Charge-density-wave (CDW) order and superconductivity coexist in the kagome metals AV_3Sb_5 (A=K, Cs, Rb), raising fundamental questions about the mechanisms driving their intertwined phases. Here we combine high-resolution inelastic X-ray scattering with first-principles calculations to uncover the origin of CDW formation in CsV_3Sb_5. Guided by structure factor analysis, we identify a soft phonon mode along the reciprocal M-L direction, with the strongest effect at the L point, where the elastic scattering intensity also grows most rapidly upon cooling. First-principles calculations incorporating lattice anharmonicity and electron-phonon coupling reproduce these observations and establish a soft-mode instability at the L point as the driving mechanism of CDW formation. Despite the weakly first-order character of the transition, our results unambiguously demonstrate that the CDW in CsV_3Sb_5 originates from a softened phonon, clarifying its microscopic origin and highlighting the central role of lattice dynamics in kagome metals.

Magnetic ordering of PbMnTeO6 crystallizing in the chiral noncentrosymmetric P312 space group has been experimentally investigated by high-resolution neutron powder diffraction (NPD) and theoretically by high-temperature expansion, density matrix renormalization group, and linear spin-wave theory approaches. Processing of low-temperature NPD data showed that, to describe the rather complex spin ordering in PbMnTeO6, two incommensurate modulations determined by the propagation vectors k1 and k2 should be used. The observed ground state with k1 = (31, 31, 0.4077) is a noncollinear 120 degrees spin structure, which is helically ordered along the layer stacking direction with a characteristic spiral period of similar to 13.2 & Aring;. Additionally, the second incommensurate modulation for magnetic structure with the propagation vector k2 = (31 + 81, 31 + 81, 82) was also observed. An effective spin model for explaining the observed magnetic characteristics in PbMnTeO6 was also constructed from numerical calculations. The dominant nearest-neighbor interaction was estimated as J1 = 12 +/- 2 K by fits of magnetic susceptibility, the magnetization curve, and the magnetic part of the specific heat between experimental and numerical results. It is shown that spiral ordering along the layers stacking direction can be attributed to frustrated interlayer couplings or the presence of an interlayer Dzyaloshinskii-Moriya interaction. Also, the incommensurate modulation of the propagation vector in the ab plane can be reproduced by introducing a subtle anisotropic distortion of triangular lattice.

Disorder in frustrated quantum systems can critically influence their magnetic ground states and drive exotic correlated behavior. In the S = 1/2 system ktenasite, Cu_2.7Zn_2.3(SO_4)_2(OH)_6·6H_2O, we show that structural disorder drives an unexpected dimensional crossover and stabilizes a rare coexistence of distinct magnetic states. Neutron diffraction reveals significant Cu/Zn mixing at the Cu2 site, which tunes the Cu^2+ sublattice from a two-dimensional scalene-distorted triangular lattice into a one-dimensional spin-chain network. Magnetic susceptibility, neutron diffraction, ac susceptibility, and specific heat measurements collectively indicate magnetic duality: a coexistence of incommensurate long-range magnetic order below T_N = 4K and a cluster spin-glass state with T_f = 3.28K at ν= 10Hz. Our findings highlight ktenasite as a rare platform where structural disorder tunes the effective dimensionality and stabilizes coexisting ordered and glassy magnetic phases, offering a unique opportunity to explore the interplay of frustration, disorder, and dimensional crossover in quantum magnets.

This study is devoted to magnetic neutron diffraction analysis (elastic neutron scattering yielding information about spin ordering in magnetic materials). Experimental approaches to magnetism are intensely developed mainly due to the discovery of new functional materials with unusual properties. Nevertheless, magnetic neutron diffraction analysis is a unique method, which makes it possible to directly determine magnetic structures (i.e., values and mutual orientations of magnetic moments and their orientations relative to the crystallographic axes). Neutron diffraction proved to be a good instrument for studying both simple commensurate magnetic structures in bulk samples and more complex noncollinear spiral magnets. Magnetic neutron diffraction is widely applied for studying magnetic structures in nanoscale multilayer film materials. However, bulk samples with a layered crystal structure, which can be considered as natural quasi-two-dimensional objects due to the weakness of interlayer couplings, are of greater interest for physics of low-dimensional magnetism. Our neutron diffraction studies on quasi-two-dimensional magnets, transition-metal oxides A3M2SbO6 (A = Li or Na, M = Co or Ni) and rare-earth intermetallic compounds La1–xTbx(Ni1–xMnx)2Si2, are briefly considered.

The magnetic structure of quasi-two-dimensional (2D) honeycomb lattice Na2Ni2TeO6 has been determined by low-temperature neutron diffraction and the crystal structure fine details at room temperature have been established by a combination of synchrotron and neutron powder diffraction. The atomic structure is described by the P6(3)/mcm space group, but the strong presence of stacking faults defects in layered ordering is found for the first time. Both magnetization and specific heat data indicates an establishment of a long-range antiferromagnetic order with T-N = 25 +/- 1 K, preceded by a short-range order on 2D honeycomb lattice at about 34 K. Determined effective magnetic moment mu(eff) = 4.35 mu(B) is in excellent agreement with numerical estimation using effective g-factor g = 2.19 directly measured by electron spin resonance. The ground magnetic state at T = 1.5 K is represented by the commensurate zigzag-type magnetic order. Magnetic moments of Ni are almost perpendicular to the honeycomb layers, which antiferromagnetically coupled along the c-direction. Besides, the coherent magnetic scattering area has a disk shape that is homogeneous over the ab plane and compressed along the c-axis that indicates 2D nature of magnetic correlations in the compound. The magnetic diffuse neutron scattering related to the presence of strong short-range spin-spin correlations above T-N was observed. Based on our experimental measurements we propose magnetic phase diagram for Na2Ni2TeO6. (C) 2019 Elsevier B.V. All rights reserved.

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate Li2MnGeO4 were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the Pmn2(1) space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector k = (1/2 1/2 1/2) has been found below T-N approximate to 8 K. Symmetry analysis revealed the model of the ground state spin structure within the C(a)c (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of 4.9 mu(B)/Mn2+ at 1.7 K, which corresponds to the saturated value for the high-spin configuration S = 5/2. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above T-N. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in Li2MnGeO4, which is symmetry consistent on a small scale with the long-range magnetic state below T-N. The reconstructed radial spin-pair correlation function S(0)S(r) displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at r approximate to 12 angstrom at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below T-N. As a result, an exhaustive picture of the gradual formation of magnetic ordering in Li2MnGeO4 is presented.

Crystallographic and magnetic properties of new layered honeycomb-lattice Li3Co2SbO6 antimonate were studied and compared with its sodium precursor Na3Co2SbO6.

An experimental study of long‐range magnetic order formation mechanisms in a layered structure with a honeycomb arrangement of the magnetic atoms Na2Ni2TeO6 is conducted. For the first time, the strong spin correlations are directly observed above the Neel temperature TN that is manifested in the presence of broad diffuse peaks on neutron diffraction patterns obtained with the XYZ polarization analysis. Due to the possibility of separating the magnetic, nuclear incoherent, and nuclear coherent contributions to the total neutron scattering cross section, it is unequivocally established that the observed diffuse scattering has magnetic nature. The spin‐pair correlation function is reconstructed by modeling diffuse neutron scattering on Na2Ni2TeO6 with reverse Monte Carlo method. The obtained results indicate 2D nature of the magnetic correlations, and moreover, the symmetry of short‐range magnetic state corresponds to long‐range zigzag‐type magnetic order in the honeycomb net, which is established earlier based on the theoretical calculations.

The magnetic structure of Li3Ni2SbO6 has been determined by low-temperature neutron diffraction, and the crystal structure has been refined by a combination of synchrotron and neutron powder diffraction. The monoclinic (C2/m) symmetry, assigned previously to this pseudohexagonal layered structure, has been unambiguously proven by peak splitting in the synchrotron diffraction pattern. The structure is based on essentially hexagonal honeycomb-ordered Ni2SbO6 layers alternating with Li-3 layers, all cations and anions being in an octahedral environment. The compound orders antiferromagnetically below T-N = 15 K, with the magnetic supercell being a 2a x 2b multiple of the crystal cell. The magnetic structure within the honeycomb layer consists of zigzag ferromagnetic spin chains coupled antiferromagnetically. The ordered magnetic moment amounts to 1.62(2) mu(B)/Ni, which is slightly lower than the full theoretical value. Upon cooling below TN, the spins tilt from the c axis, with a maximum tilting angle of 15.6 degrees at T = 1.5K. Our data imply non-negligible ferromagnetic interactions between the honeycomb layers. The observed antiferromagnetic resonance modes are in agreement with the two-sublattice model derived from the neutron data. Orthorhombic anisotropy shows up in zero-field splitting of Delta = 198 +/- 4 and 218 +/- 4 GHz. Above TN, the electron spin resonance data imply short-range antiferromagnetic order up to about 80 K.

Quasi-two-dimensional magnetism is one of the most enthralling topics of a modern solid state physics.Reduced dimension gives rise to plenty of new phenomena.One of the most intriguing case for 2D lattices is a hexagonal net of antiferromagnetically ordered spins.Minimal possible for 2D lattices coordination number (z=3), frustrated interactions of nearest neighbors with second and third neighboring spins and also a quantum fluctuations leads to a large variety of a possible ground states [1].Fine examples of realization for such structures are mixed honeycomb oxides.Such compounds are described with chemical formulas: A2M2TeO6 and A3M2XO6 where A -is alkali, M -3d-metal and X is for Sb or Bi cations.Crystal structure of these compounds is formed by alternating layers of Te/Sb/Bi and M oxygen octahedra forming a honeycomb ordering and alkali cations with different type of oxygen surrounding depending on a structural politype.So magnetic ground state in these compounds depend on the nature of cations, their spin, orbital and electronic states determined by the local environment and also by a type of superstructure ordering due to a complexity of in-and interlayer exchange interactions.Still no adequate description between crystal structure and magnetic properties for such compounds was established, so the aim of a present work was to investigate crystal and magnetic structures of Li3Ni2SbO6 and Na2Ni2TeO6 compounds.For Li3Ni2SbO6 synchrotron diffraction experiment revealed a peak splitting that finally allowed to identify true space group to be C2/m; additional diffuse scattering indicating stacking faults presence was also detected.Neutron diffraction pattern at RT for Na2Ni2TeO6 revealed an anisotropic peak broadening indirectly specifying a sample to be a possible mixture of a P6322 and P63/mcm politypes.Close to 90o values of Ni-O-Ni bond angles shows presence of weak ferromagnetic inlayer interactions according to Goodenough-Kanamori rules; distorted O-Ni-O bond angles indicate a trigonal crystal field presence at Ni sites.LT neutron powder diffraction revealed addition peaks associated with magnetic scattering appearing at temperatures below 15 and 27 K for Li3Ni2SbO6 (on the left pic.) and Na2Ni2TeO6 (on the right pic.) respectively.Magnetic structures for both compounds are determined to be a zig-zag ferromagnetic chains coupled antiferromagnetically in ab-plane.Propagation vectors are k=(1/2 1/2 0) for Li3Ni2SbO6 and k=(1/2 0 0) for Na2Ni2TeO6.For Li3Ni2SbO6 ferromagnetic coupling for chains from adjacent layers was found, revealing non-negligible interlayer interactions.With temperature decreasing Ni spins directed along c-axis, demonstrates a certain tilt aligning perpendicular to ab-plane at T = 1.5 K.For Na2Ni2TeO6 increased ionic radius of alkali cation leads to a suppression of interlayer interactions.Ferromagnetic chains are coupled antiferromagnetically, nevertheless magnetic moments exhibit an inclination indicating a presence of a small ferromagnetic component within interlayer interactions.[1]