Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.     

Anomalous states of the structure of (NH4)2SO4 crystals in the temperature range 4.2–300 K

Crystals of the (NH₄)₂SO₄ ammonium sulfate are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameters exhibits an anomalous behavior, namely, a global hysteresis, and an anomalous increase in the lattice parameter a and the unit cell volume at temperatures below the ferroelectric phase transition point (T _c=223 K). The series of superstructure reflections observed corresponds to an incommensurate composite structure. Analysis of the temperature behavior of the mismatch parameters for the matrix (host) and superstructure (guest) lattices demonstrates that the (NH₄)₂SO₄ compound undergoes a number of phase transitions, including a transition to a three-dimensionally incommensurate composite phase and transitions to commensurate (along one of the crystallographic directions) composite phases.     

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.     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

Dielectric properties of Bi4Ti3O12–SrBi4Ti4O15 intergrowth ceramics synthesized by a modified oxalate route

In this study, Bi₄Ti₃O₁₂–SrBi₄Ti₄O₁₅ (BIT–SBTi) intergrowth ferroelectric ceramics was synthesized by a modified oxalate route. The phase formation behaviour, structure, morphology and electrical properties of the intergrowth ceramics were also investigated. The phase formation takes place through intermediate phases like SrBi₂O₄ and Bi₁₂TiO₂₀. The precursor mostly changes to Bi₄Ti₃O₁₂ at 600°C and to BIT–SBTi intergrowth at 800°C. Rietveld analysis of the X-ray diffraction pattern showed that the structure of the intergrowth compound was orthorhombic with lattice parameters a = 5.4408(3), b = 5.4505(1) and c = 74.0851(4) Å. The intergrowth ferroelectrics showed a phase transition at 610°C and a frequency-stable permittivity and dielectric loss behaviour. The intergrowth ferroelectrics also showed a larger 2P_r than their constituents BIT and SBTi.     

Dielectric properties of RbCl-RbBr mixed crystals

A systematic measurement of dielectric constant and loss on RbCl-RbBr mixed crystals in various compositions has been carried out in the frequency range 100 Hz to 100 kHz and in the temperature range from room temperature to 320°C. From these measurements the static dielectric constant, the Szigeti charge, the conductivity and the activation energy for conduction are evaluated. All these properties show a nonlinear composition dependence. Semiempirical equations proposed earlier are employed to evaluate the dielectric constant as a function of composition. The validity of these relations is also discussed.     

Consequences of electronic excitations in CoFe1.90Dy0.10O4

Present work reports the irradiation induced effects in Dy³⁺ doped cobalt ferrite nanoparticles in the regime of dominant electronic excitation processes induced by 100 MeV O⁷⁺ ion irradiation. Irradiation leads to the deterioration of crystalline phase as envisaged by X-ray diffraction. Crystallite size decreases with the increase of irradiation fluence. Disappearance of certain bands in Raman spectra at higher fluence of irradiation confirms the crystalline disorder induced by electronic excitations. Fourier transform infrared spectra show onset of cation migration from tetrahedral site to octahedral site and vice versa. X-ray absorption fine structure measurements depict the preservation of valence state of metal ions after irradiation. These measurements further infer bond breaking process in irradiated materials. Magnetic measurements carried out on these materials indicate slight increase of saturation magnetization at room temperature followed by the decrease of coercive field. Obtained results are discussed on the basis of appropriate mechanism.     

Dielectric properties of Pb(Mg1/4Zn1/4Nb1/2)O11/4

Polycrystalline samples of Pb(Mg_1/4Zn_1/4Nb_1/2)O_11/4 have been synthesized by high temperature columbite precursor solid state reaction technique. Using X-ray diffraction (XRD) technique, compound formation in single phase cubic structure was observed and XRD analysis provided preliminary structural data. Detailed studies of dielectric properties of the compound reveal that this compound has high dielectric constant and diffuse phase transition in a wide range of temperatures around the Curie temperature. The charge deficiency of the compound presumably gets compensated in the high temperature columbite precursor process of sample preparation which is supported by single phasic form of the material.     

Magnetic and electronic properties of two-dimensional metal-organic frameworks TM_3(C_2NH)_(12)

The ferromagnetism of two-dimensional(2 D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2 D transition metal-organic framework materials(TM-NH MOF, TM = Sc–Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and abinitio molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2 D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni-and Zn-NH MOFs are nonmagnetic semiconductors(SM) with band gaps of 0.41 e V and 0.61 e V, respectively. Co-and Cu-NH MOFs are bipolar magnetic semiconductors(BMS), while Fe-NH MOF monolayer is a half-semiconductor(HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3 d states. This work predicts several new 2 D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.     

First-principle investigation of the electronic and magnetic properties of PbMn（SO4）2

The magnetic properties of oxide PbMn(SO₄)₂ consisted of MnO₆ octahedra which connected with each other through SO₄ tetrahedra, are well studied in experiments. In this paper, we explored its interesting electronic and magnetic properties with first-principle calculations. Our results show that all Mn ions have high spin states, namely, S = 5/2, and the magnetic couplings between NN and NNN are antiferromagnetic, which agree well with the experimental results. Besides, the surprising results of spin exchange interactions between the NN and NNN are excellently explained with extended Hückel tight-binding calculations.     

Dielectric relaxation behaviour of liquid crystals with opposite orientation of -COO ester group in molecular core

Dielectric measurements were performed for two smectogens, being structurally analogous with the opposite space orientation of the-COO ester group in the molecular core, using a HP4192A impedance analyser. Gold coated electrode cells of different thicknesses were used. Temperature dependences of the relaxation frequency fR and the inverse of the dielectric strength Δε′_┴ ⁻¹ obtained by fitting experimental values of perpendicular components, i.e., the real ε′_┴ and the imaginary ε″_┴ parts of the complex dielectric permittivity to the Cole-Cole equation as well as the investigation of modification of relaxation processes under bias were determined for the SmC_*, SmA_*, and N_* phases of studied compounds. One can conclude, on the basis of the above results, that dielectric relaxation processes observed by us in the studied compounds are similar to those of the soft and Goldstone mode typically observed by others in the SmC_*, SmA_*, and N_* phases. It is concluded from acomparison of their properties with other related compounds that the link between the biphenyl moiety and-COO ester group is closely related to the stability of smectic phases.     

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.     

Dielectric relaxation of mixed crystals of Rb1-x(NH4)xH2PO4 at microwave frequencies

Measurements of the clamped dielectric constant on mixed single crystals of Rb_1-x(NH₄)_xH₂PO₄,x=0.35, are reported up to 11 GHz. Between 24 and 60 K, the dielectric dispersion can be fitted to a Cole-Cole relaxation, the parameters of which indicate a temperature-dependent distribution of relaxation times consistent with Vogel-Fulcher freezing. Both the audio and the microwave measurements can be scaled up to 100 MHz, with a freezing temperature ofT _o8.4 K. In the GHz range, a relaxation process in addition to the low-frequency freezing mode contributes to the dielectric response.     

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)     

Dielectric and charge transport properties of the spin crossover complex [Fe(Htrz)2(trz)](BF4)

Spin crossover compounds are considered to be a viable alternative for creating display, memory and switching devices due to the bistability of their magnetic, optical, mechanical and electrical properties. This Letter presents the study of the dielectric and transport properties of the [Fe(Htrz)₂(trz)](BF₄) (Htrz = 1H‐1,2,4‐triazole) complex in a wide temperature and frequency range. Our results reveal a singular behavior of the dielectric modulus upon the spin transition in conjunction with the switching of the conductivity between the high spin and low spin states.

     

Temperature evolution of the intra-ion absorption spectra of (NH2(C2H5)2)2CoCl4 crystals in the region of their phase transitions

On the basis of spectroscopic studies of (NH₂(C₂H₅)₂)₂CoCl₄ crystals, the absorption bands corresponding to the internal electronic transitions in the Co²⁺ ion were identified. The values of the crystal field and Racah parameters were calculated. The temperature evolution of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals reveals the anomalies of their parameters at the points of phase transitions. The corresponding changes of the absorption spectra were discussed in terms of distortion of the metal-halogen complex. The temperature dependences of the absorption spectra of (NH₂(C₂H₅)₂)₂CoCl₄ crystals confirm the presence of the thermochromic phase transitions at 255 and 330?K.