Synthesis of a new potassium titanate,KTiO2(OH) by hydrothermal oxidation of Ti

Abstract

A new titanium hydroxide KTi₂O(OH) has been synthesized by hydrothermal oxidation of titanium powder in concentrated potassium hydroxide solution. A formation diagram of products synthesized in Ti-KOH-H₂O system at the region of KOH concentration from 0 to 90mol.kg-^?1-H₂O and the temperature from 150 to 350°C has been also constructed.     

The thermoelectric properties of quaternary chalcogenides

Abstract

Single phase compounds of the form 1–2₂-3–6, were fabricated using powder metallurgical techniques. Measurements were made of the thermoelectric power, Hall coefficient, electrical conductivity, microhardness and optical reflectance. The electronic properties are discussed in terms of alloy and ionized impurity scattering.     

Neutron powder diffraction studies at high pressure using a pulsed neutron source

Abstract

High-pressure neutron-diffraction experiments using the time-of-flight (TOF) method are presented. We first introduce the helium-gas pressure system at Argonne's Intense Pulsed Neutron Source (IPNS) with the Special Environment Powder Diffractometer (SEPD). The results for TIBa₂CuO₆₊₈, and La_2?x, Sr_x, CuO_4–8 are briefly reviewed. We then introduce the present situation in Japan. We have been developing a high-pressure system on high-resolution powder diffractometer (HRP) which was much improved recently and renamed VEGA at KEK. The best resolution of 0.45% was obtained at VEGA. The preliminary results for YBa₂, Cu₄O₈, and HgBa₂, CuO₄₊₈, are presented and compared with the SEPD data.     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

Synthesis of oxide ceramic powder by reaction in supercritical fluids

Abstract

The reaction of the alkoxide Mg(Al(OR)₄)₂ in supercritical fluid ethanol around 350°C and 15 MPa leads to the formation of partly crystallized submicronic powder. The dried solid particle size measured by TEM ranges from about 0.04 to 0.2 μm. The spinel MgAl₂O₄ is formed after heating the dried powder at 1200°C. Structural properties determined by X-ray diffraction and IR spectroscopy as well as thermal behaviour of these powders are compared with those of samples elaborated from sol-gel process.     

Triangular antiferromagnetic order in the honeycomb layer lattice of

ErCl₃ crystallizes in the AlCl₃-type layer structure. The crystal structure was refined in the paramagnetic state by powder neutron diffraction. The monoclinic lattice parameters at 1.5 K are a = 6.8040(3)?, b = 11.7456(5)?, c = 6.3187(3)? and . The space group is C2/m. Short-range, predominantly in-plane, magnetic ordering occurs above 350 mK up to several Kelvin. Below mK a three-dimensional antiferromagnetic order with a propagation vector of sets in. The magnetic structure of ErCl₃ was determined by powder and single-crystal neutron diffraction at temperatures down to 45 mK. The Er³⁺ ions are located on two-dimensional honeycomb layers in the a–b plane. There are two antiferromagnetically coupled triangular sublattices which form right- and left-handed helices along the c-axis. The magnetic moments are oriented in the a–b plane and amount to 3.3(1) at saturation. From the temperature dependence of the integrated neutron magnetic peak intensity a critical exponent (2) was derived for the magnetic phase transition. Received 1 December 1999 and Received in final form 21 July 2000     

A pressure induced phase transition in pyroxene

Abstract

Two monoclinic pyroxenes of composition Ca(Fe,Mg)Si₂O₆ were studied up to 10 GPa using X-ray powder diffraction and ⁵⁷Fe Mössbauerspectroscopy. The results are indicative of a phase transition at 4 GPa.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Performance of different types of detectors in a high-pressure X-ray study of phase transition

Abstract

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.     

EPR studies of highly pure,pure and doped chromium oxide powders

Abstract

EPR absorption measurements on ‘pure’, highly pure and A1₂O₃ doped Cr₂O₃ powder have been made. The EPR absorption in the ‘pure’ powders obtained below Ntel temperature is shown to be due to background magnetic impurities present in the powders and not due to superparamagnetism as suggested by earlier authors. No EPR absorption could be observed below Nkl temperature in highly pure powders (total background impurity concentration less than 5 ppm). ‘Pure’ powders or highly pure powders mixed with A1₂O₃ powder and annealed at high temperatures showed a symmetrical EPR absorption line at room temperature. The shape and the g value of this line are practically the same as those obtained for Cr³⁺ ions in Cr₂O₃ above Nee1 temperature or in other nonmagnetic crystals. It is concluded from these results that the impurities diffuse into Cr₂O₃ powder, the antiferro-magnetic coupling between some of the Cr³⁺ ions is broken and these Cr³⁺ ions become paramagnetic, even when the bulk of the material is in antiferromagnetic state. The variation of half-width of EPR lines with impurity concentration shows that the dipolar coupling between Cr³⁺ ions decreases with the increase in impurity concentration and when the impurity concentration is high the Néel temperature seems to shift to lower temperatures. A longer heat treatment of the ‘pure’ B powder resulted in the production of shining metal particles in the powder. The EPR of this powder showed excessive increase in the intensity of EPR absorption when the temperature of the powder was raised to a value just above the Néel temperature. A comparison of these reuslts with the work of earlier authors suggests that the shinning metal particles are those of chromium metal and are responsible for this increase in EPR absorption.     

CONFORMATIONAL ANALYSIS AND MOLECULAR MODELING OF CHOLESTERIC LIQUID CRYSTAL POLYESTERS BASED ON XRD,RAMAN, AND TRANSITION THERMAL ANALYSIS

Molecular modeling of the cholesteric liquid crystal polyester poly[oxy(1,2 - dodecane)oxycarbonyl-1,4-phenyleneoxycarbonyl-1,4-phenylenecarbonyloxy-1,4-phenylenecarbonyl] (PTOBDME), [C₃₄H₃₆O₈] _n , synthesized in our laboratory and thermally characterized by differential scanning calorimetry (DSC), was performed to explain both its cholesteric mesophase and 3D crystalline structure. Conformational analysis (CA) was run for the monomer both by systematic search and with molecular dynamics (MD) simulations. Minima energy conformers were “polymerized” with Cerius² and helical, cholesteric molecules were obtained in all cases. Our models agree with the chiral behavior observed by X-ray diffraction (XRD), thermooptical analysis (TOA) and circular dichroism (CD) experiments. Crystal packing of the polymer molecules were simulated in cells with parameters a and b obtained from experimental powder X-ray diffraction patterns and c calculated from the translational repetitive unit during the theoretical polymerization. Recalculated X-ray powder diffraction patterns of our models matched the observed ones. Morphology simulation from those crystal models is in good agreement with the crystals observed by optical microscopy. We have also modeled the self-associating nature of those polyesters when dispersed in aqueous media. Simulation of our models surrounded by different solvents, such as water and chloroform, were performed by calculating their interaction energies, coordination numbers, and mixing energies, applying Monte Carlo simulation techniques based on the Flory-Huggins theory. These results were compared with their experimental vibrational Fourier transform (FT)–Raman spectra in the regions in which structural marker bands of the polymer appear.     

Effect of different dielectrics on performance of sub-5.1 nm blue phosphorus Schottky barrier field-effect transistor from quantum transport simulation

Two-dimensional materials have attracted great attention because of their ultra-thin atomic layer thickness and high carrier mobility. In this work, we investigated the electronic transport of in-plane (IP) heterojunction based on Cu/Blue Phosphorus (BlueP), and the results suggest the metallization at the IP Cu/BlueP contact interface and a small Schottky barrier. Then, we investigated the performance of 5.1 nm IP BlueP Schottky barrier field-effect transistors (SBFET) with different dielectrics (SiO₂, Al₂O₃, Y₂O₃, and La₂O₃) using quantum transport simulations. The results show that IP BlueP SBFETs with four dielectrics satisfy the off-state requirement of the International Technology Roadmap for Semiconductors (ITRS) for the high-performance (HP) device. However, the on-state current of only IP BlueP SBFET with La₂O₃ satisfies the requirements of ITRS. This will provide a reference for designing BlueP SBFETs.     

Crystal structure and magnetic ordering of RNi Si compounds

The element distributions and the magnetic ordering behaviour of compounds RNi₁₀Si₂ (R = Tb, Dy, Ho, Er, Tm) have been studied by neutron powder diffraction down to temperatures of 1.6 K. The compounds crystallize in an ordered variant of the ThMn₁₂ structure type in the tetragonal space group P4/nmm. An ordered 1:1 distribution of Ni and Si on sites 4d and 4e, respectively, corresponds to a modulation vector [0, 0, 1] with respect to the space group I4/mmm of the ThMn₁₂ structure. TbNi₁₀Si₂ orders antiferromagnetically below T _N = 4.5 K with a magnetic propagation vector of [0, 0, 1/2]. The magnetic Tb moments, 8.97(2) /Tb atom at 1.6 K, are aligned along the c-axis. The Ni sites in TbNi₁₀Si₂ do not carry any ordered magnetic moments. The compounds with R = Dy, Ho, Er, and Tm are paramagnetic down to 1.6 K and 3.0 K, respectively. Received 10 July 2002 / Received in final form 12 September 2002 Published online 29 October 2002     

Phonon echo in L-alanine

Phonon echo signals have been observed in a finely-dispersed crystalline powder of the L-alanine amino acid. Measurements of the relaxation time T ₂ have revealed a phase transition in L-alanine crystals at a temperature of about 170 K. Fiz. Tverd. Tela (St. Petersburg) 40, 2119–2120 (November 1998)     

ESR evidence of a low-temperature phase transition in Na2SeO4

Abstract

ESR investigations of gamma-irradiated sodium selenate in powder form indicates a number of paramagnetic species as SeO^? ₂, SeO^? ₃, and SeO^? ₄, identified on the basis of their g-tensors. The appearance of a new ESR line at g = 2.0260 attributed to the SeO^? ₄ radical indicates a possible structural phase transition at 165 K in Na₂SeO₄.     

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa₂Cu₃O_7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andB c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.     

Magnetic and structural properties of Y2Fe15·3Si1·7 alloy under high pressure

Abstract

The pressure dependence of the crystal structure of a powder sample of Y₂Fe_15·3Si_1·7 was studied in the pressure range from 0 to 2 GPa, using the D-2 diffractometer with hydrostatic liquid pressure cell at the IVV-2M reactor (Ekaterinburg) and DN-12 time-of-flight diffractometer with a sapphire anvil cell at the IBR-2 pulsed reactor (Dubna). Also the pressure dependence of the Curie temperature T_c (P) of the alloy was studied. The correlation between the decreases of both T_c and Fe-Fe interatomic distances in “dumbbell” site 4f under pressure was obtained.     

Transmission spectra of thin CdS and CdSe crystals near higher-order isotropic points

Summary For photon energies below the absorption edge in CdS, CdSe and other II–VI crystals the polariton dispersion curves forE⊥c andE‖c (c-axis in a wurtzite crystal) corss at some points called isotropic points (IP). The occurrence of isotropic points provides the possibility of mode coupling between ordinary and extraordinary waves. Since the consequences of mode coupling on the optical properties for photon energies near the lowest IP lying much below the first excitonic state were widely discussed in recent years, more attention is now paid to isotropic points lying near the band gap and related to then=2,3, … excitonic states (?higher isotropic points?). Making use of Stahl's real density matrix approach we derive the polariton dispersion relationsk _⊥ (ω), andk _‖ (ω), for CdS and CdSe bulk crystals and determine the positions of IP's due to the crossing of theB-polariton with higherA-excitonic resonances. By the method of multiple internal reflection we calculate the transmission spectra for various crystal thicknesses (between 3 μm and 0.5 mm) and coupling mechanisms. The calculated transmission shows sharply peaked structures centred at the isotropic points.     

Orthorhombic to tetragonal phase transition in high TC superconductor YBa2Cu3O7-y and NdBa2Cu3O7-z under high pressure

Abstract

X-ray powder diffraction measurements for YBa₂Cu₃O_7-y and NdBa₂Cu₃O_7-y were made at the intense synchrotron radiation source under high pressure up to 5 GPa. These samples were wrapped tightly in platinum foil to avoid deoxidizing atmosphere. The orthorhombic to tetragonal transition temperature increases with pressure in both samples. These results are discussed on the basis of the disordering of the oxygen atoms on the chain sites.     

On the 240 K anomaly in the magnetic properties of LiNiO2

In this work, results of X-band ESR spectroscopy, ac-magnetic susceptibility and X-ray powder diffraction measurements on Li_1-xNi _1+x O₂ (x = 0.02 and x = 0.07) are presented and discussed. While the susceptibility of the compound with x = 0.02 is shown to follow a Curie-Weiss law, with a Weiss temperature of the order of 30 K, the compound with x = 0.07 is found to order ferromagnetically below K. However, an additional anomaly is observed in the magnetic properties of this latter compound at around 240 K. We attribute this anomaly to the presence of macroscopic Ni-rich regions which order ferrimagnetically below this temperature. This phenomenon is different from the bulk ferromagnetism that occurs at much lower temperatures, and allows us to discard earlier suggestions proposed in the literature in which the 240 K anomaly has been considered as denoting an intrinsic phenomenon. Received 14 May 1999 and Received in final form 5 August 1999