Thermal expansion of boric acid

Summary

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.