Phase Formation and Properties in the System Bi₂O₃:2CoO_1+x:Nb₂O₅

Abstract

Subsolidus phase relations have been determined for the Bi–Co–Nb–O system in air (750–925 °C). Ternary compound formation was limited to pyrochlore (A2B2O6O′), which formed a substantial solid solution region at Bi‐deficient stoichiometries (relative to Bi2Co2/3Nb4/3O7) suggesting that about 8 to 25 % of the A sites are occupied by Co. X‐ray powder diffraction data confirmed that all Bi–Co–Nb–O pyrochlores exhibit displacive behavior. A structural refinement of the pyrochlore Bi1.56Co0.96Nb1.48O7 using single‐crystal X‐ray diffraction data is reported with the A and O′ atoms displaced (0.34 Å and 0.45 Å, respectively) from ideal positions to96g sites and 32e sites, respectively [Fd3$\bar {S}$m (#227), a = 10.551(1) Å]. The displacive structural behavior is similar to that found in analogous Bi–M–Nb–O pyrochlores (M = Zn, Fe, Mn). Bi–Co–Nb–O pyrochlores exhibited overall paramagnetic behavior with negative Curie–Weiss temperature intercepts indicating weak antiferromagnetic interactions. At 250 K and 1 MHz the relative dielectric permittivity of the pyrochlore 0.4400:0.2100:0.3500 Bi2O3:2CoO1+x:Nb2O5 was ca. 115 with tan δ = 0.06; however, at lower frequencies the sample was conductive. Low‐temperature dielectric relaxation such as that observed for Bi1.5Zn0.92Nb1.5O6.92 and other bismuth‐based pyrochlores was not observed. Consideration of bond lengths, effective magnetic moments, and dielectric properties suggests that the average oxidation state of Co in the pyrochlore phases is close to but slightly higher than 2.0. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems

“…to a very flat, or pancake-shaped, ellipsoid. Clearly, as has also been found for BZN [7][8][9] and the other Bi-pyrochlores whose structures have been refined to date [9][10][11][12][13], the disordered A cation displacements are constrained to displace in {111} planes perpendicular to the local O 0 -A-O 0 -axis (by $0.385 Å /$0.405 Å in this case).…”

“…This has led to their designation as so-called ''misplaced-displacive'' cubic pyrochlores [10]. Structure refinements of BZN [7][8][9] and a few other such Bi-based pyrochlores [11][12][13] to date have all reported considerable displacive disorder, particularly associated with the O 0 A 2 sub-structure. Despite considerable recent efforts, however, the local crystal chemistry underlying such behaviour still remains far from well understood [7,8,14,15].…”

“…Crystal chemical interest in these Bi-based pyrochlores has simultaneously arisen as a result of careful phase analysis studies in BZN and several other such systems [9][10][11][12][13] showing that they do not form at 'sensible' stoichiometries such as Bi 2 (M II 2/3 Nb V 4/3 )O 7 or Bi 2 (M III Nb V )O 7 but rather appear to be significantly Bi-deficient with respect to these ideal stoichiometries and to require the presence of nominally far too small M 2+ or M 3+ cations on the A sites (as well as the B sites) of the ideal A 2 B 2 O 7 (or B 2 O 6 Á O 0 A 2 ) pyrochlore structure type (shown in Fig. 1).…”

Synthesis and characterization of new pyrochlore solid solution Bi1.5Sb1.5Cu1−xMnxO7

“…The copper is distributed between A and B sites like this formula (Bi 1.5 Cu 0.5 )(Sb 1.5 Cu 0.5 )O 7 . Several compounds possess this organization as the zinc pyrochlore compound with formula (Bi 1.5 Zn 0.5 )(Sb 1.5 Zn 0.5 )O 7 [12] and cobalt pyrochlore compounds with formula (Bi 1.56 Co 0.44 )(Sb 1.48 Co 0.52 )O 7 [37] or (Bi 1.56 Co 0.44 )(Nb 1.48 Co 0.52 )O 7 [38]. The substitution of copper for manganese in Bi 1.5 Sb 1.5 CuO 7 compound can form a total solid solution with formula (Bi 1.5 Cu 0.5−x/2 Mn x/2 )(Sb 1.5 Cu 0.5−x/2 Mn x/2 )O 7 (Fig.…”

Synthesis, structure and electrical properties of Mg-, Ni-codoped bismuth niobates

“…The most attention has been paid to the Zn-, Mg-containing bismuth niobates, which possess high dielectric constant (170-180) and low dielectric loss (~10 -4 ) at 1 MHz (at room temperature) [1][2][3][4][5][6][7][8][9]. To search for the same properties Fe- [10], Mn- [11][12], Co- [13], Ni- [12,[14][15], Cu- [12] and the mixed Zn-M (M -Sr [16], Ca [16][17], Mn [16,18], Ti [19][20][21][22], Sn [19,22], Zr [19,[21][22], Ce [19,22], Gd [21], Ta [23], La [24]), Mg-M (M -Sr [25], Nd [26], Cu [27]) bismuth niobates and other ones were synthesized. The improved permittivity was achieved by Ti doping of the Nb sites in the pyrochlore structure [21][22] and by Cu doping in Bi 1.5 Cu x Mg 1-x Nb 1.5 O 7 (x = 0.075) [27].…”