Polymorphie von SrTa₂O₆

Abstract

Von SrTa2O6 wurde eine neue, mit orthorhombischem CaTa2O6 strukturverwandte Tieftemperatur‐Modifikation erhalten. SrTa2O6(orh.) ließ sich bei Temperaturen unter 1150°C sowohl aus der bisher bekannten, den tetragonalen Wolframbronzen verwandten Modifikation SrTa2O6(TWB) in Gegenwart eines Transportmittels (Chlor) bzw. eines Mineralisators (B2O3‐Schmelze), wie auch durch Umsetzen von Gemengen (1:1) aus SrCO3 oder Sr(NO3)2 und Ta2O5 in geschlossenen Quarzglasampullen erhalten. Auch als Zwischenprodukt der Umsetzung entsprechender 1:2‐Gemenge bei Temperaturen unter 900°C (z. B. 840°C) tritt diese Modifikation auf. Die Gitterkonstanten von SrTa2O6(orh.) sind aufgrund von Pulverdaten: a = 11,006 Å, b = 7,638 Å, c = 5,622 Å. Beim Erhitzen auf etwa 1 220°C (an der Luft) wandelt sich SrTa2O6(orh.) in SrTa2O6(TWB) um. Eine Rückumwandlung von SrTa2O6(TWB) in SrTa2O6(orh.) ohne Mineralisator‐ oder Transportmittelzusatz war nicht zu erreichen. Mischkristalle CaxSr1−xTa2O6 der Tieftemperaturform ließen sich nicht sicher nachweisen. Die bei Mischkristallen mit 0,05 < x ≤ 0,20 (1300°C) vorliegende TWB‐Struktur ist orthorhombisch verzerrt. CaTa2O6(orh.) zeigte dagegen ab x ≥ 0,85 (1300°C) keine Entmischung.

Local structure and dielectric behavior of tetragonal tungsten bronzes &beta;-SrTa₂O₆ and &beta;&prime;-SrTa₂O₆

“…23) Similarly, in the sintering attempts, excessive heating of ¢-SrTa 2 O 6 occasionally produced intermediate phases between the orthorhombic ¢-and tetragonal ¢B-forms. Therefore, care was taken to avoid phase evolution during the HIP treatment.…”

Crystal structure analysis of tungsten bronzes β-SrTa2O6 and β′-SrTa2O6 by synchrotron X-ray and neutron powder diffraction

“…As is common for complex oxides containing d 0 tantalum, niobium, or titanium, the above Sr m Ta 2n O m þ 5n compounds have been popular choices for exploring photocatalysts or dielectrics [2][3][4][5][6][7]. One particular member, SrTa 2 O 6 , is known to form in three polymorphs designated as a-, b-, and b 0 -phases [8,9], whereas the formation of a metastable defect perovskite Sr 1/2 TaO 3 [10] has been also reported. The a-SrTa 2 O 6 , often denoted as the low-temperature form, is favored in preparations below E1100 1C, while the band b 0 -SrTa 2 O 6 form at higher temperatures, roughly above 1300 and 1500 1C, respectively.…”

“…Although the above three polymorphs of SrTa 2 O 6 have been identified more than three decades ago, their crystal structures have not yet been completely compiled. While the crystal symmetry and the atomic coordinates for a-SrTa 2 O 6 were established from a single crystal diffraction study [8], structural descriptions of the high temperature b-and b 0 -SrTa 2 O 6 phases were limited to the crystal system and the unit cell dimensions [9][10][11][12][13][14]. Moreover the reported cell parameters of band b 0 -SrTa 2 O 6 are somewhat inconsistent, as summarized in Table 1.…”

SrTa4O11: a Rietveld Refinement Using Neutron Powder Diffraction Data

“…The platinum crucible was placed in a quartz tube, evacuated and heated at 1300 K for two weeks, grinding once after one week (Bayer & Gruehn, 1984).…”

“…We report here the crystal structure of SrTa4Ol~. SrTa4Oll was prepared at low temperature by Bayer & Gruehn (1983, 1984. From X-ray diffraction patterns it was concluded that it is isostructural with CaTa4Oll (Jalmberg, 1970;Isobe, Marumo, Iwai & Kimura, 1975) but details of the structure were not known.…”