Pressure-induced phase transition in an orthorhombic C60 crystal

X-ray diffraction studies of an orthorhombic C₆₀ single crystal grown from CS₂ solution have revealed a phase transition to a monoclinic phase between 1.1 and 2.2GPa. Compressibility of three principal axes is measured up to 3GPa and found to be nearly isotropic. Its bulk modulus is obtained as 10.5±1.9GPa, and this crystal is more compressible than an fcc one. We discuss the structural characteristic differences under pressure between the orthorhombic crystal and the fcc crystal.     

Free energy and orientational phase transition in solid C60

On the basis of a microscopic model, the free energy of the orientationally disordered fcc crystalline phase of C₆₀-fullerite is derived up to fourth order in the rotational density fluctuations. The order parameter variables are symmetry adapted rotator functions which belong to thel=6 andl=10 manifolds. They are basis functions ofT _2g representations of the cubic group. The phase transition to thePa3 structure is discussed, and is found to be of first order. The orientationally ordered phase is described in terms of condensed rotator functions.     

EPR spectra of C60 anions

The broad and sharp EPR signals observed in spectra of C₆₀ and C ₆₀ ^3? are summarized and proposed assignments are discussed. The sharp signals in samples of C ₆₀ ^2? are reviewed.     

EPR study of the low temperature ferroelectric phase transition in Cu doped Rb2ZnCl4 single crystals

Temperature dependent EPR measurements on copper doped Rb₂ZnCl₄ single crystals allowed us to evidence and study the P2₁cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu²⁺ centers localized at different anionic sites, called Cu²⁺(I) and Cu²⁺(II), which are formed in this compound, only the Cu²⁺(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.     

EPR study of the ferroelectric phase transition in chromium-doped DMAAS

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr³⁺-doped dimethylammonium aluminium sulphate hexahydrate are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from the ferroelastic to the ferroelectric phase. The spin-Hamiltonian parameters have been determined for both phases. The spin-Hamiltonian parameters in the ferroelectric phase are:g=1.980±0.003,b ₂ ⁰ =(1140±15)·10^?4 cm^?1,b ₂ ² =(214±10)·10^?4 cm^?1. Remarkable EPR line width changes confirm the order-disorder character of the ferroelectric phase transition on a microscopic level and demonstrate that the dimethylammonium reorientation freezing-out is the prime reason for this transition.     

Powder neutron diffraction study of low temperature phase transition in LiKSO4

The salient features of the neutron diffraction measurements on polycrystalline sample of LiKSO₄ at 170 K and 104 K are reported. The sample undergoes a phase transformation to a single phase as against “mixed phases” reported in single crystal studies. These differences are discussed on the basis of the kinetics of nucleation and growth processes.     

铁磁电材料BiFeO_3低温拉曼谱的研究

BiFeO3是少数的在室温时具有磁电耦合效应的铁磁电材料之一,在信息存储、传感器和自旋电子器件等方面都有潜在的应用前景。在4-1100K,通过拉曼散射研究了BiFeO3单晶的性质,讨论了与结构相变相联系的磁有序。     

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.     

低温均匀相变的格子Boltzmann方法模拟

文中基于气液两相接触时的传质机理和两相格子Boltzmann理论模型,提出相界面的密度数处于气液相密度数之间,对低温液滴相变进行了模拟研究。在低温状态下,对单个液滴进行了均匀蒸发现象模拟,分析了不同过热度对蒸发速率的影响,并模拟运动液滴的相变。结果表明,所采用的格子Boltzmann方法能够有效分析模拟低温介质的相变。     

Effect of pressure on transition temperature of Rb3C60 fullerides

Electronic structure parameters play a significant role in fullerides leading to a superconducting state. Relevant electronic parameter as renormalized Coulomb repulsive parameter μ* and the attractive electron-phonon coupling strength λ are obtained within the dielectric function formalism for random phase approximation. As a first step, the superconducting transition temperature is deduced within the framework of McMillan approximation and strong coupling results using the widely spread phonon spectrum. In view of the importance of Coulomb screening for doped fullerides, the influence of pressure and volume on T _c are estimated to be within the range of experimental values. The isotope and dopant effects are also discussed. It is noticed that the high-T _c , the huge pressure effect, negative pressure derivative of T _c and positive volume derivative of T _c in alkali intercalated fullerides are dictated by the properties of Coulomb and on-ball-C₆₀ high energy intramolecular modes.     

The transition from the ordered to the merohedral disordered phase in oxygenated solid C60

C₆₀ powder, pellets and single crystals were oxygenated at 0.11 kbar and then studied with MAS ¹³C NMR, DSC and XRD. In agreement with reports by other authors, powder samples exhibited minor NMR resonances, indicative of blocking of fullerene rotation by interstitial oxygen, the distribution of which had yet to attain the equilibrium state. Also, as reported by others, DSC showed two endothermic peaks near 260 K, and both, we found, corresponded to a discontinuous change in the lattice parameter. The peak on the low-temperature side moved up in temperature with elapsed time. Pellets, in which presumably oxygen diffused easily, displayed only one peak but shifted in the same manner. Single crystals, in contrast, had only the higher-temperature “immobile” peak. We suggest that the shifting endotherm originates from the transition of oxygen-intercalated C₆₀ to a merohedral disordered phase, in which the rotation of fullerene molecules is partially impeded by oxygen. A phenomenological model is proposed for this transition.     

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.     

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C₆₀ single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at T _c =250–260 K: crystal strengthening is observed for T<T _c , and softening occurs for T>T _c . This indicates a change in the crystal lattice structure in the magnetic field.     

Wetting-to-nonwetting transition in metal-coated C60

Based on ab initio and density-functional theory calculations, an empirical potential is proposed to model the interaction between a fullerene molecule and many sodium atoms. This model predicts homogeneous coverage of C60 below eight Na atoms and a progressive droplet formation above this size. The effects of ionization, temperature, and external electric field indicate that the various, and apparently contradictory, experimental results can indeed be put into agreement.