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 $\alpha =(3.34\mathrm{}{T}^3+0.030\mathrm{}{T}^5)\times {10}^{-8\mathrm{}}$ ${\mathrm{K}}^{-1\mathrm{}}$ ($1<T<\mathrm{}6\mathrm{}$ K). When combined with other data, these results can be used to obtain Grüneisen 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 ${\mathrm{O}}_2$ impurities.