Dynamics and Thermodynamics of Crystalline Polymorphs. 3. γ‑Glycine, Analysis of Variable-Temperature Atomic Displacement Parameters, and Comparison of Polymorph Stabilities

In a series of systematic studies, we have investigated the molecular motion in crystals of the glycine polymorphs and determined their thermodynamic functions from an analysis of multitemperature atomic displacement parameters (ADPs) combined with ONIOM calculation on 15-molecule clusters. The studies are aimed at providing insight into the factors governing the relative stabilities of the α-, β-, and γ-polymorphs. This Article, the last in the series, focuses on the most stable polymorph, γ-glycine. Multitemperature diffraction data of the γ-glycine polymorph have been collected to 0.5 Å resolution between 10 and 300 K at two synchrotron beamlines, KEK Photon Factory and ID11 of the ESRF. The ADPs of γ-glycine from these sources differ significantly, as previously observed also for the other two polymorphs. A simple model of rigid body motion explains the ADPs from KEK and their temperature dependence. It provides lattice vibration frequencies that are in line with those from Raman spectroscopy. Together with the internal vibration frequencies from an ONIOM calculation, the thermodynamic functions are estimated using the Einstein, Debye, and Nernst–Lindemann models of heat capacity. The relative stabilities of the three polymorphs of glycine are discussed on the basis of the contributions to their free energies as obtained in this work and from various experimental and theoretical studies. The comparison shows that the free-energy differences are determined primarily by differences in lattice and zero-point vibrational energies

Smart Energetic Nanosized Co-Crystals: Exploring Fast Structure Formation and Decomposition

The interest in co-crystals of energetic materials is explained by the fact that they can offer better thermodynamic stability and tunable sensitivity and detonation performance. In the present work, a combination of DSC, ultrafast chip calorimetry, high-resolution X-ray powder diffraction, and nanofocus X-ray diffraction was employed to investigate the thermal behavior and structure formation in nanosized co-crystals of CL-20 with HMX and TNT prepared using Spray Flash Evaporation (SFE). The CL-20/HMX co-crystal does not reveal any thermal transitions up to the thermal decomposition. In contrast, CL-20/TNT exhibits an irreversible melting transition. Upon melting, it can rapidly crystallize on heating or, at a slower pace, at room temperature to form homocrystals of γCL-20, the polymorph stable at high temperature. These observations constitute the first evidence of a CL-20 crystallization process, which occurs from the melt and not from solution. The solid–liquid phase separation occurring during heating of a CL-20/TNT melt may explain its complex thermal decomposition process as compared to that of CL-20/HMX: the main exothermic peak of decomposition can be assigned to that of a pure CL-20

Crystallization and Texturing of Sr_<i>x</i>Ba_1–<i>x</i>Nb₂O₆ Thin Films Prepared by Aqueous Solution DepositionAn <i>In Situ</i> X‑ray Diffraction Study

Aqueous chemical solution deposition (CSD) is an environmentally friendly and highly flexible fabrication route to prepare oxide thin films. Here, we present an aqueous CSD process for ferroelectric SrxBa1–xNb2O6 (SBN) thin films on SrTiO3 (STO) single-crystal substrates. In situ synchrotron X-ray diffraction was employed to study the crystallization of the films during thermal processing with heating rates in the range 0.04–20 °C/s. Three different crystal orientations of SBN were observed based on the heating rate and the orientation of the STO substrates. SBN(001) and SBN(310) orientations were observed on STO(100), while only the SBN(311) orientation was observed on STO(110). The SBN(001) orientation was favored by an ultraslow heating rate of 0.04 °C/s, suggesting that this is the thermodynamic stable orientation. The SBN(310) orientation was kinetically favored at moderate heating rates and also promoted by increasing the Sr content in the film. A high heating rate of 20 °C/s rendered polycrystalline SBN films. It was revealed that nucleation and growth occurred via a classical Volmer–Weber (VW) growth mode and that the SBN grains preferably grow along the c-axis. The present findings demonstrate that control of nucleation and growth is a prerequisite to deposit films with different orientations and textures, which is detrimental for the film properties

Dynamics and Thermodynamics of Crystalline Polymorphs. 2. β‑Glycine, Analysis of Variable-Temperature Atomic Displacement Parameters

The molecular dynamics in the crystal and the thermodynamic functions of the β-polymorph of glycine have been determined from a combination of molecular translation-libration frequencies reflecting the temperature dependence of atomic displacement parameters (ADPs), with frequencies derived from ONIOM­(DFT:PM3) calculations on a 15-molecule β-glycine cluster. ADPs have been obtained from variable-temperature diffraction data to 0.5 Å resolution collected with X-ray synchrotron (10–300 K) and sealed tube radiation (50–298 K). At the higher temperatures, the ADPs of β-glycine from synchrotron are larger than those from sealed tube probably due to different experimental conditions. The lattice vibration frequencies from normal-mode analysis of ADPs and the internal vibration frequencies from ONIOM­(B3LYP/6-311+G­(2d,p):PM3) calculations agree with those from spectroscopy. Estimation of thermodynamic functions using the vibrational frequencies, the Einstein and Debye models of heat capacity, and the room-temperature compressibility provides <i>C</i>_<i>p</i>, <i>H</i>_vib, and <i>S</i>_vib that agree with those from calorimetry. The β-phase with higher <i>H</i> and <i>G</i> is found to be less stable than the α-phase in the temperature range of the experiment

SAPO-37 microporous catalysts: revealing the structural transformations during template removal

<p>We have studied the structural behavior of SAPO-37 during calcination using simultaneous <i>in situ</i> powder X-ray diffraction (PXRD) and mass spectroscopy (MS) in addition to <i>ex situ</i> thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). A spike in the unit cell volume corresponding to template removal (tracked using the occupancy of the crystallographic sites in the SAPO-37 cages) is revealed from the XRD data and is strongly correlated with the DSC curve. The occupancy of the different template molecules in the faujasite (FAU) and sodalite (SOD) cages is strongly related to the two mass loss steps observed in the TGA data. The templates act as a physical stabilizing agent, not allowing any substantial unit cell response to temperature changes until they are removed. The FAU cages and SOD cages have different thermal response to the combustion of each template. The FAU cages are mainly responsible for the unit cell volume expansion observed after the template combustion. This expansion seems to be related with residual coke from template combustion. We could differentiate between the thermal response of oxygen and T-atoms. The T–O–T angle between two double 6-rings and a neighboring T–O–T linkage shared by SOD and FAU had different response to the thermal events. We were able to monitor the changes in the positions of oxygen and T-atoms during the removal of TPA⁺ and TMA⁺. Large changes to the framework structure at the point of template removal may have a significant effect on the long-term stability of the material in its activated form.</p

Synthesis, Structure, and Thermoelectric Properties of α‑Zn₃Sb₂ and Comparison to β‑Zn₁₃Sb₁₀

Zn–Sb compounds (e.g., ZnSb, β-Zn₁₃Sb₁₀) are known to have intriguing thermoelectric properties, but studies of the Zn₃Sb₂ composition are largely absent. In this work, <i>α-</i>Zn₃Sb₂ was synthesized and studied via temperature-dependent synchrotron powder diffraction. The <i>α-</i>Zn₃Sb₂ phase undergoes a phase transformation to the β form at 425 °C, which is stable until melting at 590 °C. Rapid quenching was successful in stabilizing the α phase at room temperature, although all attempts to quench β-Zn₃Sb₂ were unsuccessful. The structure of α-Zn₃Sb₂ was solved using single crystal diffraction techniques and verified through Rietveld refinement of the powder data. α-Zn₃Sb₂ adopts a large hexagonal cell (<i>R</i> 3̅ space group, <i>a</i> = 15.212(2), <i>c</i> = 74.83(2) Å) containing a well-defined framework of isolated Sb^3– anions but highly disordered Zn²⁺ cations. Dense ingots of both the α-Zn₃Sb₂ and β-Zn₁₃Sb₁₀ phases were formed and used to characterize and compare the low temperature thermoelectric properties. Resistivity and Seebeck coefficient measurements on α-Zn₃Sb₂ are consistent with a small-gap, degenerately doped, <i>p</i>-type semiconductor. The temperature-dependent lattice thermal conductivity of α-Zn₃Sb₂ is unusual, resembling that of an amorphous material. Consistent with the extreme degree of Zn disorder observed in the structural analysis, phonon scattering in α-Zn₃Sb₂ appears to be completely dominated by point-defect scattering over all temperatures below 350 K. This contrasts with the typical balance between point-defect scattering and Umklapp scattering seen in β-Zn₁₃Sb₁₀. Using the Debye–Callaway interpretation of the lattice thermal conductivity, we use the differences between α-Zn₃Sb₂ and β-Zn₁₃Sb₁₀ to illustrate the potential significance of cation/anion disorder in the Zn–Sb system

Lithium Diffusion Pathway in Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) Superionic Conductor

The Al-substituted LiTi₂(PO₄)₃ powders Li_1+<i>x</i>Al_<i>x</i>Ti_2–<i>x</i>(PO₄)₃ (LATP) were successfully prepared by a water-based sol–gel process with subsequent calcination and sintering. The crystal structure of obtained samples was characterized at different temperatures using high-resolution synchrotron-based X-ray and neutron powder diffraction. Possible lithium diffusion pathways were initially evaluated using the difference bond-valence approach. Experimental 3D lithium diffusion pathway in LATP was extracted from the negative nuclear density maps reconstructed by the maximum entropy method. Evaluation of the energy landscape determining the lithium diffusion process in NASICON-type superionic conductor is shown for the first time

Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface

Single crystals of thiophene–phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas–liquid interface by using solvent–antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′-diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm², which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry <i>Cc</i> for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent–antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas–solution interface are discussed