Showing papers on "Orthorhombic crystal system published in 1986"

In situ determination of crystal structure for high pressure phase of ZrO 2 using a diamond anvil and single crystal X-ray diffraction method

Abstract: Combining a miniature diamond-anvil pressure cell with a single crystal four-circle diffractometer, the crystal structure of a synthetic ZrO2 has been studied in situ up to 51 kbar at room temperature. The space group of the unquenchable orthorhombic high pressure phase is Pbcm. The directions of the b and c axes are preserved through the transition and the transformation is displacive. The coordination configurations of the Zr atoms and oxygen atoms are the same in the high pressure and low pressure phases. The orthorhombic high pressure phase has a higher entropy than that of low pressure monoclinic phase.

The growth of large single crystals of lysozyme.

Abstract: If protein single crystals larger than those suitable for x-ray analysis are obtained, various spectroscopic, thermal, mechanical, and electrical measurements become possible. To understand the factors governing the crystal size, tetragonal lysozyme crystals were grown in batches at 15°C from solutions of different protein and salt concentrations between pH 4–7. The number and size of the crystals, and the protein concentration remaining in the supernatant, varied markedly with the initial salt amount, pH, and cation species, but large crystals always grew when the initial protein concentration (P) was in a narrow range of 2.5–3 times the crystal solubility (S). It was also shown (1) that the period before the first crystals appeared (D) varied as D ∝ (P/S)−n, where n ≃ 5, and (2) that many previous experiments used more supersaturated solutions than the optimal ones thus determined. The reason why large crystals grow only from moderately supersaturated solutions is discussed. The crystal size of the orthorhombic form grown at 40°C was less sensitive to pH and P than the tetragonal form. An effort to measure D and the solubility at 40°C revealed many differences between the two crystal forms, which we ascribe to different interactions to promote crystallization.

Phase transitions in langbeinites I: Crystal chemistry and structures of K-double sulfates of the langbeinite type M2+ + K2(SO4)3, M++ = Mg, Ni, Co, Zn, Ca

Abstract: The crystal structures of the langbeinite type M 2 + + K2(SO4)3 with M+ +=Mg, Ni, Co, Zn, Ca in their cubic phase (P 213) and Ca2K2(SO4)3 in its orthorhombic phase (P 212121) are determined. Whereas the SO4-tetrahedra in these compounds are almost undistorted, the two symmetry-independent coordination polyhedra of M+ + are highly distorted octahedra with trigonal site symmetry in P 213. The deformation of the oxygen octahedra and the off-centering of M+ + along the trigonal axis show systematic dependences on the ionic radii and the electronegativities of the M+ +-ions. The correlations are remarkably different for the two symmetry-independent M+ +-ions indicating different M+ + — O bonding.

A NEW MOLECULAR SUPERCONDUCTOR, (BEDT-TTF)2(I3)1−x(AuI2)x(x<0.02)

Abstract: A new molecular superconductor based on BEDT-TTF molecule was prepared electrochemically by using the mixed supporting electrolyte, (n-C4H9)4NI3 and (n-C4H8)4NAuI2. The superconducting transition takes place at ambient pressure at 3.6 K. X-Ray diffraction patterns show that the system can be approximately assigned to a pseudo orthorhombic one with the lattice constants of a=10.076(2), b=33.853(5), c=4.964(1) A.

Kossel Diagrams Show Electric-Field-Induced Cubic-Tetragonal Structural Transition in Frustrated Liquid-Crystal Blue Phases

Abstract: Convergent beam diffraction patterns from a bcc blue-phase (I) monocrystal in an electric field applied in the [110] direction show this twofold axis transforming through an orthorhombic distortion to become, at a critical field, the fourfold axis of a tetragonal crystal. The implication on structures of blue phase I is important. Although the transition is weakly first order because the two order parameters describing it are coupled, the diffraction pattern changes continuously through the transition.

Ternary carbides of the rare earth and iron group metals with CeCoC2- and CeNiC2-type structure

Abstract: 32 carbides RTC2 (R ≡ rare earth metal, T ≡ Fe, Co, Ni), 18 for the first time, were prepared by arc melting of the elemental components and subsequent annealing in evacuated silica tubes. Most of them crystallize in the orthorhombic CeNiC2-type structure. The carbides LnCoC2 (Ln ≡ La, Ce, Pr, Nd) have the related monoclinic CeCoC2-type structure. SmCoC2 is of CeCoC2 type at low temperature and of CeNiC2 type at high temperature. The crystal structures of DyCoC2 and DyNiC2 were refined from singlecrystal X-ray data to residuals of R = 0.012 (581 structure factors) and R = 0.023 (971 structure factors) for 17 variable parameters. The carbon atoms in these compounds form pairs. The C-C distances of 1.37 A come close to the double bond distances of 1.34 A in alkenes. The hydrolysis of DyCoC2 in diluted hydrochloric acid, however, yields only little ethene, mainly ethane and some propane and methane. The cell volumes indicate that cerium and ytterbium are essentially trivalent in these compounds, whereas the corresponding europium compounds cannot be prepared under similar conditions.

Magnetic Properties of Intermetallic Compound Mn11Ge8

Abstract: Magnetic properties of an orthorhombic intermetallic compound Mn 11 Ge 8 , which was formerly referred to as Mn 3 Ge 2 , has been investigated on a single crystal specimen at temperatures between 80 K and 800 K. The compound has features of a non-collinear antiferromagnet below T t = 150 K, and makes a spin-flopping transition when magnetized along the c axis. It becomes ferromagnetic with a first order transition at T t with the direction of easy magnetization lying along b axis up to its Curie temperature 274 K. The susceptibility follows the Curie-Weiss law above 550 K, with asymptotic Curie temperature 329 K and effective Bohr magneton number per Mn P eff = 3.26.

Crystal structures of the three forms of CsZnPO4

Abstract: Structural investigation of cesium zinc monophosphate CsZnPO4 has been carried out at three different temperatures. At room temperature, this salt is monoclinic, P21/a with a = 18.33(8), b = 5.45(4), c = 9.25(6) A, β. = 90.14(8)° and Z = 8 (form III):at 260°C, it becomes orthorhombic, Pn21a with a = 9.246(9), b = 5.468(6), c = 9.359(7) A and Z = 4 (form II),at 310°C, it is orthorhombic Pnma with a = 9.191(5), b = 5.493(5), c = 9.406(5) A = Z = 4 (form I). Crystal structures of these three forms have been solved and refined with final R values: R = .043 (at room temperature) for 2914 independent reflections, R = .041 (at T = 280°C) for 735 independent reflections, R = .042 (at T = 340°C) for 644 independent reflections.

Raman spectra of orthorhombic sulfur at 40 K

Abstract: High-quality Raman spectra of orthorhombic sulfur (S,) have been recorded at 40 K employing multiscanning conditions. Assignments are proposed for the many new features detected, especially binary and ternary overtones and combinations of the SS stretching and S-S-S bending vibrations. The anharmonicities of these particular modes are shown to be fairly small.

High-pressure crystal chemistry of phenakite (Be2SiO4) and bertrandite (Be4Si2O7(OH)2)

Abstract: Compressibilities and high-pressure crystal structures have been determined by X-ray methods at several pressures for phenakite and bertrandite. Phenakite (hexagonal, space group R\(\bar 3\)) has nearly isotropic compressibility with β=1.60±0.03×10−4 kbar−1 and β=1.45±0.07×10−4 kbar−1. The bulk modulus and its pressure derivative, based on a second-order Birch-Murnaghan equation of state, are 2.01±0.08 Mbar and 2±4, respectively. Bertrandite (orthorhombic, space group Cmc21) has anisotropic compression, with β a =3.61±0.08, β b =5.78±0.13 and β c =3.19±0.01 (all ×10−4 kbar−1). The bulk modulus and its pressure derivative are calculated to be 0.70±0.03 Mbar and 5.3±1.5, respectively.

Low-temperature structure of Mg2NiH4: Evidence for microtwinning

Abstract: v-ray powder diffraction patterns of the monoclinic low-temperature phase of Mg 2 NiH 4 were analysed in terms of a structure model which allowed for microtwinning. In contrast to previous studies, the intensity profiles of reflections that were broadened due to twinning and of reflections that had been attributed to a second, orthorhombic phase could be reproduced. A structure refinement based on this model gave a better fit ( R p = 0.12, R F = 0.04) and more accurate metal atom parameters than those reported previously. In samples which were vuenched from temperatures above 300 °C the average distance between twin planes was found to be a few lattice translation periods.

Experimental Evidence for the Existence of the Ti3B4 Phase

Abstract: Experimental evidence for the formation of Ti3B4 with an orthorhombic Ta3B4-type structure is presented This phase forms by a peritectic reaction between TiB2 and liquid at ∼2200°C and is stable to temperatures at least as low as 1690°C, the lowest temperature at which samples were annealed No evidence was found to indicate the presence of a second high-temperature form of this phase, as has been reported in the literature

Thermal fluctuations of the order parameter in charge-density waves

Abstract: Incorporating the thermal fluctuations of the phase of charge-density-wave order parameter into the theory, we extend the Fukuyama-Lee-Rice theory to finite temperatures. We find the temperature dependence of the threshold field at low temperatures (e.g., T(1/2)${T}_{c}$) is given by , where ${T}_{0}$ is a material constant independent of the impurity concentration. Furthermore, we find the characteristic Lee-Rice correlation length depends on the temperature as . The former prediction describes quite well the observed temperature dependence of the threshold field ${E}_{T}$(T) at low temperatures of ${\mathrm{NbSe}}_{3}$, orthorhombic and monoclinic ${\mathrm{TaS}}_{3}$, and (${\mathrm{TaSe}}_{4}$${)}_{2}$I. Furthermore, the present theory describes qualitatively both the observed pressure dependence of ${E}_{T}$(0) and ${T}_{0}$ orthorhombic ${\mathrm{TaS}}_{3}$ and (${\mathrm{TaSe}}_{4}$${)}_{2}$I.

Crystal growth of MgSiO3 perovskite

Abstract: Single crystal of the orthorhombic perovskite modification of MgSiO3 have been grown from a melt at 27 GPa and 1830°C. The starting material of orthoenstatite was sealed in a platinum capsule with water. Crystals of good quality up to 200 microns in size were obtained, which are promising for the structural refinement, Brillouin spectroscopy, and other measurements. Some crystals, however, exihibit twinning on the (110) plane.

A high resolution neutron diffraction study of the perovskite LaTiO3

Abstract: LaTiO3 shows a transition from temperature-independent paramagnetism to antiferromagnetism at 125 K. The temperature coefficient of the electrical resistivity changes at the same temperature from positive (i.e. metal-like) to negative (i.e. semiconductor-like). The crystal structure of LaTiO3 was investigated at five temperatures from 298 K to 10 K by time-of-flight neutron diffraction to look for any structural origin to the above phenomena. Analysis of the 298 K data confirm and improve on an existing X-ray structure determination in Pbnm. The same orthorhombic cell and space group are consistent with the data at all temperatures studied. The cell constants show anisotropy as b and c decrease with temperature and a increases slightly. There are no significant changes in Ti-O bond lengths or Ti-O-Ti angles with temperature. In the La-O polyhedron the eight nearest-neighbour bond distances decrease with decreasing temperature while the next four increase demonstrating a tendency to eightfold coordination. There is no structural anomaly of any significance near 125 K. This is not inconsistent with an itinerant antiferromagnetic model for the transition.