Structural characterization, thermal, spectroscopic and magnetic studies of the (C 3H 12N 2) 0.75[Mn 1.50IIFe 1.50III(AsO 4)F 6] and (C 3H 12N 2) 0.75[Co 1.50IIFe 1.50III(AsO 4)F 6] compounds

Abstract

The (C 3H 12N 2) 0.94[Mn 1.50Fe 1.50 III(AsO 4)F 6] and (C 3H 12N 2) 0.75[Co 1.50Fe 1.50 III(AsO 4)F 6] compounds 1 and 2 have been synthesized using mild hydrothermal conditions. These phases are isostructural with (C 3H 12N 2) 0.75[Fe 1.5 IIFe 1.5 III(AsO 4)F 6]. The compounds crystallize in the orthorhombic Imam space group. The unit cell parameters calculated by using the patterns matching routine of the FULPROOF program, starting from the cell parameters of the iron(II),(III) phase, are: a = 7.727(1) Å, b = 11.047(1) Å, c = 13.412(1) Å for 1 and a = 7.560(1) Å, b = 11.012(1) Å, c = 13.206(1) Å for 2, being Z = 8 in both compounds. The crystal structure consists of a three-dimensional framework constructed from edge-sharing [M II(1) 2O 2F 8] (M = Mn, Co) dimeric octahedra linked to [Fe III(2)O 2F 4] octahedra through the F(1) anions and to the [AsO 4] tetrahedra by the O(1) vertex. This network gives rise two kinds of chains, which are extended in perpendicular directions. Chain 1 is extended along the a-axis and chain 2 runs along the c-axis. These chains are linked by the F(1) and O(1) atoms and establish cavities delimited by eight or six polyhedra along the [1 0 0] and [0 0 1] directions, respectively. The propanediammonium cations are located inside these cavities. The thermal study indicates that the structures collapse with the calcination of the organic dication at 255 and 285 °C for 1 and 2, respectively. The Mössbauer spectra in the paramagnetic state indicate the existence of two crystallographically independent positions for the iron(III) cations and a small proportion of this cation in the positions of the divalent Mn(II) and Co(II) ones. The IR spectrum shows the protonated bands of the H 2N– groups of the propanediamine molecule and the characteristic bands of the [AsO 4] 3− arsenate oxoanions. In the diffuse reflectance spectra, it can be observed the bands characteristic of trivalent iron(III) cation and divalent Mn(II) and Co(II) ones in a distorted octahedral symmetry. The calculated Dq and B-Racah parameters for the cobalt(II) phase are 710 and 925 cm −1, respectively. The ESR spectra of compound 1 maintain isotropic with variation in temperature, being g = 1.99. Magnetic measurements for both compounds indicate that the main magnetic interactions are antiferromagnetic in nature. However, at low temperatures small ferromagnetic components are detected, which are probably due to a spin decompensation of the two different metallic cations. The hysteresis loops give values of the remnant magnetization and coercive field of 84.5, 255 emu/mol and 0.01, 0.225 T for phases 1 and 2, respectively.