Order-to-Disorder Transition and Hydrogen Bonding in the Jahn-Teller Active NH4CrF3 Fluoroperovskite.

Large quantities of high-purity NH4CrF3 have been synthesized using a wet-chemical method, and its structural chemistry and magnetic properties are investigated in detail for the first time. NH4CrF3 is a tetragonal fluoroperovskite that displays an ordering of the ammonium (NH4+) groups at room temperature and C-type orbital ordering. The ammonium groups order and display distinct signs of hydrogen bonds to nearby fluoride anions by buckling the Cr-F-Cr angle away from 180°. The ammonium ordering remains up to 405 K, much higher than in other ammonium fluoroperovskites, indicating a correlation between the flexibility of the Jahn-Teller ion, the hydrogen bond formation, and the ammonium ordering. At 405 K, an order-to-disorder transition occurs, where the ammonium groups disorder, corresponding to a transition to higher symmetry. This is accompanied by a contraction of the unit cell from breaking hydrogen bonds, similar to the phenomenon observed in water ice melting. The compound orders antiferromagnetically with a Neél temperature of 60 K, an effective paramagnetic moment of 4.3 µB, and a Weiss temperature of -33 K. An A-type antiferromagnetic structure is identified by neutron diffraction, with an ordered moment of 3.72(2) µB.

Dynamic magnetic crossover at the origin of the hidden-order in van der Waals antiferromagnet CrSBr.

The van-der-Waals material CrSBr stands out as a promising two-dimensional magnet. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below TN ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers. Furthermore, we unravel the low-temperature hidden-order within the long-range magnetically-ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal field. These take place upon cooling below Ts ≈ 100 K, until a spin freezing process occurs at T* ≈ 40 K. We argue this complex behavior to reflect a crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by its mixed-anion character. Our findings reinforce CrSBr as an important candidate for devices in the emergent field of two-dimensional magnetic materials.

Competing Magnetic Phases in LnSbTe (Ln = Ho and Tb).

The interplay between a topological electronic structure and magnetism may result in intricate physics. In this work, we describe a case of rather peculiar coexistence or competition of several magnetic phases below seemingly single antiferromagnetic transition in LnSbTe (Ln = Ho and Tb) topological semimetals, the magnetic members of the ZrSiS/PbFCl structure type (space group P4/nmm). Neutron diffraction experiments reveal a complex multi-step order below TN = 3.8 K (Ln = Ho) and TN = 6.4 K (Ln = Tb). Magnetic phases can be described using four propagation vectors k1 = (1/2 0 0) and k2 = (1/2 0 1/4) at a base temperature of 1.7 K, which transform into incommensurate vectors k1' = (1/2 - δ 0 0) and k3 = (1/2 - δ 0 1/2) at elevated temperatures in both compounds. Together with the refined models of magnetic structures, we present the group theoretical analysis of magnetic symmetry of the proposed solutions. These results prompt further investigations of the relation between the electronic structure of those semimetals and the determined antiferromagnetic ordering existing therein.

Anomalous Ferromagnetic Behavior in Orthorhombic Li3Co2SbO6.

Monoclinic Li3Co2SbO6 has been proposed as a Kitaev spin liquid candidate and investigated intensively, whereas the properties of its polymorph, the orthorhombic phase, are less known. Here we report the magnetic properties of orthorhombic Li3Co2SbO6 as revealed by dc and ac magnetic susceptibility, muon spin relaxation (µSR), and neutron diffraction measurements. Successive magnetic transitions at 115, 89, and 71 K were observed in the low-field dc susceptibility measurements. The transitions below TN (115 K) are suppressed at higher applied fields. However, zero-field ac susceptibility measurements reveal distinct frequency-independent transitions at about 114, 107, 97, 79, and 71 K. A long-range magnetic ordered state was confirmed by specific heat, µSR, and neutron diffraction measurements, all indicating a single transition at about 115 K. The discrepancy between different measurements is attributed to possible stacking faults and/or local disorders of the ferromagnetic zigzag chains, resulting in ferromagnetic boundaries within the overall antiferromagnetic matrix.

Revisiting the antiferromagnetic structure of Tb14Ag51: the importance of distinguishing alternative symmetries for a multidimensional order parameter.

The antiferromagnetic structure of Tb14Ag51 with the propagation vector [1/3, 1/3, 0] and the parent space group P6/m is revisited using both magnetic symmetry and irreducible representation arguments. A new magnetic structure under the hexagonal Shubnikov magnetic space group P6' which fits much better the experimental data is found. This new solution was obtained by constraining the spin arrangement to one of the three possible magnetic space groups of maximal symmetry that can be realized by a magnetic ordering transforming according to the four-dimensional physically irreducible representation that is known to be relevant in this magnetic phase. The refined model, parameterized under P6', implicitly includes the presence of a third harmonic with the propagation vector at the gamma point [0, 0, 0], which has an important weight in the final result. The structure consists of 13 symmetry-independent Tb magnetic moments with the same size of 8.48 (2) µB, propagating cycloidally in the ab plane. The modulation has a substantial deviation from being purely sinusoidal due to the contribution of the mentioned third harmonic.

Temperature evolution of 3d- and 4f-electron magnetic ordering in the ferrimagnetic Mn self-doped perovskite (Yb0.667Mn0.333)MnO3.

A high-pressure synthesis method was employed to prepare Mn-self-doped perovskites (R0.667Mn0.333)MnO3(R= Yb, Lu) at about 6 GPa and 1670 K. Crystal and magnetic structures of (Yb0.667Mn0.333)MnO3have been studied by combining neutron powder diffraction, magnetic susceptibility and specific heat measurements. Within the orthorhombic space groupPnma, magnetic cations are located on site 4c(A site, occupied by two thirds of Yb3+and one third of Mn2+) and on site 4b(B site, occupied by two thirds of Mn3+and one third of Mn4+). The degree of structural distortion of the MnO6octahedra follows the general trend of (R1-xMnx)MnO3compounds which shows a decrease with increasing amount of Jahn-Teller inactive Mn4+cations. Mn-Mn interactions produce a collinear ferrimagnetic structure (TC,Mn= 106 K) with ferromagnetically ordered Mn moments at the B site being coupled antiferromagnetically with ordered Mn moments at the A site. Mn-Yb interactions induce a small but non-zero ferromagnetic Yb3+moment which can explain a small decrease of the magnetic susceptibility at low temperature. Yb-Yb interactions create an antiferromagnetic structure atTN,Yb≈ 40 K. Ordered moments of the ferrimagnetic and antiferromagnetic structures are oriented perpendicular to each other within theac-plane and Yb3+moments contribute to both structures. The appearance of ordered Yb3+moments induced by Mn-Yb interactions in perovskite (Yb0.667Mn0.333)MnO3is a result of the Mn self-doping on the A site and has not been observed in the orthorhombic perovskite modification (space groupPnma) of the undoped parent compound YbMnO3, but interestingly, it also appears in the hexagonal non-perovskite modification (space groupP63cm) of YbMnO3.

A quantum liquid of magnetic octupoles on the pyrochlore lattice.

Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles1. Theories define such states using gauge fields and deconfined quasiparticle excitations that emerge from a local constraint governing the ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice2-4 in rare-earth pyrochlores. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders5-6. Here we show that the correlated ground state of a Ce3+-based pyrochlore, Ce2Sn2O7, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with a fluid-like state where degrees of freedom have a more complex magnetization density than that of magnetic dipoles. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule7-8. Our work identifies Ce2Sn2O7 as a unique example of frustrated multipoles forming a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations.

Structural Polymorphism in Na4Zn(PO4)2 Driven by Rotational Order-Disorder Transitions and the Impact of Heterovalent Substitutions on Na-Ion Conductivity.

Solid electrolytes have regained tremendous interest recently in light of the exposed vulnerability of current rechargeable battery technologies. While designing solid electrolytes, most efforts concentrated on creating structural disorder (vacancies, interstitials, etc.) in a cationic Li/Na sublattice to increase ionic conductivity. In phosphates, the ionic conductivity can also be increased by rotational disorder in the anionic sublattice, via a paddle-wheel mechanism. Herein, we report on Na4Zn(PO4)2 which is designed from Na3PO4, replacing Na+ with Zn2+ and introducing a vacancy for charge balance. We show that Na4Zn(PO4)2 undergoes a series of structural transitions under temperature, which are associated with an increase in ionic conductivity by several orders of magnitude. Our detailed crystallographic study, combining electron, neutron, and X-ray powder diffraction, reveals that the room-temperature form, α-Na4Zn(PO4)2, contains orientationally ordered PO4 groups, which undergo partial and full rotational disorder in the high-temperature ß- and γ-polymorphs, respectively. We furthermore showed that the highly conducting γ-polymorph could be stabilized at room temperature by ball-milling, whereas the ß-polymorph can be stabilized by partial substitution of Zn2+ with Ga3+ and Al3+. These findings emphasize the role of rotational disorder as an extra parameter to design new solid electrolytes.

Topological Magnetic Phase in the Candidate Weyl Semimetal CeAlGe.

We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated multi-k[over →] magnetic phases below T_{N}. The topological properties of a phase stable at intermediate magnetic fields parallel to the c axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space.

Influence of Temperature-Driven Polymorphism and Disorder on Ionic Conductivity in Li6Zn(P2O7)2.

Ionic conductivity in a compound is rooted in a delicate interplay between its crystal structure and its structural defects (vacancies, interstitials, etc.). Hence, understanding this interplay is of utmost importance to design new solid state electrolytes. To shed some light on the above query, we investigated the rich crystal chemistry of Li6Zn(P2O7)2. This compound undergoes multiple structural transitions under the influence of temperature, which increases the conductivity by several orders and lowers the activation energy. We explained this jump in conductivity by the increased disorder associated with cation mixing. Our structural exploration indicates that both the room-temperature α-polymorph and the high-temperature ζ-polymorph crystallize in a C2/ c space group but with a much smaller unit cell volume for the latter. While their structural framework based on P2O74- is similar, the ζ-polymorph presents a fully disordered Li/Zn sublattice, while it is fully ordered for the α-polymorph. Furthermore, the bond valence energy landscape calculations show that in the α-polymorph, the Li+ conduction is two-dimensional, whereas because of Li+/Zn2+ site mixing, Li+ can hop three-dimensionally in the ζ-polymorph.

Crystal and Magnetic Structures and Properties of (Lu1- xMn x)MnO3 Solid Solutions.

(Lu1- xMn x)MnO3 solid solutions, having the perovskite-type structure and Pnma space group, with 0 ≤ x ≤ 0.4 were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from Lu2O3 and Mn2O3. Their crystal and magnetic structures were studied by neutron powder diffraction. The degree of octahedral MnO6 tilting decreases in (Lu1- xMn x)MnO3 with increasing x. Only the incommensurate (IC) spin structure with a propagation vector of k = ( k0, 0, 0) and k0 ≈ 0.44 remains in (Lu0.9Mn0.1)MnO3 in the whole temperature range below the Neel temperature TN = 36 K, and the commensurate noncollinear E-type structure that has been reported in the literature for undoped o-LuMnO3 is not observed. (Lu1- xMn x)MnO3 samples with 0.2 ≤ x ≤ 0.4 have a ferrimagnetic structure with a propagation vector of k = (0, 0, 0) and ferromagnetic (FM) ordering of Mn3+ and Mn4+ cations at the B site, which are antiferromagnetically coupled to a noncollinear predominantly FM arrangement of Mn2+ at the A site. The ferrimagnetic Curie temperature, TC, increases monotonically from 67 K for x = 0.2 to 118 K for x = 0.4. Magnetic and dielectric properties of (Lu1- xMn x)MnO3 and a composition-temperature phase diagram are also reported.

Denticity and Mobility of the Carbonate Groups in AMCO3F Fluorocarbonates: A Study on KMnCO3F and High Temperature KCaCO3F Polymorph.

We report on a thorough structural study on two members of layered fluorocarbonates KMCO3F (M = Ca, Mn). The Ca-based member demonstrates a phase transition at ∼320 °C, evidenced for the first time. The crystal structure of the high temperature phase (HT-KCaCO3F) was solved using neutron powder diffraction. A new Mn-based phase KMnCO3F was synthesized, and its crystal structure was solved from electron diffraction tomography data and refined from a combination of X-ray synchrotron and neutron powder diffraction. In contrast to other members of the fluorocarbonate family, the carbonate groups in the KMnCO3F and HT-KCaCO3F structures are not fixed to two distinct orientations corresponding to mono- and bidentate coordinations of the M cation. In KMnCO3F, the carbonate group can be considered as nearly "monodentate", forming one short (2.14 Å) and one long (3.01 Å) Mn-O contact. This topology provides more flexibility to the MCO3 layer and enables diminishing the mismatch between the MCO3 and KF layers. This conclusion is corroborated by the HT-KCaCO3F structure, in which the carbonate groups can additionally be tilted away from the layer plane thus relieving the strain arising from geometrical mismatch between the layers. The correlation between denticity of the carbonate groups, their mobility, and cation size variance is discussed. KMnCO3 orders antiferromagnetically below TN = 40 K.

Coulomb spin liquid in anion-disordered pyrochlore Tb2Hf2O7.

The charge ordered structure of ions and vacancies characterizing rare-earth pyrochlore oxides serves as a model for the study of geometrically frustrated magnetism. The organization of magnetic ions into networks of corner-sharing tetrahedra gives rise to highly correlated magnetic phases with strong fluctuations, including spin liquids and spin ices. It is an open question how these ground states governed by local rules are affected by disorder. Here we demonstrate in the pyrochlore Tb2Hf2O7, that the vicinity of the disordering transition towards a defective fluorite structure translates into a tunable density of anion Frenkel disorder while cations remain ordered. Quenched random crystal fields and disordered exchange interactions can therefore be introduced into otherwise perfect pyrochlore lattices of magnetic ions. We show that disorder can play a crucial role in preventing long-range magnetic order at low temperatures, and instead induces a strongly fluctuating Coulomb spin liquid with defect-induced frozen magnetic degrees of freedom.Experimental studies of frustrated spin systems such as pyrochlore magnetic oxides test our understanding of quantum many-body physics. Here the authors show experimentally that Tb2Hf2O7 may be a model material for investigating how structural disorder can stabilize a quantum spin liquid phase.

Single crystal growth, structure and magnetic properties of Pr2Hf2O7 pyrochlore.

Large single crystals of pyrochlore [Formula: see text] were successfully grown by the floating zone technique using an optical furnace equipped with high power xenon arc lamps. Structural investigations were carried out via powder synchrotron x-ray and neutron diffraction to establish the crystallographic structure of the materials produced. The magnetic properties of the single crystals were determined for magnetic fields applied along different crystallographic axes. The results revealed that [Formula: see text] is an interesting material for further investigation as a frustrated magnet. The high quality of the crystals produced makes them ideal for detailed investigation, especially using neutron scattering techniques.

Low-Temperature Cationic Rearrangement in a Bulk Metal Oxide.

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic-scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn2 FeMoO6 at unparalleled low temperatures of 150-300 (o) C. The irreversible ionic motion at ambient pressure, as evidenced by real-time powder synchrotron X-ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni3 TeO6 -type to an ordered-ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.

Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2.

We report on the magnetic properties of CsCo2Se2 with ThCr2Si2 structure, which we have characterized through a series of magnetization and neutron diffraction measurements. We find that CsCo2Se2 undergoes a phase transition to an antiferromagnetically ordered state with a Néel temperature of [Formula: see text] K. The nearest neighbour interactions are ferromagnetic as observed by the positive Curie-Weiss temperature of [Formula: see text] K. We find that the magnetic structure of CsCo2Se2 consists of ferromagnetic sheets, which are stacked antiferromagnetically along the tetragonal c-axis, generally referred to as A-type antiferromagnetic order. The observed magnitude of the ordered magnetic moment at T = 1.5 K is found to be only 0.20(1)[Formula: see text] / Co. Already in comparably small magnetic fields of [Formula: see text] T, we observe a metamagnetic transition that can be attributed to spin-rearrangements of CsCo2Se2, with the moments fully ferromagnetically saturated in a magnetic field of [Formula: see text] T. We discuss the entire experimentally deduced magnetic phase diagram for CsCo2Se2 with respect to its unconventionally weak magnetic coupling. Our study characterizes CsCo2Se2, which is chemically and electronically posed closely to the A x Fe2-y Se2 superconductors, as a host of versatile magnetic interactions.

Candidate Quantum Spin Liquid in the Ce3+ Pyrochlore Stannate Ce2Sn2O7.

We report the low-temperature magnetic properties of Ce2Sn2O7, a rare-earth pyrochlore. Our susceptibility and magnetization measurements show that due to the thermal isolation of a Kramers doublet ground state, Ce2Sn2O7 has Ising-like magnetic moments of ∼1.18 µ_{B}. The magnetic moments are confined to the local trigonal axes, as in a spin ice, but the exchange interactions are antiferromagnetic. Below 1 K, the system enters a regime with antiferromagnetic correlations. In contrast to predictions for classical ⟨111⟩-Ising spins on the pyrochlore lattice, there is no sign of long-range ordering down to 0.02 K. Our results suggest that Ce2Sn2O7 features an antiferromagnetic liquid ground state with strong quantum fluctuations.

New synthesis route and magnetic structure of Tm2Mn2O7 pyrochlore.

In this work, we present a new chemical route to synthesize Tm2Mn2O7 pyrochlore, which a compound that is thermodynamically unstable at ambient pressure. Differently from the reported in the past high-pressure synthesis of the same compound applying oxides as starting materials, we have obtained a pure Tm2Mn2O7 phase by a converting TmMnO3 at 1100 °C and an oxygen pressure of 1300 bar. The studies of Tm2Mn2O7 performed by a high-resolution neutron powder diffraction have shown that a pure pyrochlore cubic phase Tm2Mn2O7 (space group Fd3¯m) have been obtained. Upon cooling below 25 K, there is a transition to a ferromagnetically (FM) ordered phase observed with an additional antiferromagnetic (AFM) canting, suggesting a lowering of the initial cubic crystal symmetry. The magnetic transition is accompanied by a small but very visible magnetostriction effect. Using symmetry analysis, we have found a solution for the AFM structure in the maximal Shubnikov subgroup I41/am'd'.

Full propagation-vector star antiferromagnetic order in quantum spin trimer system Ca3CuNi2(PO4)4.

We show that the antiferromagnetic structure in the quantum spin trimer system Ca3CuNi2(PO4)4 is based on both arms of propagation vector k star {[1/2, 1/2, 0], [-1/2, 1/2, 0]} of the paramagnetic space group C2/c. The structure is generated by a symmetric direction of the order parameter of two-dimensional irreducible representation of C2/c with one active magnetic mode and corresponds to the Shubnikov magnetic space group Ca2/c. We reveal the relation between representation analysis in the propagation vector formalism and Shubnikov symmetry. These types of multi-k structures are extremely rarely observed experimentally. To further prove the specific magnetic structure we have performed the calculations of the spin expectation values in the isolated Ni(2+)-Cu(2+)-Ni(2+) trimer with realistic Hamiltonian. The calculated spin values 〈SNi〉 = 0.9 and 〈SCu〉 = 0.3 are within 10% accuracy in agreement with the experiment, providing a strong complementary argument in favor of a multi-arm magnetic structure.

Localization and impact of Pb-non-bonded electronic pair on the crystal and electronic structure of Pb2YSbO6.

The synthesis and crystal structure evolution of the double perovskite Pb2YSbO6 is reported for the first time. The structure has been analyzed in the temperature range between 100 and 500 K by using a combination of synchrotron and neutron powder diffraction. This compound shows two consecutive first order phase transformations as previously observed for a subgroup of Pb2RSbO6 perovkites (R = rare earths). The thermodynamic parameters associated with the phase transitions were calculated using differential scanning calorimetry (DSC), and the role of the diverse cations of the structure was studied from DFT calculations for the room temperature polymorph. The crystal structure evolves from a C2/c monoclinic structure (a(-)b(-)b(-) tilting system in Glazer's notation) to another monoclinic P2(1)/n (a(-)a(-)b(+)) phase with an incommensurate modulation and finally to a cubic Fm3m perovskite (a(0)a(0)a(0)). The highly distorted nature of the room temperature crystal structure seems to be driven by the polarization of the Pb lone pair which shows a marked local effect in the atomic spatial arrangements. Moreover, the lone pairs have been localized from DFT calculations and show an antiferroelectric ordering along the b monoclinic axis.