September 16, 1929
Professor W.L. Bragg,
University of Manchester,
Dear Professor Bragg:
We have not very much to report as the result of the summer's work, but several
problems are coming along nicely. I am writing up my work on pseudobrookite. It has
a =9.70, b = 9.78, c=3.69 Å, space group Vh17, 4Fe2TiO3 in the unit. I verified the
structure with great care, analysing five Laue photographs completely as well as a
number of oscillation photographs. A few obstinate spots on each photograph gave great
trouble until it was finally shown that they were due to small crystals of rutile
in parallel growth with the large pseudobrookite crystal. Since the X-ray data ruled
out the formula Fe4Ti3O12, this explained the excess TiO2 over Fe2TiO5 reported for
the catural crystals used in our investigation. There are strings of titanium octahedra
along the c-axis, each attached by a corner, to the one above and the one below. Each
iron ion is 1.9 Å from four oxygens and 2.2 Å from two more, so its coordination number
is either four or six. The structure is not related to those of sillimanite and andalusite,
in which there are rutile strings of aluminum octrahedra, nor to that of brookite.
I am glad to see that you also are turning your attention to the micas, feldspars,
and zeolites. I have begun work on the zeolites too, for no particular reason except
that I had some good crystals at hand.
There are five of us doing crystal work here now. We have a pretty extensive
collection of apparatus, lacking only an ionization spectrometer, the one which Dickinson
used having been dismantled some years ago. I hope to have one next year since your
work has made quantitative data so important for structure determinations. we have
a weekly conference during the winter, with interesting discussions. they would be
much more interesting if one of your men were here to participate in them. If you
send one of your men to America on a fellowship I hope you will keep us in mind in
advising him. He would find research (as well as living) conditions here second to
none. No matter how physical his training he would not need to be frightened at coming
to a chemical laboratory.
My wife and I will bring our boy, five years old, with us to England. He is very
anxious to come - he half believes that he didn't go to Europe at one year because
he hadn't behaved! We are already as excited as he about the trip.