• M A Lonappan

      Articles written in Proceedings – Section A

    • Thermal expansion of potassium nitrate

      M A Lonappan

      More Details Abstract Fulltext PDF

      The principal coefficients of expansion have been determined for the orthorhombic crystal potassium nitrate for the range of temperature 30 to 100° C. by an X-ray method. The values are α11(b) = 22.0; α22(a) = 23.5; α33(c) = 182.6 × 10−6. The direction of the greatest expansion is perpendicular to the plane of the O3 group and the expansion is nearly constant for all directions in the plane of the O3 group. The approximate validity of the Grüneisen relation is tested by calculating the compressibility from a knowledge of the variation of the Raman frequency with temperature.

    • Thermal expansion of boric acid

      M A Lonappan

      More Details Abstract Fulltext PDF

      The principal coefficients of expansion and the orientation of the ellipsoid of expansion have been determined for orthoboric acid, a triclinic crystal, for the range of temperatures 30 to 80° C. by an X-ray method. The anisotropy of expansion is extremely large, the maximum expansion being 256 and the minimum −4, ×10−6. The direction of the maximum expansion coefficient is close to the normal to thec plane, to which all the O3 groups are closely parallel. The occurrence of a negative expansion is a special feature of this crystal, the direction of which is found to be parallel to the shortest hydrogen bonds. This is the first time that the thermal expansion of a triclinic crystal has been fully studied by an X-ray method.

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