James R. Hester

Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic‐Transition Temperature

We report a novel oxyhydride SrCrO2H directly synthesized by a high-pressure high-temperature method. Powder neutron and synchrotron X-ray diffraction revealed that this compound adopts the ideal cubic perovskite structure (Pm3̄m) with O(2-)/H(-) disorder. Surprisingly, despite the non-bonding nature between Cr 3d t(2g) orbitals and the H 1s orbital, it exhibits G-type spin ordering at T(N)≈380 K, which is higher than that of RCrO3 (R=rare earth) and any chromium oxides. The enhanced T(N) in SrCrO2H with four Cr-O-Cr bonds in comparison with RCr(3+)O3 with six Cr-O-Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry.

Oxyhydrides of (Ca,Sr,Ba)TiO3 perovskite solid solutions.

The oxyhydride solid solutions (Ca,Sr)TiO(3-x)H(x) and (Sr,Ba)TiO(3-x)H(x) have been prepared by reducing the corresponding ATiO(3) oxides with calcium hydride. Under the reaction conditions examined, a hydride content of x = 0.1-0.3 was obtained for all compositions. Compared to our previous result with BaTiO(3-x)H(x), the larger particle size in this study (20-30 μm vs 170 nm) resulted in a somewhat lower hydride amount despite prolonged reaction times. We examined changes in cell volume, octahedral tilt angle, and site occupancy of different anion sites after conversion to oxyhydrides; it appears that these oxyhydrides fit the geometrical descriptions typical for regular ABO(3) perovskites quite well. The hydrogen release temperature, previously shown to be indicative of the hydride exchange temperature, however, does not scale linearly with the A-site composition, indicating a potential effect of chemical randomness.

Room-Temperature Polar Ferromagnet ScFeO3 Transformed from a High-Pressure Orthorhombic Perovskite Phase

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R3c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 μC/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe(3+) spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.

Detailed investigations of phase transitions and magnetic structure in Fe(III), Mn(II), Co(II) and Ni(II) 3,4,5-trihydroxybenzoate (gallate) dihydrates by neutron and X-ray diffraction.

The effect of cation valency on the complex structures of divalent and trivalent transition metal gallates has been examined using a combination of neutron and synchrotron X-ray powder diffraction, single-crystal X-ray diffraction and XANES spectroscopy. In the divalent frameworks, M(C(7)H(4)O(5))·2H(2)O (M = Mn, Co and Ni), it was found that charge balance was achieved via the presence of protons on the meta-hydroxyl groups. It was also established that these compounds undergo a discontinuous phase transition at lower temperatures, which is driven by the position of the extra-framework water molecules in these materials. By contrast, in the trivalent Fe gallate, Fe(C(7)H(3)O(5))·2H(2)O, it was found that the stronger bonding between the meta-hydroxy oxygen and the cations leads to a weakening of the bond between this oxygen and its proton. This is turn is thought to lead to stronger hydrogen bonding with the extra-framework water. The lattice water is disordered in the Fe(III) case, which prevents the phase transition found in the M(II) gallates. Refinement against the neutron diffraction patterns also revealed that the relatively mild microwave synthesis of gallate frameworks in D(2)O led to an extensive deuteration of the ortho-hydrogen sites on the aromatic ring, which may suggest a more versatile method of deuterating aromatic organics. The antiferromagnetic structure of Co gallate has also been determined.

Kinetic pathways of carbon nanotube nucleation from graphitic nanofragments

A detailed analysis of nanotube (NT) nucleation from graphitic nanofragments by thermal vibration is given, outlining the role of activation energy barriers which may be significantly decreased by the interaction with amorphous carbon, fullerene-like, and carbonized metal catalyst nanoparticles. This analysis predicts the variety of carbon nanotube chiralities observed experimentally. Heat dissipation by the inert gas and the cooling rate of the metal–carbon nanoparticle are suggested to play an important role in kinetic selection between (i) a carbon nanosheet wrapping around the metal nanoparticle, (ii) NT nucleation and growth on the nanoparticle surface by the so called “root mechanism,” and (iii) the metal nanoparticle surface being covered by an amorphous carbon layer.

Synthesis and Physical Properties of the New Oxybismuthides BaTi2Bi2O and (SrF)2Ti2Bi2O with a d^{1} Square Net

We have recently reported the \(d^{1}\) square-lattice compound BaTi2Sb2O, which shows superconductivity at \(T_{\text{c}} = 1.2\) K coexisting with a charge- or spin-density wave (CDW/SDW) state. Here, we successfully prepared two new oxybismuthides, BaTi2Bi2O and (SrF)2Ti2Bi2O, as the first Pn = Bi compounds in the \(A\)Ti2 Pn 2O family. The CDW/SDW state disappeared for both compounds, presumably owing to the considerable interaction between the Ti-\(3d\) and Bi-\(6s\) orbitals. The complete suppression of the CDW/SDW instability resulted in an enhanced \(T_{\text{c}}\) of 4.6 K for BaTi2Bi2O. However, (SrF)2Ti2Bi2O exhibits no superconductivity, suggesting the importance of the interlayer interaction for superconductivity.

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X-ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C(5)H(5))(2), are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C(5)H(5) rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT-like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS-based estimates of the bond distances have accuracies comparable with those obtained using single-crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.

A validating CIF parser: PyCIFRW

PyCIFRW is a general-purpose Python package providing a simple, powerful interface for working with CIF files. Objects and methods are available for reading, writing and manipulating CIF files and dictionaries. Comprehensive validation of CIF files and dictionaries against DDL1 or DDL2 dictionaries is also possible. PyCIFRW is easily included in larger projects and is portable across a large number of platforms. Although written in an interpreted language, parsing and validation times are acceptable for most applications.

Structural disorder along the lithium diffusion pathway in cubically stabilized lithium manganese spinel. I. Synchrotron X-ray studies

Abstract Electron density distribution, EXAFS, and transmission electron microscope studies on stoichiometric Li(Mg1/6Mn11/6)O4 have revealed that: (1) the structure is essentially of spinel-type with slight diffuse scattering, (2) the Li atoms are not all located at the ideal 8a site of the Fd 3 m spinel structure, but are partially tetrahedrally distributed along the 8a–16c tie line, (3) the O atoms also exhibit a statistical distribution about their ideal positions and (4) the Mn 3d electrons are squeezed out toward the open space of the coordinating octahedra with D3d distortion. The present results indicate the possible existence of many metastable Li positions in the structure, suggestive of complicated Li atom hopping routes in conjunction with a local distortion of neighboring atoms at least up to the second shell.

High-Temperature Structural Studies of SrPbO3 and BaPbO3

High-resolution X-ray powder diffraction studies have shown BaPbO3 to transform from an orthorhombic Imma structure at room temperature, through an intermediate I4/mcm phase, to a cubic Pm3–mstructure above 500°C. The Imma to I4/mcm transition is first order and the I4/mcm–Pm3–m transition for BaPbO3 is tricritical in nature. We find no evidence for any phase transitions in SrPbO3 up to 760°C, the structure remaining in Pnma from room temperature to 760°C.