Preparation and properties of crystals of potassium and sodium azide

Abstract

When sodium or potassium azide crystals that have undergone shock or strain are observed in a polarizing microscope brightly coloured birefringent bands appear. Viewed along the c -axis these bands intersect at 120° in therhombohedral NaN 3 and at 90° in the tetragonal KN 3 . Each band is a plane extending through the crystal at an angle to the c -axis. From the thickness of the crystal and the width of the band one finds that these are (110) planes in NaN 3 and (112) planes in KN 3 , an observation verified by X-ray examination. Interference microscope measurements indicate that there is either a step or a ‘ramp’ on the surface where these planes emerge. Where the ‘ramp’ appears, lamellar twinning has occurred and the measured birefringence agrees with that calculated from the geometry and refractive indices. The steps might be unresolved ‘ramps’ or may be due to slip, a combination of slip and twinning, multiple twinning, or rapid deterioration of the ‘ramp’. Decomposition occurs readily at the intersection of these planes and the surface. Exposure to 10 7 r of γ -rays produces a deep groove at this line and many pyramidal etch pits. In addition, the methods used to grow the KN 3 and NaN 3 crystals are briefly described.