Successive magnetic transitions and multiferroicity in layered honeycomb BiCrTeO$_{6}$

Low-dimensional magnetic systems based on honeycomb lattices provide a promising platform for exploring exotic quantum phenomena that emerge from the intricate interplay of competing spin, orbital, lattice, and dipolar degrees of freedom. Here, we present a comprehensive study of the layered honeycomb lattice antiferromagnet BiCrTeO$6$ using magnetization, specific heat, muon spin–relaxation ($μ$SR) spectroscopy, dielectric, pyrocurrent, and high-resolution synchrotron X-ray diffraction (SXRD) measurements. Our results reveal an array of intriguing and strongly correlated phenomena, including two successive antiferromagnetic transitions at $T{\rm N1}\approx16$ K and $T_{\rm N2}\approx11$ K, a pronounced magnetodielectric coupling effect, and ferroelectric order at $T_{\rm N2}$. Consequently, this compound emerges as a new spin-driven multiferroic system. The SXRD analysis reveals a magnetoelastic-coupling-induced structural phase transition at $T_{\rm N2}$, characterized by a symmetry lowering from P$\bar{3}$1c (163) to P31c (159), which likely triggers the onset of ferroelectricity. In addition to its low-temperature multiferroic behavior, the system exhibits dielectric relaxor characteristics at higher temperatures within the paramagnetic region ($T<50$ K), which is intrinsically linked to the antisite disorder of Cr and Te atoms.

💡 Research Summary

In this work the authors present a comprehensive investigation of the layered honey‑comb oxide BiCrTeO₆, revealing it as a new spin‑driven multiferroic material. High‑resolution synchrotron X‑ray diffraction (SXRD) confirms that the compound crystallises in a trigonal P‾31c (No. 163) structure at room temperature, but a substantial antisite disorder between Cr³⁺ and Te⁶⁺ is present: the 2d and 2c Wyckoff sites are occupied by Cr:Te ≈ 0.62:0.38 and Te:Cr ≈ 0.62:0.38, respectively. This disorder, caused by the similar ionic radii of the two cations, creates a highly frustrated magnetic network in the two‑dimensional honey‑comb layers formed by edge‑shared CrO₆/TeO₆ octahedra.

Magnetisation measurements under 1 kOe show two clear anomalies at ≈ 16 K (TN1) and ≈ 11 K (TN2). The high‑temperature susceptibility follows a Curie–Weiss law with θCW = ‑69 K and an effective moment μeff ≈ 3.89 μB per Cr³⁺, indicating dominant antiferromagnetic (AFM) interactions. Isothermal M(H) curves remain linear and unsaturated down to 2 K, consistent with AFM order.

Specific‑heat data display a λ‑type peak at TN2 and a shoulder at TN1. After subtracting a Debye‑Einstein lattice contribution, the magnetic specific heat Cmag/T and the associated entropy change clearly mark two separate magnetic transitions, confirming that TN2 corresponds to the establishment of long‑range three‑dimensional AFM order, while TN1 likely reflects a short‑range or partially ordered state.

Zero‑field and weak‑transverse‑field μSR experiments down to 1.5 K detect two distinct internal‑field components appearing at the same temperatures, further corroborating the two-step magnetic ordering and suggesting a change in spin configuration between the two phases.

Dielectric measurements reveal strong frequency‑dependent permittivity and loss below 50 K, characteristic of a relaxor‑like response. This behavior is attributed to polar nanoregions generated by the Cr/Te antisite disorder, which creates local charge inhomogeneities. Pyrocurrent measurements show a sharp current peak at TN2 without a reversal of polarization on field cooling, indicating that ferroelectricity emerges only in the magnetically ordered state – a hallmark of spin‑driven multiferroicity.

High‑resolution SXRD performed across the low‑temperature range uncovers a subtle structural phase transition at TN2: the space group changes from P‾31c to P31c (No. 159). This symmetry lowering involves a slight distortion of the Cr/Te octahedra and activation of the stereochemically active Bi³⁺ 6s² lone pair, thereby generating a macroscopic electric polarization. The coincidence of the magnetic transition, structural symmetry breaking, and onset of ferroelectricity demonstrates a strong magneto‑elastic coupling.

In summary, BiCrTeO₆ exhibits (i) significant Cr/Te antisite disorder that enhances magnetic frustration, (ii) two successive antiferromagnetic transitions (TN1 ≈ 16 K and TN2 ≈ 11 K), (iii) a magneto‑elastic structural transition at TN2 that induces ferroelectricity, and (iv) relaxor‑type dielectric behavior at higher temperatures. The material therefore joins a very limited class of honey‑comb oxides where spin, lattice, and electric degrees of freedom are intimately intertwined, offering a promising platform for exploring field‑controlled multiferroic phenomena and for designing novel spintronic devices.