Thermal expansion of potassium nitrate
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.