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Register a new userCharge States of Ions, and Mechanisms of Charge-Ordering Transitions
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To gain insight into the mechanism of charge-ordering transitions, which conventionally are pictured as a disproportionation of an ion M as 2M$^{n+}$ $rightarrow$ M$^{(n+1)+}$ + M$^{(n-1)+}$, we (1) review and reconsider the charge state (or oxidation number) picture itself, (2) introduce new results for the putative charge ordering compound AgNiO$_2$ and the dual charge state insulator AgO, and (3) analyze cationic occupations of actual (not formal) charge, and work to reconcile the conundrums that arise. We establish that several of the clearest cases of charge ordering transitions involve no disproportion (no charge transfer between the cations, hence no charge transfer), and that the experimental data used to support charge ordering can be accounted for within density functional based calculations that contain no charge transfer between cations. We propose that the charge state picture retains meaning and importance, at least inn many cases, if one focuses on Wannier functions rather than atomic orbitals. The challenge of modeling charge ordering transitions with model Hamiltonians is discussed.
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Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the $Na_{0.41}CoO_2$. The formal valence of Co changes from +3.59 estimated from the Na content to +3.5, the same as that in $Na_{0.5}CoO_2$. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate.cite{takada1} The charge ordering is the same as that observed in $Na_{0.5}CoO_2$. It suggests that the Co valence of +3.5 is necessary for the charge ordering.
Here we report on the dynamics of the structural order parameter in a chromium film using synchrotron radiation in response to photo-induced ultra-fast excitations. Following transient optical excitations the effective lattice temperature of the film rises close to the N{e}el temperature and the charge density wave (CDW) amplitude is reduced. Although we expect the electronic charge ordering to vanish shortly after the excitation we observe that the CDW is never completely disrupted, which is revealed by its unmodified period at elevated temperatures. We attribute the persistence of the CDW to the long-lived periodic lattice displacement in chromium. The long-term evolution shows that the CDW revives to its initial strength within 1 ns, which appears to behave in accordance with the temperature dependence in equilibrium. This study highlights the fundamental role of the lattice distortion in charge ordered systems and its impact on the re-condensation dynamics of the charge ordered state in strongly correlated materials.
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 along 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.
We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)$_2$PF$_6$. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. Anomalies in the dielectric function are also observed in the spin-Peierls ground state revealing some intricate lattice effects in a temperature range where both phases coexist. The variation of the spin-Peierls ordering temperature under magnetic field appears to follow a mean-field prediction despite the presence of spin-Peierls fluctuations over a very wide temperature range in the charge ordered state of these salts.
Using first principle band structure calculations, we critically examine results of resonant x-ray scattering experiments which is believed to directly probe charge and orbital ordering. Considering the specific case of La0.5Sr1.5MnO4, we show that this technique actually probes most directly and sensitively small structural distortions in the system. Such distortions, often difficult to detect with more conventional techniques, invariably accompany and usually cause the orbital and charge orderings. In this sense, this technique is only an indirect probe of such types of ordering. Our results also provide a microscopic explanation of the novel types of charge and orbital ordering realized in this system and other doped manganites.
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