Laboratoire de Cristallographie et Sciences des Matériaux

A detailed study of doped LaMn${\mathrm{O}}_{3}$ with fixed carrier concentration reveals a direct relationship between the Curie temperature ${T}_{c}$ and the average ionic radius of the La site $〈{r}_{A}〉$, which is varied by substituting different rare earth ions for La. With decreasing $〈{r}_{A}〉$, magnetic order and significant magnetoresistance occur at lower temperatures with increasing thermal hysteresis, and the magnitude of the magnetoresistance increases dramatically. These results show that the notion of ``double exchange'' must be generalized to include changes in the Mn-Mn electronic hopping parameter as a result of changes in the Mn-O-Mn bond angle.

Complex perovskite oxides exhibit a rich spectrum of properties, including magnetism, ferroelectricity, strongly correlated electron behaviour, superconductivity and magnetoresistance, which have been research areas of great interest among the scientific and technological community for decades. There exist very few materials which exhibit multiple functional properties; one such class of materials is called the multiferroics. Multiferroics are interesting because they exhibit simultaneously ferromagnetic and ferroelectric polarizations and a coupling between them. Due to the nontrivial lattice coupling between the magnetic and electronic domains (the magnetoelectric effect), the magnetic polarization can be switched by applying an electric field; likewise the ferroelectric polarization can be switched by applying a magnetic field. As a consequence, multiferroics offer rich physics and novel devices concepts, which have recently become of great interest to researchers. In this review article the recent experimental status, for both the bulk single phase and the thin film form, has been presented. Current studies on the ceramic compounds in the bulk form including Bi(Fe,Mn)O3, REMnO3 andthe series of REMn2O5 single crystals (RE = rare earth) are discussed in the first section and a detailed overview on multiferroic thin films grown artificially (multilayers and nanocomposites) is presented in the second section.

ATOMS is a user application providing crystallographic functionality useful to x-ray absorption spectroscopists. ATOMS is also a set of reusable, object-oriented software modules written in the Perl programming language providing crystallographic functionality and access to databases of absorption coefficients and anomalous scattering factors. The main use of the ATOMS program is to generate input data for the ab initio, multiple scattering, x-ray absorption spectroscopy code FEFF. However the code offers many additional features, including useful calculations involving absorption coefficients and simulations of Diffraction Anomalous Fine-Structure (DAFS) spectra. Command line, graphical, and web-based interfaces to the code are offered as part of the standard distribution. As Perl runs on a wide variety of common computer platforms, ATOMS itself is a cross platform application. All text presented to the user can be internationalized - support for four languages is currently included in the package. Development of ATOMS is active - a FEFF interface, structure visualization, and additional crystallographic calculations are among the future developments.

We show in this Letter that heating under high pressure drives ${\mathrm{C}}_{60}$ to new distorted crystalline phases that are metastable at room temperature and pressure. We report three different distortions that are essentially characterized by two nearest neighbor distances, ${d}_{0}^{\ensuremath{'}}\ensuremath{\sim}10$ and ${d}_{0}^{\ensuremath{'}\ensuremath{'}}\ensuremath{\sim}9.2$ \AA{}. The excellent accord with theoretical calculations supports the view that these new carbon phases can be understood as the long range order polymerization of ${\mathrm{C}}_{60}$ through cycloaddition reactions that are at the origin of the shorter intermolecular distances.

Experimental support is found for the multiband model of the superconductivity in the recently discovered system ${\mathrm{MgB}}_{2}$ with the transition temperature ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39\mathrm{K}$. By means of Andreev reflection, evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps ( ${\ensuremath{\Delta}}_{S}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.8\mathrm{meV}$ and ${\ensuremath{\Delta}}_{L}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}7\mathrm{meV}$) are, respectively, smaller and larger than the expected weak coupling value. Because of the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures, but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk ${T}_{c}$ in the raw data.

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This study shows for the first time that the rhombohedral to tetragonal phase transition in Na0.5Bi0.5TiO3 (NBT) is a two step phase transition. The transformation begins by a first order phase transition involving the reconstructive transformation of the rhombohedral phase into an orthorhombic one, through the formation of an intermediate modulated phase. This phase transition begins slightly over 200 °C by the disappearance of the ferroelectric−ferroelastic domains. The intermediate modulated phase is then formed from 230 to 300 °C, the temperature at which it disappears. The modulated phase corresponds to an intergrowth of rhombohedral perovskite blocks in which Pnma orthorhombic sheets are formed by a microtwinning process of the rhombohedral phase. The intermediate orthorhombic phase is then formed at 300 °C and immediately turns to the tetragonal one. A model is presented explaining the formation of the modulated phase and the origin of the antiferroelectric and relaxor behaviors of NBT.

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound ${\mathrm{MgB}}_{2}$ directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of ${\mathrm{MgB}}_{2}$ is used as a tip in the STM experiment. The ``sample'' is chosen to be a 2H- ${\mathrm{NbSe}}_{2}$ single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages ${V}_{S}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}3.8\mathrm{mV}$ and ${V}_{L}\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}7.8\mathrm{mV}$. With rising temperature both peaks disappear at the ${T}_{C}$ of the bulk ${\mathrm{MgB}}_{2}$, a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

In this study of metallic perovskites Mn 3 MX (M=metal, X=C, N) we emphasize the influence of the (outer) electron concentration on the sequences of crystallographic transitions, on the magnetic ordering temperatures and on the constant (Pauli-type like) part of the magnetic susceptibility. A great variety of observed magnetic structures and transitions (including spin rotations) are described for carbides and nitrides. The Labbé-Jardin tight binding model which invokes a p-d II hybridization between N and Mn orbitals having their energy levels near to the Fermi surface provides the possibility of great variations of the density of states versus temperature and is in qualitative agreement with experiment. It also explains that small substitutions on X have a more drastical effect than those can M.

In this review we discuss considerations regarding the common techniques used for measuring thermoelectric transport properties necessary for calculating the thermoelectric figure of merit, <italic>zT</italic>.

We provide a comprehensive analysis of the microstructure of the porous glass, vycor. Using transmission electron microscopy, small-angle x-ray scattering, molecular adsorption, and the dynamic process of direct energy transfer, a consistent picture of the mass, pore, and interfacial features of this material is presented. From a transmission-electron-microscopy image of an ultrathin section of vycor the material appears to have a homogeneous distribution of mass with no hierarchical organization. The pore interface exhibits a roughness which is probed by both small-angle x-ray scattering and molecular adsorption. The roughness has an upper cutoff of &amp;lt;20 Å which is not resolved in the transmission-electron-microscopy image and is shown to be unimportant to the dynamics of the direct energy transfer process. The dimensionality probed by direct energy transfer is shown to be related to interfacial geometrical crossover from two dimensional to three dimensional, which is characterized by a persistent length of the interface of 45 Å.

Metal oxides (Ca 3 Co 4 O 9 , CaMnO 3 , SrTiO 3 , In 2 O 3 ), Ti sulfides, and Mn silicides are promising thermoelectric (TE) material candidates for cascade‐type modules that are usable in a temperature range of 300–1200 K in air. In this paper, we review previous studies in the field of TE materials development and make recommendations for each material regarding future research. Furthermore, the R&amp;D of TE modules composed of metal oxide materials and the prospect of their commercialization for energy harvesting is demonstrated.

The effect of crystallite sizes L smaller than 100 nm on the integrated Raman cross section Σc of the transverse optical (TO) mode of fcc silicon was studied experimentally in fully nanocrystallized thin films. The Σc/Σa (amorphous) ratio of this mode is shown to be 1 up to L=30 Å, and to decay exponentially down to 0.1 at larger L. A systematic procedure taking into account both this effect and the experimental optical absorption coefficient αexp at the excitation wavelength is then proposed for the determination of the crystalline volume fraction in mixed phase (amorphous/nanocrystalline) silicon systems by Raman measurements.

${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ presents a first order structural transition at 290 K that was known to perturb the functioning as cathode in rechargeable Li batteries. We have solved the structure at 230 K and deciphered unambiguously the nature of this phase transition. The analysis of valence bond sums shows that the transition results from a partial charge ordering: two of the five Mn sites correspond to well-defined ${\mathrm{Mn}}^{4+}$ and the other three sites are close to ${\mathrm{Mn}}^{3+}$ ions. Charge ordering is accompanied by simultaneous orbital ordering due to the Jahn-Teller effect in ${\mathrm{Mn}}^{3+}$ ions. The microscopic details obtained from the structure are crucial for understanding the electron hopping persisting below the transition.

Oxide electronic materials provide a plethora of possible applications and offer ample opportunity for scientists to probe into some of the exciting and intriguing phenomena exhibited by oxide systems and oxide interfaces. In addition to the already diverse spectrum of properties, the nanoscale form of oxides provides a new dimension of hitherto unknown phenomena due to the increased surface-to-volume ratio.

Fifty years ago, the optimization of thermoelectric devices was analyzed by considering the relation between optimal performances and local entropy production. Entropy is produced by the irreversible processes in thermoelectric devices. If these processes could be eliminated, entropy production would be reduced to zero, and the limiting Carnot efficiency or coefficient of performance would be obtained. In the present review, we start with some fundamental thermodynamic considerations relevant for thermoelectrics. Based on a historical overview, we reconsider the interrelation between optimal performances and local entropy production by using the compatibility approach together with the thermodynamic arguments. Using the relative current density and the thermoelectric potential, we show that minimum entropy production can be obtained when the thermoelectric potential is a specific, optimal value.

Magnetic properties of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ (BFO) single crystals with rhombohedral symmetry $(R3c)$ were measured in the temperature range of $4.2--350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Zero field cooled (ZFC) and field cooled magnetization curves split below $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with a sharp cusp around $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the ZFC curves revealing spin-glass behavior in BFO single crystals. The observed coercive field increases with decreasing temperature, suggesting week ferromagnetism below $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and spin glass behavior below $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The cusps around the freezing temperature $[{T}_{f}(\ensuremath{\omega})]$ which are frequency dependent confirm the spin-glass behavior in BFO single crystals with spin freezing temperature $({T}_{\mathit{SG}}=29.4\ifmmode\pm\else\textpm\fi{}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The critical exponent $z\ensuremath{\nu}=1.38$ agrees rather well with experimental values in $\mathrm{La}{\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{O}}_{3}$ $(z\ensuremath{\nu}=1.0)$ [K. De et al., J. Appl. Phys. 99, 13908 (2006)] and the mean field theory $(z\ensuremath{\nu}=2.0)$ of Kirkpatrick and Sherrington [Phys. Rev. B 17, 4384 (1970)].

In rhombohedral ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$, an improper ferroelectric polarization of magnitude $2870\text{ }\text{ }\ensuremath{\mu}\mathrm{C}\text{ }{\mathrm{m}}^{\ensuremath{-}2}$ is induced by an incommensurate helical magnetic structure that evolves below ${T}_{\mathrm{N}1}=90\text{ }\text{ }\mathrm{K}$. The electric polarization was found to be constrained to the high symmetry threefold rotation axis of the crystal structure, perpendicular to the in-plane rotation of the magnetic moments. The multiferroicity is explained by the ferroaxial coupling mechanism, which in ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$ gives rise to the largest magnetically induced, electric polarization measured to date.

Controlling Zeolite Nucleation Small zeolite crystals are of increasing interest as catalysts and for membrane separations because they allow the high selectivity of their cages to be exploited while minimizing the kinetic limitations caused by diffusion. Ng et al. (p. 70 , published online 8 December) synthesized ultrasmall crystals (6 to 15 nanometers) of the EMT zeolite, which has a low framework density and good catalytic properties for hydrocarbon “cracking” (conversion of a large hydrocarbon to smaller ones). The synthesis of EMT has normally required expensive organic templates that limit its industrial use. Careful control of the synthesis conditions, such as ratios of reactants and short bursts of microwave heating, allowed small EMT crystals to nucleate and avoid formation of zeolites with closely related structures.

Exceptional magnetic properties of magnetite, Fe3O4, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe3O4 nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe3O4 composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe3O4+δ composition, which could be attributed to the formation of Fe3O4–γ-Fe2O3 solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia.