Frustration and Dzyaloshinsky-Moriya anisotropy in the kagome francisites Cu3Bi(SeO3)2O2X (X = Br, Cl)

We investigate the antiferromagnetic canting instability of the spin-1/2 kagome ferromagnet, as realized in the layered cuprates Cu3Bi(SeO3)2O2X (X = Br, Cl). While the local canting can be explained in terms of competing exchange interactions, the direction of the ferrimagnetic order parameter fluctuates strongly even at short distances on account of frustration which gives rise to an infinite ground state degeneracy at the classical level. In analogy with the kagome antiferromagnet, the accidental degeneracy is fully lifted only by nonlinear 1/S corrections, rendering the selected uniform canted phase very fragile even for spins-1/2, as shown explicitly by coupled-cluster calculations. To account for the observed ordering, we show that the minimal description of these systems must include the microscopic Dzyaloshinsky-Moriya interactions, which we obtain from density-functional band-structure calculations. The model explains all qualitative properties of the kagome francisites, including the detailed nature of the ground state and the anisotropic response under a magnetic field. The predicted magnon excitation spectrum and quantitative features of the magnetization process call for further experimental investigations of these compounds

Frustrated spin chain physics near the Majumdar-Ghosh point in szenicsite Cu3(MoO4)(OH)4

© 2017 American Physical Society. In this joint experimental and theoretical work magnetic properties of the Cu2+ mineral szenicsite Cu3(MoO4)(OH)4 are investigated. This compound features isolated triple chains in its crystal structure, where the central chain involves an edge-sharing geometry of the CuO4 plaquettes, while the two side chains feature a corner-sharing zigzag geometry. The magnetism of the side chains can be described in terms of antiferromagnetic dimers with a coupling larger than 200 K. The central chain was found to be a realization of the frustrated antiferromagnetic J1-J2 chain model with J1≃68 K and a sizable second-neighbor coupling J2. The central and side chains are nearly decoupled owing to interchain frustration. Therefore, the low-temperature behavior of szenicsite should be entirely determined by the physics of the central frustrated J1-J2 chain. Our heat-capacity measurements reveal an accumulation of magnetic entropy at low temperatures and suggest a proximity of the system to the Majumdar-Ghosh point of the antiferromagnetic J1-J2 spin chain, J2/J1=0.5

Spin-reorientation transitions in the Cairo pentagonal magnet Bi4 Fe5 O13 F

© 2017 American Physical Society. We show that interlayer spins play a dual role in the Cairo pentagonal magnet Bi4Fe5O13F, on one hand mediating the three-dimensional magnetic order, and on the other driving spin-reorientation transitions both within and between the planes. The corresponding sequence of magnetic orders unraveled by neutron diffraction and Mössbauer spectroscopy features two orthogonal magnetic structures described by opposite local vector chiralities, and an intermediate, partly disordered phase with nearly collinear spins. A similar collinear phase has been predicted theoretically to be stabilized by quantum fluctuations, but Bi4Fe5O13F is very far from the relevant parameter regime. While the observed in-plane reorientation cannot be explained by any standard frustration mechanism, our ab initio band-structure calculations reveal strong single-ion anisotropy of the interlayer Fe3+ spins that turns out to be instrumental in controlling the local vector chirality and the associated interlayer order

Hindered magnetic order from mixed dimensionalities in CuP2O6

We present a combined experimental and theoretical study of the spin-12 compound CuP2O6 that features a network of two-dimensional (2D) antiferromagnetic (AFM) square planes, interconnected via one-dimensional (1D) AFM spin chains. Magnetic susceptibility, high-field magnetization, and electron spin resonance (ESR) data, as well as microscopic density-functional band-structure calculations and subsequent quantum Monte Carlo simulations, show that the coupling J2D40 K in the layers is an order of magnitude larger than J1D3 K in the chains. Below TN8 K, CuP2O6 develops long-range order, as evidenced by a weak net moment on the 2D planes induced by anisotropic magnetic interactions of Dzyaloshinsky-Moriya type. A striking feature of this 3D ordering transition is that the 1D moments grow significantly slower than the ones on the 2D units, which is evidenced by the persistent paramagnetic ESR signal below TN. Compared to typical quasi-2D magnets, the ordering temperature of CuP2O6 TN/J2D0.2 is unusually low, showing that weakly coupled spins sandwiched between 2D magnetic units effectively decouple these units and impede the long-range ordering

Low-Temperature Structure and Thermoelectric Properties of Pristine Synthetic Tetrahedrite Cu₁₂Sb₄S₁₃

We have examined the low-temperature crystal structure and thermoelectric properties of unsubstituted synthetic tetrahedrite, Cu₁₂Sb₄S₁₃, a parent compound for modern state-of-the-art thermoelectric materials for midtemperature heat-to-power conversion. The crystal structure, space group <i>I</i>4̅3<i>m</i>, was probed by X-ray powder diffraction with synchrotron radiation at different temperatures within the range of 10–293 K. It displays subtle changes at the temperature of the metal-to-semiconductor transition (MST) near 90 K, at which a concerted displacement of two independent atoms occurs without symmetry reduction. The displacement of the sulfur atom toward the face of the Cu₆ octahedron, the shift of the copper atom toward the triangle S₃ plane composed of two independent sulfur atoms, and a sharp elongation of the CuSb separation upon the MST largely affect the transport properties of Cu₁₂Sb₄S₁₃. It displays sharp increase in electrical resistivity and a maximum in thermopower below the MST. On the contrary, the lattice part of thermal conductivity increases smoothly in the entire temperature range. Low thermal conductivity of Cu₁₂Sb₄S₁₃ is associated with the quazi-localized out-of-plane rattling of three-coordinated copper atoms, which softens with decreasing temperature responding to subtle structural changes upon the MST

Cation Ordering and Flexibility of the BO₄^2– Tetrahedra in Incommensurately Modulated CaEu₂(BO₄)₄ (B = Mo, W) Scheelites

The factors mediating cation ordering in the scheelite-based molybdates and tungstates are discussed on the basis of the incommensurately modulated crystal structures of the CaEu₂(BO₄)₄ (B = Mo, W) red phosphors solved from high-resolution synchrotron powder X-ray diffraction data. Monoclinic CaEu₂(WO₄)₄ adopts a (3 + 1)-dimensionally modulated structure [superspace group <i>I</i>2<i>/b</i>(αβ0)­00, <i>a</i> = 5.238 73(1)­Å, <i>b</i> = 5.266 35(1) Å, <i>c</i> = 11.463 19(9) Å, γ = 91.1511(2)°, <b>q</b> = 0.56153(6)<b>a</b>* + 0.7708(9)<b>b</b>*, <i>R</i>_F = 0.050, <i>R</i>_P = 0.069], whereas tetragonal CaEu₂(MoO₄)₄ is (3 + 2)-dimensionally modulated [superspace group <i>I</i>4₁/<i>a</i>(αβ0)­00(−βα0)­00, <i>a</i> = 5.238 672(7) Å, <i>c</i> = 11.548 43(2) Å, <b>q</b>_<b>1</b> = 0.55331(8)<b>a</b>* + 0.82068(9)<b>b</b>*, <b>q</b>_<b>2</b> = −0.82068(9)<b>a</b>* + 0.55331(8)<b>b</b>*, <i>R</i>_F = 0.061, <i>R</i>_P = 0.082]. In both cases the modulation arises from the ordering of the Ca/Eu cations and the cation vacancies at the A-sublattice of the parent scheelite ABO₄ structure. The cation ordering is incomplete and better described with harmonic rather than with steplike occupational modulation functions. The structures respond to the variation of the effective charge and cation size at the A-position through the flexible geometry of the MoO₄^2– and WO₄^2– tetrahedra demonstrating an alternation of stretching the B–O bond lengths and bending the O–B–O bond angles. The tendency towards A-site cation ordering in scheelites is rationalized using the difference in ionic radii and concentration of the A-site vacancies as parameters and presented in the form of a structure map

Multiple Twinning As a Structure Directing Mechanism in Layered Rock-Salt-Type Oxides: NaMnO₂ Polymorphism, Redox Potentials, and Magnetism

New polymorphs of NaMnO₂ have been observed using transmission electron microscopy and synchrotron X-ray powder diffraction. Coherent twin planes confined to the (NaMnO₂) layers, parallel to the (101̅) crystallographic planes of the monoclinic layered rock-salt-type α-NaMnO₂ (O3) structure, form quasi-periodic modulated sequences, with the known α- and β-NaMnO₂ polymorphs as the two limiting cases. The energy difference between the polymorphic forms, estimated using a DFT-based structure relaxation, is on the scale of the typical thermal energies that results in a high degree of stacking disorder in these compounds. The results unveil the remarkable effect of the twin planes on both the magnetic and electrochemical properties. The polymorphism drives the magnetic ground state from a quasi-1D spin system for the geometrically frustrated α-polymorph through a two-leg spin ladder for the intermediate stacking sequence toward a quasi-2D magnet for the β-polymorph. A substantial increase of the equilibrium potential for Na deintercalation upon increasing the concentration of the twin planes is calculated, providing a possibility to tune the electrochemical potential of the layered rock-salt ABO₂ cathodes by engineering the materials with a controlled concentration of twins

Compressibility of BiCu₂PO₆: Polymorphism against <i>S</i> = ¹/₂ Magnetic Spin Ladders

BiCu₂PO₆ is a unique example of a <i>S</i> = ¹/₂ ladder where the magnetic exchanges are mainly confined in 1D _∞[BiCu₂O₂]³⁺ cationic ribbons, although the shortest Cu–Cu separation between them exists. Its original magnetic topology gives the most representative example of a frustrated quantum ladder to investigate the complex physics behind it. Herein, we report the synthesis and characterization of one high-pressure polymorph. In this new phase, the preservation of 1D _∞[BiCu₂O₂]³⁺ units somewhat restacked leads to the preservation of its gapped magnetic ground state and ladder topology. The comparison of both compounds highlights the start of a thermodynamic conjuncture, where both the stable ambient-pressure (AP) and metastable high-pressure (HP) forms display the same equilibrium volume and superposed volume dependence of the energy, leading to a first-order AP → HP transition undetected by differential thermal analysis

Nontrivial Recurrent Intergrowth Structure and Unusual Magnetic Behavior of Intermetallic Compound Fe_32+δGe₃₃As₂

A new phase Fe_32+δGe₃₃As₂ (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe_32+δGe₃₃As₂ crystallizes in its own structure type (space group <i>P</i>6/<i>mmm</i>, <i>Z</i> = 1, <i>a</i> = 11.919(3) Å, <i>c</i> = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe₆Ge₆ and Co₂Al₅. Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe_32+δGe₃₃As₂ undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie–Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe_32+δGe₃₃As₂ reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K

Expanding the Ruddlesden–Popper Manganite Family: The <i>n</i> = 3 La_3.2Ba_0.8Mn₃O₁₀ Member

La_3.2Ba_0.8Mn₃O₁₀, a representative of the rare <i>n</i> = 3 members of the Ruddlesden–Popper manganites A_<i>n</i>+1Mn<i>_n</i>O_3<i>n</i>+1, was synthesized in an evacuated sealed silica tube. Its crystal structure was refined from a combination of powder X-ray diffraction (PXD) and precession electron diffraction (PED) data, with the rotations of the MnO₆ octahedra described within the symmetry-adapted mode approach (space group <i>Cccm</i>, <i>a</i> = 29.068(1) Å, <i>b</i> = 5.5504(5) Å, <i>c</i> = 5.5412(5) Å; PXD <i>R</i>_F = 0.053, <i>R</i>_P = 0.026; PED <i>R</i>_F = 0.248). The perovskite block in La_3.2Ba_0.8Mn₃O₁₀ features an octahedral tilting distortion with out-of-phase rotations of the MnO₆ octahedra according to the (Φ,Φ,0)­(Φ,Φ,0) mode, observed for the first time in the <i>n</i> = 3 Ruddlesden–Popper structures. The MnO₆ octahedra demonstrate a noticeable deformation with the elongation of two apical Mn–O bonds due to the Jahn–Teller effect in the Mn³⁺ cations. The relationships between the octahedral tilting distortion, the ionic radii of the cations at the A- and B-positions, and the mismatch between the perovskite and rock-salt blocks of the Ruddlesden–Popper structure are discussed. At low temperatures, La_3.2Ba_0.8Mn₃O₁₀ reveals a sizable remnant magnetization of about 1.3 μ_B/Mn at 2 K, and shows signatures of spin freezing below 150 K