Crystal chemistry on a lattice: The case of BZN and BZN-related pyrochlores

This paper uses a diagnostic, highly structured diffuse intensity distribution to investigate the local crystal chemistry of (Bi_1.5Zn_0.5−δ)(Zn_0.5Nb_1.5)O_7−δ (BZN) as well as Sn⁴⁺ and Ti⁴⁺, B site substituted, BZN-related pyrochlore phases. The structured diffuse distribution of the B site substituted material is found to be remarkably similar to that observed for BZN itself. In the special case of (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ (BZNT), the continuous G±〈10l〉* type diffuse streaking characteristic of BZN-related pyrochlores has virtually condensed out to give just G±〈001〉* “satellite reflections” and a P-centred, close to a superstructure phase of average pyrochlore unit cell dimensions. Bond valence sum considerations are used to investigate the local crystal chemistry of this BZNT phase and to derive a plausible model for this superstructure phase. Monte Carlo modelling is used to confirm the plausibility of the model proposed. The underlying crystal chemistry of BZN and BZN-related pyrochlores is shown to result from strong local Bi/Zn ordering rules and associated large amplitude structural relaxation.