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The structural phase transitions of Pr(Sr0.1Ca0.9)2Mn2O7 have been studied by synchrotron radiation x-ray powder diffraction. Three different charge order (CO) phases are identified as a function of temperature. The three phases have a checkerboard arrangement of compressed and expanded MnO6 octahedra in the perovskite bilayers. The structural changes associated with each transition are described in terms of the mode decomposition analysis with respect to the parent tetragonal structure. The high-temperature phase exhibits an orthorhombic symmetry (Amam) due to cooperative tilts of the MnO6 octahedra that can be ascribed to the irreducible representation (irrep) X3−. On cooling, the CO1 phase with Pbnm symmetry arises from the condensation of modes belonging to the irrep DT2 together with breathing (X1+) and antiferrodistortive (M5− and Y2) modes. The Mn site is split into two nonequivalent Mn atoms (Mn1 and Mn2) with a small charge segregation, and the asymmetric environment of Mn1 atoms form zigzag chains along the doubled b axis. Further cooling leads to the formation of the CO2 phase that comes from a relative change between the directions of previous X3− and DT2 distortions, producing the simultaneous condensation of new DT1 and ferrodistortive GM5− modes. In the CO2 structure (Am2m symmetry), Mn2 atoms are split into two nonequivalent sites but with similar oxidation states. Then the checkerboard arrangement between Mn1 and Mn2 atoms is preserved but with a reorientation of the zigzag chains along the a axis. At lower temperatures, the CO3 phase arises from a new direction change of irrep X3−, leading to the simultaneous condensation of new secondary modes. Its symmetry is Pn21m, and the Mn1 site is now split, which leads to a total of four nonequivalent Mn sites in the unit cell. In this phase, more tilts of the MnO6 octahedra are allowed following the a−b−c+ tilt schema.

We here report on the study of the crystallographic and magnetic properties of layered perovskites Ca3Mn2-xRuxO7 (x≤0.9). We observe a solid solution between Mn and Ru atoms in the whole series and all samples present the same orthorhombic structure independently of the Ru content. Different magnetic structures, depending on the Ru content in the sample, have been determined using neutron powder diffraction. For low Ru doping (x≤0.1), there is a dominant G-type antiferromagnetic ordering in the perovskite bilayers but, differently from undoped Ca3Mn2O7, the magnetic moments are located on the ab plane. For higher Ru concentration (x≥0.3), the predominant G-type ordering is preserved along the y axis while an A-type component is developed along the x axis and its intensity increases as Ru content does. This component is characterized by a ferromagnetic ordering in the a direction of one of the Mn(Ru)O6 layers, coupled antiferromagnetically with the neighbor Mn(Ru)O6 layer within the same bilayer. The study of the macroscopic magnetic properties shows that ferromagneticlike correlations are enhanced with increasing Ru content as deduced from the shift to higher temperature of the onset of the magnetic transition temperature. The magnetic transitions take place in two steps. At higher temperatures (140-200 K), short-range magnetic correlations are established. Tiny spontaneous magnetization is observed in the hysteresis loops with small coercive field. At TN≈115-125K, long-range antiferromagnetic ordering is developed. The ferromagnetic component remains with a strong increase of coercivity. We discuss in the paper the possible origins of this ferromagnetic contribution.

The magnetoelectric phase diagram of A-site ordered REBaMn2O6 and A-site disordered RE0.5Ba0.5MnO3 perovskites studied in this investigation. Comparison between the X-ray diffraction patterns of the ordered REBaMn2O6 and disordered RE0.5Ba0.5MnO3 (RE: Sm and Y) samples at room temperature. Details about the experimental procedure to obtain k range extended EXAFS free of Nd and Sm self-absorption. Comparison of XANES spectra for the ordered YBaMn2O6 sample at 80 K recorded by Mn Kbeta-detected HERFD and conventional transmission modes.

We have investigated the temperature dependence of the Mn local structure in A-site ordered RBaMn2O6 (R=La, Nd, Sm, and Y) perovskites, in parallel with their disordered counterparts, R0.5Ba0.5MnO3, by means of x-ray emission (XES) and x-ray absorption spectroscopy (XAS) The end member LaBaMn2O6 shows a nearly regular MnO6 octahedron independent of temperature. With decreasing the R ionic radius in the ordered samples, the XAS results indicate that a local distortion develops in the MnO6 octahedron at the low-temperature charge-localized and polar phases. For NdBaMn2O6, this local distortion is tiny, indicating the absence of charge segregation at the Mn site. This is followed by a bigger local distortion anticipated for SmBaMn2O6 in its respective charge-localized and polar phase and finally, the biggest local distortion for the smallest A-site cation ordered compound, YBaMn2O6, for which it even persists above the polar charge-localization transition temperatures. The high-resolution XAS spectra confirm the presence of charge segregation between two nonequivalent Mn sites in the low-temperature polar phase of Sm and Y ordered samples. Thus, our XAS study suggests a displacive mechanism for the charge-localization and polar transitions in the Nd and Sm ordered samples while a combination of displacive and order-disorder contributions is revealed for YBaMn2O6. Besides, calorimetric measurements confirm the combination of the two mechanisms, order-disorder and displacive, for the ordered Sm and Y compounds. On the other hand, the A-site disordered R0.5Ba0.5MnO3 samples with R cations smaller than Nd present a significant static (temperature-independent) local disorder, which explains why polar charge-localization transitions are not developed in these samples. Finally, we correlate our results about the Mn local structure and character of the transitions with the macroscopic magnetic and electric behavior of both A-site ordered and disordered compounds.

Details about Rietveld fits of NPD patterns at 295 and 10 K, temperature dependence of the refined lattice parameters, magnetization curve for the undoped compound (Ca3Mn2O7), Curie-Weiss fits of M(T) curves, and schematic representations of different types of magnetic orderings ascribed either to GM- or Y-points.

In Sr2NiIrO6 long-range Ir-Ir antiferromagnetic exchange interactions have been reported to overcome the ferromagnetic Ni-Ir interactions hampering the otherwise expected ferromagnetic behavior. Prompted by this, a combination of x-ray absorption spectroscopy and x-ray diffraction at high pressure is used here to investigate the interplay between the magnetic structure of the Ir sublattice and lattice degrees of freedom. The compression of Sr2NiIrO6 drives an unexpected nonmonotonic change of the x-ray magnetic circular dichroism (XMCD) spectra: The intensity first decreases in the 0- to 18-GPa range, then shows an increase in the 18- to 30-GPa range and again decreases for higher pressures. The XMCD intensity, a measure of the net magnetization in the Ir sublattice, however, is found to remain very low in the whole pressure range so the observed changes do not correspond with a transition from antiferromagnetic to ferromagnetic or ferrimagnetic order. The evolution of the XMCD is better explained in terms of a weakening/strengthening of the long-range antiferromagnetic (AFM) Ir-Ir interaction between ferromagnetic planes associated with the reduction of the lattice parameters. In particular, a correlation can be established between the evolution of the b/a ratio and the weakening/strengthening of the AFM interaction.

A combination of x-ray-diffraction, x-ray absorption, x-ray magnetic circular dichroism, macroscopic magnetization, and muon-spin relaxation measurements is used to investigate the interplay between structure (both crystallography and microstructure) and magnetism in Sr2YIrO6 as a function of both chemical (Ca-doping) and physical (hydrostatic) pressure. X-ray absorption spectroscopy clearly shows that the physical pressure is more effective in modifying the structure. On the other hand, the dichroic measurements evidence a constant magnetic signal with physical pressure and strong differences with Ca-doping. Muon-spin relaxation reveals the presence of magnetic order, even when this is hidden in the magnetization data. From the combined analysis, the magnetic results are explained in terms of the presence of Ir6+ or Ir4+ ions in magnetic clusters, most likely located at and triggered by antisite disorder. The measurements under high physical pressure indicate that the magnetic state is independent of the crystallographic details such as Ir-O distances and Ir-O-Y angles.

The atomic environments involved in the magnetostriction effect in CoFe2O4 and La0.5Sr0.5CoO3 polycrystalline samples have been identified by differential extended x-ray absorption fine structure (DiffEXAFS) spectroscopy. We demonstrate that cobalt atoms at octahedral sites are responsible for their magnetostriction. The analysis of DiffEXAFS data indicates that the local-site magnetostrictive strains of Co atoms are reversed in these two oxides, in agreement with the macroscopic magnetostriction. For the CoFe2O4 spinel, a large negative strain along the (100) direction has been determined for the CoO6 octahedron causing a tetragonal contraction in contrast with the La0.5Sr0.5CoO3 perovskite, where a positive moderate strain along the (100) direction was found resulting in a tetragonal expansion. The different local-site magnetostriction is understood in terms of the different valence and spin state of the Co atoms for the two oxides. The macroscopic magnetostriction would be explained then by the relative change in volume, either contraction in CoFe2O4 or expansion in La0.5Sr0.5CoO3, when the tetragonal axis of the Co site is reoriented under an externally applied magnetic field.

Magnetoelectric materials attract a great interest due to their wide applications in spintronics. Most of them are based on transition metal oxides with polar structures showing magnetic frustration. This is the case for CaBaCo4O7, which is ferrimagnetic below Tc=64K and also shows a linear magnetoelectric coupling below this temperature. Ab-initio calculations proved that the material is pyroelectric, and the large pyroelectric currents observed were ascribed to exchange-striction effects. It adopts an orthorhombic unit cell (space group Pbn21). The crystallographic structure of CaBaCo4O7 consists of a stacking of alternating triangular (T) and kagomé (K) layers of CoO4 tetrahedra along the c axis. There are four different crystallographic sites for Co: Co1 stays at the T layer, while Co2, Co3, and Co4 are in the K layers and ferrimagnetism appears because the magnetic moments at the Co1 and Co4 sites are larger than those at the Co2 and Co3 sites. The stoichiometric formula corresponds to CaBaCo22+Co23+O7 and therefore the nominal oxidation state for Co is 2.5+. Furthermore, the substitution of Co with another transition metal may be useful to increase the transition temperature and magnetic coupling. To fully characterize the tetrahedral distortion on CaBaCo4O7 and the effects of doping with Fe and Zn on Co sublattice, we performed XAS measurements as a function of temperature on Co, Fe and Zn K edges at BM23 at the ESRF (Grenoble, France). CaBaCo4-xFexO7 (x=0.5, 1, 1.5, 2) and CaBaCo4-x-yFexZnyO7 (x=0, 1, 2 and y=1) samples were synthesized by solid state chemistry reactions and the resulting powders were pressed into pellets mixed with cellulose for optimized XAS transmission measurements. From XANES, we conclude that a mixed-valence oxidation state is found for Co atom in all the studied CaBaCo4-xMxO7 (M=Fe or Zn) samples, with a combination of Co2+ and Co3+ in their formal ionic species. Fe and Zn are incorporated as Fe3+ and Zn2+ in the whole series. The ana

The effect of substituting iron and zinc for cobalt in CaBaCo4O7 was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo4O7 decreases with increasing the content of either Fe or Zn. The samples CaBaCo3ZnO7 and CaBaCo4–xFexO7 with x ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo4–xFexO7 (0.5 ≤ x ≤ 2) samples are antiferromagnetically ordered following a new propagation vector k = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo3ZnO7 sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe3+ or Zn2+ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co2+/Co3+ valence state or, alternatively, Co2+ and Co3+ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO4 tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo4O7, revealing the occurrence of local magneto-elastic coupling.

The structural properties and the magnetic ground state of Sr2/3Ln1/3FeO3 (Ln = La, Pr, Nd) samples were studied by means of synchrotron x-ray powder diffraction, neutron powder diffraction, and Mössbauer spectroscopy. All samples exhibit a metal-insulator-like transition coupled to a magnetic arrangement at a critical temperature, TMI. The diffraction techniques reveal strong structural changes at TMI that lead to new cells with reduced symmetry at low temperature. The new symmetry of the low-temperature phase has been determined for all compounds. The space groups are P¯3c1 for La-based compound and A2/n for the rest of samples. The high-resolution x-ray patterns detected superstructure peaks that can be accounted for by a small charge disproportionation between two nonequivalent Fe sites in the low-temperature phase explained in terms of a charge density wave that propagates along one of the body diagonals of the primitive cubic cell of these compounds. Our results clearly reveal that a full charge disproportionation of Fe4+ into Fe3+ and Fe5+ is not produced. We have determined the magnetic ordering of these samples exhibiting an antiferromagnetic structure with a sixfold periodicity with respect to the primitive cubic structure. The magnetic group accounting for the magnetic arrangements was obtained by a symmetry analysis and it is C2/c (15.85) for all samples but with different unit cell depending on the type of FeO6 tilts. The collinear ordering of Fe moments is established perpendicular to the charge density wave (along the body diagonal of the primitive cubic cell) and also perpendicular to the unique monoclinic axis.

CVD-grown multiwalled carbon nanotubes were purified by applying four different treatments with increasing oxidation severity. The growing severity of the treatment results in progressive oxygen functionalization of the surface along with introduction of an increasing quantity of defects on the carbon nanotube walls. Iridium-N-heterocyclic carbene complexes were covalently anchored to those oxidized surfaces through their surface carboxylic acids via acetyl linkers. The carbon nanotube-based iridium-NHC hybrid materials developed are active in the hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source but with rather different activity. The catalytic activity of the hybrid catalysts is strongly influenced by the type and amount of oxygenated functionalization resulting from the treatment applied to the support, being the most active and the most oxidized material.

Hybrid catalysts based on Iridium N-heterocyclic carbenes anchored to carbon nanotubes (CNT) have been studied by XAFS spectroscopy. Oxidation of CNT yields a large amount of functional groups, mainly hydroxyl groups at the walls and carboxylic groups at the tips, defects and edges. Different kinds of esterification reactions were performed to functionalize oxidized CNT with imidazolium salts. Then, the resulting products were reacted with an Ir organometallic compound to form hybrid catalysts efficient in hydrogen transfer processes. XANES spectroscopy agree with the presence of Ir(I) in these catalysts and the EXAFS spectra detected differences in the local structure of Ir atoms between the initial Ir organometallic compound and the Ir complexes anchored to the CNT. Our results confirm that the halide atom, present in the Ir precursor, was replaced by oxygen from -OH groups at the CNT wall in the first coordination shell of Ir. The lability of this group accounts for the good recyclability and the good efficiency shown by these hybrid catalysts.

A proper design of heterogeneous molecular catalysts supported on carbon materials requires a systematic study of >metal-carbon support interactions> and their influence on catalytic activity. In this study, hybrid materials containing covalently anchored iridium N-heterocyclic carbene (NHC) organometallic complexes have been successfully prepared from oxidized and partially reduced carbon nanotubes (CNTs). The preparation method for these supported materials relies on the selective functionalization of the superficial hydroxylic groups using the imidazolium salt, 1-(3-hydroxypropyl)-3-methyl-1H-imidazol-3-ium chloride. The hydrogen transfer catalysis activity of these nanotube-based hybrid catalysts was tested by the reduction of cyclohexanone to cyclohexanol with 2-propanol, and the results of the tests were compared with those obtained using similar hybrid graphene-based catalysts. While EXAFS analysis revealed a common first coordination shell of the iridium atom for all the hybrid materials examined, independently of whether they were either supported on carbon nanotubes or graphene materials, catalytic activity in all the reduced materials was significantly superior. Moreover, catalytic systems based on reduced CNTs exhibited a better performance than those based on reduced graphene materials. Both these facts suggest there is a positive correlation between hydrogen transfer catalytic activity, reconstruction of the aromatic carbon structure and the smaller amount of oxygen functional groups.

We have investigated the role of Mn3+ Jahn-Teller distortion in driving ferromagnetism in the LaMn1-xScxO3 series. The replacement of Mn by Sc in LaMnO3 decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the LaScO3 sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in LaMnO3 into a nearly regular one in LaScO3. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc K edges reveal the stability of both Jahn-Teller distorted MnO6 octahedron and nearly regular ScO6 octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted MnO6 octahedra spatially ordered as in LaMnO3 and nearly regular ScO6 octahedra. This result contrasts with the pseudocubic phase and the appearance of regular MnO6 octahedra in LaMn1-xGaxO3 for x>0.5. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the Mn3+ atom is allocated. Besides, a ferromagnetic ground state is observed for x>0.5 in both series independently of the presence (or not) of Jahn-Teller distortions around Mn3+, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in LaMn1-xBxO3 (B=Sc or Ga).

We report high pressure angle-dispersive x-ray diffraction measurements on CoxFe3−xO4 (x=1, 1.5, 1.75) spinels at room temperature up to 34 GPa. The three samples show a similar structural phase transformation from the cubic spinel structure to an analogous post-spinel phase at around 20 GPa. Spinel and post-spinel phases coexist in a wide pressure range (~20–25 GPa) and the transformation is irreversible. The equation of state of the three cubic spinel ferrites was determined and our results agree with the data obtained in related oxide spinels showing the role of the pressure-transmitting medium for the accurate determination of the equation of state. Measurements releasing pressure revealed that the post-spinel phase is stable down to 4 GPa when it decomposes yielding a new phase with poor crystallinity. Later compression does not recover either the spinel or the post-spinel phases. This phase transformation induced by pressure explains the irreversible lost of the ferrimagnetic behavior reported in these spinels.

We present here an x-ray absorption study of the RFe2O4 series (LuFe2O4, YFe2O4, YbFe2O4, and LuCoFeO4) at the Fe K-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal c axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe2+ and Fe3+ does not fit to the experimental XANES of either LuFe2O4, YFe2O4, or YbFe2O4, discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 ± 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions. © 2014 American Physical Society.