410 THE CORRECTION OF MEASURED INTEGRATED BRAGG INTENSITIES 0( 4 is the corresponding value calculated for the finite aperture (0-20 scan) and it can be seen that the approximation of an intinite slit in this case gives an error of the order of 300/0 in the TDS contribution to the peak scan. The values obtained for a spherical volume (~5) are in this case also quite good approximations for 0 > 40 °; the effective scan range being + 1-47 °. A sphere of equal volume would give an effective scan range of + 1.43 ° resulting in values of 7 30/0 lower than ~5. These effective scala ranges may be compared with the true scan range of + 1.2 °; a spherical volume using the true scan range would thus give values of c~ of the order of 20% too small in this case. The error in the intensities resulting from ignoring the TDS correction would again be less than about 1% with a corresponding error in the mean temperature factor. The relative agreement between the various ~ values is also illustrated in Fig. 2.We have also estimated the 'mosaic spread' effect for the barium fluoride measurements. The full width at half height of the mcasured peaks varied from 0.2 ° up to 0.8 ° resulting in a reduction of the TDS correction between 1% and 10%. This can therefore be an important factor particularly if the width of the measured peaks changes rapidly with the scattering angle and passes through a focusing position. The structures of thiourea, SC(NH2)~,, and deuterated thiourea, SC(ND2)2, have been determined at room and liquid nitrogen temperatures from three-dimensional neutron diffraction data. No significant structural change on deuteration has been found. N-H.--S hydrogen bonds occur in both materials at both temperatures with N-S distances of 3.35-3-43/~ and N-H-S angles of 169-171 ° and, apart from these hydrogen atoms, the molecules are planar to within 0-010/~. An analysis of the thermal parameters of the atoms in each molecule in terms of rigid vibration parameters shows that at liquid nitrogen temperature the molecules are fairly rigid whereas at room temperature there are serious deviations from rigidity. Excellent agreement has been found between the thermal vibrations of the molecules at room temperature and the observed structure change to the lower ferroelectric state. A qualitative theory of the fcrroelectric nature of thiourea is proposed which explains the observed temperature variation of the spontaneous polarization and coercive field in the lower ferroelectric region, in terms of a variable molecular orientation and a single hydrogen bond which is switched from one sublattice to the other during ferroelectric reversal.

The structures of Bi3Ru3O11, a = 9.3050(2)Ǻ, Pn3, and Bi2Ru2O7, a = 10.2957(1)Ǻ, Fd3m, have been redetermined by Rietveld analysis of powder neutron diffraction data. For both compounds there is no evidence for appreciable oxygen non- stoichiometry . X-Ray photoelectron spectra of both compounds show unusual line shapes. In the case of Bi3Ru3O11 this is a result of the mixed valency and surface oxidation. In Bi2Ru2O7 final state effects appear to be important.

The high resolution neutron powder diffractometer installed on the AAEC HIFAR reactor at Lucas Heights is described. The resolution is in good agreement with predictions and, although below the most optimistic estimates, the intensities are usable. Examples are given of problems solved using the diffractometer which could not have been solved either by X-ray diffraction or by use of a conventional neutron powder diffractometer. Plans for diffractometer development are outlined.

Neutron diffraction studies have been made on the orthorhombic phase of caesium thiocyanate for temperatures below the order-disorder phase transition at 470 K. The structure, as determined at 453 K by elastic neutron diffraction, is found to be very similar to that at room temperature. This behaviour is contrasted with that of potassium thiocyanate. The temperature dependences of the acoustic phonon dispersion branches, as determined by inelastic neutron diffraction, are also presented for temperatures up to 463 K. No evidenq: of significant mode softening was observed.

The theory of the neutron Kikuchi effect has been re-examined and an exact formulation in reciprocal space derived. The conditions necessary for its observation have been considered and appropriate data from a lead single crystal have been collected on the triple-axis spectrometer at the HIFAR reactor in the laboratories of the Australian Nuclear Science and Technology Organisation. A series of constant-Q phonon scans across the predicted positions of the Kikuchi lines show reductions in the integrated phonon intensity, which appear to move as expected with changes in the scattered neutron wavevector.