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Microstructure related to the charge ordered (CO) state and metal-insulator (MI) transition in ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ was investigated by transmission electron microscopy. An in situ observation on heating from the CO state revealed a drastic change in the microstructure: the nucleation and growth of the ferromagnetic (FM) charge disordered regions proceed in the vicinity of the antiphase boundaries in the CO regions, which are closely related to the MI transition around 160 K. These transition processes suggest that an abrupt increase in the electronic conductivity during the MI transition is mainly due to the percolation mechanism. Furthermore, the FM metallic phase is found to be characterized by a spatially fine mixture of incommensurate charge ordered microdomains in the FM charge disordered phase. This result clearly demonstrates the occurrence of microscopic-scale electronic phase separation in the manganese oxides.

Structural and magnetic properties are systematically investigated for filling- and structure-controlled manganites $({\mathrm{La}}_{1\ensuremath{-}z}{\mathrm{Nd}}_{z}{)}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ $(0.35l~xl~0.50,$ $0.0l~zl~1.0)$. The ground state at a fixed x value was found to change from ferromagnetic metallic (FM) to charge-ordered insulating (CO) states with increase of z. We have found suppressed ferromagnetic moment $(\ensuremath{\sim}0.5{\ensuremath{\mu}}_{B}\char21{}2.5{\ensuremath{\mu}}_{B}$ at 1 T) at 10 K in the vicinity of the FM-CO phase boundary, and have ascribed it to the phase separation (PS) into the FM and CO states. The PS region spreads out around the FM-CO phase boundary in the ground-state phase diagram.

The temperature dependences of high-pressure Raman spectra are investigated in the mixed-valence (MV) gold complexes Cs 2 Au 2 X 6 ( X = Cl and Br), which show a pressure-induced phase transition from the MV to single-valence (SV) states. In the SV state at lower temperatures, we observe a broad Raman band at ∼400 cm -1 for X = Cl and at ∼320 cm -1 for X = Br. Judging from their frequencies as well as the significant temperature dependence, we interpret the bands in terms of the formation of the local Au- X 2 - -like state.

We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7} exhibits CE-type charge-ordered and spin-ordered states below $T_{N, CO}^A = 210$ K and below $T_{N}^{CE} \sim 145$ K, respectively. However, the volume fraction of the CE-type ordering is relatively small, and the system is dominated by the A-type spin ordering. The coexistence of the two types of ordering is essential to understand its transport properties, and we argue that it can be viewed as an effective phase separation between the metallic $d(x^{2}-y^{2})$ orbital ordering and the charge-localized $d(3x^{2}-r^{2})/d(3y^{2}-r^{2})$ orbital ordering.

Optical reflectivity spectra of La(1-x)Sr(x)MnO(3) (0

Effects of hydrostatic pressure on Raman spectra have been investigated in mixed-valence (MV) gold complexes Cs2Au2X6 (X=Cl and Br). We have observed pressure-induced deactivation of the Raman-active Au-X stretching modes at various temperatures, indicating a pressure-induced phase transition from the MV state to a single-valence (SV) state. The critical pressure at 300 K decreases from 10.6 GPa for X=Cl to 6.8 GPa for X=Br, perhaps due to reduced electron–phonon interaction in Cs2Au2Br6. We further derived comprehensive electronic phase diagrams of Cs2Au2X6 (X=Cl and Br) as a function of pressure and temperature.

Electronic structures of halogen bridged mixed-valence (MV) complexes ${\mathrm{Cs}}_{2}{\mathrm{Au}}_{2}{X}_{6} (X$=I, Br, and Cl) have been investigated by means of reflectivity measurements. We have observed five polarization-dependent optical transitions in the energy range of 0.6--5.5 eV. The lower-lying three bands below \ensuremath{\sim}3 eV are due to the intermolecular transitions from Au${X}_{2}^{\ensuremath{-}}$ to Au${X}_{4}^{\ensuremath{-}}$ molecules, while the others are assigned to the intramolecular transitions. The optical gap ${E}_{\mathrm{gap}}$ increases from 1.31 eV for X=I to 2.04 eV for X=Cl, due to enhancement of the nearest-neighboring Coulomb repulsion V and the Jahn-Teller distortion of Au${X}_{4}^{\ensuremath{-}}(S)$. We further investigated external pressure effects on the electronic state, and found pressure-induced disappearance of the Raman-active Au-Br stretching modes, which indicates an electronic phase transition from the MV state to a single-valence (SV) state.

Temperature variation of reflectivity spectra (0.6--5.0 eV) has been investigated for ferromagnetic ${\mathrm{CoS}}_{2}.$ We have found significant spectral changes at the ferromagnetic transition ${(T}_{\mathrm{C}}=120\mathrm{}\mathrm{K}):$ (1) oscillator strength of the lower-lying interband transition $(\ensuremath{\sim}1.6\mathrm{eV})$ is suppressed, and (2) the higher-lying transition $(\ensuremath{\sim}3.4\mathrm{eV})$ shifts to the low energy side. We have ascribed the spectral change to the splitting of the ${e}_{g}$ state into the up-${(e}_{g\ensuremath{\uparrow}})$ and down-spin ${(e}_{g\ensuremath{\downarrow}})$ states, or the half-metallic behavior of ${\mathrm{CoS}}_{2}.$

Anisotropic magnetic and transport properties have been investigated for single crystals of (Nd 1- z La z )Sr 2 Mn 2 O 7 . We have derived the magnetic phase diagram against the La concentration z and have found that the low-temperature state is of layered antiferromagnetic with alternating MnO 2 -sheets. The reentrant-type charge-ordered state is observed in the limited z -region around z =1.0 due to the released Jahn-Teller distortion. We further investigated magnetotransport properties for these antiferromagnetic manganites, and have found a large negative magnetoresistance, e.g. , ρ c ( H )/ρ c (0)∼0.86 and ρ ab ( H )/ρ ab (0)∼0.88 at µ 0 H =1 T.

Chemical substitution effects on the magnetic and transport properties have been investigated for single crystals of ${(\mathrm{L}\mathrm{a}}_{1\ensuremath{-}z}{\mathrm{Nd}}_{z}{)}_{1.4}{\mathrm{Sr}}_{1.6}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ with bilayer structure. We have found that the ferromagnetic ground state for $z=0.0$ is replaced by a layered antiferromagnetic state with alternating ferromagnetic bilayers beyond $zg~0.1.$ Reflecting the subtle balance of the competing interbilayer exchange couplings, application of an external magnetic field induces an antiferromagnetic-ferromagnetic transition. The transition accompanies a switching-type reduction of the current-perpendicular-to-plane resistivity, which can be viewed as a spin-valve functionality.

We have used high energy-resolution angle-resolved photoemission spectroscopy(ARPES) to map the k-dependent electronic structure of single-crystallinesamples of the manganese-based perovskites as a function of doping level, temperature,and layer number. The measured near-Fermi energy states display E vs. kand symmetry relationships which agree relatively well with the LSDA band theoryprediction through much of the Brillouin zone, and the locus of lowest energy excitations matches the predicted large Fermi surface quite well. However, thespectralfeatures are too broad to be well described as Fermi-Liquid likequasiparticles, and they are strongly suppressed from the Fermi energy, e.g., there is apseudogap in the excitation spectrum. We discuss the spectral properties interms of strong coupling to a local effect such as a lattice distortion.

We have performed direct electronic structure measurements of the colosssal magnetoresistive oxides using high energy resolution photoemission spectroscopy. A key result of these measurements is the discovery of a pseudogap which drastically supresses or completely removes the spectral weight (density of states) at the Fermi energy E F . It is shown that the pseudogap plays a dominant role in the "colossal" conductivity changes which occur across the magnetic ordering temperature. We briefly discuss possible origins for the pseudogap.

Comprehensive neutron-diffraction studies on La 2-2 x Sr 1+2 x Mn 2 O 7 ( x =0.40,0.45 and 0.48) single crystals have revealed that the low-temperature magnetic phase consists of planar ferromagnetic (FM) and A-type antiferromagnetic (AFM) components, indicating a canted AFM ordering. Upon increasing the hole concentration x , the canting angle between planes changes from 6.3° (nearly planar FM) at x =0.40 to 180° (A-type AFM) at x =0.48, while the ordering temperature of the FM component, T C , decreases from 120 K to 0 K, correspondingly. We have also discovered that the A-type AFM ordering remains above T C and shows an anomalous exponential decrease to T N ∼200 K. This newly found intermediate A-type AFM phase may play a significant role in the enhancement of CMR effects in this layered Mn perovskite system.

The k-dependent electronic structure of the low temperature ferromagnetic state of La 1.2Sr1.8Mn2O7 was measured using angle-resolved photoemission spectroscopy and calculated using the local spin density approximation (LSDA). The measured near-Fermi energy states display E vs k and symmetry relationships which agree relatively well with the LSDA prediction through much of the Brillouin zone, and the locus of lowest energy excitations matches the predicted large Fermi surface quite well. However, the spectral features are too broad to be well described as Fermi-liquid-like quasiparticles, and they are strongly suppressed from the Fermi energy, i.e., there is a pseudogap in the excitation spectrum. We discuss the spectral properties in terms of strong coupling to a local effect such as a lattice distortion. [S0031-9007(98)06522-3] PACS numbers: 71.30. + h, 71.20.Lp, 72.80.Ga, 79.60.Bm

Charge ordering and orbital ordering in ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_{4}$ have been investigated by using synchrotron x-ray diffraction. The alternating pattern of ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ in real space is observed directly by making use of their anomalous dispersion. The first clear evidence of the orbital ordering of ${e}_{g}$ electrons on ${\mathrm{Mn}}^{3+}$ is also presented based on the measurements of ATS (anisotropy of the tensor of susceptibility) reflection near the Mn $K$-absorption edge. The present observations indicate that the charge ordering and orbital ordering occur simultaneously at a transition temperature higher than that of the spin ordering.

Anisotropic magnetic and transport properties have been investigated for single crystals of layered-perovskite manganites (La 1- z Nd z ) 2-2 x Sr 1+2 x Mn 2 O 7 as a function of thendoping level x and averaged ionic radius z of the rare-earth ion. The ferromagnetic metallic (FM) state observed in La 1.2 Sr 1.8 Mn 2 O 7 ( x =0.4; z =0.0) disappears with increasing x or z , and the ground state is replaced by a layered antiferromagnetic (layered-AF) state beyond x ∼0.5. Thus obtained phase diagram is discussed in terms of variation of the e g -band structure with the band-filling (=1- x ).

High pressure Raman scattering measurements are performed on hydrogen-bonded molecular system, crystal of squaric acid H2C4O4. Specific intramolecular vibrational modes of squaric dianion C4O42- are found to disappear under pressures due to vanishment of the molecular distortion. This indicates that application of presure deformed the proton potential from double-minimum to single-minimum type. We fourther found a novel quantum paraelectric state, where deformed dianions can not show any ordering down to the lowest temperature, in the pressure range of 3.0-4.5 GPa.

Magnetic and transport properties are investigated for single crystals of layered-type\char21{}doped manganites ${R}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$ with systematic variation of the averaged radius ${r}_{R}$ of the trivalent rare-earth ions ${R}^{3+}$. La${}_{0.5}$Sr${}_{1.5}$MnO${}_{4}$ $(R=\mathrm{La})$ shows a charge-ordering (CO) transition at ${T}_{\mathrm{CO}}=230$ K. With a decrease of the in-plane Mn-O bond length ${d}_{\mathrm{Mn}\ensuremath{-}\mathrm{O}(\mathrm{in})},$ ${T}_{\mathrm{CO}}$ decreases and eventually the transition disappears for $R=\mathrm{Nd}$. Such a remarkable lattice effect on the CO transition is in sharp contrast to the negligible lattice effect in the isostructural layered nickelates.

Effects of hydrostatic pressure on the magnetic and transport properties have been investigated for melt-grown crystals of La{sub 1{minus}x}Sr{sub x}MnO{sub 3} with a finely-controlled doping level x near the metal-insulator phase boundary. Application of pressure affects the critical temperatures for the orthorhombic-rhombohedral structural transition (T{sub s}) as well as for the metal-insulator transition (T{sub MI}) that has been interpreted in terms of an ordering of the Jahn-Teller polarons. Reflecting that the kinetic energy of the e{sub g} carriers sensitively depends on the degree of the lattice distortion, T{sub s} shows an abrupt decrease under pressure when it crosses the magnetic phase boundary. On the other hand, T{sub MI} is significantly suppressed under pressure via enhanced e{sub g}-electron hopping. These pressure effects are compared with those of an external magnetic field. {copyright} {ital 1997} {ital The American Physical Society}

The metal-insulator (M-I) phase transitions relevant to charge ordering (CO) have been investigated for perovskite-type (Nd{sub 1{minus}y}Sm{sub y}){sub 1/2}Sr{sub 1/2}MnO{sub 3} (0{le}y{le}1) crystals, in which the one-electron bandwidth (W) is systematically controlled by varying the averaged ionic radius of the A site and by application of quasihydrostatic pressure (P). Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex M-I phase diagrams with temperature (T) and W (y and/or P) as the parameters. The M-I phase boundaries are associated with critically W- and T-dependent hystereses, which result in unique appearance of the metastable state. We have demonstrated the pressure-induced phase transition from the metastable ferromagnetic metal to the thermodynamically stable charge-ordered insulator for the y=0.875 crystal locating near the critical M-I phase boundary. With decrease of W, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (T{sub c}) and enhances the electron-lattice coupling. Consequently, the lattice-coupled first-order I-M transition is observed at T{sub c} in the small-W region of y{ge}0.5. It was found that application of magnetic field also induces the phase transition from the insulator with antiferromagnetic spin correlations to the ferromagnetic metal, which is accompanied by lattice-structuralmore » change. {copyright} {ital 1997} {ital The American Physical Society}« less

Lattice effects on the anisotropic magnetic and transport properties have been investigated for single crystals of (La{sub 1{minus}z}Nd{sub z}){sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} with a layered structure. The ferromagnetic transition temperature T{sub C} is suppressed from T{sub C}=130 K for z=0.0 to 80 K for z=0.2, and eventually the transition disappears beyond z=0.4. From the significant change of the magnetic anisotropy, we have found that the suppression originates from the increasing d{sub 3z{sup 2}{minus}r{sup 2}} character in the occupied e{sub g} state due to the Jahn-Teller distortion of the MnO{sub 6} octahedra. Our observations indicate that the effects of chemical substitution are qualitatively different between the cubic and layered doped manganites. {copyright} {ital 1997} {ital The American Physical Society}

The temperature dependence of absorption spectra for thin films of double-exchange ferromagnet ${R}_{0.6}$Sr${}_{0.4}$MnO${}_{3}$ has been investigated with variation of the one-electron bandwidth $W$. With decreasing temperature, which corresponds to the increase of the magnetization $M$, a large amount of spectral weight transfers from the high- $(\ensuremath{\sim}$3 eV) to low- $(l~$1 eV) energy region. We ascribe these transitions to the interband transition between the bands split by the on-site exchange interaction $J$ and the intraband transition within the lower band, respectively. This suggests that optical excitations in the visible region strongly couple with local spin alignments in these doped manganites.

We have used inelastic neutron scattering to measure the spin wave dispersion througout the Brillouin zone of the double exchange ferromagnet La0.7Pb0.3MnO3. The dispersion is well described by that for the double exchange Hamiltonian. The Curie temperature calculated for a local moment ferromagnet with the observed dispersion agrees with experiment within 15%. In the paramagnetic phase of the two-dimensional analogue La1.2Sr1.8Mn2O7, we have observed long-lived antiferromagnetic cluster coexisting with ferromagnetic fluctuations. This effect must be included in theories that attempt to explain giant magnetoresistance in manganites, at least in two dimensions.

The bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ exhibits a transition between nearly insulating paramagnetic and metallic ferromagnetic states at ${T}_{C}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}126\mathrm{K}$. We use magnetic neutron scattering to show that the paramagnetic state contains long-lived antiferromagnetic clusters coexisting with ferromagnetic critical fluctuations. The principal implication is that at least in two dimensions, Anderson localization effects must also be taken into account in attempts to explain giant magnetoresistance in manganites.

Soft X-ray magnetic circular dichroism (SXMCD) measurements are reported on the O K- and Mn L-absorption edges of La 1-x Sr x MnO 3 (x = 0.15,0.18, and 0.4) single crystals in their ferromagnetic regime. This has been done using a recently developed experimental technique for magnetizing the samples offering the possibility to get high-accuracy SXMCD effect measurements on bulk samples. The very strong doping-induced O K-edge intensity is proof of an enhanced O2p hole component of the doped holes. O K soft X-ray magnetic circular dichroism spectra give evidence for an orbital magnetic moment of this O2p component ferromagnetically coupled to the Mn moments.

Effects of hydrostatic pressure on the magnetic and transport properties of R 1- x Sr x MnO 3 have been investigated with systematic variation of the one-electron bandwidth ( W ) of a conduction electron, as well as of the doping level ( x ). The positive pressure coefficient (dln T C /d P ) of the Curie temperature is observed for all the compounds, and its absolute value increases with decrease of W , or equivalently, T C . We have derived a nearly x -independent empirical relation between the coefficient and T C in the lightly doped region ( x ≤0.4). The coefficients at x ≈1/2, however, are found to deviate from the above relation due to competing antiferromagnetic and/or charge-ordering interaction.

Effects of chemical and external pressures have been investigated on the two types of charge-ordering (CO) systems of perovskite manganites with the use of single-crystal specimens: One is ${\mathrm{Nd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{MnO}}_{3}$ with moderate CO instability occurring only near x=1/2 and the other is ${\mathrm{Pr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{MnO}}_{3}$ with stronger CO instability extending over a wide x region 0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7. We have partially substituted the Nd ions of ${\mathrm{Nd}}_{1\mathrm{/}2}$${\mathrm{Sr}}_{1\mathrm{/}2}$${\mathrm{MnO}}_{3}$ with larger La ions or applied external pressure on them with the aim of destabilizing the CO state via an increase of the 3d-electron hopping interaction. An electronic phase diagram relevant to the CO transition was derived for (${\mathrm{Nd}}_{1\mathrm{\ensuremath{-}}\mathrm{z}}$${\mathrm{La}}_{\mathrm{z}}$${)}_{1\mathrm{/}2}$${\mathrm{Sr}}_{1\mathrm{/}2}$${\mathrm{MnO}}_{3}$ by such a control of the one-electron bandwidth (W). With an increase of W, the enhanced ferromagnetic double-exchange interaction increases the Curie temperature (${\mathrm{T}}_{\mathrm{C}}$) and suppresses the charge-ordered state with a concomitant antiferromagnetic charge-exchange-type spin ordering (AF-CE). In a narrow window of z (0.4\ensuremath{\leqslant}z\ensuremath{\leqslant}0.6) or in the pressurized state for z=0.4, another type of antiferromagnetic (perhaps the A type) phase replaces the AF-CE state. Application of external pressure and resultant enhanced carrier itineracy suppresses the CO transitions also for ${\mathrm{Pr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{MnO}}_{3}$. For the x=0.30 crystal, application of pressure induces a metallic phase from the low-temperature side in the charge-ordered insulating phase. The pressure-temperature phase diagrams relating to the CO transition or the concurrent insulator-to-metal transition were shown to scale well with the magnetic-field--temperature phase diagrams.

We have investigated the electronic structure of an electron-doped double perovskite series Sr 2− x La x FeMoO 6 by photoemission spectroscopy and LDA + U band-structure calculations. Characteristic double-peak structures observed at ∼0.2 eV (Feature A) and ∼1.3 eV (Feature B) have been identified to be a Fe–Mo t 2g↓ band and a Fe e g↑ band, respectively. Upon La substitution, Feature A moves away from the Fermi level by ∼50 meV, reflecting electron-doping. At the same time, the intensity enhancement of both A and B has been observed, which can be understood in terms of the strong Hund's rule energy stabilization in 3d 5 configuration.

The cathode properties of Na1.32Mn [Fe(CN)6]0.83·3.5H2O are investigated using a thin film electrode.

Neutron scattering studies on a single crystal of the highly correlated electron system, La{sub 1{minus}{ital x}}Sr{sub {ital x}}MnO{sub 3} with {ital x}{approx_equal}0.3, have been carried out elucidating both the spin and lattice dynamics of this metallic ferromagnet. We report a large measured value of the spin wave stiffness constant, which directly shows that the electron transfer energy of the {ital d} band is large. The spin dynamics, including magnetic critical scattering, demonstrate that this material behaves similar to other typical metallic ferromagnets such as Fe or Ni. The crystal structure is rhombohedral, as previously reported, for all temperatures studied (below {approximately}425 K). We have observed superlattice peaks which show that the primary rhombohedral lattice distortion arises from oxygen octahedra rotations resulting in an {ital R}{bar 3}{ital c} structure. The superlattice reflection intensities, which are very sensitive to structural changes, are independent of temperature demonstrating that there is no primary lattice distortion anomaly at the magnetic transition temperature {ital T}{sub {ital C}}=378.1 K; however, there is a lattice contraction. {copyright} {ital 1996 The American Physical Society.}

An appreciable magnetic circular dichroism is found for the La 4d → 4f core absorption in a ferromagnetic La0.84Sr0.16MnO3. An ordered moment of the La 4f state with its spin parallel to the Mn 3d majority spin is revealed. The x-ray photoemission spectra have shown satellite structures of the La 3d, 4d and O 1s core states besides the Mn 2p and 3p states. Appreciable hybridization between the La 4f, La 5d, O 2p and Mn 3d states in the core hole excited states are clarified.

Competition between the ferromagnetic double-exchange interaction and the antiferromagnetic charge-ordering instability gives rise to thermally as well as magnetic field (H) induced insulator-metal transitions in distorted perovskites $({\mathrm{Nd}}_{1\ensuremath{-}y}{\mathrm{Sm}}_{y}{)}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3}$ with a controlled one-electron bandwidth. A systematic study on the metal-insulator phase diagram in the $T\ensuremath{-}H$ plane with varying y as well as a high-pressure study has revealed the metastable nature of the undercooled ferromagnetic metallic state.

Elastic neutron scattering measurements characterizing the charge and magnetic order in L${\mathrm{a}}_{0.5}$S${\mathrm{r}}_{1.5}$Mn${\mathrm{O}}_{4}$ are presented. Superlattice reflections corresponding to an alternating ${\mathrm{Mn}}^{+3}/{\mathrm{Mn}}^{+4}$ pattern with wave vector ${\mathbf{q}}_{\mathrm{co}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\frac{1}{2},\frac{1}{2},0)$ are observed below the charge ordering temperature, ${T}_{\mathrm{co}}\ensuremath{\approx}217$ K. Below the N\'eel temperature, ${T}_{N}\ensuremath{\approx}110$ K, the magnetic moments associated with these ions form an ordered structure with dimensions $2\sqrt{2}a\ifmmode\times\else\texttimes\fi{}2\sqrt{2}a\ifmmode\times\else\texttimes\fi{}2c$ relative to the underlying chemical cell.

Field-induced insulator-to-metal transitions have been found in ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}(0.3l~xl~0.5)$ single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., ${\mathrm{Mn}}^{3+}$/${\mathrm{Mn}}^{4+}$ ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The $x$-dependent metal-insulator phase diagrams in the $H\ensuremath{-}T$ plane indicate that a deviation of $x$ from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordered state.

We have investigated the peculiar structure of charge and spin ordering in the half-doped layered perovskite oxide La$_{1.5}$Sr$_{0.5}$CoO$_4$ by elastic neutron scattering. Two samples with different preparation histories were studied. We find that the generic features of the ordered states, such as their short-range, glassy nature and the spin incommensurability, are sample-independent. At the same time, some subtle features of the ordered phases, such as the correlation lengths, differ., Proof-corrected version, to appear in J. Appl. Phys. 95, # 11, 18141MMM (2004)

Optical study of trivalent 3 d transition metal oxide compounds LaSr M O 4 ( M =Cr, Mn, Fe and Co) with layered perovskite structure has revealed fundamental features of their electronic structures. Systematic change of the charge-transfer gap ( E gap ) with the 3 d element ( M ) can be explained by a simple ionic model. The spectrum below ∼7 eV is found to be strongly M -dependent, and is interpreted in terms of multiplet splitting of the final 3 d N +1 state.

Magnetotransport properties were investigated for a crystal of perovskite-type manganese oxide, Pr 1- x Ca x MnO 3 ( x =0.3). The single-crystal compound is insulating and below 220 K shows the charge-ordering of Mn 3+ and Mn 4+ species. Application of a magnetic field (>20 kOe) changes a canted antiferromagnetic insulating state into a ferromagnetic metallic state accompanied by change of resistivity by several orders of magnitude. At low temperatures, e.g.

O3-NaFe1/2Co1/2O2 shows initial capacity of 160 mAh/g and an average operating voltage of 3.1 V (vs. Na) with good cyclability, and is a promising candidate of the cathode materials for sodium-ion secondary batteries (SIBs). Here, we found that the cyclability of the slowly-cooled sample is much worse than that of quenched one, even though the former sample keeps the O3-type structure. The energy dispersive X-ray spectroscopy (EDX) images suggest that the slow-cooled sample (Nax Fe1–yCoy O2) is inhomogeneous in the Fe concentration (1 – y), perhaps triggered by the Na deficiency (1 – x). We ascribed the poor cyclability in the slowly-cooled sample to the concentration inhomogeneity (Δy). The Δy is further responsible for the fluctuation of the lattice constants (a and c), as revealed by the Williamson–Hall plot. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

The perovskite-type manganese oxide, ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5)$, shows a first-order phase transition from a ferromagnetic (FM) metal to antiferromagnetic (AFM) nonmetal at 140 K, which accompanies the transition to the charge-ordered state (generalized Wigner lattice). The nonmetal-to-metal transition temperature can be decreased down to near 0 K with a magnetic field up to 70 kOe. The temperature-field phase diagram for the charge-ordered state is presented together with a discussion on the mechanism of this field-induced phase transition.

Insulator-metal phenomena depending on band filling (doping degree), temperature, and external magnetic field have been investigated for prototypical double-exchange ferromagnets, namely, crystals of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ (0\ensuremath{\le}x\ensuremath{\le}0.6). The electronic phase diagram in the plane of the temperature vs nominal hole concentration (x) has been deduced from the magnetic and electrical measurements on the melt-grown crystals. Around the ferromagnetic transition temperature ${\mathit{T}}_{\mathit{C}}$, large negative magnetoresistance was observed. Irrespective of temperature, reduction of the resistivity is scaled with the field-induced magnetization (M) as -\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}=C(M/${\mathit{M}}_{\mathit{s}}$${)}^{2}$ for M/${\mathit{M}}_{\mathit{s}}$\ensuremath{\lesssim}0.3, where ${\mathit{M}}_{\mathit{s}}$ is the saturated magnetization. The coefficient C strongly depends on x, i.e., C\ensuremath{\approxeq}4 near the compositional insulator-metal phase boundary (${\mathit{x}}_{\mathit{c}}$\ensuremath{\sim}0.17), but decreases down to \ensuremath{\approxeq}1 for xg=0.4, indicating the critical change of the electronic state.

Giant magnetotransport phenomena have been investigated for single crystals of perovskite-type La1−xSrxMnO3 and related compounds. Magnetoresistance (MR) with negative sign shows a universal curve as a function of the magnetization, the body of which is quantitatively accounted for by the mean-field theory of the Kondo lattice model with strong ferromagnetic (Hund) coupling. Furthermore, magnetic field-induced structural change has been found between the two types of slightly distorted perovskite structure perhaps due to the strong coupling between the electronic and lattice degrees of freedom in these hole-doped Mn-oxides.

Dielectric and thermodynamic properties have been investigated for isotopically mixed crystals of K3D1−x Hx(SO4)2, which are composed of structurally isolated dimeric units SO 4 − ... H-SO 4 − . The critical temperature Tc for the antiferroelectric phase transition decreases with proton concentration (x) and eventually the phase transition vanishes at xc ∼ 0.66 ± 0.04. The x-dependent behavior of Tc, the dielectric constant e(T) and the specific heat C(T) are well accounted for by the mean-field theory for the transverse Ising model, indicating an important role of the proton tunneling on the phase transition and related properties. These features are compared with conventional hydrogen-bonded dielectrics with infinite networks of the hydrogen bonds.

Manganese hexacyanoferrates (Mn-HCFs) with a jungle-gym-type structure are promising cathode materials for Li+/Na+ secondary batteries (LIBs/SIBs). Here, we investigated the diffusion constants D Li/D Na of Li+/Na+ against the Li+/Na+ concentration x Na/x Li and temperature (T) of A 1.32Mn[Fe(CN)6]0.833.6H2O (A = Li and Na). We evaluated the activation energy of D Li/D Na against x Na/x Li. We found that steeply increases with x Na from 0.41 eV at x Na = 0.69 to 0.7 eV at 1.1. The increase in is ascribed to the occupancy effect of the Na+ site. The increase in is suppressed, probably because the number of Li+ sites is three times that of Na+ sites.

Electrical resistivity (\ensuremath{\rho}) and magnetic susceptibility (\ensuremath{\chi}) were measured for single crystals of ${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Sr}}_{1+\mathit{x}}$${\mathrm{MnO}}_{4}$ (0.0\ensuremath{\le}x\ensuremath{\le}0.7), which were grown by the floating-zone method with varying the nominal hole concentration (x). With hole doping, critical temperature (${\mathit{T}}_{\mathit{N}}$) for the antiferromagnetic phase transition decreases and eventually a spin-glass phase appears for x\ensuremath{\ge}0.2, perhaps due to the competition between the generic antiferromagnetic superexchange interaction and the ferromagnetic double-exchange interaction. At x\ensuremath{\approxeq}1/2, real-space ordering of the doped holes occurs at \ensuremath{\sim}220 K, accompanying a steep increase of \ensuremath{\rho} as well as a suppression of \ensuremath{\chi}.