• S Ramaseshan

Articles written in Proceedings – Section A

• The crystal forms of the Panna diamonds

• Mahadevite—A new species of mica

The physical properties and chemical composition of a species of mica from the Warangal district in Hyderabad have been studied. Thick sheets are bronze-coloured and opaque, while thin sheets transmit light of a greenish yellow tint, the pleochroism being weak but distinct. The optic axial angle is 13°48′. Th plane of the optic axes is perpendicular to the plane of symmetry. The composition is SiO2 38·98%, Al2O3 29·94%, Fe2O3 and FeO 4·12%, Mgo 13·2%, K2O 9·6% water 3·8%. On the basis of Pauling’s model of the mica structure, the numbers of atoms of various elements in six, and twelve co-ordination positions are calculated. Approximately equal number of Al and Mg atoms are found in six co-ordination. The mica appears to be of a type intermediate between muscovite and phlogopite.

• The crystal forms of diamond and their significance

• The faraday effect in diamond

• The cleavage properties of diamond

Calculations of the cleavage energies of various planes in diamond indicated the presence of cleavages other than those already known. A careful goniometric study of several crystal fragments in Sir C. V. Raman’s collection revealed numerous other cleavages. The presence of the following cleavage has been definitely established: (111), (221), (110), (322), (331), (211) and (332). The (111) cleavage was found to be by far the most perfect and most abundant, while (221) and (110) cleavages were not uncommon. It is suggested that the perfection of the (111) cleavage is not merely because that plane has the minimum cleavage energy but also due to the fact that on either side of the plane of easy cleavage lie layers of atoms having three times the cleavage energy. It was also found that diamond definitely has curved fractures and the theory developed in the earlier part of the paper accounts for the existence of these.

• A theory of the crystal forms of diamond

• Determination of the magneto-optic anomaly of some glasses

• The magnetic properties of iron-pyrites

• Faraday effect in some cubic crystals

• The faraday effect and magneto-optic anomaly of some cubic crystals

Using a Lippich double field polarimeter together with a spectrograph, the Faraday rotation for crystals of NaCl, KCl, KBr, KI, NH4Cl, NH4Br, NaClO3, KAl(SO4)2 12H2O and NH4Al(SO4)2 12 H2O have been measured for the wavelengths λ 5893, λ 5780, λ 5461, λ 4358, λ 4046 and λ 3665. The magnetic rotation for concentrated solutions of these crystals in water were also measured. The magneto-optic anomaly for the crystals and solutions has been calculated from the measurements of the dispersion values. It is found that in the case of crystals, γ(NaCl)&gt;γ(KCl)&gt;γ(KBr)&gt;γ(KI) and γ(NH4Cl)&gt;γ(NH4Br). The γ for the solutions except in the case of NaClO3 and the alums is always greater than that for water. The γ values for the ions in solution are higher than those for the ions in the crystal. The presence of covalent bonds tends to diminish the value of γ [γ(NaClO3 crystal)=33% and γ(Alum)=52%]. The γ value for a 50% solution of NaClO3 is 72%.

• The Christiansen experiment with spherical particles

• Diffraction of light by transparent spheres and spheroids: The fresnel patterns

The paper describes and discusses the diffraction effects observed when a beam of light traverses a transparent sphere immersed in a liquid of slightly lower index and emerges therefrom. The two most interesting features are firstly, a concentration of intensity along the periphery of the emerging light beam which is evidently in the nature of a caustic and secondly, a concentration of intensity along the axial ray which is in the nature of a continuous focus. These two features are each accompanied by a set of interference-rings and these appear superposed on each other. Significant alterations appear in these features when the particle has a spheroidal shape. With a birefringent sphere, two sets of caustics are, in general, observed. 12 Photographs illustrate the paper. *** DIRECT SUPPORT *** A00OCO56 00002

• The magneto-optic anomaly of electrolytes in aqueous solution - I. Alkali halides

The Verdet constant and dispersions of 4-normal solutions of seventeen univalent halides have been measured. From these, the values of the magnetooptic anomaly have been estimated. The magneto-optic anomaly of the free ions of the alkali halides as distinct from that of the solution have been evaluated. It is found that the value of the anomaly of the free ions having the inert gas configuration is very near the theoretical value of 1. The ionic dispersivity and rotativity of the individual ions have been estimated. It is found that the additivity rule is approximately valid. The pure acids, HCl, HBr and HI, have values of the anomaly much lower than those for their solutions. This has been attributed to the presence of covalent linkages in the acids in the pure state. The anomaly for the rare gases neon and argon are very nearly unity. It is found that the values of molecular refractivity, dispersion, rotativity, rotation and the magneto-optic anomaly of the ions in solution are higher than those for the ions in the crystalline state.

• The magneto-optic anomaly of electrolytes in aqueous solution—II

The paper records the magneto-optic rotation and the dispersion for a large number of ionic solutions. From these data the magneto-optic anomaly for these solutions have been evaluated. The ionic rotation for the different substances have also been estimated. Amongst the substances studied are halides, nitrates, sulphates, hydroxides, chlorates, bromates and iodates of monovalent and divalent elements. The anomaly in some pure acids have also been determined. The magneto-optic anomaly for about twenty-three gases and vapours have been calculated from the data given in Landolt and Bornstein’s tables. It is found that the presence of ions with covalent linkages decreases the value of the anomaly for water, the decrease being greater if the strength of the covalent bond is greater. In the case of gases also the value of the anomaly is lower for gases in which the strength of the bonds is greater. There are some vapours that do not follow this rule.

• Faraday effect and birefringence—I

Approximate formulæ correlating the decrease in Faraday rotation with birefringence have been derived and these have been verified by experiments on strained glasses and plastics. It is found that for small values of birefringence, the decrease in the Faraday rotation is proportional to the square of the total birefringence. For values of the birefringence of λ/30, the decrease in the rotation is less than 1%. The Verdet constant in birefringent solids can be accurately determined provided sufficiently thin specimens are taken such that the total birefringence is small. Finally it is shown that the decrease in the Faraday rotation can itself be used as an accurate measure of the birefringence in strained solids.

• Faraday effect and birefringence—II - Corundum

The Verdet constant and the magneto-optic anomaly for corundum for light travelling along the optic axis have been determined for the wavelengths λ5,893, λ5,461 and λ4,358. The Verdet constant has the values V5893=0·0210′, V5461=0·0240′ and V4358=0·0381′ minutes per cm. per Oersted andγ has a value of 0·64. Accurate measurements have also been made for directions of propagation inclined to the optic axis and it is found that the Verdet constant does not sensibly change for an inclination of about 15° away from the optic axis.

• Paramagnetic resonance in some cupric salts

Paramagnetic resonance absorption of 3 cm. wave in eleven copper salts has been studied. Of these four show paramagnetic absorption;g factors and half-widths obtained are given. Single crystals of cupric ammonium chloride and cupric ammino-nitrate have been studied for anisotropy ing factor and line-shape parameters. Results regarding cupric ammonium chloride indicate that exchange interaction may not be isotropic.

• Investigation of the degree of perfection of a crystal by means of polarized X-rays

The paper describes an investigation of the intensity of Bragg reflection when the incident X-rays are polarized and the azimuth of the electric vector is varied with respect to the plane of reflection. It is observed, using natural and ground (211) faces of NaNO3, that the variation of intensity with azimuth of polarization is different for a mosaic and a perfect crystal. Such a difference is in fact to be expected from theoretical considerations. The actual behaviour of both the ground and the natural faces was found to be intermediate between what is predicted by theory for an ideally perfect and an ideally mosaic crystal. By comparing the observed azimuthal variation of the integrated reflection with the theoretical expectation for the two limiting cases, it is possible to assess the degree of perfection of the crystal.

• A magneto-optic method for the determination of piezo-optic coefficients

A simple magneto-optic method for the measurement of the relative retardation produced on stressing an isotropic solid is described. The method is based on the determination of the true and apparent magneto-optic rotation of the isotropic solid in the unstressed and the stressed states. The method is found to be useful in the measurement of the dispersion of the piezo-optic coefficients with wavelentgh.

• Faraday rotation in calcite

The Faraday rotation has been measured in calcite parallel to the optic axis for the wavelengths λ 5893, λ 5780, λ 5461, λ 4358 and λ 4047 Å. A dispersion formula for the magnetic rotation has been fitted up using the same absorption frequencies that are used in refractive dispersion. It is shown that the magneto-optic anomaly factors for the various absorption frequencies are different and the value of the anomaly factor for the absorption wavelength λ 1535, is very much lower than that for the other absorption wavelengths. (γ1535 = 0·20, γ1000 = γ500 = 0·825.) This paper also reports the values of the Verdet constant of NaClO3 for a few wavelengths from λ 5780 to λ 3650 Å. A numerical error in the earlier measurements has been noticed and rectified here. In this case also a dispersion formula for magnetic rotation has been proposed and it is found that for this crystal γ1850 = 0·46 while γ900 = 0·74.

• The use of anomalous scattering for the determination of crystal structures-KMnO4

The decrease in the scattering factor when the incident radiation is close to the absorption edge of an atom has been utilized for determining the structure of the centrosymmetric crystal KMnO4. Multiple film zero-layer Weissenberg photographs were taken with CuKα, CoKα and FeKα radiations, all of which are quite close to the K-absorption edge of the Mn atom. After reducing these data to the same relative scale, an Anomalous Difference Patterson (A.D.P.) map was computed with (|FCu|2 - |FFe|2) as coefficient and it was found to have striking resemblance to the Isomorphous Difference Patterson map made with (|FCuKMnO4|2 - |FCuKClO4|2) as coefficient. The structure was recovered from the A.D.P. diagram by means of the Buerger Vector shift method. An interesting offshoot of this technique is that the position of the excited atom could be directly determined by making a Patterson with (|FCu| - |FFe|)2 as coefficient. From the knowledge of the position of the anomalously scattering atom and also its scattering factors for the different radiations, it was possible to assign the signs of as many as 50% of the reflexions. Commencing with these the structure was determined and the atomic parameters were refined. The final R-factor for the (hol) and(hko) projections were respectively 0·145 and 0·153. The parameters found are appreciably different from those reported earlier.

• The absolute configuration of echitamine iodide by the X-ray technique

The absolute configuration of echitamine iodide has been determined by the Bijvoet technique, making use of the intensity differences betweenhkl and$$\bar h \bar k \bar l$$ reflections due to the anomalous scattering of CuKa radiation by the iodine atom. The various steps in the procedure are discussed in detail in this paper.

• A low temperature attachment to the weissenberg goniometer

A simple cooling attachment to “Unicam S-35” goniometer has been described. The attachment permits the removal of the camera without disturbing the cooling arrangement. A simple method of growing single crystalsin situ in the goniometer has also been discussed.

• An apparatus for low and high temperature optical goniometry of hygroscopic crystals

Crystals which are hygroscopic or which exist only at low temperatures can be conveniently studied on an optical goniometer with the attachment and method described. The apparatus is simple in design and uses a current of air (or gas) at suitable temperature blowing on the crystal mounted on a goniometer head inside a glass chamber.

• Rapid alignment of crystals in x-ray cameras

The misalignment in the setting of a crystal in the Weissenberg goniometer is determined from the measurement of the displacement of the spots from the central line atθ=±45° with the arcs set in the parallel-perpendicular position (Hendershot, 1937) or in the 45° position (Davis, 1950). When reflections are not present atθ=±45° a graphical straight line method has been suggested by Garaycocheaet al. (1961).

Another graphical method called the “Ellipse method” suggested in the paper does not involve too much calculation after the measurement and hence is easy in application, accurate in the determination of errors and economical in time.

• Rotating elliptic analysers for the automatic analysis of polarised light—part I

An elliptic analyser, consisting of a birefringent element and a linear element, can be converted into a rotating elliptic analyser by rotating one or both the elements. The Poincaré sphere could be scanned, with such a device, along a meridian, a latitude circle, a great circle of any arbitrary inclination to the equator or any oblique path. Continuous analysis of polarised light can be accomplished by using such an analyser. The principles of some of the possible methods of analysis are presented. The problem of analysis of partially polarised light is also briefly discussed. The speed of analysis is important in analysing changing states of polarisation and depends on the speed of rotation of the elliptic analyser. It is pointed out here that fast rates of scanning are possible by using an electro-optic cell as a birefringent element and rotating the field that causes birefringence.

• Crystal co-ordination of the barium ion

A systematic study has been made of the crystal co-ordination of the barium ion in various compounds whose structures have been solved. Apart from the more common co-ordination polyhedra which are enumerated in text-books, a number of new polyhedra have been identified, particularly in cases where the co-ordination numbers are unusual, such as ten or eleven. According to the radius-ratio rule of Pauling, a co-ordination number of nine or ten is normally expected for the barium ion. The present investigations, however, reveal that it shows a variety of co-ordinations with ligancies from six up to twelve. Some of the factors that might possibly enter in explaining this wide range of co-ordination numbers are discussed. It appears as though the part played by the Ba2+ ion in deciding the structure is secondary, limiting itself only to occupying vacant spaces provided by other atoms in the crystal.

• Rotating elliptic analysers and automatic analysis of polarised light—Part II

The various main arrangements leading to a few typical rotating elliptie analysers are discussed. Simple devices for the determination of I, M, C, S of completely and partially polarised light beams are given. The possibility of using a rotating elliptic analyser having an electro-optic cell, for the determination of the Stokes parameters of light beams whose state of polarisation is rapidly changing is pointed out. The accuracy of the Stokes parameter method is analysed and it is found that this method leads to λp andωp values which are subject to an error smaller than one minute of arc (intensity measurement accurate to one in ten thousand). Some of the other methods of using the rotating elliptic analysers are also discussed. It is pointed out that the Stokes parameter method and the interference method, using the concept that orthogonal states do not interfere, are the most promising methods for the analyses of polarised light.

• Piezo-rotatory coefficients and stress-induced optical activity

This paper reports the methods of obtaining the components of the fourth rank axial piezo-rotatory tensor in different crystal classes. The methods of recovering the two piezo-rotatory coefficients R11 and R12 of isotropic optically active glasses, the three coefficients R11, R12 and R44 of cubic crystals belonging to the point group 432 and the four coefficients R11, R12, R13 and R44 of crystals like NaClO3 (point group 23) have been described in detail. The non-centrosymmetric class 43 m. which is not optically active has one non-vanishing piezo-rotatory coefficient, showing that stress induces optical activity in it. A method of retrieving this coefficient has also been described. In most of the other point groups, the difficulty of measuring optical activity in directions other than those of the optic axes severely limits the number of coefficients that can be extracted. The paper also touches upon some interesting methods of obtaining the components of the gyration tensor in non-enantiomorphic optically active crystals.

• # Proceedings – Section A

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