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
Shahzad, A.; Vijaya P.; Nagarajan. Hexagonally Ordered KLaF4 Host: Phase-Controlled Synthesis and Luminescence Studies. J. Am. Chem. Soc., 2012, 10.1021