A complex rare-earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high-resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-ray diffraction experiments were confirmed for Ce10W22O81. A C2/c monoclinic superstructure, with cell parameters a = 7.8, b = 36.1, c = 22.2 Å and β = 100.2°, was shown and attributed to a partial oxidation of Ce(3+) leading to interstitial oxygen ions.
Hexagonal ferrites represent an extensive family of mixed-layer magnetic materials with periods up to 1500 A along the stacking direction, probably constituting the largest unit cells in the inorganic realm. The (TS)(n)T subfamily includes P3m1 and R3m structures that can be derived from Y ferrite Ba(2)M(2)Fe(12)O(22) (M = Zn, Fe, Co, Mg, Mn) by introducing stacking faults. A unified (3 + 1)-dimensional superspace model is proposed for all members of the (TS)(n)T family. The model belongs to the superspace group X3m1(00gamma) with X = {(1/3, 2/3, 0, 1/3), (2/3, 1/3, 0, 2/3)}, has a unit cell of the basic structure with a = 5.88, c = 4.84 A and modulation vector q = 4n+3/9n+6 c*, where n is rational for periodic structures and irrational for the aperiodic ones. The model was tested on calculated data of one of the principal members of the (TS)(n)T family, the Y ferrite (n = infinity). The fit obtained with the superspace model was excellent. The model allowed a reduction of refinable parameters by 19% with respect to the ordinary refinement without a significant increase of the refinement R values.
Crystals of the esterase EstB were obtained at 277 K with the hanging-drop technique from polyethylene glycol 4000 solutions containing 2-propanol at pH 7.5. The crystals belong to the trigonal space group P3(1)21 (or P3(2)21) with cell dimensions a = b = 82.9 and c = 193.4 A (at 100 K). The crystals diffract beyond a resolution of 2.0 A.
X-ray diffraction data to 1.60 angstrom resolution have been collected from monoclinic crystals of papain. The monoclinic model was derived from the orthorhombic one by molecular replacement, X-ray restrained molecular-dynamics simulation and least-squares refinement. Refinement against 1.60 angstrom data produced a model with reasonable sterochemistry and an R factor of 16.0%. The X-ray structures of orthorhombic and monoclinic papain are compared. The two structures are similar, the r.m.s. deviation between the two structures is 1.049 angstrom (mean difference 0.531 angstrom). The monoclinic model is shown to shift considerably (r.m.s. 3.083 angstrom) during refinement which indicates that bias due to the starting model may reasonably be expected to be low; in addition, solvent structure is independently determined. Tightly bound solvent occupies the same position in both structures and weakly bound solvent structures (high temperature factors) are different. Differences in protein structure are attributable to different crystal contacts, different covalent modification of the active-site cysteine, or different interpretation of weak density. The temperature factors for both structures show similar trends.
The defect crystal structures of eta-, gamma- and theta-alumina obtained from dehydroxylation of well crystallized bayerite and boehmite have been derived from the analysis of their X-ray powder diffraction patterns and from the Rietveld refinement of their neutron powder diffraction patterns. Profile analysis of the various reflection zones in these defect spinel structures shows different coherent domain sizes which can be associated with the tetrahedral and octahedral aluminium and the oxygen sublattices. These observations have been used to define the nature of the crystal structures, and to give insight into the transformation mechanisms. The very large surface energies of these phases are evident in the observation of a nearly three-coordinated surface Al atom in the eta phase, and are the reason for the stability of the defect spinel structures of the transition aluminas. The reduction of surface area and ordering of the tetrahedral Al sublattice which occurs on heating causes the spinel framework to collapse so that the structure, which exhibits tetragonal character at the early stage of the transition, settles into monoclinic theta-alumina displacively at the later stage, and eventually transforms to hexagonal corundum reconstructively. Thus theta-alumina should be considered the ultimate rather than the intermediate structural form into which the transition aluminas could evolve on the way to corundum. The overall crystal structure of the transition aluminas should therefore be viewed intrinsically as spinel deformed rather than as tetragonally deformed. Crystal data at room temperature: eta-alumina, cubic, Fd3m, a = 7.914 (2) angstrom, R(B) = 6.24, R(p) = 6.50, R(w) = 8.43%; gamma-alumina, cubic, Fd3m, a = 7.911 (2) angstrom, R(B) = 10.53, R(p) = 7.61, R(w) = 10.25%; theta-alumina, monoclinic, C2/m, a = 11.854 (5), b = 2.904 (1), c = 5.622 angstrom, beta = 103.83 (7)-degrees, R(B) = 15.02, R(p) = 9.37, R(w) = 11.93%.
