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Charge-density-wave transitions, phase diagram, soft phonon and possible electronic nematicity: a thermodynamic investigation of BaNi2(As,P)2

A detailed investigation of BaNi2(As,P)2 single crystals using high-resolution thermal-expansion, heat-capacity,Young-modulus and resistivity measurements is presented. The experimental data are complemented by density-functional calculations. The phase diagram of BaNi2(As,P)2 is shown to be much richer than suggested by the original data of Kudo et al. [Phys. Rev. Lett. 109, 097002 (2012)]. The transition to the commensurate charge-density-wave (C-CDW) is always preceded by a four-fold symmetry-breaking transition associated with the long-range ordering of a strongly fluctuating unidirectional incommensurate charge-density wave (I-CDW). Significant precursors above the I-CDW and C-CDW transitions are seen in the thermal expansion and resistivity and are particularly evident in the temperature dependence of the $c/a$ ratio of the lattice parameters. Heat-capacity measurements of the crystals with a higher P content and a higher critical temperature of 3.2 K uncover a Debye-like behavior of a soft-phonon mode with a very low Debye temperature of roughly 15 K. Associated with this soft phonon are unusually large thermal-expansion anomalies, resulting in logarithmically diverging uniaxial phonon Grueneisen parameters. Young-modulus data of these higher-Tc crystals exhibit a significant softening in both B1g and B2g channels, which is argued to be incompatible with nematic criticality and is rather associated with a broad phase transition to an hitherto unknown structure. Possible origins of the increase in the superconducting critical temperature with P-substitution are discussed.