JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 1987 – 1995 Preparation and characterization of new niobophosphate glasses in the Li 2 O-Nb 2 O 5 -CaO-P 2 O 5 system ITALO ODONE MAZALI ∗ Laborat ´ orio de Qu´ ımica do Estado S ´ olido, Instituto de Qu´ ımica, UNICAMP, P.O.B. 6154, 13084-971 Campinas, SP, Brazil LUIZ CARLOS BARBOSA Instituto de F´ısica Gleb Wataghin, UNICAMP, P.O.B. 6165, 13083-970 Campinas, SP, Brazil OSWALDO LUIZ ALVES ‡ Laborat ´ orio de Qu´ ımica do Estado S ´ olido, Instituto de Qu´ ımica, UNICAMP, P.O.B. 6154, 13084-971 Campinas, SP, Brazil E-mail: oalves@iqm.unicamp.br; http://lqes.iqm.unicamp.br In the present work we describe the synthesis, spectroscopy, thermal and chemical durability properties of the vitreous system Li 2 O-Nb 2 O 5 -CaO-P 2 O 5 (LNCP). Investigations of the short-range order by Fourier transform infrared, Raman, UV-VIS and 31 P MAS-NMR spectroscopies suggest that the network former glass consists of Nb octahedra linked to pyro/orthophosphate units through Nb O P bonds. The presence of modiﬁer cations (Li + and Ca 2+ ) promotes depolymerization of the P O P chains, yielding pyro/orthophosphate units. The presence of this kind of structure accounts for the improvement of the chemical durability at low pH when the Nb content in the LNCP glass composition is high. The density and linear refractive indices of LNCP glasses increased linearly as the Nb 2 O 5 /P 2 O 5 molar ratio increased, as a consequence of P 2 O 5 substitution by Nb 2 O 5 as the glassformer. The dependence of the glass transition temperature, the softening temperature and the crystallization temperature on the Nb 2 O 5 /P 2 O 5 ratio exhibits the same behavior. On the other hand, the thermal expansion coefﬁcient decreases with the increased Nb 2 O 5 /P 2 O 5 ratio. C  2004 Kluwer Academic Publishers 1. Introduction Phosphate glasses have received considerable atten- tion in the past few years due to the synthesis of new glass compositions with high chemical stability. The improvement of chemical stability [1–3] stimulated the application of phosphate glasses in several ﬁelds of materials science, such as fast ionic conductors [4], semiconductors [5], photonic materials [6–9], hermetic seals [10], rare-earth ion host solid state lasers [11] and biomedical materials [12]. Calcium niobophos- phate glasses can be used as precursors to porous glass- ceramics with potential applications in the preparation of integrated chemical systems and nanocomposites [13]. Recently, we reported the fabrication of high per- formance waveguides by ion exchange (Ag + /Na + ) in a Na 2 O-P 2 O 5 -PbO-Nb 2 O 5 glass substrate [14], which is chemically more durable than commercially available glasses [3]. The properties of the phosphate glasses are deter- mined by the bond polarizability, which depends on ∗ Dedicated to the memory of Italo Mazali. ‡ Author to whom all correspondence should be addressed. their microscopic structures [7] and the network former. For example, the Na 2 O-TiO 2 -P 2 O 5 system presents a high non-linear refractive index that is explained in terms of modiﬁcation of the glass-forming network de- termined by the polarizability of the cation and anion bonds and by the presence of Ti in tetrahedral and/or octahedral sites [6–8]. Raman spectra of niobium sili- cate glass indicate that even a small addition of Nb 2 O 5 causes a strong polarized band to appear in the re- gion of 800–900 cm −1 [8]. This band has been as- cribed to Nb 5+ in an octahedra site. Lines has studied the inﬂuence of the empty d-orbital on the linear and nonlinear refractive indices of transparent transition- metal oxides, based on a bond-orbital theory, and that these results reveal that the most inﬂuential ions for the objective of increasing refractive indices are Ti 4+ , Nb 5+ and W 6+ and these are viable additives in this context [15, 16]. Niobophosphate glass compositions such as P 2 O 5 -Nb 2 O 5 -V 2 O 5 -TiO 2 and P 2 O 5 -Nb 2 O 5 - V 2 O 5 -Fe 2 O 3 , investigated by vibrational spectroscopy, 0022–2461 C  2004 Kluwer Academic Publishers 1987