Octahedral tilting dominated phase transition in compressed double perovskite Ba$_2$SmBiO$_6$

Abstract

The comprehension of structural behaviors in double perovskites is crucial for their functional optimization, especially when applying external regulations. Here, to inquire about potential structures with better magnetic performance, high-pressure phase transformation in double perovskite Ba$_2$SmBiO$_6$ was first investigated up to 50 GPa via in situ high-pressure x-ray diffraction and Raman spectroscopy. A pressure-induced phase transition from cubic Fm-3m to orthorhombic Pnma is discovered at 4.8 GPa, accompanied by the splitting of the diffraction peaks. Above 19.8 GPa, the new phase becomes distorted as shown by the peak recombination and broadening. The variation of Raman spectra also confirms the formation and distortion of the high-pressure phase during compression, through the evolution of Bi–O stretching, Bi–O bending, octahedral rotation, and Ba-sites translation modes. The analysis of tilt angles and distortion factor evinced that the octahedral BiO$_6$ tilting is the key factor for the phase transition occurrence. Based on the Mulliken populations analyses, the Bi–O bonds undergo a covalent-ionic-antibonding transition across the phase transition under compression. Our exploration of the phase transition mechanism guides the modulation of the magnetic and electronic properties under extreme conditions