EN4GINEERING RESEARCH INST ITUTE UNIVERSITY OF MICHIGAN ANN ARBOR QUARTERLY REPORT NO. 2 ON INFRARED STUDIES OF CRYSTALS II (Period: 15 August to 15 November 1954) By G. B. B. M. SUTHERLAND Principal Investigator and C. Y. PAN LIANG Project 2235 SIGNAL CORPS, DEPARTMENT OF THE ARMY CONTRACT DA 36-039 sc-56736 SC PROJECT 152B, DA PROJECT 3-99-15-022 SQUIER SIGNAL LABORATORY, FORT MONMOUTH, N. J. DECEMBER, 1954

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN TABLE OF CONTENTS Page I. PURPOSE OF THE RESEARCH 1 II. ABSTRACT 1 III. PUBLICATIONS 1 IV. FACTUAL DATA A. Barium Titanate 1 B. Brucite. 2 C. Micas 2 D. Gypsum 4 V. CONCLUSIONS 4 VI. FUTURE PROGRAM 5 VII. PERSONNEL 5 ii

I- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN - I I QUARTERLY REPORT NOo 2 ON INFRARED STUDIES OF CRYSTALS II Io PURPOSE OF THE RESEARCH The general purpose of this research is to complete the investigations started in May 1951 under Contract DA 36-039 sc-56736 on the infrared spectra and structure of barium titanate, brucite, mica and gypsumo IIo ABSTRACT The effect of temperature on the spectrum of barium titanate has been studied. No new work has been done on brucitoe The possibility of removing certain ambiguities in- the location of the hydrogen atoms in muscovite and biotite has been investigated. A start has been made on the general interpretation of the absorption spectra of the micas in the range 2 to lOO0o The interpretation of the spectra of gypsum is proceeding. III, PUBLICATIONS None of the work done during the period covered by this report has yet been published. The following publications concerning work done on the preceding contract have now appeared: "The Problem of the Two Types of Diamond". Go Be Be M. Sutherland, Do Eo Blackwell & Wo Go Simeral, Nature 174, 901 (1954)o "The Infrared Spectrum of Barium Titanate"t Ro To Mara, Go Bo B Mo — Sutherland & Ho Vo Tyrell, Physo Rev, 96, 801 (1954)o IVo FACTUAL DATA Ao Barium Titanate The spectrum of a powdered sample of barium titanate has been obtained at a temperature above 150O C, (io.e well over the Curie temperature of 120~ Co) using the K Br disk techniques It I I.1

i - ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN appears that there is an increase in the absorption intensity on the long wave side of the absorption band centered near 520 em-lo It has not yet been determined whether the change occurs at exactly 1200 Co In the light of some recent work in Switzerland, it appears that the Curie temperature is much higher than 120~ Co for very small crystals of barium titanateo The indications are that the surface layers of the crystal may have a much higher Curie temperature than the interior of the crystal. This means that there is no sharp transition point, the transition from the tetragonal form to the cubic form being smeared out over a temperature range which increases with decreasing particle size, B. Brucite Nothing new to report. C, Micas The orientation of the OH groups in muscovite and biotite is still occupying our attention, It was mentioned in our last report (Final Report, October 1954, po 43) that the sign of the "a coordinate" of the hydrogen atom in the first layer must be positive. However, this conclusion depended on an arbitrary convention for the sign of the angle of refraction (r)o If the sign of r is reversed, then "al" would have to be negative, We have therefore tried to see whether this ambiguity can be resolved by examining the environment of the hydrogen atom and using physical considerationso In muscovite the two sheets of atoms adjacent to a o sheet containing an OH group are composed of Si ions (about lo6 A on one side of the OH sheet) and Al ions (about 1.2 2 on the other side of the OH sheet) as shown in Fig.o l We have already used physical considerations in concluding that the OH group in the first layer points towards the sheet of Si ions and away from the sheet of Al ions (ioe the sign of z is positive, cfo, Final Report, October 1954, po 43), The dichroic measurements have enabled us to find the angle the projection of the OH direction makes with the a and b axes, but since the signs of a and b are unknown the hydrogen atom may be in any one of the four quadrants, io e at A, B, C, or D in Fig. 1, If we assume that the equilibtri um position of the hydrogen atom.: is- determined by its electrostatic interaction with the surrounding ions, then the position A is to be preferred since the hydrogen is now placed much more symmetrically with respect to the Al ions than if it occupies position B, C or Do All four positions are virtually equivalent with respect to the Si ions. According to the conventions we have adopted, this means that the sign of "a" is positive so our previous conclusion is unchanged, Furthermore, this establishes the sign of "b" as negative. In the case of biotite, the situation is much more complex. The exact locations of the Fe and Mg ions in the sheet.... I a2

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN *OH Si a Fig. 1 The projection of OH Group (No. 1 and 2 in Fig. 5, p. 33, Final Report, October 1954) on cleavage plane in Muscovite..0 a..$ Silicon on the Upper Sheet OH Group on the Middle Sheet Aluminum on the Lower Sheet *oH -i. Fig. 2 The projection of OH Group on cleavage plane in Biotite, 0 O.' Fe, Silicon on the Upper Sheet OH Group on the Middle Sheet Mg on the Lower Sheet.3

7- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN - adjacent to the OH sheet have not been determined. The infrared results indicate that the hydrogen positions in biotite should conform to the Cqh space group. Taking the Fe/Mg ratio as exactly 2/1, it is possible to place these ions in the sheet so that the unit cell conforms either to the C~h or to the Cgh space group. Since the hydrogens belong to the CGh space group, it seems more reasonable to make the Fe and Mg ions conform to the same space groupo This leads to the positions of the Fe and Mg ions shown in Fig. 2. Again we have four possible positions for the OH direction corresponding to the hydrogen atom being at A, B, C or D, So far we have found no satisfactory way of deciding between these four possibilities. Hitherto we have concentrated our attention almost exclusively on the interpretation of one band in the spectra of micas, viz. that due to the OH stretching frequency near 2.8io We have recently started to consider the overall interpretation of the mica spectra. This has been done using two approaches. The first was to compare carefully the spectra of all the micas examined over as wide a spectral range as possibleo This involved some more experimental work to fill in gaps which became apparent and to check some of the earlier observational work, It is found that the spectra of the various micas have certain features in common, e.g. 5 strong bands in the range between 4V and 355, The structural feature common to all micas is the existence of linked Si04 tetrahedra, and these 5 bands probably all originate from vibrations of these silicate groups. The second approach has therefore been to survey previous infrared work on silicates and see to what extent previous work on the silicates can be used in interpreting the common features of the spectra of the micas. This work is now in progress but it would be premature to report on it at this stageo D. Gypsum The computational work involved in transforming the reflection spectra to give absorption coefficients for the intense bands of gypsum has been completed, The interpretation of the spectrum is proceeding satisfactorily. Since this is a very complex problem and impossible to summarise, this work will eventually form the subject of a separate technical report. Vo CONCLUSIONS A. Barium Titanate It appears that there are small but real changes in the spectrum of barium titanate as it passes through the Curie temperature. The investigation of these changes will be difficult because in the powder form there may be complicating factors due to surface layer effects.

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN Be Brucite Nothing new to report. C. Micas Although it has been possible to establish the positions of the hydrogen atoms in muscovite, it may not be possible to do so for biotite with the same degree of certainty. D. Gypsum Still too early to draw any important conclusions. VI. FUTURE PROGRAM A. Barium Titanate Work on the temperature effect will be extended to include single crystals and comparison with SrTiO3 B. Brucite No new work contemplated, Co Micas The main effort will be on the general interpretation of the spectra of the micas, Some time will also be devoted to the preparation of earlier work for publication. D. Gypsum Interpretation of the spectrum, especially location of the H20 molecules. VIIo PERSONNEL The following have been engaged on the work reported here: Professor G.BBoM. Sutherland, Director (part time) Mrs. C. Y. Pan Liang (half time) Mr. M. Hass (part time) Mr. A. Dockrill (technician - part time) Mr. H. Diamond (part time - unpaid) -5