Evolution of Charge Density Waves and Crystal Structure in BaNi2(As1-xPx)2

BaNi2As2, is the nickel-based nonmagnetic superconducting analogue of the iron-based compound BaFe2As2, which shares the same room temperature crystal structure (SG: I4/mmm). [1,2]. Recent, x-ray diffraction studies have provided evidence for the presence of an incommensurate charge density wave order (I-CDW) that sets in slightly above a 2nd order structural transition Ts1 (~142 K) from tetragonal to orthorhombic. This is followed by an abrupt 1st order structural transition Ts2 (137 K) to a triclinic crystal structure (SG: P1̅), as well as a new long-range commensurate charge density wave (C-CDW) [3,4]. One of the key open issues is the role of the crystal structure and its influence on the interplay between the observed electronic orders. Chemical substitution of arsenic for phosphorus, results in the suppression of the triclinic phase and the C-CDW, albeit with a substantial enhancement of the superconducting transition temperature from 0.6 K to 3.5 K [2]. However, the impact of physical pressure in combination with chemical pressure on the crystal lattice of BaNi2(As1-xPx)2 accompanied by the electronic orders has not been explored so far.
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In this contribution, we will report on a systematic x-ray diffraction study of the high-pressure phase diagram of high-quality BaNi2(As1-xPx)2 single crystals up to 13 GPa in a Diamond Anvil Cell, measured at the Synchrotron Facilities (ESRF & DESY) (Fig. 1). A cascade of new superstructures and several polymorphic phase transitions in BaNi2(As1-xPx)2 are observed and will be discussed in detail.
Acknowledgements: The authors are grateful to Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - TRR 288 for funding this project.
[1] F Ronning, N Kurita, E.D Bauer et al., J. Phys: Condens. Matter 20 (2008) 342203.
[2] K. Kudo, M. Takasuga, Y. Okamoto, Z. Hiroi, et al., Phys. Rev. Lett. 109 (2012) 097002.
[3] S. Lee, G. de la Peña, A.X.-L. Sun et al., Phys. Rev. Lett. 122, (2019) 147601.
[4] M. Merz, L. Wang, Th. Wolf et al., Phys. Rev. B 104, (2021) 184509.
Fig.1: