Crystal Structure, Magnetic Order, and Vibrational Behavior in Iron Rare-Earth Borates

Abstract

Crystals of Fe3R(BO3)4 (R = Y, La, Nd) have been grown. Their structure and properties show close relations. The structures of these materials have been established by single-crystal X-ray diffraction. They crystallize in the huntite, CaMg3(CO3)4, structure type, trigonal system, space group R32 (No. 155), Z = 3, with unit-cell parameters (Å) a = 9.592(1), 9.578(1), and 9.563(2) and c = 7.597(7), 7.605(3), and 7.575(2), for R = La, Nd, and Y0.5Bi0.5, respectively. They contain RO6 trigonal prisms and smaller FeO6 octahedra forming layers and giving rise to Fe−R distances of ≃3.78 Å. RO6 polyhedra are interconnected within the layers by corner sharing with triangular BO3 groups of two types and FeO6 octahedra. FeO6 octahedra form 1D helicoidal chains with relatively short Fe−Fe distances, ≃3.18 Å. The temperature dependence of the magnetic susceptibility from 350 to 1.8 K for Fe3R(BO3)4 shows two maxima at about 260 and 35 K and, for R = Nd, another maximum at 6 K. The presence of magnetic order is attributed to magnetic Fe−Fe or Fe−O−Fe interactions. Infrared spectra in the range 4000−240 cm-1 show the ν1, ν3, and ν4 IR-active bands of the B(1)O33- triangular groups as well as ν2 of less symmetrical B(2)O33-.