VUV Optical Properties of Rare Earth Doped YPO₄ Prepared by Different Routes

Abstract

The optical properties of nanocrystalline YPO 4 :Ln 3+ (Ln = Eu, Sm, Tb) prepared via co-precipitation are compared to larger crystallites of YPO 4 :Ln 3+ prepared via traditional solid state reaction. In larger crystals (∼330 nm) a distinct peak is observed at 150 nm in the excitation spectra, the intensity of which decreases markedly in smaller crystals (∼20 nm). Using excitation and reflectance spectroscopy, host-to-activator energy transfer efficiencies were calculated for Y 1-x PO 4 :Ln x 3+ (0.01 ≤ x ≤ 0.10). From the transfer efficiency data, we estimate that trapping by Eu 3+ and Sm 3+ is at least five times more efficient than trapping by Tb 3+ for excitation at the band edge. The fraction of energy lost to the surface or grain boundaries for excitation at 150 nm and 138 nm is also estimated. We propose that in the samples prepared via co-precipitation, an amorphous phase forms at grain boundaries that is responsible for the loss of efficiency under 150 nm excitation. Rare-earth doped YPO 4 has been studied extensively, both for potential technological applications and in more fundamental investigations of electron transport and trapping processes.1-4 Nakazawa reported on the vacuum ultraviolet (VUV) spectroscopy of Ln 3+ doped microcrystalline YPO 4 , 4 and assigned excitations due to host, 4f -5d, and charge transfer transitions for all of the lanthanides (except Pm and Lu). Additional studies of YPO 4 include one by Lai et al. that described a precipitation route to ∼20 nm YPO 4 :Eu 3+ , but only the UV optical properties are discussed.5 Di et al. have compared the VUV optical properties of YPO 4 :Tb 3+ prepared via solid state reaction and co-precipitation. 6 However, unlike in the work presented here, the high calcination temperatures applied to the co-precipitated samples led to materials that were similar in size to solid state samples but with improved morphology. Van Pieterson et al. obtained high-resolution excitation spectra of YPO 4 doped with both light 7 and heavy 8 lanthanides at 6 K. They assigned spectral features and compared the experimentally observed spectra with energy level calculations for the 4f n-1 5d 1 states. Makhov et al. investigated the optical properties of YPO 4 :Nd 3+ under VUV synchrotron radiation over a temperature range of 9-300 K. Their work included the assignment of various hostsensitized excitation mechanisms that will be discussed more later. 9Several groups have also reported on the optical properties of doubledoped YPO 4 .10-12 Much of that work was interpreted in the context of the lanthanide energy level scheme for YPO 4 reported by Bos et al. 13 and Dorenbos and Bos, 14 in which the Ln 2+ and Ln 3+ ground state energies are placed relative to the host valence and conduction bands. Of interest to the current study is the observation that the excitation spectra of some lanthanides (particularly Sm, Eu, Gd and Tm) contain a distinct, intense peak for excitation right at the band edge of YPO 4 (∼150 nm). Since the focus of prior work was on assigning peaks in ...

Review—Progress in Understanding Host-Sensitized Excitation Processes in Luminescent Materials

“…In a second study, YPO 4 doped with Sm 3+ , Eu 3+ and Tb 3+ was prepared via a solid state route (resulting in crystallite sizes of about 330 nm) and compared to samples prepared via a co-precipitation route that yielded crystallite sizes of around 20 nm. 77 The 20 nm crystallites exhibited values of S loss of 0.31-0.42, while the 330 nm crystallites gave S loss values of 0.73-0.94. A sample of the data is presented in Figure 13, which compares the reciprocal plots obtained for 20 and 330 nm YPO 4 :Eu 3+ .…”