Presence Of Anion Vacancies Research Articles

Structural, optical and magnetic characteristics of Mg2+- doped (Ba0.94Sr0.06)(Fe0.80Al0.20-xMgx)O3-ẟ (x = 0–0.20) oxides

Perovskite-type ABO3 oxides have attracted immense attention in recent technologies e.g., gas sensors, solar cells, fuel cells, oxygen-permeable membranes, etc. An attempt has been made here to synthesize (Ba0.94Sr0.06)(Fe0.80Al0.20-xMgx)O3-ẟ (x = 0–0.20) oxides using sol-gel method and characterized with regards to phase, Raman and magnetic properties. X-ray diffraction-cum-Rietveld refinement shows the structural transformation from cubic (Pm3m) to hexagonal (P63/mmc) via cubic (Pm3m) + rhombohedral (R3c) and increase of cubic lattice parameter ac = 4.0369–4.1296 Å with magnesium. FT-IR spectra exhibit different vibrational bands at 548, 555, 638, 667, and 692 cm−1, representing the metal-oxygen (M-O-M) bond vibrations (M - Fe, Mg, Al), deformation of MO6 octahedral, and internal changes in bond lengths. No Raman band was appeared for cubic phase with x=0–0.10. Bands between ∼ 250–450 cm−1 assign the tilting, rotation, and asymmetric bending vibrations of Fe – O, Mg – O and Al – O bonds and asymmetric bending mode of (Fe/Mg/Al)O6 octahedra. Oxygen bending vibration, anti-stretching mode and presence of oxygen vacancies lie in the bands of range ∼498–656 cm−1. Maximum coercivity (Hc ∼ 1081 Oe) and remanent magnetization (Mr ∼ 0.033 emu/g) have been observed for x = 0 and found similar variation with ‘x’. The saturation magnetization (Ms) and anisotropic constant (K1) values lie as Ms = ∼ 0.678 – 0.754 emu/g and K1 = ∼ 1.38× 104 – 1.53× 104 erg/cm3, respectively with magnesium (x). Presence of anion vacancies and high coercivity make the materials suitable for industrial applications.

The impact of anionic vacancies on the mechanical properties of NbC and NbN: An ab initio study

The development of super-hard materials has recently focused on systems containing heavy transition metal and light non-metallic elements. Niobium carbides and nitrides have previously been identified as potential candidates, however, the volatility of carbon and nitrogen during synthesis make them prone to formation of anionic vacancies, which have the ability of changing the electronic structure, dynamical stability and adversely affecting the mechanical properties. This study presents ab initio Density Functional Theory calculations that probe the occurrence of anionic vacancies as a function of concentration, thereafter, pertinent mechanical properties are investigated. Our results showed that the presence of anionic vacancies in NbC and NbN tend to deteriorate the mechanical properties and ultimately the mechanical hardness due to vacancy softening that can be attributed to defect induced covalent to metallic bond transition. Further, it was observed that anionic vacancies in NbC tend to modify its toughness; in particular, NbC in ZB structure becomes brittle while NbC in WZ phase becomes ductile in presences of C vacancies of up to 6%. The dynamical stability of NbC in RS, ZB, and WZ phases and NbN in WZ phase were found to be insensitive to the presence C and N vacancies, while NbN in RS and ZB becomes dynamically stable with the introduction of N vacancies. In addition, the toughness of NbN was found to be insensitive to defect concentration of even up to 8%. Consequently, stringent control of anionic defects during synthesis of NbC and NbN is critical for realization of the desired mechanical response that can make these materials ideal for super-hard and related applications.

Surprising formation of a rhodochrosite-like (MnCO3) phase on Co-Zn-Mn sintered spinels upon storage at room temperature and ambient air

A detailed XPS investigation, supported by XRD analysis, was performed on Co-Zn-Mn oxide-based spinels as a function of cobalt concentration and calcination temperature (723 and 973K). Results revealed for the first time that a recarbonation occurred under ambient conditions but only for the Co-containing materials calcined at 973K. Indeed, such effect was not observed for the homologous preparations at 723K. This evidence was quite surprising because the solids calcined at 973K, for which a sintering occurred as reflected by the low surface areas, were supposed to be almost inert under ambient conditions. Such an intriguing recarbonation chemistry, leading even to a bulky rhodochrosite-like (MnCO3) phase in the cobalt most enriched samples, arises from the combined effect of the calcination treatment at high temperature and the presence of cobalt ions. The spinels calcined at higher temperature are characterized by a high surface defectivity, due to anionic vacancies produced by a partial self-reduction of a fraction of Mn3+ ions that occurs only at 973K but not at 723K. Consequently, the presence of anionic vacancies and the incomplete coordination of the outermost layers promoted a higher reactivity of the solids calcined at 973K with CO2 and moisture from ambient air. However, a carbonate phase was absent in the sample at 973K with no cobalt. This suggests that the presence of cobalt is another crucial factor for the recarbonation process to take place. Cobalt ions appear to play a catalytic role. An attempt is made to provide a reasonable explanation of this intriguing recarbonation chemistry, trying to shed some more light on the rather complex chemistry behind the attack of moisture and CO2 to oxide solid surfaces. These results, as a whole, may provide new insight on phenomena observed in the case of some Mn-based catalysts, as the severe deactivation by traces of moisture occurring in hopcalite during CO oxidation at RT. They may also give an additional perspective to environmental processes that involves the interaction of moisture and CO2 with the surface of Mn-based solids or other materials as environmental interfaces.

Spectra of divalent samarium in LaF3 crystals

The optical spectra and electrical conductivity of LaF3 crystals containing Sm2+ ions are studied. Groups of emission lines at 560–620, 650–690, and 680–770 nm at 7.9 K are produced by transitions from states 5 D 2, 5 D 1, and 5 D 0 to states 7 F j of Sm2+ ions. Radiative transitions from state 5 D 0 are characterized by a decay time of 8.9 ms and temperature quenching in the range of 70–160 K. A linear dependence of the crystals’ electrical conductivity on the magnitude of absorption bands of Sm2+ ions is observed, due the presence of anionic vacancies that compensate for the low charge of Sm2+ ions. The long-wavelength absorption band at 600 nm that is not observed in the excitation spectra is produced by the transition from 4f Sm2+ to the 1s level of anion vacancies. Absorption bands with wavelengths below 520 nm are due to 4f-5d transitions in the Sm2+ ions.

The formation of surface stable anion vacancy states at CeO2 ultra-small crystallite dimensions

The ceria–terbia solid solution system has been studied in order to assess whether ultra-small crystallites have any propensity to form stable anion vacancies at their surface. This has been suggested by many authors and has become generally accepted. The presence of anion vacancies should be manifest in lattice expansion brought about by the presence of associated Ce3+ sites. Using X-ray diffraction (XRD), no such lattice expansion was observed here and instead a lattice contraction with reduced crystallite size was observed. Further, X-ray photoelectron spectroscopy (XPS) indicates there is little evidence for the association of any vacancies with cerium cations.

Neutron powder diffraction as a characterization tool of solid oxide fuel cell materials

Abstract Neutron diffraction is a powerful tool for the characterization of materials and, particularly, oxides. Oxide materials find applications in solid oxide fuel cells (SOFCs) as solid electrolytes as well as anode and cathode materials. As a structural probe, neutrons are specially suitable for the crystallographic study of oxides, given the comparable scattering factors of O and other heavier elements, allowing its precise localization in the crystal structure. Many problems can be addressed by neutrons, related to the octahedral tilting in perovskites, phase transitions, order–disorder phenomena, presence of anionic vacancies, etc. Neutrons make possible an accurate determination of the thermal factors and provide a visualization of the diffusion paths in ionic conductors. Neutrons allow the localization of light atoms such as hydrogen, and make possible the distinction between neighbouring elements, typically Fe and Mn. In this work we will describe some recent applications of this technique in the field of solid electrolytes and electrode materials, including some examples from our group.

Synthesis and Characterization of a MetastableLnTh2F11Series (Ln=La–Lu, Y) with Cationic and Anionic Disorder: Crystal Structure of SmTh2F11

A series of apparently stoichiometric and metastable phases of compositionLnTh2F11(Ln=La–Lu) was prepared for the first time. The crystal structure of SmTh2F11was determined from single-crystal data with direct methods and refined toR1=0.043 and wR2=0.107 [space group:Pnma;a=861.0(2) pm,b=413.7(1) pm,c=722.5(2) pm;Z=1.33]. This new structure type is characterized by a (Th+Sm) disorder on the 4ccationic site and by the presence of anionic vacancies on three of the four 4canionic sites. The idealMF4structure is a three-dimensional network of interconnectedMF11polyhedra but the coordination number of the cations is locally reduced to eight for Sm3+. The structural relationship with the LaF3(tysonite) type is discussed.

Exchange of metal atoms in solid mixed-valence TlI3TlIIICl6. Conductivity and positron annihilation investigations

Physical techniques have been applied to derive information on isotopic exchange in mixed-valence Tl4Cl6. Using the positron as a probe, it has been found that cation-vacancy formation is relatively easy, with an activation energy of 0.35 eV. Conductivity measurements have shown that Tl4Cl6 is a good ionic conductor in the temperature range where exchange proceeds (470–550 K) with an activation energy for conduction of 0.69 eV. Comparing these energies with the activation energy derived in a previous chemical study shows that the exchange process is triggered by the presence of anion vacancies.

Thermal depolarization in ammonium chloride crystals, I

The thermal depolarization of crystals of NH 4Cl polarized above the phase transition temperature (242.5K) by an applied electric field was studied in 13 crystals prepared under a variety of conditions in order to examine the effects of doping with divalent anions or cations and of acid or alkaline conditions during crystal growth. All the features found in crystals grown from solution also occur in a crystal grown from the vapor. We may conclude, therefore, that the depolarization effects observed are properties of crystalline NH 4Cl and are not due to water or urea occluded during crystal growth. Four main depolarizations were observed: one of these is ascribed to space charge and the other three are attributed to dipole relaxation processes. The activation energies for these are 0.58, 0.73, and 0.85 eV, respectively. The various doping experiments lead to the conclusion that one of these is favored by acid conditions and the presence of cation vacancies and the other two by alkaline conditions and the presence of anion vacancies. Specific models for the dipolar defects are formulated and discussed.