High-resolution measurements of the thermal expansion of superconducting Co-doped BaFe₂As₂

High-resolution thermal expansion measurements of single crystalline BaFe_1.84Co_0.16As₂ and BaFe_1.77Co_0.23As₂ in the temperature range 5<T<300 K are reported. The thermal expansion is highly anisotropic, with the largest expansion along the c axis. Distinct anomalies are present at the normal-to-superconducting phase-transition temperature T_c; the phase transition appears to be continuous. No structural transitions are observed over the temperature range of our measurements. The thermal expansion data and heat-capacity data acquired on the same specimens are used to estimate the volumetric pressure derivative of T_c using the Ehrenfest relation.