Disentangling the phase sequence and correlated critical properties in Bi_0.7La_0.3FeO₃ by structural studies

This work addresses the study of the high-temperature phase sequence of Bi0.7La0.3FeO3 by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that Bi0.7La0.3FeO3 exhibits an incommensurate modulated orthorhombic Pn21a(00γ)000 structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above T1=543K, the low-temperature modulated Pn21a(00γ)000 evolves monotonically into a fractionally growing Pnma structure up to TN=663K. At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated Pn21a structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The Pn21a space group is obtained from the Pnma one through the Γ4- polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below TN, revealed by the relation between the weak ferromagnetism of the canted iron spins and the FeO6 octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (β) value of 0.38. The lower limit of the phase coexistence temperature T1=543K, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.