CeRuSn exhibits an extraordinary room temperature structure at 300~K with coexistence of two types of Ce ions, namely trivalent Ce$^{3+}$ and intermediate valent Ce$^{(4-delta)+}$, in a metallic environment. The ordered arrangement of these two Ce ty
pes on specific crystallographic sites results in a doubling of the unit cell along the $c$-axis with respect to the basic monoclinic CeCoAl-type structure. Below room temperature, structural modulation transitions with very broad hysteresis have been reported from measurements of various bulk properties. X-ray diffraction revealed that at low temperatures the doubling of the CeCoAl type structure is replaced by a different modulated ground state, approximating a near tripling of the basic CeCoAl cell. The transition is accompanied by a significant contraction of the $c$ axis. We present new x-ray absorption near-edge spectroscopy data at the Ce L$_{3}$ absorption edge, measured on a freshly cleaved surface of a CeRuSn single crystal. In contrast to a previous report, the new data exhibit small but significant variations as function of temperature that are consistent with a transition of a fraction of Ce$^{3+}$ ions to the intermediate valence state, analogous to the $gamma rightarrow alpha$ transition in elemental cerium, when cooling through the structural transitions of CeRuSn. Such results in a valence-modulated state.
At ambient temperatures, CeRuSn exhibits an extraordinary structure with a coexistence of two types of Ce ions in a metallic environment, namely trivalent Ce3+ and intermediate valent Ce(4-x)+. Charge ordering produces a doubling of the unit cell alo
ng the c-axis with respect to the basic monoclinic CeCoAl type structure. Below room temperature, a phase transition with very broad hysteresis has been observed in various bulk properties like electrical resistivity, magnetic susceptibility, and specific heat. The present x-ray diffraction results show that at low temperatures the doubling of the CeCoAl type structure is replaced by an ill-defined modulated ground state. In this state, at least three different modulation periods compete, with the dominant mode close to a tripling of the basic cell. The transition is accompanied by a significant contraction of the c axis. XANES data suggest that the average Ce valence remains constant, thus the observed c axis contraction is not due to any valence transition. We propose a qualitative structure model with modified stacking sequences of Ce3+ and Ce(4-x)+ layers in the various modulated phases. Surprisingly, far below 100 K the modulated state is sensitive to x-ray irradiation at photon fluxes available at a synchrotron. With photon fluxes of order 10E12/s, the modulated ground state can be destroyed on a timescale of minutes and the doubling of the CeCoAl cell observed at room temperature is recovered. The final state is metastable at 10 K. Heating the sample above 60 K again leads to a recovery of the modulated state. Thus, CeRuSn exhibits both thermally and x-ray induced reversible transformations of the Ce3+/Ce(4-x)+ charge ordering pattern. Such a behavior is unique among any know intermetallic compound.
Synchrotron X-ray diffraction investigations of two single crystals of Na_xCoO_2 from different batches with composition x = 0.525-0.530 reveal homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53 0) at room-temperature. The
incommensurate (qq0) superstructure exists between 220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a broad plateau at the latter value between 260 K and 360 K. On cooling, unusual reversible phase segregation into two volume fractions is observed. Below 220 K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50 superstructure, while a second volume fraction with x = 0.55 exhibits another commensurate superstructure, presumably with a 6a x 6a x c hexagonal supercell. We argue that the commensurate-to-incommensurate transition is an intrinsic feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.
X-ray resonant magnetic scattering studies of rare earth magnetic ordering were performed on perovskite manganites RMnO3 (R = Dy, Gd) in an applied magnetic field. The data reveal that the field-induced three-fold polarization enhancement for H || a
(H approx. 20 kOe) observed in DyMnO3 below 6.5 K is due to a re-emergence of the Mn-induced Dy spin order with propagation vector k(Dy) = k(Mn) = 0.385 b*, which accompanies the suppression of the independent Dy magnetic ordering, k(Dy) = 1/2 b*. For GdMnO3, the Mn-induced ordering of Gd spins is used to track the Mn-ordering propagation vector. The data confirm the incommensurate ordering reported previously, with k(Mn) varying from 0.245 to 0.16 b* on cooling from T_N(Mn) down to a transition temperature T. New superstructure reflections which appear below T suggest a propagation vector k(Mn) = 1/4 b* in zero magnetic field, which may coexist with the previously reported A-type ordering of Mn. The Gd spins order with the same propagation vector below 7 K. Within the ordered state of Gd at T = 1.8 K we find a phase boundary for an applied magnetic field H || b, H = 10 kOe, which coincides with the previously reported transition between the ground state paraelectric and the ferroelectric phase of GdMnO3. Our results suggest that the magnetic ordering of Gd in magnetic field may stabilize a cycloidal ordering of Mn that, in turn, produces ferroelectricity.
