Complex terbium luminescence centers in spectral transformers based on CaSO$_4$

The blue and green terbium luminescence excitation spectra (5–15 eV) at a temperature of 9 or 300 K have been measured for a set of CaSO$_4$:Tb$^{3+}$ phosphors with calcium and terbium ions characterized by fundamental features, which were synthesized by the solid-state reaction method with variations in the composition of the raw material, the type of terbium compounds, and the concentration of introduced terbium. A comprehensive analysis of these data, emission spectra, and decay kinetics (with the rising stage) of the terbium luminescence has revealed that the main luminescence centers in an efficient phosphor with 4 at% Tb$^{3+}$ each contain four closely spaced Tb$^{3+}$ ions in calcium positions, as well as Na$^+$ ions and hydrogen-containing radicals. In such complex luminescence centers, cooperative nonradiative resonant transitions in neighboring terbium ions leads to a nearly complete concentration of the terbium luminescence in the green spectral region $({}^5D_4\to{}^7F_J)$. The quantum yield of the green luminescence in CaSO$_4$ : Tb$^{3+}$ (4 at%) exceeds unity, $QY >$ 1, in the region of direct excitation of Tb$^{3+}$ into the 5d state ($\sim$ 5.9 eV) or excitation of oxyanions near the terbium ions ($\sim$ 8.1 eV).