RMCProfile7: reverse Monte Carlo for multiphase systems.

This work introduces a completely rewritten version of the program RMCProfile (version 7), big-box, reverse Monte Carlo modelling software for analysis of total scattering data. The major new feature of RMCProfile7 is the ability to refine multiple phases simultaneously, which is relevant for many current research areas such as energy materials, catalysis and engineering. Other new features include improved support for molecular potentials and rigid-body refinements, as well as multiple different data sets. An empirical resolution correction and calculation of the pair distribution function as a back-Fourier transform are now also available. RMCProfile7 is freely available for download at https://rmcprofile.ornl.gov/.

Melt-quenched glass formation of a family of metal-carboxylate frameworks.

Metal-organic framework (MOF) glasses are an emerging class of glasses which complement traditional inorganic, organic and metallic counterparts due to their hybrid nature. Although a few zeolitic imidazolate frameworks have been made into glasses, how to melt and quench the largest subclass of MOFs, metal carboxylate frameworks, into glasses remains challenging. Here, we develop a strategy by grafting the zwitterions on the carboxylate ligands and incorporating organic acids in the framework channels to enable the glass formation. The charge delocalization of zwitterion-acid subsystem and the densely filled channels facilitate the coordination bonding mismatch and thus reduce the melting temperature. Following melt-quenching realizes the glass formation of a family of carboxylate MOFs (UiO-67, UiO-68 and DUT-5), which are usually believed to be un-meltable. Our work opens up an avenue for melt-quenching porous molecular solids into glasses.

Electronic origin of negative thermal expansion in samarium hexaboride revealed by X-ray diffraction and total scattering.

Samarium hexaboride, SmB6, is a negative thermal expansion (NTE) material whose structure is similar to other known NTE materials such as the family of Prussian blues. In the Prussian blues, NTE is due to a phonon mechanism, but we recently showed from DFT calculations that this is unlikely in SmB6 (Li et al., Phys. Chem. Chem. Phys. 2023, 25, 10749). We now report experimental X-ray diffraction and pair distribution function analysis of this material in the temperature range 20-300 K. The interatomic distances shown by both methods are consistent with the NTE instead arising from an electronic effect, by which the samarium atoms lose electrons and thus have a smaller ionic radius as the temperature increases.

Observation of the mixed magneto-optical Kerr effects using weak measurement.

In this paper, we use weak measurement to study the mixed magneto-optical Kerr effects (MOKEs) in a magnetic single layer of Co and in organic/Co bilayer films. The relationship between the amplified shift and the MOKE parameters is theoretically established and then experimentally observed as a function of both the magnetization intensity and the magnetization direction in magnetic thin films with an arbitrary magnetization. Furthermore, we experimentally observe a magnetic coupling at the organic/ferromagnetic interface. Due to this being a robust coupling, there is a significant dependence of the amplified shift with the thickness of the organic layer. This work has value for its application in the magnetic measurement technique for the more general case based on weak measurement.

Phonon mechanism for the negative thermal expansion of zirconium tungstate, ZrW2O8.

Negative thermal expansion (NTE) in ZrW2O8 was investigated using a flexibility analysis of ab initio phonons. It was shown that no previously proposed mechanism adequately describes the atomic-scale origin of NTE in this material. Instead it was found that the NTE in ZrW2O8 is driven, not by a single mechanism, but by wide bands of phonons that resemble vibrations of near-rigid WO4 units and Zr-O bonds at low frequency, with deformation of O-W-O and O-Zr-O bond angles steadily increasing with increasing NTE-phonon frequency. It is asserted that this phenomenon is likely to provide a more accurate explanation for NTE in many complex systems not yet studied.

The contribution of phonons to the thermal expansion of some simple cubic hexaboride structures: SmB6, CaB6, SrB6 and BaB6.

We have performed first-principles calculations of the structure and lattice dynamics in the metal hexaborides SmB6, CaB6, SrB6 and BaB6 using Density Functional Theory in an attempt to understand the negative thermal expansion in the first of these materials. The focus is on the role of Rigid Unit Modes involving rotations of the B6 octahedra similar to the rotations of structural polyhedra connected by bonds in Zn(CN)2, Prussian Blue and Si(NCN)2. However, it was found that there is very low flexibility of the network of connected B6 octahedra, and the lattice dynamics do not support negative thermal expansion except possibly at very low temperature. Thus the negative thermal expansion observed in SmB6 probably has an electronic origin.

The Rigid Unit Mode model: review of ideas and applications.

We review a set of ideas concerning the flexibility of network materials, broadly defined as structures in which atoms form small polyhedral units that are connected at corners. One clear example is represented by the family of silica polymorphs, with structures composed of corner-lined SiO4tetrahedra. TheRigid Unit Mode(RUM) is defined as any normal mode in which the structural polyhedra can translate and/or rotate without distortion, and since forces associated with changing the size and shape of the polyhedra are much stronger than those associated with rotations of two polyhedra around a shared vertex, the RUMs might be expected to have low frequencies compared to all other phonon modes. In this paper we discuss the flexibility of network structures, and how RUMs can arise in such structures, both in principle and in a number of specific examples of real systems. We also discuss applications of the RUM model, particularly for our understanding of phenomena such as displacive phase transitions and negative thermal expansion in network materials. .

Unique features of the structural phase transition in acetylene showing simultaneous characteristics of reconstructive, displacive and order-disorder.

We have studied the two phases of the molecular crystal acetylene, C2H2, using calculations of the lattice dynamics by Density Functional Theory methods. together with the use of classical molecular dynamics (MD) simulation methods. The two phases share the same simple face-centred cubic lattice arrangement of the molecular centres of mass, but with different molecular orientations. We show that the higher-temperature phase has lower phonon frequencies and hence higher entropy, giving thermodynamic stability at higher temperature. The calculated lattice dynamics of this phase show instabilities associated with phonons involving pure rotations of the molecules. The MD results show large amplitudes of librational motion in this phase. The MD simulations also showed a potential phase transition to a structure of tetragonal symmetry. The picture that emerges is that the phase transition in acetylene is a very rare example of one that encompasses elements of three types of transition: displacive, order-disorder and reconstructive.

Neutron powder-diffraction study of phase transitions in strontium-doped bismuth ferrite: 1. Variation with chemical composition.

We report results from a study of the crystal and magnetic structures of strontium-doped BiFeO3using neutron powder diffraction and the Rietveld method. Measurements were obtained over a wide range of temperatures from 300-800 K for compositions between 10%-16% replacement of bismuth by strontium. The results show a clear variation of the two main structural deformations-symmetry-breaking rotations of the FeO6octahedra and polar ionic displacements that give ferroelectricity-with chemical composition, but relatively little variation with temperature. On the other hand, the antiferromagnetic order shows a variation with temperature and a second-order phase transition consistent with the classical Heisenberg model. There is, however, very little variation in the behaviour of the antiferromagnetism with chemical composition, and hence with the degree of the structural symmetry-breaking distortions. We therefore conclude that there is no significant coupling between antiferromagnetism and ferroelectricity in Sr-doped BiFeO3and, by extension, in pure BiFeO3.

Orientational disorder in sulfur hexafluoride: a neutron total scattering and reverse Monte Carlo study.

The orientational disorder in crystalline sulfur hexafluoride, SF6, has been studied using a combination of neutron total scattering and the reverse Monte Carlo method. Analysis of the atomic configurations has shown the extent of the disorder through the evaluation of the S-F bond orientational distribution function, consistent with, but improving upon, the results of earlier neutron powder diffraction data. The correlations between orientations of neighbouring molecules have been studied through analysis of the distributions of F-F distances, showing that nearest-neighbour F-F close contacts are avoided, consistent with previous molecular dynamics simulation results. The results present a new case study of the application of neutron total scattering and the reverse Monte Carlo methods for the study of orientational disorder, where in this instance the disorder arises from orientational frustration rather than from a mismatch of molecular and site symmetries.

Origin of Ferroelectricity in Two Prototypical Hybrid Organic-Inorganic Perovskites.

Hybrid organic-inorganic perovskite (HOIP) ferroelectrics are attracting considerable interest because of their high performance, ease of synthesis, and lightweight. However, the intrinsic thermodynamic origins of their ferroelectric transitions remain insufficiently understood. Here, we identify the nature of the ferroelectric phase transitions in displacive [(CH3)2NH2][Mn(N3)3] and order-disorder type [(CH3)2NH2][Mn(HCOO)3] via spatially resolved structural analysis and ab initio lattice dynamics calculations. Our results demonstrate that the vibrational entropy change of the extended perovskite lattice drives the ferroelectric transition in the former and also contributes importantly to that of the latter along with the rotational entropy change of the A-site. This finding not only reveals the delicate atomic dynamics in ferroelectric HOIPs but also highlights that both the local and extended fluctuation of the hybrid perovskite lattice can be manipulated for creating ferroelectricity by taking advantages of their abundant atomic, electronic, and phononic degrees of freedom.

Neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide.

We report the results of a neutron powder diffraction study of the phase transitions in deuterated methylammonium lead iodide, with a focus on the system of orientational distortions of the framework of PbI6octahedra. The results are analysed in terms of symmetry-adapted lattice strains and normal mode distortions. The higher-temperature cubic-tetragonal phase transition at 327 K is weakly discontinuous and nearly tricritical. The variations of rotation angles and spontaneous strains with temperature are consistent with a standard Landau theory treatment. The lower-temperature transition to the orthorhombic phase at 165 K is discontinuous, with two systems of octahedral rotations and internal distortions that together can be described by 5 order parameters of different symmetry. In this paper we quantify the various symmetry-breaking distortions and their variation with temperature, together with their relationship to the spontaneous strains, within the formalism of Landau theory. A number of curious results in the low-temperature phase are identified, particularly regarding distortion amplitudes that decrease rather than increase with lowering temperature.

The ferroelastic phase transition in hydrogen cyanide studied by density functional theory.

We report calculations of the crystal structures and lattice dynamics of the tetragonal and orthorhombic phases of the molecular crystal hydrogen cyanide, HCN, using density functional theory methods. By treating negative pressure as a proxy for raising temperature we show that the ferroelastic phase transition involves softening of a transverse acoustic mode, and confirm that the phase transition is discontinuous. Analysis of the complete phonon spectrum shows that the acoustic modes are responsible both for the very large thermal expansion seen in HCN and also for the thermodynamic driving force for the phase transition.

Orientational order and phase transitions in deuterated methane: a neutron total scattering and reverse Monte Carlo study.

We report a study of the orientational order and phase transitions in crystalline deuterated methane, carried out using neutron total scattering and the reverse Monte Carlo method. The resultant atomic configurations are consistent with the average structures obtained from Rietveld refinement of the powder diffraction data, but additionally enable us to determine the C-D bond orientational distribution functions (ODF) for the disordered molecules in the high-temperature phase, and for both ordered and disordered molecules in the intermediate-temperature phase. We show that this approach gives more accurate information than can been obtained from fitting a bond ODF to diffraction data. Given the resurgence of interest in orientationally-disordered crystals, we argue that the approach of total scattering with the RMC method provides a unique quantification of orientational order and disorder.

Analysis of the atomic structure of CdS magic-size clusters by X-ray absorption spectroscopy.

Magic-size clusters are ultra-small colloidal semiconductor systems that are intensively studied due to their monodisperse nature and sharp UV-vis absorption peak compared with regular quantum dots. However, the small size of such clusters (<2 nm), and the large surface-to-bulk ratio significantly limit characterisation techniques that can be utilised. Here we demonstrate how a combination of EXAFS and XANES analyses can be used to obtain information about sample stoichiometry and cluster symmetry. Investigating two types of clusters that show sharp UV-vis absorption peaks at 311 nm and 322 nm, we found that both samples possess approximately 2 : 1 Cd : S ratio and have similar nearest-neighbour structural arrangements. However, both samples demonstrate a significant departure from the tetrahedral structural arrangement, with an average bond angle determined to be around 106.1° showing a bi-fold bond angle distribution. Our results suggest that both samples are quasi-isomers - their core structures have identical chemical compositions, but different atomic arrangements with distinct bond angle distributions.

Colossal Pressure-Induced Softening in Scandium Fluoride.

The counterintuitive phenomenon of pressure-induced softening in materials is likely to be caused by the same dynamical behavior that produces negative thermal expansion. Through a combination of molecular dynamics simulation on an idealized model and neutron diffraction at variable temperature and pressure, we show the existence of extraordinary and unprecedented pressure-induced softening in the negative thermal expansion material scandium fluoride ScF_{3}. The pressure derivative of the bulk modulus B, B^{'}=(∂B/∂P)_{P=0}, reaches values as low as -220±30 at 50 K, and is constant at -50 between 150 and 250 K.

Negative thermal expansion of cubic silicon dicarbodiimide, Si(NCN)2, studied by ab initio lattice dynamics.

We report an ab initio calculation of crystal structure and lattice dynamics of cubic silicon dicarbodiimide, Si(NCN)2, using Density Functional Theory methods. The calculations reveal a low-energy spectrum of rigid unit modes that are shown to be associate with negative thermal expansion. Comparisons are drawn with the closely-related materials Zn(CN)2, and the cubic-cristobalite phase of SiO2. Instabilities in the spectrum of rigid unit modes point to the existence of disorder of the positions and orientations of the dicarbodiimide molecular anions.

Neutron scattering study of the orientational disorder and phase transitions in barium carbonate.

Orientational disorder of the molecular [Formula: see text] anions in BaCO3, which occurs naturally as the mineral witherite, has been studied using a combination of neutron total scattering analysed by the reverse Monte Carlo method and molecular dynamics simulations. The primary focus is on the phase transition to the cubic phase, which assumes a rocksalt structure (Strukturbericht type B1) with highly disordered orientations consistent with the mismatch between the site ([Formula: see text]) and molecular (3/m) symmetries. Both experiment and simulation show a high degree of disorder, with the C-O bond orientation distribution never exceeding 25% variation from that of a completely uniform distribution, although there are differences between the two methods regarding the nature of these variations. Molecular dynamics simulations are also reported for the analogous phase transitions in the very important mineral calcite, CaCO3. The combination of the simulations and comparison with BaCO3 shows that the properties of calcite at all temperatures within its stability field are affected mostly by the onset of orientational disorder associated with the high-temperature cubic phase, even though this lies outside the stability field of calcite. This is a new understanding of calcite, which previously had been interpreted purely in terms of the phase transition to an intermediate partially-disordered phase. Finally, we also found that witherite itself appears to support the development of orientational disorder on heating, with the simulations showing a sequence of phase transitions that explain the much larger thermal expansion of one axis.

First-Principles Many-Body Nonadditive Polarization Energies from Monomer and Dimer Calculations Only: A Case Study on Water.

The many-body polarization energy is the major source of nonadditivity in strongly polar systems such as water. This nonadditivity is often considerable and must be included, if only in an average manner, to correctly describe the physical properties of the system. Models for the polarization energy are usually parametrized using experimental data, or theoretical estimates of the many-body effects. Here we show how many-body polarization models can be developed for water complexes using data for the monomer and dimer only using ideas recently developed in the field of intermolecular perturbation theory and state-of-the-art approaches for calculating distributed molecular properties based on the iterated stockholder atoms (ISA) algorithm. We show how these models can be calculated, and we validate their accuracy in describing the many-body nonadditive energies of a range of water clusters. We further investigate their sensitivity to the details of the polarization damping models used. We show how our very best polarization models yield many-body energies that agree with those computed with coupled-cluster methods, but at a fraction of the computational cost.

Adjustable uniaxial zero thermal expansion and zero linear compressibility in unique hybrid semiconductors: the role of the organic chain.

Zero thermal expansion (ZTE) and zero linear compressibility (ZLC) are unique and rare properties. Materials combining ZTE and ZLC will have promising prospects. A novel route is proposed in this work to design the coexistence of uniaxial-ZTE and ZLC based on layered hybrid semiconductors [ZnTe(L)0.5] [L = N2H4, ethylenediamine (en), propyldiamine (pda)]. In the framework of [ZnTe(L)0.5], the organic chain contains the attractive and repulsive interactions that arise from the different organic components. It is demonstrated that changing the length of the organic chain can effectively regulate the interaction between different organic components and then achieve the uniaxial-ZTE and ZLC or the desired thermal expansion and compression behaviors. The origin of the coexistence of abnormal axial responses has been traced from thermodynamic formalisms, model Grüneisen parameters and specific vibration modes. It is found that low-energy phonons play an important internal role in realizing the multi-peculiar properties.