Divergent-beam technique used in a SEM to measure the cell parameters of isotopically distinct samples of LiF over the temperature range ∼15–375 K

The cell parameters for LiF containing 90.4% 6Li and 50.2% 6Li were determined from a series of divergent-beam photographs taken at approximately 25 K intervals in the ∼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 6Li90.4%  = 4.0272 (2) and 6Li50.2%.  = 4.0266 (2) A. If it is assumed that the stated isotopic compositions are correct, linear extrapolation to end-member cell parameters gives 6Li100%  = 4.0273 (2) and 6Li0%  = 4.0259 (2) A. 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.