J. Phys.: Condens. Matter 8 (1996) 9059–9069. Printed in the UK An optical study of silicate glass containing Cr 3+ and Cr 6+ ions M Casalboni†, V Ciafardone†, G Giuli‡, B Izzi†, E Paris‡ and P Prosposito§ † Dipartimento di Matematica e Fisica, e Istituto Nazionale di Fisica della Materia, Universit` a di Camerino, Via Madonna delle Carceri, 62032 Camerino (MC), Italy ‡ Dipartimento di Scienze della Terra, e Istituto Nazionale di Fisica della Materia, Universit` a di Camerino, Via Gentile III da Varano, 62032 Camerino (MC), Italy § Dipartimento di Fisica, e Istituto Nazionale di Fisica della Materia, Universit` a di Roma Tor Vergata, Via della Ricerca Scientifica, 1, 00133 Roma, Italy Received 15 April 1996, in final form 25 July 1996 Abstract. The optical properties of chromium-doped sodium silicate glass synthesized under oxidizing conditions have been investigated. The method of preparation provides a large concentration of Cr 6+ ions, but absorption and emission spectra showed that a small amount of Cr 3+ (and Cr 5+ ) ions are always present. The Dq -, B- and C-parameters of the crystal- field theory for the Cr 3+ ion were obtained, confirming the glassy nature of the sample. Cr 3+ luminescence was observed in the near-infrared region, even at room temperature, confirming the low value of the crystal-field Dq . Moreover, the high ultraviolet transparency of this matrix allows observation of a well resolved absorption spectrum of the Cr 6+ ion. 1. Introduction In the last three decades a great effort has been devoted to the study of glasses containing transition metal and rare-earth element impurities [1–3]. One of the most investigated impurity ions is Cr 3+ , and the large number of review articles and papers testifies to the high level of interest in this field [4–6], even in connection with the development of lasers. Currently, the basic features of Cr 3+ in a large number of crystalline matrices are so well established that this ion is extensively used as a probe for studying the structure and the local symmetry of new and exotic materials. Moreover, in applications ranging from new optical material research to optoelectronic and integrated optical devices, there is an increasing interest in glasses, due to their lower production costs with respect to single crystals. In this regard an interesting point is whether the host matrix is fully glassy or some crystallites are present. In this regard also, the study of the optical properties of the Cr 3+ ions can help to solve the problem. In fact, the optical properties of ion impurities in glasses show differences with respect to the corresponding crystalline matrices. In particular, the emission properties in the glass are characterized by broader emission spectra, a radiation lifetime with a non-exponential decay law and a peculiar temperature dependence of the quantum efficiency [7]. In nature chromium is known to occur both as Cr 6+ and Cr 3+ . In minerals, the first is found to be tetrahedrally coordinated by oxygen as [CrO 4 ] 2− or [Cr 2 O 7 ] 2− with Cr–O mean distances ranging from 1.60 to 1.67 ˚ A. Cr 3+ favours instead distorted octahedrally 0953-8984/96/469059+11$19.50 c  1996 IOP Publishing Ltd 9059