Th. Proffen

PDFFIT, a program for full profile structural refinement of the atomic pair distribution function

The program PDFFIT is designed for the full profile structural refinement of the atomic pair distribution function (PDF). In contrast to conventional structure refinement based on Bragg intensities, the PDF probes the local structure of the studied material. The program presented here allows the refinement of atomic positions, anisotropic thermal parameters and site occupancies as well as lattice parameters and experimental factors. By selecting individual atom types one can calculate partial and differential PDFs in addition to the total PDF. Furthermore one can refine multiple data sets and/or multiple structural phases. The program is controlled by a command language which includes a Fortran style interpreter supporting loops and conditional statements. This command language is also used to define the relation between refinement parameters and structural or experimental information, allowing virtually any constraint to be implemented in the model. PDFFIT is written in Fortran-77, and the source code and documentation are available via the World Wide Web.

DISCUS: a program for diffuse scattering and defect-structure simulation

The program DISCUS is a versatile tool for the analysis of diffuse scattering and for defect structure simulations. The model structure can be created from an asymmetric unit of a unit cell or a complete structure can be read from a file. A Fortran77 style interpreter that includes IF statements and various loops combined with predefined defect types like thermal displacements, waves and microdomains allows one to create all sorts of defect structures. The Fourier-transform segment of the program allows one to calculate neutron as well as X-ray intensities including isotropic temperature factors and anomalous scattering. The calculation of the inverse and difference Fourier transform as well as the Patterson function is also implemented. A model structure can be `fitted to observed diffuse scattering data by reverse Monte Carlo (RMC) simulations. The RMC segment allows one to model displacive as well as occupational disorder. The program is completely written in Fortran77 and the source code is available via the World Wide Web.

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.

PDFgetN: a user-friendly program to extract the total scattering structure factor and the pair distribution function from neutron powder diffraction data

The total scattering structure factor, S(Q), and the atomic pairdistribution function (PDF) are usually obtained from powder diffraction data employing either neutrons or X-rays (Billinge & Thorpe, 1998). Many steps have to be performed to convert the measured diffraction data to S(Q) data, which are Fourier transformed to obtain the real-space PDF (Billinge & Thorpe, 1998). A weakness of current analysis programs is that the parameters used to obtain a particular S(Q) are not saved automatically, making it dif®cult to reproduce a particular S(Q) from raw data. Also, many existing neutron PDF data analysis programs do not propagate random errors. This is becoming more important as S(Q) and PDF data are increasingly being modeled using regression.

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.

PDFgetX: a program for obtaining the atomic pair distribution function from X-ray powder diffraction data

For many materials with disorder, the local structure information is important in understanding their physical properties. However, the conventional crystal structure re®nement by the Rietveld method only allows the determination of the long-range average structure. The pair distribution function (PDF), which can be obtained from powder diffraction data, allows one to extract both local and average structural information (Petkov et al., 1999; Billinge & Thorpe, 1998). We present here a program, PDFgetX, for obtaining the PDF from Xray powder diffraction data. A similar program to process neutron powder diffraction data has already been published (Peterson et al., 2000).

Evidence for charge localization in the ferromagnetic phase of La{sub 1-x}Ca{sub x}MnO{sub 3} from high real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of

Lightly stuffed pyrochlore structure of single-crystalline Yb2Ti2O7grown by the optical floating zone technique

{mathrm{La}}_{1ensuremath{-}x}{mathrm{Ca}}_{x}{mathrm{MnO}}_{3}

Measurement of the local Jahn-Teller distortion in LaMnO{sub 3.006}

(x=0.12,