Decoupling Lattice and Magnetic Instabilities in Frustrated CuMnO2.

The AMnO2 delafossites (A = Na, Cu) are model frustrated antiferromagnets, with triangular layers of Mn3+ spins. At low temperatures (TN = 65 K), a C2/m → P1̅ transition is found in CuMnO2, which breaks frustration and establishes magnetic order. In contrast to this clean transition, A = Na only shows short-range distortions at TN. Here, we report a systematic crystallographic, spectroscopic, and theoretical investigation of CuMnO2. We show that, even in stoichiometric samples, nonzero anisotropic Cu displacements coexist with magnetic order. Using X-ray/neutron diffraction and Raman scattering, we show that high pressures act to decouple these degrees of freedom. This manifests as an isostuctural phase transition at ∼10 GPa, with a reversible collapse of the c-axis. This is shown to be the high-pressure analogue of the c-axis negative thermal expansion seen at ambient pressure. Density functional theory (DFT) simulations confirm that dynamical instabilities of the Cu+ cations and edge-shared MnO6 layers are intertwined at ambient pressure. However, high pressure selectively activates the former, before an eventual predicted reemergence of magnetism at the highest pressures. Our results show that the lattice dynamics and local structure of CuMnO2 are quantitatively different from nonmagnetic Cu delafossites and raise questions about the role of intrinsic inhomogeneity in frustrated antiferromagnets.

Spin-chain correlations in the frustrated triangular lattice material CuMnO2.

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO2, which consists of a distorted lattice of Mn3+ spins with single-ion anisotropy. Physical property measurements on CuMnO2 are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to ∼25 meV above T N = 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

Is artificial intelligence magic dust for big-science facilities?

AI is no magic dust: for it to become a true discovery accelerator, much work is needed to make it transparent and robust.

Response to comment on 'Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers'.

A response is given to comments by Bürgi & Macchi [IUCrJ (2018), 5, 654-657] about Belo et al. [IUCrJ (2018), 5, 6-12.].

Hydrogen bonds in crystalline d-alanine: diffraction and spectroscopic evidence for differences between enantiomers.

Enantiomeric amino acids have specific physiological functions in complex biological systems. Systematic studies focusing on the solid-state properties of d-amino acids are, however, still limited. To shed light on this field, structural and spectroscopic studies of d-alanine using neutron powder diffraction, polarized Raman scattering and ab initio calculations of harmonic vibrational frequencies were carried out. Clear changes in the number of vibrational modes are observed as a function of temperature, which can be directly connected to variations of the N-D bond lengths. These results reveal dissimilarities in the structural properties of d-alanine compared with l-alanine.

Big-Science facilities in Europe need greater coordination of resources.

The leading role in science played by crystallography is heavily dependent on Big-Science facilities. The need for Europe-wide coordination of operational resources in Big Science is discussed with particular reference to neutron sources.

Charge order at the frontier between the molecular and solid states in Ba3NaRu2O9.

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.

Solitonic lattice and Yukawa forces in the rare-earth orthoferrite TbFeO3.

The random fluctuations of spins give rise to many interesting physical phenomena, such as the 'order-from-disorder' arising in frustrated magnets and unconventional Cooper pairing in magnetic superconductors. Here we show that the exchange of spin waves between extended topological defects, such as domain walls, can result in novel magnetic states. We report the discovery of an unusual incommensurate phase in the orthoferrite TbFeO(3) using neutron diffraction under an applied magnetic field. The magnetic modulation has a very long period of 340 Å at 3 K and exhibits an anomalously large number of higher-order harmonics. These domain walls are formed by Ising-like Tb spins. They interact by exchanging magnons propagating through the Fe magnetic sublattice. The resulting force between the domain walls has a rather long range that determines the period of the incommensurate state and is analogous to the pion-mediated Yukawa interaction between protons and neutrons in nuclei.

Polymorphism and piezochromicity in the three-dimensional network-based phosphate RbCuPO4.

Rubidium copper phosphate, RbCuPO(4), forms two room-temperature polymorphs that have been investigated with neutron powder diffraction. Polymorph (II) can be converted quantitatively into (I) by grinding the material or by pelletization, and the phase transition is accompanied by a significant colour change from very pale green to sky blue. Polymorph (II) can be obtained essentially free of (I) by quenching from 723 K. Each polymorph shows two unique Cu atoms: in (I) both sites are four-coordinate in a 2:1 ratio, whereas in (II) the atoms are four- and five-coordinate in a 1:1 ratio. In each case these sites are linked by phosphate tetrahedra to form three-dimensional frameworks based on the 4(2)6(3)8-a four-connected net. The Rb atoms are hosted in the six- and eight-ring channels that are similar to those observed in zeolite ABW. The (II) --> (I) phase transition is also accompanied by a volume reduction of 2.1% even though the average coordination of the Cu atoms also falls. The structures of the polymorphs are critically examined and compared with those of KNiPO(4) and KCuPO(4) in terms of hexagonal close packing containing ordered phosphate arrays. As a result of buckling of the six-ring layers, one-dimensional chains of dimerized copper polyhedra are identified in (II), chains that become trimers with mirror symmetry in (I).

Raman and neutron scattering study of partially deuterated L-alanine: evidence of a solid-solid phase transition.

Raman and neutron experiments using specific isotope labeling were combined in order to study the dynamics and structure of L-alanine. Inelastic neutron and Raman scattering data of C(2)H(4)(ND(2))CO(2)D are discussed in relation to the doubling of the lattice parameter a observed by means of neutron powder diffraction in C(2)D(4) (NH(2))CO(2)H. The major changes accompanying the phase transition are found in the vibrational frequencies involving the torsional vibration tau(CO(2)(-)), which is clearly affected by the hydrogen bonds between the protons of the ammonium group and the oxygen atoms of the carboxylate group. At lower temperatures the rearrangement of identifiable hydrogen bonds induces changes in the bending vibration delta(ND(3)), confirming some orientational disorder.

Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2.

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

Metal-insulator transition and orbital order in PbRuO3.

Anomalous low temperature electronic and structural behavior has been discovered in PbRuO3. The structure [space group Pnma, a=5.563 14(1), b=7.864 68(1), c=5.614 30(1) A] and metallic conductivity at 290 K are similar to those of SrRuO3 and other ruthenate perovskites, but a sharp metal-insulator transition at which the resistivity increases by 4 orders of magnitude is discovered at 90 K. This is accompanied by a first-order structural transition to an Imma phase [a=5.569 62(1), b=7.745 50(1), c=5.662 08(1) A at 25 K] that shows a coupling of Ru4+ 4d orbital order to distortions from Pb2+ 6s6p orbital hybridization. The Pnma to Imma transition is an unconventional reversal of the group-subgroup symmetry relationship. No long range magnetic order is evident down to 1.5 K. Calculations show that Pb 6s6p and Ru 4d orbital hybridization and strong spin-orbit coupling are significant.

Dynamics of Water in NaxCoO2.yH2O.

Incoherent inelastic neutron scattering experiments were performed on Na0.7CoO2 and Na0.28CoO2.1.3H2O in order to understand how the dynamics of the hydrogen-bond network of water is modified in the triangular crystalline lattice NaxCoO2.yH2O. Using quasi-elastic neutron scattering (QENS), we were able to differentiate between two types of proton dynamics: a fast process (due to water strongly bound into the sodium cobalt oxyhydrate structure during the hydration process) and a slow process (likely attributable to a collective motion). High-resolution QENS experiments, carried out on Na0.28CoO2.1.3H2O, show that, at temperatures above 310 K, the water dynamics can be well-described by a random jump diffusion model characterized by a diffusion constant equal to 0.9 x 10(-9)m2/s, which is significantly lower than the rate of diffusion for bulk water. Furthermore, our results indicate that, at room temperature, the sodium ions have no influence on the rotational dynamics of the "fast" water molecules.

Hydrogen in N-methylacetamide: positions and dynamics of the hydrogen atoms using neutron scattering.

This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O...H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior of the phonon vibrations together with the observation of NTE, could be justified by the "vibrational" polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally, this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.

Structural isotopic effects in the smallest chiral amino acid: observation of a structural phase transition in fully deuterated alanine.

A first study of possible changes instigated by deuteration in amino acids was carried out using neutron diffraction, inelastic neutron scattering, and Raman scattering in l-alanine, C2H4(NH2)COOH. Careful analysis of the structural parameters shows that deuteration of l-alanine engenders significant geometric changes as a function of temperature, which can be directly related to the observation of new lattice vibration modes in the Raman spectra. The combination of the experimental data suggests that C2D4(ND2)COOD undergoes a structural phase transition (or a structural rearrangement) at about 170 K. Considering that this particular amino acid is a hydrogen-bonded system with short hydrogen bonds (O...H approximately 1.8 A), we evoke the Ubbelohde effect to conclude that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions. The structural differences suggest distinct relative stabilities for the hydrogenous and deuterated l-alanine.