Solid-state Density Functional Theory Simulations Research Articles

Charge-clustering induced fast ion conduction in 2LiX-GaF3: A strategy for electrolyte design.

2LiX-GaF3 (X = Cl, Br, I) electrolytes offer favorable features for solid-state batteries: mechanical pliability and high conductivities. However, understanding the origin of fast ion transport in 2LiX-GaF3 has been challenging. The ionic conductivity order of 2LiCl-GaF3 (3.20 mS/cm) > 2LiBr-GaF3 (0.84 mS/cm) > 2LiI-GaF3 (0.03 mS/cm) contradicts binary LiCl (10-12 S/cm) < LiBr (10-10 S/cm) < LiI (10-7 S/cm). Using multinuclear 7Li, 71Ga, 19F solid-state nuclear magnetic resonance and density functional theory simulations, we found that Ga(F,X)n polyanions boost Li+-ion transport by weakening Li+-X- interactions via charge clustering. In 2LiBr-GaF3 and 2LiI-GaF3, Ga-X coordination is reduced with decreased F participation, compared to 2LiCl-GaF3. These insights will inform electrolyte design based on charge clustering, applicable to various ion conductors. This strategy could prove effective for producing highly conductive multivalent cation conductors such as Ca2+ and Mg2+, as charge clustering of carboxylates in proteins is found to decrease their binding to Ca2+ and Mg2+.

Low-Frequency Raman Spectroscopy of Pure and Cocrystallized Mycophenolic Acid.

The aqueous solubility of solid-state pharmaceuticals can often be enhanced by cocrystallization with a coformer to create a binary cocrystal with preferred physical properties. Greater understanding of the internal and external forces that dictate molecular structure and intermolecular packing arrangements enables more efficient design of new cocrystals. Low-frequency (sub-200 cm-1) Raman spectroscopy experiments and solid-state density functional theory simulations have been utilized together to investigate the crystal lattice vibrations of mycophenolic acid, an immunosuppressive drug, in its pure form and as a cocrystal with 2,2'-dipyridylamine. The lattice vibrations primarily consist of large-amplitude translations and rotations of the crystal components, thereby providing insights into the critical intermolecular forces governing cohesion of the molecular solids. The simulations reveal that despite mycophenolic acid having a significantly unfavorable conformation in the cocrystal as compared to the pure solid, the cocrystal exhibits greater thermodynamic stability over a wide temperature range. The energetic penalty due to the conformational strain is more than compensated for by the strong intermolecular forces between the drug and 2,2'-dipyridylamine. Quantifying the balance of internal and external energy factors in cocrystal formation indicates a path forward in the development of future mycophenolic acid cocrystals.

Low-Frequency Vibrational Spectroscopy and Quantum Mechanical Simulations of the Crystalline Polymorphs of the Antiviral Drug Ribavirin.

Crystal polymorphism is a common phenomenon in pharmaceutical solids and a critical issue when considering the formulation of therapeutics since multiple polymorphs may form during drug manufacturing. Low-frequency vibrational spectroscopy is sensitive to polymorphic content, and in this work, terahertz time-domain spectroscopy and low-frequency Raman spectroscopy were utilized in the study of crystalline ribavirin, a widely applicable antiviral. Characteristic spectra with numerous peaks in the sub-200 cm–1 region were obtained of the more common polymorph of ribavirin (Form II). Solid-state density functional theory (ss-DFT) simulations were then used to optimize the crystal structure of this polymorph and calculate the frequencies and spectral intensities of the lattice vibrations in the low-frequency region. The near-harmonic thermal behavior of the sample with cooling enabled excellent agreement between experiment and theory to be achieved, emphasizing the quality of the applied model, and the observed spectral peaks could be assigned to specific atomic motions in the solid. Form I and Form II polymorphs of ribavirin were both investigated with ss-DFT to understand the different aspects governing the relative stabilities of these solids. The ss-DFT simulations of the polymorph energies revealed that Form II is more stable at all temperatures due to a stronger cohesive energy than Form I; however, ribavirin in Form I has a significantly lower conformational energy. The finding of monotropism appears to conflict with the reported enantiotropism of the ribavirin polymorphs but ultimately confirms that crystal defects in the real samples greatly affect the thermodynamic relationship of the crystals.

Understanding the Metastability of Theophylline FIII by Means of Low-Frequency Vibrational Spectroscopy

While theophylline has been extensively studied with multiple polymorphs discovered, there is still currently no conclusive structure for the metastable theophylline form III. In this present work, by combining more widely used techniques such as X-ray diffraction and thermogravimetric analysis with more emerging techniques like low-frequency Raman and terahertz time-domain spectroscopy, to analyze the structure and dynamics of a crystalline system, it was possible to provide further evidence that the form III structure has a theophylline monohydrate structure with the water molecules removed. Solid-state density functional theory simulations were paramount in proving that this proposed structure is correct and explain how vibrational modes within the crystal structures feature and govern polymorphic transitions and the metastable form III. Through the insight provided by both simulated and experimental results, it was possible to decisively conclude the elusive crystal structure of theophylline form III. It was also shown that the correct space group for theophylline monohydrate is not P21/n but, in fact, Pc.

Crystalline Molecular Standards for Low-Frequency Vibrational Spectroscopies

The sub-200 cm−1 (sub-6 THz) vibrations of molecular crystals provide identifying features that are characteristic of each solid sample under study. These distinctive vibrational spectra have driven the development of new techniques and instrumentation in analytical spectroscopy. As terahertz time-domain spectroscopy and low-frequency Raman spectroscopy become increasingly prevalent in non-specialist laboratories, the need for a common set of spectral standards for use across these techniques becomes imperative. To meet this need, α-lactose monohydrate, biotin, and L-cystine are proposed here as molecular standards to evaluate instrument performance with both terahertz and Raman spectroscopies, as well serve as benchmarks for quantum mechanical simulations and analyses of these spectra. These substances all reveal a series of readily discernable peaks across the low-frequency region and also over a range of temperatures (295–50 K) making them even more useful. The often overlooked aspect of detailed spectral interpretation and assignment is directly addressed with rigorous solid-state density functional theory simulations of the three compounds based on a standard computational framework. By investigating these proposed molecular crystal standards with commonly available experimental and theoretical approaches, a set of realistic performance expectations can be achieved for both commercial instrumentation and software being used in low-frequency vibrational spectroscopy.

Identifying and explaining vibrational modes of quinacridones via temperature-resolved terahertz spectroscopy: absorption experiments and solid-state density functional theory simulations.

Quinacridone and its substituted analogs are pigments widely used in art and industry. The temperature dependence of the crystal structures of two quinacridone polymorphs (β and γ), along with the common variant 2,9-dimethylquinacridone, were investigated using powder X-ray diffraction and terahertz spectroscopy. These were then compared with solid-state density functional theory simulations of both structures and vibrations. X-ray patterns were collected at eight temperatures in the range 13-298 K and terahertz spectra at fifteen temperatures in the range 20-300 K. Simulations were at absolute zero and at appropriate expansions to model room temperature. It was found that some of the powder X-ray diffraction features in only β-quinacridone (15.7°, 19.7° and 31.2° at 13 K) underwent anomalous shifting with temperature change. We attribute this to the unique coplanar hydrogen bonding pattern of β-quinacridone compared to the other solids, with the unusual diffraction peaks originating from crystallographic planes perpendicular to the a axis intermolecular hydrogen bonds. This observation coincides with a contraction of the a axis with heating and results from its relatively weak N-HO hydrogen bonds and significant C-HH-C repulsions. Associated with this anomalous contraction, for β-quinacridone only spectral peaks are seen to increase in energy with temperature.

Quantitative Analysis of Minium and Vermilion Mixtures Using Low-Frequency Vibrational Spectroscopy.

Low-frequency vibrational spectroscopy offers a compelling solution for the nondestructive and noninvasive study of pigments in historical artifacts by revealing the characteristic sub-200 cm-1 spectral features of component materials. The techniques of terahertz time-domain spectroscopy (THz-TDS) and low-frequency Raman spectroscopy (LFRS) are complementary approaches to accessing this spectral region and are valuable tools for artifact identification, conservation, and restoration. In this investigation of historical pigments, pure and mixed samples of minium (Pb3O4) and vermilion (HgS) were studied using a combination of THz-TDS and LFRS experiments to determine the limits of detection (LOD) and quantitation (LOQ) for each compound with both methods. The measurements were also supported using solid-state density functional theory simulations of the pigment structures and vibrations, enabling spectral peaks to be assigned to specific atomic motions in these solids. The THz-TDS LOD was found to be similar for both minium and vermilion at 6% by mass on average. In comparison, LFRS was found to be more sensitive to both pigments, particularly to the presence of vermilion with an LFRS LOD of 0.2%. These results demonstrate that low-frequency vibrational spectroscopy can be used for successful quantitative analysis of pigment mixtures and provide reliable new data for use in heritage science.

Anomalous Temperature Dependence of the Lowest-Frequency Lattice Vibration in Crystalline γ-Aminobutyric Acid.

Crystalline γ-aminobutyric acid (GABA) exhibits unusual thermal behavior in a low-frequency lattice vibration that occurs at 37.2 cm-1 at 290 K but decreases dramatically by 34.0% when the sample is cooled to 78 K. Lattice vibrations in molecular crystals are indicators of intermolecular force characteristics, and the extraordinary temperature sensitivity of this vibration offers new insight into the local environment within the solid. Solid-state density functional theory simulations of the GABA crystal have found this anomalous frequency shift is based in unexpected differences in the strengths of the intermolecular hydrogen bonds that are cursorily the same. This was accomplished through mapping of the potential energy surfaces governing the terahertz-frequency motions of the GABA solid and use of the quasi-harmonic approximation to model the response of all the lattice vibrations to temperature-induced unit cell volume changes brought about through the anharmonic character of the intermolecular interactions. The analysis reveals that the vibration in question is rotational in nature and involves the significant distortion of a specific weak intermolecular N-H···O hydrogen bond in the crystal that results in its unique thermal response.

Terahertz Spectroscopy and Quantum Mechanical Simulations of Crystalline Copper-Containing Historical Pigments.

Terahertz spectroscopy, a noninvasive and nondestructive analytical technique used in art conservation and restoration, can provide compelling data concerning the composition and condition of culturally valuable and historical objects. Terahertz spectral databases of modern and ancient artists' pigments exist but lack explanations for the origins of the unique spectral features. Solid-state density functional theory simulations can provide insight into the molecular and intermolecular forces that dominate the observed absorption features as well as reveal deviations from simple harmonic vibrational behavior that can complicate these spectra. The characteristic terahertz spectra of solid azurite, malachite, and verdigris are presented here, along with simulations of their crystalline structures and sub-3.0 THz lattice vibrations. The powerful combination of theory and experiment enables unambiguous spectral assignment of these complex materials and highlights the challenges that anharmonic peak broadening in organic-containing materials may present in the construction of reference pigment databases.

Sodium, Potassium, and Calcium in Europa: An Atomic Journey through Water Ice

Abstract Sodium and potassium are known to be present as neutral elements in the exosphere of Europa. The question of the origin of these alkalis—endogenous or exogenous—remains open. They have been ascribed to exogenous contamination due to volcanism from nearby Io, or the accretion of meteorites and dust. However, these mechanisms fail to fit the observed sodium-to-potassium ratio. Sodium and potassium have also been considered to originate from Europa’s putative subsurface ocean, generated by past rock-water leaching. The latter scenario implies a journey of the ions and atoms throughout the ice covering Europa. This raises questions about their stability into the bulk as well as on top of ice. These questions are addressed with first principle periodic solid-state density functional theory simulations describing the relative propensities of sodium, potassium, and calcium for being trapped in the bulk. The evolution of the ionic character of these atoms is followed by means of a topological analysis as they come up to the surface of the ice crust. We find that the metals, almost totally ionized in the ice bulk (net charge ∼+0.8) where they are stabilized by ∼1 eV or more, recover a quasi-neutrality (net charge ∼+0.2) when weakly adsorbed at the surface by ∼0.15 eV. Our results are consistent with the assumption that sodium and potassium observed in Europa exosphere come from the sputtering of materials issued from the underlying ocean and exposed by resurfacing events. They also suggest that calcium should be searched for by future missions.

Predicting the structures and associated phase transition mechanisms in disordered crystals via a combination of experimental and theoretical methods.

Disordered materials make up a large proportion of condensed phase systems, but the difficulties in describing their structures and molecular dynamics limit their potential applications. Disordered crystalline systems, also known as plastic crystals, offer a unique perspective into these factors because the systems retain a degree of crystallinity, reducing the degrees of freedom that must be explored when interpreting the results. However, while disordered crystals do diffract X-rays, it is difficult to fully resolve meaningful crystalline structures, with the best scenario resulting in lattice parameters. In this study, we use a combination of experimental terahertz time-domain spectroscopy, and theoretical solid-state ab initio density functional theory and molecular dynamics simulations to fully elucidate the structures and associated dynamics of organic molecular solids. The results highlight that this combination provides a complete description of the energetic and mechanistic pathways involved in the formation of disordered crystals, and highlights the importance of low-frequency dynamics in their properties. Finally, with structures fully determined and validated by the experimental results, recent progress into anharmonic calculations, namely the quasi-harmonic approximation method, enables full temperature and pressure-dependent properties to be understood within the framework of the potential energy hyper-surface structure.

Examination of l-Glutamic Acid Polymorphs by Solid-State Density Functional Theory and Terahertz Spectroscopy.

The ability of l-glutamic acid to crystallize in two different forms has long been the subject of study due to its commercial importance. While a solvent-mediated phase transformation between the α and β polymorphs is the prevailing theory, recent reports indicate a thermal solid-solid transformation between the two may be possible. However, determining accurate thermodynamic stabilities of these crystals has been challenging. Here new low-temperature single-crystal X-ray diffraction data coupled to solid-state density functional theory simulations have enabled a detailed description to be achieved for the energetic parameters governing the stabilization of the two l-glutamic acid solids. The temperature-dependent Gibbs free-energy curves show that α-glutamic acid is the preferred form at low temperatures (<222 K) and the β form is most stable at ambient temperatures. Terahertz time-domain spectroscopy was utilized to evaluate the quality of the intermolecular force modeling as well as to provide characteristic low-frequency spectral data that can be used for quantification of polymorph mixtures or crystal growth monitoring.

Origins of the Relative Stabilities of Anhydrous and Hydrated d-Mannitol Crystals.

d-Mannitol, a common bulking agent used in the pharmaceutical and food industries, is known to crystallize in three anhydrous polymorphs (α, β, and δ) and a hemihydrate form. In this work, solid-state density functional theory simulations and experimental terahertz spectroscopy were used to evaluate the stabilities, thermodynamic relationships, and spectral signatures of these various solids. The thermodynamic analyses of the anhydrous polymorphs identified enantiotropic transitions for β-δ at approximately room temperature and α-δ near the α melting point, but showed α and β to be monotropically related. Computational refinement of the d-mannitol hemihydrate crystal structure and comparison of its Gibbs free energy to the anhydrous forms showed the hemihydrate to be stable only at low temperatures (<253 K), in agreement with experiment, and reveals its conversion to the δ polymorph upon dehydration. Quantification of the intramolecular and intermolecular energies within these solids highlights the competition that exists between molecular conformation and cohesion energies and how a balance of these forces dictates the observed behaviors.

ChemInform Abstract: Density Functional Theory in the Solid State

AbstractReview: 81 refs.

Uncovering the Terahertz Spectrum of Copper Sulfate Pentahydrate.

Terahertz vibrational spectroscopy has evolved into a powerful tool for the detection and characterization of transition metal sulfate compounds, specifically for its ability to differentiate between various hydrated forms with high specificity. Copper(II) sulfate is one such system where multiple crystalline hydrates have had their terahertz spectra fully assigned, and the unique spectral fingerprints of the forms allows for characterization of multicomponent systems with relative ease. Yet the most commonly occurring form, copper(II) sulfate pentahydrate (CuSO4·5H2O), has proven elusive due to the presence of a broad absorption across much of the terahertz region, making the unambiguous identification of its spectral signature difficult. Here, it is shown that the sub-100 cm(-1) spectrum of CuSO4·5H2O is obscured by absorption from adsorbed water and that controlled drying reveals sharp underlying features. The crystalline composition of the samples was monitored in parallel by X-ray diffraction as a function of drying time, supporting the spectroscopic results. Finally, the terahertz spectrum of CuSO4·5H2O was fully assigned using solid-state density functional theory simulations, helping attribute the additional absorptions that appear after excessive drying to formation of CuSO4·3H2O.

Solid-State Physics Perspective on Hybrid Perovskite Semiconductors

In this review we examine recent theoretical investigations on 2D and 3D hybrid perovskites (HOPs) that combine classical solid-state physics concepts and density functional theory (DFT) simulations as a tool for studying their optoelectronic properties. Such an approach allows one to define a new class of semiconductors, where the pseudocubic high-temperature perovskite structure plays a central role. Bloch states and k.p. Hamiltonians yield new insight into the influence of lattice distortions, including loss of inversion symmetry, as well as spin–orbit coupling. Electronic band folding and degeneracy, effective masses, and optical absorption are analyzed. Concepts of Bloch and envelope functions, as well as confinement potential, are discussed in the context of layered HOP and 3D HOP heterostructures. Screening and dielectric confinements are important for room-temperature optical properties of 3D and layered HOP, respectively. Nonradiative Auger effects are analyzed for the first time close to the ele...

Terahertz Vibrational Modes of the Rigid Crystal Phase of Succinonitrile

Succinonitrile (N ≡ C-CH2-CH2-C ≡ N), an orientationally disordered molecular plastic crystal at room temperature, exhibits rich phase behavior including a solid-solid phase transition at 238 K. In cooling through this phase transition, the high-temperature rotational disorder of the plastic crystal phase is frozen out, forming a rigid crystal that is both spatially and orientationally ordered. Using temperature-dependent terahertz time-domain spectroscopy, we characterize the vibrational modes of this low-temperature crystalline phase for frequencies from 0.3 to 2.7 THz and temperatures ranging from 20 to 220 K. Vibrational modes are observed at 1.122 and 2.33 THz at 90 K. These modes are assigned by solid-state density functional theory simulations, corresponding respectively to the translation and rotation of the molecules along and about their crystallographic c-axis. In addition, we observe a suppression of the phonon modes as the concentration of dopants, in this case a lithium salt (LiTFSI), increases, indicating the importance of doping-induced disorder in these ionic conductors.

Density functional theory in the solid state

Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure-property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program.

Rational Design of Monocrystalline (InP)yGe5–2y/Ge/Si(100) Semiconductors: Synthesis and Optical Properties

In this work, we extend our strategy previously developed to synthesize functional, crystalline Si(5-2y)(AlX)y {X = N,P,As} semiconductors to a new class of Ge-III-V hybrid compounds, leading to the creation of (InP)(y)Ge(5-2y) analogues. The compounds are grown directly on Ge-buffered Si(100) substrates using gas source MBE by tuning the interaction between Ge-based P(GeH3)3 precursors and In atoms to yield nanoscale "In-P-Ge3" building blocks, which then confer their molecular structure and composition to form the target solids via complete elimination of H2. The collateral production of reactive germylene (GeH2), via partial decomposition of P(GeH3)3, is achieved by simple adjustment of the deposition conditions, leading to controlled Ge enrichment of the solid product relative to the stoichiometric InPGe3 composition. High resolution XRD, XTEM, EDX, and RBS indicate that the resultant monocrystalline (InP)(y)Ge(5-2y) alloys with y = 0.3-0.7 are tetragonally strained and fully coherent with the substrate and possess a cubic diamond-like structure. Molecular and solid-state ab initio density functional theory (DFT) simulations support the viability of "In-P-Ge3" building-block assembly of the proposed crystal structures, which consist of a Ge parent crystal in which the P atoms form a third-nearest-neighbor sublattice and "In-P" dimers are oriented to exclude energetically unfavorable In-In bonding. The observed InP concentration dependence of the lattice constant is closely reproduced by DFT simulation of these model structures. Raman spectroscopy and ellipsometry are also consistent with the "In-P-Ge3" building-block interpretation of the crystal structure, while the observation of photoluminescence suggests that (InP)(y)Ge(5-2y) may have important optoelectronic applications.

A solid-state density functional theory investigation of the effect of metal substitution (Metal = Mn, Cd, Co) on the terahertz spectra of isomorphous molecular metal 5-(4-pyridyl)tetrazolato complexes

The crystal structure and experimental terahertz spectroscopy of an isomorphous series [Mn(C6H4N5)2(H2O)4]⋅2H2O (Mn-4PT), [Co(C6H4N5)2(H2O)4]⋅2H2O (Co-4PT), and [Cd(C6H4N5)2(H2O)4]⋅2H2O (Cd-4PT) were compared using solid-state density functional theory (DFT) simulations. The effect of the central metal atom was investigated to determine the influence on the low energy lattice and molecular vibrations exhibited in the region from 10 to 100cm−1, known as the terahertz (THz) region. Using solid-state DFT the normal modes of these THz vibrations were determined and it was shown that the mass and size of the metal center has a large effect in this region. Each complex exhibited common vibrational modes involving whole ligand motion around the central metal atom. These vibrations were found to shift to lower frequencies with a drastic mass increase; however, this trend is reversed with the smaller mass change between the manganese and cobalt due to the stronger cobalt–nitrogen bond compared to the manganese–nitrogen bond.