Spin Exchange and Magnetic Dipole–Dipole Interactions Leading to the Magnetic Superstructures of MAs2O6 (M = Mn, Co, Ni)

ISODISTROTLaMTeO6 (M = Ga3+ and Mn3+): the critical role of electronic configuration and cation ordering in crystal structures

In this work, PbSb2O6-type oxides LaMTeO6 (M = Ga3+ and Mn3+) were synthesized and structurally characterized by Rietveld refinements against high-resolution X-ray powder diffraction data. The Ga3+/Te6+ partial ordering within the honeycomb-like two-dimensional [GaTeO6]3- anionic layer leads to the loss of the inversion center between Ga3+ and Te6+; however the inversion center on the 3̄-roto-inversion axis is preserved, thereby resulting in a 2-fold PbSb2O6-type superstructure by doubling the c-axis associated with a structural symmetry descending from the original P3̄1m to P3̄1c symmetry. In contrast, LaMnTeO6 (P21/c) adopts a monoclinically distorted 4-fold superstructure with lattice dimensions of a ≈ aH, b ≈ √3aH, c ≈ 2cH, where aH and cH represent the lattice parameters of trigonal PbSb2O6. The formation of this P21/c-superstructure is attributed to the combination of complete Mn3+/Te6+ ordering and the first-order Jahn-Teller distortion of Mn3+ with the electronic configuration of d4. Such a monoclinic distortion can effectively lift the Mn3+ spin moments arranged on the triangular sublattice, resulting in a sharp peak for antiferromagnetic transition, which is in stark contrast to subtle magnetic transitions for PbSb2O6-type tellurates AMn(VI)TeO6 (A = alkaline earth and Pb2+) and LnCrTeO6 (Ln = rare earth) with higher structural symmetry. Our findings highlight that the electronic configuration effects of M-cations play a critical role in controlling the structure symmetry of LaMTeO6, providing a strategy to fine-tune the crystal structures and physical properties.

Successive short- and long-range magnetic ordering in rosiaite-type CoGeTeO6 prepared by ion-exchange reaction

The missing member of the rosiaite family, CoGeTeO₆, was synthesized by mild ion-exchange reactions and characterized by magnetization M and specific heat C_p measurements. It exhibits a successive short- and long-range magnetic ordering at T_short-range ≈ 45 K and T_N = 15 K, respectively. Based on these measurements, the magnetic H-T phase diagram was established, showing two antiferromagnetic phases separated by a spin-flop transition. The reason why the pronounced short-range correlation occurs at a temperature nearly three times higher than T_N was found by evaluating the Co-O⋯O-Co exchange interactions using energy-mapping analysis. Although CoGeTeO₆ has a layered structure, its magnetic structure consists of three-dimensional antiferromagnetic lattices made up of rhombic boxes of Co²⁺ ions. The experimental data obtained at high temperatures agree well with the computational results by treating the Co²⁺ ions of CoGeTeO₆ as S = 3/2 ions, but the heat capacity and magnetization data were obtained at low temperatures by treating the Co²⁺ ion as a J_eff = 1/2 ion.

Crystal and Magnetic Structures of Melilite-Type Ba2MnSi2O7

Melilite-type Ba₂MnSi₂O₇ was synthesized by a standard powder solid-state reaction route, and its magnetic properties were studied at low temperature. The magnetic structure was found to be C-type pointing along the c axis from neutron powder diffraction, which is different from the G-type ordering previously reported in all other 2-2-4-2 melilites with manganese as the B'-site transition metal. Ab initio (density functional theory) and magnetic dipole-dipole calculations were used to understand the magnetic structure by determining the spin supersuperexchange parameters as well as the relative influence of spin-orbit coupling and the magnetic dipole-dipole interactions.

Three-dimensional magnetic interactions in quasi-two-dimensional PdAs2O6

Millimeter-sized PdAs₂O₆ single crystals are grown using the vapor transport technique. The magnetic order at [Formula: see text] K is studied by measuring magnetic properties, specific heat, and neutron single crystal diffraction. The anisotropic magnetic susceptibility and a metamagnetic transition observed in magnetic fields above 20 kOe suggest that the magnetic moment lies in the ab plane, consistent with the magnetic structure determined by neutron single crystal diffraction. Below 140 K, Pd²⁺ ions order ferromagnetically in the ab plane but antiferromagnetically along the crystallographic c axis. The ordered moment is refined to be 2.09(2) [Formula: see text]/Pd²⁺ using the fitted magnetic form factor of Pd²⁺ . A weak λ-type anomaly around T _N was observed in specific heat and the magnetic entropy change across T _N is 1.72 J mol^-1 K.This small entropy change and the temperature dependence of the magnetic susceptibility support the presence of short range correlations in a wide temperature range [Formula: see text] 250 K. The comparison with SrRu₂O₆ suggests that the magnetic interactions in PdAs₂O₆ are dominated by Pd-(O-[Formula: see text]-O)-Pd super-superexchange and three dimensional despite the quasi-two-dimensional arrangement of magnetic ions. The comparison with NiAs₂O₆ suggests that increasing covalency of isostructural compounds is an effective approach to design and to discover new materials with higher magnetic order temperatures in the localized regime.

Structure and Magnetic Behavior of Layered Honeycomb Tellurates, BiM(III)TeO6 (M = Cr, Mn, Fe)

New layered honeycomb tellurates, BiM(III)TeO₆ (M = Cr, Mn, Fe) were synthesized and characterized. BiM(III)TeO₆ (M = Cr, Fe) species crystallize in a trigonal space group, P3̅1c (No. 163), of edge-sharing M³⁺/Te⁶⁺O₆ octahedra, which form honeycomb-like double layers in the ab plane with Bi³⁺ cations located between the layers. Interestingly, the structure of BiMnTeO₆ is similar to those of the Cr/Fe analogues, but with monoclinic space group, P2₁/c (No. 14), attributed to the strong Jahn-Teller distortion of Mn³⁺ cations. The crystal structure of BiM(III)TeO₆ is a superstructure of PbSb₂O₆-related materials (ABB'O₆). The Cr³⁺ and Fe³⁺ cations are ordered 80% and 90%, respectively, while the Mn³⁺ ions are completely ordered on the B-site of the ABB'O₆ structure. BiCrTeO₆ shows a broad antiferromagnetic transition (AFM) at ∼17 K with a Weiss temperature (θ) of -59.85 K, while BiFeTeO₆ and BiMnTeO₆ show sharp AFM transitions at ∼11 K with θ of -27.56 K and at ∼9.5 K with θ of -17.57 K, respectively. These differences in the magnetic behavior are ascribed to the different concentration of magnetic nearest versus next-nearest neighbor interactions of magnetic cations due to the relative differences in the extent of M/Te ordering.

PbMn(IV)TeO6: A New Noncentrosymmetric Layered Honeycomb Magnetic Oxide

PbMnTeO6, a new noncentrosymmetric layered magnetic oxide was synthesized and characterized. The crystal structure is hexagonal, with space group P6̅2m (No. 189), and consists of edge-sharing (Mn(4+)/Te(6+))O6 trigonal prisms that form honeycomb-like two-dimensional layers with Pb(2+) ions between the layers. The structural difference between PbMnTeO6, with disordered/trigonal prisms of Mn(4+)/Te(6+), versus the similar chiral SrGeTeO6 (space group P312), with long-range order of Ge(4+) and Te(6+) in octahedral coordination, is attributed to a difference in the electronic effects of Ge(4+) and Mn(4+). Temperature-dependent second harmonic generation by PbMnTeO6 confirmed the noncentrosymmetric character between 12 and 873 K. Magnetic measurements indicated antiferromagnetic order at T(N) ≈ 20 K and a frustration parameter (|θ|/T(N)) of ∼2.16.