The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study
The temperature dependence of the crystal structures of the low-temperature α-phase and the intermediate-temperature β-phase of crystalline malononitrile, CH2(CN)2, have been determined using Rietveld refinement on neutron powder diffraction data. The results confirm that the α-phase has the same structure as the high-temperature γ-phase, and the refined structure of the β-phase is in agreement with previous structure determination using single-crystal x-ray diffraction. The variations of the structure with temperature due to the phase transitions are identified and analysed in terms of molecular rotations and spontaneous strains.
Across the structural re-entrant transition in BaFe2(PO4)2: influence of the two-dimensional ferromagnetism.
BaFe2(PO4)2 was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO6 octahedra containing high-spin Fe(2+) ions (S = 2) are isolated by PO4 groups and Ba(2+) cations. BaFe2(PO4)2 has a trigonal R-3 structure at room temperature but adopts a triclinic P-1 structure below 140 K due to the Jahn-Teller (JT) instability arising from the (t2g)(4)(eg)(2) configuration. The triclinic crystal structure was refined to find significantly distorted Fe(2+)O6 octahedra in the honeycomb layers while the distortion amplitude QJT was estimated to 0.019 Å. The JT stabilization energy is estimated to be ∼7 meV per formula unit by DFT calculations. Below ∼70 K, very close to the ferromagnetic transition temperature Tc = 65.5 K, the structure of BaFe2(PO4)2 returns to a trigonal R-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe(2+), as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above Tc , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe2(PO4)2 is uniaxial (g⊥ = 0).
Magnetic properties of Lu2Fe17 crystals
Abstract We present the results of crystal structure and magnetization studies of three single crystals grown from melts of different nominal compositions in the vicinity of the Lu2Fe17 stoichiometry. The ferromagnetic state of the crystals is strongly dependent on the real sample composition and the local atomic ordering. A drastic difference is observed in the behaviour of the ferro–antiferromagnetic transition, even in crystals for which almost identical composition and crystal parameters were determined by microprobe and X-ray analysis.