July 15, 1929
Professor W.L. Bragg,
University of Manchester,
Dear Professor Bragg:
I have decided definitely to stay in Pasadena now, although I was very much interested in the possibility of going to Harvard.
But staying here means that I shall have the opportunity of coming to Manchester for a few months. We can leave Pasadena early
in March, 1939. It will still be necessary for me to spend a month in Berkeley then; I have a connection with the University
of California requiring that I give lectures in chemistry and physics for a month every year. Hence we can't arrive in Manchester
until about the end of April. I suppose you close up sometime in July, or perhaps June. I would like to stay in Manchester
as long as work is going on, and then spend a couple of months on the Continent; I plan to visit old friends - Sommerfeld,
Ewald, Debye, Mark - and to meet some people whom I' have never seen, such as Niggli.
No special apparatus would be needed for me. I should like to see you apparatus and perhaps help in making some measurements;
possible I shall have a crystal from which I should like some quantitative data. But most of all I want to talk with you and
the men in your laboratory.
I thank you very much for the manuscript you sent me. Your general description of your method is very interesting, and the
work itself very valuable. I was also very much interested in your Faraday Society address, with its description of silicate
structures. The cyanite structure pleases me, satisfying the electrostatic valence rule so well as it does. Sillimanite
Bragg-2 7-1 5-29
and andalusite as worked out by Taylor seem alright too. The electrostatic valence rule isn't rigorous, and could perhaps
be restated in the following way: In a crystal containing anions in an approximately close-packed arrangement ΣSi should be
equal to S for every anion (or very nearly equal to S); if the anions are not close-packed, those having fewer than 12 anion-contacts
should have ΣSi less than S, the others having ΣSi greater than S.
I am pleased too with Naray-Szabo's staurolite structure, which also satisfies the rule. You may be interested in a method
of distinguishing between (OH)^∞ and (OHO)^-3 groups in crystals containing O and H. If for every H there is one I in the
crystal with Σs=1, the existence of hydroxyl ion is indicated; but if for every H there are two O's with Σs=3/2, we may accept
this as showing that the two O's are joined by a hydrogen bond, as are the two F's in the acid flouride ion, FHF^∞. Examples
of the first class, with OH^∞, are topaz and the members of the chondrodite series; of the second, with OHO^-3, staurolite
and diaspore, A1HO2, göthite, FeHO2, manzanite, MnHO2, whose structure Mr. Ewing and I have just worked out. Now we can see
that a necessary condition for the isomorphous replacement of OH^∞ by F^∞ in a crystal is that the crystal belongs to the
first class, and as a matter of fact the mineralogists report this replacement in topaz and chondrodite, etc., but not in
staurolite and diaspore, etc.
I met Warren in Boston, and enjoyed talking with him. I'm hoping that after his year with you he will come to Pasadena.
We have been working away at several oxides and hope to have several new structures before long. We are making use of
the same method as for brookite - to find a coordinated structure with the correct size of unit and space-group symmetry,
to distort the polyhedra in order to predict parameter values as closely as possible, and then consider small changes in the
parameters to get the best agreement between structure factors and visually
estimated intensities. Sturdivant is getting along well with tantalite, Ta2FeO6, which has a structure closely related to
one which we mention in the brookite paper as not being the structure of brookite.
My wife and boy are looking forward to coming to Manchester, and join me in sending greetings to you and Mrs. Bragg.