Incommensurate structures in highly ordered complex perovskites Pb(${\mathrm{Co}}_{1/2}$${\mathrm{W}}_{1/2}$)${\mathrm{O}}_3$ and Pb(${\mathrm{Sc}}_{1/2}$${\mathrm{Ta}}_{1/2}$)${\mathrm{O}}_3$

A transmission electron microscopy study of two B-site order perovskites Pb(${\mathrm{Co}}_{1/2}$ ${\mathrm{W}}_{1/2}$)${\mathrm{O}}_3$ and Pb(${\mathrm{Sc}}_{1/2}$ ${\mathrm{Ta}}_{1/2}$)${\mathrm{O}}_3$ has revealed evidence for an incommensurate phase within the antiferroelectric and ferroelectric phase, respectively. Direct observations by dark field techniques have revealed discommensurate structures in both Pb(${\mathrm{Sc}}_{1/2}$ ${\mathrm{Ta}}_{1/2}$)${\mathrm{O}}_3$ and Pb(${\mathrm{Co}}_{1/2}$ ${\mathrm{W}}_{1/2}$)${\mathrm{O}}_3$. A possible ‘‘lock-in’’ transition is found in Pb(${\mathrm{Sc}}_{1/2}$ ${\mathrm{Ta}}_{1/2}$)${\mathrm{O}}_3$, but the incommensurate to commensurate transition seems to be dependent on the degree of B-site order.