Transmission Electron Microscopy Studies of NaLaMgWO₆: Spontaneous Formation of Compositionally Modulated Stripes

Abstract

Transmission electron microscopy studies of the perovskite NaLaMgWO 6 reveal the formation of a complex, compositionally modulated structure. Annular dark field scanning transmission electron microscopy images and scanning transmission electron microscopy-electron energy-loss spectroscopy scans show that this modulation involves a repeating pattern of La-rich and La-poor stripes, each stripe 6 a p or approximately 24 A wide (where a p is the edge length of the simple cubic perovskite unit cell). High-resolution transmission electron microscopy images clearly show, and electron diffraction patterns confirm, a periodicity of 12 a p along either the [100] p or [010] p direction. Available evidence suggests a spontaneous separation into stripes that possess the nominal stoichiometry, NaLaMgWO 6, alternating with Na-poor/La-rich stripes that have a stoichiometry of (La x Na 1-3 x )LaMgWO 6. X-ray powder diffraction measurements are insensitive to this intricate structural complexity, which may be a more widespread feature of (A (+)Ln (3+))MM'O 6 perovskites than previously appreciated.

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

“…Following that report similar modulations were reported in a family of B 3 þ (Al, Mn, Cr) substituted Li systems and, more recently, in three different Li-free alkali-containing perovskites based on Na þ and K þ [2,[4][5][6][7]. While chemically similar, the modulations observed for each system are unique in at least the magnitude of the periodic repeat.…”

“…Recent studies have revealed that several (AA 0 )BO 6 ordered, alkali-based mixed A-site perovskites exhibit unusual nanoscale modulations in composition and structure [1][2][3][4][5][6][7]. The first observation of A-site driven modulations was made on compositions in the lithium-based (Nd 2/3 À x Li 3x )TiO 3 solid solution system [1].…”

Nanocheckerboard modulations in (NaNd)(MgW)O6

“…They also share several similarities to the striped patterns observed in ͑NaLa͒͑MgW͒O 6 which were argued to result from an ordered phase separation of Na-rich, ͑NaLa͒͑MgW͒O 6 , and Na-deficient, La 4/3 ͑MgW͒O 6 , end member phases. 5 The nanoscale structures in both of those systems also involve a periodic twinning of the a − a − c 0 ͑Glazer notation͒ octahedral tilt system, the same tilt system reported for ͑NaNd͒͑MgW͒O 6 . 8,9 With these similarities it is tempting to conclude the modulations reported here for ͑NaNd͒͑MgW͒O 6 involve periodic phase separation into ͑NaNd͒͑MgW͒O 6 domains separated by Na-deficient regions of Nd 4/3 ͑MgW͒O 6 .…”

“…For ͑NaLa͒͑MgW͒O 6 the modulation occurs in one dimension leading to a striped pattern of light and dark regions with a repeat distance of 12a p ; compositional analyses indicated separation into Narich ͓͑NaLa͒͑MgW͒O 6 ͔ and Na-deficient ͓La 4/3 ͑MgW͒O 6 ͔ phases. 5 HRTEM images of ͑KLa͒͑MnW͒O 6 provided evidence for checkerboard type modulations in another A + / A 3+ mixed A-site system. 6 This paper focuses on the perovskite ͑NaNd͒͑MgW͒O 6 , one of several ͑AЈAЉ͒͑BЈBЉ͒O 6 perovskites with both A-and B-site ordering.…”

Nanolamellar magnetoelectric BaTiO3–CoFe2O4 bicrystal

“…[4][5][6] We used this approach to improve on previous eutectoid 7 and other [8][9][10][11] syntheses of CFO/BTO composites. Avellaneda and Harshe 12 already demonstrated that solidification of eutectic BTO-CFO leads to lamellar microstructures.…”