Thermal Expansion Coefficients for Solid Argon from 1 to 25 K

Measurements of the coefficient of linear thermal expansion accurate to 1% are reported for free-standing samples of solid argon for temperatures from 1 to 25 K. These data were obtained using a differential parallel-plate-capacitor technique and at low temperatures can be represented by α=(3.34T³+0.030T⁵)×10^-8 K^-1 ($1<T<6$ K). When combined with other data, these results can be used to obtain Gr\"uneisen parameters which are systematically 10% lower than those reported for anharmonic theories. Preliminary data also showed a low-temperature thermal expansion anomaly which is believed due to the spin-split ground state of O₂ impurities.