The structure of -doped oxy-fluoride transparent glass-ceramics studied by Raman scattering

¹ Novel Photonic Glasses Research Group, Centre for Advanced Materials University of Nottingham - Nottingham NG7 2RD, UK
² CNR-IFN, Istituto di Fotonica e Nanotecnologie - 38050 Povo (TN), Italy
³ INFM, Dipartimento di Fisica, Università di Trento - 38050 Povo (TN), Italy
⁴ Departamento de Engenharia de Materiais / INESC-ID, Instituto Superior Técnico Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Received: 19 August 2002
Accepted: 9 September 2003

Abstract

We show that the structure of transparent oxy-fluoride glass-ceramics formed by heat treatment of glasses of typical composition 32():9():31.5():18.5(): 5.5():3.5() mol% consists of diameter, -doped, β- nano-crystals embedded in a silica-based glass network and connected to it via non-bridging and anions, or fluorine linkages such as -- and --. It is proposed that the glass network structure is mostly chain-like and dominated by tetrahedra with two bridging and two non-bridging and/or atoms (Q² units). tetrahedra with zero and one bridging (Q⁰ and Q¹ units, respectively) are also present in the glass structure, in the approximate proportion , a characteristic which appears to be of primary importance. The flexible, chain-like glass-network, with many broken bonds, results in easy accommodation of the -doped nano-crystals, which are grown by heat-treatment of the precursor glass. The boson peak in the Raman spectrum of the precursor glass decreases in intensity upon ceramming and is partly converted to narrow crystalline peaks at lower frequency, consistent with the precipitation of crystalline nano-particles. It is suggested that the boson peak involves localized vibrations of broken or stretched - bonds. The mean free path for these vibrations increases with ceramming, which involves partial crystallization of the glass network, resulting in a shift of the boson peak vibrations to lower-frequency crystalline peaks.