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.     

Raman study of KMnF3 perovskite crystals doped by Na+

A study of the influence of cationic Na⁺ substitution in the archetype KMnF₃ perovskite crystal was performed using the Raman method. The Raman spectra of mixed K_1-xNa_xMnF₃ crystals with x = 0.029, 0.048 and 0.065 were recorded versus temperature and fully interpreted in terms of a “one mode” behaviour. In addition to the soft mode not completely vanishing close to Tc, attention was especially paid to evidence of static and dynamical disorder. From this point of view the behaviour of the hard Raman modes versus temperature has been studied together with two unexpected Raman bands in the cubic phase. The interpretation has been made within the more general framework of structural disorder existing in such perovskites with anisotropic interactions.     

Theoretical study of mechanisms for NCO with CH3 reaction

A detailed computational study has been performed at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) level for the NCO with CH₃ reaction by constructing singlet and triplet potential energy surfaces (PES). The results show that the title reaction is more favorable for the singlet PES than the triplet PES. On the singlet PES, the dominant channel is the barrierless addition of the O or N atom to the C atom of the methyl group to form CH₃NCO (IM1) and CH₃OCN (IM2). On the triplet PES, the favorable channel is the barrierless addition of the N atom to the C atom of the methyl group to form an intermediate CH₃NCO (³IM2), which then undergoes a N–C bond scission process to give out CH₃N + CO.     

Optical study of the phase transitions in LiKSO4

The birefringence of LiKSO₄ has been measured over the range 27–700°C. The change in birefringence with heating and cooling is seen to be very different. Observations have been made on domains in (001) and (100) plates near the phase transitions.     

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.     

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

The structural phase transitions and the electrical behaviour of the complex perovskite PbLu_1/2Nb_1/2O₃ have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/₂Nb1/₂O₃. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.     

X-ray study on the phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals

The structural phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals below room temperature have been studied by X-ray diffraction. Three phases are observed in the temperature range from 25°C down to — 160°C. The space groups in three different phases are identified as C2/c, P2/n (or Pn), and C2 for (NH₄)₃H(SO₄)₂ and (ND₄)₃D(SO₄)₂ crystals. No isotope effect on the structural phase transitions in these crystals could be detected by these studies. The occurrence of structural phase transitions caused by the reorientation of SO₄ groups and/or the shift of oxygen atoms from the sulfate atom in the SO₄ group are suggested from the diffraction photographs.     

BiFeO_3结构性质与相转变的第一性原理研究

采用基于密度泛函理论的第一性原理计算,系统地研究了BiFeO3的7种不同空间群(R3c,R3m,P4mm,Cm,Pm3m,R3m和R3c)结构及其转变关系.结果表明,铁电相R3c结构是基态,不同结构之间也存在着一定的转变关系,其变化主要包括两种形式,在[111]方向上Bi3+相对FeO6八面体存在一定的位移和FeO6八面体绕[111]极化轴的反铁扭曲旋转.此外,还得出BiFeO3的薄膜结构受到衬底结构的作用会导致其从三方相(R3c)向四方相(P4mm)转变.     

Investigation of the phase transitions of ferroelectric KIO3 and KNbO3 crystals by optical diffraction

Optical diffraction is reviewed as a technique for investigation of the phase transitions in crystals with a multidomain structure. It has been used to study the phase transitions in KIO₃ and KNbO₃ single crystals. Strong optical diffraction bands resulted from electric domains in KNbO₃ crystals and their change with temperature were observed when a laser beam passed through the crystals. The diffraction patterns observed changed abruptly at 427°C, 223°C, and -50°C respectively, at which KNbO₃ crystals undergo structural phase transitions. It is considered that the change of the diffraction patterns with temperature is due to change of the electric domains in the crystals.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

Half-metallic ferromagnetism in cubic perovskite type NdInO3

ABSTRACT

Based on the full-potential linearised augmented plane wave plus local orbitals (FP-L/APW?+?lo) method within the density functional theory (DFT), the structural, electronic, and magnetic calculations of the cubic oxide perovskite NdInO₃ compound have been done under the generalised gradient approximation (GGA). The exchange and correlation (XC) potential is defined as GGA framework in the analyses of structural properties, while both GGA and GGA?+?U (U is the Hubbard correlation term) approximations are taken to treat the electronic and magnetic properties. It is found that ferromagnetic (FM) configuration is reported as the most stable ground state of the cubic NdInO₃ material; however, the equilibrium lattice parameters such as lattice constant (a₀ ), bulk modulus (B₀ ), its first-pressure derivative (B’), and the minimum of total energy (E₀ ) are given in paramagnetic (PM), ferromagnetic (FM), and anti-ferromagnetic (AFM) states. The spin-polarized electronic structure calculations (band structure and density of states) of the cubic oxide perovskite NdInO₃ compound verify the half-metallic feature due to the spin-up case which has the metallic nature, whereas the spin-down case presents the semiconducting character. Moreover, the magnetic properties show the integer value of the total magnetic moment for the studied compound (3μ_B ), where it is manly contributed by Nd atoms with apparition of weak local magnetic moments in non magnetic In and O sites.     

钙钛矿晶体的电荷复合动力学研究

本文利用紫外吸收光谱和稳态荧光光谱技术结合理论模型,研究了钙钛矿材料CH₃NH₃PbI₃晶体在光激发过程中的电荷复合动力学行为,进而获得晶体的扩散长度. 电荷载体的扩散长度是判断光电材料的重要参数. 研究通过合成两种不同缺陷态浓度的CH₃NH₃PbI₃晶体,测量这两种晶体在0.019∽4.268 μJ/cm²的激光激发下的时间分辨荧光光谱,利用动力学模型对光谱进行拟合,可以获得每个晶体的掺杂浓度,空穴浓度以及电荷复合参数. 将这些参数结合已有公式,最终可获得每个晶体的电荷载体的扩散长度.     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Activated carbon as back contact for HTM-free mixed cation perovskite solar cell

Carbon materials have potential applications in perovskite solar cell because of their excellent electronic properties and low cost. In this paper, we report, for the first time, activated carbon as a back contact for hole transport layer-free mixed halide perovskite solar cells. The ability of activated carbon to form conducting chain-like structure when dispersed in a polymeric solution makes it a possible candidate for back contact. A composite of activated carbon and PEMA was optimized with varying concentration. Mixed cation was used as a perovskite absorber and was analysed for its structural and optical properties. The fabricated devices were studied for their electrical performance. They were also subjected to stability study and showed promising results.     

1H NMR study of [N(CH3)4]2ZnCI4 at high pressures and low temperatures

Wide-line proton NMR studies on polycrystalline tetramethylammonium tetrachlorozincate have been carried out at high hydrostatic pressures up to 15 kbar in the temperature range 77-300 K and at ambient pressure down to 4.2 K. A second-moment transition is observed to occur starting around 161 K, the temperature for the V-VI phase transition. This transition temperature is seen to have a negative pressure coefficient up to 2 kbar, beyond which it changes sign. At 77 K the second moment decreases to 4 kbar and then increases again as a function of pressure. The results are explained in terms of the dynamics of the N(CH₃)₄ groups.     

Studies of phase transitions and ferroelastic domain walls in Pb5Al3F19 crystals by optical measurements

Three phase transitions between 100 and 400 K were previously shown by X-ray diffraction measurements. Such results have been confirmed by optical measurements on crystals. The thermal dependence of the birefringence has been determined. A new phase transformation has been shown at around 590 K not only by optical study but also by dielectric measurements. Ferroelastic domain-walls have been visualized. The sequence transition has been discussed taking into account the various space groups.     

Theory of the successive phase transitions in KNbO3

The microscopic mechanism of the successive cubic-tetragonal-orthorhombic-rhombohedral phase transitions in KNbO₃ is discussed quantitatively from the microscopic free energy based upon the mean field approximation where the Nb ions are displaced to create spontaneous deformations. From the calculation of the microscopic free energy, it is shown that the order of the phase transitions and the experimental values of the transition entropy in KNbO₃ are well explained by this model.     

Study of electrical conductivity changes and phase transitions in Co3O4 doped ZrO2

The electrical conductivity of ZrO₂ doped with Co₃O₄ has been measured at various temperatures for different molar ratios. The conductivity increases due to the migration of vacancies created by doping. The conductivity is also found to increase with rise in temperature up to 120°C, and after attaining a maximum the conductivity decreases due to a collapse of the lattice framework. A second rise in conductivity around 460°C in all the compositions confirms the phase transition in ZrO₂ from monoclinic to tetragonal symmetry. X-ray powder diffraction and DTA studies were carried out for confirming the doping effects and the transition in ZrO₂.     

The inversion potential for NH3 and its isotopic species

Usingab initio data of the potential surface for the ground state of NH₃ a function is obtained to represent, with a very high precision, the potential along the inversion coordinate. The inversion spectra of NH₃, ND₃, NH₂D and NHD₂ are found by numerically solving the Schrödinger equation for this potential. Comparison is made of the calculated inversion frequencies and the experimental values, and the molecular constants of NH₃ are also compared to those found in the literature.