Crystallization Kinetics Research Articles

Tailoring drug release from long-acting contraceptive levonorgestrel intrauterine systems

The wide array of polydimethylsiloxane (PDMS) variants available on the market, coupled with the intricate combination of additives in silicone polymers, and the incomplete understanding of drug release behavior make formulation development of levonorgestrel intrauterine systems (LNG-IUSs) formidable. Accordingly, the objectives of this work were to investigate the impact of excipients on formulation attributes and in vitro performance of LNG-IUSs, elucidate drug release mechanisms, and thereby improve product understanding. LNG-IUSs with a wide range of additives and fillers were prepared, and in vitro drug release testing was conducted for up to 12 months. Incorporating various additives and/or fillers (silica, silicone resins, silicone oil, PEG, etc.) altered the crystallization kinetics of the crosslinked polymer, the viscosity, and the microstructure. In addition, drug-excipient interactions can occur. Interestingly, additives which increased matrix hydrophobicity and hindered PDMS crystallization facilitated dissolution and permeation of the lipophilic LNG. The influence of additives and lubricants on the mechanical properties of LNG-IUSs were also evaluated. PDMS chemical substitution and molecular weight were deemed to be most critical polymer attributes to the in vitro performance of LNG-IUSs. Drugs with varying physicochemical characteristics were used to prepare IUSs, modeling of the release kinetics was performed, and correlations between release properties and the various physicochemical attributes of the model drugs were established. Strong correlations between first order release rate constants and both drug solubility and Log P underpin the partition and diffusion-based release mechanisms in LNG-IUSs. This is the first comprehensive report to provide a mechanistic understanding of material-property-performance relationships for IUSs. This work offers an evidence-based approach to rational excipient selection and tailoring of drug release to achieve target daily release rates in vivo. The novel insights gained through this research could be helpful for supporting development of brand and generic IUS products as well as their regulatory assessment.

Single Crystal Seed Induced Epitaxial Growth Stabilizes α-FAPbI3 in Perovskite Solar Cells.

FAPbI3 stands out as an ideal candidate for the photoabsorbing layer of perovskite solar cells (PSCs), showcasing outstanding photovoltaic properties. Nonetheless, stabilizing photoactive α-FAPbI3 remains a challenge due to the lower formation energy of the competitive photoinactive δ-phase. In this study, we employ tetraethylphosphonium lead tribromide (TEPPbBr3) single crystals as templates for the epitaxial growth of PbI2. The strategic use of TEPPbBr3 optimizes the evolution of intermediates and the crystallization kinetics of perovskites, leading to high-quality and phase-stable α-FAPbI3 films. The TEPPbBr3-modified perovskite exhibits optimized carrier dynamics, yielding a champion efficiency of 25.13% with a small voltage loss of 0.34 V. Furthermore, the target device maintains 90% of its initial PCE under maximum power point (MPP) tracking over 1000 h. This work establishes a promising pathway through single crystal seed based epitaxial growth for achieving satisfactory crystallization regulation and phase stabilization of α-FAPbI3 perovskites toward high-efficiency and stable PSCs.

Solidification process of ABO3-type perovskites: Kinetic two-phase growth method with optimized potential

The functional properties of ABO3-type perovskite materials have garnered extensive attention, yet their solidification properties have remained challenging to investigate due to high-temperature environments and characterization limitations. In this study, molecular dynamics simulations are employed to comprehensively explore the melting, quenching, and crystal growth processes of SrTiO3 (STO) structural evolution. Through iterative fitting and optimization of ion effective charge, a set of potential functions capable of accurately simulating the high-temperature melting of STO are derived. The melting point obtained for STO (2403 K) using the two-phase coexistence method closely corresponds to the empirical value (2333 K), affirming the precision of the optimized potential. Quenching the molten STO yields an amorphous structure characterized primarily by Ti-O six-fold coordination, which increased by 13.09% when reduced to 300 K at a cooling rate of 0.1 K/ps. Notably, the results for different cooling rates revealed that slower cooling rates and lower temperatures yielded more ordered amorphous structures. To circumvent the formation of amorphous states during crystal growth of perovskite materials, we have developed a kinetic two-phase growth (KTPG) method. This approach regulates the cooling rate within a solid-liquid two-phase system, maintaining constant low supercooling at the interface to mimic STO crystal growth kinetics. Cooling at 0.01 K/ps to 1760 K leads to a notable transformation, with the percentage of Ti-O six-fold coordination reaching 90%. This signifies substantial progress in achieving crystal growth through this method, highlighting its efficacy in facilitating crystal formation from the melt phase.

Additive Manufacturing of Composite Polymers: Thermomechanical FEA and Experimental Study.

This study presents a comprehensive approach for simulating the additive manufacturing process of semi-crystalline composite polymers using Fused Deposition Modeling (FDM). By combining thermomechanical Finite Element Analysis (FEA) with experimental validation, our main objective is to comprehend and model the complex behaviors of 50 wt.% carbon fiber-reinforced Polyphenylene Sulfide (CF PPS) during FDM printing. The simulations of the FDM process encompass various theoretical aspects, including heat transfer, orthotropic thermal properties, thermal dissipation mechanisms, polymer crystallization, anisotropic viscoelasticity, and material shrinkage. We utilize Abaqus user subroutines such as UMATHT for thermal orthotropic constitutive behavior, UEPACTIVATIONVOL for progressive activation of elements, and ORIENT for material orientation. Mechanical behavior is characterized using a Maxwell model for viscoelastic materials, incorporating a dual non-isothermal crystallization kinetics model within the UMAT subroutine. Our approach is validated by comparing nodal temperature distributions obtained from both the Abaqus built-in AM Modeler and our user subroutines, showing close agreement and demonstrating the effectiveness of our simulation methods. Experimental verification further confirms the accuracy of our simulation techniques. The mechanical analysis investigates residual stresses and distortions, with particular emphasis on the critical transverse in-plane stress component. This study offers valuable insights into accurately simulating thermomechanical behaviors in additive manufacturing of composite polymers.

Isothermal crystallization of polyethylene droplets within the self-nucleated isotactic polypropylene matrix of a 70/30 iPP/PE immiscible blend via fast scanning chip calorimetry (FSC)

The escalating volume of waste related to polyolefins poses recycling challenges. Given the mixed composition of the post-consumer feedstock, studying blends of virgin polyolefins is a fruitful strategy to simulate such compositions. For the first time, this study investigates crystallization, self-nucleation (SN), and kinetics of a 70/30 iPP/PE blend, characterised by a "sea-island" morphology, using fast scanning calorimetry (FSC). Preliminary experiments on different chip sensors proved the technique's sensitivity and representativeness for polymer blends, even with minimal sample quantities. Application of the self-nucleation thermal protocol (employing fast scanning rates, from 500 to 1000 °C/s) to the iPP matrix phase revealed insights into the characteristic SN domains. The crystallization kinetics of PE-dispersed droplets within a SN iPP matrix was also investigated. Results indicate a 30 °C crystallization window for PE droplets within the self-nucleated iPP matrix, expanding the understanding of crystallization rates (up to four decades) compared to standard DSC measurements.

Freshwater and minerals recovery from synthetic produced water by membrane distillation/membrane crystallization processes

The purpose of this work is to study the influence of oil and surfactant present in produced water on the membrane distillation and membrane crystallization performance. The latter is evaluated in terms of permeate flux, quality of permeate and sodium chloride crystallization kinetics. Polypropylene and Hyflon AD40H/PVDF composite membranes with 0.2 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mathrm{\\mu m}$$\\end{document} pore size were used in the investigation. The tests were carried out in direct contact configuration using two synthetic feed solutions: the first one without oil and surfactant, and the second one with oil and surfactant. The achieved results showed a permeate flux reduction of 20 and 40% for PP and AD40H, respectively, in membrane distillation and of 35% in membrane crystallization. These results may be attributed to the interaction between the salts and surfactant, which led to the deterioration of the membrane performance. Despite this, high salt rejection factors greater than 99.9% and total carbon rejections ranging between 80 and 90% indicated the good potential of membrane distillation technology for the treatment of produced water. Moreover, good quality crystals and high total water recovery factor (97%) were achieved using the membrane crystallization process. Nevertheless, the presence of oil and surfactant in the feed caused an increase in the induction time compared to the system without oil and surfactant. In addition, in the performed experiments, simple physical cleaning with distillate water was sufficient to recover the initial trans-membrane flux of the membranes.

A DSC study of undercooling thermodynamics and crystallization kinetics for liquid Ag-Si alloys

The undercooling thermodynamics and crystallization kinetics of pure silver, hypoeutectic Ag-2%Si, and eutectic Ag-3.1%Si alloys were explored with differential scanning calorimetry. Liquid silver was highly undercooled by an extent of 200 K (0.16Tm) within molten B2O3 denucleating agent. The nucleating incubation time and crystallization kinetics were investigated under isothermal condition. It was found that the crystal incubation time of nucleation generally decreased with enhanced undercooling. The analyses based on classical nucleation theory showed that heterogeneous nucleation was still dominant even at substantial undercoolings. The transformed solid fraction was derived to elucidate the crystallization kinetics of liquid silver by Johnson-Mehl-Avrami equation. The Avrami exponent was revealed to vary with liquid undercooling and crystallization extent. High cooling rate and denucleating agent were two effective ways to achieve large undercoolings for liquid Ag-Si alloys. The primary (Ag) dendrites were refined with increased undercoolings in hypoeutectic Ag-2%Si alloy. Irregular (Ag)+(Si) eutectics were the typical solidification microstructures for eutectic Ag-3.1%Si alloy at small undercoolings, while primary (Ag) dendrites displayed a remarkable volume fraction at extended undercoolings.

Yield enhancement and phase behaviors of cyclic peptide drug crystallization in the presence of polyethylene glycol additive

Crystallization of peptide drugs typically requires a lengthy period to attain reasonable yields due to slow crystallization kinetics of macromolecules. Herein we investigated the use of polyethylene glycol (8 kDa) (PEG 8k) as an additive to enhance the yield of salting-out crystallization of cyclic peptide drug vancomycin (Van). Our preliminary study showed the addition of PEG 8k increased the 24-h yield from 46 wt% to 70 wt% attributed to the polymer’s volume-exclusion effects. PEG 8k addition, however, resulted in modified Van crystal habits, thereby phase behavior study was performed at different pH, salt, polymer, and Van concentrations. The phase behavior study showed PEG 8k and salt concentrations must be meticulously determined to produce stable Van crystals because their excessive concentrations resulted in heavy precipitates. Different crystal habits (e.g., acicular, blade-like) were produced with PEG 8k additive depending on the pH, distinct from octahedral crystals produced without additive. Bulk crystallization with PEG 8k additive resulted in platy Van crystals with higher crystallinity, lower thermal stability and purity, but with similar dissolution rates compared to octahedral crystals prepared without PEG 8k. PEG 8k addition had catalytic effects on the salting-out yield whereby lower salt concentration was needed. In short, PEG 8k addition enhanced the yield without compromising Van crystals’ qualities.

Studying crystallisation processes using electron microscopy: The importance of sample preparation.

We present a comparison of common electron microscopy sample preparation methods for studying crystallisation processes from solution using both scanning and transmission electron microscopy (SEM and TEM). We focus on two widely studied inorganic systems: calcium sulphate, gypsum (CaSO4·2H2O) and calcium carbonate (CaCO3). We find significant differences in crystallisation kinetics and polymorph selection between the different sample preparation methods, which indicate that drying and chemical quenching can induce severe artefacts that are capable of masking the true native state of the crystallising solution. Overall, these results highlight the importance of cryogenic (cryo)-quenching crystallising solutions and the use of full cryo-TEM as the most reliable method for studying the early stages of crystallisation.

Ductile–brittle indentation fracture transitions in hard coatings

The article has provided a novel way of finding the probability of fracture ( Pf) based on modifications of some established concepts. It has also discussed ductile to brittle as well as brittle to ductile transitions in the same coating/substrate system subject to differences in crystallisation, indenter sharpness, sliding modes and dislocation kinetics. The indentation positioning and nanocrystalline features cause ambiguity in the results and need meticulous analysis. As a result of increased dislocation movement, the crystalline portions toughened in contrast to the amorphous parts, which were more brittle. The Pf, which varies and reaches a maximum of 52% in the amorphous or near-amorphous area, was calculated using the Weibull distribution. Transitions between ductility and brittleness can be seen in sliding indentations. Evidence of adhesive failure, which required better coating component inspection because it happened significantly earlier, was shown. Finite-element modelling was used to analyse the stress and provide information on dislocation motions and the impact of indenter shape on fracture. The outcomes are advantageous for the production of devices based on nano/micro-electro-mechanical systems.

Kinetics of the plastic crystal transition in neopentyl glycol

The thermal hysteresis exhibited in plastic crystal compounds greatly reduces their cyclic efficiency, limiting their potential for replacing current environmentally harmful refrigerants. A mechanistic understanding of the origins of this hysteresis has yet to be established. Here, we systematically investigate the transformation kinetics of the model plastic crystal, neopentyl glycol (NPG), through microscopic and calorimetric techniques. We reveal an asymmetry between the forward (heating) and reverse (cooling) transitions. We also demonstrate that the forward transformation is rate-limited by the rate of growth of rotationally disordered domains. In contrast, the reverse transformation is rate-limited by the nucleation of the ordered crystal domain, demonstrated by the sharp exothermic peaks in calorimetry and rapid self-nucleation phenomena observed optically. This nucleation limitation is largely responsible for the large thermal hysteresis in NPG, which we observe to be as large as 16.7 °C for an approximately 10 mg sample cooled at 0.5 °C min−1. These findings demonstrate the underlying origin of the thermal hysteresis and introduce a direction to mitigate hysteresis in plastic crystal transformations.

Evolution of interfacial phases between Al alloy and high entropy alloy during annealing

While the interfacial phases between Al and high-entropy alloy (HEA) share the same types as that between Al and steel, it has been found previously that the thermal stability of the Al-HEA interfacial phases is significantly higher than that of the Al-steel interfacial phases. To further elucidate the reason for the exceptional thermal stability of Al-HEA interfacial phases, this study investigated the evolution of the interfacial phases formed between Al alloy and FeCoCrNiMn HEA during annealing. An Al-FeCoCrNiMn thin film diffusion couple was extracted from a friction stir lap welding (FSLW) joint by focused ion beam and then annealed at 400 °C. An amorphous layer is formed initially at the interface due to the high strain rate deformation during FSLW. The amorphous layer remained even after prolonged annealing at 400 °C (∼0.72 Tm of Al alloy) for 60 min with very limited thickening (from 50 nm to 55 nm), showing remarkable thermal stability at the relatively high temperature. Al13Fe4-type intermetallic compound (IMC) with the Fe site occupied by Fe, Co, Cr, Ni and Mn first nucleates at the interface between Al alloy and amorphous layer. With the annealing time extended to 180 min, the amorphous layer is partially transformed into Al5Fe2-type IMC. The rate constant for the thickening of the reaction layer at the Al/FeCoCrNiMn interface is 0.03 μm∙min1/2, which is much smaller than (only 0.02 times) that at the Al/steel interface under similar annealing condition. The high thermal stability of Al-HEA interfacial layer is thus attributed to the slow kinetics in crystallization of the initially formed amorphous layer, the barrier effect of amorphous layer for interdiffusion, and the reduced diffusivity inside of IMCs caused by significant fluctuations in lattice potential energy due to high-density doping with HEA elements. This finding provides new insights into the fabrication of thermally stable Al-HEA hybrid structures suitable for service conditions involving high-temperature exposure.

Glass science behind lithium silicate glass-ceramics

ObjectivesLithium silicate-based glass ceramics have evolved as a paramount restorative material in restorative and prosthetic dentistry, exhibiting outstanding esthetic and mechanical performance. Along with subtractive machining techniques, this material class has conquered the market and satisfied the patients’ needs for a long-lasting, excellent, and metal-free alternative for single tooth replacements and even smaller bridgework. Despite the popularity, not much is known about the material chemistry, microstructure and terminal behaviour. MethodsThis article combines a set of own experimental data with extensive review of data from literature and other resources. Starting at manufacturer claims on unique selling propositions, properties, and microstructural features, the aim is to validate those claims, based on glass science. Deep knowledge is mandatory for understanding the microstructure evolution during the glass ceramic process. ResultsFundamental glass characteristics have been addressed, leading to formation of time-temperature-transformation (TTT) diagrams, which are the basis for kinetic description of the glass ceramic process. Nucleation and crystallization kinetics are outlined in this contribution as well as analytical methods to describe the crystalline fraction and composition qualitatively and quantitatively. In relation to microstructure, the mechanical performance of lithium silicate-based glass ceramics has been investigated with focus on fracture strength versus fracture toughness as relevant clinical predictors. ConclusionFracture toughness has been found to be a stronger link to initially outlined manufacturer claims, and to more precisely match ISO recommendations for clinical indications.

Concurrent effect of shear flow and CNT presence on crystallization behavior and electrical properties of polypropylene based nanocomposites: A comprehensive structure-property investigation

The loading of nanoparticle in the polymer matrix affects their response to shear flows applied during the fabrication process. With this in mind, in the present study, the concurrent effect of carbon nanotube (CNT) presence and shear application on the rheological and electrical properties as well as on the crystallization behavior of polypropylene (PP) based nanocomposites has been investigated. The results of rheological analyses showed that the solid-like behavior increases in proportion to the CNT content, and a remarkable increase in the solid-like behavior is obtained at a CNT content of 2 wt%. The isothermal crystallization results corroborated that crystallization temperature increases proportionally with the CNT content, however does not change with shearing. The non-isothermal crystallization findings confirmed that crystallization kinetics promotes with increasing CNT content, and this effect becomes more pronounced with shearing. The results of thermal analysis confirmed that the melting temperature decreases slightly, and the crystallinity remains almost unchanged with increasing CNT content up to 2%. However, when the CNT content rose up to 4%, the crystallinity decreased significantly due to confined crystallization. When this nanocomposite was subjected to shearing, the crystallinity increased. The results of electrical conductivity measurement revealed that the conductivity increases with increasing CNT content, while it decreases with shearing and the vulnerability of nanocomposites to shearing decreases with increasing CNT content.

Effect of mango butter on the physicochemical properties of beeswax-Moringa seed oil-based oleogels for topical application

Objective The purpose of this research was to determine any connections between the characteristics of oleogels made of beeswax and the impact of mango butter. Methods Oleogel was prepared through inverted tube methods, and optimized through oil binding capacity. Other evaluations like bright field and polarized microscopy, Fourier-transform infrared (FTIR) spectroscopy, crystallization kinetics, mechanical study, and X-ray diffractometry (XRD). The drug release kinetic studies and in vitro antibacterial studies were performed. Results FTIR study reveals that the gelation process does not significantly alter the chemical composition of the individual components. Prepared gel exhibiting fluid-like behavior or composed of brittle networks is particularly vulnerable to disruptions in their network design. The incorporation of mango butter increases the drug permeation. In-vitro microbial efficacy study was found to be excellent. Conclusion The studies revealed that mango butter can be used to modify the physico-chemical properties of the oleogels.

Crystallization Kinetics of Perovskite Films by a Green Mixture Antisolvent for Efficient NiOx-Based Inverted Solar Cells.

Environment-friendly antisolvents are critical for obtaining highly efficient, reproducible, and sustainable perovskite solar cells (PSCs). Here, we introduced a green mixture antisolvent of ethyl acetate-isopropanol (EA/IPA) to finely regulate the crystal grain growth and related film properties, including the morphology, crystal structure, and chemical composition of the perovskite thin film. The IPA with suitable content in EA plays a key role in achieving a smooth and compact high-quality perovskite thin film, leading to the suppression of film defect-induced nonradiative recombination. As a result, the PSCs based on the EA/IPA (5:1) antisolvent showed a power conversion efficiency of 22.9% with an open-circuit voltage of 1.17 V.

In Situ Synthesized Low-Dimensional Perovskite for >25% Efficiency Stable MA-Free Perovskite Solar Cells.

Developing an additive to effectively regulate the perovskite crystallization kinetics for the optimized optoelectronic properties of perovskite film plays a vital role in obtaining high efficiency and stable perovskite solar cells (PSCs). Herein, a new additive is designed and directly synthesized in perovskite precursor solution by utilizing an addition reaction between but-3-yn-1-amine hydrochloride (BAH) and formamidinium iodide. It is found that its product may control the intermediate precursor phase for regulating perovskite nucleation, leading to advantageous 2D perovskite to induce growth of perovskite along the preferred [001] orientation with not only released lattice strain but also strong interaction with perovskite to passivate its surface defects. By taking advantage of the above synergistic effects, the optimized PSC delivers an efficiency of 25.19% and a high open-circuit voltage (VOC) of 1.22 V. Additionally, the devices demonstrate good stability, remaining over 90% of their initial efficiencies under ambient atmosphere conditions for 60 days, high temperature of 85°C for 200 h, or maximum power point tracking for 500 h.

Ultrafast-degradable and super-elastic PBAT/polybutylene succinate foam with stable cellular structure and enhanced thermal insulation

PBAT is considered a leading eco-friendly polymer known for its exceptional biodegradability and mechanical properties. However, the production of high-performance PBAT foam remains challenging owing to poor foaming ability and intrinsic shrinkage. Herein, this study presents a flexible approach to prepare super-elastic and ultrafast-degradable PBAT/polybutylene succinate (PBS) foams achieved by microcellular foaming with CO2 & N2 as co-blowing agents. Firstly, the SEM, FTIR, as well as XPS spectra, confirmed a uniform and miscible distribution of PBS. Further, the crystallization kinetics and rheological analysis demonstrated that PBS effectively promoted crystallization and molecular chain entanglement. Thus, the PBAT/PBS foam exhibited significantly refined cellular structure, higher expansion ratio of 20.3, and restricted shrinkage of less than 4 %. More importantly, compared with neat PBAT foam, the compression strength of PBAT/PBS foam was dramatically enhanced by 47.7 %, and the energy loss coefficient was pronouncedly reduced by 58.8 % under the expansion ratio of 15.0. Meanwhile, the PBAT/PBS foam showed a thermal conductivity as low as 35.9 mW/m·K, enhanced hydrophobicity, and an exceptionally rapid degradation ratio. Considering the eco-friendly and flexible characteristics of this process, anti-shrinkage and ultrafast-degradable PBAT/PBS foams with improved mechanical and thermal insulation performance present promising prospects to replace the nondegradable polymer foams.

Tetracyclic aromatic lactam-substituted tin-free donors for high-efficiency ternary organic solar cells via regulation of pre-aggregation and crystallization kinetics

Organic solar cells (OSCs) are rapidly developed for next-generation photovoltaic technologies due to low-cost and solution-processability etc. However, so far high-efficiency organic photovoltaic materials are typically prepared by multi-step Stille reactions, which are neither cost-efficient nor atom-economic, even raise safety concerns and incompatible with large-scale process. Large-scale industrial production of photovoltaic materials through simple, cost-effective, and green chemistry remains to be addressed. In this work, we present a case where two isomeric small-molecule donors based on polycyclic aromatic lactams (PAL) unit, named oPh2F and pPh2F with wide-bandgap and high-crystallinity, were designed and synthesized by atom-economic direct C-H activation strategy, modulates pre-aggregation and crystallization kinetics of host system PM6:Y6, induce distinct morphology evolution and effectively boost photovoltaic performance. The best power conversion efficiency (PCE) of 18.40 % for pPh2F-based ternary devices are found to be a fruit of improved light-harvesting ability, favorable vertical phase separation with enhanced crystallinity, the separated crystallization process of host materials and the higher and more balanced charge transport. To our knowledge, this study achieved the highest PCE among PM6:Y6-based ternary OSCs containing tin-free small-molecule donor, which not only highlights the great potential of PAL-based photovoltaic materials, but also promotes the integration of renewable energy with green chemistry.

Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6.

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.