The low-temperature thermal expansion and specific heat of glassy B2O3and two glassy sodium borates

The results of thermal expansion measurements using a three-terminal capacitance dilatometer for pure glassy B 2 O 3 (B 2 O 3 ) 0.99 (Na 2 O) 0.01 and (B 2 O 3 ) 0.90 (Na 2 O) 0.10 are reported in the temperature range 2010K. The isothermal compressibility has also been measured at room temperature. The linear term in the thermal expansion, attributed to the effect of two-level systems, is analyzed in comparison with that for other borate glasses-(B 2 O 3 ) 1-x (Na 2 O) x for x=0.06, 0.16 and 0.25-studied previously. The authors arrive at the conclusion that a negative cubic term in the thermal expansion gives rise to a negative expansivity in all borate glasses at the lowest temperatures (around 3K) and that the linear term characteristic of disordered systems is positive and grows with increasing Na 2 O content. They argue that this is due to the change in short-range coordination produced in B 2 O 3 glass by addition of Na 2 O. The first measurements of the specific heat of sodium boron oxide glass are reported at low temperatures.