Nanoscale modulations in (KLa)(CaW)O6 and (NaLa)(CaW)O6

Complex nanoscale modulations are identified in two new A-site ordered perovskites, (KLa)(CaW)O6 and (NaLa)(CaW)O6. In (KLa)(CaW)O6, selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) show an incommensurate nanocheckerboard modulation with ∼9.4×9.4ap periodicity (ap≈4 Å for the cubic perovskite aristotype). For (NaLa)(CaW)O6 a one-dimensional modulation is observed with a ∼16(1 1 0)ap repeat; the 〈1 1 0〉 orientation of the nanostripes is different from the 〈1 0 0〉 stripes observed in other mixed A-site systems. Studies using high temperature x-ray diffraction suggest the formation of the complex modulations is associated with small deviations from the ideal 1:1:1:1 stoichiometry of the (A+La3+)(CaW)O6 phases. Z-contrast images acquired on an aberration-corrected microscope provide evidence for deviations from stoichiometry with a ∼1:15 periodic arrangement of La4/3(CaW)O6:(NaLa)(CaW)O6 nano-phases.