An optically detected electron-paramagnetic-resonance study of the luminescent triplet state of fresnoite,Ba2TiSi2O8

The luminescent center of the fresnoite crystal has been studied by electron paramagnetic resonance with optical detection at temperatures between 0.5 and 10 K. The results prove that the strong blue-green luminescence originates from a spin triplet state that is centered on the Ti ion. The C₄ symmetry of the Ti site is lost on excitation through a static Jahn-Teller effect, and eight sets of equivalent, strongly inhomogeneously broadened optically detected EPR signals appear. The signals are described by a spin hamiltonian withS=1, |D| ≈ 70 GHz, |E|=3.02 GHz, g_zz=1.93, and g_xx=g_yy ≡ g₃=2.0023. The lifetimes of the three sublevels aret _x =3.4,t _y=30,t _z=23 ms; the dominant radiative decay is from τ_x and polarized perpendicular to thec axis.