Phase Transition In NH Research Articles

Raman and X-ray Investigations of Ferroelectric Phase Transition in NH4HSO4

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH(4)HSO(4) single crystals in the temperature range 77-298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as Pc, which has been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO(4)(-) ions and that below 143 K is driven by the change in both HSO(4)(-) and NH(4)(+) ions.

Calculation of the specific heat from the Raman frequency shifts close to the tricritical and second order phase transitions in [formula omitted

This study gives our calculation for the specific heat using our Raman frequency shifts of the ν 5 ( 174 cm - 1 ) mode close to the tricritical ( P = 1.6 kbar ) and second order ( P = 2.8 kbar ) phase transitions in NH 4 Cl . For this calculation we use as the values of the critical exponent for the specific heat, a = 0.11 ( T < T c and T > T c ) for the tricritical ( T c = 257.17 K ) , and a = 0.18 ( T < T c ) , a = 0.64 ( T > T c ) for the second order ( T c = 268 K ) phase transitions, which we deduce from our analysis of the Raman frequency shifts of this mode studied here, on the basis of an Ising model. Our calculated specific heats agree well with those measured thermodynamically for both tricritical and second order phase transitions in NH 4 Cl .

Magnetic Field- and Pressure-Induced Quantum Phase Transitions in NH4CuCl3

Quantum antiferromagnet NH 4 CuCl 3 shows magnetization plateaus at 1/4 and 3/4 of the saturation magnetization. Specific heat measurements at ambient pressure and magnetization measurements under hydrostatic pressure were performed on NH 4 CuCl 3 in magnetic fields. From specific heat measurements, we obtained the detailed magnetic phase diagram below 9 T, which includes the lower edge field H c1 of 1/4 plateau. The phase boundary just below H c1 can be described by a power low, H c1 - H N (T) T Φ with Φ = 2, where H N (T) is the transition field at temperature T. Magnetic susceptibility measurements under pressure revealed reentrant quantum phase transitions, such that the gapless ground state at zero field changes into gapped state with increasing pressure, and further into gapless state again.

Frequency shifts of the disorder-induced modes close to the tricritical phase transistion in NH3Cl

We have studied the Raman frequencies of the disorder-induced modes of v 7 (93 cm - 1 ) and v s (144 cm - 1 ) close to the tricritical phase transition in NH 4 Cl (P 1.6 kbar, T c 257 K). Using our observed Raman data for these phonon modes, we have obtained the value of the exponent a 0.3 for the frequency shifts, which can describe the critical behaviour of the thermal expansivity α p and also of the specific heat C p . Our exponent value can be compared with the value of 0.1 predicted by the 3d Ising model.

A study of phase transitions in NH4Br using charge transfer bands of Cu2+centres as probes

Charge transfer (CT) spectra due to small amounts ( approximately 100 p.p.m.) of (CuBr4(NH3)2)2- centres in the NH4Br lattice have been investigated in the 300-14 K temperature range. When upon cooling the gamma to delta phase transition occurs the assigned eu( pi + sigma ; Br) to alg* and eu( sigma + pi ;Br) to alg* peaks experience abrupt red shifts of 700 and 1200 cm-1 respectively, stressing the great sensitivity of CT peaks for detecting that transition. Those red shifts support the assertion that when the gamma to delta transition is produced the Cu2+-Br- distance increases though the average lattice parameter of NH4Br decreases abruptly by 2.4 pm. This is the first time that the first-order sluggish gamma to delta transition in NH4Br and the hysteresis effects involved have been clearly detected using an impurity as probe. Moreover CT bands also detect the phase transition at 234 K and the possibility for the crystal to remain in a metastable tetragonal phase below 78 K.

The Role of Hydrogen Bonds in Antiferroelectricity of NH4H2PO4

From a microscopic viewpoint on the role of two kinds of hydrogen bonds, antiferroelectric phase transition in NH 4 H 2 PO 4 is re-examined. The role played by the protons of NH 4 ions is explicitely taken into account, and it is shown that the hydrogen bonding effect of NH 4 ions with neighbouring oxygens may be renormalized into the effective antiferroelectric interaction between protons which form the network of hydrogen bonds connecting PO 4 groups.

Raman scattering study of phase transitions in NH4SCN

The Raman spectra (30–3300 cm−1) of a single crystal and of polycrystalline forms of NH4SCN are presented in the temperature range 135 to −175 °C. The polarization behavior of the Raman bands reveals the antipolar arrangements of SCN− ions in the room temperature phase. Above 86.5 °C the spectra do not show polarization behavior due to the formation of polycrystalline domains. Other changes observed in the entire spectrum at this phase transition are attributed to reorientation of SCN− ions between two equivalent configurations. At the next phase transition around 112 °C the lattice modes of SCN− ions are affected. The characteristics are those of a second-order displacive phase transition. A new phase transition observed at −70 °C is characterized by the strengthening of the hydrogen-bond networks in the lattice.

Effect of Hydrostatic Pressure on the Antiferroelectric Phase Transitions in Ammonium Dihydrogen Arsenate NH4H2AsO4and Deuterated Analogue

Effect of hydrostatic pressure on the antiferroelectric phase transitions in NH 4 H 2 AsO 4 and deuterated analogue (deuterium concentration ∼80%) was studied by dielectric constant measurements at high pressures up to about 0.8 GPa. The transition temperature linearly decreases with increasing pressure with the ratios of -19.7 K GPa -1 and -14.5 K GPa -1 for NH 4 H 2 AsO 4 and the deuterated compound, respectively. The pressure effect is compared with previously reported results of other KH 2 PO 4 -type ferro- and antiferroelectric crystals.

Investigation of phase transitions in NH4HSeO4crystals by pyroelectric method

Abstract The pyroelectric properties of NH4HSeO4 crystals were measured in the temperature range of 100–300 K. A new phase transition was found at 109 K. It was noted that 80% deuteration of the NH4HSeO4 crystals caused these crystals to become non-ferroelectric.

The detection of phase transitions in NH4Sm(SO4)2∙4H2O single crystals using the electron paramagnetic resonance of Gd3+ impurity ions

The 9.4 GHz electron paramagnetic resonance (epr) of Gd3+ impurity ions in NH4Sm(SO4)2∙4H2O single crystals has been studied at 101–297 K. The 294 K epr spectra show that the Gd3+ impurity ion substitutes for the Sm3+ host ion in two magnetically nonequivalent complexes corresponding to the tetramolecular unit cell. These spectra were analyzed using an orthorhombic symmetry spin-Hamiltonian. The observation of resolved Gd3+ impurity ion spectra in the presence of paramagnetic Sm3+ ions has been interpreted in terms of a random modulation of the interaction between the Gd3+ impurity and Sm3+ host ions by rapid spin–lattice relaxation of the Sm3+ ions. The spin–lattice relaxation time of the Sm3+ ions has been estimated at 294 K from the impurity ion epr linewidths and is found to be consistent with an Orbach resonance process. The value for τ1, has also been computed for Nd3+ ions in NH4Nd(SO4)2∙4H2O from the epr linewidth data in the literature. It has been shown for the first time that NH4Sm(SO4)2∙4H2O single crystals have the phase transition sequence[Formula: see text]

The order-disorder phase transition in NH4Cl: a new investigation by means of optical second harmonic generation at atmospheric pressure

The temperature dependence of the second harmonic intensity SH(T) at a wavelength of 0.532 mu m has been measured across the weakly discontinuous phase transition of NH4Cl. It is shown that the tensor component d14 of the non-linear quadratic susceptibility, which is proportional to square root SH(T), can be taken as an order parameter eta for the phase transition. The experimental data are well described by equations derived from the Landau free energy expansion of the form: F=F0+1/2A2 eta 2+1/6A6 eta 6+1/8A8 eta 8, recently proposed by Benguigui (1976) for tetracritical systems. It is further shown, that the theoretical order parameter data, calculated from the Landau equations using the best fitting parameters of the experimental data, can themselves be fitted very precisely to a 'critical' power law. In this way the low values for the 'critical' exponent beta of the order parameter, reported in literature for NH4Cl, can be explained.

Evidence for zero‐field fluctuations in Cr3+ near the phase transition in NH4Al(SO4)2 12 H20

AbstractThe effect of the phase transition of NH4Al(SO4)2 12 H2O on the EPR of the Cr3+ ion is studied. The axial zero‐field tensor shows a strong linear temperature dependence with a change in slope occurring at −160°C which correlates to the onset of a marked broadening of the high and low field resonances as well as with an unusual increase in the intensity of the center resonance. The effects are attributed to fluctuations in the local symmetry of the H2O molecules about the Cr3+ ion. The strong temperature dependence of D is argued to be due to an unusually large coupling of the paramagnetic ion to local vibrations of the complex.

Kinetics of NH 3 decomposition on iron at high temperatures

The kinetics of NH 3 decomposition on polycrystalline iron wires between 600 and 1250 K and pressures between 0.05 and 1 Torr are measured and correlated with examination by SEM and AES. Reproducible rates are obtained in pure NH 3 which can be fit accurately at all temperatures and pressures by a Langmuir-Hinshelwood (LH) unimolecular rate expression with reaction and desorption activation energies of 49,600 and 39,600 cal/mole, respectively. Hydrogen alters the decomposition rate to a higher value which for high hydrogen pressures is independent of P H 2 and can also be fit by a LH expression. Results are interpreted in terms of surface nitride in pure NH 3 and nitride-free iron in hydrogen. Reaction in NH 3 produces much more rapid and extensive facetting than does heating in vacuum, and repeated cycling through the α-γ phase transition in NH 3 produces even more extensive facetting of the surface. This is apparently caused by preferential attack of NH 3 at dislocations induced at the surface by the phase transformation. No discontinuity in rate was observed in crossing the transition temperature.