High Pressure Research Vol. 26, No. 4, December 2006, 345–348 Pressure-induced luminescence from broadband to narrow-line emission in Cr 3+ -doped LiCaAlF 6 at room temperature M. N. SANZ-ORTIZ†, F. RODRÍGUEZ*†, I. HERNÁNDEZ†, R.VALIENTE‡ and S. KÜCK§ †DCITIMAC; ‡Dpto. Física Aplicada, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain §Physikalisch-Technische Bundesanstalt, Optics Division,Working Group 4.13: Laser Radiometry, Bundesallee 100, 38116 Braunschweig, Germany (Received 30 May 2006; revised 29 September 2006; in final form 20 October 2006) This work investigates pressure-induced excited-state-crossover phenomena in Cr 3+ -doped LiCaAlF 6 at room temperature. The aim is to find the structural requirements yielding the change in the Cr 3+ photoluminescence (PL) behaviour from a single broadband emission, which is located at 1.62 eV (765 nm) at ambient conditions, to a ruby-like narrow-line emission at 1.87 eV (662 nm). For this purpose, emission and excitation spectroscopy as well as lifetime measurements as a function of pressure were performed in the 0–35GPa range. Interestingly, we have been able to transform the broadband Cr 3+ PL at ambient pressure into a ruby-like emission at 28 GPa. This behaviour, together with the variations of the 2 E and 4 T 2 excited-state energies, and the PL lifetime with pressure, can be explained on the basis of the electron–ion coupling associated with the 4 T 2 and 2 E states. Keywords: Spectroscopy; High pressure; Photoluminescence; Cr 3+ ; Excited-state crossover 1. Introduction Pressure-induced excited-state-crossover (ESCO) in Cr 3+ -doped materials is a rather intriguing phenomenon that has motivated spectroscopists to find out new ways to modify and control the photoluminescence (PL) properties of materials. Besides applications as tuneable solid-state lasers and PL materials [1, 2], fluoroelpasolites [3], garnets [4] and nio- bates [5, 6] are attractive host materials for Cr 3+ to investigate ESCO processes since they provide crystal-fields (CF) around Cr 3+ , which are close to the 2 E ↔ 4 T 2 crossing point. Each excited state exhibits a rather different PL due to the different electron-lattice coupling. The spin-allowed 4 T 2 → 4 A 2 transition, whose energy depends linearly on the CF splitting 10 Dq, is strongly coupled to the lattice providing short-lived broadband PL. Nevertheless, the *Corresponding author. Email: rodriguf@unican.es High Pressure Research ISSN 0895-7959 print/ISSN 1477-2299 online © 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/08957950601104237