Pressure-induced phase transitions in gypsum

Abstract

We report high-pressure Raman scattering spectroscopy and energy dispersive X-ray diffraction investigations on gypsum, CaSO₄ · 2H₂O, at room temperature in a diamond cell. With increasing pressure, measurements indicate that CaSO₄ · 2H₂O undergoes two stages of crystalline-state phase transitions at 5 and 9 GPa, and then converts to a disordered phase above 11 GPa. The structures of the three high-pressure phases of gypsum have not been determined yet. These phases are tentatively named as “post-gypsum-I” (PG-I), “post-gypsum-II” (PG-II) and “disordered” according to the sequence of their appearance with pressure.

Gypsum shows anisotropic compressibility along three crystallographic axes with b > c > a below 5 GPa. The difference in the behavior of the two OH stretching modes in gypsum is attributed to the different reduction rate in the hydrogen bonding distances by the anisotropic axial compressibility.     

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     

Iron Precursor Decomposition in the Magnesium Combustion Flame: A New Approach for the Synthesis of Particulate Metal Oxide Nanocomposites

Powders of Fe–Mg–O nanocomposite particles have been grown using a novel chemical vapor synthesis approach that employs the decomposition of a metalorganic precursor inside the metal combustion flame. After annealing in controlled gas atmospheres composition distribution functions, structure and phase stability of the obtained magnesiowüstite nanoparticles are measured with a combination of techniques such as inductively coupled plasma‐optical emission spectroscopy, energy dispersive X‐ray spectroscopy, X‐ray diffraction, and scanning and transmission electron microscopy. Complementary Mössbauer spectroscopy measurements reveal that depending on Fe loading and temperature of annealing either metastable and superparamagnetic solid solutions of Fe³⁺ ions in periclase (MgO) or phase separated mixtures of MgO and ferrimagnetic magnesioferrite (MgFe₂O₄) nanoparticles can be obtained. The described combustion technique represents a novel concept for the production of mixed metal oxide nanoparticles. Adressing the impact of selected annealing protocols, this study underlines the great potential of vapor phase grown non‐equilibrium solids, where thermal processing provides means to trigger phase separation and, concomitantly, the emergence of new magnetic properties.     

Temperature and magnetic field dependence of the elastic properties of the HoxTb1−xCo2 Laves phase compounds

Ultrasonic sound velocity measurements have been carried out in order to determine the adiabatic compressibility and elastic moduli of the pseudobinary Ho_xTb_1?xCo₂ Laves phase compounds. The anomalies, associated with the Curie temperatures and the presence of various spin reorientations, allowed the determination of the spin orientation diagram of the system. The similarity of this spin orientation diagram with that previously established of the Ho_xTb_1?xFe₂ system strongly supports the single rare earth ion model for the magnetic anisotropy behavior of the compounds studied. Measurements carried out in an external magnetic field revealed the presence of a significant ΔE effect.     

Theoretical stability,thin film synthesis and transport properties of the Mon +1GaCn MAX phase

The phase stability of Mo_{n +1}GaC_n has been investigated using ab‐initio calculations. The results indicate stability for the Mo₂GaC phase only, with a formation enthalpy of –0.4 meV per atom. Subsequent thin film synthesis of Mo₂GaC was performed through magnetron sputtering from elemental targets onto Al₂O₃ [0001], 6H‐SiC [0001] and MgO [111] substrates within the temperature range of 500 °C and 750 °C. High structural quality films were obtained for synthesis on MgO [111] substrates at 590 ºC. Evaluation of transport properties showed a superconducting behavior with a critical temperature of approximately 7 K, reducing upon the application of an external magnetic field. The results point towards the first superconducting MAX phase in thin film form. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Preparation and Properties of Iron and Iron Oxide Nanocrystals in MgO Matrix

We have prepared α-iron and magnetite (Fe₃O₄) nanoparticles in MgO matrix from a mixture of nanocrystalline Fe₂O₃ with Mg(H,O) powders calcinated in hydrogen. This procedure yielded spherical magnetic nanoparticles embedded in MgO. Transmission electron microscopy and Mössbauer spectroscopy were used for structure and phase analysis. The measurements of magnetic properties showed increased coercivity of the nanocomposite samples.     

High-pressure X-ray diffraction study of dimedone

Abstract

Dimedone (i.e. 5.5-dimethyl-I,3-cyclohexanedione) crystals, C₈HI₁₂O₂, have been studied at high pressures by X-ray diffraction using a Merrill-Bassett diamond-anvil cell. The unit-cell dimensions have been measured to 1.20(5) GPa and the structure has been determined at 0.95(5) GPa. The crystal compressibility is strongly anisotropic and non-linear, relatively strong compressibility of the crystals is observed along the helices of the hydrogen-bonded molecules. Small anomalous changes of the unit-cell dimensions are observed between 0.1 and 50 MPa. The main structural changes are compression of intermolecular contacts, but also an alongation of the O=C bond—accompanied with the compression of the hydrogen bond involving the carbonyl oxygen atom—has been observed. This elongation is consistent with similar effects reported on compression of the hydrogen bonds in 1,3-cyclohexanedione and 2-methyl-1,3-cyclopentanedione. Crystal data for the dimedone structure at 0.95 GPa: monoclinic, P2₁/c, a=9.909(6), b= 6.505(3), c=12.313(6) Å, β=14.51°, V=722.1(5) Å, Z=4, R=0.139 for 336 independent reflections.     

Quadrupole relaxation enhancement and polarisation transfer in DMSO solution of [Bi(NO3)3(H2O)3]*18-crown-6 in solid state

ABSTRACT

¹H spin-lattice relaxation studies have been performed for pure [Bi(NO₃)₃(H₂O)₃]*18-crown-6 in powder and its solution in dimethyl sulfoxide (DMSO). The experiments have been carried out in the frequency range of 10?kHz–30?MHz and the temperature range of 240–277?K; at 277?K the solution is already frozen. The ¹H relaxation of pure [Bi(NO₃)₃(H₂O)₃]*18-crown-6 has been interpreted in terms of three dynamical processes. Quadrupole Relaxation Enhancement effects have been observed in the frozen DMSO solution of [Bi(NO₃)₃(H₂O)₃]*18-crown-6. The specific mechanisms of the ¹H spin-lattice relaxation enhancement have been discussed distinguishing between effects caused by time independent (residual) and fluctuating ¹H-²⁰⁹Bi dipole-dipole interactions.     

Calculation on the CO2 influences on the structure,stability of (MgO)m (m = 1–6) clusters

ABSTRACT

The concentration of carbon dioxide (CO₂) has a significant influence on the morphology of thermal decomposition products of magnesite. So, structures, stabilities and adsorption mechanisms of (MgO)_m (m?=?1–6) clusters by one or two CO₂ molecules were calculated by the GGA-PW91 method. The results show that the stability of the considered clusters is (MgO)_m(CO₂)₂ clusters > (MgO)_m(CO₂) clusters > (MgO)_m clusters by the average binding energy. Certain low-lying isomers of (MgO)_m(CO₂) and (MgO)_m(CO₂)₂ clusters which have an isolated O atom are deviating from the cluster center which possess higher kinetic activity. (MgO)_m clusters prefer to adsorb a CO₂ molecule, while (MgO)₃(CO₂) clusters prefer to adsorb a CO₂ molecule rather than the neighbors. Magnesite is difficult to transit to (MgCO₃)₂ clusters at room temperature. However, magnesite will spontaneously transit to (MgO)₂ clusters and further transit to MgO crystal which need to adsorb more energy at 700?K.     

Effect of oxidizing anneals on thermochemically reduced MgO:Ni crystals

Oxidizing anneals have been carried out to test the stability of nickel precipitates previously formed in MgO:Ni by thermochemical reduction. The annealed crystals have been examined by transmission electron microscopy (TEM) and cathodoluminescence (CL). Both techniques, TEM and CL, reveal that the nickel precipitates become unstable under oxidizing annealing at temperatures 1223 K for 30 min. Cathodoluminescence results are analyzed on the basis of anion vacancy complexes and Ni²⁺ transitions.     

Ferromagnetism in amorphous MgO

Abstract

We report, for the first time, the atomic structure of amorphous MgO based on ab initio molecular dynamics simulations. We find that its main building blocks are four-fold and five-fold coordinated configurations, similar to those formed in the liquid state. Its average coordination is estimated to be about 4.36. The amorphous form having a perfect stoichiometry has a band gap energy of 2.4 eV. On the other hand, Mg vacancies induce an insulator to metal transition and ferromagnetism in amorphous MgO whilst O vacancies do not cause such a transition, implying that the magnetism in amorphous MgO is related to the non-stoichiometry and Mg vacancies. With the application of pressure, the stoichiometric and non-stoichiometric (Mg vacancies) models undergo a phase transformation into a rocksalt state, suggesting that the electronic structure of the initial configurations has no influence on the resulting high-pressure phase in amorphous MgO.     

The structure of a propagating MgAl2O4/MgO interface: linked atomic- and μm-scale mechanisms of interface motion

To understand how a new phase forms between two reactant layers, MgAl₂O₄ (spinel) has been grown between MgO (periclase) and Al₂O₃ (corundum) single crystals under defined temperature and load. Electron backscatter diffraction data show a topotaxial relationship between the MgO reactant and the MgAl₂O₄ reaction product. These MgAl₂O₄ grains are misoriented from perfect alignment with the MgO substrate by ~2–4°, with misorientation axes concentrated in the interface plane. Further study using atomic resolution scanning transmission electron microscopy shows that in 2D the MgAl₂O₄/MgO interface has a periodic configuration consisting of curved segments (convex towards MgO) joined by regularly spaced misfit dislocations occurring every ~4.5 nm (~23 atomic planes). This configuration is observed along the two equivalent [1 0 0] directions parallel to the MgAl₂O₄/MgO interface, indicating that the 3D geometry of the interface is a grid of convex protrusions of MgAl₂O₄ into MgO. At each minimum between the protrusions is a misfit dislocation. This geometry results from the coupling between long-range diffusion, which supplies Al³⁺ to and removes Mg²⁺ from the reaction interface, and interface reaction, in which climb of the misfit dislocations is the rate-limiting process. The extra oxygen atoms required for dislocation climb were likely derived from the reactant MgO, leaving behind oxygen vacancies that eventually form pores at the interface. The pores are dragged along by the propagating reaction interface, providing additional resistance to interface motion. The pinning effect of the pores leads to doming of the interface on the scale of individual grains.     

Characterisation of geometric isomers of europium chlorides with 2,2′-bipyridine based on X-ray diffraction,luminescence and quantum chemical data

A low-temperature high-resolution luminescence study of the EuCl₃bpy₂(H₂O) _n isomers has been carried out. The Eu³⁺ luminescence spectra of all geometric isomers were recorded over the spectral range which includes transitions from the ⁵D₀ excited state to the ⁷F_0–4 ground state manifolds and from the ⁵D₁ excited state to the ⁷F_0–2 ground state manifolds. Analysis of the Eu³⁺ transitions observed in the luminescence spectra shows that the Eu³⁺ ion occupies a spectroscopic site symmetry that approaches a C_{2 v} symmetry with distortion towards C₂ or lower symmetry. The structural features and distortions of the Eu³⁺ coordination polyhedron in these geometric isomers were described based on the X-ray crystallographic data as well. The splitting patterns and energies found of the ⁷F_0–4 manifolds have been used to calculate the crystal field parameters (CFPs) of the Eu³⁺ ions in these geometric isomers. In addition the mutual influence of the ligands as well as the relative stability of geometric isomers of the [EuCl₃bpy₂(H₂O)₂]⁺ cation in the gas phase was analyzed within DFT calculations.     

Etude par resonance paramagnetique electronique des verres du systeme B2O3-Na2O-Na3PO4

An EPR study of fast Na⁺ ion vitreous conductors of the system B₂O₃?Na₂O?Na₃PO₄ has been carried out. After X-ray irradiation two types of paramagnetic centers have been observed. The first one is of boron-oxygen hole center (B.O.H.C.) type and similar to that earlier observed for other alkali borate glasses. The second one is of PO₄^2? type. A simulation of its spectrum has been achieved and a defect model discussed.     

Double-stranded helical lanthanide(III) supramolecular networks with rigid diimine ligands: Synthesis, structure, and physical properties

Two lanthanide coordination complexes [Nd(NO₃)₃(CH₃OH)₂(4,4′-bipy)₂] (1) (4,4′-bipy=4,4′-bipyridine) and [4,4′-Hbipy][La(NO₃)₄(H₂O)₂(4,4′-bipy)] (2), with a salt of cationic diprotonated 4,4′-bipy, [2(4,4′-H₂bipy)][4(NO₃)] (3), have been identified and isolated from a methanol solution of Ln(NO₃)₃·6H₂O, 4,4′-bipyridine and pyrazine in 1:2:1 ratio. Their structures have been determined by single-crystal X-ray diffraction analyses, which reveal that 1 has an interesting three-dimensional supramolecular architecture containing 2₁ double-stranded helical chains through hydrogen bonding and π–π interactions, while 2 and 3 have well defined infinite chiral 3D open networks that undergo self-interpenetration. The electrospray ionization mass spectra (ESI-MS) indicate that the covalent complex has higher stability than the electrostatic bonding one. ESI-MS/MS of these ions reveal that the Ln–O bond forms a stronger coordinated bonding than that of Ln–N system and the nitrate anion remains bound to the lanthanide centers after complete dissociation in methanol solution.     

Orientational order in three nO.m liquid crystalline compounds

Refractive index and density measurements have been carried out on three nO.m liquid crystalline compounds, namely, 4O.2, 4O.3 and 1O.10 belonging to the N-(p-n-alkoxybenzylidene)-p-n-alkylaniline series. From the data, the orientational order parameter has been estimated using Vuks and Neugebauer local field models. Furthermore, the orientational order parameter has been calculated directly from refractive index data employing the Vuks scaling factor method, Neugebauer f(B) parameter, effective geometry parameter and a method proposed by Kuczyński et al. It is observed that order parameter values estimated from different methods agree well near the nematic–isotropic transition and diverge as the nematic phase attains equilibrium. The temperature gradient of refractive indices and the nematic crossover temperatures have also been estimated for these compounds. The results obtained are compared and discussed.     

High pressure studies using synchrotron radiation and tungsten carbide anvil cells

Abstract

High pressure studies have been carried out on a range of materials using the high pressure facility at the UK Synchrotron Radiation Source (SRS). Custom-built Bridgman, Drickamer and belt-type cells (Häusermann et al., 1989) were used for this work. The compressibility of copper has been measured on the NaCl scale up to 100 kbar in the Bridgman and Drickamer type cells. The data obtained with the two cells are in agreement, but differ from those found in the literature.     

Switching Model with Two Habitats and a Predator Involving Group Defence

Abstract

Switching model with one predator and two prey species is considered. The prey species have the ability of group defence. Therefore, the predator will be attracted towards that habitat where prey are less in number. The stability analysis is carried out for two equilibrium values. The theoretical results are compared with the numerical results for a set of values. The Hopf bifuracation analysis is done to support the stability results.     

Effect of growth defects on microwave properties in epitaxial Ba0.5Sr0.5TiO3 thin films grown on (001) MgO by pulsed laser deposition

Ba_0.5Sr_0.5TiO₃ (BSTO) films have been grown heteroepitaxially on (001) MgO substrates by pulsed laser deposition (PLD) to fabricate microwave phase shifters for the wide frequency range 45 MHz–50 GHz. Both as-grown and ex situ annealed films have a cube on cube epitaxial relationship with ?100?BSTO//?100?MgO. Threading dislocations are the dominant defects, mostly with Burgers vectors b = ?101?. Growth at 10^?1 mbar oxygen pressure, compared to 10^?4 mbar, resulted in significantly better properties. Ex situ annealing of the film grown at 0.1 mbar resulted in a reduction of 40% in threading dislocation density and a 40% increase in dielectric tunability.     

Structural and optical properties of InSe under pressure

Abstract

We have investigated the high pressure behavior of InSe by x-ray powder diffraction and optical measurements. The rhombohedral γ-polytype of InSe (space group R3m) exhibits a strongly anisotropic compressibility characteristic of the layer-type structure. Mode Gruneisen parameters of intralayer modes have been determined by Raman scattering. At 10.3(5) GPa InSe undergoes a phase transition to the rocksalt structure, which remains stable up to at least 30 GPa. Optical reflectivity measurements show the cubic high pressure phase to have metallic character.