TABLE OF CONTENTS The lasing of trivalent chromium ions in a chryzoberyll crystal (alexandrite, Cr 3+:BeAl 2O 4) was reported for the first time on R-lines ( 2 E ? 4 A 2) in Ref. 33, and for 4 T 2 ? 4 A 2 electron-vibrational transitions 4 T 2 ? 4 A 2 in the investigations carried out in Ref. 34. In the alexandrite crystal, part of the Al 3+ions is isomorphously substituted by Cr 3+ ions. Alexandrite is characterised by high heat conductivity (23 W/m deg) which is still only half the heat conductivity of the ruby crystal, and the optical resistance to radiation of alexandrite is not inferior to that of the ruby crystal. Alexandrite is a biaxial crystal with a rhombic structure and its refractive index in the individual axes is: n a = 1.796, n b = 1.738 and n c = 1.746. The Moose hardness is 8.5, density 3.7 g/cm 4. The coefficients of thermal expansion of alexandrite are 7.0 (7.9; 9.5) 10 -6 deg -1, and the thermo-optical constant is dn/ dT = 9.4 (8.3) 10 -6 deg -1. The melting point of alexandrite is 1870 C, and the crystal is characterised by isotropic expansion with increasing temperature. A wider application of the alexandrite crystal is prevented by both the toxicity of beryllium (i.e. the component of alexandrite) and by the relatively complicated technology...
