Heat Capacities of Cubic, Monoclinic, and Vitreous Arsenious Oxide from 5 to 360°K

Experimentally determined heat capacities, Cp, precise to better than 0.1%, and derived thermal data are reported for cubic and monoclinic crystalline and vitreous arsenious oxide at temperatures from 5 to 360°K. Cp for the glass is higher than for either crystal at all temperatures. Cp's for the cubic crystal are lower than those for the monoclinic between about 20 and 310°K and higher at temperatures below and above this range. Cp / T3 for the glass is still increasing with decreasing temperature at 5°K, although Cp / T3 for both crystals peaks a few degress above this. Cp's reported here for the monoclinic crystal are grossly different from those previously reported by Stranski's school. The corresponding entropy difference between the two crystals at 298.15°K reported here is 5.9 ± 0.2 J deg−1·mole−1, whereas that previously reported from Cp measurements by Stranski's school was 14.4 J deg−1·mole−1. Combined with a value newly obtained elsewhere for the enthalpy difference between the two crystals at 298.15°K, the value reported here for the entropy difference yields a transition temperature between them of 311 ± 30°K, which is in the range currently cited on other bases. Cp values reported here for the cubic crystal and the glass are consistently 4%–5% lower than those previously reported by Tarasov's school, suggesting a systematic proportional effect of the magnitude in those previous values. The proportionality of Cp to T for the cubic crystal from 65 to 160°K, previously erroneously attributed to the monoclinic crystal by Tarasov on the basis of Anderson's Cp data, are discussed.