The formation of a diamond layer on a carbide substrate during diamond interaction with Si,WC and Co

Abstract

A diamond layer was formed on a carbide substrate in an irregular temperature field at high pressures (HP). A gradient scheme of HP cell set-up has been developed, which provides for a simultaneous impregnation of opposite planes of a diamond layer by components that differ in melting temperature. The cell temperature field has been calculated and physico-mechanical properties of the obtained composite material have been studied.     

A XANES study of the Cu K-edge in A2CuCl4 perovskite layers under pressure. Influence of antiferrodistortive structure

Abstract

The temperature dependence of ice resistivity R has been investigated in the diamond anvil cell (DAC) of “rounded cone-plane” type. Mean pressure is about 20 GPa, temperature interval is 4,2–273 K. Measuring R it has been detected that ice changes to metallic conductivity type state between 220–260 K. At 25 K the temperature dependence of R shows the transport current dependence.     

Equations of state of MgO at high pressure and temperature

Abstract

A synchrotron X-ray diffraction study on MgO has been done at simultaneous high pressure and temperature. The lattice parameter of MgO has been measured up to a static pressure of 6 GPa and a temperature of 1273 K, using a large volume pressure cell and energy-dispersive synchrotron X-ray powder diffraction. The compression was made following six high-temperature isotherms. A Vinet equation of state was used to fit the experimental P-V-T data. The Vinet's model compares very well with the experimental data above the Debye temperature (760 K) and allows the use of MgO as an alternative internal pressure calibrant for experiments at high temperature.     

Sulphur electric properties at superhigh pressure around the polymerization temperatures

Abstract

The electrical resistivity temperature dependence of S has been studied in the diamond anvil cell (DAC) of the “rounded cone-plane” type. The pressure is about 50 GPa. The experimental data analysis shows the polymerization between 380–410 K. The electrical resistivity temperature dependence is distorted by a number of maxima, some of them being shifted along the temperature axis towards the lower temperature and disappear with increasing transport current.     

Single crystal x-ray diffraction with a synchrotron source in a mdac at low temperature

Abstract

A new diamond anvil cell and a helium flow cryostat have been developed for x-ray diffraction on single crystals at low temperatures and high pressures using the white radiation of a synchrotron beam. This novel instrument especially enables continuous change of sample temperature and pressure without any adjustment of alignment. A minimum temperature of 46 K has been reached.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’, Daresbury Laboratory 20-21 July 1991     

Pressure dependence of crystal structure and molecular packing in anthracene

Anthracene molecular crystal has been investigated up to a pressure of 10.5 GPa at room temperature using variable shape variable size Monte Carlo simulations in an isothermal–isobaric ensemble. We have reported various structural quantities, such as cell parameters and unit cell volume, as a function of pressure and compared them with the experimental results [J. Chem. Phys. 119, 1078 (2003)]. The pressure dependence of angles θ, δ and χ which describe the relative packing of molecules in the crystal has been calculated. We report that anthracene molecular crystal does not exhibit any first order phase transition up to a pressure of 10.5 GPa which is consistent with the experimental observations by Oehzelt et al. [Phys. Rev. B 66, 174104 (2002)]. The calculated equation of state (EOS) has been fitted to a Murnaghan-type EOS with good agreement. The calculated bulk modulus and the pressure derivative of bulk modulus are 8.2 GPa and 8.9 respectively which are in agreement with the experimentally calculated values.     

On the microstructure of the charge density wave observed in La1− x Ca x MnO3

We have used low temperature (90?K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La_{1? x} Ca _x MnO₃. It has been stated that Mn³⁺ and Mn⁴⁺ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5?µm in La_0.48Ca_0.52MnO₃ had a wavevector which varied on a scale of tens of nanometres with an average of ?q??=?0.450a * and a standard deviation Δq?=?0.004?a* in its magnitude and Δθ?=?0.56° in its direction at 90?K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90?K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30?K above the ‘charge ordering temperature’.     

Thermo-elastic property of Ca(OH)2 portlandite

An equation of state (EoS) for Ca(OH)₂ portlandite has been obtained through measurements of pressure and temperature dependence of volume by means of in-situ X-ray observation. The bulk modulus and its pressure derivative at zero pressure calculated using third-order Birch-Murnaghan's equation of state is 33.1 GPa and 4.2 at 300 K, respectively. The unit cell parameters and the volumes have been also determined at 573 K and 673 K. Temperature derivatives of the bulk modulus and its pressure derivative have been calculated to be ?0.022 GPa/K and 0.0072 K^?1, respectively. Thermal expansion coefficient of portlandite has been calculated from the EoS. The pressure dependence of entropy has been obtained from the present thermo-elastic parameters.     

High pressure raman study of PbMoO4

Abstract

Raman spectra of PbMoO₄ have been measured up to 31 GPa in a diamond anvil cell (DAC). Two new phases were found at 10 and 16 GPa pressures at room temperature.     

Compressibility of a zinc single crystal under confining pressure

Abstract

In the present study, the two compressibility moduli of a zinc single crystal were measured at room temperature up to 18 GPa. The results are compared with those obtained by Bridgman from a different experimental method. This study required a gaseous hydrostatic confining pressure, and then it has been necessary to develop an original diamond anvil cell with a large optical aperture allowing the use of sophisticated video imaging and image analysis techniques. A model is proposed for the zinc volume-pressure relation.     

Evolution of ( La 1 - y Pr y ) 0.7 Ca 0.3 MnO 3 crystal structure with A-cation size, temperature, and oxygen isotope substitution

The atomic structure of ( La _{1 - y} Pr _y ) _0.7 Ca _0.3 MnO ₃ compound with 0.5≤ y ≤1 has been systematically studied by neutron powder diffraction in the temperature range from 15 to 293 K. For composition with y = 0.75, the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes and the other one 75% enriched by ¹⁸ O. The room temperature structural characteristics of the series, including cell volume, average Mn-O bond distance, and average Mn-O-Mn bond angle, are the linear functions of the < r _A >. Temperature dependencies of these parameters are quite smooth, except for the point T = T _FM , where a jump like changes occur. The isotope enriched samples have been found identical in crystal and magnetic structure down to the temperature of transition of the sample with ¹⁶ O into the metallic ferromagnetic phase. It confirms that different transport and magnetic properties of the samples with ¹⁶ O and ¹⁸ O at low temperature are driven by the different oxygen atoms dynamics solely. Temperature dependencies of the CO and AFM diffraction peak intensities and of the peak widths for compositions close to the metal-insulator boundary ( y ≈ 0.75) indicate the macroscopically phase separated AFM-dielectric + FM-metallic state below T _FM . Received 28 April 2000     

High pressure cell for neutron diffraction investigations

Abstract

The design of a titanium-zirconium clamped cylinder-piston type pressure cell for neutron diffraction investigations under hydrostatic pressure up to 10 kbars without supports is described. It is the first time that Freon-11 has been used as a hydrostatic pressure transmitting medium. The following results carried out at the room temperature are presented: the discovery of the transition from I to 111 phase in a LiKSO₄ single crystal and the results of the investigation on the influence of pressure upon the structure of the YBaCuO HTSC obtained by means of the powder diffraction method.     

Temperature and pressure dependencies of the crystal structure of the organic superconductor (TMTSF) 2 ClO 4

The crystal structure of (TMTSF)₂ClO₄ has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of b ^* /2 superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV. Received 10 April 2000 and Received in final form 27 September 2000     

High pressure neutron diffraction using the paris-edinburgh cell: Experimental possibilities and future prospects

Abstract

High pressure neutron diffraction using the Paris-Edinburgh cell has attracted considerable interest ever since it has been shown that full structural data can be obtained at pressures up to 10 GPa. In this paper we will focus on the current state of this technique. Specifically, we report on new experimental possibilities concerning: i) access to “ultrahigh” pressures beyond 20 GPa, ii) experiments at variable temperatures down to 100 K, and iii) experiments on single crystals in inelastic neutron scattering. Current attempts to increase the pressure and temperature range are discussed.     

Magnetic phase transition and magnetic structure of ground state in CsDyW O

Magnetic phase transition in the CsDyW₂O₈ magnet has been studied by means of low temperature specific heat C ( T ) measurements. The magnetic ordering temperature of the Dy³⁺ sublattice was established to be 1.34 K. The experimental results indicate on the antiferromagnetic character of interactions between Dy³⁺ ions. The behavior of the C ( T ) dependencies above and below T _N is discussed in frames of different theoretical models. The measurements data on temperature and field dependencies of magnetization are used to calculate the exchange and dipole-dipole interactions energy and to determine the possible magnetic structure of the ground state. Received 7 January 2002 / Received in final form 15 May 2002 Published online 7 September 2002     

On the high-pressure phase transition in

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO₄ at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a =7.306?, b =5.887? and c =5.124?. Received: 7 October 1997 / Received in final form: 17 November 1997 / Accepted: 18 November 1997     

Melting curve of NaCl0.5Br0.5 up to 4.5 GPa

Abstract

The melting curve of NaCl_0.5Br_0.5 has been measured under pressure up to 4.5 GPa. The melting temperatures of Ag and NaCl have been used to determine the pressure in the sample at its melting temperature.     

Optical studies of the ferroelastic phase transitions in KFe(MoO4)2

ABSTRACT

The ferroelastic phase transitions in KFe(MoO₄)₂ have been studied by means of polarized light microscopy. The crystal undergoes a sequence of ferroelastic phase transitions. It has been found that the second transition consists of two transitions separated by the temperature interval of about 0.4 K. Both these transitions are of the first order and are evidenced through a phase front passing, without the domain structure rebuilding. The disposition of optical indicatrix axes n_g, n_m has been established, and the birefringence has been measured in the plane (0001) in the temperature range covering all ferroelastic phases. From temperature studies of the morphic birefringence, a critical exponent of the order parameter has been estimated.     

Laser-induced fluorescence measurements of a 3 Π metastable carbon monoxide in a high enthalpy flow

Excitation of carbon monoxide molecules has been carried out in a cold cell and in a low-pressure plasma jet using an ArF narrow-band excimer laser. The different excitation models are discussed and the relevance of atomic carbon absorption into the laser cavities is pointed out. Excitation spectra of Cameron bands have been obtained in a room-temperature cell and compared with calculated spectra. A value of the constant σ related to the interaction strength between a ³ Π( v = 2) state and its neighbouring singlet states is derived: 0≤σ≤0.05. The fluorescence spectrum following broad band excitation of CO has been observed both in UV and visible. Similar experiments carried out in a high enthalpy flow have allowed to point out the presence of a ³ Π metastable carbon monoxide. A method for relative measurements of this species concentration is proposed. Received 9 April 2001 and Received in final form 13 June 2001