Studies of structure, thermal, and electrical properties for Cs 5 H 3 (SO 4 ) 4 crystal

Electrical conductivity, differential scanning calorimetry, and X-ray diffraction measurements were performed on a pentacesium trihydrogen tetrasulfate, Cs₅H₃(SO₄)₄, crystal. The transition entropy at a superionic phase transition and the activation energy of proton migrations in the superionic phase were determined to be 58.2 J K⁻¹ mol⁻¹ and 0.48 eV, respectively. The crystal structure of Cs₅H₃(SO₄)₄ at room temperature was refined. The electrical conduction in Cs₅H₃(SO₄)₄ was discussed with the refined structure.     

Mixed alkali metal selenate proton conductors: Phase transitions and crystal structure of [Rb0.54(Nh4)0.46]3H(SeO4)2

Optical birefringence, calorimetric, thermal expansion, powder and single crystal X-ray diffraction investigations of mixed proton conductors [Rb_1-x(NH₄)_x]₃H(SeO₄)₂ were performed with the aim of studying the influence of partial substitution of cations on the superprotonic phase transition, on the atomic structure and on other characteristic features of this type of crystals.     

Nd1.85Ce0.15CuO4－δ单晶c-轴方向电阻的低场行为

采用自助熔剂缓冷法成功地生长出了Nd_1.85Ce_{0. 15}CuO_4-δ单晶，其零场下零电阻温度约为21K. 在0—0.5T范围内分别测量了磁场平行和垂直样品表面的电阻转变曲线以及0.5T不同角度下的电阻转变曲线. 结果显示磁场平行和垂直样品表面时的转变温度T_p随磁场的变化均服从H=H₀(1-T_p(h)/T_p(0))²关系. 0.5T 关键词： 1.85Ce_0.15CuO_4-δ单晶')" href="#">Nd_1.85Ce_0.15CuO_4-δ单晶 输运性质     

Characterization of organic–inorganic hybrid layered perovskite and intercalated compound (n-C12H25NH3)2ZnCl4

We report on some electrical properties and solid–solid phase transitions of organic–inorganic hybrid layered halide perovskite and intercalated compound (n-C₁₂H₂₅NH₃)₂ZnCl₄ which is one member of the long-chain compounds of the series (n-C_nH_2n+1NH₃)_2,(n = 8–18). The complex dielectric permittivity ?*(ω,T) and the ac conductivity σ (ω,T) were measured as functions of temperature 100 K < T < 390 K and frequency 5 kHz < f < 100 kHz. Moreover, the differential scanning calorimetery and the differential thermal analysis thermograms were performed. The analysis of our data confirms the existence of a structural phase transition at T ≈ (362?±?2) K, where the compound changes its state from intercalation to non-intercalation with a drastic increase in the c-axis by about 16.4%.

The behavior of the frequency-dependent conductivity follows the Jonscher universal power law: σ (ω, T) α?^s(?,T). The mechanism of electrical conduction in the low-temperature phase (phase II) can be described as quantum mechanical tunneling model.     

Dielectric behavior and low temperature phase transition in NH4IO3

The electrical properties namely ac conductivity σ(ω,?T) and the complex dielectric permittivity (ε*) are measured at selected frequencies (5–100?kHz) as function of temperature (95?K?T?4IO₃. The ferroelectric hysteresis loops and the X-ray diffraction pattern are also measured. The analysis of the data indicates that the compound undergoes a structural phase transition at ～103?K and the behavior of σ(ω,?T) obeys the power law. The trend of the temperature dependence of the angular frequency exponent s (0?s?4IO₃; (2) the data indicate that the compound undergoes a structural phase transition at 103?K; (3) the originality of this transition has been confirmed by X-ray diffraction; (4) no evidence for the existence of a ferroelectric transition at 103?K as mentioned earlier; and (5) the quantum mechanical tunneling is proposed as the main mechanism of the electric conduction.     

Quasi-two-dimensional magnetism in (CnH2n+1NH3)2CuCl4 studied by electron paramagnetic resonance

The quasi-two-dimensional magnetism in the layered transition metal compound (C_nH_2n+1NH₃)₂CuCl₄ (n=10, 14) was investigated by means of electron paramagnetic resonance (EPR) and superconducting quantum interference device measurements. As a result, the high temperature magnetic phase transitions were reflected in the EPR parameters in a sensitive manner.     

X-ray diffraction investigation of the thermochromic phase transition in an [NH2(C2H5)2]2CuCl4 crystal

The unit cell parameters of an [NH₂(C₂H₅)₂]₂CuCl₄ crystal are determined using x-ray diffraction analysis, and the thermal expansion coefficients along the principal crystallographic directions are calculated in the temperature range 100–330 K. The behavior of the intensities of the diffraction reflections from the (100), (010), and (001) crystallographic planes is studied in the vicinity of the thermochromic phase transition temperature. The occurrence of a first-order phase transition in the [NH₂(C₂H₅)₂]₂CuCl₄ crystal at T ≈ 324 K is confirmed experimentally.     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

NQR Study of Phase Transition and Cationic Motion in 4-NH2C5H4NHBiBr4·H2O

Four ⁸¹Br NQR lines in 4-NH₂C₅H₄NHBiBr₄·H₂O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr₄ ⁻) _n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T ₁ temperature dependence of ⁸¹Br NQR follows the usual T ² law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T ₁ vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T ₁ of ⁸¹Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH₂C₅H₄H⁺ ions. Echo signals of the ⁸¹Br NQR could not be detected above 190 K owing to poor S/N with very short T ₂.     

1H relaxation study in (NH4)2SbF5

¹H spin-lattice relaxation rate (T ₁ ⁻¹ ) has been measured using inversion recovery technique in polycrystalline (NH₄)₂SbF₅ system in the temperature range 140–400 K. From the plot of log (M ₀−M) againstτ, we have estimated two differentT ₁ corresponding to two inequivalent ammonium ions in the unit cell. Temperature-dependence ofT ₁ in each case exhibits features of double minima indicating the influence of different correlation times corresponding to different types of motion. Activation energies at different temperature regions have been estimated. Some features of dynamics of motion of the different groups of ions across the phase transitions have been discussed.     

Co(S1-xSex)2系统中的铁磁量子相变

针对Co(S_1-xSe_x)₂系统在x=0.11附近发生的铁磁金属到顺磁金属相变,制备了一系列不同Se替代浓度的多晶样品.通过对其结构和电阻率-温度ρ(T)关系的系统观测,结果发现,样品铁磁相变温度T_C随着Se替代浓度x值的增加,以(1-x)^1/2关系单调下降,其二级铁磁相变转变为一级相变 关键词： 量子相变 自旋量子涨落 1-xSe_x)₂')" href="#">Co(S_1-xSe_x)₂     

Luminescence characteristics of Na21(SO4)7F6Cl:Cu+ phosphor

Copper-doped Na₂₁(SO₄)₇F₆Cl phosphor was synthesized via the conventional wet chemical method. The synthesis was carried using CuCl₂ and Cu (NO₃)₂·3H₂O as dopants in two different steps successively. The formation and phase purity of the compound were revealed by the X-ray diffraction pattern. Functional groups of the prepared phosphor were observed in the FT–IR spectrum. The emission along with excitation spectra were followed to explore the luminescence attributes. Photoluminescence (PL) emission spectrum of the material synthesized using CuCl₂ as the dopant was observed at 358?nm due to 3d^l0?3d⁹4s transitions when excited around 247?nm for various copper concentrations. Efficient blue emissions were obtained at peaks 423 and 469?nm for materials synthesized using Cu (NO₃)₂·3H₂O as the dopant, when monitored at 357?nm excitation. The Commission Internationale de I’Eclairage chromaticity coordinates for different copper concentrations were calculated for the emission around 423?nm. TL glow curves of Na₂₁(SO₄)₇F₆Cl:Cu phosphor for different dopant concentrations, irradiated with 100?Gy gamma dose, were studied and hence the trap parameters, namely order of kinetics (b), activation energy (E) and frequency factor (s) associated with the most intensive glow peak of Na₂₁(SO₄)₇F₆Cl:Cu phosphor were determined by using Chen’s Peak shape method. The results indicate that Na₂₁(SO₄)₇F₆Cl:Cu⁺ is a potential novel blue-emitting lamp phosphor and may be quite suitable for use in dosimetry of ionizing radiations.     

Spectral characterization and energy levels of Cr:Sc2(MoO4)3 crystal

This paper reports the spectral properties and energy levels of Cr³⁺:Sc₂(MoO₄)₃ crystal. The crystal field strength Dq, Racah parameter B and C were calculated to be 1408 cm⁻¹, 608 cm⁻¹ and 3054 cm⁻¹, respectively. The absorption cross sections σ_α of ⁴A₂→⁴T₁ and ⁴A₂→⁴T₂ transitions were 3.74×10⁻¹⁹ cm² at 499 nm and 3.21×10⁻¹⁹ cm² at 710 nm, respectively. The emission cross section σ_e was 375×10⁻²⁰ cm² at 880 nm. Cr³⁺:Sc₂(MoO₄)₃ crystal has a broad emission band with a broad FWHM of 176 nm (2179 cm⁻¹). Therefore, Cr³⁺:Sc₂(MoO₄)₃ crystal may be regarded as a potential tunable laser gain medium.     

Characteristics of the Li<Superscript>+</Superscript>-Ion Conductivity of Li<Subscript>3</Subscript>R<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> Crystals (R = Fe,Sc) in the Superionic State

The characteristics of Li+-ion conductivity σdc of structural γ modifications of Li₃R₂(PO₄)₃ compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R₂P₃O₁₂]_∞^3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li₃R₂(PO₄)₃ (ΔH_σ = 0.31 ± 0.03 eV) is lower than in γ-Li₃Fe₂(PO₄)₃ (ΔH_σ = 0.36 ± 0.03 eV). The conductivity of γ-Li₃Fe₂(PO₄)₃ (σ_dc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li₃R₂(PO₄)₃. A decrease in temperature causes a structural transformation of Li₃R₂(PO₄)₃ from the superionic γ modification (space group Pcan) through the intermediate metastable β modification (space group P2₁/n) into the “dielectric” α modification (space group P2₁/n). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li₃Fe₂(PO₄)₃ σdc gradually decreases in the temperature range T_SIC = 430–540 K, whereas in Li₃R₂(PO₄)₃ σdc undergoes a jump at T_SIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and ΔH_σ values and the thermodynamics and kinetics of the superionic transition for Li₃R₂(PO₄)₃ are discussed.     

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract

DTA, structural and electric conductivity investigations were made for (NH₄)₄H₂(SeO₄)₃ single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (～4.10^?3 Ω^?1 cm^?1) and a low activation energy (0.11 eV).     

Lattice site dependent cathodoluminescence behavior and surface chemical changes in a Sr5(PO4)3F host

Eu activated Sr₅(PO₄)₃F phosphor powders have been subjected to the electron bombardment at 2 keV (10 μA) at an oxygen pressure of 1×10⁻⁶ Torr. The synthesized Sr₅(PO₄)₃F phosphor was identical to the hexagonal apatite structure, with the Sr present at two different sites C_s (S1) and C₃ (S2) in the Sr₅(PO₄)₃F host, as inferred from the crystallographic study. Cathodoluminescence (CL) and Auger electron spectroscopy of the phosphor excited by the same electron beam were used to monitor changes in the surface state during prolonged electron bombardment. A direct correlation between the surface reactions and the degradation of the CL brightness was observed. Both C and F were depleted from the surface during electron bombardment. The postulated mechanism for the electron stimulated chemical reactions on the phosphor surface is electron beam dissociation of molecular species to atomic species, which subsequently react with C to form volatile compounds CO₂, CH₄, etc. and with Sr₅(PO₄)₃F to form a non luminescence layer of metal oxides of Sr and P.