Structure and microstructure of the high pressure synthesised misfit layer compound [Sr2O2][CrO2]1.85

Abstract

a b s t r a c tThe strontium chromium oxide [Sr 2 O 2 ][CrO 2 ] 1.85 misfit layer compound has been synthesised at highpressure and high-temperature conditions. Electron diffraction patterns and high-resolution transmission electron microscopy images along [001] show the misfit character of the different layers composing the structure with a supercell along the incommensurate parameter bE7b 1 E13b 2 . The modulated crystal structure has been refined within the superspace formalism against single-crystal X-ray diffraction data, employing the (3+1)-dimensional superspace group C 0 nmb(0s 2 0)0 0 s. The compound has a composite structure with lattice parameters a 1 ¼ 5.182(1) Å , b 1 ¼ 5.411(1) Å , c 1 ¼ 18.194(3) Å for the first, SrO, subsystem and the same a and c, but with b 2 ¼ 2.925(1) Å for the second, CrO 2 , subsystem. The layer stacking is similar to that of orthorhombic PbS(TiS 2 ) 1.18 , but with a much stronger intersubsytem bonding in the case of the oxide. The intersubsystem lattice mismatch is mainly handled by displacement modulations of the Sr atoms, correlated with modulations of the valence, the coordination and the anisotropic displacement parameters.

Structural investigations of ferecrystals [(SnSe)<inf>1+&#x03B4;</inf>]<inf>m</inf>[TSe<inf>2</inf>]<inf>n</inf> (T = Mo, Ta) by means of transmission electron microscopy

“…3, which suggest the average coherence length along z within the SnSe component increases with m, but is relatively unaffected by n for MoSe 2 . Turbostratic disorder has been observed earlier in ferecrystals of various compositions [1][2][3][4][5][6][7] and is one of the differentiating characteristics between ferecrystals and MLCs [12][13]: the latter intergrowths exhibit long range crystalline order, whereas the former do not. The higher defect density and higher degree of rotational disorder in MoSe 2 are also clearly observed in Fig.…”

“…2) and 3R rhombohedral structure with space group R3m (not shown). (1,1) ferecrystals resemble so called misfit layer compounds (MLC) which are typical for many natural and synthetic chalcogenides [12,13]. MLCs consist of the alternate stacking of two types of layers along the caxis, MX and TX 2 with X = S, Se, O; M = Sr, Ca, Bi, Pb, Ba, and T = Nb, Ta, Ti, Co, Rh, Cr.…”

“…Similar to (1,1) ferecrystals, MLCs have two types of layers with different chemical compositions, and distinct individual periodicities parallel to the layers. MLC sub-structures often possess a common reciprocal lattice plane (a*, c*) in one direction [13], while in the perpendicular direction b*, collinear lattice parameters have an incommensurate length ratio. For example, in the Nb or Ta ternary rare-earth chalcogenides, the MX layer frequently is a two atoms thick distorted rock salt layer oriented along [001], or another pseudoquadratic structure.…”

High-pressure polymorph ofLuFe2O4with room-temperature antiferromagnetic order

“…It is more realistic, however, to consider that the "true" oxygen network is more complex, with a likely distribution of Fe-O distances on the different Fe sites, a problem that is beyond the power of powder diffraction. The layered structure of LuFe 2 O 4 -hp described here can be compared with the misfit crystal structures of cobaltites [30][31][32], or chalcogenides [33], sometimes synthesized under high pressure [34]. The oxides belonging to this structural family result from the intergrowth of triangular CdI 2 -and rectangular RS-type layers (occupied by Co or Cr and alkaline earth, post-transition metals or lanthanides, respectively) and are characterized by strong structural modulations, owing to the discrepancy between the periodicities of the two subsystems.…”

High spatial resolution studies of phase transitions within organic aperiodic crystals

“…Phase transitions within quasicrystals have not really been analyzed in the literature. More works have been dedicated to incommensurate composite crystals [7,[15][16][17][18][19]. A peculiar case corresponds to host/guest intergrowth nanotubular structures, which have a sole incommensurate direction, called c here.…”