Dielectric properties of (NH4)2SO4 crystals in the range of electronic excitations

Spectra of the real and imaginary parts of the pseudo‐dielectric permittivity, 〈?₁〉(E) and 〈?₂〉(E), of ferroelectric ammonium sulfate crystals, (NH₄)₂SO₄, have been measured in the range of electronic excitations 4.0 to 9.5 eV by ellipsometry using synchrotron radiation. Temperature dependences of the corresponding susceptibilities, 〈χ₁〉(T) and 〈χ₂〉(T), obtained for the photon energy E = 8.5 eV, related to excitations of oxygen p‐electrons, reveal sharp peak‐like temperature changes near the Curie point T_C = 223 K. The large temperature‐dependent increase of the imaginary part of the susceptibility χ₂(T), together with a simultaneous decrease of the real part of the susceptibility χ₁(T), take place at the phase transition. These anomalies have been ascribed mainly to the SO₄ group of the crystal structure.     

Anomalous behavior of the elastic and inelastic properties in the ferroelectric phase of single-crystal (NH4)2SO4

The anomalous changes of the low-frequency elastic and inelastic properties of single-crystal (NH₄)₂SO₄ accompanying the phase transition from the paraelectric to the ferroelectric phase have been studied by the reverse torsion-pendulum method at 223 K and in the temperature region where the spontaneous polarization changes sign. Fiz. Tverd. Tela (St. Petersburg) 40, 2202–2205 (December 1998)     

EPR evidence for a weak first order behavior of the ferroelectric transition in (NH4)2SO4

EPR line-broadening effects have been analyzed for the low (∼ 10¹⁰ Hz) rotational fluctuations of NH₃ ⁺ radicals in (NH₄)₂SO₄ crystals near their ferroelectric transition. Critical fluctuations together with coexistence of the paraelectric and ferroelectric phases have been observed and interpreted in terms of a weak first order, Landau-type model for the phase transition.     

Dielectric and calorimetric investigations of KNO3 in pores of nanoporous silica matrices MCM-41

The temperature dependences of the linear dielectric permittivity, the third harmonic amplitude, and the heat capacity of nanoporous silica matrices MCM-41 with cellular channels (3.7 and 2.6 nm in diameter) filled with KNO₃ have been investigated in comparison with those obtained for bulk potassium nitrate. Measurements have been performed during heating and cooling in the range from room temperature to 463 K. Anomalies corresponding to structural phase transitions have been observed. A significant broadening of the temperature region of the existence of the ferroelectric phase III of potassium nitrate upon cooling has been revealed. This broadening increases with a decrease in the size of pores. It has been shown that, in the nanocomposites with potassium nitrate, the ferroelectric phase can also be formed during heating. The efficiency of observation of the third harmonic generation for studying nanocomposites with the ferroelectric phase has been demonstrated.     

Chalcogen based treatment of InAs with [(NH4)2S/(NH4)2SO4]

A sulphur based chemical, ([(NH₄)₂S/(NH₄)₂SO₄]) to which S has been added not previously reported for the treatment of (111)A InAs surfaces is introduced and benchmarked against the commonly used passivants Na₂S·9H₂O and ((NH₄)₂S + S), using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It has been found that the native oxide layer present on the InAs surface is more effectively removed when treated with ([(NH₄)₂S/(NH₄)₂SO₄] + S) than with ((NH₄)₂S + S) or Na₂S·9H₂O. AES depth profiles of the sulphurized layers revealed the formation of a thin (less than 8.5 nm) In–S surface layer for both ((NH₄)₂SO₄ + S) and ([(NH₄)₂S/(NH₄)₂SO₄] + S) treatments. No evidence for the formation of As―S bonds was found. Treatment with ([(NH₄)₂S/(NH₄)₂SO₄] + S) also affected a significant improvement compared to the more established sulphur treatments in the surface morphology of the otherwise poor as-received n-InAs (111)A surface.     

Dielectric studies of nanoporous alumina films filled with the Rochelle salt

Dielectric studies of nanoparticles of the Rochelle salt embedded in pores of porous alumina have been performed in the temperature range from 80 K to the decomposition temperature of the bulk Rochelle salt. It has been revealed that the permittivity exhibits an anomaly corresponding to the lower structural transition to the paraelectric phase, whereas the upper ferroelectric transition is shifted above the decomposition temperature in agreement with the recently published data. The temperature of the lower transition for nanoparticles in pores is found to decrease by 10 K. Possible physical factors that can be responsible for the broadening of the region of existence of the ferroelectric phase have been discussed.     

On the low temperature phase transition in (NH4)2SO4

An ultracryostat and multidecameter were used to determine the temperature dependence of the dielectric constant ?′ and dielectric loss ?″ over a wide range of frequencies of single crystals and polycrystalline samples of (NH₄)₂SO₄ in the region of the low temperature phase transition. A sharp increase was observed in the values of ?′ and ?″ at about ?50°C. In addition, a dielectric dispersion was detected and found to be more pronounced in the high temperature phase. This dispersion was attributed to piezoelectric resonance. The observed sudden increase in the values of the dielectric constant and dielectric loss below ? 50°C was attributed to the ferroelectric nature of the low temperature phase of (NH₄)₂SO₄.A DTA thermogram showed a sharp peak at ? 50°C which indicated that the phase transition is one of first order type. A TMA thermogram showed that this transformation was associated with a rapid increase in the expansion coefficient. Such an increase in the lattice parameter might be attributed to the enhanced rotation of electric dipoles associated with the distorted NH₄⁺ and SO₄^2? ions. The distortion of both the ammonium and sulfate ions in addition to their expected orientational motion are suggested to be responsible for the ferroelectric behaviour of ammonium sulfate below ?50°C.A transition to a metastable hexagonal state at about ?40°C is thought to occur, and this transformation is found to be irreversible.     

Impact of ethylene carbonate on electrical properties of PVA/(NH4)2SO4/H2SO4 proton-conductive membrane

A new proton-conductive membrane (PCM) based on poly (vinyl alcohol) and ammonium sulfate (NH₄)₂SO₄ complexed with sulfuric acid and plasticized with ethylene carbonate (EC) at different weight percent were prepared by casting technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD patterns of all the prepared polymer electrolytes reveal the amorphous nature of the films. ac conductivity and dielectric spectra of the electrolyte were studied with changing EC content from weight 0.00 to 0.75 g. A maximum conductivity of 7.3 × 10⁻⁵ S cm⁻¹ has been achieved at ambient temperature for PCM containing 0.25 g of ethylene carbonate. The electrical conductivity σ, dielectric constant ε′ and dielectric loss ε″ of PCM in frequency range (100 Hz to 100 KHz), and temperature range (300–400 K) were carried out. Measurement of transference number was carried out to investigate the nature of charge transport in these polymer electrolyte films using Wagner’s polarization technique. Transport number data showed that the charge transport in these polymer electrolyte systems was predominantly due to ions. The electrolyte with the highest electrical conductivity was used in the fabrication of a solid-state electrochemical cell with the configuration (Mg/PCM/PbO₂). Various cell parameters ldensity, and current density were determined. The fabricated cells gave capacity of 650 μAh and have an internal resistance of 11.6 kΩ.     

Proton N.M.R. study of the dynamics of the ammonium ion in ferroelectric langbeinite, (NH4)2Cd2(SO4)3

The proton N.M.R. lineshape of polycrystalline Langbeinite, (NH₄)₂Cd₂(SO₄)₃, has been studied in the temperature range 300 K to 1·8 K. The resonance line is motionally narrowed over the entire temperature range, and the low temperature proton line shows clear evidence for tunnelling motion of the ammonium ion between spin-symmetry states. From a computer simulation of the lineshape, we obtain an estimate for the tunnelling splitting parameter, J, of the torsional ground state of the ammonium ion, as 375 ± 125 gauss. For an undistorted tetrahedral crystal field this corresponds to a tunnelling splitting Δ = 4J = 6·3 ± 2·1 MHz.

Pulsed proton N.M.R. studies have also been carried out on the above compound at 30·8 MHz and 48·2 MHz and the spin-lattice relaxation time (T ₁) has been measured by the π - t - π/2 pulse sequence as a function of temperature down to 77 K. At 30·8 MHz, a T ₁ minimum of 13 ms occurs at 105·8 K, and is ascribed to random reorientations of the NH₄ ⁺ ion. An activational energy barrier of 0·74 ± 0·1 kcal/mole and an associated pre-exponential factor of 8·0 × 10^-13 s are calculated for the above motional process, and the value of the activation energy is correlated with the tunnelling splitting of the torsional ground state.

An anomaly in T ₁ has been observed at the ferroelectric Curie point (95 K), indicating the order-disorder nature of the transition. This is the first experimental evidence relating to the nature of the transition in Langbeinite.     

Optical studies of the ferroelastic phase transitions in Rb3H(SeO4)2, (NH4)3H(SO4)2 and (NH4)3H(SeO4)2

Optical observations of growth twins and ferroelastic domains and measurements of the rotation of the optical indicatrix were carried out for Rb₃H(SeO₄)₂ and (NH₄)₃H(SO₄)₂ using an optical microscope. Taking into account the symmetry reduction from the rhombohedral (Rm) to the monoclinic phase (B2/a) the occurrence of domains and growth twins can be well described. The orientations of oblique ferroelastic walls are well determined by the spontaneous strains ^s e ₁₁ and ^s e ₂₃ at room temperature.     

Conductivity and dielectric relaxation phenomena in (NH4)2SO4 single crystal

AC impedance measurements have been carried out on (NH₄)₂SO₄ single crystals for the temperatures from 300 to 473 K and frequency range between 100 Hz and 4 MHz. The results reveal two distinct relaxation processes in the sample crystal. One is the dipolar relaxation with a peak at frequency slightly higher than 4 × 10⁶ Hz. The other is the charge carrier relaxation at lower frequencies. The frequency dependence of conductivity is described by the relation σ(ω) = Bωⁿ, and n = 1.32 is obtained at temperatures below 413 K. This value drops to 0.2 and then decreases slightly with increasing temperature. The dipolar response of the (NH₄)₂SO₄ single crystal under an ac field is attributed to the reorientation of dipoles. The contribution of charge carriers is increasing substantially with increasing temperature at temperatures above 413 K. The temperature variation of conductivity relaxation peaks follows the Arrhenius relation. The obtained activation energy for migration of the mobile ions for (NH₄)₂SO₄ single crystal was 1.24 eV in the temperature range between 433 and 468 K in this intrinsic region. It is proposed that the NH₄⁺ in the sample crystal has the contribution to the electrical conduction.     

Electrooptical properties of ferroelectric Gd2(MoO4)3

On the basis of the symmetry of the free energy function describing properly the transition of gadolinium molybdate into the ferroelectric state, macroscopic relations are derived for the spontaneous birefringence and for the temperature dependence of the electrooptic coefficientsz _ij . Since the transition parameters correspond to lattice vibrational modes with finitek, the resulting formulae differ considerably from those for classical ferroelectrics with transition parameterP. Experimental data are presented for the temperature dependence of freez ₆₃ andz ₄₁, clampedz ₆₃ and free permittivity for a multidomain and single domain crystal. These results and earlier data on spontaneous birefringence compare well with theory.     

Light scattering studies of phase transitions of (NH4)2SO4-type crystals

The mechanism of the ferroelectric transition of pure and rubidium-doped ammonium sulfate is investigated by means of Raman, Brillouin and dielectric measurements. The study contributes to spectroscopic evidences of temperature anomalies of lattice vibrations in the paraelectric and ferroelectric range. The different behaviour of rubidium-doped crystals with respect to their dielectric and elastic properties at the transition is interpreted as an indication that different order parameters might become active in this first order transformation.     

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.     

1H NMR study of proton dynamics in (NH4)3H(SO4)2

Proton dynamics in (NH₄)₃H(SO₄)₂ has been studied by means of ¹H solid-state NMR. The ¹H magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT) and at Larmor frequency of 400.13 MHz. ¹H static NMR spectra were measured at 200.13 MHz in the range of 135–490 K. ¹H spin-lattice relaxation times, T₁, were measured at 200.13 and 19.65 MHz in the ranges of 135–490 and 153–456 K, respectively. The ¹H chemical shift for the acidic proton (14.7 ppm) indicates strong hydrogen bonds. In phase III, NH₄⁺ reorientation takes place; one type of NH₄⁺ ions reorients with an activation energy (E_a) of 14 kJ mol^− 1 and the inverse of a frequency factor (τ₀) of 0.85 × 10^− 14 s. In phase II, a very fast local and anisotropic motion of the acidic protons takes place. NH₄⁺ ions start to diffuse translationally, and no proton exchange is observed between NH₄⁺ ions and the acidic protons. In phase I, both NH₄⁺ ions and the acidic protons diffuse translationally. The acidic protons diffuse with parameters of E_a = 27 kJ mol^− 1 and τ₀ = 4.2 × 10^− 13 s. The translational diffusion of the acidic protons is responsible for the macroscopic proton conductivity, as the NH₄⁺ translational diffusion is slow and proton exchange between NH₄⁺ ions and the acidic protons is negligible.