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Structural Phase Transition (structural + phase_transition)
Selected AbstractsChemInform Abstract: Low Temperature Structural Phase Transition of Ba3NaIr2O9.CHEMINFORM, Issue 23 2009Hans-Conrad zur Loye Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Structural Phase Transition of Gd3RuO7.CHEMINFORM, Issue 13 2008Nobuo Ishizawa Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Pressure-Induced Structural Phase Transition in a Ferromagnet CrTe.CHEMINFORM, Issue 20 2001T. Eto Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Structural Phase Transitions of Aliphatic Nylons Viewed from the Simultaneous Measurements of WAXD and SAXSMACROMOLECULAR SYMPOSIA, Issue 1 2006Kohji Tashiro Abstract Structural changes occurring in the high temperature region of doubly-oriented nylon 10/10 sample have been investigated through the temperature-dependent simultaneous measurements of wide-angle and small-angle X-ray scatterings, and the results were compared with the infrared spectral data as well as the molecular dynamics simulation results. In the Brill transition region of 150,180,°C the methylene segments are conformationally disordered with keeping the intermolecular hydrogen bonds. During this phase transition the stacked lamellar structure did not change very much: the lamellae are tilted by ca. 34° from the draw axis and the long period is almost 160 Å. In the temperature region immediately below the melting point the molecular chains were found to be contracted by ca. 10% the original repeating period and the intermolecular hydrogen bonds were almost broken, causing the violent rotational and translational motions of the chains around the chain axis. At the same time the long period increased remarkably from 160 Å to 410 Å and the originally tilted lamellae stood up in parallel to the draw axis. [source] Observation and Characterization of Structural Phase Transitions by X-Ray Powder DiffractionPARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 6 2005Wulf Depmeier Abstract Some of the basic properties of powder diffraction are summarized after a short introduction into the theory. Then it is described phenomenologically how various kinds of structural transformations affect the powder diffraction pattern. This is achieved on emphasizing that diffraction studies on powders are statistically more sound than those on single crystals, thus allowing the reliable determination of the generic properties of a given crystal structure. [source] ChemInform Abstract: Structural Phase Transitions in Ag2Se (Naumannite).CHEMINFORM, Issue 12 2008Heinrich Billetter Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] ChemInform Abstract: Structural Phase Transitions in CaC2.CHEMINFORM, Issue 4 2002Michael Knapp Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Structural phase transition in [NH3(CH2)5NH3]BiCl5: thermal and vibrational studiesJOURNAL OF RAMAN SPECTROSCOPY, Issue 11 2005H. Jeghnou Abstract An x-ray diffraction study carried out on the pentylenediammonium pentachlorobismuthate (III) compound [NH3(CH2)5NH3]BiCl5 indicated that it is orthorhombic with a P212121(Z = 4) space group. Differential scanning calorimetry (DSC), differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were also made, which showed that this compound undergoes a unique phase transition at high temperatures. The infrared and Raman spectra recorded at room temperature were interpreted on the basis of data published in the literature. The Raman bands observed below 300 cm,1 are very characteristic of the anions, which are made of isolated (BiCl52, )n units composed of BiCl63, octahedra. Raman spectra recorded at various temperatures confirmed the contribution to the phase-transition mechanism of both the anion and cation motions. Copyright © 2005 John Wiley & Sons, Ltd. [source] New Organic Nonlinear Optical Polyene Crystals and Their Unusual Phase Transitions,ADVANCED FUNCTIONAL MATERIALS, Issue 11 2007O-P. Kwon Abstract A series of new nonlinear optical chromophores based on configurationally locked polyenes (CLPs) with chiral pyrrolidine donors are synthesized. All CLP derivatives exhibit high thermal stability with decomposition temperatures Td at least > ,270,°C. Acentric single crystals of enantiopure D - and L -prolinol-based chromophores with a monoclinic space group P21 exhibit a macroscopic second-order nonlinearity that is twice as large than that of analogous dimethylamino-based crystal. This is attributed to a strong hydrogen-bonded polar polymer-like chain built by these molecules, which is aligned along the polar crystallographic b -axis. Five ,-phase CLP crystals with different donors grown from solution exhibit a reversible or irreversible thermally induced structural phase transition to a ,-phase. These phase transitions are unusual, changing the crystal symmetry from higher to lower at increasing temperatures, for example, from centrosymmetric to non-centrosymmetric, enhancing their macroscopic second-order nonlinear optical properties. [source] Differential scanning calorimetric and Raman studies of a phase transition in [C3H7NH3]2SiF6JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2004H. Jeghnou Abstract A new structural phase transition was detected at low temperatures in [C3H7NH3]2SiF6 by means of differential scanning calorimetry; the basic thermodynamic data were determined for this transition. The Raman spectra of this compound were recorded from ambient temperature through the phase transition at 223 K. The observed phase transition may be of the order,disorder type and probably of first-order character. Copyright © 2004 John Wiley & Sons, Ltd. [source] Mechanistic comparison of blood undergoing laser photocoagulation at 532 and 1,064 nmLASERS IN SURGERY AND MEDICINE, Issue 2 2005John F. Black PhD Abstract Background and Objectives We seek to compare and contrast the mechanisms of blood photocoagulation under 532 and 1,064 nm laser irradiation in vitro in order to better understand the in vivo observations. We also seek to validate a finite element model (FEM) developed to study the thermodynamics of coagulation. Study Design/Materials and Methods We study the photocoagulation of whole blood in vitro at 532 and 1,064 nm using time-domain spectroscopic and optical coherence tomography (OCT)-based imaging techniques. We model the coagulation using an FEM program that includes the latent heat of vaporization (LHV) of water, consideration of the pulse shape of the laser, and the bathochromic shift in the hemoglobin absorption spectrum. Results We find significant similarities in the spectroscopic, chemical, and structural changes occurring in hemoglobin and in the blood matrix during photocoagulation despite the very large difference in the absorption coefficients. The more uniform temperature profile developed by the deeper-penetrating 1,064 nm laser allows us to resolve the structural phase transition in the red blood cells (going from biconcave disc to spherocyte) and the chemical transition creating met-hemoglobin. We find that the RBC morphology transition happens first, and that the met-Hb transition happens at a much higher temperature (,>,90°C) than is found in slow bath heating. The FEM analysis with the LHV constraint and bathochromic shift predicts accurately the imaging results in both cases, and can be used to show that at 1,064 nm there is the potential for a runaway increase in absorption during the laser pulse. Conclusions Photothermally mediated processes dominate the in vitro coagulation dynamics in both regimes despite the difference in absorption coefficients. There is a significant risk under 1,064 nm irradiation of vascular lesions in vivo that the dynamic optical properties of blood will cause runaway absorption and heating. This may in turn explain some recent results at this wavelength where full-thickness burns resulted from laser treatment. Lasers Surg. Med. 36:155,165, 2005. © 2005 Wiley-Liss, Inc. [source] Polarization effects and phase equilibria in high-energy-density polyvinylidene-fluoride-based polymersACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2010V. Ranjan Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar , and polar , phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF,TeFE is stabilized in the , phase, whereas the , phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations ( 12%) undergo a transition from the , to the , phase until a breakdown field of ~600,MV,m,1 is reached. These structural phase transitions can be exploited for efficient storage of electrical energy. [source] Analysis of phase transition and expansion behaviour of Al2(WO4)3 by temperature-regulated X-ray diffractionPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2008Takuya Hashimoto Abstract X-ray diffraction measurements of Al2(WO4)3 were carried out under various temperatures from 123 K to 373 K. Diffraction patterns above 273 K and below 253 K could be successfully explained as orthorhombic Pbcn (No. 60) and monoclinic P21/n (No. 14) with superstructure, respectively, which were in agreement with the results of convergent-beam electron diffraction. Thermal expansion of low-temperature monoclinic phase and slight shrinkage of high-temperature orthorhombic phase were observed from the temperature dependence of lattice constants and molar volume, showing agreement with the result of dilatometry. It is revealed that abrupt expansion of Al2(WO4)3 at the structural phase transition is attributed to an abrupt increase of the c -axis, which is parallel to the 21 screw axis and perpendicular to the n-glide plane. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure calculations of europium chalcogenides EuS and EuSePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2007D. Rached Abstract We have performed ab-initio self-consistent calculations on the full-potential linear muffin-tin orbital method with the local-density approximation and local spin-density approximation to investigate the structural and electronic properties of EuS and EuSe in its stable (NaCl-B1) and high-pressure phases. The magnetic phase stability was determined from the total energy calculations for both the nonmagnetic (NM) and magnetic (M) phases. These theoretical calculations clearly indicate that both at ambient and high pressures, the magnetic phase is more stable than the nonmagnetic phase. The transition pressure at which these compounds undergo the structural phase transition from NaCl-B1 to CsCl-B2 phase is calculated. The elastic constants at equilibrium in both NaCl-B1 and CsCl-B2 structures are also determined. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Groupoid of orientational variantsACTA CRYSTALLOGRAPHICA SECTION A, Issue 1 2006Cyril Cayron Daughter crystals in orientation relationship with a parent crystal are called variants. They can be created by a structural phase transition (Landau or reconstructive), by twinning or by precipitation. Internal and external classes of transformations defined from the point groups of the parent and daughter phases and from a transformation matrix allow the orientations of the distinct variants to be determined. These are algebraically identified with left cosets and their number is given by the Lagrange formula. A simple equation links the numbers of variants of the direct and inverse transitions. The equivalence classes on the transformations between variants are isomorphic to the double cosets (operators) and their number is given by the Burnside formula. The orientational variants and the operators constitute a groupoid whose composition table acts as a crystallographic signature of the transition. A general method that determines if two daughter variants can be inherited from more than one parent crystal is also described. A computer program has been written to calculate all these properties for any structural transition; some results are given for Burgers transitions and for martensitic transitions in steels. The complexity, irreversibility and entropy of fractal systems constituted by orientational variants generated by thermal cycling are briefly discussed. [source] On the polymorphism of benzocaine; a low-temperature structural phase transition for form (II)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2009Eric J. Chan A low-temperature structural phase transition has been observed for form (II) of benzocaine (BZC). Lowering the temperature doubles the b -axis repeat and changes the space group from P212121 to P1121 with , now 99.37 °. The structure is twinned, the twin rule corresponding to a 21 screw rotation parallel to a. The phase transition is associated with a sequential displacement parallel to a of zigzag bi-layers of ribbons perpendicular to b*. No similar phase transition was observed for form (I) and this was attributed to the different packing symmetries of the two room-temperature polymorphic forms. [source] Dynamic proton disorder and the II,I structural phase transition in (NH4)3H(SO4)2ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009Yoo Jung Sohn X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction methods were used to investigate triammonium hydrogen disulfate (NH4)3H(SO4)2 (TAHS) in the temperature range between 293 and 493,K. The temperature-dependent X-ray powder diffraction measurements show a clear hysteresis of the I II phase transition of TAHS with transition temperatures of Tup = 412.9,(1),K on heating and of Tdown = 402.6,(1),K on cooling. From the existence of hysteresis and from the jump-like changes of the lattice parameters, the I II phase transition of TAHS is considered to be first order. With DSC/TG measurements we confirmed that there is only one phase transition between 293 and 493,K. Through careful investigation on single crystals of TAHS using neutron diffraction, the correct space group (C2/c) of room-temperature TAHS-II phase was confirmed. Crystal structure analysis by single-crystal neutron diffraction showed a strongly elongated displacement ellipsoid of the proton which lies in the middle of the (SO4)H(SO4) dimer with local symmetry. The protons of the NH4 groups also show strongly enlarged anisotropic mean-square displacements. These findings are interpreted in terms of a characteristic proton disorder in the TAHS-II phase. [source] Structural behaviour of synthetic Co2SiO4 at low temperaturesACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2008Andrew Sazonov Synthetic Co2SiO4 has an olivine structure with isolated SiO4 groups (space group Pnma) and shows magnetic ordering below 50,K. Single-crystal neutron diffraction was applied to determine precise crystal structure parameters at low temperatures. No structural phase transition was revealed in the temperature range 2.5,300,K. Lattice parameters were determined by high-resolution X-ray powder diffraction between 15 and 300,K. There is a clear evidence of an anomalous thermal expansion related to the magnetic phase transition which can be attributed to magnetostriction. [source] Tl2CO3 at 3.56 GPaACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2008A. Grzechnik The crystal structure of thallium carbonate, Tl2CO3 (C2/m, Z = 4), is stable at least up to 3.56,GPa, as demonstrated by hydrostatic single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. Our results contradict earlier observations from the literature, which found a structural phase transition for this compound at about 2,GPa. Under atmospheric conditions, all atoms except for one O atom reside on the mirror plane in the high-pressure structure. The compression mainly affects the part of the structure where the nonbonded electron lone pairs on the Tl+ cations are located. [source] High-Spin- and Low-Spin-State Structures of [Fe(chloroethyltetrazole)6](ClO4)2 from Synchrotron Powder Diffraction DataCHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2006Eva Dova Dr. Abstract The spin-crossover complex [Fe(teec)6](ClO4)2 (teec = chloroethyltetrazole) exhibits a 50,% incomplete spin crossover in the temperature range 300,30 K. Time-resolved synchrotron powder diffraction experiments have been carried out to elucidate its structural behavior. We report crystal structure models of this material at 300 K (high spin) and 90 K (low spin), as solved from synchrotron powder diffraction data by using Genetic Algorithm and Parallel Tempering techniques and refined with Rietveld refinement. During short synchrotron powder diffraction experiments (five minutes duration) two distinguishable lattices were observed the quantities of which vary with temperature. The implication of this phenomenon, that is interpreted as a structural phase transition associated with the high-to-low spin crossover, and the structural characteristics of the high-spin and low-spin models are discussed in relation to other compounds showing a similar type of spin-crossover behavior. [source] Thermal and EPR investigations of thallium gallium disulphide single crystalCRYSTAL RESEARCH AND TECHNOLOGY, Issue 7 2009M. Acikgoz Abstract In this research, the results of the differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) investigations of TlGaS2 single crystal are presented. Specific heat capacity (Cp) anomalies of layered TlGaS2 have been obtained by using a new DSC technique for such crystals. Remarkable heat capacity anomalies have been revealed at the temperatures of 137.7 K, 174.5 K and 238.5 K. It is found that the anomalies appear at maximum with a small deviation (by 3-4%) from the regular values, and Cp discontinuity amounted to approximately 5%. Additionally, EPR spectra of Fe doped TlGaS2 single crystals have been recorded at various temperatures down to 6 K for different orientations of the applied magnetic field. Transformations of present EPR spectra are not sufficient for the confirmation of structural phase transitions, in contrast to the cases in iso structural TlInS2 and TlGaSe2 compounds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008L. Cheng Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source] Polarization effects and phase equilibria in high-energy-density polyvinylidene-fluoride-based polymersACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2010V. Ranjan Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar , and polar , phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF,TeFE is stabilized in the , phase, whereas the , phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations ( 12%) undergo a transition from the , to the , phase until a breakdown field of ~600,MV,m,1 is reached. These structural phase transitions can be exploited for efficient storage of electrical energy. [source] 23Na and 87Rb relaxation study of the structural phase transitions in the Tutton salts Na2Zn(SO4)2·6H2O and Rb2Zn(SO4)2·6H2O single crystalsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2010Ae Ran Lim Abstract The NMR spectrum, spin,lattice relaxation times, T1, and the spin,spin relaxation times, T2, for the 23Na and 87Rb nuclei in Tutton salts Na2Zn(SO4)2·6H2O and Rb2Zn(SO4)2·6H2O single crystals were determined by using an FT NMR spectrometer. The NMR spectrum and relaxation times for the 23Na and 87Rb nuclei in two crystals undergo significant changes near TC. These changes close to TC were the only detectable result of the structural transformation; the change is due to the fact that the electric field gradient value becomes 0. Especially, T1 and T2 for 87Rb in the Rb2Zn(SO4)2·6H2O crystals at high temperature have similar values on the order of milliseconds, which is indicative of a liquid-like system. The motion giving rise to this liquid-like behavior is probably related to superionic motion at high temperature. [source] Synthesis of MnGeO3 polycrystalline and single-crystal samples and comparative analysis of their magnetic propertiesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2009N. V. Sapronova Abstract MnGeO3 single crystals have been grown by a flux method. The obtained MnGeO3 is orthorhombic; a Pbca space group does not undergo any structural phase transitions in the range from room temperature to 900 °C. Magnetic measurements carried out for the first time on the MnGeO3 single crystal have revealed higher values (TN = 38 K and , = ,100 K) as compared to the data for polycrystalline samples reported in the literature (TN = 10 K and 14 K, , = ,54 K and ,46 K). These magnetic parameters for polycrystalline samples synthesized by us are close to the literature data. A Mössbauer spectrum taken at T = 300 K for a sample containing 5% Fe257O3 shows that manganese ions, Mn2+, occupy two nonequivalent positions and iron is included in a sublattice as Fe2+ and distributed among two positions substituting Mn2+. In this study, the magnetic characteristics are shown to be sensitive even to minor impurity amounts. The MnGeO3 magnetic structure and one of possible reasons causing the effect of impurities on the MnGeO3 magnetic properties are considered in the framework of a simple indirect coupling model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optical properties and structural phase transitions of lead-halide based inorganic,organic 3D and 2D perovskite semiconductors under high pressurePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2004K. Matsuishi Abstract Optical absorption, photoluminescence and Raman scattering of lead-halide based inorganic,organic perovskite semiconductors were measured under quasi-hydrostatic pressure at room temperature. For the 3D perovskite semiconductor, (CH3NH3)PbBr3, the free exciton photoluminescence band exhibits red-shifts with pressure, and jumps to a higher energy by 0.07 eV at 0.8 GPa, which is associated with a phase transition from a cubic to an orthorhombic structure confirmed by Raman scattering. Above the phase transition pressure, the exciton band shows blue-shifts with further increasing pressure, and eventually disappears above 4.7 GPa. The results are compared with those for the 2D perovskite semiconductor, (C4H9NH3)2PbI4. First principles pseudopotential calculations were performed to investigate changes in octahedral distortion and electronic band structures with pressure. The calculations have explained the origins of the intriguing changes in the electronic states with pressure in view of bonding characters between atomic orbitals in octahedra. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Neutron powder diffraction study of the anion-deficient La0.70Sr0.30MnO3.00-, manganitesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2009S. V. Trukhanov Abstract Using powder neutron diffraction method the magnetic structure of the anion-deficient La0.70Sr0.30MnO3.00-, (, = 0, 0.15; 0.20) solid solutions at different temperatures has been defined. It is established that the magnetic structure changes with oxygen vacancies concentration. For the investigated samples the structural phase transitions have not been detected. The obtained data confirm the conception of the magnetic phase state forming for the Sr-doped anion-deficient manganites. In according to this conception in case of orbital ordering absence the lowering of the coordination of the magnetic ion results to change of the indirect superexchange interactions Mn3+ -O-Mn3+ sign from positive to negative. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2001A general nomenclature applicable to the phases that form in any sequence of transitions in the solid state has been recommended by an IUCr Working Group [Acta Cryst. (1998). A54, 1028,1033]. The six-field notation of the first Report, hereafter I, was applied to the case of structural phase transitions, i.e. to transformations resulting from temperature and/or pressure changes between two crystalline (strictly periodic) phases involving modifications to the atomic arrangement. Extensive examples that illustrate the recommendations were provided. This second Report considers, within the framework of a similar six-field notation, the more complex nomenclature of transitions involving magnetic phases, incommensurate phases and transitions that occur as a function of composition change. Extension of the nomenclature to the case of phases with less clearly established relevance to standard schemes of transition in equilibrium systems, namely polytype phases, radiation-induced and other transient phases, quasicrystalline phases and their transitions is recommended more tentatively. A uniform notation for the translational periodicity, propagation vector or wavevector for magnetic and/or incommensurate substances is specified. The notation adopted for incommensurate phases, relying partly on the existence of an average structure, is also consistent with that for commensurate phases in a sequence. The sixth field of the nomenclature is used to emphasize the special features of polytypes and transient phases. As in I, illustrative examples are provided for each category of phase sequence. [source] Pressure-induced phase transitions in l -alanine, revisitedACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2010N. A. Tumanov The effect of pressure on l -alanine has been studied by X-ray powder diffraction (up to 12.3,GPa), single-crystal X-ray diffraction, Raman spectroscopy and optical microscopy (up to ,,6,GPa). No structural phase transitions have been observed. At ,,2,GPa the cell parameters a and b become accidentally equal to each other, but without a change in space-group symmetry. Neither of two transitions reported by others (to a tetragonal phase at ,,2,GPa and to a monoclinic phase at ,,9,GPa) was observed. The changes in cell parameters were continuous up to the highest measured pressures and the cells remained orthorhombic. Some important changes in the intermolecular interactions occur, which also manifest themselves in the Raman spectra. Two new orthorhombic phases could be crystallized from a MeOH/EtOH/H2O pressure-transmitting mixture in the pressure range 0.8,4.7,GPa, but only if the sample was kept at these pressures for at least 1,2,d. The new phases converted back to l -alanine on decompression. Judging from the Raman spectra and cell parameters, the new phases are most probably not l -alanine but its solvates. [source] Structural changes of hexamethylenetetramine and undecanedioic acid co-crystal (HMT-C11) as a function of the temperatureACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2003Carlos Basílio Pinheiro HMT-C11 belongs to the family of adducts formed by the co-crystallization of N4(CH2)6 molecules (hexamethylenetetramine or HMT) and aliphatic dicarboxylic acids HOOC(CH2)n, 2COOH with 5 ,n, 13 (Cn). The adducts exhibit a layered structure in which the packing between HMT and Cn is determined by strong hydrogen bonds. The compounds in this family studied so far present thermotropic structural phase transitions and, depending on the chain length, disordered, twinned and modulated phases. The structure refinement of HMT-C11 based on X-ray diffraction experiments indicates three distinct phases from the melting point down to liquid nitrogen temperature: phase I is not crystalline; phase II is disordered (stacking fault) and its average structure is described in space group Bmmb; phase III is partially disordered and its symmetry is P21/c. The systematic study of the structure evolution of phase III upon cooling revealed that the disorder has a dynamic character (anharmonicity). The main structural change observed from 293,K down to 93,K is the increase of the tilting angle of the C11 chains relative to the layer plane and the rotation of the HMT molecules. Both HMT and C11 behave like rigid bodies in the temperature range investigated. The quality of the refinements leads to a conclusive model for the O,H,N hydrogen bonds linking HMT and C11. [source] | ||||||||||||||||