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E.J.C., O.M., and C.P. acknowledge financial support from the Leverhulme Trust Research Project Grant No. RPG-2017-109. O.M. is grateful for funding via the Leverhulme Trust Early Career Fellowship ECF-2021-170. A.S.G. acknowledges funding through an EPSRC Early Career Fellowship EP/ T011130/1. A.S.G. and H.T. acknowledge funding through the Humboldt Foundation and the Max Planck Institute for Solid State Research. The authors thank the Science and Technology Facilities Council for beamtime allocated at ISIS through proposal RB1990046 (DOI: 10. 5286/ISIS.E.RB1990046) and the Swiss Muon Source at the Paul Scherrer Institute through proposal numbers 20150959 and 20211440. The authors are grateful for access to the MPMS3 instrument at The Royce Discovery Centre at the University of Sheffield (EPSRC grant no. EP/R00661X/1) and the PPMS instrument at the University of St. Andrews (EPSRC grant no. EP/T031441/1). Ba2CuTeO6 has attracted significant attention as it contains a two-leg spin ladder of Cu2+ cations that lies in close proximity to a quantum critical point. Recently, Ba2CuTeO6 has been shown to accommodate chemical substitutions, which can significantly tune its magnetic behavior. Here, we investigate the effects of substitution for non-magnetic Zn2+ impurities at the Cu2+ site, partitioning the spin ladders. Results from bulk thermodynamic and local muon magnetic characterization on the Ba2Cu1 – xZnxTeO6 solid solution (0 ≤ x ≤ 0.6) indicate that Zn2+ partitions the Cu2+ spin ladders into clusters and can be considered using the percolation theory. As the average cluster size decreases with increasing Zn2+ substitution, there is an evolving transition from long-range order to spin-freezing as the critical cluster size is reached between x = 0.1 to x = 0.2, beyond which the behavior became paramagnetic. This demonstrates well-controlled tuning of the magnetic disorder, which is highly topical across a range of low-dimensional Cu2+-based materials. However, in many of these cases, the chemical disorder is also relatively strong in contrast to Ba2CuTeO6 and its derivatives. Therefore, Ba2Cu1 – xZnxTeO6 provides an ideal model system for isolating the effect of defects and segmentation in low-dimensional quantum magnets. Publisher PDF

We present a comprehensive structural and magnetic characterization of the barlowite family of S = 1/2 kagomé magnets, Cu4(OH)6FX, where X = Cl, Br, or I. Through high-resolution synchrotron X-ray and neutron powder diffraction measurements, we reveal two sources of structural complexity within this family of materials, namely, compositional disorder of the halide species that occupy sites in between the kagomé layers and the positional disorder of the interlayer Cu2+ ions that persists well into the Pnma structural ground state. We demonstrate that understanding these inherent structural disorders is key as they correlate with the degree of partial order in the magnetic ground states of these quantum frustrated magnets.

Na-for-Rb cation exchange followed by K-for-Na cation exchange of RbNdM2O7 (M = Nb, Ta) yields the corresponding, metastable KNdM2O7 phases. Synchrotron X-ray and neutron powder diffraction data, combined with powder SHG data, reveal that the KNdM2O7 phases adopt a polar structure (space group Im2m) consisting of NdM2O7 perovskite double sheets stacked in a (0, 1/2, z) manner with K+ cations ordered within the 6-coordinate prismatic interlayer sites. The perovskite double sheets adopt an (a0b+c0/a0-b+c0) tilting distortion; however, unlike other A′AB2O7 phases, this distortion is not the origin of the noncentrosymmetric structure, which is attributed to a second-order Jahn–Teller distortion of the MO6 units. First-principles density functional theory (DFT) calculations confirm that the polar Im2m phase is more stable than the corresponding centrosymmetric alternative. The role of the A′- and A- cations in directing the stacking patterns and tilting distortions of A′AB2O7 phases is discussed with reference to hybrid improper ferroelectric behavior.

Recently, there has been much interest in hybrid improper ferroelectrics materials that adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related structures. Using a combination of synchrotron X-ray powder diffraction (XRD) and high-resolution neutron powder diffraction data, the temperature-dependent phase transitions of a series of n = 2 Dion–Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, a–a–c+/–(a–a–c+) distorted I2cm phase to an antipolar, a–b0c–/–(a–b0)c– distorted Cmca phase at T = 790 and 500 K for M = Nb and Ta, respectively. There is a subsequent transition to an a0a0c–/a0a0–c– distorted I4/mcm structure at 865 and 950 K for M = Nb and Ta, respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast, CsNdM2O7 undergoes a transition from a polar, a–a–c+ distorted P21am structure to an antipolar, a–b0c– distorted C2/m phase at T = 625 and 330 K for M = Nb and Ta, respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 and 820 K for M = Nb and Ta, respectively. A plot of Tc against the relative stability of the 4 polar Dion–Jacobson phases compared to the corresponding aristotype P4/mmm structures (calculated from first-principles density functional theory (DFT)) yields a strong linear relation, suggesting that Tc is not proportional to the enthalpy change at the ferroelectric phase transition.

A flurry of recent theoretical studies have predicted the existence of new polar materials among several families of layered perovskites, including the double-layered Dion-Jacobson phases. These predictions have opened up exciting new opportunities for both fundamental studies of the crystal chemistry of Dion-Jacobson phases and their application as components in next-generation memories and low-power electronic devices. However, with some rare exceptions, all known double-layered Dion-Jacobson phases are nonpolar. We use an integrated theoretical-experimental approach to show that several Dion-Jacobson phases that have previously been synthesized and characterized as nonpolar are in fact polar. Additional theoretical calculations reveal that the polar phases of these materials emerge through a hybrid improper or trilinear coupling mechanism. Finally, our work has highlighted the critical role of neutron diffraction in characterizing the structures of double-layered Dion-Jacobson phases, which are typified by subtle oxygen atom displacements not easily resolved using even synchrotron X-ray diffraction.