms; B and y (which are assumed to be independent of pressure) vary with the temperature. The linear isobaric coefficient of thermal expansion is fairly well represented along an isobar by α L = α lim ̃L + Aε−x, where ε ≡ | T−T c |/T c and Tc is the transition temperature for the given isobar. The quantity α lim ̃L has been assigned the constant value 5.0 × 10−5deg−1 for all isobars; A and x (which are assumed to be independent of temperature) vary with the pressure. The exponents x and y, which range from 0.4 to 0.9, are much larger than those associated with an ideal Ising lattice.