Time-resolved Neutron Powder Diffraction - Thermodiffractometry

Introduction

1. A high neutron flux at the sample position. For a given source (reactor) this can be achieved by moving the instrument as close as possible to the core and/or by using a vertically focusing, highly efficient monochromator. One of the most efficient monochromator materials is highly oriented pyrolytic graphite (HOPG) with typical mosaicity between 0.5 and 1°; it is particularly useful at long wavelengths (e.g. lambda ~ 2.5 Å). For shorter wavelengths common choices are germanium or copper crystals with a properly adapted mosaic spread.
2. A stationary, large, uniform and curved PSD to measure simultaneously complete diffraction patterns without any movement of the detector. A high efficiency of neutron detection is also essential which, in the case of a gas detector, will require the use of ³He-based mixtures instead of BF3.

Thermodiffractometry

The dehydration of WO3.3H2O.

click on figure for more details (fig.1)

This sequence of dehydration/Phase transformation was studied on D1B with a hydrate sample which was a loosely packed cylinder of powder WO3.3H2O with h = 50mm, diameter = 10mm) held under a vacuum(p ~ 10^-4 torr) using a temperature resolution of 5° Centigrade (delta t = 12min). The figure below shows a part of the pattern in the temperature range 100-640° Centigrade; from a 3-D plot such as this, one can immediately notice that the compound completely dehydrates before it transforms into a HTB structure (in other words, the transformation HYD > HTB occurs in two steps; first dehydration, then reconstructive phase transition).

click on figure for more details (fig.2)

A more detailed picture of the mechanism of the transformations is obtained by examining the thermal evolution of the Bragg reflections:

(A) Cell parameters (fig.3). The cell dimensions in the plane of the hexagonal layers (a,b) decrease slightly during dehydration (the coordination number of the tungsten atoms noted W2 changes from 6 to 5) whereas the c axis (whose length is governed only by the coordination polyhedra of the second kind of cation W1) shows only a normal thermal expansion.

click on figure for more details (fig.3)

(B) Intensities (fig.4). The 00l reflections are almost unaffected by the HYD > HTB > PTB transformations. On the contrary, after the first transformation the h00 and 0k0 peaks are enhanced by a factor of 3 as expected from the change of symmetry (orthorhombic to hexagonal); all hkl reflections vanish during dehydration.

click on figure for more details (fig.4)

(C) Line breaths (fig.5). This is the most revealing feature of the HYD > HTB phase transformation. Indeed the FWHM of most reflections is not modified; one only notices a slight narrowing of these lines (note, however, that all peaks of HYD are already much larger than the instrumental resolution; this simply results from the poor crystallinity of the starting material). By way of contrast the 020 reflection of HYD splits into two gaussians centered at the same theta angle with intensity in the ratio 1/2. The stronger reflection has a FWHM of 0.29° (i.e. only slightly larger than the instrument resolution) whereas the weaker one is extremely broad (about 0.82°) This corresponds to an average crystal dimension of about 200Å along this direction (020 in HYD. i.e. one of the 100 directions in HTB). This observation has an obvious bearing on the mechanism of the transformation from anhydrous HYD into HTB and suggests that it proceeds (at least in part) through a shear of the HYD structure in a plane perpendicular to its b axis.

click on figure for more details (fig.5)

These results provide a good illustration of the power of thermodiffractometry to shed light on both structural and textural aspects of transformation in solids.