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. - Graphical abstract: 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 and high-resolution transmission electron microscopy show a two-dimensional, nanocheckerboard modulation. For (NaLa)(CaW)O6 a one-dimensional modulation is observed; the 〈1 1 0〉 orientation of the nanostripes is different from the 〈1 0 0〉 stripes observed in other mixed A-site systems. Highlights: ► Two new A-site ordered perovskites were synthesized, (KLa)(CaW)O6 and (NaLa)(CaW)O6. ► Unusual 1D and 2D nanoscale patterns were observed. ► Tolerance factor shown to be not enough to predict the observed morphologies. ► High temperature x-ray diffraction data suggests a loss of stoichiometry is related to the modulations. ► Z-contrast imaging provides direct evidence for non-stoichiometry and a new model.