Crystal Cohesive Energy Research Articles

"Whole-Body" Fluorination for Highly Efficient and Ultra-Stable All-Inorganic Halide Perovskite Quantum Dots.

Inherent "soft" ionic lattice nature of halide perovskite quantum dots (QDs), triggered by the weak Pb-X (X=Cl, Br, I) bond, is recognized as the primary culprit for their serious instability. A promising way is to construct exceedingly strong ionic interaction inside the QDs and increase their crystal cohesive energy by substituting the interior X- with highly electronegative F- , however, which is challenging and hitherto remains unreported. Here, a "whole-body" fluorination strategy is proposed for strengthening the interior bonding architecture of QDs, wherein the F- are uniformly distributed throughout the whole nanocrystal encompassing both the interior lattice and surface, successfully stabilizing their "soft" crystal lattice and passivating surface defects. This approach effectively mitigates their intrinsic instability issues including light-induced phase segregation. As a result, light-emitting devices based on these QDs exhibit exceptional efficiency and remarkable stability.

“Whole‐Body” Fluorination for Highly Efficient and Ultra‐Stable All‐Inorganic Halide Perovskite Quantum Dots

AbstractInherent “soft” ionic lattice nature of halide perovskite quantum dots (QDs), triggered by the weak Pb−X (X=Cl, Br, I) bond, is recognized as the primary culprit for their serious instability. A promising way is to construct exceedingly strong ionic interaction inside the QDs and increase their crystal cohesive energy by substituting the interior X− with highly electronegative F−, however, which is challenging and hitherto remains unreported. Here, a “whole‐body” fluorination strategy is proposed for strengthening the interior bonding architecture of QDs, wherein the F− are uniformly distributed throughout the whole nanocrystal encompassing both the interior lattice and surface, successfully stabilizing their “soft” crystal lattice and passivating surface defects. This approach effectively mitigates their intrinsic instability issues including light‐induced phase segregation. As a result, light‐emitting devices based on these QDs exhibit exceptional efficiency and remarkable stability.

Different Conformations of 2'-Deoxycytidine in the Gas and Solid Phases: Competition between Intra- and Intermolecular Hydrogen Bonds.

Computational results have been reported for 2'-deoxycytidine (dC), its gas phase isomers, tautomers, and their conformers, as well as for the crystalline phase. In addition to the neutral gas phase molecules, we have also considered associated radical anions and cations. The structural calculations were performed at the density functional and MP2 levels of theory. Vertical electron ionization energies and excess electron binding energies were determined using electron propagator theory. The α-anomer proved to be more stable by a fraction of kcal/mol than the biologically relevant canonical β-anomer. The conformational space of canonical dC has been systematically probed. dC in the crystalline phase or DNA structures favors canonical anti conformations. These structures were used in past computational studies to model gas phase characteristics of dC. Our findings indicate, however, that the gas phase dC favors syn conformations. It has repercussions for earlier interpretations of gas phase experimental results based on these computational results. The thermodynamic dominance of syn conformations results from the formation of an intramolecular O5'-H13···O2 hydrogen bond. The IR spectra of the most stable syn and anti canonical conformers differ markedly in the region of frequencies corresponding to NH/OH stretching modes. The MP2 value of deprotonation enthalpy of dC of 1411.7 kJ/mol is in very good agreement with the experimental value of 1409 ± 2.5 kJ/mol. The most stable valence anions are characterized by electron vertical detachment energies (VDE) in the 0.8-1.0 eV range, in good agreement with the experimental VDE of 0.87 eV. The barrier for the glycosidic bond cleavage is significant in the neutral canonical dC, 40.0 kcal/mol, and it is reduced to 22 and 16 kcal/mol for the anionic and cationic radicals of dC, respectively. The cleavage reaction is exothermic by 4 kcal/mol for dC- and endothermic by 7 and 9 kcal/mol for dC+ and dC, respectively. We decomposed the crystal cohesive energy into repulsive one-body terms associated with the syn-anti conformational changes, and the attractive intermolecular interaction term. We exposed that the syn-anti conformational changes are very favorable for intermolecular interactions; in particular they make the imino-amino side of the cytosine residue accessible to intermolecular interactions.

Assessment of Different Quantum Mechanical Methods for the Prediction of Structure and Cohesive Energy of Molecular Crystals.

A comparative assessment of the accuracy of different quantum mechanical methods for evaluating the structure and the cohesive energy of molecular crystals is presented. In particular, we evaluate the performance of the semiempirical HF-3c method in comparison with the B3LYP-D* and the Local MP2 (LMP2) methods by means of a fully periodic approach. Three benchmark sets have been investigated: X23, G60, and the new K7; for a total of 82 molecular crystals. The original HF-3c method performs well but shows a tendency at overbinding molecular crystals, in particular for weakly bounded systems. For the X23 set, the mean absolute error for the cohesive energies computed with the HF-3c method is comparable to the LMP2 one. A refinement of the HF-3c has been attempted by tuning the dispersion term in the HF-3c energy. While the performance on cohesive energy prediction slightly worsens, optimized unit cell volumes are in excellent agreement with experiment. Overall, the B3LYP-D* method combined with a TZP basis set gives the best results. For cost-effective calculations on molecular crystals, we propose to compute cohesive energies at the B3LYP-D*/TZP level of theory on the dispersion-scaled HF-3c optimized geometries (i.e., B3LYP-D*/TZP//HF-3c(0.27) also dubbed as SP-B3LYP-D*). Besides, for further benchmarking on molecular crystals, we propose to combine the three test sets in a new one denoted as MC82.

Boosting the accuracy and speed of quantum Monte Carlo: Size consistency and time step

Diffusion Monte Carlo (DMC) simulations for fermions are becoming the standard to provide high quality reference data in systems that are too large to be investigated via quantum chemical approaches. DMC with the fixed-node approximation relies on modifications of the Green function to avoid singularities near the nodal surface of the trial wavefunction. We show that these modifications affect the DMC energies in a way that is not size-consistent, resulting in large time-step errors. Building on the modifications of Umrigar {\em et al.} and of DePasquale {\em et al.} we propose a simple Green function modification that restores size-consistency to large values of time-step; substantially reducing the time-step errors. The new algorithm also yields remarkable speedups of up to two orders of magnitude in the calculation of molecule-molecule binding energies and crystal cohesive energies, thus extending the horizons of what is possible with DMC.

Substituent and Solvent Effects on Intermolecular Interactions in Crystals of N-Acylhydrazone Derivatives: Single-Crystal X-ray, Solid-State NMR, and Computational Studies

New crystalline forms of hydrated and anhydrous N-acylhydrazones are reported. The studied crystal structures were determined by single-crystal X-ray diffraction at 90 or 100 K. Transferred aspherical atom model (TAAM) structure refinements were performed with the aid of the most recent version of the University at Buffalo Databank (UBDB). The resulting crystal structures were analyzed in terms of molecular conformations, intermolecular interaction energies, and crystal packing motifs. For this purpose, solid-state NMR studies and theoretical calculations were conducted supplementarily. It was found that all studied hydrazones adopt the E configuration around the azine N–N bond and imino N═C function in the solid state, whereas the hydrazide N–N–C═O moiety exhibits the E and Z arrangement in the N-acyl and N-aroyl derivatives, respectively. The constrained energy scans confirmed the E conformation of the hydrazide unit and the E arrangement of pyridine and hydrazone N atoms as the most stable ones. The association modes in the studied crystals are dominated by strong hydrogen bonds of the N–H···O or N–H···N-type involving the amide group as a proton donor. Consequently, as indicated by lattice energy calculations, a significant increase in the crystal cohesive energy per asymmetric unit is observed when water molecules are incorporated into the crystal structure, because this enables efficient saturation of the hydrogen bond acceptor and donor atoms. On the other hand, a substantial contribution of π···π stacking interactions to the overall stabilization of the crystal nets was also found. Thus, when more bulky phenyl substituents are introduced, the cohesive energy becomes more favorable.

Incrementally Corrected Periodic Local MP2 Calculations: I. The Cohesive Energy of Molecular Crystals.

A method for accurate calculations of the cohesive energy of molecular crystals is presented. The cohesive energy is evaluated as a sum of several components. The major contribution is captured by periodic Hartree-Fock (HF) coupled with the local Møller-Plesset perturbation theory of second order (LMP2) with a triple-ζ basis set. Post-MP2 corrections and corrections for the basis set incompleteness are calculated from inexpensive incremental calculations with finite clusters. This is an essential improvement with respect to the periodic LMP2 method and allows for results of benchmark quality for crystalline systems. The proposed technique is superior to the standard incremental scheme as concerns the cluster size and basis set convergence of the results. In contrast to the total energy or electron correlation energy, which are evaluated in standard incremental calculations, post-MP2 and basis set corrections are rather insensitive to approximations and converge quickly both in terms of the order of the increments and the number of terms at a given order. Evaluation of the incremental corrections within the sub-kJ/mol precision requires computing very few of the most compact two-center and three-center non-embedded clusters, making the whole correction scheme computationally inexpensive. This method as well as alternative routes to compute the cohesive energy via the incremental scheme are tested on two molecular crystals: carbon dioxide (CO2) and hydrogen cyanide (HCN).

Understanding the Formation of Pentagonal Cyclic Twinned Crystal from the Solvent Dependent Assembly of Au Nanocrystals into Their Colloidal Crystals

Due to the presence of high-density twinned-defects and diffuse elastic strain, pentagonal cyclic twinning (PCT) structures may have more fascinating properties than the corresponding single crystalline structure. Although there are a lot of reports concerning the PCT nanocrystals of many monometals and bimetal alloys, the established growth mechanisms of PCT structures are still not straightforward and usually inconsistent with each other. In this Letter, using dodecanethiol-capped Au nanocrystals (NCs) as building blocks, taking self-assembly of Au NCs into their colloidal crystals as the crystallization model, we found the competition of crystal cohesive energy and surface free energy plays an important role in the formation of PCT colloidal crystal. By rationally selecting the solvent to tailor the crystal cohesive energy, the structure of the colloidal crystals can be tuned. PCT structures are more likely to form when the interaction between building blocks is weak, while the single crystal structure is formed when the interaction is strong. The results demonstrate that the thermodynamic factors are the origin of pentagonal cyclic twinning.

DEPENDENCY OF THERMO-PHYSICAL PROPERTIES ON SURFACE BONDING OF BN NANO-CRYSTAL

In this paper we have studied the variation of normalized per-atom pair cohesive (binding) energy and melting temperature with size and number of atom-pairs in the BN nano-crystal by simple model approach. In small nano-particles, large proportions of their atoms reside either at or near the surface, and those in clusters are basically all on the surface. So, we have to study the surface configuration in detail that leads the constancy of the crystals. Even for bulk materials, parameters that ensure this constancy involving normalized cohesive energy, surface reconstruction, iconicity, bulk structure, hybridization and charge balance, melting point. It is worthwhile to say that many of these factors are quite interdependent. These all factors influence the whole cohesive energy of crystals and give important information about the deflections from inverse size dependence. The per-atom-pair binding energy and melting temperature of BN nano-crystal is a quadratic function of the inverse of the crystal size. The binding energy and melting temperature comes near their bulk value with increasing the crystal size and same as the bulk material when the crystal size is above than 100 nm.

Effect of the La addition content on valence electron structure and properties of ZrB2 ceramics

ZrB2 is a combined bonding compound composed of strong covalent bonds which make it difficult to sinter and densify. Thus, rare earth or other metal elements are usually used to be sintering additives to improve its sintering properties. To forecast properties of ZrB2 solid solutions with addition of lanthanum, their valence electron structure (VES) was calculated by using the empirical electron theory (EET) of solids and molecules, and the effect of lanthanum with various proportion on the VES and properties of ZrB2 ceramics has been studied. The results show that with the increase of the lanthanum addition content, the hybridization steps of Zr and B atoms of ZrB2 solid solutions are still A16 and 5, respectively. The hybridization step of lanthanum is always A1. The covalent electron numbers and bonding energy of the strongest bonds of the ZrB2 matrix decrease with the lanthanum addition content increase. These suggest that the addition of lanthanum will improve the fracture toughness and decrease the hardness, crystal cohesive energy and melting point of ZrB2. In a word, its sintering properties are improved, and its densities are increased.

Calculation of crystal cohesive energy of ZrB2 compound

The valence electron structure (VES) of ZrB2 was set up with the bond length difference (BLD) method based on empirical electron theory (EET) of solids and molecules, and there were 43 potential hybridization combinations. Based on the calculation result of the melting point, the 16th hybridization step of Zr atom and fifth hybridization step of B atom are ascertained as the final hybridization combination. Therefore, the covalent electron number and the bonding energy of the strongest bond (the B-B bonds), the theoretical melting point, and the crystal cohesive energy of the ZrB2 compound can be figured out.

Valence electron structure and properties of the ZrO2

To reveal the properties of ZrO2 at the atom and electron levels, the valence electron structures of three ZrO2 phases were analyzed on the basis of the empirical electron theory of solids and molecules. The results showed that the hybridization levels of Zr and O atoms in the m-ZrO2 were the same as those in the t-ZrO2, while those in the c-ZrO2 rose markedly. The electron numbers and bond energies on the strongest covalent bonds in the m-ZrO2 phase were the greatest, the values were 0.901106 and 157.5933 kJ/mol, respectively. Those in the t-ZrO2 phase took second place, which were 0.722182 and 123.9304 kJ/mol, and those in the c-ZrO2 phase were the smallest, which were 0.469323 and 79.0289 kJ/mol. According to the product of the bond energy on the strongest covalent bond and equivalent bond number (this value reflected the crystal cohesive energy), the order from the greatness to smallness was the c-ZrO2> t-ZrO2 > m-ZrO2. This showed that the m-phase bonds were the tightest, their energy was the smallest, the crystal cohesive energy of the m-phase was the largest, and the m-phase existed most stably at room temperature. So it must need energy or higher temperature to take apart the stronger covalent bonds to form a new phase.

Structural stability and site preference of Pr(Fe, M)12 and Pr(Fe, M)12Nx (M = Ti, V, Cr, Mo)

A systemic study of the structural properties of Pr(Fe, M)12 and Pr(Fe, M)12Nx (M = Ti, V, Cr, Mo) compounds has been performed by a series of interatomic pair potentials obtained through the lattice inversion method. Adding a ternary element Ti, V, Cr or Mo makes the crystal cohesive energy of Pr(Fe, M)12 decrease markedly, which indicates that Ti, V, Cr or Mo helps stabilize the crystal structure. The calculated preferentially occupation site of the stabilizing element M is found to be the 8i site. The interstitial N atoms only occupy 2b interstitial sites in Pr(Fe, M)12. The calculated results correspond well to available experimental data. The method utilized in the present investigation provides an effective approach to predict different properties of Pr(Fe, M)12 and Pr(Fe, M)12Nx.

Atomistic simulation for the phase stability, site preference and thermal expansion of YFe 12− xT x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W)

The effect of the ternary element on the structural properties of YFe 12− x T x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W) has been studied by using interatomic pair potentials based on ab initio method and lattice inversion equations. Calculated results show that adding ternary element T makes the crystal cohesive energy of YFe 12− x T x decrease markedly, which indicates that T helps stabilize the 1:12 phases. The ternary elements T prefer to occupy the 8i sites in these compounds. The calculated results are found in good agreement with the values deduced from experiments. Furthermore, we have calculated the thermal expansion of YFe 11V compounds with the ThMn 12 structure. The method utilized in the present investigation offers a rather easy and direct way to study the structural properties of YFe 12− x T x .

Atomistic investigation on site preference and lattice vibrations of Sm(Co,M) 12 (M = Cr, Ti, V, Nb, Fe)

The phase stability and site preference of the intermetallics Sm(Co,M) 12 (M = Cr, Ti, V, Nb, Fe) are evaluated by using interatomic pair potentials based on Chen's lattice inversion method. The calculated results show that substituting either Cr, Ti, V, Nb or Fe atoms makes the crystal cohesive energy of Sm(Co,M) 12 decrease markedly, proving that these atoms can stabilize Sm(Co,M) 12 with ThMn 12 structure. The calculated lattice constants coincide quite well with the experimental data. The order of site preference of these ternary elements T is 8 i, 8 j and 8 f with the occupation of 8 i corresponding to the greatest energy decreases. The properties related to lattice vibration, such as the phonon densities of states, specific heat and vibrational entropy, are also evaluated.

Atomistic simulation of rare earth compounds Sm 2Fe 17−xT x (T = Ti, V, Cr, Mo, Ni)

By using the interatomic pair potential obtained with the lattice inversion method, the structural stability of Sm 2Fe 17− x T x (T = Ti, V, Cr, Mo, Ni) of the rhombohedral Th 2Zn 17-type structure is studied. Calculated results show that adding Ti, V, Cr or Mo atoms makes the crystal cohesive energy of Sm 2Fe 17− x T x decrease markedly, proving that these atoms can stabilize Sm 2Fe 17− x T x structure. The calculated lattice constants coincide quite well with experimental values. The ternary element T preferentially substitutes for Co in the 6 c site only . The calculated results also show that Ni does not stabilize the system with the structure of Th 2Zn 17-type. Moreover, the phonon densities of states are first evaluated for the Sm 2Fe 17− x T x (T = Ti, V, Cr, Mo) compounds with the rhombohedral Th 2Zn 17-type structure. This may be an attempt to predict the thermodynamic properties for the rare earth materials with complex structures.

Theoretical Investigation on the Phase Stability and Site Preference of R(Co,T)12 and R(Co,T)12Nx(R=Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T=Mo, Mn, Ni)

The phase stability and site preference of some ternary elements of ThMn 12 type rare-earth intermetallic compounds R(Co,T) 12 and R(Co,T) 12 N x are evaluated based on ab initio converted interatomic potentials. The calculated results show that adding either Mo or Mn makes the crystal cohesive energy decrease markedly. It proved that these elements can stabilize R(Co,T) 12 with ThMn 12 structure. The systemic energy is the lowest when T atoms occupy symmetrically 8i sites. For T=Mo or Mn, the doped nitrogen atom does not affect the order of site preference of these stabilizing elements. Moreover, the nitrogen atom occupies 2b sites. According to the calculated results, it can be seen that lattice parameters and X-rays are in good agreement with experiment. All the above results indicate that the ab initio converted pair potentials are effective for the calculation of these kinds of anisotropy materials and some related nitrides.

THEORETICAL INVESTIGATION ON THE PHASE STABILITY AND SITE PREFERENCE OF R(Co,T)12 AND R(Co,T)12Nx (R = Y, Ce, Pr, Nd, Sm, Gd, Tb, Ho, Er, Dy, T = Mo, Mn, Ni)

The phase stability and site preference of some ternary elements of ThMn12 type rare-earth intermetallic compounds R ( Co , T )12 and R ( Co , T )12 N x are evaluated based on ab initio converted interatomic potentials. The calculated results show that adding either Mo or Mn makes the crystal cohesive energy decrease markedly. It proved that these elements can stabilize R ( Co , T )12 with the ThMn12 structure. The systemic energy is the lowest when T atoms occupy symmetrically 8i sites. For T = Mo or Mn, the doped nitrogen atom does not affect the order of site preference of these stabilizing elements. Moreover, the nitrogen atom occupies 2b sites. According to the calculated results, it can be seen that lattice parameters and X-ray patterns are in good agreement with experiment. All the above results indicate that the ab initio converted pair potentials are effective for the calculation of these kinds of anisotropy materials and some related nitrides.

Modeling of melting enthalpy of organic nanowires

AbstractA model is developed to interpret size‐dependent melting enthalpy of nanocrystals based on consideration of the effect of surface bond number and bond strength on cohesive energy of crystals and the corresponding liquids. The model prediction corresponds to the experimental results of organic nanowires well.

Theoretical study of the phase stability and site preference for R3(Fe,T)29 (R=Nd, Sm; T=V, Ti, Cr, Cu, Nb, Mo, Ag)

The phase stability of intermetallics R3(Fe,T)29 with Nd3(Fe,Ti)29 structure and site preference of some 3d or 4d transition elements T were investigated in molecular static and molecular dynamic methods with a series of ab initio pair potentials obtained though the lattice inversion method. Calculated results show that adding either Cr, Mo, Ti, V or Nb atoms makes the crystal cohesive energy of R3(Fe,T)29 decrease markedly, proving that these atoms can stabilize R3(Fe,T)29 with the structure of Nd3(Fe,Ti)29, even though the R3Fe29 crystal structure is itself metastable. The calculated lattice parameters are in good agreement with the experimental data. The degree of the decrease in cohesive energy corresponds with the species and occupation sites of the ternary atoms. The order of site preference of these stabilizing elements T is 4i2, 4i1 and 4g with the occupation of 4i2 corresponding to the greatest energy decrease. The calculated result further shows that the addition of Cu or Ag cannot play a role in stabilizing the structure. These calculated results correspond well to available experiments. Supported by the pair potentials, calculated structures are stable within a certain temperature range and the space group of the final structure remains unchanged with respect to a variety of initial deformations. So it was confirmed that there exist a series of R3(Fe,T)29 compounds with the stable structure of Nd3(Fe,Ti)29 in the R–Fe–T systems. The process of the evolution from the RFe5 structure to metastable R3Fe29 was well explained too with the pair potential in this paper. All these prove the effectiveness of ab initio pair potentials obtained through the lattice inversion method in the description of rare-earth materials.