INCORPORATION OF OH SPECIES IN FLUOROZIRCONATE GLASSES - NATURE AND INFLUENCE ON PHYSICAL-PROPERTIES

The main purpose of the present study is to correlate the presence of controlled amounts of residual OH groups with the values of several ph ysical properties, namely, the characteristic temperatures, linear the rmal expansion coefficient, density, refractive index, microhardness, ultrasonic velocities, elastic constants and infrared transmission of ZrF4-based fluoride glasses. A series of glass samples were prepared i n the ZrF4-BaF2-LaF3-AlF3-NaF system, with increasing NaF contents. Ba tches for the reference (dry) glass samples were first dried in a vacu um oven and were then melted in Pt crucibles in a dry box filled with high purity N-2. Weakly hydrated glasses were prepared according to a similar procedure, but the batches were kept in water vapor saturated atmosphere for at least 24 h before fusion. Strongly hydrated glasses were melted in a fume hood, under a water vapor saturated stream of N- 2. For levels of OH incorporation up to 8 X 10(-4) mol l(-1) for ZBLA, 1.0 X 10(-3) mol l(-1) for ZBLAN(6.6) and 1.5 X 10(-3) mol l(-1) for ZBLAN(17.8) the density, refractive index and glass transition tempera ture, T-g, decrease with increasing OH content (by 0.8%, 0.25% and 3.0 %, respectively) is similar to what has been observed for v-SiO2. At O H levels greater than 8 x 10(-4) mol l(-1) for ZBLA, 1.0 X 10(-3) mol l(-1) for ZBLAN(6.6) and 1.5 X 10(-3) mol l(-1) for ZBLAN(17.8), an in crease in physical properties, such as the density, refractive index a nd T-g with increasing OH content, was noticed (of 0.8%, 0.4% and 2.8% , respectively), similar to what has been observed in silicate glasses . This behavior strongly suggests the presence of hydrogen bonded Zr-O H species. A structural model is proposed for a typical fluorozirconat e ZBLAN glass, based on structural data and physico-chemical propertie s. The model allows a correlation between the effect of residual OH gr oups and the values of some of the most representative properties of t hese glasses.