Über Ordnungs-Unordnungsphänomene bei Sauerstoff perowskiten vom Typ A2+3B2+M5+2O9

An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:2 B site ordered A3CoNb2O9 perovskites (A=Ca2+, Sr2+, Ba2+)

“…2c) could also be indexed on a metrically hexagonal unit cell, with a h ¼ 3:9913O2 ¼ 5:6445ð7Þ Å , c h ¼ 4:0073O3 ¼ 6:9408ð12Þ Å . Such a cell is in reasonably good agreement with the triclinic cell reported by Treiber and Kemmler-Sack [16] (a ¼ b ¼ 5:64; c ¼ 6:94 Å , a ¼ 90:3; b ¼ 89:7; g ¼ 120 ) but not with the metrically cubic 3.93 Å cell reported by Yin et al [7]. At the highest values of 2y there was some possible indication of even further lowering of the metric symmetry in the form of broadened lines and/or additional partial splittings of the parent perovskite lines (see, e.g., Fig.…”

“…1) that might have been expected given earlier reports [14][15][16]. The Ba(Co 1/3 Nb 2/3 )O 3 and Sr(Co 1/3 Nb 2/3 )O 3 compounds were thus reported to be metrically cubic and of Pm % 3m space group symmetry (a ¼ a p ¼ 4:09 and 3.93 Å , respectively, subscript p for the underlying perovskite parent sub-structure) while the Ca(Co 1/3 Nb 2/3 )O 3 compound was reported as being metrically monoclinic [7] (aBa p ¼ 3:911; bBa p ¼ 3:892; cBa p ¼ 3:911 Å , b ¼ 91 16 0 ) although no space group was given.…”

Physical properties of BaMg1/3Nb2/3O3–BaCo1/3Nb2/3O3 solid solutions

“…The increase of the lattice parameters with an increase in cobalt concentration can be easily explained by the difference in the value of ionic radii of the two ions involved in the substitution ( VI R Mg 2C Z 0:660 A, VI R Mg 2C Z 0:720 A) [9]. It has been observed [10], that the BCN material sintered at 1400 8C exhibits 1:2 type ordered structure (Table 1) and that this ordering disappears at higher temperatures (above1500 8C the structure becomes disordered cubic perovskite). In certain synthesis conditions this order-disorder transition use to be reversible.…”

Influence of the calcium excess in the structural and spectroscopic properties of the complex perovskite Ba3CaNb2O9

“…Nonetheless, a very clear trend between the energy of vibration and the (pseudocubic) lattice parameter can be observed for the series studied here and more complex perovskites as shown in Fig. 5 with data from references [13][14][15][16].…”

“…Our XRD studies show clearly the formation of the perovskite phase with a gradual increase in the pseudo-cubic cell [13][14][15][16] parameter. The data only gives us qualitative information about the presence of both phases and a transition from hexagonal to cubic in going from x00 to x00.25.…”