Abstract: By means of neutron diffraction the low-temperature crystal structure of NaBD4 has been determined. At 10 K the lattice parameters are a = 4.332(1) Å and c = 5.869(1) Å. Deuterium is found in a tetrahedral arrangement [sites (8g)] around B. The symmetry corresponds to space group P42/nmc. For room temperature the structure model for NaBD4 of Davis and Kennard with disordered deuterium distributed over two sites has been revised to space group Fm-3 m. Thus the 190 K phase transition known from specific heat measurements is of order-disorder type, caused by reorientations of BD4 tetrahedra.
Abstract: This work is a preliminary kinetic study of the crocidolite decomposition followed in situ at high temperature using real time conventional powder diffraction and DTA in the temperature range 720-795 °C. The data analysis using the Avrami models indicates that the rate limiting step of the reaction is monodimensional ion diffusion (n=0.5) with an activation energy of 129 (10)kcal/mole.
Abstract: The HFC-134/NaY binding has been monitored by temperature dependent X-ray powder diffraction. Diffraction patterns were obtained as a function of different loading levels to investigate the rearrangement of the cations and of the HFC-134 molecules, in the temperature range 100-230 K. In situ real-time data were collected using a Translating Imaging Plate System (TIPS) at the NSLS (USA). Rietveld structure refinements were performed for the HFC-134 (32 molecules per unit cell [m/uc = molecules per unit cell] or 4 molecules per supercage [m/sc = molecules per
supercage]) using data collected from 100 to 230 K. A phase transition is observed at about 180 K (A to B phase transition) where a disordering of the HFC-134 molecule takes place and modification of its surrounding due to Na+ migrations is observed. These results are preliminary and the experiment with the 32 m/uc should be repeated in order to assess whether the observed effect is reproducible.
Abstract: Lamellar calciumaluminatehydrates are products of the hydration of ordinary Portland
cement and play an important role in its hardening process. Due to the fixation ability of inorganic ions the technical features of this lamellar phases are important as reservoir minerals for pollutants (storage minerals). The lamellar calciumaluminatechromatehydrate Monochromate with the idealized formula C3A.Al2O3.CaCrO4.Ca(OH)2.nH2O has been investigated using a Philips
Diffraction System X'Pert with the X'Celerator detector to measure the kinetics of the changes in interlayer dimensions in the lamellar calciumaluminatechromatehydrate at room temperature in air. A certain amount of water is weakly fixed in the interlayer depending on the interlayer anion, temperature and relative humidity of the atmosphere. Processes like ion exchange or water-loss in the interlayer cause a variation of the crystal dimension in c-direction. These "processes" can be very fast, therefore an ultra fast data collection and measurement times less than 2 minutes from 2° 2Θ to 70° 2Θ were essential.
Abstract: The evolution of the crystal structures as a function of temperature of the new solid-
solution KTiOPO4 (KTP) isomorphs, namely K1-xNbxTi1-xOPO4 and K1-xSbxTi1-xOPO4 have been studied by neutron powder diffraction below and above the ferroelectric phase transition temperatures. On the basis of the experimental data obtained the cation distribution, among the sublattices of this structure, and the distortion of the framework at the phase transition, have been discussed. The possible correlation between the non-linear optical properties and the dominant structural features tailored by compositional variation has also been studied.
Abstract: Synthetic manganeous Layered Double Hydroxides of the Hydrotalcite- and Shigaite-
type were prepared by the coprecipitation method. The thermal behaviour of two compounds, [Mn6Al3(OH)18] [(SO4)2Na·12H2O] and [Mn6Al3(OH)18] [Cl·4H2O] was investigated by several experimental techniques. Thermal analysis and in-situ X-ray diffractometry show several dehydration stages of the synthesis products in the temperature range of 25 < T < 170°C followed by decomposition at temperatures above 200°C.
Abstract: Abstract. Lithium-manganese spinels obtained by sol-gel method were studied by XRD, impedance spectroscopy, electric conductivity and thermogravimetric analysis (TGA). Heating of low-temperature spinels up to 630° C leads to the formation of spinels with lattice constants close to stoichiometric high-temperature spinels, which exhibit no phase transition around room temperature and have good electric conductivity.
Abstract: The calcination of the zeolite ZSM-5 was studied by in situ X-ray synchrotron powder diffraction. Two samples were studied: One conventionally synthesized (CONV) and one synthesized in a charcoal matrix according to the confined space synthesis method (CSS). The powder diffraction data were collected between 375 K and 975 K in steps of approximately 8 K/5 min. and evaluated by Rietveld refinements. Both sets of powder diffraction patterns displayed the
characteristic intensity changes for calcination, but neither of them showed the expected orthorhombic to monoclinic phase transition. Refinements of the extra-framework occupancy showed that the calcination starts at about 475 K and is essentially completed at 675 K for CSS, while the CONV calcination starts at 525 K and ends at 650 K. The refinement results agree very well with the TGA results. The carbon matrix of the CSS sample is combusted between 700 and 850 K. The unit cell dimensions continuously decrease from 475 to 975 K. An initial slow 1.2 % reduction between 375 and 875 K is followed by a fast 1.2 % reduction between 875 and 950 K for the CSS sample. This second unit cell reduction step is unrelated to the calcination process and does not involve any weight loss. It is suggested that this step is a recrystallisation/annealing process reducing the amount of defects and strain.
Abstract: Solid solutions of GaxCoFe1-xCrO4 ( x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were obtained by solid state reactions in air at 1623K. X-ray and neutron powder diffraction measurements have been performed on the materials in order to characterize them. Rietveld refinement of neutron powder diffraction data showed cubic symmetry corresponding to the space group Fd-3m for all of the samples in the series. The results reveal that Ga has been substituted for Fe selectively. Ga exclusively occupies the octahedral site up to x = 0.8, while at x =1.0 a small amount of Ga (5%)enters into the tetrahedral site in the compound. Cr, Fe and Co cations are distributed between the tetrahedral 8 a and octahedral 16 d sites. Their proportion at each site varies with composition. The fractional coordinates of oxygen are almost the same in all the samples while the lattice parameter shows a slight decrease with increasing Ga content. The magnetic structure at room temperature was ferrimagnetic for the composition x = 0.0 and x = 0.2.