INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 14 (2002) 12383–12389 PII: S0953-8984(02)37388-0 Structure determination and a vibrational study for the hexagonal elpasolite Cs 2 NaGaF 6 :Cr 3+ H N Bordallo 1,2 , X Wang 1 , K M Hanif 3 , G F Strouse 3 , R J M da Fonseca 4 , L P Sosman 4 and A Dias Tavares Jr 4 1 Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL 60439, USA 2 Hahn-Meitner-Institut-Berlin, SF-1 Gleinicker Straße, 100 D-14109, Berlin, Germany 3 Department of Chemistry, UC Santa Barbara, Santa Barbara, CA 93106, USA 4 Inst´ ıtuto de F´ ısica, UERJ, Rua S˜ ao Francisco Xavier, 524, Rio de Janeiro, 20550, Brazil E-mail: bordallo@hmi.de Received 23 May 2002 Published 15 November 2002 Online at stacks.iop.org/JPhysCM/14/12383 Abstract Single crystals of 0.5% Cr 3+ -doped Cs 2 NaGaF 6 were studied by means of x-ray diffraction and polarized Raman scattering. The crystal exhibits a unique stacking interaction, with a hexagonal structure with R ¯ 3m symmetry. Polarized Raman spectra recorded at 16 and 300 K reveal sidebands that are ascribed to the local vibrational of the [CrF 6 ] coordination unit, which differ markedly from those of the Cs 2 NaGaF 6 host lattice. The results are compared to findings for other elpasolite lattices, and the room temperature luminescence quantum yields are discussed in terms of a delicate balance between electron–phonon strength and lattice distortion. 1. Introduction Numerous studies have been dedicated to the optical properties of Cr 3+ as an impurity ion in an extended lattice [1], and more particularly in the development of solid-state lasers. The most prominent example is that of ruby, i.e. Cr 3+ in Al 2 O 3 , where extensive studies of the optical properties led to the development of the first laser ever operated [2]. In the research on broadband lasers, particular attention has been focused on Cr 3+ -doped crystalline hosts [3–7]. In this context, Cr-doped elpasolite provides an excellent model system in which to investigate crystal-field and vibronic coupling effects. In this system the electron–phonon (el– ph) interaction between the impurity centre and the host lattice is weaker than in oxide hosts, and a broad band dominates the emission spectrum. This is a fundamental property for tunable laser applications, given that the broader the emission band, the larger the tuning range. Recently [7], we observed at 300 K a broad vibronic band that extends from the visible to the near-infrared spectral region in Cs 2 NaMF 6 :Cr 3+ (M = Ga and Al) emission spectra. As 0953-8984/02/4712383+07$30.00 © 2002 IOP Publishing Ltd Printed in the UK 12383