Ba Phase Research Articles

A novel layered aluminate with a Kagome arrangement: A 1.5+ xSb +5−2 xAl 5O 11.5 (A = Sr, Ba)

The A 1.5+ x Sb +5−2 x Al 5O 11.5 (A, x = Sr, 0.18 for A = Sr; and x = 0 for A = Ba) phases crystallize in a hexagonal cell ( P6 3/ mmc – no. 194) with a = b = 5.61549(4) Å, c = 31.4421(4) Å for the Sr phase and a = b = 5.64390(6) Å, c = 32.1594(5) Å for the Ba phase. Their crystal structure was determined by electron diffraction and X-ray powder diffraction. Its original layered structure presents both close-packed double hexagonal blocks and lacunar pillars layers. It consists of Al Kagome layers connected through Al 2O 7 pillars and bordered by mixed layers made from SbO 6 and AlO 4 polyhedra. The A 2+ ions lie between the Al 2O 7 pillars and between the mixed layers. High resolution electron microscopy comforts the structural model and shows the regularity of the layers' stacking mode.

In situ FT-IR and reactivity study of NOxstorage over Pt–Ba/Al2O3catalysts

The mechanism of NOx storage on Pt–Ba/Al2O3 and, for comparison, on Ba/Al2O3 catalysts was investigated. In situ FT-IR spectroscopy was used to follow temporal changes in the types and relative amounts of surface species formed at three different temperatures (423, 523, 623 K) during the reaction with NO, NO/O2, NO2 or NO2/O2 atmospheres. At 623 K the NOx storage capability was also investigated by a mass-spectrometry transient response method (TRM) as a complementary technique. The results show that in absence of oxygen appreciable amounts of NO are adsorbed only on the Pt–Ba/Al2O3 system mainly as ionic nitrites. When oxygen is present, NO is stored according to two different mechanisms, both promoted by Pt. The first occurs through the formation of surface nitrites that evolve to nitrates; the second occurs through the NO oxidation to NO2 over Pt, followed by NO2 migration to the Ba phase and nitrate formation. When NO2 is used with or without O2, an extensive storage in the form of ionic nitrates occurs both on Ba/Al2O3 and Pt–Ba/Al2O3 catalysts at all the temperatures studied, without the formation of nitrites. NO2 storage occurs initially through a very fast mechanism, giving nitrates without NO release. In parallel, it proceeds via a disproportionation reaction of NO2 to nitrate and NO. Both mechanisms are not promoted by Pt.

Metallic and nonmetallic double perovskites: A case study ofA2FeReO6(A=Ca, Sr, Ba)

We have investigated the structure and electronic properties of ferrimagnetic double perovskites, A2FeReO6 (A= Ca, Sr, Ba). The A=Ba phase is cubic (Fm3m) and metallic, while the A=Ca phase is monoclinic (P21/n) and nonmetallic. 57Fe Mossbauer spectroscopy shows that iron is present mainly in the high-spin (S=5/2) Fe3+ state in the Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. It is argued that a direct Re t2g - Re t2g interaction is the main cause for the metallic character of the Ba compound; the high covalency of Ca-O bonds and the monoclinic distortion (which lifts the degeneracy of t2g states) seem to disrupt the Re-Re interaction in the case of the Ca compound, making it non-metallic for the same electron count.

Characteristics of the surface layer of barium strontium titanate thin films deposited by laser ablation

Ba 0.5 Sr 0.5 TiO 3 (BST) thin films grown on Si by an in situ ultraviolet-assisted pulsed laser deposition (UVPLD) technique exhibited significantly higher dielectric constant and refractive index values and lower leakage current densities than films grown by conventional PLD under similar conditions. X-ray photoelectron spectroscopy (XPS) investigations have shown that the surface layer of the grown films contained, besides the usual BST perovskite phase, an additional phase with Ba atoms in a different chemical state. PLD grown films always exhibited larger amounts of this phase, which was homogeneously mixed with the BST phase up to several nm depth, while UVPLD grown films exhibited a much thinner (∼1 nm) and continuous layer. The relative fraction of this phase was not correlated with the amount of C atoms present on the surface. Fourier transform infrared spectroscopy did not find any BaCO3 contamination layer, which was believed to be related to this new phase. X-ray diffraction measurement showed that although PLD grown films contained less oxygen atoms, the lattice parameter was closer to the bulk value than that of UVPLD grown films. After 4 keV Ar ion sputtering for 6 min, XPS analysis revealed a small suboxide Ba peak for the PLD grown films. This finding indicates that the average Ba–O bonds are weaker in these films, likely due to the presence of oxygen vacancies. It is suggested here that this new Ba phase corresponds to a relaxed BST surface layer.

Nonepitaxial sputter synthesis of aligned strontium hexaferrite, SrO⋅6(Fe2O3), films

Films of strontium hexaferrite were synthesized through rf diode sputter deposition, nonepitaxially onto polycrystalline Al2O3 substrates. So far such films for the analogous Ba phase have been synthesized mostly through epitaxial growth onto suitably chosen matching substrates. Crystallization of the strontium hexaferrite phase was achieved for deposition temperatures as low as 500 °C. For nonepitaxially synthesized 2–3-μm-thick polycrystalline films, a fairly high degree of variable and controlled c-axis orientation of the crystallites has been obtained. The x-ray-diffraction traces of films with a strong c-axis in-plane alignment showed predominantly (110)-type reflections. The film coercivities were increased by postdeposition annealing in O2. The remnant to saturation flux density ratio of such in-plane anisotropy films for in-plane measurements was 0.62, with an in-plane coercivity ≊2.3 kOe.

Core and valence XPS spectra of clean, cleaved single crystals of YBa 2Cu 3O 7

Small single crystals of YBa 2Cu 3O 7 with sharp superconducting transitions at 91 K were cleaved at 300 K in UHV (10 −9 torr) and their XPS spectra were measured using small spot (300 μm dia.) Al Kα radiation. The cleaved surfaces showed no initial contamination, as determined by a lack of C(1s) intensity. Only one Ba core level feature is found to be characteristic of the bulk superconductor (Ba(5p 3 2 ), Ba(4d 5 2 ), and Ba(3d 5 2 ) at 12.0 eV, 87.0 eV, and 777.4 eV respectively). A second Ba feature at ~1 eV higher binding energy, found on some cleaved surfaces, represents a surface Ba phase. For the intrinsic surface the O(1s) spectrum is dominated by a peak at 528.0 eV and contains at least two unresolved components. Any large intensity at higher binding energy (530 to 533 eV are often reported) represents reactions at the surface. A relatively large intensity is observed at E F for high quality single crystals. This intensity and the Ba, Y and Cu core level features are stable for a few hours following the cleave at 300 K. There no evidence for oxygen loss at 300 K as judged by the constant O(1s) intensity. These results are compared to results in the literature, where it is often claimed that the higher binding energy feature relates to the intrinsic superconductor. We believe this claim is erroneous, as is the claim that oxygen is lost rapidly from the superconductor in vacuum if the sample is not maintained at 20 K. Finally, the Cu(2p 3 2 ) spectrum of the intrinsic superconductor's surface is very similar to that of Cu 2+ in CuO, whereas data reported in literature often show a strong Cu 1+ component.

The Phase Transitions in Ammonium Hydrogen Sulfate

It was experimentally confirmed by observations of stress-strain hysteresis loop that the room temperature paraelectric phase of NH 4 HSO 4 is ferroelastic. This observation undoubtedly suggests the existence of the prototypic crystal structure from which the paraelectric-ferroelastic phase is derived by the phase transition. By adopting the Pnma phase as a prototype the phase transitions to the room temperature B2 1 /a and then to the ferroelectric Ba phase are group-theoretically analysed. The results are in satisfactory agreement with observations.