RAMAN-SCATTERING STUDY AND LATTICE-DYNAMICS INVESTIGATION OF THE NAMNF3 PEROVSKITE

In the framework of the study of structural phase transitions in fluoroperovskites AMF3, sodium manganese fluoride NaMnF3 is especially interesting because it is a rare distorted perovskite that can be obtained easily as a monodomain sample in an orthorhombic low-temperature phase. X-ray experiments were performed to confirm this quasimonodomain character. Raman spectra of NaMnF3 were recorded and interpreted between 40 and 573 K. From group-theory analysis it was shown that the orthorhombic-cubic high-temperature phase transition which occurs in NaMnF3 can be imputed to the condensation of vibrational modes located at the Γ, R, M, and X points of the first cubic Brillouin zone. Assuming that the Raman modes which are not responsible for the transition are temperature independent and with help from compatibility diagrams, frequencies of high-symmetry zone boundary modes in the high-temperature phase (cubic symmetry) were deduced only from experimental data obtained in the orthorhombic phase. Consequently these data were used to adjust the parameters of a rigid-ion model in the ideal cubic phase. The calculated phonon spectrum and the phonon density of states were deduced. The calculated cubic elastic constants C11, C12, C44 are consistent with the elastic constants measured by Brillouin scattering in the quasicubic approximation. Moreover, the reported Raman study provides evidence for the occurrence of a two-magnon scattering due to antiferromagnetic character below TN =60 K with persistence of a broad line above the Néel temperature. © 1995 The American Physical Society.