Divergent-beam technique used in a SEM to measure the cell parameters of isotopically distinct samples of LiF over the temperature range approximately 15-375 K.
The cell parameters for LiF containing 90.4% (6)Li and 50.2% (6)Li were determined from a series of divergent-beam photographs taken at approximately 25 K intervals in the approximately 15 to 375 K range. The divergent-beam photographs were generated in a Stereoscan S4 scanning electron microscope fitted with a liquid-helium cold finger and a Dingley Kossel camera. Lattice parameters can be determined from the ratios of lines joining conic intersections, and such data, measured on films taken at known temperatures, were correlated with equivalent data from computer-generated divergent-beam patterns using appropriate cell parameters; our cell parameters at 298 K are (6)Li(90.4%) = 4.0272(2) and (6)Li(50.2%) = 4.0266(2) Å. If it is assumed that the stated isotopic compositions are correct, linear extrapolation to end-member cell parameters gives (6)Li(100%) = 4.0273 (2) and (6)Li(0%) = 4.0259(2) Å. Since for a cubic crystal only one lattice-parameter measurement is required from each photograph, the usefulness of the ratio approach was tested by comparing values of the lattice parameter deduced from many such ratios obtained from photographs with no symmetry and with mirror symmetry.
Crystallography and Mineral Physics Unit, Research School of Geological and Geophysical Sciences, Birkbeck and University College London, Gower Street, London WCIE 6BT, England. m.mendelssohn@ucl.ac.uk
MJ
Mendelssohn
HJ
Milledge