The ambient-pressure cubic Na₃Ir₃O₈ phase has been observed to undergo a symmetry-breaking transformation around 10 GPa [1]. Structure determination proved to be a challenge due to the large system size and the low symmetry of the new ground state. We performed extensive ab initio evolutionary searches [2] without any input from the high-pressure experiment and identified a complex monoclinic phase with 56 atoms per unit cell that agreed well with the collected XRD patterns. According to our ab initio calculations, the monoclinic phase features a dimerized Ir hyperkagome framework and a lower bulk modulus compared to that of the starting cubic phase. Study of the compound's electronic structure revealed significant convergence problems in the DFT+U approach.

[1] F. Sun, H. Zheng, Y. Liu, E. D. Sandoval, C. Xu, J. Xu, C. Q. Jin, C. J. Sun, W. G. Yang, H. K. Mao, J. F. Mitchell, A. N. Kolmogorov, and D. Haskel, Phys. Rev. B 98, 085131 (2018)
[2] https://github.com/maise-guide/maise