First-principles study of elastic, dielectric, and vibrational properties of orthoferrites RFeO₃ (R = Ho, Er, Tm and Lu)

We perform first-principle calculations on a series of orthoferrites RFeO₃ (R = Ho, Er, Tm and Lu) to reveal the dependence of the mechanical, dielectric and vibrational properties on the size of the R elements. We prove that the orthoferrites RFeO₃ are mixtures of covalent and ionic bonds and have great capacity to resist the interior fracture and plastic deformation under the outside stress. In the assignment of the Raman spectra, the evolutionary relationship of lattice distortion, which is closely related to two soft mode frequencies, is quantified. We first report that the vibrational modes around 7.64 THz is sensitive to the radius of the rare earth and can prominently increase the vibration diversity of the center iron atom. Our results also demonstrate that not only the soft-mode frequencies but also the Born effective charges and dielectric constants are slightly sensitive to the atomic geometry. And this conclusion can be extended to the whole lanthanide system. We hope that our work can provide reference data for structural investigation and property measurements of the whole orthorhombic RFeO₃ family. (paper)