Degradation of the structure of specimens of the intermetallide Ni3Al obtained by self-propagating high-temperature synthesis

Specimens of Ni₃Al obtained by self-propagating high-temperature synthesis (SHS) are subjected to an experimental study of the influence of the structural homogeneity on the recrystallization under conditions of diffusional contact with steel at T=1473 K. It is found that the homogenization of SHS-Ni₃Al with a nonuniform structure in diffusional contact with steel is accompanied by recrystallization induced by the diffusion of iron atoms. A comparative analysis of the mechanisms of diffusion of impurity elements in SHS-Ni₃Al shows that the diffusional mobility at the migrating boundaries of the SHS-Ni₃Al under conditions of diffusion-initiated recrystallization is three orders of magnitude higher than at stationary boundaries. Zh. Tekh. Fiz. 67, 57–60 (January 1997)     

Phase relations in the Fe-P system at high pressures and temperatures from ab initio computations

ABSTRACT

Based on the first-principles calculations within the density functional theory and crystal structure prediction algorithms iron phosphide phases stable under pressure of the Earth’s core and temperatures up to 4000?K were determined. A new low-temperature modification FeP-P2₁/c stable above ～75?GPa was predicted. Fe₂P with the allabogdanite structure has been established to be stable in the low-temperature region at ambient conditions. At 750?K it transforms into the barringerite structure. The transition from Fe₃P with schreibersite structure to Fe₃P-Cmcm was observed at 27?GPa, and the phase transition boundary is nearly isobaric. Fe₂P and FeP are thermodynamically stable at the Earth’s inner core pressures and 0?K according to the obtained results, whereas Fe₃P stabilizes with respect to decomposition to Fe?+?Fe₂P at high temperatures above ～3200?K.     

High-pressure single-crystal studies of MnTiO3

Abstract

Recent high-pressure single-crystal x-ray diffraction experiments have provided new information on the crystal chemistry of MnTiO₃ and have provided insight into polymorphic transitions among phases of this composition. The stable polymorph of MnTiO₃ at room P and T has the ilmenite structure. At high P and T, MnTiO₃ ilmenite (MnTiO₃ II) transforms to a LiNbO₃ structure through a cation reordering process, and the quenched LiNbO₃-structure phase transforms to a perovskite structure as the pressure is again increased. This transition is unique in that twinned MnTiO₃ II crystals transform under pressure to untwinned crystals having the perovskite structure. The back-transformation of perovskite to the LiNbO₃ structure as pressure is released is similar to that observed previously for the rutile-type dioxides TiO₂ and SnO₂.     

Structure changes in vacancy-rich titanium monoxide at high pressures and high temperatures

Abstract

Powdered samples of TiO_0.82, TiO_1.04 and TiO_1.25 having the cubic rocksalt-type structure with high concentration of vacancies randomly distributed were held at combined conditions of high pressure, ranging from 1 GPa to 8 GPa, and high temperature, ranging from 973 K to 1173 K, and structural changes occurring were investigated by synchrotron radiation diffraction at the conditions and by conventional X-ray diffraction after the samples were brought back to ambient condition. Pressure has been shown to suppress formation of ordered arrangements of vacancies in all the samples and lead to precipitation of a hexagonal δ-Ti₃O₂ in TiO_0.82 and TiO_1.04 and precipitation of a corundum-type Ti₂O₃ in TiO_1.25. Irreversible change in the lattice parameter of the remaining rocksalt-type structure has been observed which is due to partial annihilation of vacancies under pressure.     

A high pressure Raman study of terbium molybdate

Abstract

Tb₂(MoO₄)₃ has been studied by Raman spectroscopy under hydrostatic pressure up to 9 GPa at room temperature. The measurements reveal two phase transitions, one at around 2 GPa and another one above 5 GPa. The first phase transition is associated with an increase in the coordination number of Mo while the second is probably a transition to an amorphous phase in which only a wide band originating from Mo-O vibrations remains. This behaviour is irreversible as the Raman spectrum of the initial structure is not recovered at atmospheric pressure.     

Kinetic model and mechanism of Y3Al5O12 formation in hydrothermal and thermovaporous synthesis

Abstract

Kinetics and mechanism of fine-crystalline yttrium-aluminum garnet (YAG) formation under hydrothermal and thermovaporous treatment were investigated. It was synthesized from the stoichiomctric mixture of oxides in the temperature range 200-400°C at pressures of water vapor 4.0-26MPa. It was established that in quasiequilibrium with water vapor conditions the synthesis of YAG proceeds with formation of intermediate substance with Y(OH)₃ structure and amorphous aluminous component. The diffusion of this aluminous component into the Y(OH)₃ matrix resulted in the reorganization of oxygen sublattice accompanied with dehydroxylation. The kinetics of YAG formation is described by the equation of solid-phase transformation with the limiting stage of nucleation. Synthesized YAG contains 5-7% of water, which corresponds to a hydrogarnet structure. The study of luminescence properties of YAG doped with Nd³⁺ or Cr³⁺ ions has allowed to determine the positions of hydroxyl groups and oxygen vacancies in the structure.     

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     

Superhyperfine structure of the EPR spectra of Nd<Superscript>3+</Superscript> and U<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The superhyperfine structure of the EPR spectra of impurity Nd³⁺ and U³⁺ ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals has been observed and discussed. In LiYF₄: Nd (g _| = 1.987, g _⊥ = 2.554) and LiTmF₄: Nd, the superhyperfine structure is observed at the orientation of the external magnetic field B in parallel to the c axis of the crystals and consists of nine components with a splitting of ∼15.4 MHz. In LiYF₄: U (g _| = 1.149, g _⊥ = 2.508) and LiLuF₄: U, the superhyperfine structure is observed at both B | c and B ⊥ c and consists of nine and eleven components, respectively, with a splitting of ∼21.5MHz. It should be noted that the resolution of the superhyperfine structure of the EPR spectrum of LiLuF₄: U³⁺ becomes significantly higher with a deviation from the orientation B ⊥ c.     

First principles study of electronic structure and optical properties of CaTiO3

The electronic-energy band structure, site and angular momentum decomposed density of states (DOS) and charge-density contours of perovskite CaTiO ₃ are calculated by the first principles tight-binding linear muffin-tin orbitals method with atomic sphere approximation using density functional theory in its local density approximation. The calculated band structure shows an indirect (R-Γ) band gap of 1.5 eV. The total DOS as well as the partial density of states (PDOS) are compared with the experimental photoemission spectra. The calculated DOS are in reasonable agreement with the experimental energy spectra and the features in the spectra are interpreted by a comparison of the spectra with the PDOS. The origin of the various experimentally observed bands have been explained. From the DOS analysis, as well as charge-density studies, we conclude that the bonding between Ca and TiO ₃ is mainly ionic and that the TiO ₃ entities bond covalently. Using the projected DOS and band structure we have analyzed the interband contribution to the optical properties of CaTiO ₃ . The real and imaginary parts of the dielectric function and hence the optical constants such as refractive index and extinction coefficient are calculated. The calculated spectra are compared with the experimental results for CaTiO ₃ and are found to be in good agreement with the experimental results. The effective number of electrons per unit cell participating in the interband transitions are calculated. The role of band structure calculation as regards the optical properties of CaTiO ₃ is discussed. Received 1 February 2000 and Received in final form 21 July 2000     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

Spectroscopic Studies of the Stability of Doped Polythiophenes Under Air

Abstract

The stability of the doped state of conductive polythiophenes under air was investigated by the use of Infrared (IR) and UV-visible (UV) spectroscopies. Poly(3-methyl thiophene) doped with BF₃-ethyl ether (BFEE). shows higher stability due to its stereoregularity-chain structure. While the doped polythiophene (PT) degrades easily under the moisture.     

Structures and stability of N13 cluster

Quantum chemical studies for the nine N₁₃ isomers with low spin were carried out at the UHF/6-31G*, UB3LYP/6-31G* and UMP2/6-311G* levels of theory. The most stable N₁₃ isomer is structure 1 with C_2v symmetry, which contains two separated pentazole rings connected by three nitrogen atoms. The results show that structures 1, 2, 3, 4 and 5 which are composed of the pentazole ring are more stable than the open chain structure 6 and structures 8 and 9 which contain the four- and six-membered rings. This suggests that the pentazole ring is a fundamental stable structural unit, which is applicable not only for the even-numbered nitrogen clusters, but also for the large odd-numbered clusters.     

Electronic structure and physical properties of bcc selenium under high pressure

Abstract

Here we report a theoretical calculation of the band structure and superconductivity of Se in the bcc phase. The energy band structure and the effect of pressure on the band structure is obtained by means of the Linear Muffin-Tin Orbital method within the atomic sphere approximation. The superconducting transition temperature (T_c) is calculated using McMillan's formula and we predict the value of T_c at 115.3 Gpa as 2.3 K. Further increase in presssure decreases the T_c values. The normal state electrical resistivity at 115.3 Gpa is 1.43 fl cm, with further increase in pressure the resistivity decreases, which is a typical behaviour of number of elemental metals under pressure.     

Effects of Gd<Superscript>3+</Superscript> doping on structural and dielectric properties of PZT (Zr:Ti = 52:48) piezoceramics

The purpose of this research is to study the effect of doping Gd into Pb(Zr_0.52Ti_0.48)O₃ ceramics prepared by solid state reaction. X-ray diffraction patterns show that all PGZT samples are of tetragonal structure and the highest doping should be no more than 2 mole % Gd at which the unreacted oxides start to appear. The electron spin resonance (ESR) spectra of PGZT's indicate that Gd³⁺ can enter both A site of the perovskite structure instead of only A site as widely believed. The ESR peaks resonance shift towards low fields as the concentration is higher, which is due to the change in crystal field experienced by Gd³⁺ ions. At x =0.001, 0.005 and 0.01 dopings, two sets of powder ESR signals arising from Gd³⁺ (4f⁷, spin 7/2) ions at A site. The first set shows some fine structure having strong absorption peaks centered at 76.26 mT (g = 8.550). The second is a seven-peak spectrum centered at 206.01 mT (g = 3.165), which belongs to the Gd³⁺ ions at B sites. Furthermore, the overlapped ESR strong absorption peaks from 309.17 mT to 314.49 mT (g = 2.2818-2.1087) belong to Gd³⁺ of unreacted Gd₂O₃. The local environments of Gd³⁺ ions were verified from the calculated ESR spectra using appropriate spin Hamiltonian parameter, i.e. gyromagnetic tensor g, zero-field splitting D and hyperfine tensor A.     

X-ray study of SnO2 under high pressure and temperature generated with sintered diamond anvils

Abstract

In-situ X-ray diffraction technique using synchrotron radiation was applied for polymorphic transitions in SnO₂ under high pressure and temperature generated with 6–8 type double-stage multianvils made of sintered diamond. At 23.6 GPa, the mixed phases of rutile-type structure (R-SnO₂) and columbite-type (c-SnO₂) were heated: at 700°C peaks of fluorite-type structure (F-SnO₂) began to appear, and at 1000°C, C-SnO₂ and 5m later R-SnO₂ disappeared completely and a single phase of F-SnO₂ was recognized, indicating a direct transition from R-SnO₂ to F-SnO₂. The volume reduction of 5.3% was obtained for C-SnO₂→F-SnO₂ transition.     

Flow equations for electron-phonon interactions: phonon damping

We report Nuclear Magnetic Resonance studies of CeAuAl₃ and LaAuAl₃ at frequencies between 1.30 and 76.91 MHz and at temperatures from up to . CeAuAl₃ is a new heavy-electron compound which orders antiferromagnetically at . For the two inequivalent Al sites of CeAuAl₃, the transferred hyperfine couplings and are relatively small compared to those in other Ce compounds. The nuclear quadrupolar coupling constants e2qQ / h are 2.9 MHz and 7.8 MHz, respectively. We suggest that the magnetic structure of the ordered state below is of a simple spiral type with the ordered Ce moments arranged ferromagnetically within the ( ab ) planes of the tetragonal crystal lattice. Based on this magnetic structure we estimate a 25% reduction of the Ce moments, most likely due to Kondo screening. At high temperatures, in the paramagnetic state of CeAuAl₃, the spin-lattice relaxation rate T1 ^-1 is dominated by the fluctuations of the localised Ce moments. At , T1 ^-1 is , more than two orders of magnitude larger than for the reference compound LaAuAl₃. For temperatures lower than T1 ^-1 decreases as a function of ( T / H ) and below , displays a linear-in-T behaviour, strongly enhanced compared to the reference compound LaAuAl₃. Received: 5 May 1998 / Received in final form: 16 July 1998 / Accepted: 20 July 1998     

Simple cubic arsenic under pressure

Abstract

We report here a calculation of the band structure and superconductivity of Arsenic in the simple cubic phase under pressure. The effect of pressure on the band structure is obtained using Andersen's linear muffin-tin orbital method under atomic sphere approximation. McMillan's formula is used to calculate the superconducting transition temperature (T_c). The theoretically calculated valve of T_c in sc phase at 26.6 GPa is 3.62 K. Further increase in pressure decreases the T_c values.     

Lung MRI assessment with high-frequency noninvasive ventilation at 3 T

PurposeTo investigate three MR pulse sequences under high-frequency noninvasive ventilation (HF-NIV) at 3 T and determine which one is better-suited to visualize the lung parenchyma.MethodsA 3D ultra-short echo time stack-of spirals Volumetric Interpolated Breath-hold Examination (UTE Spiral VIBE), without and with prospective gating, and a 3D double-echo UTE sequence with spiral phyllotaxis trajectory (3D radial UTE) were performed at 3 T in ten healthy volunteers under HF-NIV. Three experienced radiologists evaluated visibility and sharpness of normal anatomical structures, artifacts assessment, and signal and contrast ratio computation. The median of the three readers‘scores was used for comparison, p < .05 was considered statistically significant. Incidental findings were recorded and reported.ResultsThe 3D radial UTE resulted in less artifacts than the non-gated and gated UTE Spiral VIBE in inferior (score _{3D radial UTE} = 3, slight artifact without blurring vs. score _{UTE Spiral VIBE non-gated and gated} = 2, moderate artifact with blurring of anatomical structure, p = .018 and p = .047, respectively) and superior lung regions (score _{3D radial UTE} = 3, vs. score _{UTE Spiral VIBE non-gated} = 2.5, p = .48 and score _{UTE Spiral VIBE gated} = 1, severe artifact with no normal structure recognizable, p = .014), and higher signal and contrast ratios (p = .002, p = .093). UTE Spiral VIBE sequences provided higher peripheral vasculature visibility than the 3D radial UTE (94.4% vs 80.6%, respectively, p < .001). The HF-NIV was well tolerated by healthy volunteers who reported on average minor discomfort. In three volunteers, 12 of 18 nodules confirmed with low-dose CT were identified with MRI (average size 2.6 ± 1.2 mm).ConclusionThe 3D radial UTE provided higher image quality than the UTE Spiral VIBE. Nevertheless, a better nodule assessment was noticed with the UTE Spiral VIBE that might be due to better peripheral vasculature visibility, and requires confirmation in a larger cohort.     

Electronic structure and metal-insulator transition in SrTi 1 - x Ru xO 3

We studied the changes in the electronic structure of SrTi_1-xRu_xO₃ across the metal-insulator transition. The parent compound, SrTiO₃, is a well known diamagnetic insulator; whereas the doped compound, SrTi_1-xRu_xO₃, becomes a ferromagnetic metal above x _C = 0.35. The techniques used in the study were photoemission (PES) and O 1 s X-ray absorption (XAS) spectroscopy. The experimental spectra were analyzed in terms of band structure and Hubbard model calculations. The PES and XAS spectra of SrTi_1-xRu_xO₃ show the Ru 4 d bands growing in the band gap of SrTiO₃ . The analysis in terms of the Hubbard model indicates that the Ti 3 d and Ru 4 d bands are mostly decoupled. This suggests that the metal-insulator transition is a percolation transition like that of metals embedded in a rare gas matrix. Electron correlation effects are present in this system, but they do not seem to play a major role in the transition. Received 10 September 2001     

Pressure-induced disordering and phase transformations in Eu2Zr2O7 pyrochlore

ABSTRACT

The structural properties of pyrochlore Eu₂Zr₂O₇ under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu₂Zr₂O₇ undergoes a reversible structural change around 21.2?GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5?GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the <Zr–O> bonds should play a more important role than the <Eu–O> bonds in the process of the phase transformation.