Phonon softening and possible nematicity in BaNi₂(As_1-xP_x)₂

BaNi₂As₂ undergoes a series of charge-ordering phase transitions upon cooling, first from a tetragonal to an incommensurately charge-ordered orthorhombic phase and then to a commensurate triclinic phase [1]. The suppression of these phase transition by either P-substitution of As, or by Sr-substitution of Ba leads to an increase of Tc up to about 3.5 K, which has been attributed to either soft phonon behavior [2] or B1g nematic fluctuations [3].  Here we study P-substituted BaNi₂As₂ using high-resolution thermal expansion, Young's modulus, heat capacity and transport measurements, from which a detailed phase diagram is obtained.  Both the phonon softening (via a Grueneisen analysis) and the question of nematicity (via Young's modulus data) will be addressed.

[1] M. Merz, et al., arXiv:2012.05024.

[2] K. Kudo, et al., Phys. Rev. Lett. 109, 097002 (2012).

[3] C. Eckberg et al., Nat. Phys. 16, 346 (2020).