Excimer-like Yb3+-pair and photoluminescence spectra of Yb3+-molybdates

We found a broad cooperative luminescence (CL) band due to Yb-pair at 470-520 nm in the up-conversion spectra of an Yb3+-molybdate, LiYb(MoO4)2 which was excited at 975 nm. Evidence is presented for reasonability of the assignment of the blue-green emission to CL. The emission from Yb-pair usually leads us to expectation in observation of its absorption from Yb-pair. Any absorption due to Yb-pair, however, was not found although several challenges were made. It is suggested that absence of the Yb-pair absorption is due to presence of excimer-like Yb-pair in LiYb(MoO4)2. Unusually much weaker CL intensity was observed than the up-converted emission from RE in very lightly RE-doped LiYb(MoO4)2 even at high 975 nm laser power, where RE = Tm3+, Eu3+, Er3+, Tb3+, and Ho3+. Discussion is made on the reason. The up-conversion measurement for Yb3+-compounds is useful (1) for detection of unwanted impurities even in carefully purified chemical compounds and (2) for generation of intense up-converted green-red emissions from RE3+-doped LiYb(MoO4)2.