Magnetism and Structural Chemistry of the n = 1 Ruddlesden−Popper Phases La₄LiMnO₈ and La₃SrLiMnO₈

Abstract

Polycrystalline samples of La4LiMnO8 and La3SrLiMnO8 have been studied by a combination of X-ray diffraction (XRD), neutron diffraction (ND), 6Li MAS NMR, electron microscopy (EM), and magnetometry. Room-temperature XRD and ND measurements suggest that both compounds have the K2NiF4 structure, with a disordered arrangement of Li and Mn over the six-coordinate sites. However, MAS NMR and EM demonstrate the presence of local 1:1 Li:Mn order on these sites, and EM shows that although cation order is well-developed in each xy sheet of corner-sharing octahedra, the sheets are stacked randomly along z. The structures are best described as paracrystalline, and many of the concepts of conventional crystallography are inapplicable. Magnetometry and low-temperature ND experiments show that, despite their paracrystallinity, the two compounds are ordered antiferromagnetically with susceptibility maxima at 26 and 18 K, respectively, and with ordered magnetic moments of 3.61(6) and 2.3(1) muB per Mn cation at 2 K. Anisotropic peak broadening reveals a 2D character in the magnetic behavior of both compounds, and La3SrLiMnO8 is well-modeled as a quadratic layer S = 3/2 Heisenberg antiferromagnet.

Understanding the NMR shifts in paramagnetic transition metal oxides using density functional theory calculations

“…36,37 In LiNiO 2 , a local distortion was seen by x-ray absorption analysis, 38 exhibit cation ordering in the perovskitelike sheets of the K 2 NiF 4 structure, but that the stacking of the sheets is disordered along ͓001͔ in both these compounds. 41 The lithium first coordination sphere is identical in the ordered and disordered structures, the lithium ion interacting through 180°oxygen bonds with four transition metal ions ͑Fig. 2͒.…”

“…1͒. These two interactions are termed the 90°and 180°( M -O-Li͒ interactions, respectively, because of the angle of the M-O-Li bond.In order to also consider M-O-Li interactions with different geometries, we chose to study the La 4 LiM O 8 (M ϭCr, Mn, Ni͒ phases, which are isostructural to K 2 NiF 4 [39][40][41]. To our knowledge, only the La 4 LiMnO 8 (Mn 3ϩ ) and La 4 LiNiO 8 (Ni 3ϩ ) materials have been synthesized, but as Cr 3ϩ is isoelectronic to Mn 4ϩ , and as the Mn 4ϩ -containing material La 3 SrLiMnO 8 has been synthesized,40 we also studied the hypothetical La 4 LiCrO 8 material by first principles calculations.…”

“…The charge difference between the Li and Ni or Mn ions leads to a strong ordering interaction, and hence, the La 4 LiM O 8 (M ϭNi, Mn͒ phases exhibit a chessboard-type Li/M ordering in the xy plane, leading to a ͱ2ϫͱ2ϫ1 supercell of the original K 2 NiF 4 tetragonal unit cell ͑Fig. 2͒ [39][40][41]. The symmetry of these phases is then A-centered orthorhombic (Ammm).…”

Robustness of superconductivity to structural disorder inSr0.3(NH2)y(NH3)1Foronda1, Ghannadzadeh2, Sedlmaier3 et al. 2015Phys. Rev. B9040View full textAdd to dashboardCite

“…Attempts to explore the diffuse scattering in more detail using transmission electron microscopy were unsuccessful due to the decomposition of these samples in the electron beam. This description of the structure in which well-defined layers are stacked in a disordered manner may be described as paracrystalline [19]. The refinements produced a Sr:Fe ratio of 0.15:1 and an N:Fe ratio of 0.5:1.…”

The first layered analogue of Sr2FeMoO6; the structure and electronic properties of Sr4FeMoO8

“…We have chosen to examine analogues of the perovskite in the Ruddlesden-Popper (RP) family of materials, where cation-order between octahedrally coordinated metals has been observed in a number of systems. [15][16][17] The RP structures contain a variable number (n) of layers of perovskite structure separated by a layer of rock salt, as shown for n 5 1 in Fig. 1, which can give rise to highly anisotropic electronic properties.…”