The nonlinear Zeeman and parastriction effects in luminescence spectra of image crystals

Splitting of the first excited image doublet state of image ions in the image crystals in external magnetic fields up to 5.5 T has been studied using measurements of luminescence spectra at low temperatures. In the magnetic field perpendicular to the image-symmetry axis of the crystal lattice, the splitting of the non-Kramers doublet is caused by the joint action of the nonlinear Zeeman effect and the piezospectroscopic effect related to parastriction. Experimental dependences of the doublet splitting on the magnetic field strength and direction are in good correspondence with the theoretical estimations obtained in the framework of the semiphenomenological crystal field model. It is shown that spectroscopic data present direct information on the forced magnetostriction in the basal plane of uniaxial Van Vleck paramagnets that contain non-Kramers rare earth ions. The measured concentration dependence of the doublet splitting can be explained by taking into account variations of the elastic constants with the concentration of image ions.