Selective formation of metastable polymorphs in solid-state synthesis

Metastable polymorphs often result from the interplay between thermodynamics and kinetics. Despite advances in predictive synthesis for solution-based techniques, there remains a lack of methods to design solid-state reactions targeting metastable materials. Here, we introduce a theoretical framework to predict and control polymorph selectivity in solid-state reactions. This framework presents reaction energy as a rarely used handle for polymorph selection, which influences the role of surface energy in promoting the nucleation of metastable phases. Through in situ characterization and density functional theory calculations on two distinct synthesis pathways targeting LiTiOPO4, we demonstrate how precursor selection and its effect on reaction energy can effectively be used to control which polymorph is obtained from solid-state synthesis. A general approach is outlined to quantify the conditions under which metastable polymorphs are experimentally accessible. With comparison to historical data, this approach suggests that using appropriate precursors could enable targeted materials synthesis across diverse chemistries through selective polymorph nucleation.

Deciphering strontium sulfate precipitation via Ostwald's rule of stages: From prenucleation clusters to solution-mediated phase tranformation

Multiple-step nucleation pathways have been observed during mineral formation in both inorganic and biomineral systems. These pathways can involve precursor aqueous species, amorphous intermediates, or metastable phases. Despite the widespread occurrence of these processes, elucidating the precise nucleation steps and the transformation mechanisms between each step remains a challenging task. Using a suite of potentiometric, microscopic, and spectroscopic tools, we studied the nucleation pathway of SrSO₄ as a function of the physico-chemical solution parameters. Our observations reveal that below a threshold supersaturation, nucleation is driven by bound species, akin to the prenucleation cluster model, which directly leads to the formation of the stable phase celestine, SrSO₄. At higher supersaturations, this situation is altered, with nucleation dominated by the consumption of free ions. Importantly, this change in nucleation mechanism is coupled to the formation of a hemihydrate metastable phase, SrSO₄ · 1/2H₂O, which eventually transforms into celestine, adhering to Ostwald's rule of stages. This transformation is a solution-mediated process, also occurring in the presence of a fluid film and is controlled by the physico-chemical parameters of the surrounding environment. It proceeds through the dissolution of the metastable phase and the de novo crystallization of the final phase. Overall, our results reveal that ion association taking place during the prenucleation stage dictates whether the nucleation pathway goes through an intermediate phase or not. This also underlines that although Ostwald's rule of stages is a common process, it is not a prerequisite for mineral formation-even in systems where it can occur.

MXene Analogue: A 2D Nitridene Solid Solution for High-Rate Hydrogen Production

Electrocatalysts for high‐rate hydrogen evolution reaction (HER) are crucial to clean fuel production. Nitrogen‐rich 2D transition metal nitride, designated “nitridene”, has shown promising HER performance because of its unique physical/chemical properties. However, its synthesis is hindered by the sluggish growth kinetics. Here for the first time using a catalytic molten‐salt method, we facilely synthesized a V−Mo bimetallic nitridene solid solution, V_0.2Mo_0.8N_1.2, with tunable electrocatalytic property. The molten‐salt synthesis reduces the growth barrier of V_0.2Mo_0.8N_1.2 and facilitates V dissolution via a monomer assembly, as confirmed by synchrotron spectroscopy and ex situ electron microscopy. Furthermore, by merging computational simulations, we confirm that the V doping leads to an optimized electronic structure for fast protons coupling to produce hydrogen. These findings offer a quantitative engineering strategy for developing analogues of MXenes for clean energy conversions.

Geoinspired syntheses of materials and nanomaterials

The search for new materials is intimately linked to the development of synthesis methods. In the current urge for the sustainable synthesis of materials, taking inspiration from Nature's ways to process matter appears as a virtuous approach. In this review, we address the concept of geoinspiration for the design of new materials and the exploration of new synthesis pathways. In geoinspiration, materials scientists take inspiration from the key features of various geological systems and processes occurring in nature, to trigger the formation of artificial materials and nanomaterials. We discuss several case studies of materials and nanomaterials to highlight the basic geoinspiration concepts underlying some synthesis methods: syntheses in water and supercritical water, thermal shock syntheses, molten salt synthesis and high pressure synthesis. We show that the materials emerging from geoinspiration exhibit properties differing from materials obtained by other pathways, thus demonstrating that the field opens up avenues to new families of materials and nanomaterials. This review focuses on synthesis methodologies, by drawing connections between geosciences and materials chemistry, nanosciences, green chemistry, and environmental sciences.

Characterization and Crystal Nucleation Kinetics of a New Metastable Polymorph of Piracetam in Alcoholic Solvents

A new polymorph of the drug active pharmaceutical ingredient piracetam (Form VI) has been discovered and characterized by X-ray powder diffraction (PXRD), solid-state Raman, attenuated total reflectance infrared spectroscopy, and differential scanning calorimetry. The PXRD diffractogram of Form VI shows a distinct peak at 24.2° (2θ) that distinguishes it from the previously known polymorphs and solvates. Form VI is metastable with respect to the previously known polymorphs Form II and Form III; in ethanol solution at 288 K, Form VI transforms into Form II within 15 min, while in isopropanol solution Form VI is kinetically stable for at least 6 h. A total of 1200 crystal nucleation induction time experiments of piracetam in ethanol and isopropanol solutions have been conducted, in sets of 40-80 repeat experiments carried out at different temperatures and solute concentrations. Each solution nucleated as a single polymorph, and each set of repeat experiments resulted in different proportions of Form II, Form III, and Form VI, with Form VI dominating at low nucleation temperatures and Form II at higher nucleation temperatures. The induction time data for Form VI at 288 K have been evaluated within the framework of the classical nucleation theory. At equal driving force, nucleation of Form VI is less obstructed in ethanol than in isopropanol, as captured by a lower interfacial energy and higher pre-exponential factor in ethanol. The proportion of Form VI obtained at a comparable driving force increases in the order ethanol < isopropanol.

Methods to stabilize aqueous supercooling identified by use of an isochoric nucleation detection (INDe) device

Stable aqueous supercooling has shown significant potential as a technique for human tissue preservation, food cold storage, conservation biology, and beyond, but its stochastic nature has made its translation outside the laboratory difficult. In this work, we present an isochoric nucleation detection (INDe) platform for automated, high-throughput characterization of aqueous supercooling at >1 mL volumes, which enables statistically-powerful determination of the temperatures and time periods for which supercooling in a given aqueous system will remain stable. We employ the INDe to investigate the effects of thermodynamic, surface, and chemical parameters on aqueous supercooling, and demonstrate that various simple system modifications can significantly enhance supercooling stability, including isochoric (constant-volume) confinement, hydrophobic container walls, and the addition of even mild concentrations of solute. Finally, in order to enable informed design of stable supercooled biopreservation protocols, we apply a statistical model to estimate stable supercooling durations as a function of temperature and solution chemistry, producing proof-of-concept supercooling stability maps for four common cryoprotective solutes.

Ultrasound-assisted theophylline polymorphic transformation: Selective polymorph nucleation, molecular mechanism and kinetics analysis

In this paper, the ultrasound-assisted solvent-mediated polymorphic transformation of theophylline was explored in detail. The induction time and reconstruction time were significantly decreased by ultrasound, thereby decreasing the total transformation time and promoting the transformation process. The ultrasound-promoted efficiency of nucleation was different in three alcoholic solvents, which was difficult to explain by traditional kinetic effects. To resolve the above confusion, binding energies calculated by Density Functional Theory were applied to explore the relationship between the ultrasound-promoted efficiency of nucleation and solute-solvent interactions. Then, a possible molecular self-assembly nucleation pathway affected by ultrasound was proposed: the ultrasound could change and magnify the crucial effect of the specific sites of solute-solvent interactions in the nucleation process. Finally, the transformation kinetics with different effective ultrasonic energies was quantitatively analyzed by Avrami-Erofeev model, indicating that the dissolution element in the rate-limiting step was gradually eliminated by higher ultrasonic energy. Fortunately, the elusive crystal form V could be easily obtained by the ultrasound-assisted polymorph transformation. This proved to be a robust method to produce high purity form V of theophylline. The outcome of this study demonstrated that the proper ultrasonic irradiation had the potential to produce specific polymorphs selectively.

Τhe role of surface energy in the apparent solubility of two different calcite crystal habits

The interplay between polymorphism and facet-specific surface energy on the dissolution of crystals is examined in this work. It is shown that, using cationic additives, it is possible to produce star-shaped calcite crystals at very high supersaturations. In crystallization processes following the Ostwald rule of stages these star-shaped crystals appear to have higher solubility than both their rhombohedral counterparts and needle-shaped aragonite crystals. The vapour pressures of vaterite, aragonite, star-shaped calcite and rhombohedral calcite crystals are measured using thermogravimetric analysis and the corresponding enthalpies of melting are obtained. Using inverse gas chromatography, the surface energy of the aforementioned crystals is measured as well and the surface energy of the main crystal facets is calculated. Combining the effect of facet-specific surface energies and the enthalpies of melting on a modified version of the classical solubility equation for regular solutions, it is proved that the star-shaped calcite crystals can indeed have higher apparent solubility than aragonitecrystals.

Quantitation of Commercially Available API Solid Forms by Application of the NMR-qSRC Approach: An Optimization Strategy Based on In Silico Simulations

Physical forms of active pharmaceutical ingredients (APIs) play a crucial role in drug discovery since 85% of API molecules exhibit polymorphism and sometimes complicated phase behavior, often resulting in important differences in the respective biochemical and physical properties. Characterization and quantitation of the different forms are becoming more and more essential in the pharmaceutical industry: once these characteristics are known, it is easier to choose the best solid form for development, formulation, manufacturing, and storage. Time domain-nuclear magnetic resonance (TD-NMR) has recently been used to develop a quantitation protocol for solid mixtures, named qSRC, based on the linear combination of T₁ saturation recovery curves (SRCs) collected on a bench-top instrument. Despite its potentials and ease of use, a limited number of application cases have been reported in the literature since its development and many aspects remain to be clarified for the technique to be adopted as a robust routinely industrial analytical tool. In the present work, the reliability of the qSRC approach has been studied by focusing on the role played by key experimental variables, including mixture composition, signal-to-noise ratio, and T₁ differences. In silico simulations were carried out for a wide range of theoretical cases to predict the expected level of accuracy obtainable for a given sample-parameter acquisition set and to clearly define the range of applicability of the method. Results of the simulation are presented alongside a comparison with three real-case studies of commercially available APIs: piroxicam, naproxen sodium, and benzocaine.

Observation of crystallisation dynamics by crystal-structure-sensitive room-temperature phosphorescence from Au(I) complexes

The aggregation behaviour of Au(I) complexes in condensed phases can affect their emission properties. Herein, aggregation-induced room-temperature phosphorescence (RTP) is observed from the crystals of trinuclear Au(I) complexes. The RTP is highly sensitive to the crystal structure, with a slight difference in the alkyl side chains causing not only a change in the crystal structure but also a shift in the RTP maximum. Furthermore, in nanocrystals, reversible RTP colour changes are induced by phase transitions between crystal polymorphs during crystal growth from solution or the pulverisation of bulk crystals. The colour change mechanism is discussed in terms of intermolecular interactions in the crystal structure of the luminescent aggregates. The results suggest that the behaviour in nanocrystals may differ from that in bulk crystals. These insights will advance the fundamental understanding of crystallisation mechanisms and may aid in the discovery of new materials properties for solids with nano- to micrometre sizes.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

Visualising early-stage liquid phase organic crystal growth via liquid cell electron microscopy

Here, we show that the development of nuclei and subsequent growth of a molecular organic crystal system can be induced by electron beam irradiation by exploiting the radiation chemistry of the carrier solvent. The technique of Liquid Cell Electron Microscopy was used to probe the crystal growth of flufenamic acid; a current commercialised active pharmaceutical ingredient. This work demonstrates liquid phase electron microscopy analysis as an essential tool for assessing pharmaceutical crystal growth in their native environment while giving insight into polymorph identification of nano-crystals at their very inception. Possible mechanisms of crystal nucleation due to the electron beam with a focus on radiolysis are discussed along with the innovations this technique offers to the study of pharmaceutical crystals and other low contrast materials.

Strategy to control magnetic coercivity by elucidating crystallization pathway-dependent microstructural evolution of magnetite mesocrystals

Mesocrystals are assemblies of smaller crystallites and have attracted attention because of their nonclassical crystallization pathway and emerging collective functionalities. Understanding the mesocrystal crystallization mechanism in chemical routes is essential for precise control of size and microstructure, which influence the function of mesocrystals. However, microstructure evolution from the nucleus stage through various crystallization pathways remains unclear. We propose a unified model on the basis of the observation of two crystallization pathways, with different ferric (oxyhydr)oxide polymorphs appearing as intermediates, producing microstructures of magnetite mesocrystal via different mechanisms. An understanding of the crystallization mechanism enables independent chemical control of the mesocrystal diameter and crystallite size, as manifested by a series of magnetic coercivity measurements. We successfully implement an experimental model system that exhibits a universal crystallite size effect on the magnetic coercivity of mesocrystals. These findings provide a general approach to controlling the microstructure through crystallization pathway selection, thus providing a strategy for controlling magnetic coercivity in magnetite systems.

Transient metastable intermediates play an important role in the crystallization process. Here, the authors unveil the microstructural changes in magnetite mesocrystals that depend on the intermediate polymorphism and the universal crystallite size effect on the magnetic coercivity of mesocrystals.

Toward materials-by-design: achieving functional materials with physical and chemical effects

Advances in renewable and sustainable energy technologies critically depend on our ability to rationally design and process target materials with optimized performances. Advanced material design and discovery are ideally involved in material prediction, synthesis and characterization. Control of material crystallization enables the rational design and discovery of novel functional inorganic materials in multi-scale. Material processing can be adjusted by various physical fields and chemical effects at different energy states. Material microstructure, architecture and functionality can thus be modified by multiple design methodologies. In this review, we show typical examples using physical and chemical methods to shape inorganic functional materials and evaluate their specific applications in Na-air batteries, Li-ion batteries and supercapacitors. Furthermore, this review also provides insight into the understanding of synthesis-structure relationship of inorganic functional materials.

2D Monte Carlo Simulation of Patchy Particles Association and Protein Crystal Polymorph Selection

Typically, protein crystals inherit the polymorphic form selected by nuclei arising in the solution. However, a transition of a polymorphic form may also occur at a later crystal growth stage. Unfortunately, due to the molecular-scale processes involved, the earliest stages of protein crystal nucleation and polymorph selection remain poorly understood. This paper attempts to elucidate the polymorph selection and crystal growth process in proteins (and colloidal crystals) using 2D Monte Carlo simulations and a computational model with short-range attraction for ‘protein-like’ patchy particles (PPs) of a specific patch geometry, bond width and strength. A relatively narrow temperature range is established whereby parts of the PPs monomers arrange initially in a rapidly growing unstable rhombohedral lattice (Rh). Stable trimers form simultaneously from the monomers remaining in the solution and monomers released from the Rh lattice. These trimers serve as building blocks of a more stable Kagome trihexagonal lattice (TriHex), which appears after a prolonged simulation time. The step-by-step scenario of this polymorphic transition and the specific role of PPs’ geometric and interaction anisotropies are discussed in detail.

Non-equilibrium crystallization pathways of manganese oxides in aqueous solution

Aqueous precipitation of transition metal oxides often proceeds through non-equilibrium phases, whose appearance cannot be anticipated from traditional phase diagrams. Without a precise understanding of which metastable phases form, or their lifetimes, targeted synthesis of specific metal oxides can become a trial-and-error process. Here, we construct a theoretical framework to reveal the nanoscale and metastable energy landscapes of Pourbaix (E-pH) diagrams, providing quantitative insights into the size–dependent thermodynamics of metastable oxide nucleation and growth in water. By combining this framework with classical nucleation theory, we interrogate how solution conditions influence the multistage oxidation pathways of manganese oxides. We calculate that even within the same stability region of a Pourbaix diagram, subtle variations in pH and redox potential can redirect a non-equilibrium crystallization pathway through different metastable intermediates. Our theoretical framework offers a predictive platform to navigate through the thermodynamic and kinetic energy landscape towards the rational synthesis of target materials.

Multistage crystallization of transition metal oxides often proceeds through a number of metastable intermediates. Here, the authors shed light on this phenomenon by extending Pourbaix diagrams to capture the metastable and nanoscale thermodynamics of manganese oxide nucleation.

Understanding crystallization pathways leading to manganese oxide polymorph formation

Hydrothermal synthesis is challenging in metal oxide systems with diverse polymorphism, as reaction products are often sensitive to subtle variations in synthesis parameters. This sensitivity is rooted in the non-equilibrium nature of low-temperature crystallization, where competition between different metastable phases can lead to complex multistage crystallization pathways. Here, we propose an ab initio framework to predict how particle size and solution composition influence polymorph stability during nucleation and growth. We validate this framework using in situ X-ray scattering, by monitoring how the hydrothermal synthesis of MnO2 proceeds through different crystallization pathways under varying solution potassium ion concentrations ([K+] = 0, 0.2, and 0.33 M). We find that our computed size-dependent phase diagrams qualitatively capture which metastable polymorphs appear, the order of their appearance, and their relative lifetimes. Our combined computational and experimental approach offers a rational and systematic paradigm for the aqueous synthesis of target metal oxides.

Polyamorphism and frustrated crystallization in the acid–base reaction of magnesium potassium phosphate cements

The kinetics of MgO dissolution explain the amorphous–amorphous transformation and frustrated crystallization of reaction products in Mg-phosphate cements.

Revealing the roles of solvation in D-mannitol's polymorphic nucleation

Using the different solvating powers of solvents, molecular distribution within solutions can be changed, leading to distinct solvation patterns that ultimately affect polymorphic outcomes.

Insights into the polymorphic transformation mechanism of aluminum hydroxide during carbonation of potassium aluminate solution

Polymorphic transformation from bayerite to gibbsite is discussed kinetically and structurally. Possibility of crossovers in Al(OH)₃ polymorphs stability is investigated for the first time.