Abstract: 1. Why Manganites Are Interesting.- 2. The Discovery of Manganites and the Colossal Magnetoresistance Effect.- 3. Phase Diagrams and Basic Properties of Manganites.- 4. Preliminary Theoretical Considerations: Coulombic and Jahn Teller Effects.- 5. Models for Manganites.- 6. The One-Orbital Model: Phase Diagram and Dominant Correlations.- 7. Monte Carlo Simulations and Application to Manganite Models.- 8. Mean-Field Approximation.- 9. Two-Orbitals Model and Orbital Order.- 10. Charge Ordering: CE-States, Stripes, and Bi-Stripes.- 11. Inhomogeneities in Manganites: The Case of La1?xCaxMnO3.- 12. Optical Conductivity.- 13. Glassy Behavior and Time-Dependent Phenomena.- 14. Inhomogeneities in La1?xSrxMnO3 and Pr1?xCaxMnO3.- 15. Inhomogeneities in Layered Manganites.- 16. An Elementary Introduction to Percolation.- 17. Competition of Phases as the Origin of the CMR.- 18. Pseudogaps and Photoemission Experiments.- 19. Charge-Ordered Nanoclusters above TC: the Smoking Gun of Phase Separation?.- 20. Other Compounds with Large MR and/or Competing FM AF Phases.- 21. Brief Introduction to Giant Magnetoresistance (GMR).- 22. Discussion and Open Questions.- References.

Abstract: We observed Verwey transition in very small (6–14 nm) amine-coated octahedral magnetite (Fe3O4) nanoparticles that is not present in spherical similarly sized (4–13 nm) nanoparticles. Electron microscopy shows that octahedral nanoparticles have {111} facets with better cationic coordination symmetry as their surface. Spherical shape illustrates conventional superparamagnetic behavior; on the contrary, a characteristic Verwey transition near 120 K is prominent in field-cooled/zero-field-cooled curves of octahedral nanoparticles. Higher saturation magnetization in octahedral nanoparticles indicates lesser surface spin disorder and well-established anisotropy. Better surface coordination offers a reduced number of oxygen vacancies at the surface and, therefore, better stoichiometry results in a Verwey transition in octahedral nanoparticles. Electrical resistivity measurements show a sharp change in resistance for octahedral particles below the Verwey transition temperature which is completely hindered in sph...

Abstract: The electronic phase transition has been considered as a dominant factor in the phenomena of colossal magnetoresistance, metal-insulator transition, and exchange bias in correlated electron systems. However, the effective manipulation of the electronic phase transition has remained a challenging issue. Here, the reversible control of ferromagnetic phase transition in manganite films through ionic liquid gating is reported. Under different gate voltages, the formation and annihilation of an insulating and magnetically hard phase in the magnetically soft matrix, which randomly nucleates and grows across the film instead of initiating at the surface and spreading to the bottom, is directly observed. This discovery provides a conceptually novel vision for the electric-field tuning of phase transition in correlated oxides. In addition to its fundamental significance, the realization of a reversible metal-insulator transition in colossal magnetoresistance materials will also further the development of four-state memories, which can be manipulated by a combination of electrode gating and the application of a magnetic field.

Abstract: We have carried out a systematic experimental investigation to address the question why thin films of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (magnetite) generally have a very broad Verwey transition with lower transition temperatures as compared to the bulk. We observed using x-ray photoelectron spectroscopy, x-ray diffraction, and resistivity measurements that the Verwey transition in thin films is drastically influenced not only by the oxygen stoichiometry but especially also by the substrate-induced microstructure. In particular, we found (1) that the transition temperature, the resistivity jump, and the conductivity gap of fully stoichiometric films greatly depends on the domain size, which increases gradually with increasing film thickness, (2) that the broadness of the transition scales with the width of the domain size distribution, and (3) that the hysteresis width is affected strongly by the presence of antiphase boundaries. Films grown on MgO (001) substrates showed the highest and sharpest transitions, with a 200 nm film having a ${\mathit{T}}_{V}$ of 122 K, which is close to the bulk value. Films grown on substrates with large lattice constant mismatch revealed very broad transitions, and yet all films show a transition with a hysteresis behavior, indicating that the transition is still first order rather than higher order.

Abstract: In a multiorbital model of the cuprate high-temperature superconductors soft antiferromagnetic (AF) modes are assumed to reconstruct the Fermi surface to form nodal pockets. The subsequent charge ordering transition leads to a phase with a spatially modulated transfer of charge between neighboring oxygen p_x and p_y orbitals and also weak modulations of the charge density on the copper d_{x^2-y^2} orbitals. As a prime result of the AF Fermi surface reconstruction, the wavevectors of the charge modulations are oriented along the crystalline axes with a periodicity that agrees quantitatively with experiments. This resolves a discrepancy between experiments, which find axial order, and previous theoretical calculations, which find modulation wavevectors along the Brillouin zone (BZ) diagonal. The axial order is stabilized by hopping processes via the Cu4s orbital, which is commonly not included in model analyses of cuprate superconductors.

Abstract: In the high-temperature ($T_{c}$) cuprate superconductors, increasing evidence suggests that the pseudogap, existing below the pseudogap temperature $T$*, has a distinct broken electronic symmetry from that of superconductivity. Particularly, recent scattering experiments on the underdoped cuprates have suggested that a charge ordering competes with superconductivity. However, no direct link of this physics and the important low-energy excitations has been identified. Here we report an antagonistic singularity at $T_{c}$ in the spectral weight of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+{\delta}}$ as a compelling evidence for phase competition, which persists up to a high hole concentration $p$ ~ 0.22. Comparison with a theoretical calculation confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two intertwined phases and the complex phase diagram near the pseudogap critical point.

Abstract: NaFePO4 is known as a promising intercalation cathode material for sodium-ion batteries. During the electrochemical reaction, an intermediate phase forms with a composition close to Na≈2/3FePO4, whose crystal structure is defined with a supercell that results from both Na/vacancy and charge ordering. In this work, we present a detailed study of this superstructure through synchrotron powder X-ray diffraction and electron microscopy studies.

Abstract: Recent experiments in the underdoped regime of the hole-doped cuprates have found evidence for an incommensurate charge density wave state. We present an analysis of the charge ordering instabilities in a metal with antiferromagnetic correlations, where the electronic excitations are coupled to the fractionalized excitations of a quantum fluctuating antiferromagnet on the square lattice. The resulting charge density wave state emerging out of such a fractionalized Fermi-liquid (FL*) has wavevectors of the form $(\pm Q_0,0), (0,\pm Q_0)$, with a predominantly $d$-form factor, in agreement with experiments on a number of different families of the cuprates. In contrast, as previously shown, the charge density wave instability of a nearly antiferromagnetic metal with a large Fermi surface, interacting via short-range interactions, has wavevectors of the type $(\pm Q_0,\pm Q_0)$. Our results show that the observed charge density wave appears as a low-energy instability of a fractionalized metallic state linked to the proximity to an antiferromagnetic insulator, and the pseudogap regime can be described by such a metal at least over intermediate length and energy scales.

Abstract: High resolution polar Kerr effect measurements were performed on La1.875Ba0.125CuO4 single crystals revealing that a finite Kerr signal is measured below an onset temperature TK that coincides with the charge ordering transition temperature TCO. We further show that the sign of the Kerr signal cannot be trained with the magnetic field, is found to be the same on opposite sides of the same crystal, and is odd with respect to strain in the diagonal direction of the unit cell. These observations are consistent with a chiral "gyrotropic" order above Tc for La1.875Ba0.125CuO4; similarities to other cuprates suggest that it is a universal property in the pseudogap regime.

Abstract: From x-ray magnetic circular dichroism experiments performed at low temperature on Cr2AlC and Cr2GeC thin films, it is evidenced that Cr atoms carry a net magnetic moment in these ternary phases. It is shown that the Cr magnetization of the Al-based compound nearly vanished at 100 K in agreement with what has been recently observed on bulk. X-ray linear dichroism measurements performed at various angles of incidence and temperatures clearly demonstrate the existence of a charge ordering along the c axis of the structure of Cr2AlC. All these experimental observations support, in part, theoretical calculations claiming that Cr dd correlations have to be considered to correctly describe the structure and properties of these Cr-based ternary phases.

Abstract: We study the extended Hubbard model on the triangular lattice as a function of filling and interaction strength. The complex interplay of kinetic frustration and strong interactions on the triangular lattice leads to exotic phases where long-range charge order, antiferromagnetic order, and metallic conductivity can coexist. Variational Monte Carlo simulations show that three kinds of ordered metallic states are stable as a function of nearest neighbor interaction and filling. The coexistence of conductivity and order is explained by a separation into two functional classes of particles: part of them contributes to the stable order, while the other part forms a partially filled band on the remaining substructure. The relation to charge ordering in charge transfer salts is discussed.

Abstract: Magnetite, Fe$_3$O$_4$, is the first magnetic material discovered and utilized by mankind in Ancient Greece, yet it still attracts attention due to its puzzling properties. This is largely due to the quest for a full and coherent understanding of the Verwey transition that occurs at $T_V=124$ K and is associated with a drop of electric conductivity and a complex structural phase transition. A recent detailed analysis of the structure, based on single crystal diffraction, suggests that the electron localization pattern contains linear three-Fe-site units, the so-called trimerons. Here we show that whatever the electron localization pattern is, it partially survives up to room temperature as short-range correlations in the high-temperature cubic phase, easily discernible by diffuse scattering. Additionally, {\it ab initio} electronic structure calculations reveal that characteristic features in these diffuse scattering patterns can be correlated with the Fermi surface topology.

Abstract: The critical cooling rate Rc above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system θ-(BEDT-TTF)2X. X-ray diffraction experiments revealed that θ-(BEDT-TTF)2TlCo(SCN)4 exhibits a horizontally charge-ordered state, and kinetic avoidance of this state requires rapid cooling of faster than 150 K/min. This value is markedly higher than that reported for two other isostructural θ-type compounds, thus demonstrating the lower charge-glass-forming ability of X = TlCo(SCN)4. In accounting for the systematic variations of Rc among the three θ-(BEDT-TTF)2X, we found that stronger charge frustration leads to superior charge-glass former. Our results suggest that charge frustration tends to slow the kinetics of charge ordering.

Abstract: We present a volume-sensitive high-energy x-ray diffraction study of the underdoped cuprate high temperature superconductor La2-xSrxCuO4 (x = 012, Tc=27 K) in applied magnetic field Bulk short-range charge stripe order with propagation vector q_ch = (0231, 0, 05) is demonstrated to exist below T_ch = 85(10) K and shown to compete with superconductivity We argue that bulk charge ordering arises from fluctuating stripes that become pinned near boundaries between orthorhombic twin domains

Abstract: We address the origin of the recently discovered close correspondence between the charge ordering wave vectors and the momentum-space separation between the tips of the Fermi arcs seen in angle-resolved photoemission measurements in underdoped high-temperature superconducting cuprates. This observation has been interpreted as a signature of charge order forming as an instability of pre-existing Fermi arcs of a different origin. We calculate the Fermi surface spectral weight for a charge density-wave model, considering a Fermi surface, charge ordering wave vectors and short correlation lengths similar to those found experimentally. We show that the observation of wave vectors spanning the tips of remnant Fermi surface sections is a natural consequence of a Fermi surface having been reconstructed by charge order. The presence of short-range charge order therefore cannot be ruled out as a potential origin of the observed Fermi arcs.

Abstract: Charge ordering and oxygen vacancy ordering are revealed in REBaMn2O6-δ (RE = Gd, Tb) oxides with perovskite-related structures. Electron diffraction and transmission electron microscopy results indicate a modulation of the crystal structure. The average oxidation state of Mn and the oxygen stoichiometry are determined by means of electron energy-loss spectroscopy, giving a REBaMn2O5.75 general formula. A 1:3 Mn4+:Mn3+ charge ordering model is confirmed by neutron powder diffraction, and oxygen vacancies-Mn3+ association is suggested by pair distribution function analysis. Direct imaging of the oxygen sublattice is obtained by phase image reconstruction. Location of the oxygen vacancies in the anion sublattice is achieved by analysis of the intensity of the averaged phase image. Both ionic conduction and multiferroic behavior are predicted from the crystal structures of these oxides.

Abstract: In attempt to search for an improved material preparation technique, Y2NiMnO6 dielectric material is prepared by a one-step thermal decomposition route where a solution of stoichiometric mixtures of metal acetates is directly heated. Structural characterization by X-ray diffraction and electron diffraction shows that the samples were successfully prepared at relatively low temperature comparing to a standard solid state synthesis. Results from several techniques including thermal analysis, electron microscopy, and X-ray absorption are used to investigate compound formation. It is revealed that metal acetates decompose at 300–350 °C resulting in mixture of several metal oxide intermediates which continue to react to form the desired product. Y2NiMnO6 nanoparticles are first obtained at 800 °C. Later, these nanoparticles agglomerate and grow at higher temperature and/or longer heating time to give larger particle size and more crystallinity. Although the starting reagent contains Mn in 2+ oxidation state, X-ray absorption near edge structure analysis indicates that the obtained Y2NiMnO6 contain Mn and Ni in 4+/3+ and 2+ oxidation states, respectively. Ceramic sample shows large dielectric constant of about 6,000–7,000 at 30–120 °C at 1 kHz. Dielectric constant and dielectric response of the sample are consistent with those reported in other works where different synthetic techniques were used. The activation energy of dielectric relaxation is similar to the energy required to transfer electrons between Ni2+ and Mn4+, thus the observed large dielectric constant is intrinsically related to electronic ferroelectricity due to charge ordering of Ni2+ and Mn4+.

Abstract: The critical cooling rate $R_{\rm c}$ above which charge ordering is kinetically avoided upon cooling, which results in charge-glass formation, was investigated for the geometrically frustrated system $\theta$-(BEDT-TTF)$_2X$. X-ray diffraction experiments revealed that $\theta$-(BEDT-TTF)$_2$TlCo(SCN)$_4$ exhibits a horizontally charge-ordered state, and kinetic avoidance of this state requires rapid cooling of faster than 150 K/min. This value is markedly higher than that reported for two other isostructural $\theta$-type compounds, thus demonstrating the lower charge-glass-forming ability of $X$ $=$ TlCo(SCN)$_4$. In accounting for the systematic variations of $R_{\rm c}$ among the three $\theta$-(BEDT-TTF)$_2X$, we found that stronger charge frustration leads to superior charge-glass former. Our results suggest that charge frustration tends to slow the kinetics of charge ordering.

Abstract: EuGa4 and EuAl4 crystallize in the BaAl4-type tetragonal structure. The antiferromagnetic order was observed at 15 and 16.5K in EuGa4 and EuAl4, respectively.1,2) The magnetic susceptibilities in EuGa4 and EuAl4 well follow the Curie–Weiss law with effective moments of 7.97 and 8.02 B, respectively, which are consistent with the theoretical value of 7.94 B for a Eu-divalent electronic state.1,3) In the electrical resistivity and thermoelectric power measurements under pressure, a shoulderlike change was observed below about 100K at 1GPa in EuGa4. Moreover, a similar resistivity hump was observed below TCDW 1⁄4 140K at ambient pressure in EuAl4. The authors in Ref. 2 considered that this transition corresponds to the emergence of a charge density wave (CDW). In this study, we measured the magnetoresistance and Hall effect to clarify whether this is indeed based on CDW transition. Single crystals of EuAl4 were grown by the Al self-flux method. The transverse magnetoresistance and Hall resistivity were measured with an AC resistance bridge. The magnetic field H was applied along the [010] and [001] directions. The electrical current flow J was parallel to the [100] direction. Figure 1 shows a Kohler plot for the transverse magnetoresistance in the configurations of H k 1⁄2001 and J k 1⁄2100 . Kohler’s rule, i.e., = 1⁄2 ðHÞ ð0Þ = ð0Þ 1⁄4 fðH= ð0ÞÞ, holds when the temperature dependences of the scattering relaxation time are the same for all carriers. The = above 150K, namely, at 150, 160, and 200K, is well scaled on a single curve, which indicates that Kohler’s rule is satisfied above TCDW. On the other hand, the = below 140K deviates from Kohler’s rule, indicating that the carrier density and/or the scattering mechanism change below TCDW. In the two-band model, the transverse magnetoresistance and Hall resistivity are expressed as xx 1⁄4 ex hxð ey þ hyÞ þ ð exR 2 h þ hxReÞH ð ex þ hxÞð ey þ hyÞ þ ðRe þ RhÞH2 ; ð1Þ yx 1⁄4 ðRe hx hy þ Rh ex eyÞ þ ReRhðRe þ RhÞH 2 ð ex þ hxÞð ey þ hyÞ þ ðRe þ RhÞH2 H; ð2Þ where ei ( hi) and Re (Rh) are the resistivity along the i 1⁄4 xor y-direction and the Hall coefficient of the electron (hole) band, respectively.4) The Hall coefficient of each band is inversely proportional to the carrier density as Re 1⁄4 1=nee and Rh 1⁄4 1=nhe, where ne and nh are the electron and hole densities, respectively. The resistivity of each band is ei 1⁄4 1=ne eie ( hi 1⁄4 1=nh hie), where ei ( hi) is the mobility of the electron (hole). The transverse magnetoresistance increases quadratically, = H, for a compensated metal (ne 1⁄4 nh) at a high field limit (!c 1), whereas it tends to saturate for an uncompensated metal (ne 61⁄4 nh). Here, !c is the cyclotron frequency and is the scattering relaxation time. !c is roughly estimated using the relation = ð!c Þ2, which is obtained from the free-electron model. There is one EuAl4 unit in a body-centered tetragonal primitive cell. Considering that Eu is divalent, as proved by the magnetic susceptibility data,3) EuAl4 is a compensated metal with an even number of valence electrons. The experimentally obtained H-dependent magnetoresistance of up to = 2:5 10 2 (!c 0:16), shown as the solid curve in Fig. 1, reveals that EuAl4 is a compensated metal above TCDW. On the other hand, the = below TCDW gradually deviates at higher fields from the initial H dependence, as shown by the broken curve in Fig. 1 and the broken line in the inset of Fig. 1, at 100K. The deviation at 100K occurs at = 8 10 3 (!c 0:09). This behavior corresponds to the tendency toward saturation at higher fields, although the high field limit (!c 1) is not satisfied. Therefore, the transverse magnetoresistance indicates that EuAl4 is a compensated metal above TCDW and changes into an uncompensated metal below TCDW. The present experimental result and the hump of μ just below TCDW indicate that ne or nh starts to decrease below TCDW. Figure 2 shows the temperature dependence of the Hall coefficient RH determined at 0 T, shown by solid circles for H k 1⁄2010 and by solid squares for H k 1⁄2001 . The phase transitions at TCDW 1⁄4 140K and TN 1⁄4 14K are well observed. RH is temperature independent and negative above TCDW for both field directions. This means that the anomalous Hall coefficient, which is related to the magnetic susceptibility, is negligible in EuAl4. The negative RH for the compensated metal indicates e > h. The absolute value of RH increases with decreasing temperature below TCDW. The Hall resistivity is proportional to the magnetic field above TCDW, for example, at 200K, as shown in the inset of Fig. 2. On the other hand, the Hall resistivity gradually deviates downward from the linear field dependence with decreasing 0.06

Abstract: We demonstrate interface energy level engineering, exploiting the modification in energy band structure across Verwey transition temperature (TV) of Fe3O4, in a Fe3O4(111)/Alq3/Co spin-valve (SV). I-V characteristics exhibit a transition in conduction mode from carrier injection to tunneling across TV of Fe3O4 electrode. Both giant magneto-resistance (GMR) and tunneling MR (TMR) have been observed in a single SV, below and above TV, respectively. We have achieved room-temperature SV operation in our device. We believe that the tuning of charge gap at Fermi level across TV resulting in a corresponding tuning of conduction mode and a unique cross over from GMR to TMR.

Abstract: Perovskite-type manganites, such as Pr1−xCaxMnO3, La1−xCaxMnO3 and La1−xSrxMnO3 solid solutions, are set forth as a case study of ferroelectricity formation mechanisms associated with the appearance of site- and bond-centered orbital ordering which breaks structural inversion symmetry. Even though the observation of macroscopic ferroelectricity may be hindered by the finite conductivity of manganites, polarization can still exist in nanoscale volumes. We use Piezoresponse Force Microscopy to probe local bias induced modifications of electrical and electromechanical properties at the manganite surface. Clear bias-induced piezocontrast and local hysteresis loops are observed for La0.89Sr0.11MnO3 and Pr0.60Ca0.40MnO3 compounds providing convincing evidence of the existence of locally induced polar states well above the transition temperature of the CO phase, while the reference samples without CO behavior show no ferroelectric-like response. Such coexistence of ferroelectricity and magnetism in manganites due to the charge ordering (CO) under locally applied electric field opens up a new pathway to expand the phase diagrams of such systems and to achieve spatially localized multiferroic effects with a potential to be used in a new generation of memory cells and data processing circuits.

Abstract: An efficient synthesis route is proposed to obtain single phase powder ceramic of CaFe5O7. This complex structure can be described as an intergrowth between one CaFe2O4 unit and n = 3 slices of FeO Wustite-type structure. A detailed structural study has been carried out at room temperature combining transmission electron microscopy (TEM) observations (ED, HREM), scanning transmission electron microscopy (STEM-HAADF), and X-ray diffraction data. The analysis of these data has revealed an unexpected supercell with a monoclinic symmetry. From the hkl conditions deduced from the electron diffraction study and the analysis of X-ray diffraction data by simulated annealing, a structural model considering the centrosymmetric P21/m setting can be proposed. In addition the first magnetic and electrical transport measurements are reported showing a sharp peak in magnetic susceptibility and a strong localization around 360 K, associated to a structural change from monoclinic setting to orthorhombic one.

Abstract: We report electrical features and magnetoresistance behavior of the oxygen deficient La0.4Ca0.6MnO3-δ perovskites (δ = 0, 0.15, and 0.2). These samples will be referred to as S0, S15, and S20, respectively. The dependence of electrical transport on temperature and magnetic field is systematically investigated between 2 K and 400 K in magnetic field ranging up to 5 T. The parent compound shows a stable charge ordering/antiferromagnetic state with a semiconductor-like behavior in all considered temperature range. The variable range hopping and thermally activated hopping models are found to fit well with the electrical resistivity data at low and high temperatures, respectively. Oxygen deficiency tends to weaken the charge ordering and induce ferromagnetism and metallicity at low temperature. Metal insulator transition appears at higher fields for lower oxygen deficit (S15 sample) and without field for the S20 sample. The resistivity data for S15 sample are discussed in the framework of the variable-range hopping model. Abnormal transport properties were observed in the S20 sample, characterized by the double metal-insulator transitions and low minimum behavior. These results are discussed in terms of phenomenological percolation model, based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. While the parent compound shows no magnetoresistance, a large magnetoresistance is observed in the deficient samples at low temperature reaching 90% and 75% at 2 T for S15 and S20 samples, respectively. Noticeably, these values reached 98% and 91% at 5 T. The appearance of colossal magnetoresistance is attributed to the spin dependent hopping between spin clusters and/or ferromagnetic domains.

Abstract: Magnetite films are grown on Si(100) substrate using pulsed laser deposition technique. The deposition is performed at three fixed oxygen pressures of 0.4 × 10−6, 4.0 × 10−6, and 10 × 10−6 Torr keeping other conditions fixed. Raman and x-ray photoemission spectroscopies confirm formation of single magnetite phase for the first two pressures and a mixed phase composed of magnetite and hematite for the highest pressure. A trivial change in oxygen stoichiometry of magnetite is found crucial for the multiferroic orders at room temperature and below the charge ordering temperature. Possible mechanisms of multiferroic orders driven by change in oxygen stoichiometry are discussed.

Abstract: The series of modular compounds [BanCo2+nO3n+2][BaCo6O9] (n = 1 to 3) including experimental and hypothetical terms, was investigated using DFT calculations and several experimental results. A systematic evolution of the electronic and magnetic states was evidenced along the series leading to ordered CoII/CoIIIversus mixed CoIII/IV charge segregation in two distinct structural motifs. In essence, using different packing modes within the labile [BanCo2+nO3n+2] block, we have systematized the spin state dependence on the CoO6 connectivity, i.e. corner-sharing (HS states) against face-sharing (LS states). We also show that the electronic and magnetic features of the [BaCo6O9] blocks do not vary trough the series, (i.e. HS-CoII and LS-CoIII charge ordering) whereas the [BanCo2+nO3n+2] blocks hold drastic changes from n = 1 to 3. In particular, the later carries a mixed III/IV cobalt charge for n ≥ 2. It leads to a triple valence cobalt state. For n = 2, we experimentally observe at 4 K a superstructure (2a, 2c) superstructure accompanied by a perfect CoII/CoIII/CoIV charge ordering. The charge ordering occurs at Tt = 160 K and is accompanied by a transition in the electronic transport leading to a 2D-VRH behaviour below Tt.