Search

Organometallic-sandwich salts are well-known materials that undergo order–disorder phase transitions, leading to a high-temperature phase characterized by the total or partial disorder of ionic species. Their potential for barocaloric applications has not previously been explored. Here, we focus on two salts derived from metallocenes with the formula [Cp2M][PF6] where Cp: cyclopentadienyl anion (C5H5)− and M: Fe3+ or Co3+. These molecular salts exhibit two solid–solid phase transitions, shifting from a well-ordered crystalline phase at low temperatures to an orientationally disordered phase in the case of the Fe-compound, and a partially disordered phase in the case of the Co-compound above room temperature. We find a significant entropy change (∼40 J K−1 kg−1) and a moderate volume change (∼2%) associated with these phase transitions. Additionally, we observe that these transitions are highly sensitive to external applied pressure, leading to substantial barocaloric effects (exceeding 20 K kbar−1). Very interestingly, we obtain substantial values for reversible adiabatic temperature change (ΔTrev > 10 K) under an applied pressure as low as 1 kbar, comparable to those observed in the most promising barocaloric materials. We prove that the unique structure and chemical bonding of these sandwich organometallic cations are responsible for their interesting and unusual barocaloric response. These findings position the metallocenium family as promising candidates for eco-friendly solid-state refrigeration technologies. [ABSTRACT FROM AUTHOR]

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/. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The phase transitions and orientational order of the methylammonium molecular ion in a deuterated sample of [CH3NH3]PbI3 have been studied using neutron total scattering and Reverse Monte Carlo methods. From analysis of the C–N bond orientational distribution functions, obtained both by forming histograms and by analysis using orthonormal spherical functions, we showed that the molecular ions were found to have an almost completely uniform distribution of orientations in the high-temperature cubic phase. The C–N bonds then show progressively increasing order on cooling within the intermediate phase, and complete order in the low-temperature phase. Analysis of the network of connected PbI6 octahedra has shown that the rigidity of the PbI6 octahedra is similar to that in the related cubic perovskite CsPbI3, and is intermediate between the oxide perovskite SrTiO3 and the more flexible ScF3. Both the fluctuations of the I–Pb–I right angle and the orientations of the PbI6 show normal variation with temperature across the range of temperatures of all three phases. The Pb–I bond has been found to be highly anharmonic, comparable to the same bond in CsPbI3. We find no evidence for static ferroelectric distortions at any temperature, but dynamic fluctuations in the local dipole moment are found to be larger than in some other materials. [ABSTRACT FROM AUTHOR]

The lithium-ion conducting solid electrolyte Li7La3Zr2O12 has one crystalline phase that has the cubic garnet structure, and this phase has one of the highest room temperature lithium-ion conductivities of any oxide. Neutron and X-ray total scattering data, analysed by the Reverse Monte Carlo method, have been used to examine the distribution of lithium ions over a wide range of temperatures, and to explore aspects of the stability and flexibility of the basic oxide structure. We compare the results with those from supporting molecular dynamics simulations. Unlike previous work, our method requires no prior assumptions about the Li ion distribution. Using total scattering rather than solely Bragg scattering, we obtain much higher real-space resolution than previously available. The combination of X-ray and neutron scattering allows detailed characterisation of both the mobile and immobile sublattices. Only a small fraction of Li ions are mobile at any one time; diffusion occurs primarily by movement between Li2 sites via empty Li1 sites. [ABSTRACT FROM AUTHOR]

Nowadays around 46% of food production around the world requires refrigeration, which is generally provided either by active vapour-compression (based on refrigerants with liquid–gas transitions) or passive cold-storage (based on solid-to-liquid phase change materials, SL-PCMs). However, in order to avoid fluid losses during the transitions, new thermomaterials with solid–solid transitions are desired for both applications. In this work, we find that [DBA][BF4] (DBA = di-n-butylammonium) is a promising thermomaterial with solid–solid phase transitions. This compound presents thermal properties of great interest not only for active barocaloric refrigeration, but also for passive cold-storage, which make this a unique multipurpose thermomaterial. The observed cold-storage capacity is very close to that of commercial SL-PCMs (E ∼ 135 kJ kg−1), while the pressure-induced thermal changes (ΔS ∼ [200–270] J K−1 kg−1) are superior to those of most barocaloric materials, operating under lower pressures (p ∼ [500–1000] bar). Moreover, the operating temperature range of this material is very adequate for food preservation (250–310 K), which is a great advantage over most barocaloric materials. Beyond the thermal properties, we perform deep structural characterization, which reveals a progressive structural disorder of the [DBA]+ cations and [BF4]− anions as the origin of such thermal properties, which will help the future rational design of enhanced thermomaterials. [ABSTRACT FROM AUTHOR]

The field of relaxor ferroelectrics has long been dominated by ceramic oxide materials exhibiting large polarisations with temperature and frequency dependence. Intriguingly, the dense metal–organic framework (MOF) [NH3NH2]Mg(HCO2)3 was reported as one of the first coordination frameworks to exhibit relaxor-like properties. This work clarifies the origin of these relaxor-like properties through re-examining its unusual phase transition using neutron single crystal diffraction, along with solid-state NMR, quasielastic neutron scattering and dielectric spectroscopy studies. This reveals that the phase transition is caused by the partial reorientation of NH3NH2 within the pores of the framework, from lying in the planes of the channel at lower temperatures to along the channel direction above the transition temperature. The transition occurs via a dynamic process such that the NH3NH2 cations can slowly interconvert between parallel and perpendicular orientations, with an estimated activation energy of 50 kJ mol−1. Furthermore these studies are consistent with proton hopping between the hydrazinium cations oriented along the channel direction via a proton site intermediate. This study suggests the ferroelectric properties of [NH3NH2]Mg(HCO2)3 are likely driven by a hydrogen bonding mechanism. The relaxor behaviour is proposed to be the result of polar regions, which likely fluctuate due to increased cation dynamics at high temperature. The combination of cation reorientation and proton hopping fully describes this material's relaxor-like behaviour, suggesting a route to the future design of non-oxide-based relaxor ferroelectrics. [ABSTRACT FROM AUTHOR]

The hybrid perovskite copper(ii) guanidinium formate, C(NH2)3[Cu(HCO2)3], is a close analog of the inorganic material KCuF3, a well-studied exemplar of the Heisenberg spin- 1 2 antiferromagnetic chain. However, the polymorphism known in KCuF3, based on different orbital ordering of the d9 Cu2+ ions, has never been observed in C(NH2)3[Cu(HCO2)3]. We use density-functional theory calculation to construct a hypothetical polymorph of the hybrid perovskite analogous to the known structure of KCuF3. This polymorph is very similar in geometric and magnetic structures to the known phase of C(NH2)3[Cu(HCO2)3], but it is no longer polar. It has only a marginally higher internal energy than the known phase but also a lower vibrational entropy and, hence, higher free energy. Furthermore, stacking faults are far more energetically costly in C(NH2)3[Cu(HCO2)3] than in KCuF3. These results together help to explain why KCuF3 is experimentally polymorphic but C(NH2)3[Cu(HCO2)3] is not. The detailed comparison between analogous inorganic and hybrid materials illustrates the subtle differences that can arise from replacing atomic ions with molecular ions and that may be exploited in crystal engineering of these materials. [ABSTRACT FROM AUTHOR]

High-entropy order–disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K−1 kg−1. Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this difference in the dynamics to the contrast between an 'interlocked' state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials. [ABSTRACT FROM AUTHOR]

Using calculations of the phonon distribution in ScF3 across the whole of reciprocal space, we show that the important phonons for the negative thermal expansion in this material are those associated with the rigid unit modes (RUMs) and associated quasi-RUMs. We discuss the role of the bond-bending flexibility within the ScF6 octahedra, and how this enables other phonons to make an additional but ultimately much weaker contribution to negative thermal expansion. These results inform recent discussions on the role of correlated atomic motions in giving rise to negative thermal expansion in ScF3. [ABSTRACT FROM AUTHOR]

The deceptively simple inorganic salt ammonium sulfate undergoes a ferroelectric phase transition associated with a very large entropy change and both electrocaloric and barocaloric functionality. While the structural origins of the electrical polarisation are now well established, those of the entropy change have been controversial for over 50 years. This question is resolved here using a combination of density‐functional theory phonon calculations with inelastic neutron scattering under variable temperature and pressure, supported by complementary total and quasielastic neutron scattering experiments. A simple model of the entropy in which each molecular ion is disordered across the mirror plane in the high symmetry phase, although widely used in the literature, proves to be untenable. Instead, the entropy arises from low‐frequency librations of ammonium ions in this phase, with harmonic terms that are very small or even negative. These results suggest that, in the search for molecular materials with functionality derived from large entropy changes, vibrational entropy arising from broad energy minima is likely to be just as important as configurational entropy arising from crystallographic disorder. [ABSTRACT FROM AUTHOR]

The vibrational properties of crystalline bulk materials are well described by Debye theory, which successfully predicts the quadratic ω2 low-frequency scaling of the vibrational density of states. However, the analogous framework for nanoconfined materials with fewer degrees of freedom has been far less well explored. Using inelastic neutron scattering, we characterize the vibrational density of states of amorphous ice confined inside graphene oxide membranes and we observe a crossover from the Debye ω2 scaling to an anomalous ω3 behaviour upon reducing the confinement size L. Additionally, using molecular dynamics simulations, we confirm the experimental findings and prove that such a scaling appears in both crystalline and amorphous solids under slab-confinement. We theoretically demonstrate that this low-frequency ω3 law results from the geometric constraints on the momentum phase space induced by confinement along one spatial direction. Finally, we predict that the Debye scaling reappears at a characteristic frequency ω× = vL/2π, with v the speed of sound of the material, and we confirm this quantitative estimate with simulations. A description of the vibrational properties of amorphous ice confined in graphene oxide membranes, as an exemplary nanoconfined material, is presented. Inelastic neutron scattering experiments and molecular dynamics simulations show anomalous deviations from standard bulk behavior. [ABSTRACT FROM AUTHOR]

Barocaloric effects in a layered hybrid organic–inorganic compound, (C10H21NH3)2MnCl4, that are reversible and colossal under pressure changes below 0.1 GPa are reported. This barocaloric performance originates in a phase transition characterized by different features: A strong disordering of the organic chains, a very large volume change, a very large sensitivity of the transition temperature to pressure and a small hysteresis. The obtained values are unprecedented among solid‐state cooling materials at such low pressure changes and demonstrate that colossal effects can be obtained in compounds other than plastic crystals. The temperature‐pressure phase diagram displays a triple point indicating enantiotropy at high pressure. [ABSTRACT FROM AUTHOR]

Highly stable ferroelectrics with reversible high-temperature phase transitions and switchable nonlinear optical behaviour are much coveted targets for emerging optoelectronic applications. Here, we demonstrate a cyano-bridged perovskite [(CH3)3NOH]2[KCo(CN)6] (TMAO-Co), a new analogue of the multi-axial ferroelectric [(CH3)3NOH]2[KFe(CN)6] (TMAO-Fe) with improved thermal stability and enhanced second-order nonlinear optical response. Indeed, for TMAO-Co the Curie temperature (Tc) is shifted to a higher value of ca. 416 K (improvement by ca. 10 K versusTMAO-Fe); the separation between Tc and the decomposition threshold is 46 K. TMAO-Co is a biaxial ferroelectric as revealed by P(E) hysteresis loop measurements along the a and c crystallographic directions with spontaneous polarization values of 0.9 and 0.63 μC cm−2 at 293 K, respectively. The SHG response of TMAO-Co is two times higher than that of TMAO-Fe. The improved stability of TMAO-Co to thermal and optical loads allowed for demonstration of bistable switching of nonlinear optical response between SHG-on and SHG-off states by temperature sweeping. Structurally, TMAO-Co reproduces the unusual characteristics of TMAO-Fe, i.e. the first-order phase transition between (polar) monoclinic to (nonpolar) cubic phases involving bond switching and is assisted by the pronounced increase of disorder of the TMAO cations above Tc. Combined temperature-resolved Raman and infrared spectroscopic measurements were employed to track the symmetry increase above Tc, which is primarily associated with changes in hydrogen-bonding. Consistent with the bond-switching character of the phase transitions, a pronounced shift to higher wavenumbers is observed for the O–H stretching modes. The DFT calculations demonstrate that the system's polarization along the a-axis mostly comes from the rotation of the [(CH3)3NOH]2 cluster, while the atomic displacement of the framework contributes largely to that along the c axis. [ABSTRACT FROM AUTHOR]

Reorientation of organic cations in the cubic interstices of cyanoelpasolite molecular perovskites results in a variety of structural phase transitions, but far less is known about these cations' dynamics. We report quasielastic neutron scattering from the materials (C3H5N2)2K[MIII(CN)6], M = Fe,Co, which is directly sensitive to the rotation of the imidazolium ion. The motion is well described by a circular three-site hopping model, with the ion rotating within its plane in the intermediate-temperature phase, but tilting permanently in the high-temperature phase. Thus the two rhombohedral phases, which are crystallographically rather similar, have markedly different dynamics. The activation energy of rotation is about 10 kJ mol−1 and the barrier between orientations is 6 kJ mol−1. Our results explain two anomalous features in these materials' dielectric constants. [ABSTRACT FROM AUTHOR]

Correction for 'Local structure and lithium-ion diffusion pathway of cubic Li7La3Zr2O12 studied by total scattering and the Reverse Monte Carlo method' by Haolai Tian et al., J. Mater. Chem. A, 2023, 11, 25516–25533, https://doi.org/10.1039/D3TA04495E. [ABSTRACT FROM AUTHOR]

A combination of single-crystal X-ray diffraction and variable-temperature powder studies is employed to study the zero thermal expansion taking place in a flexible, stable framework of tetramethylammonium copper(I) zinc(II) cyanide. The observed behavior of these compounds is shown to be a result of the presence of the low-energy vibrational modes.

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, their phase transformations under pressure are more complex and less well understood. We have studied the structures of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Under pressure, these materials transform to a rhombohedral phase isostructural to cadmium guanidinium formate. This transformation accommodates the reduced cell volume while preserving the perovskite topology of the framework. Using density-functional theory calculations, we show that this behaviour is a consequence of the hydrogenbonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression within their plane. Our results demonstrate more generally that identifying suitable host-guest hydrogen-bonding geometries may provide a route to engineering hybrid perovskite phases with desirable crystal structures. [ABSTRACT FROM AUTHOR]

The Prussian blue (cyanide-bridged, ordered double perovskite) analogue potassium imidazolium hexacyanoferrate, (C3N2H5)2K[Fe(CN)6], contains imidazolium cations encapsulated within a metal-cyanide framework. These are free to rotate in the intermediate- and high-temperature phases, but freeze into fixed orientations in the low-temperature phase. The phase transition between intermediate- and low-temperature phases thus causes a substantial change in this material's dielectric constant. However, the detailed cation dynamics, and in particular how they differ between intermediate- and high-temperature phases, remain unclear. We report here total neutron scattering measurements on a perdeuterated sample of this material. Reverse Monte Carlo modelling reveals that the intermediate-temperature phase is associated with a stiffening of the metal-cyanide framework compared to either of the other phases. This shows that the dynamics responsible for the phase transitions involve competition between the energetic penalty for bending the metal-cyanide links and the benefit of host–guest hydrogen bonding. Our results demonstrate both that disordered framework materials have important local structure that is not visible to Bragg scattering, and that there is a crucial link between this structure and the dynamics that give rise to exploitable electric properties. [ABSTRACT FROM AUTHOR]

Materials in the family of Prussian blue analogues (C3H5N2)2K[ M(CN)6], where C3H5N2 is the imidazolium ion and M=Fe, Co, undergo two phase transitions with temperature; at low temperatures the imidazolium cations have an ordered configuration ( C2/ c), while in the intermediate- and high-temperature phases (both previously reported as [ABSTRACT FROM AUTHOR]

Carbon capture and storage (CCS) offers a possible solution to curb the CO2 emissions from stationary sources in the coming decades, considering the delays in shifting energy generation to carbon neutral sources such as wind, solar and biomass. The most mature technology for post-combustion capture uses a liquid sorbent, amine scrubbing. However, with the existing technology, a large amount of heat is required for the regeneration of the liquid sorbent, which introduces a substantial energy penalty. The use of alternative sorbents for CO2 capture, such as the CaO–CaCO3 system, has been investigated extensively in recent years. However there are significant problems associated with the use of CaO based sorbents, the most challenging one being the deactivation of the sorbent material. When sorbents such as natural limestone are used, the capture capacity of the solid sorbent can fall by as much as 90 mol% after the first 20 carbonation–regeneration cycles. In this study a variety of techniques were employed to understand better the cause of this deterioration from both a structural and morphological standpoint. X-ray and neutron PDF studies were employed to understand better the local surface and interfacial structures formed upon reaction, finding that after carbonation the surface roughness is decreased for CaO. In situ synchrotron X-ray diffraction studies showed that carbonation with added steam leads to a faster and more complete conversion of CaO than under conditions without steam, as evidenced by the phases seen at different depths within the sample. Finally, in situ X-ray tomography experiments were employed to track the morphological changes in the sorbents during carbonation, observing directly the reduction in porosity and increase in tortuosity of the pore network over multiple calcination reactions. [ABSTRACT FROM AUTHOR]

We report total neutron scattering measurements on the metal–organic perovskite analogue dimethylammonium manganese(ii) formate, (CD3)2ND2[Mn(DCO2)3]. Reverse Monte Carlo modelling shows that, in both the disordered high-temperature and ordered low-temperature phases, the ammonium moiety forms substantially shorter hydrogen bonds (N…O = 2.4 Å and 2.6 Å) than are visible in the average crystal structures. These bonds result from a pincer-like motion of two adjacent formate ions about the dimethylammonium ion in such a way that the framework can adjust independently to the positions of nearest-neighbour dimethylammonium ions. At low temperatures the shortest hydrogen bond is less favourable, apparently because it involves a greater distortion of the framework. Furthermore, in the high-temperature phase, in addition to the three disordered nitrogen positions expected from the average crystal structure, there appear to be also smaller probability maxima between these positions, corresponding to orientations in which the dimethylammonium is hydrogen-bonded to the two oxygen atoms of a single formate ion. The spontaneous strain across the phase transition reveals a contraction of the framework about the dimethylammonium cation, continuing as the material is cooled below the transition temperature. These results provide direct evidence of the local atomic structure of the guest-framework hydrogen bonding, and in particular the distortions of the framework responsible for the phase transition in this system. [ABSTRACT FROM AUTHOR]

We study the temperature dependence of the pressure-induced softening in the negative thermal expansion material Zn(CN)2 using neutron powder diffraction and molecular dynamics simulations. Both the simulation and experiment show that the pressure-induced softening only occurs above a minimum temperature and also weakens at high temperatures. [ABSTRACT FROM AUTHOR]

A simple test based on Bayesian statistics for the presence of minority populations in single-crystal structure refinement is presented. The test is illustrated by analysis of photocrystallographic experiments on single crystals of ruthenium-sulfur-dioxide-based complexes. In data sets collected after irradiation with light, conventional refinements of the populations of different metastable states to values below 4% are shown to be statistically significant. The results also confirm that the photo-induced states are absent from data collected in the dark. [ABSTRACT FROM AUTHOR]

We report a first-principles study of hybrid organic–inorganic perovskites with formula [A]Cu(H2POO)3 (A = triazolium (Trz) and guanidinium (Gua), and H2POO− = hypophosphite), and [HIm]Cu(HCO2)3 (HIm = imidazolium cation, HCO 2 − = formate). The triazolium hypophosphite and the formate have been suggested as possible ferroelectrics. We study the fully relaxed structures with different magnetic orderings and possible phonon instabilities. For the [Trz]Cu hypophosphite, the Trz cation is shown to induce large octahedral distortions due to the Jahn-Teller effect, with Cu-O long-bond ordering along two perpendicular directions, which is correlated with antiferromagnetic ordering and strongly one-dimensional. We find that the structure is dynamically stable with respect to zone-center distortions, but instabilities appear along high symmetry lines in the Brillouin zone. On the other hand, for the [HIm]Cu formate, large octahedral distortions are found, with large Cu-O bonds present in half of the octahedra, in this case along a single direction, and correspondingly, the magnetism is almost two-dimensional. [ABSTRACT FROM AUTHOR]

The photoinduced O-bound coordination mode in RuSO2 complexes, previously observed only at 13 K, has been generated at 100 K in tetraammineaqua(sulfur dioxide)ruthenium(II) (±)-camphorsulfonate. This coordination state, often denoted MS1, decays to the η2-bound MS2 state, with an estimated half-life of 3.4(8) h and a long-lived population of 2.9(4)% at 120 K. [ABSTRACT FROM AUTHOR]

Rigid‐unit modes, which allow coordination polyhedra to remain undistorted and hence cost little energy, provide a way of understanding many important physical properties. Campbell et al. [Acta Cryst. (2018). A74, 408–424] have developed an elegant new algebraic approach to finding these distortion modes. [ABSTRACT FROM AUTHOR]

By considering the lattice dynamics between bond strain and transverse vibration, we establish a common modelfor ceramic and hybrid materials that exhibit negative thermal expansion (NTE). We show that pressure-inducedsoftening, increase in NTE with pressure, and variation of NTE with temperature all arise naturally from thesame mechanical model that gives rise to the NTE itself. [ABSTRACT FROM AUTHOR]