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New Mechanism for Ferroelectricity in the Perovskite Ca 2– x Mn x Ti 2 O 6 Synthesized by Spark Plasma Sintering
Perovskite oxides hosting ferroelectricity are particularly important materials for modern technologies. The ferroelectric transition in the well-known oxides BaTiO 3 and PbTiO 3 is realized by softening of a vibration mode in the cubic perovskite structure. For most perovskite oxides, octahedral-site tilting systems are developed to accommodate the bonding mismatch due to a geometric tolerance factor t = (A–O)/[√2(B–O)] < 1. In the absence of cations having lone-pair electrons, e.g., Bi 3+ and Pb 2+ , all simple and complex A-site and B-site ordered perovskite oxides with a t < 1 show a variety of tilting systems, and none of them become ferroelectric. The ferroelectric CaMnTi 2 O 6 oxide is, up to now, the only one that breaks this rule. It exhibits a columnar A-site ordering with a pronounced octahedral-site tilting and yet becomes ferroelectric at T c ≈ 650 K. Most importantly, the ferroelectricity at T < Tc is caused by an order–disorder transition instead of a displacive transition; this character may be useful to overcome the critical thickness problem experienced in all proper ferroelectrics. Application of this new ferroelectric material can greatly simplify the structure of microelectronic devices. However, CaMnTi2O6 is a high-pressure phase obtained at 7 GPa and 1200 °C, which limits its application. Here we report a new method to synthesize a gram-level sample of ferroelectric Ca 2–x Mn x Ti 2 O 6 , having the same crystal structure as CaMnTi 2 O 6 and a similarly high Curie temperature. The new finding paves the way for the mass production of this important ferroelectric oxide. In conclusion, we have used neutron powder diffraction to identify the origin of the peculiar ferroelectric transition in this double perovskite and to reveal the interplay between magnetic ordering and the ferroelectric displacement at low temperatures.
Anharmonic contribution to the stabilization of Mg(OH) 2 from first principles
Geometrical and vibrational characterization of magnesium hydroxide was performed using density functional theory. Four possible crystal symmetries were explored: P$\bar{3}$ (No. 147, point group -3), C2/m (No. 12, point group 2), P3m1 (No. 156, point group 3m) and P$\bar{3}$m1 (No. 164, point group -3m) which are the currently accepted geometries found in the literature. While a lot of work has been performed on Mg(OH) 2 , in particular for the PP$\bar{3}$m1 phase, there is still a debate on the observed ground state crystal structure and the anharmonic effects of the OH vibrations on the stabilization of the crystal structure. In particular, the stable positions of hydrogen are not yet defined precisely, which have implications in the crystal symmetry, the vibrational excitations, and the thermal stability. Previous work has assigned the P$\bar{3}$m1 polymorph as the low energy phase, but it has also proposed that hydrogens are disordered and they could move from their symmetric position in the P$\bar{3}$m1 structure towards P$\bar{3}$. Here, we examine the stability of the proposed phases by using different descriptors. We compare the XRD patterns with reported experimental results, and a fair agreement is found. While harmonic vibrational analysis shows that most phases have imaginary modes at 0 K, anharmonic vibrational analysis indicates that at room temperature only the C2/m phase is stabilized, whereas at higher temperatures, other phases become thermally competitive.
Host–guest selectivity in a series of isoreticular metal–organic frameworks: observation of acetylene-to-alkyne and carbon dioxide-to-amide interactions
In order to develop new porous materials for applications in gas separations such as natural gas upgrading, landfill gas processing and acetylene purification it is vital to gain understanding of host–substrate interactions at a molecular level. Herein we report a series of six isoreticular metal–organic frameworks (MOFs) for selective gas adsorption. These materials do not incorporate open metal sites and thus provide an excellent platform to investigate the effect of the incorporation of ligand functionality via amide and alkyne groups on substrate binding. By reducing the length of the linker in our previously reported MFM-136, we report much improved CO2/CH4 (50 : 50) and CO2/N2 (15 : 85) selectivity values of 20.2 and 65.4, respectively (1 bar and 273 K), in the new amide-decorated MOF, MFM-126. The CO2 separation performance of MFM-126 has been confirmed by dynamic breakthrough experiments. In situ inelastic neutron scattering and synchrotron FT-IR microspectroscopy were employed to elucidate dynamic interactions of adsorbed CO2 molecules within MFM-126. Upon changing the functionality to an alkyne group in MFM-127, the CO2 uptake decreases but the C2H2 uptake increases by 68%, leading to excellent C2H2/CO2 and C2H2/CH4 selectivities of 3.7 and 21.2, respectively. Neutron powder diffraction enabled the direct observation of the preferred binding domains in MFM-126 and MFM-127, and, to the best of our knowledge, we report the first example of acetylene binding to an alkyne moiety in a porous material, with over 50% of the acetylene observed within MFM-127 displaying interactions (<4 A) with the alkyne functionality of the framework.
MnFe0.5Ru0.5O3: an above-room-temperature antiferromagnetic semiconductor
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High-pressure polymorphism in L -threonine between ambient pressure and 22 GPa
The crystal structure of L -threonine has been studied to a maximum pressure of 22.3 GPa using single-crystal X-ray and neutron powder diffraction. The data have been interpreted in the light of previous Raman spectroscopic data by Holanda et al. (J. Mol. Struct. (2015), 1092, 160–165) in which it is suggested that three phase transitions occur at ca. 2 GPa, between 8.2 and 9.2 GPa and between 14.0 and 15.5 GPa. In the first two of these transitions the crystal retains its P2 1 2 1 2 1 symmetry, in the third, although the unit cell dimensions are similar either side of the transition, the space group symmetry drops to P2 1 . The ambient pressure form is labelled phase I, with the successive high-pressure forms designated I', II and III, respectively. Phases I and I' are very similar, the transition being manifested by a slight rotation of the carboxylate group. Phase II, which was found to form between 8.5 and 9.2 GPa, follows the gradual transformation of a long-range electrostatic contact becoming a hydrogen bond between 2.0 and 8.5 GPa, so that the transformation reflects a change in the way the structure accommodates compression rather than a gross change of structure. Phase III, which was found to form above 18.2 GPa in this work, is characterised by the bifurcation of a hydroxyl group in half of the molecules in the unit cell. Density functional theory (DFT) geometry optimisations were used to validate high-pressure structural models and PIXEL crystal lattice and intermolecular interaction energies are used to explain phase stabilities in terms of the intermolecular interactions.
Deep eutectic solvent route synthesis of zinc and copper vanadate n-type semiconductors – mapping oxygen vacancies and their effect on photovoltage
Ternary metal oxides M 2 V 2 O 7–δ (M = Zn and Cu) were synthesized by dissolving binary metal oxide precursors in an environmentally benign deep eutectic solvent (DES), which is a eutectic mixture of a hydrogen bond donor and acceptor, followed by annealing in an open crucible. The unique reaction environment provided by the evolved ammonia allows for stabilization of oxygen vacancies and reduced oxidation states of metal ions within an oxide matrix without the need for any post-treatment with flammable reducing agents. According to comprehensive characterization, including X-ray photoelectron spectroscopy (XPS), neutron powder diffraction, and UV-vis spectroscopy, oxygen deficiency is accompanied by reduced oxidation states of metal centers (Cu + or V 4+ ), resulting in oxides with mixed-valence metal oxidation states. The amount of oxygen vacancies can be tuned by changing the annealing temperature providing control over band gaps of ternary metal oxides and mid-gap states from reduced metal centers. All synthesized vanadates are n-type semiconductors based on negative photovoltages obtained from surface photovoltage spectroscopy (SPS). A decay of the photovoltage with increasing annealing temperatures is attributed to electron trapping and electron/hole recombination at V 4+ and Cu + mid-gap states. This work shows for the first time the impact of oxygen vacancies on the electronic structure of DES synthesized oxides for solar energy conversion applications.
The influence of the 6s 2 configuration of Bi 3+ on the structures of A′BiNb 2 O 7 (A′ = Rb, Na, Li) layered perovskite oxides
Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. Here, the ‘hybrid improper’ mechanism – in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure – offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb 2 O 7 , LiBiNb 2 O 7 and NaBiNb 2 O 7 , which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi 3+ cations which are often observed to stabilize acentric crystal structures due to their 6s 2 electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb 2 O 7 and LiBiNb 2 O 7 adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi 3+ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi 3+ cations with Nd 3+ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb 2 O 7 (space group P2 1 2 1 2 1 ) differs significantly from the centrosymmetric structure of NaNdNb 2 O 7 , which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi3+ cations.
Towards a mechanistic understanding of the sol–gel syntheses of ternary carbides
Sol–gel chemistry, while being extremely established, is to this day not fully understood, and much of the underlying chemistry and mechanisms are yet to be unraveled. Here, in this work, we elaborate on the sol–gel chemistry of Cr 2 GaC, the first layered ternary carbide belonging to the MAX phase family to ever be synthesized using this wet chemical approach. Leveraging a variety of both in- and ex situ characterization techniques, including X-ray and neutron powder diffraction, X-ray absorption fine structure analyses, total scattering analyses, and differential scanning calorimetry coupled with mass spectrometry, in-depth analyses of the local structures and reaction pathways are elucidated. While the metals first form tetrahedrally and octahedrally coordinated oxidic structures, that subsequently grow and crystallize into oxides, the carbon source citric acid sits on a separate reaction pathway, that does not merge with the metals until the very end. In fact, after decomposing it remains nanostructured and disordered graphite until the temperature allows for the reduction of the metal oxides into the layered carbide. Based on this, we hypothesize that the method is mostly applicable to systems where the needed metals are reducible by graphite around the formation temperature of the target phase.
Hydrogen Storage with Aluminum Formate, ALF: Experimental, Computational, and Technoeconomic Studies
Long-duration storage of hydrogen is necessary for coupling renewable H2 with stationary fuel cell power applications. In this presentation, I will discuss how aluminum formate, Al(HCOO)3 (ALF), which adopts an ReO3-type structure, is shown to have remarkable H2 storage performance at non-cryogenic (> 120 K) temperatures and low pressures. The most promising performance of ALF is found between 120 K and 160 K and at 10 bar to 20 bar. The talk will cover and illustrate the H2 adsorption performance of ALF over the 77 K to 296 K temperature range using gas isotherms, in situ neutron powder diffraction, and DFT calculations, as well as technoeconomic analysis (TEA), illustrating ALF’s competitive performance for long-duration storage versus compressed hydrogen and leading metal–organic frameworks. In the TEA, it is shown that ALF’s storage capacity, when combined with a temperature/pressure swing process, has advantages versus compressed H2 at a fraction of the pressure (15 bar versus 350 bar). Given ALF’s performance in the 10 bar to 20 bar regime under moderate cooling, it is particularly promising for use in safe storage systems serving fuel cells, and is currently the only MOF that works in this moderate temperature range/ low pressure regime to be cost competitive with compressed H2 gas for large scale H2 storage.[1]
Unusual zwitterionic catalytic site of SARS–CoV-2 main protease revealed by neutron crystallography
The main protease (3CL M pro ) from SARS–CoV-2, the etiological agent of COVID-19, is an essential enzyme for viral replication. 3CL M pro possesses an unusual catalytic dyad composed of Cys 145 and His 41 residues. A critical question in the field has been what the protonation states of the ionizable residues in the substrate-binding active-site cavity are; resolving this point would help understand the catalytic details of the enzyme and inform rational drug development against this pernicious virus. Here, we present the room-temperature neutron structure of 3CL M pro , which allowed direct determination of hydrogen atom positions and, hence, protonation states in the protease. We observe that the catalytic site natively adopts a zwitterionic reactive form in which Cys 145 is in the negatively charged thiolate state and His 41 is doubly protonated and positively charged, instead of the neutral unreactive state usually envisaged. The neutron structure also identified the protonation states, and thus electrical charges, of all other amino acid residues and revealed intricate hydrogen-bonding networks in the active-site cavity and at the dimer interface. The fine atomic details present in this structure were made possible by the unique scattering properties of the neutron, which is an ideal probe for locating hydrogen positions and experimentally determining protonation states at near-physiological temperature. Our observations provide critical information for structure-assisted and computational drug design, allowing precise tailoring of inhibitors to the enzyme's electrostatic environment.
Chemical complexity induced local structural distortion in NiCoFeMnCr high-entropy alloy
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Compression mechanisms of ferroelectric PbTiO 3 via high pressure neutron scattering
Switchable atomic displacements generate electric dipole moments in ferroelectric materials utilized in many contemporary devices. Lead titanate, a perovskite oxide with formula PbTiO 3 , has been referred to as a textbook example of a prototype displacive ferroelectric and is a testing platform of widely used models of piezoelectric response of complex solid-solutions. PbTiO 3 has been addressed by experimental and computational studies, often with apparently conflicting conclusions. To date, hydrostatic pressure experiments have been interpreted in terms of a model in which the dipole moments gradually diminish with increasing pressure until a transition to a cubic phase, characterized by a zero average dipole moment, occurs. Additionally, the model unrealistically assumes an even compression of the crystal. Here we show by high-pressure neutron powder diffraction measurements that a fast and slow shrinkage of 12-oxygen cages around Pb and octahedra around Ti, respectively, takes place. A phase diagram consolidating earlier and present results is given.
First-principles calculation of Hubbard U for Terbium metal under high pressure
Abstract Using density functional theory (DFT) and linear response approaches, we compute the on-site Hubbard interaction U of elemental Terbium (Tb) metal in the pressure range ∼ 0–65 GPa. The resulting first-principles U values with experimental crystal structures enable us to examine the magnetic properties of Tb using a DFT+U method. The lowest-energy magnetic states in our calculations for different high-pressure Tb phases—including hcp, α -Sm, and dhcp—are found to be compatible with the corresponding magnetic ordering vectors reported in experiments. The result shows that the inclusion of Hubbard U substantially improves the accuracy and efficiency in modeling correlated rare-earth materials. Our study also provides the necessary U information for other quantum many-body techniques to study Tb under extreme pressure conditions.
Canted antiferromagnetism and spin reorientation in corner-shared single chain quasi-one-dimensional Ba 2 FeSe 3
Here, we report the canted antiferromagnetic (AFM) structure together with a spin reorientation in a single chain quasi-one-dimensional (Q-1D) iron chalcogenide Ba 2 FeSe 3 . Ba 2 FeSe 3 crystallizes in Pnma (No. 62) orthorhombic structure with linear single iron chains consisting of corner-shared distorted FeSe 4 tetrahedra along the 𝑏 axis. Ba 2 FeSe 3 is a narrow-gap semiconductor and orders AFM below 60 K. Modeling of neutron powder diffraction data reveals a canted AFM ground state of magnetic space group 𝑃𝑎21/𝑐 (BNS No. 14.80) with commensurate propagation vector 𝐤 =(0, $\frac{1}{2}$, 0), where the Fe ion spins are AFM aligned with up-down-up-down (↑−↓−↑−↓) sequence along the Q-1D chain direction of the 𝑏 axis. In the magnetically ordered state, the canting of magnetic moments reorients from the 𝑎𝑐 plane to the 𝑎𝑏 plane below 30 K, with a 10° tilting angle toward the 𝑎 axis, and the magnetic moment does not induce a net moment in either orientation. The density functional theory results indicate that an ↑−↓−↑−↓ AFM state is stabilized along the chain direction. In this work, we elucidate the unique canted AFM of the iron chalcogenide and pave the way for searching exotic physics in Q-1D Ba 2 FeSe 3 .
Noncollinear magnetic structure and anisotropic magnetoelastic coupling in cobalt pyrovanadate $\mathrm{Co_2V_2O_7}$
$\mathrm{Co_2V_2O_7}$ was recently reported to exhibit remarkable magnetic-field-induced magnetization plateaus and ferroelectricity, but its magnetic ground state remains ambiguous. Magnetometry measurements and time-of-flight neutron powder diffraction (NPD) have been employed to study the structural and magnetic properties of $\mathrm{Co_2V_2O_7}$, which includes two nonequivalent Co sites. Upon cooling below the Néel temperature $T_N = 6.0(2)$ K, we observe magnetic Bragg peaks at 2 K in NPD, which indicates the formation of long-range magnetic order of $\mathrm{Co^{2+}}$ moments. After symmetry analysis and magnetic structure refinement, we demonstrate that $\mathrm{Co_2V_2O_7}$ possesses a complicated noncollinear magnetic ground state with Co moments mainly located in the b-c plane and forming a noncollinear spin-chain-like structure along the c -axis. The ab initio calculations demonstrate that the noncollinear magnetic structure is more stable than various ferromagnetic states at low temperature. The noncollinear magnetic structure with a canted ↑↑↓↓ spin configuration is considered to be the origin of magnetoelectric coupling in$\mathrm{Co_2V_2O_7}$ because the inequivalent exchange striction induced by the spin-exchange interaction between the neighboring spins could be the driving force of ferroelectricity. Finally, it is also found that the deviation of lattice parameters a and b is opposite below $T_N$, while the lattice parameter c and $β$ stay almost constant below $T_N$, evidencing the anisotropic magnetoelastic coupling in $\mathrm{Co_2V_2O_7}$.
Neutron scattering study of the quasi-one-dimensional antiferromagnet Ba 2 CoSi 2 O 7
Magnetization and neutron scattering measurements have been carried out on an antiferromagnet Ba 2 CoSi 2 O 7 . The observed magnetic excitation is almost dispersionless, and the neutron intensity is only modulated along the [101] direction. The dispersionless magnetic excitation suggest that the Ba 2 CoSi 2 O 7 system is a quasi-one-dimensional antiferromagnet. Classical spin-wave theory for a one-dimensional antiferromagnet can explain the dispersionless spin excitation. Additionally, the magnetic structure determined by the measurement of the neutron powder diffraction is consistent with no observation of the multiferroic property in this system.
Electronic and magnetic properties of stoichiometric CeAuBi 2
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