A. Le Bail

Ab-initio structure determination of LiSbWO6 by X-ray powder diffraction

The crystal structure of LiSbWO6 is solved from X-ray powder diffraction data. The cell is orthorhombic (space group Pbcn, Z = 4) with a = 4.6664(1) A, b = 17.4435(5) A, c = 4.9941(2) A. Initial positional parameters for antimony and tungsten were obtained from direct methods applied on 307 reflexions which were derived from a modified Rietveld analysis working without structural model. The structure was refined using Rietveld profile refinement principles ; the final reliability factors (%) are Rx = 2.1, RWP = 6.5. The structure derives from PbO2-α by a tripling of the b axis of PbO2 with a full cationic order, unlike the columbite structure which is related to PbO2-α by a tripling of the a-axis.

Size-strain line-broadening analysis of the ceria round-robin sample

The results of both a line-broadening study on a ceria sample and a size–strain round robin on diffraction line-broadening methods, which was sponsored by the Commission on Powder Diffraction of the International Union of Crystallography, are presented. The sample was prepared by heating hydrated ceria at 923 K for 45 h. Another ceria sample was prepared to correct for the effects of instrumental broadening by annealing commercially obtained ceria at 1573 K for 3 h and slowly cooling it in the furnace. The diffraction measurements were carried out with two laboratory and two synchrotron X-ray sources, two constant-wavelength neutron and a time-of-flight (TOF) neutron source. Diffraction measurements were analyzed by three methods: the model assuming a lognormal size distribution of spherical crystallites, Warren–Averbach analysis and Rietveld refinement. The last two methods detected a relatively small strain in the sample, as opposed to the first method. Assuming a strain-free sample, the results from all three methods agree well. The average real crystallite size, on the assumption of a spherical crystallite shape, is 191 (5) A. The scatter of results given by different instruments is relatively small, although significantly larger than the estimated standard uncertainties. The Rietveld refinement results for this ceria sample indicate that the diffraction peaks can be successfully approximated with a pseudo-Voigt function. In a common approximation used in Rietveld refinement programs, this implies that the size-broadened profile cannot be approximated by a Lorentzian but by a full Voigt or pseudo-Voigt function. In the second part of this paper, the results of the round robin on the size–strain line-broadening analysis methods are presented, which was conducted through the participation of 18 groups from 12 countries. Participants have reported results obtained by analyzing data that were collected on the two ceria samples at seven instruments. The analysis of results received in terms of coherently diffracting, both volume-weighted and area-weighted apparent domain size are reported. Although there is a reasonable agreement, the reported results on the volume-weighted domain size show significantly higher scatter than those on the area-weighted domain size. This is most likely due to a significant number of results reporting a high value of strain. Most of those results were obtained by Rietveld refinement in which the Gaussian size parameter was not refined, thus erroneously assigning size-related broadening to other effects. A comparison of results with the average of the three-way comparative analysis from the first part shows a good agreement.

Monte Carlo indexing with McMaille

A Monte Carlo code for indexing powder diffraction patterns is presented. Cell parameters are generated randomly and tested against an idealized powder profile generated from the extracted d’s and I’s. Limits with this program in solving problems associated with zeropoint errors and impurity lines are examined. Most problems (V 3 , cell parameters 2 GHz processor); more time is needed for triclinic cases. Attempts are shown to be successful for the indexation of two-phase samples in simple cases (combining orthorhombic or higher symmetries).

Effects of high pressure treatment (100–200 MPa) at low temperature on turbot (Scophthalmus maximus) muscle

Turbot fillets were subjected to high pressure treatments at 100, 140, 180 and 200 MPa for 15 and 30 min at 4°C. The influence of such treatments on the lipid and protein stability and on color was studied. It appeared that color, protein stability and lipid oxidation phenomena were influenced both by the pressure level and pressure holding time. The oxidative stability of lipids was particularly affected from 180 MPa as measured by the thiobarbituric acid number. Differential scanning calorimetry showed a full denaturation of myosin at 200 MPa and the appearance of a new structure from the treatment at 100 MPa for 30 min.

A new study of the structure of LaNi5D6.7 using a modified Rietveld method for the refinement of neutron powder diffraction data

Abstract The Rietveld profile analysis method has been extended using a Fourier analysis of line profiles on the basis of the Warren-Averbach method for separating size and strain effects. Assuming a given size distribution (Cauchy) and an adjustable strain variation in space, this method allows the simultaneous determination of the structural parameters and the size and strain parameters of the sample. It has been used for analysing the neutron diffraction pattern of β-LaNi 5 deuteride which exhibits a strongly anisotropic broadening. The mean coherency domain was chosen as an ellipsoid of revolution along the c axis. The structure is described in the space group P 6 3 mc as well as the P 31 c space group with five types of interstitial sites (three in the hypothetical fully ordered LaNi 5 D 7 phase). The strain parameter, i.e. the distribution of atomic positions around their equilibrium positions, varies by a factor of 2.5 while the mean particle size changes from 80 A to more than 2500 A in [ hk 0] and [00 l ] directions respectively.

Modelling the silica glass structure by the Rietveld method

Abstract To refine amorphous structures like crystalline ones is impossible. This statement needs now some reconsideration in the case of silica glass. Starting with a microstrained crystalline model deriving from the α-carnegieite structure, atomic coordinates refinements by the Rietveld method prove to be possible. The credibility of the study is supported by the simultaneous fit of neutron and X-ray diffraction data. The agreement R χ factors are the best ever obtained with a small-size model built exclusively from [SiO 4 ] tetrahedra linked by corners. However it is concluded that ‘best’ remains insufficient.

Crystal structure of the metastable form of aluminum trifluoride β-AlF3 and the gallium and indium homologs

Abstract The crystal structure of the metastable phase β-AlF3, which is related to the hexagonal tungsten bronze structure, has been solved by X-ray powder and single-crystal diffraction methods. The crystal habit is pseudo-hexagonal with systematic twinning (rotation of 120° around the c axis), but the true symmetry is orthorhombic with space group Cmcm, Z = 12, a = 6.931(3), b = 12.002(6), c = 7.134(2), A (R = 0.044 and Rw = 0.051) from 929 independent reflections). The network is built from very regular AlF6 octahedra rotated by approximately 7.2° from the positions of the ideal HTB structure. A similar network, with the same propagation of the tilting, was observed in the compound (H2O)0.33FeF3 and in the metastable polymorphs of CrF3 and of VF3. Our reinvestigation of the structures of β-GaF3 and β-InF3 using powder data shows that they are isotypic with the aluminum compound, with a = 7.210(1), b = 12.398(2), c = 7.333(1) and a = 7.875(2), b = 13.499(4), c = 7.956(2), A, respectively.

Freezing and ice crystals formed in a cylindrical food model: part I. Freezing at atmospheric pressure

Cylindrical gelatin gels were frozen at atmospheric pressure with different operating conditions (air-blast freezing at different air temperatures and brine freezing). A method to calculate a local freezing rate was proposed to take into account the variation of freezing rate as a function of the radius. A linear evolution of the local freezing rate according to the radius was observed whatever the freezing process was. Frozen gels were freeze-dried and sliced perpendicularly to the heat flux. The ice crystal marks were measured according to the radial position with image analysis software. Each radial distribution of ice crystal size was characterised by the mean representative diameter. A linear regression permitted to link the ice crystal mean representative diameter to the radial position. On addition, the variation in the mean diameter with the local freezing rate was fitted by a power law.

Optimization of Gluten-Free Formulations for French-Style Breads

The formulation of gluten-free bread, which will be suitable for patients with coeliac disease, was optimized to provide bread similar to French bread. The effects of the presence of hydrocolloids and the substitution of the flour basis by flour or proteins from different sources were studied. The added ingredients were (1) hydrocolloids (carboxymethylcellulose [CMC], guar gum, hydroxypropylmethylcellulose [HPMC], and xanthan gum), and (2) substitutes (buckwheat flour, whole egg powder, and whey proteins). The bread quality parameters measured were specific volume, dry matter of bread, crust color, crumb hardness, and gas cell size distribution. Specific volume was increased by guar gum and HPMC. Breads with guar gum had color characteristics similar to French bread. Hardness decreased with the addition of hydrocolloids, especially HPMC and guar. Breads with guar gum had the most heterogeneous cell size distribution, and guar gum was therefore selected for further formulations. Bread prepared with buckwheat flour had improved quality: an increased specific volume, a softer texture, color characteristics, and gas-cell size distribution similar to French bread. Bread with 1.9% guar gum (w/w, total flour basis) and 5% buckwheat flour (of all flours and substitutes) mimicked French bread quality attributes.

Influence of the freezing condition on the baking performances of French frozen dough

Freezing affects the baking performance of frozen bread dough. The volume of the dough during proofing is related to two main factors. First, yeast is affected by freezing. A slow freezing rate is usually recommended in the literature so as to preserve its activity. Second, the gluten network is damaged during freezing. This latter phenomenon affects the ability of the dough to retain CO2 and thus minimizes the bread volume. The objective of this study was to evaluate the relative importance of these two phenomena. Sticks of French bread dough were frozen under selected conditions. A fermentometer was used to measure CO2 volume during proofing. The gluten network performance was evaluated from the dough volume variation during proofing and from the baked volume. The results showed that yeast activity was strongly related to the freezing rate and to the location inside the dough.