Violet phosphorus, or Hittorf’s phosphorus, was first obtained in 1865 by both sublimation and lead flux growth [1], its structure was revealed in 1969[2]. The interest to the violet phosphorus has recently returned as to the parent of 2D violet phosphorene, promising for advanced optoelectronic. Presently, the information on the elasticity and, more generally, lattice dynamics, is scarce.

Our first step was to establish the robust constrains on the elasticity of violet phosphorus and derived materials (i.e. phosphorene), in a way we explored previously for graphite [3] and BN [4]. As crystalline quality of commercially available crystals was clearly compromising for the inelastic x-ray scattering experiment, we have attempted to grow high quality crystals of violet phosphorus in-house. The result serendipitously brought something beyond the expectations.

Using the inelastic x-ray scattering (IXS) at ID28 beamline of ESRF, we were indeed able to determine the main components on the elastic tensor of violet phosphorus from the phonon dispersion. It is worth noting that thus obtained elastic moduli approach the intrinsic values, as the IXS signal is essentially collected from the domains of coherent scattering.

But – the extensive screening of violet phosphorus crystals, collected from the synthesis following the described protocol [5], at the diffraction/diffuse scattering station of the same beamline ID28, revealed crystals with unusual monoclinic angles and lattice dimensions. Within the same batch two new polymorphs of phosphorus were found beyond the “normal” violet phosphorus – and structures solved. All three polymorphs use the same unique building block, assembled in different ways.

Strong anisotropy and the domination of “intralayer” elastic moduli allow us to evaluate the components of elastic tensor for new polymorphs/polytypes. Simultaneously they serve as robust constraints for the intrinsic elasticity of phosphorenes, providing reference values for the modelling.

[1] W. Hittorf, Ann. Phys. 202, 193–228 (1865)

[2]  H. Thurn, H. Krebs, Acta Crystallographica B. 25, 125–135 (1969)

[3] A. Bosak, M. Krisch, M. Mohr, J. Maultzsch, C. Thomsen, Phys. Rev. B 75, 153408 (2007)

[4] A. Bosak, J. Serrano, M. Krisch, K. Watanabe, T. Taniguchi, H. Kanda, Phys. Rev.B 73, 041402 (2006)

[5] L. Zhang, M. Gu, L. Li, X. Zhao, C. Fu, T. Liu, X. Xu, Y. Cheng, J. Zhang, Chem. Mater. 32, 7363–7369 (2020)