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Straightforward procedures for the generation of rhodium(I) κ

This article reports the first in situ diffraction result collected under applied electric field on a hybrid improper ferroelectric which shows a subtle yet robust preference for a switching mechanism that proceeds via an unwinding of the octahedral rotation mode., Improper ferroelectric mechanisms are increasingly under investigation for their potential to expand the current catalogue of functional materials whilst promoting couplings between ferroelectricity and other technologically desirable properties such as ferromagnetism. This work presents the results of an in situ synchrotron X-ray diffraction experiment performed on samples of Ca2.15Sr0.85Ti2O7 in an effort to elucidate the mechanism of hybrid improper ferroelectric switching in this compound. By simultaneously applying an electric field and recording diffraction patterns, shifts in the intensity of superstructure peaks consistent with one of the switching mechanisms proposed by Nowadnick & Fennie [Phys. Rev. B, (2016), 94, 104105] are observed. While the experiment only achieves a partial response, comparison with simulated data demonstrates a preference for a one-step switching mechanism involving an unwinding of the octahedral rotation mode in the initial stages of switching. These results represent some of the first reported experimental diffraction-based evidence for a switching mechanism in an improper ferroelectric.

We undertake a detailed high-resolution diffraction study of a novel plain band insulator, La$_2$MgO$_4$, which may be viewed as a structural surrogate system of the undoped end-member of the high-T$_c$ superconductors, La$_{2-x-y}$A$^{2+}_x$RE$^{3+}_y$CuO$_{4}$ (A = Ba, Sr, RE= Rare Earth). We find that La$_2$MgO$_4$ exhibits the infamous low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) phase transition that has been linked to the suppression of superconductivity in a variety of underdoped cuprates, including the well known La$_{2-x}$Ba$_{x}$CuO$_4$ ($x=0.125$). Furthermore, we find that the LTO-to-LTT phase transition in La$_2$MgO$_4$ occurs for an octahedral tilt angle in the 4 $^{\circ}$ to 5 $^{\circ}$ range, similar to that which has previously been identified as a critical tipping point for superconductivity in these systems. We show that this phase transition, occurring in a system lacking spin correlations and competing electronic states such as charge-density waves and superconductivity, can be understood by simply navigating the density-functional theory ground-state energy landscape as a function of the order parameter amplitude. This result calls for a careful re-investigation of the origins of the phase transitions in high-T$_c$ superconductors based on the hole-doped, $n = 1$ Ruddelsden-Popper lanthanum cuprates., 6 pages, 4 figures

We present the synthesis of a novel binary metal oxide material: Ba

We present the synthesis of a novel binary metal oxide material: Ba7Mn4O15. The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100–300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr7Mn4O15 that forms its own structural class. However, we find that Ba7Mn4O15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr7Mn4O15.

We report four new A-site vacancy ordered thiocyanate double double perovskites, Image ID:d0sc06619b-t1.gif, A = K+, NH4+, CH3(NH3)+ (MeNH3+) and C(NH2)3+ (Gua+), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders suggest a new route to non-centrosymmetric perovskites, and mean this family of materials could contain excellent candidates for piezo- and ferroelectric applications.

We use a symmetry-motivated approach to analyse neutron pair distribution function data to investigate the mechanism of negative thermal expansion (NTE) in ReO$_3$. This analysis shows that the local structure of ReO$_3$ is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring type deformations than the octahedra in the related compound ScF$_3$. These results support the idea that structural flexibility is an important factor in NTE materials, allowing the phonon modes that drive a volume contraction of the lattice to occupy a greater volume in reciprocal space. The lack of flexibility in ReO$_3$ restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. In addition, we investigate the thermal expansion properties of the material at high temperature and do not find the reported second NTE region. Finally, we show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the observed $P4/mbm$ high pressure phase of ReO$_3$. The result indicates that the motions associated with rigid unit modes are highly anisotropic in these systems., 9 pages, 6 figures

We report four new A-site vacancy ordered thiocyanate double double perovskites, , A = K+, NH4+, CH3(NH3)+ (MeNH3+) and C(NH2)3+ (Gua+), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders suggest a new route to non-centrosymmetric perovskites, and mean this family of materials could contain excellent candidates for piezo- and ferroelectric applications., Metal thiocyanate hybrid perovskites can have multiple simultaneous complex orderings and combining these orderings appropriately can produce non-centrosymmetric structures.

Journal of applied crystallography 54(5), 1514 - 1520 (2021). doi:10.1107/S1600576721008499, A novel symmetry-adapted pair distribution function analysis (SAPA) method for extracting information on local distortions from pair distribution function data is introduced. The implementation of SAPA is demonstrated in the TOPAS-Academic software using the freely available online software ISODISTORT, and scripts for converting the output from ISODISTORT to a SAPA input file for TOPAS are provided. Finally, two examples are provided to show how SAPA can evaluate the nature of both dynamic distortions in ScF$_3$ and the distortions which act as an order parameter for the phase transitions in BaTiO$_3$., Published by Wiley-Blackwell, [S.l.]

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the 38th doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn3A′Mn4BO12), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn3+ and charge disordered (CD) Mn3.5+ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn3.5+ with Mn3+ provides a natural mechanism though which long range OO can melt, giving way to a conducting state., Nature Communications, 12 (1), ISSN:2041-1723

We report four new A-site vacancy ordered thiocyanate double double perovskites,A1􀀀xfNi[Bi(SCN)6]1􀀀x3 }, A = K+, NH4+, CH3(NH3)+ (MeNH3+) and C(NH2)3+ (Gua+), including the first examples of thiocyanate perovskites containing organic A-site cations. We show, using a combination of X-ray and neutron diffraction, that the structure of these frameworks depends on the A-site cation, and that these frameworks possess complex vacancy-ordering patterns and cooperative octahedral tilts distinctly different from atomic perovskites. Density functional theory calculations uncover the energetic origin of these complex orders and allow us to propose a simple rule to predict favoured A-site cation orderings for a given tilt sequence. We use these insights, in combination with symmetry mode analyses, to show that these complex orders offer a new route to non-centrosymmetric perovskites which render them as excellent candidates forpiezo- and ferroelectric applications.

Why is it that ABO3 perovskites generally do not exhibit negative thermal expansion (NTE) over a wide temperature range, whereas layered perovskites of the same chemical family often do? It is generally accepted that there are two key ingredients that determine the extent of NTE: the presence of soft phonon modes that drive contraction (have negative Gruneisen parameters); and anisotropic elastic compliance that predisposes the material to the deformations required for NTE along a specific axis. This difference in thermal expansion properties is surprising since both ABO3 and layered perovskites often possess these ingredients in equal measure in their high-symmetry phases. Using first principles calculations and symmetry analysis, we show that in layered perovskites there is a significant enhancement of elastic anisotropy due to symmetry breaking that results from the combined effect of layering and condensed rotations of oxygen octahedra. This feature, unique to layered perovskites of certain symmetry, is what allows uniaxial NTE to persist over a large temperature range. This fundamental insight means that symmetry and the elastic tensor can be used as descriptors in high-throughput screening and to direct materials design. Symmetry breaking is a major cause of uniaxial negative thermal expansion (NTE) in layered perovskites. Researchers at Imperial College, the University of Warwick and the University of Kent have discovered the atomic factors causing perovskite oxides, materials with the general formula ABO3, to show very different thermal expansion properties from their layered counterparts, known as Ruddlesden-Popper oxides. Using first-principles calculations, the researchers found that in layered perovskites certain structural motifs can deviate from a perfectly symmetric arrangement, whereas in regular perovskites they are more firmly fixed. This feature, prevalent in layered perovskites, is what allows uniaxial negative thermal expansion, i.e., the contraction of the material with increased temperature, to persist over a large temperature range. This fundamental insight could aid high-throughput screening for new NTE materials and help direct future materials design.

We use a symmetry-motivated approach to analyzing x-ray pair distribution functions to study the mechanism of negative thermal expansion in two ${\mathrm{ReO}}_{3}$-like compounds: ${\mathrm{ScF}}_{3}$ and ${\mathrm{CaZrF}}_{6}$. Both average and local structures suggest that it is the flexibility of $M$-F-$M$ linkages $(M$ = Ca, Zr, Sc) due to dynamic rigid and semirigid ``scissoring'' modes that facilitates the observed negative thermal expansion (NTE) behavior. The amplitudes of these dynamic distortions are greater for ${\mathrm{CaZrF}}_{6}$ than for ${\mathrm{ScF}}_{3}$, which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexibility is enhanced in ${\mathrm{CaZrF}}_{6}$ due to the rocksalt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ${\mathrm{ScF}}_{3}$ anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behavior in this material.

By tuning the tolerance factor, \(t\), of the Ruddlesden–Popper oxide Ca\(_2\)MnO\(_4\) through isovalent substitutions, we show that the uniaxial coefficient of linear thermal expansion (CLTE) of these systems can be systematically changed through large negative to positive values. Highresolution X-ray diffraction measurements show that the magnitude of uniaxial negative thermal expansion (NTE) increases as \(t\) decreases across the stability window of the NTE phase. Transitions to phases with positive thermal expansion (PTE) are found to occur at both the high-\(t\) and low-\(t\) limits of stability. First-principles calculations demonstrate that reducing \(t\) enhances the contribution to thermal expansion from the lowest frequency phonons, which have the character of octahedral tilts and have negative mode Gruneisen parameter components along the NTE axis. By tuning \(t\) to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise NTE with a CLTE of -8.1 ppm/K at 125 K. We also illustrate, at the other end of the phase diagram, that an enhancement in compliance of these materials associated with the rotational instability provides another mechanism by which NTE could be yet further enhanced in this and related systems.

Hybrid improper ferroelectrics (HIFs) have been intensely studied over the last few years to gain understanding of their temperature induced phase transitions and ferroelectric switching pathways. Here we report a switching from first- to second-order phase transition pathway for topical HIFs $Ca_{3-x}Sr_xTi_2O_7$, which is driven by the differing entropies of the phases that we identify as being associated with the dynamic motion of octahedral tilts and rotations. A greater understanding of the transition pathways in this class of layered perovskites, which host many physical properties that are coupled to specific symmetries and octahedral rotation and tilt distortions -- such as superconductivity, negative thermal expansion, fast ion conductivity, ferroelectricity, among others -- is a crucial step in creating novel functional materials by design.

We report the high pressure synthesis of a layered perovskite Ca2GeO4 which is found to have the Ruddlesden–Popper structure with I41/acd symmetry. Consonant with\ud our recent predictions [Ablitt et al., npj Computational Materials, 2017, 3, 44], the phase displays pronounced uniaxial negative thermal expansion over a large temperature range. Negative thermal expansion that persists over a large temperature range is very unusual in the perovskite structure, and its occurrence in this instance can be understood to arise due to both soft lattice vibrations associated with a phase competition and the unusually compliant nature of this structure, which effectively couples thermal expansion in the layer plane to lattice contractions perpendicular to the layering direction via a “corkscrew” mechanism.

S.R.P. and J.W.G.B. acknowledge the EPSRC (grant EP/N01717X/1) and Leverhulme Trust (grant RPG-2012-576) for financial support and the STFC for the provision of beam time at ISIS. F.D.M. acknowledges the EPSRC for financial support (grant EP/P024637/1). M.S.S. acknowledges the Royal Society for a University Research Fellowship (grant UF160265). The phonon-glass electron crystal concept is one of the key guiding principles for the development of efficient thermoelectric materials. Here, we confirm that SrTiO 3 becomes a phonon-glass for large numbers of A-site vacancies in the Sr 1-x La 0.67x → 0.33x TiO 3 series and show that its electron crystal properties are stymied by the presence of a core-shell grain structure. Thermal conductivity, heat capacity, and neutron powder diffraction, complemented by representational analysis and phonon calculations, were used to investigate the thermal transport. This reveals that the heat carrying modes are dominated by Sr motions and that these become more localized upon the introduction of the A-site vacancies, consistent with the observed phonon-glass state. Impedance spectroscopy and direct current electrical measurements were used to probe the electrical properties of insulating and conducting samples. This reveals the coring of grains due to oxidation on cooling from sintering temperatures. The resultant insulating shell limits the thermoelectric power factor to S 2 /ρ = 0.45 mW m -1 K -2 and the figure-of merit to ZT = 0.15 at 900 K for Sr 0.20 La 0.53 → 0.27 Ti 0.95 Nb 0.05 O 3?δ . The thermal properties of these materials are, therefore, controlled by an intrinsic feature of the microstructure (i.e., the A-site vacancies), whereas the electrical properties are grain boundary limited, which in principle can be controlled independently to raise S 2 /ρ and ZT. Postprint

Using classical Monte Carlo simulations, we study a simple statistical mechanical model of relevance to the emergence of polarisation from local displacements on the square and cubic lattices. Our model contains two key ingredients: a Kitaev-like orientation-dependent interaction between nearest neighbours, and a steric term that acts between next-nearest neighbours. Taken by themselves, each of these two ingredients is incapable of driving long-range symmetry breaking, despite the presence of a broad feature in the corresponding heat capacity functions. Instead each component results in a "hidden" transition on cooling to a manifold of degenerate states, the two manifolds are different in the sense that they reflect distinct types of local order. Remarkably, their intersection---\emph{i.e.} the ground state when both interaction terms are included in the Hamiltonian---supports a spontaneous polarisation. In this way, our study demonstrates how local ordering mechanisms might be combined to break global inversion symmetry in a manner conceptually similar to that operating in the "hybrid" improper ferroelectrics. We discuss the relevance of our analysis to the emergence of spontaneous polarisation in well-studied ferroelectrics such as BaTiO$_3$ and KNbO$_3$., 8 pages, 8 figures

It is of great interest to design and make materials in which ferroelectric polarisation is coupled to other order parameters such as lattice, magnetic and electronic instabilities. Such materials will be invaluable in next-generation data storage devices. Recently, remarkable progress has been made in understanding improper ferroelectric coupling mechanisms that arise from lattice and magnetic instabilities. However, although theoretically predicted, a compact lattice coupling between electronic and ferroelectric (polar) instabilities has yet to be realised. Here we report detailed crystallographic studies of a novel perovskite Hg$^{\textbf{A}}$Mn$^{\textbf{A'}}_{3}$Mn$^{\textbf{B}}_{4}$O$_{12}$ that is found to exhibit a polar ground state on account of such couplings that arise from charge and orbital ordering on both the A' and B-sites, which are themselves driven by a highly unusual Mn$^{A'}$-Mn$^B$ inter-site charge transfer. The inherent coupling of polar, charge, orbital and hence magnetic degrees of freedom, make this a system of great fundamental interest, and demonstrating ferroelectric switching in this and a host of recently reported hybrid improper ferroelectrics remains a substantial challenge., 9 pages, 7 figures

A symmetry-motivated approach for designing perovskites with ferroic and magnetoelectric couplings is proposed. The results highlight which kinds of magnetic orderings and structural distortions need to coexist within the same structure to produce the desired couplings., A group-theoretical approach is used to enumerate the possible couplings between magnetism and ferroelectric polarization in the parent perovskite structure. It is shown that third-order magnetoelectric coupling terms must always involve magnetic ordering at the A and B sites which either transforms both as R-point or both as X-point time-odd irreducible representations (irreps). For fourth-order couplings it is demonstrated that this criterion may be relaxed allowing couplings involving irreps at X-, M- and R-points which collectively conserve crystal momentum, producing a magnetoelectric effect arising from only B-site magnetic order. In this case, exactly two of the three irreps entering the order parameter must be time-odd irreps and either one or all must be odd with respect to inversion symmetry. It is possible to show that the time-even irreps in this triad must transform as one of: X1 +, M3,5 − or R5 +, corresponding to A-site cation order, A-site antipolar displacements or anion rocksalt ordering, respectively. This greatly reduces the search space for type-II multiferroic perovskites. Similar arguments are used to demonstrate how weak ferromagnetism may be engineered and a variety of schemes are proposed for coupling this to ferroelectric polarization. The approach is illustrated with density functional theory calculations on magnetoelectric couplings and, by considering the literature, suggestions are given of which avenues of research are likely to be most promising in the design of novel magnetoelectric materials.

New CaCrO3−δ superstructure phases (δ = 0.33, 0.4, and 0.5) have been discovered through ‘soft’ low temperature reduction of the perovskite CaCrO3 made at ‘hard’ high pressure–temperature conditions. Their structures form a related series based on stacking of octahedral and tetrahedral chromium oxide layers.

Cation ordering in the 1322 perovskite CaCu3Fe2Nb2O12 has been investigated by synchrotron X-ray diffraction structure analysis, high-resolution electron microscopy, and magnetic measurements. All of the samples have full 1:3 ordering of Ca and Cu cations at the perovskite A-sites, but the Fe and Nb cations at the B sites show different degrees of ordering depending on the synthesis conditions. A slow-cooled sample has almost complete rock-salt-type 1:1 cation ordering at the B sites, but a rapidly cooled sample has no long-range B-cation arrangement. The ordered compound shows a large saturated magnetization (5.7 μB/formula unit (fu)) below 170 K due to the ferrimagnetism of the A′-site Cu2+ and B-site Fe3+ spins, whereas the B-site disordered form has a small magnetization (

Magnetite (Fe3O4) is the original magnetic material and the parent of ferrite magnets, with modern applications ranging from spintronics to MRI contrast agents. At ambient temperatures magnetite has a cubic spinel-type crystal structure, but it undergoes a complex structural distortion and becomes electrically insulating below the 125 K Verwey transition. The electronic ground state of the Verwey phase has been unclear for over 70 years as the low temperature structure was unknown, but the full superstructure was recently determined by high energy microcrystal x-ray diffraction. An analysis of 168 frozen phonon modes in the acentric (and hence multiferroic) low temperature magnetite structure is presented here. Differences between the amplitudes of centric and acentric branches of, X and W modes all contribute to the significant off-center atomic distortions in the low temperature structure.

The significance of an algorithm developed by H. T. Stokes & B. J. Campbell [Acta Cryst. (2017), A73, 4-13] is discussed. The algorithm promises to be a key tool for understanding the structure-property relationships of the many technologically important materials that display incommensurate modulations in their atomic and/or magnetic structure.\ud

The cooperative tilting distortions of n = 2 Ruddlesden-Popper oxides can be utilized to break the inversion symmetry of the host lattice and induce ferroelectric behavior. Unfortunately the desired a-a-c+/a-a-c+ structural deformation is only stabilized in phases with extremely small structural tolerance factors, limiting the chemical scope of this symmetry breaking approach. Here we describe the influence of topochemical fluorination on the structural distortions of n = 2 Ruddlesden-Popper oxides and demonstrate that the conversion of La3Ni2O7 to La3Ni2O5.5F3.5 breaks the inversion symmetry of the perovskite double layers which constitute the Ruddlesden-Popper framework, by driving a change from an a-a-c0/a-a-c0 distortion in the parent phase to an a-a-c+/a-a--(c+) distortion in the oxide-fluoride. In this instance, the symmetry breaking distortions of adjacent acentric perovskite sheets are antialigned, and as a result the inversion symmetry of the host lattice is broken locally, but not globally, resulting in an antiferroelectric structure. The breaking of local inversion symmetry in layered perovskite phases, in the absence of second-order Jahn-Teller active "distortion centers", is an important step toward the realization of ferroelectric and multiferroic behavior in phases of this structure type.

The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

The thermal evolution of electronic order in the complex Verwey ground state of magnetite $(\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4})$ has been determined through 22 high-accuracy synchrotron x-ray structure refinements using three $10--40\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ grains of stoichiometric magnetite. A robust fitting function is introduced to extract values of order parameterlike quantities at zero temperature and at the upper limit of the Verwey phase ${T}_{\mathrm{u}}=123.4\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The low-temperature structural distortion is found to be almost frozen below the Verwey transition but small changes in lattice and local mode amplitudes and Fe-Fe distances reveal an increase in electron localization on cooling. These distortions confirm that electron localization within trimerons is the driving force behind the Verwey transition. Electron localization is also revealed by anomalous decreases in the largest principal thermal displacement factors of Fe cations as electron-phonon decoupling occurs on cooling.

By using a symmetry motivated basis to evaluate local distortions against pair distribution function data, we show without prior bias, that the off-center Ti displacements in the archetypal ferroelectric BaTiO_{3} are zone centered and rhombohedral-like across its known ferroelectric and paraelectric phases. We construct a simple Monte Carlo model that captures our main experimental findings and demonstrate how the rich crystallographic phase diagram of BaTiO_{3} emerges from correlations of local symmetry-breaking distortions alone. Our results strongly support the order-disorder picture for these phase transitions, but can also be reconciled with the soft-mode theory of BaTiO_{3} that is supported by some spectroscopic techniques.

The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

X-ray diffraction is used to show that the structural distortion of magnetite below 125 kelvin is to a first approximation caused by charge ordering of its constituent iron ions, but that the localized electrons are distributed over three iron sites to form ‘trimeron’ quasiparticles. In a letter to Nature in 1939, Evert Johannes Willem Verwey described the first example of a low-temperature charge-ordering transition in a solid — the Verwey transition — in the mineral magnetite, Fe3O4 (see go.nature.com/3h2pp1 ). This phenomenon has since been observed in other transition metal oxides, yet despite decades of study, the precise structure of the charge-ordered state in magnetite has remained elusive. The complex structural distortions that characterize the Verwey state have now been determined, revealing an anomalous shortening of some inter-atomic distances. These are suggestive of an unusual charge configuration in which the localized electrons are each distributed over three neighbouring Fe sites. The mineral magnetite (Fe3O4) undergoes a complex structural distortion and becomes electrically insulating at temperatures less than 125 kelvin. Verwey proposed in 1939 that this transition is driven by a charge ordering of Fe2+ and Fe3+ ions1, but the ground state of the low-temperature phase has remained contentious2,3 because twinning of crystal domains hampers diffraction studies of the structure4. Recent powder diffraction refinements5,6,7 and resonant X-ray studies8,9,10,11,12 have led to proposals of a variety of charge-ordered and bond-dimerized ground-state models13,14,15,16,17,18,19. Here we report the full low-temperature superstructure of magnetite, determined by high-energy X-ray diffraction from an almost single-domain, 40-micrometre grain, and identify the emergent order. The acentric structure is described by a superposition of 168 atomic displacement waves (frozen phonon modes), all with amplitudes of less than 0.24 angstroms. Distortions of the FeO6 octahedra show that Verwey’s hypothesis is correct to a first approximation and that the charge and Fe2+ orbital order are consistent with a recent prediction17. However, anomalous shortening of some Fe–Fe distances suggests that the localized electrons are distributed over linear three-Fe-site units, which we call ‘trimerons’. The charge order and three-site distortions induce substantial off-centre atomic displacements and couple the resulting large electrical polarization to the magnetization. Trimerons may be important quasiparticles in magnetite above the Verwey transition and in other transition metal oxides.

Transuranic radionuclides such as uranium tend to be a pervasive environmental contaminant. It is absorbed through the intestine or a lung, deposited in the tissues, predominantly kidney and bone, and is carcinogenic. A novel nanosensor system has been developed for voltammetric tracing of environmental uranium contamination. The sensor consists of an organophosphorous ligand, (t-butylphenyl)-N,N-di-(isobutyl) carbamoylmethylphosphineoxide (CMPO) functionalized superparamagnetic core–shell magnetic nanoparticles and magnet based electrodes. It exploits the natural affinity of uranium for phosphate molecules to fabricate a highly specific and reproducible sensor. The small dimension along with a dramatically increased contact surface has lead to a faster response and higher sensitivity. The system uses an external magnetic field gradient for preconcentration and removal of the analyte from the surrounding aqueous media. The redox properties of the analyte are exploited for enumeration of variables by electrochemical techniques such as square wave voltammetry. The detection limit of the system is observed to be in parts-per-billion (ppb) of the uranyl concentration.

The gold complexes Au(C≡CC6H4C≡CC6H4Me)(PPh3) (3) and {Au(PPh3)}2(μ-C≡CC6H4C≡CC6H4C≡CC6H4C≡C) (6), prepared from the reaction of AuCl(PPh3) with the corresponding terminal or trimethylsilyl protected alkynes, react readily with Ru3(CO)10(μ-dppm) to afford phenylene ethynylene derivatives featuring the Ru3(μ-AuPPh3)(μ-C2R)(CO)7 cluster “end-caps”. The hydrido cluster Ru3(μ-H)(μ-C2C6H4C≡CC6H4Me)(CO)7 (4a) has also been obtained. There are significant differences in the absorption spectra of the organic precursors, the gold complexes and the clusters indicate a mixing of electronic states between the cluster and phenylene ethynylene moieties, while the presence of the Ru3 and in particular Ru3(μ-AuPPh3) cluster end-caps leads to a quenching of the phenylene ethynylene centred emission. The crystallographically determined structures of 3, 4a and Ru3(μ-AuPPh3) (μ-C2C6H4C≡CC6H4Me)(CO)7 (4b) are reported.

We present new results on the microscopic nature of the ferroelectricity mechanisms in ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ca}}_{3}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. To the first approximation, we confirm the hybrid improper ferroelectric mechanism recently proposed by Benedek and Fennie for these Ruddlesden-Popper compounds. However, in ${\mathrm{Ca}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ we find that there is a complex competition between lattice modes of different symmetry which leads to a phase coexistence over a large temperature range and the ``symmetry trapping'' of a soft mode. This trapping of the soft mode leads to a large uniaxial negative thermal expansion (NTE) reaching a maximum between 250 and 350 K ($3.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\text{ }\text{ }{\mathrm{K}}^{\ensuremath{-}1}$) representing the only sizable NTE reported for these and related perovskite materials to date. Our results suggest a systematic strategy for designing and searching for ceramics with large NTE coefficients.

3C1:2-Ba3NaRu2O9 is obtained by pressurized sintering of 6H-Ba3NaRu2O9 in a two-stage Walker-type press (900 °C, 9 GPa).

Strong correlation between spins and conduction electrons is key in spintronic materials and devices. A few ferro- or ferrimagnetic transition metal oxides such as La 1-x Sr x MnO 3, Fe 3 O 4, CrO 2 and Sr 2 FeMoO 6 have spin-polarized conduction electrons at room temperature, but it is difficult to find other spin-polarized oxides with high Curie temperatures (well above room temperature) and large magnetizations for spintronics applications. Here we show that an A- and B-site-ordered quadruple perovskite oxide, CaCu 3 Fe 2 Re 2 O 12, has spin-polarized conduction electrons and is ferrimagnetic up to 560 K. The couplings between the three magnetic cations lead to the high Curie temperature, a large saturation magnetization of 8.7a μ B and a half-metallic electronic structure, in which only minority-spin bands cross the Fermi level, producing highly spin-polarized conduction electrons. Spin polarization is confirmed by an observed low-field magnetoresistance effect in a polycrystalline sample. Optimization of CaCu 3 Fe 2 Re 2 O 12 and related quadruple perovskite phases is expected to produce a new family of useful spintronic materials.

A-site-ordered perovskite Ce1/2Cu3Ti4O12 has been found to crystallize in two different forms, one with random and the other with ordered Ce/vacancy distribution at the A site of the prototype AA'3B4O12 structure. The random phase is isostructural with CaCu3Ti4O12, and the ordered phase is a new ordered derivative of the AA'3B4O12-type perovskite with two crystallographically distinct Cu sites. Although both phases form a G-type antiferromagnetic arrangement of Cu(2+) spins below 24 K, their magnetisms are quite different. A typical antiferromagnetic transition is observed in the random phase, whereas a small ferromagnetic moment appears below 24 K in the ordered phase, which rapidly decreases upon further cooling. A mean-field approximation approach revealed that this unusual behavior in the ordered phase is an L-type ferrimagnetism driven by the nonequivalent magnetizations of the two ferromagnetic Cu(2+) spin sublattices in the G-type spin structure. This unusual ferrimagnetism is a direct consequence of the Ce/vacancy ordering.

The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the ${B}^{\ensuremath{'}}=\mathrm{La}$ and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the ${B}^{\ensuremath{'}}$ cation. An anomalous 120 K structural transition in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$ is attributed to Jahn-Teller distortion, evidenced by large-amplitude ${\ensuremath{\Gamma}}_{6}$ deformations of the NdO${}_{6}$ octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$. Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and $\frac{1}{3}$-magnetization steps in the magnetic hysteresis loop.

A-site-ordered perovskites La1-xNaxMn3Ti4O12 with (x = 0, 0.5, 1) were synthesized under high-pressure and high-temperature conditions, and their crystal structures and magnetic properties were investigated. Ti/Mn occupancies at the B site were also examined by using resonant X-ray diffraction, and the results confirm that they are stoichiometric with high Ti/Mn site selectivity. Substitution of Na+ for some of the La3+ changes the valence state of the A'-site Mn but not of the B-site Ti, and thus the compositions of the synthesized samples are expressed as La1-xNaxMn{(5+2x)/3}+3 Ti4+4O12. The magnetic properties change from spin-glass-like to antiferromagnetic to spin-glass-like behavior according to the valence states of A'-site Mn.

We show that the valence electrons of Ba3NaRu2O9, which has a quasimolecular structure, completely crystallize below 210 K. Using an extended Hubbard model, we show that the charge ordering instability results from long-range Coulomb interactions. However, orbital ordering, metal-metal bonding, and formation of a partial spin gap enforce the magnitude of the charge separation. The striped charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition with a quasidegenerate charge-melted phase under photoexcitation at low temperature. Our results establish a broad class of simple metal oxides as models for emergent phenomena at the border between the molecular and solid states.

Electronic structure calculations of the Verwey ground state of magnetite, Fe3O4, using density functional theory with treatment of on-site Coulomb interactions (DFT+U scheme) are reported. These calculations use the recently-published experimental crystal structure coordinates for magnetite in the monoclinic space group Cc. The computed density distribution for minority spin electron states close to the Fermi level demonstrates that charge order and Fe2+-orbital order are present at the B-type lattice sites to a first-approximation. However, Fe2+/Fe3+ charge differences are diminished through weak bonding interactions of the Fe2+-states to specific pairs of neighboring iron sites that create linear, three-B-atom trimeron units that may be regarded as 'orbital molecules'. Trimerons are ordered evenly along most Fe atom chains in the Verwey structure, but more complex interactions are observed within one chain., 13 pages, 4 figures. Changes for version 2: Fig. 4 and corresponding discussion extended