The newly developed GSAS‐II software is a general purpose package for data reduction, structure solution and structure refinement that can be used with both single‐crystal and powder diffraction data from both neutron and X‐ray sources, including laboratory and synchrotron sources, collected on both two‐ and one‐dimensional detectors. It is intended that GSAS‐II will eventually replace both the GSAS and the EXPGUI packages, as well as many other utilities. GSAS‐II is open source and is written largely in object‐oriented Python but offers speeds comparable to compiled code because of its reliance on the Python NumPy and SciPy packages for computation. It runs on all common computer platforms and offers highly integrated graphics, both for a user interface and for interpretation of parameters. The package can be applied to all stages of crystallographic analysis for constant‐wavelength X‐ray and neutron data. Plans for considerable additional development are discussed.

A set of general guidelines for structure re®nement using the Rietveld (whole-pro®le) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld re®nement are discussed with a view to guiding newcomers in the ®eld. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features speci®c to data from neutron (both constant-wavelength and time-of-¯ight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) re®nement of pro®le parameters, (v) Fourier analysis with powder diffraction data, (vi) re®nement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.

A new dedicated high-resolution high-throughput powder diffraction beamline has been built, fully commissioned, and opened to general users at the Advanced Photon Source. The optical design and commissioning results are presented. Beamline performance was examined using a mixture of the NIST Si and Al(2)O(3) standard reference materials, as well as the LaB6 line-shape standard. Instrumental resolution as high as 1.7 x 10(-4) (DeltaQQ) was observed.

No abstract

A dedicated high-resolution high-throughput X-ray powder diffraction beamline has been constructed at the Advanced Photon Source (APS). In order to achieve the goals of both high resolution and high throughput in a powder instrument, a multi-analyzer detector system is required. The design and performance of the 12-analyzer detector system installed on the powder diffractometer at the 11-BM beamline of APS are presented.

The mathematical functions necessary for Rietveld refinement of time-of-flight neutron powder diffraction patterns from spallation sources are developed and a computer program for least-squares analysis is described. The results of Rietveld refinements of nickel and a low-carbon steel are described and discussed. The method fully exploits the high resolution (Ad/d ,--0"3---0.5%) available with powder diffractometers currently in operation on these sources and examples are given of precise determination of atom coordinates, thermal parameters, lattice parameters and the detection of small strains.

A generalized spherical‐harmonic description of the texture for polycrystalline materials has been implemented in a multiple‐phase/multiple‐data‐set Rietveld refinement code. It has been tested using two sets of neutron time‐of‐flight data taken from a standard calcite sample previously used for a round‐robin study [Wenk (1991). J. Appl. Cryst.24, 920–927] and has been shown to give similar texture results as those obtained from individual pole figures. Simultaneous refinement of the calcite crystal structure including anisotropic thermal parameters gives results essentially identical to a recent single‐crystal X‐ray study.

Precise single-crystal X-ray structure refinements of three hollandite-type minerals have allowed a detailed study of the hoUandite structure to be made. The minerals hollandite [(Bao.75Pbo.16Nao.loKo.04)(Mn,Fe, A1)s(O,OH)16], cryptomelane [(Ko.94Nao.25Sro.13-Bao.lo)(Mn,Fe,A1)s(O,OH)16], and priderite [(Ko.9o-Ba0.35) (Ti,Fe,Mg)sO16] were refined to residuals of R = 0.0165 (599 observations, 48 parameters), R = 0.0299 (623 observations, 53 parameters), and R = 0.0096 (316 observations, 29 parameters) respectively. The first two structures are monoclinic (I2/m) and priderite is tetragonal (I4/m). The symmetry of hollandite compounds depends on the ratio of the average ionic radius of the octahedral cations to that of the tunnel cations. Structures in which this ratio is >0.48 distort, reducing the tunnel volume, and thereby lowering the symmetry from tetragonal to monoclinic. The position occupied by a tunnel cation is determined primarily by the size of the cation. Relatively small cations, such as Ba 2+ in priderite and Pb 2+ in hollandite, displace from the special position, 2(a), to more stable sites that are at the sum of the ionic radii from the nearest O atoms. This study also indicates that the reduced form of Mn in hoUandite and cryptomelane is Mn3+; bond lengths calculated from the refinements suggest that Mn 3+ is more easily accommodated in the structures than the larger Mn 2+.