Local and average structures and magnetic properties of Sr2FeMnO(5+y), y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the effect of the A-site cation.

Sr(2)FeMnO(5+y) was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm ̅3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a(0) = 3.89328(1) Å for argon (y = 0.0) and a(0) = 3.83075(3) Å for air (y = 0.5). In contrast, Ca(2)FeMnO(5) retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr(2)FeMnO(5.0) oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr(2)FeMnO(5.0)(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 Å. Mössbauer spectroscopy results indicate more than one Fe site for Sr(2)FeMnO(5+y)(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This is confirmed by an L-edge XANES study. Mn is almost entirely in the 3+ state for Sr(2)FeMnO(5.0)(Ar), whereas Mn(4+) is predominantly present for Sr(2)FeMnO(5.5)(air). Magnetic susceptibility data show zero-field-cooled/field-cooled (ZFC/FC) divergences near 50 K for the Ar sample and 25 K for the air sample, whereas Ca(2)FeMnO(5) is long-range G-type antiferromagnetically ordered at 407(2) K. Hyperfine magnetic splitting, observed in temperature-dependent Mössbauer measurements, indicates short-range magnetic correlations that persist up to 150 K for Sr(2)FeMnO(5.0)(Ar) and 100 K for Sr(2)FeMnO(5.5)(air), well above the ZFC/FC divergence temperatures. Neutron diffraction data confirm the absence of long-range magnetic ordering at room temperature and 4 K for Sr(2)FeMnO(5.0)(Ar) but indicate the presence of domains with short-range G-type order at 4 K with an average dimension of ∼50 Å (y = 0); thus, this material is actually a superparamagnet rather than a true spin glass. In sharp contrast, corresponding data for Sr(2)FeMnO(5.5)(air) show mainly a very weak magnetic Bragg peak, indicating that ∼4% of the sample has G-type antiferromagnetic ordering at 4 K.

PDFfit2 and PDFgui: computer programs for studying nanostructure in crystals.

PDFfit2 is a program as well as a library for real-space refinement of crystal structures. It is capable of fitting a theoretical three-dimensional (3D) structure to atomic pair distribution function data and is ideal for nanoscale investigations. The fit system accounts for lattice constants, atomic positions and anisotropic atomic displacement parameters, correlated atomic motion, and experimental factors that may affect the data. The atomic positions and thermal coefficients can be constrained to follow the symmetry requirements of an arbitrary space group. The PDFfit2 engine is written in C++ and is accessible via Python, allowing it to inter-operate with other Python programs. PDFgui is a graphical interface built on the PDFfit2 engine. PDFgui organizes fits and simplifies many data analysis tasks, such as configuring and plotting multiple fits. PDFfit2 and PDFgui are freely available via the Internet.

A search for disorder in the spin glass double perovskites Sr(2)CaReO(6) and Sr(2)MgReO(6) using neutron diffraction and neutron pair distribution function analysis.

The geometrically frustrated, B-site ordered, S = 1/2, double perovskites Sr(2)CaReO(6) and Sr(2)MgReO(6), which show spin frozen magnetic ground states, have been investigated using neutron powder diffraction (ND) and neutron pair distribution function (NPDF) analysis in a search for evidence for atomic positional disorder. For both materials, data were taken above and below the spin freezing temperatures of ∼ 14 K and ∼ 45 K for the CaRe and MgRe phases, respectively. In both cases the fully B-site ordered model was in excellent agreement with the data, both ND and NPDF, at all temperatures studied. Thus, the structure of these materials, from the average and the local perspectives, is very well described by the fully B-site ordered model, which raises questions concerning the origin of the spin glass ground state. These results are compared with those for the spin glass pyrochlore Y(2)Mo(2)O(7) and other B-site ordered double perovskites.

Uranium surroundings in borosilicate glass from neutron and x-ray diffraction and RMC modelling.

Neutron and high-energy x-ray diffraction measurements have been performed on multi-component 55SiO(2)·10B(2)O(3)·25Na(2)O·5BaO·ZrO(2) borosilicate host glass loaded with 30 wt% UO(3). Both the traditional Fourier transformation technique and the reverse Monte Carlo simulation of the experimental data have been applied to get structural information. It was established that the basic network structure consists of tetrahedral SiO(4) units and of mixed tetrahedral BO(4) and trigonal BO(3) units, similar to the corresponding host glass. Slight changes have been observed in the oxygen surroundings of the Na and Zr modifier cations; both the Na-O and Zr-O distances decrease and a more compact short-range structure has been obtained compared to the host glass. For the U-O correlations two distinct peaks were resolved at 1.84 and 2.24 Å, and for higher distances intermediate-range correlations were observed. Significant correlations have been revealed between U and the network former Si and B atoms. Uranium ions take part in the network forming, which may be the reason for the observed good glassy stability and hydrolytic properties.

Local structure of La1-xSrxCoO3 determined from EXAFS and neutron pair distribution function studies.

The combined local structure techniques, extended x-ray absorption fine structure and neutron pair distribution function analysis, have been used for temperatures 4< or =T< or =330 K to rule out a large Jahn-Teller (JT) distortion of the Co-O bond in La1-xSrxCoO3 for a significant fraction of Co sites (x< or =0.35), indicating few, if any, JT-active, singly occupied e_{g} Co sites exist.

An experimental and theoretical study of the variation of 4f hybridization across the La1-xCexIn3 series.

Crystal structures of a series of La(1-x)Ce(x)In(3) (x = 0.02, 0.2, 0.5, or 0.8) intermetallic compounds have been investigated by both neutron and X-ray diffraction, and their physical properties have been characterized by magnetic susceptibility and specific heat measurements. Our results emphasize atypical atomic displacement parameters (ADP) for the In and the rare-earth sites. Depending on the x value, the In ADP presents either an "ellipsoidal" elongation (La-rich compounds) or a "butterfly-like" distortion (Ce-rich compounds). These deformations have been understood by theoretical techniques based on the band theory and are the result of hybridization between conduction electrons and 4f-electrons.

Understanding the insulating phase in colossal magnetoresistance manganites: shortening of the Jahn-Teller long-bond across the phase diagram of La1-xCaxMnO3.

The detailed evolution of the magnitude of the local Jahn-Teller (JT) distortion in La(1-x)Ca(x)MnO3 is obtained across the phase diagram for 0< or =x< or =0.5 from high-quality neutron diffraction data using the atomic pair distribution function method. A local JT distortion is observed in the insulating phase for all Ca concentrations studied. However, in contrast with earlier local structure studies, its magnitude is not constant, but decreases continuously with increasing Ca content. This observation is at odds with a simple small-polaron picture for the insulating state.

Orbital correlations in the pseudocubic O and rhombohedral R phases of LaMnO3.

The local and intermediate structure of stoichiometric LaMnO3 has been studied in the pseudocubic and rhombohedral phases at high temperatures (300-1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis was carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high-temperature orthorhombic (O) and rhombohedral (R) phases, the MnO6 octahedra are still fully distorted locally. More importantly, the intermediate structure suggests the presence of local ordered clusters of diameter approximately 16 A ( approximately 4 MnO6 octahedra) implying strong nearest-neighbor JT antiferrodistortive coupling. These clusters persist well above the JT transition temperature even into the high-temperature R phase.

Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis.

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

Role of the lattice in the gamma-->alpha phase transition of Ce: a high-pressure neutron and x-ray diffraction study.

The temperature and pressure dependence of the thermal displacements and lattice parameters were obtained across the gamma-->alpha phase transition of Ce using high-pressure, high-resolution neutron and synchrotron x-ray powder diffraction. The estimated vibrational entropy change per atom in the gamma-->alpha phase transition, DeltaS(gamma-alpha)(vib) approximately (0.75+/-0.15)k(B), is about half of the total entropy change. The bulk modulus follows a power-law pressure dependence that is well described using the framework of electron-phonon coupling. These results clearly demonstrate the importance of lattice vibrations, in addition to the spin and charge degrees of freedom, for a complete description of the gamma-->alpha phase transition in elemental Ce.