Modulation Of Atomic Positions Research Articles

Modulation of atomic positions in CaCu x Mn7−x O12 (x ≤ 0.1)

The modulation of atomic positions in CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) was studied using synchrotron radiation powder diffraction below 250 and 220 K, respectively. The copper-rich member CaCu(x)Mn(7-x)O12 (x = 0.23) does not show any modulation of the atomic positions at temperatures as low as 10 K. Using low-temperature neutron powder diffraction the modulation of the magnetic moments of Mn ions in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) has been investigated. Long-range modulated magnetic ordering in CaCu(x)Mn(7-x)O12 (x = 0, 0.1 and 0.23) is observed below 90.4, 89.2 and 78.1 K. (0,0,q(p)) and (0,0,q(m)) are the propagation vectors describing the modulations of the atomic positions and the magnetic moments. For CaCu(x)Mn(7-x)O12 (x = 0 and 0.1) the magnetic modulation and atomic modulation lengths are related by a factor of 2 satisfying the relation (1-q(p)) = 2(1-q(m)).

Cooperative ordering of holes and spins in La2NiO4.125.

${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4.125}$ exhibits a cooperative ordering of dopant-induced holes and Ni spins below a transition temperature of 110 K. There is also an ordering of interstitial oxygens that occurs near room temperature. We present a comprehensive analysis of experimentally observed superlattice intensities in terms of structural models, justifying previous identifications. The model for the interstitial order involves a 3a\ifmmode\times\else\texttimes\fi{}5b\ifmmode\times\else\texttimes\fi{}5c unit cell with an ideal interstitial density of \ensuremath{\delta}=2/15 per formula unit. In the spin and charge ordered state, the magnetic moments are sinusoidally modulated within the ${\mathrm{NiO}}_{2}$ planes. The moments point transverse to the modulation direction, with nearest neighbors antiparallel, and have a maximum amplitude that is g80% of that observed in undoped ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. A corresponding modulation of atomic positions within ${\mathrm{NiO}}_{2}$ planes involves breathing-mode distortions consistent with a modulation of the charge density. The results are compared with theoretical models and with experimental observations on related systems.

Low energy electron diffraction and X-ray photoelectron diffraction study of the Cs/Si(001) surface: dependence on Cs coverage

Cs adsorption on a wide-terrace single-domain Si(001) surface has been studied by low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) as a function of Cs coverage (θ) and substrate temperature ( T S). For T S's of ~ 110 and ~ 320 K, several surface periodicities are found which have not been reported before for this system. Azimuthal Cs3d XPD patterns have been measured for most of the ordered surfaces. An XPD structure analysis is made for a streak × 3 surface at θ = 1 6 ML, in which a dilute arrangement of Cs atoms is deduced. An XPD structure analysis is made for the surfaces at θ = 1 3 ML taking into account a reversible transition between 2 × 6 and 2 × 3 for T S's of ∼ 110 and ∼ 320 K, respectively. A Cs dimer model is deduced in which a modulation of atomic positions into a 2 × 6 surface is averaged in the 2 × 3 surface but is fixed in a regular way in the 2 × 6 surface.