Precision Measurements of Crystals of the Alkali Halides

Crystal structure and dimensions for all the alkali halides.-The Hull powder method of obtaining x-ray diffraction patterns was used. By comparing each diffraction pattern directly with the pattern of NaCl simultaneously recorded on the other half of the film, the grating space of each crystal powder was measured in terms of the side of the unit cube of NaCl, assumed to be 2.814 A, with a precision of about 0.1 per cent. The average results for the sides of the unit cubes are as follows: <colspec colname="Col1"/><colspec colname="Col2"/><colspec colname="Col3"/><colspec colname="Col4"/><colspec colname="Col5"/><colspec colname="Col6"/> <entry align="align" colname="colname" colsep="colsep" rowsep="rowsep" valign="valign"/> Li. Na. K. Rb. Cs. Fluorides........ 2.007 2.310 2.664 3.663 3.004 Chlorides........ 2.566 Assumed 3.138 3.267 4.118 Bromides........ 2.745 2.968 3.285 3.418 4.287 Iodides......... 3.537 3.231 3.525 3.655 4.558 The arrangement of the ions is simple cubic except for RbF, CsCl, CsBr and CsI for which it is body-centered cubic. The relative intensities of the lines reflected from various planes and the corresponding computed interplanar distances are tabulated for each halide. It was observed that Cs and I ions are as equal in diffracting power as are the atoms of Mo, Ta and W, which crystallize with the same structure as CsI, and that K and Cl ions also have equal diffracting power. Crystal densities of all the alkali halides, computed from x-ray data.-The results, accurate to within about 0.3 per cent, are as follows, assuming the density of NaCl to be 2.163: <colspec colname="Col1"/><colspec colname="Col2"/><colspec colname="Col3"/><colspec colname="Col4"/><colspec colname="Col5"/><colspec colname="Col6"/> <entry align="align" colname="colname" colsep="colsep" rowsep="rowsep" valign="valign"/> Li. Na. K. Rb. Cs. Fluorides........ 2.007 2.310 2.664 3.663 3.004 Chlorides........ 2.566 Assumed 3.138 3.267 4.118 Bromides........ 2.745 2.968 3.285 3.418 4.287 Iodides......... 3.537 3.231 3.525 3.655 4.558 For the chlorides and iodides, except LiI, the values agree with those obtained by other methods within 0.2 per cent on the average, whereas for the others the values are higher by 0.5 to 1.5 per cent for the bromides and 1 to 9 per cent for the fluorides.