Strong-Coupling Superconductivity in BaPd2As2 Induced by Soft Phonons in the ThCr2Si2-Type Polymorph

The specific heat of two polymorphs of BaPd2As2 was measured. The ThCr2Si2-type polymorph (space group I4/mmm, \(D_{4h}^{17}\), No. 139) is a previously reported superconductor with a transition temperature \(T_{\text{c}} \simeq 3.5\) K, while the CeMg2Si2-type polymorph (P4/mmm, \(D_{4h}^{1}\), No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr2Si2-type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg2Si2-type. Moreover, a large specific-heat jump at Tc indicated that the superconductivity of ThCr2Si2-type is a strong-coupling type, which is likely derived from soft phonons.