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

Investigation of the Structure of the Modulated Doubly Ordered Perovskite NaLaCoWO6 and Its Reversible Phase Transition with a Colossal Temperature Hysteresis

“…This refinement yielded the average structure of these compounds, but electron diffraction revealed the existence of superstructures in these phases. Such superstructures have also been observed in some other doubly ordered perovskites, such as NaLaMgWO 6 , NaNdMgWO 6 , KLaMnWO 6 , NaCeMnWO 6 , NaPrMnWO 6 , KLaCaWO 6 , and NaLaCaWO 6 . − A large variety of superstructure patterns and periodicities appeared among these compounds, for instance, commensurate stripes for NaLaMgWO 6 , or commensurate chessboard patterns for KLaMnWO 6 and incommensurate stripes and chessboard patterns in NaPrMnWO 6 . Different scenarios have been proposed to explain the origin of the superstructures: compositional modulation, octahedral tilting, and ionic displacements accompanied by chemical ordering (e.g., NaNdMgWO 6 ) or not (e.g., NaLaMgWO 6 ).…”

Expanding the Doubly Cation Ordered AA′BB′O6 Perovskite Family: Structural Complexity in NaLaInNbO6 and NaLaInTaO6

“…In either case it seems apparent that a twinning of the out-of-phase components of the tilt system is present. Some studies have suggested that a variation in the A -site occupancies occurs with the same periodicity for certain compositions. , The most recent studies suggest that the composition is different at the twin boundaries, although the nature of these compositional modulations is still not well-understood and is a matter of ongoing debate. , A wide variety of superstructure sizes and dimensions have been observed. NaLaMgWO 6 and NaLaCoWO 6 have been found to have one-dimensional stripe patterns with periodicities of 12 a p , where a p is the length of the basic perovskite subcell (∼3.9 Å). ,, KLaMnWO 6 and NaNdMgWO 6 have two-dimensional checkerboard patterns with periodicities of 10 a p and 14 a p , respectively. , NaCeMnWO 6 , NaPrMnWO 6 , and KLaCaWO 6 are found to have incommensurately modulated structures. , NaLaCaWO 6 , NaPrCoWO 6 , and NaNdCoWO 6 have a different stripe pattern which goes along the ⟨110⟩ direction instead of the ⟨100⟩ direction and has a periodicity of 16 a p in all three cases. , …”

“…38,39 The most recent studies suggest that the composition is different at the twin boundaries, although the nature of these compositional modulations is still not wellunderstood and is a matter of ongoing debate. 40,41 A wide variety of superstructure sizes and dimensions have been observed. NaLaMgWO 6 and NaLaCoWO 6 have been found to have one-dimensional stripe patterns with periodicities of 12a p , where a p is the length of the basic perovskite subcell (∼3.9 Å).…”

“…44,45 NaLaCaWO 6 , NaPrCoWO 6 , and NaNdCoWO 6 have a different stripe pattern which goes along the ⟨110⟩ direction instead of the ⟨100⟩ direction and has a periodicity of 16a p in all three cases. 40,42 An important lesson that has emerged from these studies is that standard X-ray powder diffraction (XRD) cannot reliably determine the structure of a doubly ordered AA′BB′O 6 perovskite. The superstructures that sometimes form as a result of subtle variations in the tilting of the light oxygen octahedra are undetectable by laboratory based XRD and sometimes even by synchrotron XRD.…”

Coupled Compositional and Displacive Modulations in KLaMnWO6 Revealed by Atomic Resolution Imaging

“…Ordering at both A and B sites is a common mechanism to accommodate cations with different radii and/or oxidation states. In this context, simultaneous layered ordering of the A-site cations and rock-salt-type ordering with second-order Jahn–Teller distortions of the B-site cations stabilizes quintinary AA′BB′O 6 systems. , In addition to the coupled chemical ordering on the A and B sites, several compositions present an even more complex modulation of the crystal structure, including NaLaMgWO 6 , KLaMnWO 6 , NaCe­(Pr)­MnWO 6 , NaLaCoWO 6 , K­(Na)­LaCaWO 6 , and NaNdMgWO 6 . This modulation is also found in quaternary perovskites with layered ordering of the A sites such as Nd 2/3– x Li 3 x TiO 3 and La 1/3– x Li 3 x NbO 3 , − suggesting a complicated interplay of structural aspects that is not fully understood.…”