X-ray scattering study of the phase transitions in crystalline fullerene-biphenyl C 60 [(C 6 H 5 ) 2 ]

The phase diagram of the newly synthesized mixed crystal C₆₀-biphenyl is investigated as a function of temperature by single-crystal X-ray scattering. Diffuse scattering investigations evidencing complex disorder and local order effects are presented. Two phase transitions leading to two different doublings of the high temperature unit cell are observed, at 212 K and 147 K. The first transition is attributed to the ordering of twisted biphenyls, which couples to the orientational ordering of the C₆₀ molecules as the temperature decreases. Full ordering of the C₆₀ molecules is achieved below 100 K only, in the low temperature phase. The rich phase diagram of C₆₀-biphenyl is due to the interplay between fullerene and biphenyl ordering phenomena. Received 31 August 2001 and Received in final form 4 December 2001     

Multi-k multipole order analyzed by resonant X-ray scattering in the electric quadrupole transition

We introduce a convenient expression of the resonant X-ray scattering (RXS) amplitude, which is appropriate for the investigation of the multipole ordering phase in localized f electron system. The expression is applied to the calculation of the RXS spectra in the electric quadrupole transition around the Np M_2,3 edges from the longitudinal triple-k antiferrooctupolar ordering phase in NpO₂. The results demonstrate a possibility that analyzing the energy profile of the RXS spectrum is effective to identify the type of the order parameter.     

Analytic approach to the orientational ordering in the high pressure phase II of o-D2 and p-H2

Possible kinds of orientational order in the high pressure phase II of o-D₂ and p-H₂ are considered as bifurcations of solutions of some nonlinear equations. Before, we found this approach to be fruitful in the case of orientational ordering in o-H₂ and p-D₂. A solution with four sublattices in the hcp structure is obtained. The order parameter of this solution abruptly decreases with temperature and becomes zero in phase I. The curve of the phase boundary T_c(P) is calculated.     

Spinodal ordering in Cu3Au

Results from, X-ray diffuse scattering and inelastic neutron scattering demonstrate the presence of long-lived fluctuations in order up to 5.0°C above the first-order phase transition, T_c = 383.2°C. The temperature for continuous ordering has been determined to be 358.2°C. The coherent phase boundary is 3–13°C below T_c.     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

Spin-phonon interactions near the one-dimensional antiferromagnetic ordering in CsNiCl3

The velocity of 10 MHz and 27 GHz longitudinal phonons propagating along the c-axis in the one-dimensional antiferromagnet CsNiCl₃ has been measured using ultrasonic and Brillouin light scattering techniques, respectively. An anomalous change in phonon velocity near the one-dimensional magnetic ordering temperature due to the spin-phonon interaction has been observed in both cases.     

Increase of the paramagnetic-to-antiferromagnetic transition temperature of Cr2O3 by a magnetic field

The ordering temperature of antiferromagnetic Cr₂O₃ increases with magnetic field H, for H perpendicular to the easy axus c. This result disagree with mean-field theory, but is consistent with recent predictions. Results for H∥c (including the region near the bicritical point) are also presented.     

Raman scattering study of LiKSO4—phases II and III

Detailed Raman scattering investigation of LiKSO₄ in phases II and III across the transition temperatureT _c ? 700 K is reported. Abrupt change in frequency and line width of the external and internal modes have been observed. Analysis of the results suggests lithium positional disorder and sulphate orientational disorder in the high temperature phase (II). The results also throw some light on the existence of twin domains in the crystal.     

Ordering phenomena in C<Subscript>60</Subscript>-tetraphenylphosphonium bromide

The fulleride salt C₆₀-tetraphenylphosphonium bromide is investigated as a function of temperature by single crystal X-ray diffuse scattering and diffraction. At room temperature, the C₆₀ orientational disorder is found to be more complex than previously expected. Moreover, a structural phase transition, due to the C₆₀ orientational ordering, is evidenced around 120 K. Its relation with the stabilization of a static Jahn-Teller effect is discussed. Received 3 November 1999     

Low-frequency and long-wavelength anomalies of the integral quantum Hall effect

We present model calculations of the orientational potential of reference molecules sited at both of the two crystallographic positions of the cubic δ phase of nitrogen, displaying either spherical (2a-sites) or disclike (6c-sites) disorder. The single particle orientational potential obtained exhibits characteristic differences for the two crystallographic sites. The local energy barriers for the reorientation of the molecules were used to construct the p - T phase boundary line, first, between the δ and ? phase and, second between the phase with an almost free molecular reorientation and the one with orientational localization within the gδ phase observed in recent Raman investigations of solid nitrogen.     

On the quadrupole glass phase in solid hydrogen

A possible character of the orientational ordering in solid molecular hydrogen at T < 0.37 K and with orthoconcentrations 0.12 < c < 0.53 is discussed. On the basis of the assumption on the homogeneous arrangement of orthohydrogen molecules on the average in a parahydrogen matrix, thermodynamic properties of magnetic glass-type phases and unordered magnet are analyzed by the mean-field theory.     

The phase diagram of (NH4I)x(KI)1−x

The χ,T phase diagram of (NH₄I)_x(KI)_1?x has been determined using neutron diffraction experiments and dielectric spectroscopy. At low temperatures and with decreasing χ, the sequence γ, β, ε and glass phase has been detected. The critical concentration χ_c≈0.55 separates the glassy phase with frozen-in orientational disorder from the ε phase which reveals long-range orientational order. Close to χ_c our experiments reveal evidence for two subsequent glass transitions.     

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering

At T_c = 370 K squaric acid undergoes a phase transition, which was studied in detail by neutron scattering. The transition is essentially two-dimensional with β = 0.137 ± 0.010. Diffuse scattering above T_e is found to be anisotropic. From “diffuse crystallography” it is concluded that proton disorder is not the dominant source of diffuse scattering. Individual molecules retain their low-temperature shape in the high-temperature phase.     

Soft mode and magnetic phase transition in PrNi

The spectrum of magnetic excitations in a single crystal of intermetallic compound PrNi was studied by inelastic neutron scattering. Experiment showed the substantial softening of some collective magnetic excitation modes near the ferromagnetic ordering temperature T _c≈20 K. The result is analyzed within the framework of a model that describes the magnetic phase transition in systems with induced magnetic moment.     

Types of defect ordering in undoped and lanthanum-doped Bi2201 single crystals

Undoped and lanthanum-doped Bi2201 single crystals having a perfect average structure have been comparatively studied by x-ray diffraction. The undoped Bi2201 single crystals exhibit very narrow satellite reflections; their half-width is five to six times smaller than that of Bi2212 single crystals grown by the same technique. This narrowness indicates three-dimensional defect ordering in the former crystals. The lanthanumdoped Bi2201 single crystals with x = 0.7 and T _c = 8–10 K exhibit very broad satellite reflections consisting of two systems (modulations) misoriented with respect to each other. The modulation-vector components of these two modulations are found to be q ₁ = 0.237b* + 0.277c* and q ₂ = 0.238b* + 0.037c*. The single crystals having a perfect average structure and a homogeneous average distribution of doping lanthanum consist of 70-to 80-Å-thick layers that alternate along the c axis and have two different types of modulated superlattice. The crystals having a less perfect average structure also consist of alternating layers, but they have different lanthanum concentrations. The low value of T _c in the undoped Bi2201 single crystals (9.5 K) correlates with three-dimensional defect ordering in them, and an increase in T _c to 33 K upon lanthanum doping can be related to a thin-layer structure of these crystals and to partial substitution of lanthanum for the bismuth positions.     

Galvanomagnetic properties of atomically disordered Sr<Subscript>2</Subscript>RuO<Subscript>4</Subscript> single crystals

The effect of neutron-bombardment-induced atomic disorder on the galvanomagnetic properties of Sr₂RuO₄ single crystals has been experimentally studied in a broad range of temperatures (1.7–380 K) and magnetic fields (up to 13.6 T). The disorder leads to the appearance of negative temperature coefficients for both the in-plane electric resistivity (ρ_a) and that along the c axis (ρ_c), as well as the negative magnetoresistance Δρ, which is strongly anisotropic to the magnetic field orientation (H ∥ a and H ∥ c), with the easy magnetization direction along the c axis and a weak dependence on the probing current direction in the low-temperature region. The experimental ρ_a(T) and ρ_c(T) curves obtained for the initial and radiation-disordered samples can be described within the framework of a theoretical model with two conductivity channels. The first channel corresponds to the charge carriers with increased effective masses (～10m _e , where m _e is the electron mass) and predominantly electron-electron scattering, which leads to the quadratic temperature dependences of ρ_a and ρ_c. The second channel corresponds to the charge carriers with lower effective masses exhibiting magnetic scattering at low temperatures, which leads to the temperature dependence of the ρ_{a, c}(T) ∝ 1/T type.     

C60 molecular configurations leading to ferromagnetic exchange interactions in TDAE*C60

The charge-transfer salt tetrabis(dimethylamino)ethylene-fullerene (C₆₀), or TDAE*C₆₀, is a rare exception among pure organic crystalline systems, because it shows a transition to a ferromagnetic (FM) state with fully saturated s=1/2 molecular spins at a respectable T _c=16 K. In spite of extensive experimental and theoretical work over the past ten years, the origin of the ferromagnetism in TDAE*C₆₀ has remained a mystery. To resolve this problem, we performed a comparative structural study of two different magnetic forms of TDAE*C₆₀ crystals, one of which is magnetic and the other is nonmagnetic at low temperatures, and fully correlated their structural properties (particularly, the intermolecular orientations) with their magnetic properties. We identified the relative orientation of C₆₀ molecules along the c axis as the primary variable that controls the ferromagnetic order parameter and showed that both FM and low-temperature spin-glass-like ordering are possible in this material, depending on the orientational state of the C₆₀ molecules. Thus, we resolved the apparent contradictions intrinsic to different macroscopic measurements and opened a path to a microscopic understanding of p-electron FM exchange interactions.     

Character of the N D -D h (0, d) phase transition in diskotic liquid crystals

The first experimental data on the orientational ordering of a diskotic reentrant nematic N _D are presented. The data show that the phase transition N _D -D _h (0, d) is a strong first-order transition with a large jump ΔS≃0.2 in the orientational order parameter S of the molecules. This indicates an anomalously strong coupling between the columnar and orientational ordering of the molecules and explains the absence of fluctuational divergence of the elastic moduli K ₁₁ and K ₂₂ in the nematic phase near this transition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 1, 29–32 (10 January 1996)     

Effect of doping with iron on the charge ordering in La0.33Ca0.67Mn1−y Fe y O3 (y = 0, 0.05) manganites

The formation and specific features of the superstructure in La_0.33Ca_0.67Mn_1?y Fe _y O₃ (y = 0, 0.05) manganites doped with iron are investigated using transmission electron microscopy. The electron diffraction patterns of the manganites are studied in the temperature range 90–300 K, and the high-resolution electron microscope images recorded at temperatures of 91–92 K are analyzed. In both manganites, the structural transition that is accompanied by the formation of the superstructure and which is directly observed from the appearance of additional peaks in the electron diffraction patterns occurs at a temperature that is in close agreement with the charge ordering temperature T _CO determined from the temperature dependences of the magnetization M(T). In the temperature range 90 < T < 200 K, the undoped compound has a commensurate superstructure characterized by the vector q = 1/3a* and triple the unit cell «3a × b × c» (where a ≈ b ≈ √2a _c , c ≈ 2a _c , and a _c ～ 3.9 Å is the lattice parameter of a simple perovskite). The doping with iron (5 at. %) brings about a decrease in the charge ordering temperature T _CO by 50 K and the formation of an incommensurate structure for which the magnitude of the vector q is smaller by approximately 15%. The unit cell of the superstructure in the iron-doped compound is not triple the unit cell but involves defects of ordering, such as quadrupling of the unit cell, numerous translations by a _c √2 along the a direction, and dislocation-type defects in the stripe structure of the charge ordering. These pseudoperiodic defects lead to a decrease in the magnitude of the vector q and are responsible for the incommensurability of the structure. A decrease in the charge ordering temperature T _CO due to the doping with iron and the incommensurability of the superstructure correlate with the change in the concentration of Mn³⁺ Jahn-Teller ions as a result of their replacement by Fe³⁺ non-Jahn-Teller ions.     

Fluctuations of orientational order in a uniaxial nematic liquid crystal with biaxial molecules and its response to an external field

Homogeneous thermal fluctuations of the orientational order parameters S and G of biaxial molecules in a uniaxial nematic liquid crystal are investigated in the framework of the molecular-statistical theory. It is demonstrated that the molecular biaxiality significantly affects the order parameters S and G, their temperature dependences in the nematic phase, the amplitude and the temperature dependence of the order parameter fluctuations in the nematic and isotropic phases, and the character of the transition from the nematic phase to the isotropic liquid phase. It is established that the fluctuations of the parameters S and G in the nematic phase are related to the temperature dependences of S and G and the susceptibilities χ_S and χ_G of the nematic liquid crystal to external fields, which leads to a change in the parameters S and G at a fixed director orientation. Explanations are offered for the known experimental data on the orientational ordering of biaxial molecules under the action of external fields in the isotropic phase of nematic liquid crystals.