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An Unexpected Crystal‐Chemical Principle for the Pyrochlore Structure

“…[16] Similar cubic pyrochlores have been reported with Nb instead of Ru; i. e., Bi 1.5 MNb 1.5 O 7 (M = Cu, Mg, Mn, Ni), which would require half of the small M cations to be mixed with Bi 3+ on the A sites. [61] These re- ports do not include detailed X-ray powder diffraction data; however, all of these cubic pyrochlores should exhibit displacive disorder in the A 2 OЈ network to provide chemical accommodation of the small B-type cations -their X-ray powder diffraction patterns should exhibit weak forbidden reflections such as the 442, which can be indexed using the reported Fd3m cubic unit cells, but which serve as (easily overlooked) diagnostic flags for the displacement of atoms to lower-symmetry positions in the space group.…”

Structures, phase transformations, and dielectric properties of (1−x)Bi2Zn2/3Nb4/3O7–xBi1.5NiNb1.5O7 pyrochlore ceramics prepared by aqueous sol–gel method

“…Members [17][18][19]. The crystal structure of the a-BZN, BNN and BMN pyrochlores are cubic structure with space group Fd3m, while it shows to adopt an orthorhombic distorted pyrochlore structure in the b-BZN ceramic [17,19].…”

“…The crystal structure of the a-BZN, BNN and BMN pyrochlores are cubic structure with space group Fd3m, while it shows to adopt an orthorhombic distorted pyrochlore structure in the b-BZN ceramic [17,19]. Note that these two phase pyrochlores have opposite sign of the temperature coefficient of dielectric constant (s e ): negative and positive, respectively, therefore, temperature stable ceramics can be fabricated by mixing them in different stoichiometric proportions sintered at suitable temperatures.…”

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].…”