Physical properties of superhard and ultrahard fullerites created from solid C60 by high-pressure-high-temperature treatment

60 in the pressure range and the temperature range were studied. These properties are the specific gravity, specific heat in the range , sound velocities, elastic moduli, electrical properties, resistance to uniaxial stress and stability against oxidation. These properties are different from those of diamond and other carbon forms. The hardness of ultrahard fullerites exceed the hardness of diamond. The structures of the samples are described on the basis of X-ray data. Received: 27 September 1996/Accepted: 19 December 1996     

Antiphase domain growth in BCC metallic alloys via vacancies

We study by Monte Carlo simulation the influence of vacancies on the growth of antiphase domains after a quench of a BCC binary alloy. We find a powerlaw regime (R(t)t ^x withx depending on temperature and larger than the Allen-Cahn value 0.5. We have verified the scaling of the structure factor and performed a finite size analysis of the excess energy evolution and vacancy diffusion.     

New types of short-range orientational ordering in ortho-para hydrogen monolayers

Orientational ordering in monolayers of solid hydrogen is discussed in view of recent experimental findings in NMR studies of (ortho)_c-(para)_1−c-hydrogen mixtures on boron nitride substrate reported for ortho-H₂ concentrations 0.35≤c≤0.92 and temperatures 0.14≤T≤1.80 K. Analysis of the temperature-concentration behavior for the observed NMR frequency splitting is given on the basis of a two-dimension (J=1)_c-(J=0)_1−c-rotor model for which the fundamental parameters, quadrupolar coupling constant Γ₀=0.50±0.03 K and the crystal-field amplitude V₀=0.70±0.10 K, are derived from experiment. The two distinct para-rotational (PR) short-range ordered structures driven by positive and negative crystalline fields are described in terms of the local alignment and orientation of the polar principal axis. It is shown that they are rather different from the ferromagnetic-type ordering suggested earlier by Harris and Berlinsky as a unique PR structure. The common quadrupolar glass and a new hindered rotor phases found below the 2D site percolation threshold c_p=0.72 are also discussed.     

NMR and magnetic studies on 7 K anomaly in Tl3H(SO4)2

Measurements of the spin-lattice relaxation time, NMR absorption line and magnetization have been carried out on the Tl₃H(SO₄)₂ crystal below 50 K. The anomaly at around 7 K was: (1) the spin-lattice relaxation times of ¹H and ²⁰⁵Tl nuclei increase steeply with decreasing temperature below 7 K, (2) the NMR absorption lines below 7 K shift to the high-magnetic field side in comparison with that above 7 K, and (3) the ¹H NMR line width exhibits a drastic increase of the line width with decreasing temperature below 7 K. These results indicate that the magnetic dipole fluctuation of the proton changes at 7 K. On the other hand, there are no remarkable anomalies of magnetic susceptibility at around 7 K. From these results it is deduced that the anomaly at around 7 K is caused by the change in quantum mechanical process of the proton from proton tunneling to zero-point vibration of hydrogen in the hydrogen bond with the decrease of temperature.     

Preparation and crystal structure of C60S16

A new fullerene-containing van der Waals-compound, C₆₀S₁₆, has been synthesized and its crystal structure determined from single-crystal X-ray diffraction data collected at room temperature. The compound is monoclinic, space group C 2/c with a=20.867(4) Å, b=21.062(4) Å, c=10.508(2) Å, =111.25(7)° and four formula units per cell. The C₆₀ molecules form a three-dimensional framework with one-dimensional channels along c which contain crown-shaped S₈ rings. The structure has been determined by direct methods and has been refined to atomic resolution on the C₆₀ molecule. The two independent C-C bond distances, averaged under the idealized point symmetry of the free C₆₀ molecule, amount to 1.340(8) Å and 1.448(8) Å, corresponding to a bond alternation as large as 0.108(8) Å. The bond lengths are compared with the results of theoretical calculations of the molecular structure of C₆₀ as well as with bond lengths from various experimental sources.     

The effect of a strain induced field on the dielectric property of a ferroelectric bilayer system

The Transverse Ising Model (TIM) based on the effective-field theory has been developed to study the physical properties of the ferroelectric bilayer system BaTiO₃/SrTiO₃ (BTO/STO), based on the differential operator technique. The effect of strain on the interfacial layers between two different slabs (A and B) can be described by the effective built-in field E₂. The ferroelectric behavior of a bilayer system is strongly influenced by strain and associated with slab thickness. The phase transition temperature shifts toward a higher value on increasing the slab thickness. The susceptibility strongly depends on both the strength of strain and the slab thickness. The height of the peak from the plot of susceptibility against temperature decreases on increasing the slab thickness. The pyroelectric coefficient changes into a round peak at the transition temperature that is different from the sharp peak in the absence of external and strain-induced fields.     

Thermodynamic studies of successive phase transitions in BaZnGeO4 and a new solid phase below 186.1 K

The successive phase transitions of BaZnGeO₄ have been studied on meltsolidified samples. A new solid phase (named phase VI) has been found below 186.1 K in samples of large particle size (diameter:D0.1 mm). The higher temperature crystalline phase V can be supercooled easily down to liquid helium temperature. On heating, however, it transforms into phase VI above 95 K in a slow exothermic process. Heat capacities have been measured by adiabatic calorimetry between 14 and 300 K. The enthalpy and entropy of the V–VI phase transition are 187.1 Jmol^–1 and 0.971 J K^–1 mol^–1, respectively. The corresponding data for the IV–V phase transition at 199.8 K are 229.3 J mol^–1 and 1.168 JK^–1 mol^–1. The phase VI does not appear in samples of smaller particle size (D0.1 mm).     

Hetero-Epitaxial Diamond Single Crystal Growth on Surface of cBN Single Crystals at High Pressure and High Temperature

We report a new diamond synthesis process in which cubic boron nitride single crystals are used as seeds, FesoNi20 alloy powder is used as catalyst/solvent and natural flake-like graphite is used as the carbon source. The samples are investigated using laser Raman spectra and x-ray diffraction （XRD）. Morphology of the sample is observed by a scanning electron microscope （SEM）. Based on the measurement results, we conclude that diamond single crystals have grown on the cBN crystal seeds under the conditions of high temperature 1230℃ and high pressure 4.8 GPa. This work provides an original method for synthesis of high quality hereto-semiconductor with cBN and diamond single crystals, and paves the way for future development.     

Critical Exponents for the Re-entrant Phase Transitions in the Three-Dimensional Blume-Emery-Grifilths Model on the Cellular Automaton

The critical behaviour of the three-dimensional Blume-Emery-Griffiths （BEG） model ＆ investigated at D/ J = O, -0.25 and -1 in the range of-1 ≤ K/J ≤ 0 for J -- 100. The simulations are carried out on a simple cubic lattice using the heating algorithm improved from the Creutz cellular automaton （CCA ） under periodic boundary conditions. The universality of the model are obtained for re-entrant and double re-entrant phase transitions which occur at certain D/J and K/J parameters, with J and K representing the nearest-neighbour bilinear and biquadratic interactions, and D being the single-ion anisotropy parameter. The values of static critical exponents β, γ and v are estimated within the framework of the finite-size scaling theory. The results are compatible with the universal Ising critical behaviour for all continuous phase transitions in these ranges.     

Percolation properties of the antiferromagnetic Blume-Capel model in the presence of a magnetic field

The problem of order-order and order-disorder transitions in the system described by the 2D antiferromagnetic Blume-Capel model in the presence of a magnetic field is studied by the Wang and Landau flat-histogram simulation method and by the classical Monte Carlo. Anomalous thermodynamic characteristics in low temperatures indicate different type orderings in finite temperatures. The existence of pure antiferromagnetic phases as well as mixed state is shown by detailed phenomenological analysis of the system. The border lines on the phase diagram between various orderings are determined by the complementary microscopic study of the percolation problem for c(2×2) elementary structures of antiferromagnetic ordered phases. This new approach has also shown a full agreement between the percolation threshold for the cluster of mixed phase and the critical temperature of the ordered system.     

Magnetic and martensitic transitions in Ni-Fe-Ga alloy

The ferromagnetic shape memory alloy with nominal composition Ni₅₄Fe₁₉Ga₂₇ is investigated by Ac susceptibility and resistivity measurements. The alloy shows long-range ferromagnetic order below 290 K. The anomaly due to the martensitic transition is observed in the susceptibility and resistivity data in the temperature range around 220 K, which is associated with clear thermal hysteresis. Minor hysteresis loop technique was used to investigate the phase coexistence across the martensitic transition, and our analysis indicate that both martensite and austenite phases mutually coexist in the region of hysteresis.     

Finite-Size Scaling Analysis of a Three-Dimensional Blume--Capel Model in the Presence of External Magnetic Field

The Blume-Capel model in the presence of external magnetic field H has been simulated using a cellular automaton algorithm improved from the Creutz cellular automaton in three-dimension lattice. The field critical exponent 5 is estimated using the power law relations and the finite size scaling functions for the magnetization and the susceptibility in the range -0.1≤ h = H/J ≤0. The estimated value of the field critical exponent 5 is in good agreement with the universal value （δ = 5） in three dimensions. The simulations are carried out on a simple cubic lattice under periodic boundary conditions.     

Stability conditions for fermionic Ising spin-glass models in the presence of a transverse field

The stability of a spin-glass (SG) phase is analyzed in detail for a fermionic Ising SG (FISG) model in the presence of a magnetic transverse field Γ. The fermionic path integral formalism, replica method and static approach have been used to obtain the thermodynamic potential within one step replica symmetry breaking ansatz. The replica symmetry (RS) results show that the SG phase is always unstable against the replicon. Moreover, the two other eigenvalues λ_± of the Hessian matrix (related to the diagonal elements of the replica matrix) can indicate an additional instability to the SG phase, which enhances when Γ is increased. Therefore, this result suggests that the study of the replicon cannot be enough to guarantee the RS stability in the present quantum FISG model, especially near the quantum critical point. In particular, the FISG model allows changing the occupation number of sites, so one can get a first order transition when the chemical potential exceeds a certain value. In this region, the replicon and the λ_± indicate instability problems for the SG solution close to all ranges of a first order boundary.     

Undercooling and metastable phase formation in a Bi95Sb5 melt

High undercoolings have been obtained in bulk Bi₉₅Sb₅ alloy melts by the cyclic superheating and cooling technology. The highest undercooling that was achieved in this paper is 121 K. The influence of various processing factors on the undercooling behavior is examined. Undercooling of 121 K leads to the formation of a metastable solid phase with the tetragonal crystal structure. The phase selection and the metastable phase formation have been discussed based on the classical nucleation theory. A criterion that contains the relative melting temperature, the relative molar volume of the solid, the relative structure-dependent factor, and the undercooling has been developed to interpret the formation of the metastable tetragonal phase. Received: 12 January 2000 / Accepted: 28 March 2000 / Published online: 13 July 2000     

Kinetic and thermodynamic investigation of cBN formation in the system BN-Mg3N2

3 N₂BN system are presented and discussed. In particular, a temperature region slightly above the lower limit of the formation region characterized by fast transformation processes and submicron cBN grains strongly agglomerated is investigated in more detail concerning thermodynamics and kinetics. The catalyst/solvent active in that temperature interval is Mg₃BN₃ previously formed at about 5.5 G Pa and 1170 K in a solid state reaction. The cBN formation proceeds as precipitation from a eutectic melt formed by the intermediate compound and BN. These results are used to improve the previously proposed phase diagram for the Mg₃N₂BN system. The kinetics is discussed based on the usual Avrami theory. It follows that the processes may be described by nearly instantaneous nucleation at the beginning of the reaction with large rate. Growth processes are soon inhibited due to strong impingement of the formed nuclei and grains so that an Avrami exponent n in the neighbourhood of 1 results. This exponent, as well as the rate constant k summarizing the physical mechanisms of nucleation and growth, show a temperature dependence. Received: 6 January 1997/Accepted: 16 May 1997     

Crystal and electronic structure of solid C76

The crystal and electronic structure of solidC ₇₆ has been studied using transmission electron microscopy and electron energy-loss spectroscopy in transmission. C₇₆ forms a close packed structure with an average facecentred cubic symmetry. From valence band and core electron excitations information on the dielectric function and the unoccupied density of states has been obtained.     

The potts glass in uniform and random fields: a monte carlo investigation

As a simple model of an anisotropic orientational glass with short range forces, the 3-state Potts model on the simple cubic lattice with nearest neighbor interactions drawn from a Gaussian distribution is considered. With Monte Carlo methods we study the response of the system to a uniform “field” which favors one of the states. This is motivated by experiments which apply stress that favors one molecular orientation of the quadrupolar glass. The responsem to that fieldh=H/k _BT is analyzed in terms of an expansionm= χ₁ h+χ₁ h ²+χ₁ h ³+..., where χ₁ is the linear susceptibility, and χ₂,χ₁₃ are nonlinear susceptibilities. Unlike the case of spin glasses, where the spin inversion symmetry of the system in the absence of fields implies χ₂≡0,χ₂ is nonzero here and diverges to −∞ at the zero temperature transition of the model, while χ₃ diverges to +∞ as in spin glasses. At inifinite temperature, however, χ₁=1/3, χ₂=1/18 and χ₃=-1/54, i.e. the nonlinear susceptibilities have a different sign as at low temperature. In contrast, a random field does not induce a uniform order parameterm but only a glass order parameterq. The temperature dependence of this glass order parameterq(T) shows for intermediate field strength order parameterq(T) shows for intermediate field strength a maximum of the slopedq(T)/dT very similar to corresponding experiments.     

Raman spectra of single-crystal C60

We have analysed the Raman spectra of C₆₀ single crystals between room temperature and 10K and studied the temperature-induced phase transition in this material. The spectra show crystal field splitting of the internal Raman modes but no evidence for a line shift near the phase transition. The photo-induced transformation of the crystals and its implication on the interpretation of the Raman spectra is discussed. In the low temperature phase we observed two lines at 30 cm^–1 and 41 cm^–1 which we assign to the librational modes of the crystal.     

Order disorder transitions in Ising models in transverse fields with second neighbour interactions

The (ferromagnetic) order-disorder transitions in a class of Ising models with second neighbour interaction in transverse fields is studied using the path integral method. Within the limitations of the method, the critical fields at zero temperature are estimated for different systems.