Magnetic structure and spin dynamics of quasi-one-dimensional spin-chain antiferromagnet BaCo2V2O8

We report a neutron diffraction and muon spin relaxation muSR study of static and dynamical magnetic properties of BaCo2V2O8, a quasi-one-dimensional spin-chain system. A proposed model for the antiferromagnetic structure includes: a propagation vector k_AF = (0, 0, 1), independent of external magnetic fields for fields below a critical value H_c(T). The ordered moments, of 2.18 \mu_B per Co ion, are aligned along the crystallographic c-axis. Within the screw chains, along the c axis, the moments are arranged antiferromagnetically. In the basal planes the spins are arranged ferromagnetically (forming zig-zags paths) along one of the axis and antiferromagnetically along the other. The temperature dependence of the sub-lattice magnetization is consistent with the expectations of the 3D Ising model. A similar behavior is observed for the internal static fields at different muon stopping sites. Muon time spectra measured at weak longitudinal fields and temperatures much higher than T_N can be well described using a single muon site with an exponential muon spin relaxation that gradually changes into an stretched exponential on approaching T_N. The temperature-induced changes of the relaxation suggest that the Co fluctuations dramatically slow down and the system becomes less homogeneous as it approaches the antiferromagnetic state.Comment: 7 pages, 9 figure

Pressure-induced phase transition and band gap decrease in semiconducting β-Cu2V2O7

The understanding of the interplay between crystal structure and electronic structure in semiconductor materials is of great importance due to their potential technological applications. Pressure is an ideal external control parameter to tune the crystal structures of semiconductor materials in order to investigate their emergent piezo-electrical and optical properties. Accordingly, we investigate here the high-pressure behavior of the semiconducting antiferromagnetic material β-Cu2V2O7, finding it undergoes a pressure-induced phase transition to γ-Cu2V2O7 below 4000 atm. The pressure-induced structural and electronic evolutions are investigated by single-crystal X-ray diffraction, absorption spectroscopy and ab initio density functional theory calculations. β-Cu2V2O7 has previously been suggested as a promising photocatalyst for water splitting. Now, these new results suggest that β-Cu2V2O7 could also be of interest with regards to barocaloric effects, due to the low phase -transition pressure, in particular because it is a multiferroic material. Moreover, the phase transition involves an electronic band gap decrease of approximately 0.2 eV (from 1.93 to 1.75 eV) and a large structural volume collapse of approximately 7%.The authors acknowledge financial support from the Spanish Research Agency (AEI) and Spanish Ministry of Science and Investigation (MCIN) under projects PID2019106383GBC41/ C43/C44 (DOI: 10.13039/501100011033), and projects PGC2018-101464−B-I00 and PGC2018-097520-A-I00 (cofinanced by EU FEDER funds). The authors acknowledge financial support from the MALTA Consolider Team network, under project RED2018-102612-T. R.T. acknowledges funding from the Spanish Ministry of economy and competitiveness (MINECO) via the Juan de la Cierva Formación program (FJC2018-036185-I). J.G.P. thanks the Servicios Generales de Apoyo a la Investigación (SEGAI) at the University of La Laguna. A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025, and the authors would also like to thank them for funding under the Grant Prometeo/2018/123 (EFIMAT). The authors also thank ALBA synchrotron light source for funded experiment under proposal numbers 2020074389 and 2020074398 at the MSPD-BL04 beamline

A New One-Dimensional Spin Chain System Co₃(BPO₄)₂(PO₄)(OH)₃ Showing 1/3 Magnetization Plateau

A new borophosphate Co₃(BPO₄)₂­(PO₄)­(OH)₃ was synthesized by a conventional hydrothermal method. The titled compound crystallizes in the monoclinic system with space group <i>Cc</i>, which exhibits a typical spin-chain structure. In the structural framework, Co²⁺ ions form a zigzag chain via edge-sharing oxygen atoms, and further the zigzag Co-chains are separated by linear [B₂P₂O₈]_∞ chains. Magnetic measurements confirm that Co₃(BPO₄)₂­(PO₄)­(OH)₃ possesses an antiferromagnetic ordering at <i>T</i>_N = 12 K, while a 1/3 plateau is observed in the magnetization curve at 2 K. The possible spin arrangements for antiferromagnetic, ferrimagnetic, and ferromagnetic states are also suggested

Layered Cu₇(TeO₃)₂(SO₄)₂(OH)₆ with Diluted Kagomé Net Containing Frustrated Corner-Sharing Triangles

The half-spin Kagomé antiferromagnet is one of the most promising candidates for the realization of a quantum spin liquid state because of its inherent frustration and quantum fluctuations. The search for candidates for quantum spin liquids with novel spin topologies is still a challenge. Herein, we report a new diluted Kagomé lattice in Cu₇(TeO₃)₂(SO₄)₂(OH)₆, showing a 9/16-depleted triangle lattice, where the corner-sharing triangle units [Cu₅(OH)₆O₈] are separated by CuO₂(OH)₂. Magnetic measurements show that the title compound does not exhibit long-range antiferromagnetic order down to 2 K, suggesting strong spin frustration with <i>f</i> > 19

Layered Cu₇(TeO₃)₂(SO₄)₂(OH)₆ with Diluted Kagomé Net Containing Frustrated Corner-Sharing Triangles

The half-spin Kagomé antiferromagnet is one of the most promising candidates for the realization of a quantum spin liquid state because of its inherent frustration and quantum fluctuations. The search for candidates for quantum spin liquids with novel spin topologies is still a challenge. Herein, we report a new diluted Kagomé lattice in Cu₇(TeO₃)₂(SO₄)₂(OH)₆, showing a 9/16-depleted triangle lattice, where the corner-sharing triangle units [Cu₅(OH)₆O₈] are separated by CuO₂(OH)₂. Magnetic measurements show that the title compound does not exhibit long-range antiferromagnetic order down to 2 K, suggesting strong spin frustration with <i>f</i> > 19

K₄Fe₄P₅O₂₀: A New Mixed Valence Microporous Compound with Elliptical Eight-Ring Channels

A new small-pore compound K₄Fe₄P₅O₂₀ was obtained by conventional solid-state reaction in a closed crucible. The crystal structure is constructed by Fe₄P₅O₂₀ units forming chains along the <i>c</i> axis and elliptical eight-ring channels on the <i>a</i>–<i>b</i> plane in which K cations locate inside. Such structural characteristics seem to be quite similar to those seen in the natrolite family. However, Fe ions in K₄Fe₄P₅O₂₀ have trigonal–bipyramidal instead of common tetrahedral coordination. Furthermore, our experimental results combined from magnetic susceptibility and ⁵⁷Fe Mössbauer spectrum measurements show mixed valence Fe³⁺/Fe²⁺ in the titled material. To the best of our knowledge, this is the first example that contains mixed valence iron ions in a so-called natrolite framework