Optical properties of chromium(III) in trigonal KAl(MoO4)2 and monoclinic NaAl(MoO4)2 hosts

Infrared, Raman, absorption, excitation and emission spectra as well as electron spin resonance spectra were measured for KCrxAl1−x(MoO4)2 and NaCrxAl1−x(MoO4)2 crystals (x varies from 0% to 2%) at temperatures ranging from 15 to 300 K. A very rich vibronic structure of the emission band was explained and assigned to the respective vibrational modes. One Cr3+ center characterized by 2.1 and 2.8 ms lifetimes (at 15 K) for the sodium and potassium derivative, respectively, for the 2E→4A2 transition was identified for both the crystals. The phase transition from trigonal to monoclinic space group was discovered at about 60 K for KAl(MoO4)2 : Cr3+ crystal. The local structure of the Cr3+ surrounding is discussed in terms of the spectroscopic results and the crystal field parameters are derived for both the materials.