Several materials properties of rutile TiO 2 (together with its other polymorphs) have been widely investigated in view of both fundamental and practical interest. It is a wide band gap (E g ¼ 3.05 eV) semiconductor that finds application, among others, as a photocatalyst for splitting water [1] into H 2 and O 2 and remediation of organic pollutants. [2] Among the lesswell-known properties of TiO 2 is its abnormally large static dielectric permittivity that shows strong frequency dependence as well as the associated soft A 2u mode that decreases with decreasing temperature. [3,4] The latter, however, never becomes completely soft, even at 0 K. Accordingly, rutile is classified as an incipient ferroelectric. We prepared M 0 TiTaO 6 (M 0 ¼ Al, Cr, Fe) oxides by conventional solid-state synthesis starting from pure binary oxides. Powder X-ray diffraction (XRD) showed formation of rutile structure for all three M 0 TiTaO 6 compounds. All the reflections in the patterns (Fig. 1) could be indexed to the tetragonal rutile structure. The structural parameters derived from Rietveld refinement of the powder XRD data are given in Table 1. We see that while the c/a ratio remains nearly the same as in the parent rutile TiO 2 , the a and c parameters of M 0 TiTaO 6 show variations that are consistent with the average ionic radii of the metal atoms. We find that the average M-O distances in FeTiTaO 6 are slightly larger than those in rutile, consisting of 4 short and 2 long M-O bonds (elongated MO 6 octahedron); curiously, for M 0 ¼ Al and Cr oxides we find 4 long and 2 short M-O bonds (compressed MO 6 octahedron).We have recorded neutron diffraction (ND) patterns of M 0 ¼ Cr, Fe at T ¼ 300, 150, and 12 K, in an attempt to characterize further the structure of these oxides. We see no evidence for a long-range magnetic ordering down to 12 K in
The meta-stable orthorhombic phase of lead meta-niobate (PbNb 2 O 6 ), as obtained by quenching, is piezoelectric, but difficult to prepare in pure state as a few competing phases like the non-piezoelectric rhombohedral PbNb 2 O 6 tend to form during the preparation. However, it can have an attractively high Curie temperature above 570 • C. High Curie temperature piezoelectric sensors are desirable for imaging and other applications in certain industrial and research sectors. Presently, both phases have been synthesized as single phase samples as shown here from structural Rietveld analysis of their XRD. Characterization of both compounds by FTIR and XRD have been carried out and presented.
A high-temperature neutron diffraction study has been carried out on La(0.75)Sr(0.25)CrO(3) compound in the temperature range 300-1400 K. On doping the parent compound LaCrO(3) with Sr at the La site, the orthorhombic (Pbnm) to rhombohedral ([Formula: see text]) structural transition shifts to lower temperatures. From quantitative Rietveld analysis it is found unequivocally that there is a two-phase coexistence (orthorhombic and rhombohedral phases with ∼89 and 11 weight%, respectively) in the temperature range 300-470 K and a three-phase coexistence (with a new cubic phase with space group Pm3m) in the temperature range 480-1400 K. The weight percentages of the orthorhombic, rhombohedral and cubic phases were found to be ∼49%, 37% and 14%, respectively, in the temperature range 480-1300 K, while over 1350-1400 K, the average weight percentages of orthorhombic, rhombohedral and cubic phases were found to be ∼41%, 41% and 18%, respectively. The coefficients of volume thermal expansion and linear thermal expansion have been determined for all three phases. The importance of the present study has been discussed for practical applications of the studied compound in solid oxide fuel cells.
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