title: Hexagonally ordered KLaF4 host: phase-controlled synthesis and luminescence studies.
abstract: Experiments resulting in the successful synthesis of hexagonally ordered KLaF(4) 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 KLaF(4). While La(O(i)Pr)(3) and La(acac)(3) yielded hexagonal KLaF(4) by their reaction with KF in methanol at 65 °C, LaCl(3) favored only the formation of cubic KLaF(4) at 25 °C (room temperature). Size-induced phase transformation from cubic KLaF(4) to its hexagonal polymorph has been proposed for the reactions involving La(acac)(3) and La(O(i)Pr)(3) and KF. Rietveld refinement of the powder X-ray diffraction pattern of the hexagonally ordered KLaF(4) 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 KLaF(4) (determined from its Raman spectrum) suggests its potential as a host for optically active elements with the possibility of minimized nonradiative processes. The hexagonal KLaF(4) sample was doped with Er(3+) ion (3 mol %) and systematically investigated by diffuse reflectance, normal emission, and upconversion studies. Strong green emission ((4)S(3/2), (2)H(11/2) → (4)I(15/2)) has been observed upon 980 and 460 nm excitation. A highly transparent light-emitting polymer [poly(methyl methacrylate)] composite containing hexagonal KLaF(4):Er(3+) phosphor has also been effectively demonstrated for many potential applications.
authors.affiliation: Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India.
authors.forename: Shahzad
authors.initials: S
authors.lastname: Ahmad
authors.forename: G Vijaya
authors.initials: GV
authors.lastname: Prakash
authors.forename: R
authors.initials: R
authors.lastname: Nagarajan