Local crystal chemistry, induced strain and short range order in the cubic pyrochlore (Bi1.5−αZn0.5−β)(Zn0.5−γNb1.5−δ)O(7−1.5α−β−γ−2.5δ) (BZN)

Local Structure of Relaxor Dielectric Ceramics

“…Breaking the ideal cubic pyrochlore structure up in this way is nonetheless a valid and quite useful means of investigating the crystal chemistry of the overall structure. Withers et al, 2004). Clearly there is considerable scope for local chemical and associated displacive disorder in these systems.…”

“…Clearly there is considerable scope for local chemical and associated displacive disorder in these systems. Average structure refinements of these bismuth-based niobate pyrochlores shows that they are all of the same cubic pyrochlore average structure type with similar cubic lattice parameters [a = 10.5465(9)-10.5633(9) Å for BZN Withers et al, 2004), a = 10.5354(2) Å for BNN and a = 10.5607 (5) Å for BMN (Nguyen et al, 2007b), a = 10.5255(1) Å for BFN (Somphon et al, 2006), a = 10.792(2) Å for BIN and a = 10.660(9) Å for BSN (Liu et al, 2009)]. In addition, they all show considerable displacive disorder in the O'A 2 tetrahedral structure, usually modelled in terms of a split atom approach (see Fig.3 above).…”

“…<044>* in Fig. 5a [see ref (Withers et al, 2004) for details]. Furthermore, the diffuse streaks only run through the G±<001>* type positions and not through the Fd3m allowed Bragg reflections.…”

“…ref (Brese & O'Keeffe, 1991). For this bond valence sum calculation, a nominal stoichiometry of (Bi 1.5 Zn 0.5 )(Zn 0.5 Nb 1.5 )O 7 is assumed, ignoring the known slight non-stoichiometry Withers et al, 2004). The average occupancy of the octahedral B si t e i s t h u s 3 / 4 Nb + 1 / 4 Z n w h i l e t h a t o f t h e A site is 3 / 4 Bi + 1 / 4 Zn.…”

“…6b), however, does not allow the Bi ions to follow the O' shifts and hence leads to a significantly under-bonded Bi ion and thus is energetically extremely unfavourable (see ref. (Withers et al, 2004) for details). This provides a clear crystal chemical rationale as to why 1 / 4 <112> type Zn-Zn separation vectors should be strongly favoured and 1 / 2 <110> type Zn-Zn separation vectors completely avoided in BZN-related pyrochlores.…”