Charge transfer and f–f emission of trivalent ytterbium observed in double phosphates MIMIII(PO4)2 (MI = Na, Rb; MIII = Lu, Y)

Spectroscopic properties of Yb3+-doped MI3MIII(PO4)2 and (MI = an alkali metal: Na, Rb; MIII = trivalent cation Lu or Y) are reported. In order to study their optical characteristics, the absorption, emission and luminescence excitation spectra have been measured at 4, 10 and 293 K in the VUV to IR range. Systematic comparison of the absorption spectra, intensity of the f–f transitions and emission efficiency of ytterbium in all the types of the investigated compounds have been made. Luminescence of the samples has been excited using various excitation lines, including the synchrotron radiation. Strong fundamental 2F5/2 → 2F7/2 emission in the IR range has been observed. The double peak spectrum typical for the Yb3+ charge transfer (CT) luminescence has been recorded after synchrotron radiation excitation only in the sodium compounds. The possible process of the CT emission quenching in rubidium ytterbium double phosphates is considered. Possible mechanism of the energy transfer to the Yb3+ 4f states is discussed. Detailed analysis of the spectra of Yb3+-doped sodium and rubidium lutetium double phosphates has been performed to determine the energy levels in these hosts and to compare their optical behaviour with analogous sodium and rubidium yttrium double phosphates. The IR spectra have been used to assign the electronic and vibronic components. The tentative energy level scheme of the 2F7/2 ground and 2F5/2 excited levels is proposed.