Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Infrared spectra of solid germane

An IR spectroscopic study has shown that GeH₄ molecules are situated on one set of sites of C, symmetry in phase IV. The III–IV transition at 63 K is apparently a second-order phase transition. In phases II and III the molecules are on sites of C_3v. or C₃ symmetry. The II–III phase transition was observed at 67 K. In solid GeD₄, phase transitions were observed at 68.5 and 77 K. In phases II and III the site symmetry is C_I. The II–III phase transition in GeD₄ is apparently second order. There is evidence that the ν₁, vibration of GeD₄ is IR active in the solid state.     

Phase transition in C60-2-thioxo-l, 3dithiole cycloadduct

Quadratic electrooptic coefficient (QEOC) measurements have been made for fullerene C₆₀) and its derivative C₆₀-2-thioxo-l,3-dithiole. An anomaly in C_p has been revealed at about — 60°C that corresponds to the appearance of a structural phase transition. Correlation with a temperature anomaly in the QEOC is revealed. This leads to the conclusion that the QEOC for fullerene depends mainly on vibrational and rotational contributions. The contribution arising from charge distribution asymmetry is most important in the fullerene derivative. The observed temperature anomalies of the QEOC result in the possibility of using the QEOC as a sensitive tool for observing low-temperature phase transitions in the fullerene derivatives.     

Investigations of a quasi two-dimensional Heisenberg antiferromagnetic system. Nondeuterated and deuterated alkyl-ammonium-tetrachloromanganate

By neutron diffraction and susceptibility measurements the crystallographic and magnetic structures of (CH₃NH₃)₂MnCl₄ and (CD₃ND₃)₂MnCl₄ have been investigated. These compounds belong to a system of quasi two-dimensional Heisenberg antiferromagnets. At 96K a structural first order phase transition from the tetragonal high-temperature structure to an orthorhombic low-temperature structure was found. A magnetic phase transition from a two-dimensional antiferromagnetic preorder to a three-dimensional magnetic order occurs at 44.5K. The three-dimensional magnetic structure is characterized by antiferromagnetic (MnCl₄)^2- — layers perpendicular to the c-axis, with a ferromagnetic coupling between interacting next nearest (MnCl₄)^2- layers. The magnetic moments of the Mn-ions lie in the antiferromagnetic planes.     

Acoustophosphorescence double resonance in impurity C70 fullerene crystal

An acoustophosphorescent double resonance is investigated in terms of a low-temperature spin-phonon interaction mechanism determined by a coupling of translational and rotational molecular motions. Numerical values are obtained for the transition times of direct single-phonon processes in high, as well as zero, magnetic fields for a triplet excited isotopic impurity in crystalline C₇₀ fullerene. The dependence of the transition rate on the propagation direction of the acoustic wave is analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 39, 1122–1124 (June 1997)     

A molecular dynamics study of carbon dioxide in water: diffusion,structure and thermodynamics

Atomistic simulations are reported of a model of CO₂ in water. CO₂ is modelled by partial charges and Lennard-Jones interaction sites on each atom; the SPC/E model for water is used. Good agreement with experiment is found for the translational diffusion constants. The variation of the dynamics with the potential parameter was investigated. As expected, the orientational correlation times increase as the magnitude of the quadrupole moment is increased, but the translational diffusion constants are found to be surprisingly insensitive to the magnitude of the CO₂ quadrupole moment. The translational friction coefficient was resolved into electrostatic, Lennard-Jones and cross-terms; the Lennard-Jones contribution is found to be the largest. Varying the Lennard-Jones size parameter affects both translational and reorientational motion. In order to try to understand these results further, the variation of solvation free energy was investigated and the solvent structure around carbon dioxide was examined as the electrostatic and Lennard-Jones parameters were changed. The temperature dependence of the self-diffusion constant of pure SPC/E water was determined.     

Raman scattering and X-ray diffraction study of structural phase transitions in the perovskite-type layer compound (C3H7NH3)2CdCl4

The structural phase transitions of (C₃H₇NH₃)₂CdCl₄ (PACC) have been studied by means of Raman scattering. X-ray diffraction and DSC measurements It is shown that the order-disorder phase transition Abma ? Pbca occurs at 156 K and not at 183 K as previously proposed by Chapuis (Acta CrystB34, 1506 (1978)) Apparently, the transition at 183 K does not change the Abma space group; it is suggested that it could be related to the occurrence of an incommensurate phase Another structural transformation of PACC is detected at 114 K, but no conclusion can be given as far as the structure of the low-temperature phase is not determined.     

Relaxation contribution to shear moduli of the low-temperature phase of solid C60

The relaxation contribution of the molecular reorientation in an elastic field of an acoustic wave to the effective shear moduli is calculated in the framework of the phenomenological two-level model of orientational states in the low-temperature phase of solid C₆₀. The polarity of the rotation axis of C₆₀ molecules and the possible existence of orientational domains in the structure of the low-temperature phase are taken into consideration. The estimates obtained are compared with the available experimental data.     

C60单晶生长及特性的研究

用气相生长法生长了最大线度约为１．５ｍｍ的C₆₀单晶。单晶Ｘ射线衍射分析结果表明，该C₆₀单晶为ｆｃｃ结构，晶格常数为１．４１３ｎｍ，在２５１Ｋ附近与相变有关的电导率的尖锐跳变表明，C₆₀单晶具有很高的纯度和良好的均匀性。观察到在２７４Ｋ附近激活能的变化，这可能是另一个相变的反映。 关键词：     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

The influence of oxygen deficiency on the magnetic and electric properties of La<Subscript>0.70</Subscript>Ba<Subscript>0.30</Subscript>MnO<Subscript>3 −γ</Subscript> (0≤γ≤0.30) manganite with a perovskite structure

The crystal structure and the magnetic and electric properties of La_0.70Ba_0.30MnO_{3 ? γ} manganite (0≤γ≤0.30) with a perovskite structure were studied experimentally depending on the concentration of oxygen vacancies. The stoichiometric La_0.70Ba_0.30MnO₃ compound (γ = 0) had cubic unit cell symmetry, which did not change as oxygen deficiency increased up to γ=0.30. A decrease in the content of oxygen in the compound under study caused the occurrence of several sequential magnetic phase transitions in the ground state, from the ferromagnetic state at γ=0 through the cluster spin glass state (γ=0.15) to the antiferromagnetic state (γ=0.30) with the presence of a small ferromagnetic component. The specific electric resistance grew to become activation in character at γ=0.11, and the metal-semiconductor transition disappeared as oxygen deficiency increased. The magnetoresistance of anion-deficient compositions included (1) magnetoresistance close to the temperature of the transition to the magnetically ordered state and (2) low-temperature magnetoresistance. The magnetoresistance peak at T_C disappeared as γ increased (γ=0.11), whereas the low-temperature magnetoresistance component first increased to attain a maximum of about 34% at γ=0.15 and then decreased. The results of experimental studies were used to construct a magnetic phase diagram. These results could be interpreted within the framework of superexchange magnetic ordering theory. The suggestion was made that Mn³⁺-O-Mn³⁺ indirect exchange interactions were positive in the orbitally disordered phase only when manganese was in octahedral coordination, whereas these interactions became negative if at least one of the Mn³⁺ ions was five-coordinate or had a smaller coordination number.     

Complete pentagon orientational ordering in C60 fullerite charged with NO

Solid C₆₀ was stored in NO under high pressure, and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C NMR MAS spectra are composed of a primary resonance at 143.7 ppm accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase transition temperature at 260 K in pure C₆₀ and to substantially reduce the drop Δ?′ at the phase transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P-orientation below T _c was calculated to be larger than 11/12. These results show that a completely P-oriented phase occurred in (NO)_0.1C₆₀ and that this phase is favored by a negative pressure on the C₆₀ lattice exerted by NO, as well as by the electrostatic interaction between the two.     

Phase transformations in Pr1–x Sr x CoO3

The elastic properties and crystal structure of the Pr_1?x Sr _x CoO₃ system are studied. Two types of crystal structure transitions are found. For the composition x = 0.5, the monoclinic phase transforms to a rhombohedral one in the high-temperature transition (T ≈ 310 K), while the unit cell symmetry remains monoclinic though the unit cell parameters change drastically in the low-temperature transformation (T ≈ 110 K). It is suggested that the high-temperature transition is caused by the dimensional effect, while the low-temperature transition is associated with the presence of praseodymium ions actively involved in chemical bonding.     

Structure, dielectric and magnetodielectric properties of Bi1-xGdxFeO3 Ceramics

Different magnetodielectric effects were observed in Bi1-xGdxFeO3 ceramics depending on gadolinium content. A positive one was observed in the samples with x ≤0.10 at 295K and 16K, and a negative one in the sample with x = 0.4 at 16 K. Structure analysis by x-ray diffraction (XRD) reveals that the samples crystallize in the R3c structure (ferroelectrics) for x<0.08 and in the Pbnm structure (paraelectrics) for x≥0.3 at room temperature. Temperature-dependent dielectric response and x-ray diffraction confirm the occurrence of a structural transition in the Pbnm phase at low temperature for the samples with x ≤0.4. While the positive magnetodielectric effects can be attributed to a coupling of magnetic and crystallographic structures of the R3c phase, the observed negative magnetodielectric effect in the Pbnm phase can be associated with a low-temperature modification of the Pbnm structure. The observed dualsigned magnetodielectric effects suggest that the Bi1-xGdxFeO3 oxides are a good prototype for understanding the magnetodielectric coupling mechanism in this kind of materials.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

An i.r. study of heavy phosphine in the solid state

The near i.r. spectra of PD₃ and of PHD₂; isolated as a point defect in PD₃ have been obtained. Analysis of these spectra indicates that PD₃ exists in at least two crystalline forms. In the low-temperature form the molecules probably occupy two sets of sites of C_s. symmetry. A first-order phase transition accompanied by hysteresis occurs at 50 K and 60 K. This is most probably an order-disorder transition, because the spectrum of the high-temperature phase is that of a disordered crystal.     

A weakly coupled magnetic sublattice in NaMnFeF6, observed by Mössbauer spectroscopy and magnetic measurements

The thermal variations of the hyperfine fields of the two iron sites (1a, 2d) of NaMnFeF₆ (S.G.P321) indicate a weak magnetic coupling of the (2d) sublattice to the net magnetization, and suggest a magnetic phase transition near 32 K. The Mössbauer and magnetization data are consistent with a ferrimagnetic structure (T_N=44.6 ± 0.5 K) with low-temperature compensation point.     

Ion Energy Distribution in a Radiofrequency C4F8

The dependence of ion energy distribution (IED) of planar radiofrequency driven inductively coupled plasma source on pressure and power is analyzed by using plasma monitor. The transition from capacitive coupling (E mode) to inductive coupling (H mode) is observed between 100 and 200 W for C₄F₈ gas pressure in the range from 0.8 to 8 Pa. In the H mode, the concentration of light ions is higher due to an increase in dissociation and ionization rate of C₄F₈. In the E mode, the IED consists of large peak (round 20 eV) with an appearing saddle structure (0.8 P), whereas at higher pressure (8 P), the IED exhibits an important contribution near zero energy, indicating collisional sheath. In the H mode, in most cases, the IED consists of one peak which is narrower for higher mass.