Research on the
lanthanide-doped upconversion (UC) energy systems continues to be a vibrant and
growing interdisciplinary field, essentially in two directions. One of
them is tuning of the optical properties such as the high UC efficiency and
emission profile in well-established energy UC systems, adopting different
synthetic strategies, surface modification, and multicolor emission
optimization. The other direction has been controlling the size, shape, and
phase purity of the crystals of these systems from the applications point of
view. In addition, the search for newer host fluoride lattices that are more
effective for the UC process is also being pursued with the concomitant aim of
understanding its role.
Experiments resulting in the successful
synthesis of hexagonally ordered KLaF4 have been described for the first time.
Syntheses from three different lanthanum precursors and KF under nonaqueous
conditions and at atmospheric pressure are presented. The temperature, time of
the fluorination reactions, and lanthanum precursor influenced the formation of
hexagonal KLaF4. While La(OiPr)3 and La(acac)3 yielded hexagonal KLaF4 by their
reaction with KF in methanol at 65 °C, LaCl3 favored only the formation of
cubic KLaF4 at 25 °C (room temperature). Size-induced phase transformation from
cubic KLaF4 to its hexagonal polymorph has been proposed for the reactions
involving La(acac)3 and La(OiPr)3 and KF. Rietveld refinement of the powder
X-ray diffraction pattern of the hexagonally ordered KLaF4 was successfully
carried out in space group P6̅2m (No. 189) with lattice constants a = 6.5842(3)
Å and c = 3.8165(3) Ǻ. A relatively lower effective phonon energy of 262 cm–1
observed for the hexagonally ordered KLaF4 (determined from its Raman spectrum)
suggests its potential as a host for optically active elements with the
possibility of minimized nonradiative processes. The hexagonal KLaF4 sample was
doped with Er3+ ion (3 mol %) and systematically investigated by diffuse
reflectance, normal emission, and upconversion studies. Strong green emission
(4S3/2, 2H11/2 → 4I15/2) has been observed upon 980 and 460 nm excitation. A
highly transparent light-emitting polymer [poly(methyl methacrylate)] composite
containing hexagonal KLaF4:Er3+ phosphor has also been effectively demonstrated
for many potential applications
Fuente:
Shahzad, A.; Vijaya P.; Nagarajan. Hexagonally
Ordered KLaF4 Host: Phase-Controlled Synthesis and Luminescence Studies. J. Am.
Chem. Soc., 2012, 10.1021
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