High-temperature Diffraction Study Research Articles

Высокотемпературные дифракционные исследования процесса расслоения марганец-галлиевой шпинели состава Mn1.5Ga1.5O4

Стабильность смешанного оксида Mn1.5Ga1.5O4 со структурой шпинели, полученного в инертной среде (1000 °C, Ar), исследовали на воздухе в широком температурном интервале 30—1000 °C с использованием термогравиметрии и высокотемпературной рентгеновской дифракции. При нагреве наблюдаются обратимые процессы расслоения исходной шпинели. От 30 до 600 °C в поверхностный слой исходной шпинели Mn1.5Ga1.5O4 присоединяется кислород, благодаря чему образуется новая фаза, отличающаяся от "родительского" оксида стехиометрией по кислороду (формируются катионные вакансии). Продукт расслоения представляет собой два оксида Mn1.5Ga1.5O4 и Mn1.5–xGa1.5–x[·]xO4. Выше 600 °C, наоборот, происходит потеря кислорода, уменьшается концентрация катионных вакансий в Mn1.5–xGa1.5–x[·]xO4 и идет обратный процесс кристаллизации однофазного оксида. При 1000 °C вновь образуется одна фаза шпинели, близкая по составу к исходной "родительской" фазе Mn1.5Ga1.5O4. При охлаждении вновь наблюдается расслоение этой фазы вследствие присоединения кислорода.

In situ high-temperature diffraction study of the perovskite-brownmillerite phase transition in SrCo0.8Fe0.2O2.5 in isostoichiometric mode

In situ high-temperature diffraction study of the perovskite-brownmillerite phase transition in SrCo0.8Fe0.2O2.5 in isostoichiometric mode

Chemical ordering rather than random alloying in SbAs

The semimetallic group-V elements display a wealth of correlated electron phenomena due to a small indirect band overlap that leads to relatively small, but equal, numbers of holes and electrons at the Fermi energy with high mobility. Their electronic bonding characteristics produce a unique crystal structure, the rhombohedral A7 structure, which accommodates lone pairs on each site. Here, we show via single-crystal and synchrotron x-ray diffraction that antimony arsenide (SbAs) is a compound and the A7 structure can display chemical ordering of Sb and As, which were previously thought to mix randomly. Formation of this compound arises due to differences in electronegativity that are common to IV-VI compounds of average group V such as GeTe, SnS, PbS, and PbTe, and also ordered intraperiod compounds such as CuAu and NiPt. High-temperature diffraction studies reveal an order-disorder transition around 550 K in SbAs, which is in stark contrast to IV-VI compounds GeTe and SnTe that become cubic at elevated temperatures but do not disorder. Transport and infrared reflectivity measurements, along with first-principles calculations, confirm that SbAs is a semimetal, albeit with a direct band separation larger than that of Sb or As. Because even subtle substitutions in the semimetals, notably Bi${}_{1\ensuremath{-}x}$Sb${}_{x}$, can open semiconducting energy gaps, a further investigation of the interplay between chemical ordering and electronic structure on the A7 lattice is warranted.

Experimental charge density study of (E)-4-(2,4-diisopropylphenyl)-4-oxo-2-butenoic acid

Oxygen-ionic conductors are very important materials for various kinds of applications such as fuel cells and oxygen-gas sensors. In 1995, Nakayama found new type of oxygen-ionic conductor; Ln-Si-O apatites (Ln = Lanthanoids). [1,2] These compounds show better oxygen ionic conductivity in middle range of temperature compared with other famous oxygen-ionic conductors (ZrO2 and/or CeO2-based materials). Recently, Yoshioka reported that Mg-doped apatites show the best ionic conduction. [3] However, in all of the apatite system, details of mechanism of ionic conduction are still unknown. Therefore, we carried out high-temperature diffraction study using synchrotron radiation to suppose mechanism from a change of electronic density. In this study, we selected 20 % Mg-doped compound, which showed the highest conductivity. Intensity data was measured by powder method at BL-19XU beam line, SPring-8, Japan, and the temperature range was from room temperature to 1283 K. The crystal structures were refined by Rietveld method using RIETAN-FP [4], the electron distributions were investigated by whole-pattern fitting approach based on the maximum-entropy method (MEM) using PRIMA [5], and the structure and electron distributions were drawn by VESTA [6,7]. In whole temperature range, this compound belongs to P63/m. The Rietveld refinement suggested an interstitial oxygen site O5 located at (-0.01,-0.003,-0.002). Electron density map derived from the MEM analyses revealed that oxide ions O4 located at 2a site were diffused along the caxis. In addition, a short-range curved diffusion path through the interstitial site was clearly visualized between two oxygen atoms O3 within the SiO4 tetrahedra.

D087 Phase instability of SrXLa1−XCoO3−D: a high temperature diffraction study

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A high-temperature diffraction study of the solid solution CaTiOSiO4-CaTiOGeO4

The structure of CaTiOGeO 4 (CTGO) has been refined using single crystal X-ray diffraction data. CTGO is isostructural with titanite, CaTiOSiO 4 . The displacive P 2 1 / a - A 2/ a phase transition analogous to titanite has been studied by in situ heating X-ray powder diffraction and transmission electron microscopy. The transition is accompanied by the disappearance of superstructure reflections, k + l = 2 n + 1, which are replaced by diffuse scattering for T > T c . The diffuse scattering is seen as streaks along b* in high-temperature TEM SAD. Lattice parameters as a function of temperature and composition were determined by X-ray powder diffraction between room temperature and a maximum of 1123 K. Strain analysis of CTGO indicates a transition temperature of T c = 588 ± 4 K and the additional occurrence of an isosymmetric anomaly at T i = 800 ± 25 K. There is complete solid-solution along the join CaTiO(Ge x Si 1− x )O 4 . The lattice parameters across the solid solution vary continuously and the structural phase transitions were identified based on the determination of spontaneous strain associated with the transitions. The e 11 and e 13 components dominate the strain tensor. All compositions across the solid solution exhibit close to tricritical behavior and nearly constant scalar strain.

High-temperature X-ray investigation of natural columbites

Structural investigations at high temperature were carried out on natural columbite samples across the join Fe(Nb0.95Ta0.05)2O6–Mn(Nb0.95Ta0.05)2O6. The samples were preliminarily annealed to attain the complete cation-ordered state and avoid the superimposition of the effects of cation ordering during high-temperature studies. Unit-cell parameters of three columbites with different XFe content were measured at regular intervals in the temperature range 25–900 °C using single-crystal X-ray diffraction techniques. The crystal structures of completely ordered ferrocolumbite and manganocolumbite were also refined from intensity data collected at room temperature, 300 and 600 °C. Structural thermal expansion coefficients show positive, linear expansion of a, b, c lattice constants and cell volume. In general, slightly higher expansion occurs along a and c directions. However, anisotropy decreases sharply with decreasing Fe content. Reversibility of thermal expansion in the investigated temperature range was checked by high-temperature diffraction studies under heating-up and cooling-down conditions. Impurities do not play an important role in thermal expansion of columbites; expansion coefficients measured on two crystals of the same sample characterized by different Ti content are in fact almost identical. Structural changes with temperature essentially affect bond lengths: volumes of both A and B octahedral sites increase linearly with temperature, whereas interpolyhedral geometrical parameters do not vary significantly.

High-precision parallel-beam X-ray system for high-temperature diffraction studies

A recently developed Rigaku parallel-beam X-ray diffraction system equipped with a parabolic graded-multilayer mirror in the incident beam and a parallel-slits analyzer in the diffracted beam was used for precision high-temperature diffraction studies. The lattice parameters a and c of α-Al2O3 at room temperature and up to 1473 K were determined with precision in the range of 0.6–7.3×10−5. The thermal expansion coefficients for a and c agreed with literature values to better than 3%. The system was used successfully also to determine the Debye characteristic temperature of Si and to study structural phase transition of LaCoO3 from rhombohedral at room temperature to cubic at 1700 K.

High temperature thermal expansion of mullite: an in situ neutron diffraction study up to 1600°C

Structural thermal expansions of undoped and Cr-doped 3/2 mullites (10.8 wt.% Cr2O3) were measured with in situ neutron (25–1600°C) and X-ray synchrotron (25–1000°C) diffraction techniques. Heat treatments between 25 and 300°C cause little and non-linear structural expansions. Above 300°C and up to 1000°C the mullites display linear and low increase of the lattice constants. The mean values of undoped mullite are slightly higher (αav≈ 5.45×10−6) than those of Cr-doped mullite (αav≈ 4.95×10−6), thus confirming earlier in situ high-temperature diffraction data. Above about 1000°C the expansion behaves discontinuously. Structural expansions of undoped and Cr-doped mullite strongly increase yielding similar mean expansion coefficients (αav≈ 7.50×10−6) for both materials. The highest increase of lattice constants has been determined in b axis direction throughout 1600°C in undoped and Cr-doped mullite. High temperature diffraction studies under heating-up and cooling-down conditions yielded reversible expansion effects, including the anomalous behaviour at about 1000°C.

A high temperature diffraction study of synthetic titanite catiosio4

The results of a high temperature single crystal X-ray diffraction study of synthetic titanite within the stability field of the A2/a (C2/c) paraphase (T > 500 K) are reported. The structure has been refined using a conventional model and one in which the Ca atom is disordered over two positions. A break in thermal expansion near T c = 825 K correlates with an effective volume contraction of the Ti octahedron. When refined with split Ca position a reorientation of the Ca displacement vector at T c is found, resulting in a more symmetrical structural arrangement of the disordered Ca cations with respect to the surrounding Ti cations. In the conventional model this reorientation is seen as a break in the thermal elongation of the shortest Ti-Ca distance. The observed temperature of the isosymmetrical structural instability is in agreement with previous observations based on Hard Mode IR and Raman spectroscopic measurements. A possible mechanism causing the observed structural changes and similarities to the thermal behaviour of the iso-structural malayaite, CaSnSiO5, are discussed.

Powder Diffraction Studies in the YONO3-Y2O3 System

The crystal structure of yttrium oxide nitrate and the microstructure of the oxide obtained from its thermal decomposition have been investigated by modern powder diffraction methods. Sequential high-temperature diffraction studies of the thermal decomposition of two precursors of yttrium oxide nitrate, i.e., neutral yttrium nitrate and yttrium hydroxide nitrate, are reported. The structure of YONO3 has been determined ab initio from conventional powder diffractometry using monochromatic X-rays. The pattern was indexed by the successive dichotomy method yielding tetragonal unit cell dimensions: a = 3.8590(1) Å and c = 9.7161(6) Å. The space group is P4/nmm and Z = 2. Most of the atoms were located by direct methods and the remaining ones from a Fourier map. Refinement of the complete diffraction profile parameters converged to final agreement factors Rp = 0.12, Rwp = 0.16 and RF = 0.039. The structure is closely related to the matlockite PbFCl-type structure. The microstructure of ex-oxide-nitrate yttrium oxide has been analysed by total pattern fitting techniques, from which strain-free coherently diffracting nanodomains with a rectangular parallelepipedic shape have been obtained.

High-temperature diffraction study of LnCoO 3 perovskites: A high-order electronic phase transition

A powder X-ray diffraction study of selected orthorhombic LnCoO 3 (Ln = Nd, Gd, Dy, and Ho) perovskites has been performed between 300 and 1098 K. The room-temperature result shows that polyhedral distortion increases with decreasing Ln ionic radius. The distortions for both the Co octahedra and Ln dodecahedra increase from NdCoO 3 to HoCoO 3, and the average Co-O bond length remains unchanged. Although there are no symmetry-related structural changes up to 1098 K, it was observed that, with increasing temperature, the polyhedral distortion first decreases up to T 1, (for Nd T 1 ⩽ 300 K, which increases gradually with Ln atomic number, and for Ho T 1 = 730 K), and then increases to T 2 ( T 2 = 730 K for Nd and ⩾ 1100 K for Ho), after which the distortion decreases again. This change in distortion is also accompanied by a large unit cell volume increase. These observations are attributed to a high-order cobalt electronic phase transition, in which the localized cobalt 3 d electrons in t 2 g and e g orbitals gradually transform within the temperature range between T 1 and T 2 to an itinerant state forming the π ∗5σ ∗13 d electron band with σ ∗ half-filled.

A simple heater attachment to precession camera for high temperature diffraction studies up to 1000°C

A simple heating attachment suitable for taking x-ray precession photographs at high temperatures is described. It consists of a platinum wire-wound heater, split at the middle for sample insertion and x-ray irradiation. The heater is mounted on a quartzsupport and is independant of both the goniometer head and collimator. This makes crystal mounting and alignment at high temperature as easy as at room temperature and precession photographs up to precession angle\(\bar \mu = 30^ \circ \)=30° can be taken without any interference from the heater assembly, up to 1000°C.