The phonon dispersion in terbium iron borate TbFe3(BO3)4 has been measured by inelastic neutron scattering in a temperature range 180 < T < 350 K through the displacive structural transition at TS = 192.5 K and studied by ab initio calculations. Significant, but not complete, softening of the transverse acoustic (TA) branch has been observed at the corner of the Brillouin zone (Λ point) at temperatures T TS, in full agreement with theoretical calculations. The TA soft mode undergoes considerable broadening at the Λ point near the transition temperature that can be attributed to the anharmonic interference between transverse acoustic and optical modes.

Single crystals of solid solutions of HoFe3–x Ga x (BO3)4 with x = 0, 0.5, 1, 1.5, and 3 were obtained using flux synthesis. The conditions of the synthesis are described in detail. The structural properties of each of the synthesized samples were studied using X-ray powder diffraction analysis at several temperature points (303, 403, and 503 K). The structural parameters of the obtained samples and the “pure” compounds HoFe3(BO3)4 and HoGa3(BO3)4 were compared. The Raman spectra of the obtained solid solutions HoFe3–x Ga x (BO3)4 were studied in a wide temperature range (T = 10–400 K). The vibrational spectra and eigenvectors of the HoFe3Ga­(BO3)4 and HoGa3(BO3)4 in R32 phase and HoFe3Ga­(BO3)4 in P3121 phase were calculated within density functional theory. The features of the Raman spectra of HoFe2Ga­(BO3)4, HoFe2.5Ga0.5(BO3)4, HoFe3(BO3)4 crystals associated with the R32 → P3121 structural phase transition, which have a strong dependence on the degree of substitution x, were investigated. Peculiarities of the Raman spectra, which are associated with magnetic ordering in HoFe1.5Ga1.5(BO3)4, HoFe2Ga­(BO3)4, and HoFe2.5Ga0.5(BO3)4 crystals, were detected.

The GdFe3(BO3)4 crystal has attracted great interest as a magnetic-field-induced multiferroic. In this paper, we show that the multiferroic properties in this crystal can be induced by high pressure. At high pressures up to 50 GPa, created in diamond anvil cells, the structural and vibrational (phonon) properties of the GdFe3(BO3)4 crystal were studied. The structural phase transition was detected at about 23–25 GPa by Raman and synchrotron Mössbauer (NFS) spectroscopy. First-principle calculations of the crystal lattice dynamics at pressures below and above the structural transition were carried out. It was established that at pressures above the structural transition, the space group R32 of GdFe3(BO3)4 is changed to the polar space group R3, and the crystal becomes a ferroelectric. At the R32 → R3 transition, the displacement of the boron ion B(2) and oxygen O results in the formation of boron–oxygen B(2)­O4 tetrahedrons instead of the plane BO3 triangles. Meanwhile, the triangle oxygen environment of boron in the site B(1) remains unchanged. The nearest environment of the gadolinium ion also changes significantly. Instead of six oxygen ions in the R32 phase, the nearest surroundings of Gd in the R3 phase consist of nine oxygen ions forming a complex polyhedron. A large hysteresis of the transition indicates that this crystal remains a ferroelectric with a decrease in pressure to about ambient pressure.

In this report we prepared for the first time the orthorhombic Dy 1-x Ho x MnO 3 single crystals with x = 0, 0.1, 0.2, 0.3, and 0.4 using the flux technique. The post-growth processing and chemical and structural characterization of the synthesized samples were performed. Also we examined the samples obtained by their magnetic properties and the magnetic anisotropy in wide ranges of temperatures and magnetic fields.