Leeds University Library

Pyrites

Pyrites, FeS2

Figure 5 shows a typical example of (WHB's) experimental records. The data (for reflection from the (110) plane of FeS2 (pyrites) using an X-ray bulb with a Rh (rhodium) target) is set out in sets of five (unheaded) columns.

For clarity of interpretation, we reproduced seven lines from the left-hand set of data, together with interpretative headings for each column of figures.

Angular measurements
Ionisation(intensity) measurements
(glancing) Crystal (θ)
Ionization chamber (2θ)
background
reflection
difference
10
18.1
207
250
43
9.5
"
207
328
121
9.4
"
204
333
  129*
9.2
"
210
302
92
9.3
"
210
328
118
9.4
17.8
206
310
104
-
18.5
208
298
90

The first column gives the angle θ between the crystal face and incident X-ray beam. The second column is the angular setting of the ionization chamber, 2θ. As can be seen, these angular settings are not coupled - the angles in the second column are not precisely double those of the first column. Whereas nowadays a diffractometer would sweep through the θ-2θ angular range, continuously recording the change in intensity, the Braggs had to record each intensity measurement separately.

For each small angular adjustment of the crystal (θ) and ionisation chamber (2θ), the background ionisation current was recorded (third column) - presumably a shutter was interposed - then the ionisation current from the crystal reflection (fourth column) and the difference (fifth column).

The starred row of data indicates what WHB considered to be the optimum angular settings of the crystal and ionization chamber - but as can be seen the difference is not very marked and the measurements would be repeated for different ionization chamber slit widths and different settings of the X-ray bulb.

Similar experimental data for second-order (higher-angle) reflections from the (110) plane of FeS2 are given in the columns of data on the right-hand side of p18 and data for reflections from the (100) and (111) planes are given on pages 16, 17 and 19.

Of all this data, the important measurements which the Braggs made use of in the determination of the crystal structure of FeS2 were the 'starred' values for the angular settings (2θ) of the ionization chamber rather than the angular settings (θ) of the crystal face which, they realised, may deviate slightly from that of the reflecting planes.

This starred data (2θ and intensity measurements) is collected together (WLB handwriting) in a sketch-table on p20, as shown in Figure 6.

This data is reproduced as (part of) Table 1 in WLB's paper 'Analysis of Crystals in the X-ray Spectrometer'(16) and also in the book 'X-rays and Crystal Structure'(8) in which the observed and calculated intensities are compared, as shown in Figure 7a.

In short, here we see, in the notebook records, the full experimental data upon which the Braggs determined the crystal structure of pyrites in the summer of 1913 and which, as WLB recalled long afterwards in, The Development of X-ray Analysis(18), "provided the greatest thrill". This is because, unlike the crystal structures on the halides, diamond, fluorspar (CaF2) and zinc blende, CaF2, the positions of the sulphur atoms were not completely determined by the symmetry elements. Their exact positions were deduced, by the Braggs, from the observed and calculated intensity measurements.

The notebook, it should be stressed, does not include the details of the Braggs' analysis, neither on pyrites nor any of the other crystal structures. It does, however, contain fragmentary notes. For example, on p 23 [with respect to the structure of cuprite, Cu20] WHB records: "in comparing with NaCl we will suppose the Cu atoms are arranged in a face centred cubic lattice. There will be 4 Cu's and 2 O's to each large cube. In the case of NaCl we have put 4 Na's and 4 Cl's in each large cube. Thus we should compare CuO½ to NaCl o..... [indecipherable]".