Spin–glass Magnetism in RFeTi2O7 (R=Lu and Tb) Compounds

AbstractThe compounds RFeTi2O7 (R=Lu and Tb) crystallize at room temperature in centrosymmetric orthorhombic space group Pcnb. There are five non-equivalent positions of the iron ions: the two positions, Fe’ and Fe”, in the octahedron consisting of the Fe’ tetrahedron and Fe” five-vertex polyhedron and the three positions, Fe1, Fe2 and Fe3 in the mixed Fe-Ti octahedra [1]. The populations of the mixed Fe-Ti sites are different. The crystal structure features lead to atomic disorder in the distribution of the magnetic ions in this compound. From low temperature heat capacity, magnetization and frequency dependent ac susceptibility we conclude that both compounds undergo a spin glass transition at TSG=4.5 and 6K for R =Lu and Tb, respectively. Since Lu is not magnetic, in RFeTi2O7 the spin glass behavior is caused by the disordered distribution of the magnetic Fe3+ ions in the different crystallographic positions. The substitution of the magnetic and highly anisotropic Tb ion instead of Lu increases TSG because of the additional Tb-Fe exchange interaction, while the critical exponent of the frequency dependence on temperature hardly varies. The spin glass behavior in these crystalline compounds is caused by the presence of competitive interactions that lead to frustration

Spin state crossover in Co3BO5

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100-250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state. © 2021 American Physical Society

Synthesis and thermal transformation of a neodymium(III) complex [Nd(HTBA)₂(C₂H₃O₂)(H₂O)₂]·2H₂O to non-centrosymmetric oxosulfate Nd₂O₂SO₄

<div><p>Neodymium complex [Nd(HTBA)₂(C₂H₃O₂)(H₂O)₂]_n·2nH₂O (<b>1</b>) (H₂TBA = 2-thiobarbituric acid, C₄H₄N₂O₂S) has been synthesized in an aqueous solution at 80–90 °C. The crystal structure of <b>1</b> has been determined by the Rietveld method in space group P2₁/n, <i>a</i> = 8.5939(2), <i>b</i> = 22.9953(5), <i>c</i> = 10.1832(2) Å, <i>β</i> = 112.838(1)°, <i>Z</i> = 4, and <i>R</i> = 0.0181. In <b>1,</b> the Nd(III) is coordinated by four μ₂-HTBA^– ions through O, three oxygens from two μ₂-η² : η¹-bridging CH₃COO^– anions, and two terminal waters with a tri-capped trigonal prism structure. The prisms form an edge-contact pair through two O from two acetates. The pairs are connected by HTBA^– and form a 3-D framework. The principle product of thermal decomposition of <b>1</b> at >750 °C is Nd₂O₂SO₄ (<b>2</b>). The crystal structure of <b>2</b> has been obtained in space group I222, <i>a</i> = 4.1199(4), <i>b</i> = 4.2233(4), <i>c</i> = 13.3490(12) Å, <i>Z</i> = 2, and <i>R</i> = 0.0246. The structure is related to an orthorhombic structure type of M₂O₂SO₄ (M = Ln) compounds. In <b>2</b>, the Nd³⁺ is coordinated by six oxygens in a trigonal prism. Each NdO₆ prism links with two SO₄ tetrahedra by nodes, with four other NdO₆ prisms by edges, and with four other NdO₆ prisms by nodes, and the units form the 3-D frame. In the frame, the layers of SO₄ tetrahedra are alternated by two NdO₆ prism layers.</p></div