X rays and crystal structure . we are led tosuppose that this series has a structure as representedin Fig. 28a. The two kinds of particles represent atoms of metal(K or Na) and halogen (CI, Br, I). Consideringthe points of one kind alone, the whites for example,it is clear that they lie on a face-centred lattice, suchas is hinted at by the spectra of rock salt. If theatoms are identical as regards their behaviour toX-rays, the structure reduces to a straightforwardcubic one. The arrangement of the atoms in planes is showndiagrammatically by Fig. 2 8b. The planes (100)contain atoms of both kind
X rays and crystal structure . we are led tosuppose that this series has a structure as representedin Fig. 28a. The two kinds of particles represent atoms of metal(K or Na) and halogen (CI, Br, I). Consideringthe points of one kind alone, the whites for example,it is clear that they lie on a face-centred lattice, suchas is hinted at by the spectra of rock salt. If theatoms are identical as regards their behaviour toX-rays, the structure reduces to a straightforwardcubic one. The arrangement of the atoms in planes is showndiagrammatically by Fig. 2 8b. The planes (100)contain atoms of both kinds. As we are poinp- tocompare KCl and NaCl, let us call the metal can then denote the constitution of the (100)planes in both cases by saying they contain R . (no) planes are built up in the same way, but ANALYSIS OF CRYSTAL STRUCTURE 95 their distance apart is smaller in the ratio i •. (ill) planes are of a different nature, successive planes contain R atoms alone and CIatoms alone (110) (I I) NaCL NaCl NaClf^(ioo) = o. NaCL NaCL NaCL NaCl Na CL Na CL Nd^(110) = a/v/2 o?(ni) = 2aj^Fig. 28. If we reflect the X-rays from the planes (111) inFig. 28B, the true distance ^(„,) is that from likeplane to like. The angle of the first reflection isgiven by the equation X = 2(7^(„,)Sin 0. But halfwaybetween the planes of composition CI are spaced 96 ANALYSIS OF CRYSTAL STRUCTURE planes of composition R. These reflect waves whichare, for the first reflection, just out of step with { opposite phase to) the waves reflected from theCI planes. Their effect is to tend to destroy thefirst reflection, and in fact every reflection of the oddorder, while they strengthen the reflections of evenorder. This effect is apparent in the spectrum of(ill) NaCl, where the planes containing sodium ofatomic weight 23 alternate with the planes contain-ing chlorine of atomic weight In KC1(,„) theatomic weights (39 and ) are so close that thespe
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