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Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an x-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below T(C) = 35 K. The charge ordering pattern is the fully charged zigzag-type ladder with the unit cell 2ax2bx4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as the average valence V(4.5+/-delta(c)/2) is observed below T(C), showing continuous variation of delta(c) as a function of temperature.

Antiferroquadrupolar (AFQ) ordering has been conjectured in several rare-earth compounds to explain their anomalous magnetic properties. No direct evidence for AFQ ordering, however, has been reported. Using the resonant x-ray scattering technique near the Dy L(III) absorption edge, we have succeeded in observing the AFQ order parameter in DyB2C2 and analyzing the energy and polarization dependence. The much weaker coupling between the orbital degrees of freedom and the lattice in 4f electron systems than in 3d compounds makes them an ideal platform to study orbital interactions originating from electronic mechanisms.

A lightly doped manganite ${\mathrm{La}}_{0.88}{\mathrm{Sr}}_{0.12}{\mathrm{MnO}}_{3}$ exhibits a phase transition at ${T}_{\mathrm{OO}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}145\phantom{\rule{0ex}{0ex}}\mathrm{K}$ from a ferromagnetic metal ( ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}172\phantom{\rule{0ex}{0ex}}\mathrm{K}$) to a novel ferromagnetic insulator. We identify that the key parameter in the transition is the orbital degree of freedom in ${e}_{g}$ electrons. By utilizing the resonant x-ray scattering, orbital ordering is directly detected below ${T}_{\mathrm{OO}}$, in spite of a significant diminution of the cooperative Jahn-Teller distortion. The experimental features are well described by a theory treating the orbital degree of freedom under strong electron correlation. The present studies uncover a crucial role of the orbital degree of freedom in the metal-insulator transition in lightly doped manganites.

We have investigated the Co-3d spin-orbital state in a thin film of perovskite LaCoO3 to clarify the origin of strain induced spontaneous magnetization (T(C)=94 K) by means of x-ray diffraction, optical spectroscopy, and magnetization measurements. A lattice distortion with the propagation vector (1/4 -1/4 1/4) and an anomalous activation of optical phonons coupled to Co-3d orbital are observed below 126 K. Combined with the azimuthal angle analysis of superlattice reflection, we propose that the ordering of Co-3d orbital promoted by an epitaxial strain produces a unique ferrimagnetic structure.

Ferroelectric polarization of 6.3 μC cm(-2) is induced by the neutral-to-ionic transition, upon which nonpolar molecules of electron donor tetrathiafulvalene (TTF) and acceptor p-chloranil (CA) are incompletely ionized to ±0.60e and dimerized along the molecular stacking chain. We find that the ferroelectric properties are governed by intermolecular charge transfer rather than simple displacement of static point charge on molecules. The observed polarization and poling effect on the absolute structural configuration can be interpreted in terms of electronic ferroelectricity, which not only exhibits antiparallel polarity to the ionic displacement but also enhances the polarization more than 20 times that of the point-charge model.

Synchrotron x-ray diffraction experiment shows that the metal-insulator transition occurring in a ferromagnetic state of a hollandite ${\mathrm{K}}_{2}{\mathrm{Cr}}_{8}{\mathrm{O}}_{16}$ is accompanied by a structural distortion from the tetragonal $I4/m$ to monoclinic $P{112}_{1}/a$ phase with a $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}\ifmmode\times\else\texttimes\fi{}1$ supercell. Detailed electronic structure calculations demonstrate that the metal-insulator transition is caused by a Peierls instability in the quasi-one-dimensional column structure made of four coupled Cr-O chains running in the $c$ direction, leading to the formation of tetramers of Cr ions below the transition temperature. This provides a rare example of the Peierls transition of fully spin-polarized electron systems.

We report a ferroelectric transition driven by the off-centering of magnetic Mn(4+) ions in antiferromagnetic Mott insulators Sr(1-x)Ba(x)MnO(3) with a perovskite structure. As x increases, the perovskite lattice shows the typical soft-mode dynamics, as revealed by the momentum-resolved inelastic x-ray scattering and far-infrared spectroscopy, and the ferroelectricity shows up for x ≥ 0.45. The observed polarization is comparable to that for a prototypical ferroelectric BaTiO(3). We further demonstrate that the magnetic order suppresses the ferroelectric lattice dilation by ∼70% and increases the soft-phonon energy by ∼50%, indicating the largest magnetoelectric effects yet attained.

The present resonant x-ray scattering has been performed on a monoclinically split single domain of ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$. The observation of a critically enhanced contrast between ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{5+}$ ions has led us to the unequivocal conclusion of the charge-order pattern of the low-temperature phase of ${\mathrm{NaV}}_{2}{\mathrm{O}}_{5}$ below ${T}_{\mathrm{c}}=35\text{ }\text{ }\mathrm{K}$. In spite of the possible four types of configuration of the zigzag-type charge-order patterns in the $ab$ plane ($A,{A}^{\ensuremath{'}},B$ and ${B}^{\ensuremath{'}}$), the stacking sequence along the $c$ axis is determined as the $AA{A}^{\ensuremath{'}}{A}^{\ensuremath{'}}$ type by comparison with model calculations.

The present resonant x-ray scattering has been performed on a monoclinically split single domain of NaV(2)O(5). The observation of a critically enhanced contrast between V(4+) and V(5+) ions has led us to the unequivocal conclusion of the charge-order pattern of the low-temperature phase of NaV(2)O(5) below T(c) = 35 K. In spite of the possible four types of configuration of the zigzag-type charge-order patterns in the ab plane (A,A',B and B'), the stacking sequence along the c axis is determined as the AAA'A' type by comparison with model calculations.

The "devil's staircase"-type phase transition in the quarter-filled spin-ladder compound NaV2O5 has been discovered at low temperature and high pressure by synchrotron radiation x-ray diffraction. A large number of transitions are found to successively take place among higher-order commensurate phases with 2a x 2b x zc type superstructures. The observed temperature and pressure dependence of modulation wave number q(c), defined by 1/z, is well reproduced by the axial next nearest neighbor Ising model. The q(c) is suggested to reflect atomic displacements presumably coupled with charge ordering in this system.

Etude cinetique de la nucleation et de la croissance pour une transition du premier ordre appartenant au systeme monodimensionnel a non-conservation des parametres d'ordre, dans un compose d'insertion rubidium-graphite. Caracterisation du volume de la phase en croissance et de l'evolution de la taille du noyau vis-a-vis du temps