Thermodynamic properties and enhancement of diamagnetism in nitrogen doped lutetium hydride synthesized at high pressure.

Nitrogen doped lutetium hydride has drawn global attention in the pursuit of room-temperature superconductivity near ambient pressure and temperature. However, variable synthesis techniques and uncertainty surrounding nitrogen concentration have contributed to extensive debate within the scientific community about this material and its properties. We used a solid-state approach to synthesize nitrogen doped lutetium hydride at high pressure and temperature (HPT) and analyzed the residual starting materials to determine its nitrogen content. High temperature oxide melt solution calorimetry determined the formation enthalpy of LuH1.96N0.02 (LHN) from LuH2 and LuN to be -28.4 ± 11.4 kJ/mol. Magnetic measurements indicated diamagnetism which increased with nitrogen content. Ambient pressure conductivity measurements observed metallic behavior from 5 to 350 K, and the constant and parabolic magnetoresistance changed with increasing temperature. High pressure conductivity measurements revealed that LHN does not exhibit superconductivity up to 26.6 GPa. We compressed LHN in a diamond anvil cell to 13.7 GPa and measured the Raman signal at each step, with no evidence of any phase transition. Despite the absence of superconductivity, a color change from blue to purple to red was observed with increasing pressure. Thus, our findings confirm the thermodynamic stability of LHN, do not support superconductivity, and provide insights into the origins of its diamagnetism.

Melting temperature prediction using a graph neural network model: From ancient minerals to new materials.

The melting point is a fundamental property that is time-consuming to measure or compute, thus hindering high-throughput analyses of melting relations and phase diagrams over large sets of candidate compounds. To address this, we build a machine learning model, trained on a database of ∼10,000 compounds, that can predict the melting temperature in a fraction of a second. The model, made publicly available online, features graph neural network and residual neural network architectures. We demonstrate the model's usefulness in diverse applications. For the purpose of materials design and discovery, we show that it can quickly discover novel multicomponent materials with high melting points. These predictions are confirmed by density functional theory calculations and experimentally validated. In an application to planetary science and geology, we employ the model to analyze the melting temperatures of ∼4,800 minerals to uncover correlations relevant to the study of mineral evolution.

Thermochemical Properties of High Pressure Neodymium Monoxide.

Neodymium monoxide (NdO) is a metastable rare earth oxide material with a unique electronic structure, which has potential applications across various fields such as semiconductors, energy, catalysis, laser technology, and advanced communications. Despite its promising attributes, the thermodynamic properties of NdO remain unexplored. In this study, high pressure, high temperature phases of neodymium monoxide (NdO, with a rocksalt structure) and body-centered cubic (bcc) Nd metal were synthesized at 5 GPa and 1473 K. X-ray photoelectron spectroscopy (XPS) measurements indicate that the Nd 3d peak shifts to higher energy in NdO relative to Nd2O3, suggesting the possibility of complex electronic states in NdO. Formation enthalpies for the reaction 1/3Nd2O3 + 1/3bcc Nd = NdO obtained from high temperature solution calorimetry in molten sodium molybdate and for the reaction dhcp Nd (metal) = bcc Nd (metal) from differential scanning calorimetry are 25.98 ± 8.65 and 5.2 kJ/mol, respectively. Utilizing these enthalpy values, we calculated the pressure-temperature boundary for the reaction 1/3 bcc Nd + 1/3Nd2O3 = NdO, which has a negative P-T slope of -1.68× 10-4 GPa/K. These insights reveal the high pressure behavior of NdO and neodymium metal, underscoring their potential utility in technological applications.

Cooperative formation of porous silica and peptides on the prebiotic Earth.

Modern technology has perfected the synthesis of catalysts such as zeolites and mesoporous silicas using organic structure directing agents (SDA) and their industrial use to catalyze a large variety of organic reactions within their pores. We suggest that early in prebiotic evolution, synergistic interplay arose between organic species in aqueous solution and silica formed from rocks by dynamic dissolution-recrystallization. The natural organics, for example, amino acids, small peptides, and fatty acids, acted as SDA for assembly of functional porous silica structures that induced further polymerization of amino acids and peptides, as well as other organic reactions. Positive feedback between synthesis and catalysis in the silica-organic system may have accelerated the early stages of abiotic evolution by increasing the formation of polymerized species.

Thermochemistry of hybrid materials.

This paper is based on a lecture Navrotsky gave honouring the memory of Paul McMillan. It summarizes our recent findings in the thermodynamics of hybrid materials including metal organic frameworks (MOFs), polymer-derived ceramics (PDCs) and ionic organic-inorganic compounds. This work describes the main structure types and their corresponding thermodynamic stability, obtained from calorimetric measurements in our laboratory. The effects of linker substituent and framework topology on the thermodynamic stability of isostructural zeolitic imidazolate frameworks and other MOFs are discussed. The paper documents the effects of interdomain interaction and bonding speciation on the thermodynamic stability of various PDC compositions, including SiC, SiOC and SiCN systems. The paper further describes effects of different cations on the thermodynamic stability of selected ionic organic-inorganic compounds. Similarities and differences among these materials are emphasized. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Water uptake and energetics of the formation of barium zirconate based multicomponent oxides.

A group of multi-component oxides based on BaZrO3 have been prepared using a solid-state reaction method and examined in terms of their water uptake and thermodynamics of formation. Depending on the type and amount of acceptor substitution, the synthesized compounds exhibit various proton defect concentrations, reaching up to 0.2 mol/mol for a compound containing 10 different elements in the B-sublattice, where 50% of them are acceptors. For the most promising materials, van't Hoff plots were created and the enthalpies and entropies of hydration were calculated. At higher temperatures, these parameters do not differ from the values for the reference yttrium doped barium zirconate. However, at lower temperatures they are more negative, indicating a more exothermic process of proton incorporation.

Greigite (Fe3S4) is thermodynamically stable: Implications for its terrestrial and planetary occurrence.

Iron sulfide minerals are widespread on Earth and likely in planetary bodies in and beyond our solar system. Using measured enthalpies of formation for three magnetic iron sulfide phases: bulk and nanophase Fe3S4 spinel (greigite), and its high-pressure monoclinic phase, we show that greigite is a stable phase in the Fe-S phase diagram at ambient temperature. The thermodynamic stability and low surface energy of greigite supports the common occurrence of fine-grained Fe3S4 in many anoxic terrestrial settings. The high-pressure monoclinic phase, thermodynamically metastable below about 3 GPa, shows a calculated negative P-T slope for its formation from the spinel. The stability of these three phases suggests their potential existence on Mercury and their magnetism may contribute to its present magnetic field.

Acid-Base Properties of Oxides Derived from Oxide Melt Solution Calorimetry.

The paper analyzes the relationships among acid-base interactions in various oxide systems and their thermodynamics. Extensive data on enthalpies of solution of binary oxides in oxide melts of several compositions, obtained by high-temperature oxide melt solution calorimetry at 700 and 800 °C, are systematized and analyzed. Oxides with low electronegativity, namely the alkali and alkaline earth oxides, which are strong oxide ion donors, show enthalpies of solution that have negative values greater than -100 kJ per mole of oxide ion. Their enthalpies of solution become more negative with decreasing electronegativity in the order Li, Na, K and Mg, Ca, Sr, Ba in both of the commonly used molten oxide calorimetric solvents: sodium molybdate and lead borate. Oxides with high electronegativity, including P2O5, SiO2, GeO2, and other acidic oxides, dissolve more exothermically in the less acidic solvent (lead borate). The remaining oxides, with intermediate electronegativity (amphoteric oxides) have enthalpies of solution of between +50 and -100 kJ/mol, with many close to zero. More limited data for the enthalpies of solution of oxides in multicomponent aluminosilicate melts at higher temperature are also discussed. Overall, the ionic model combined with the Lux-Flood description of acid-base reactions provide a consistent and useful interpretation of the data and their application for understanding the thermodynamic stability of ternary oxide systems in solid and liquid states.

Entropy Stabilization Effects and Ion Migration in 3D "Hollow" Halide Perovskites.

A recently discovered new family of 3D halide perovskites with the general formula (A)1-x(en)x(Pb)1-0.7x(X)3-0.4x (A = MA, FA; X = Br, I; MA = methylammonium, FA = formamidinium, en = ethylenediammonium) is referred to as "hollow" perovskites owing to extensive Pb and X vacancies created on incorporation of en cations in the 3D network. The "hollow" motif allows fine tuning of optical, electronic, and transport properties and bestowing good environmental stability proportional to en loading. To shed light on the origin of the apparent stability of these materials, we performed detailed thermochemical studies, using room temperature solution calorimetry combined with density functional theory simulations on three different families of "hollow" perovskites namely en/FAPbI3, en/MAPbI3, and en/FAPbBr3. We found that the bromide perovskites are more energetically stable compared to iodide perovskites in the FA-based hollow compounds, as shown by the measured enthalpies of formation and the calculated formation energies. The least stable FAPbI3 gains stability on incorporation of the en cation, whereas FAPbBr3 becomes less stable with en loading. This behavior is attributed to the difference in the 3D cage size in the bromide and iodide perovskites. Configurational entropy, which arises from randomly distributed cation and anion vacancies, plays a significant role in stabilizing these "hollow" perovskite structures despite small differences in their formation enthalpies. With the increased vacancy defect population, we have also examined halide ion migration in the FA-based "hollow" perovskites and found that the migration energy barriers become smaller with the increasing en content.

Thermochemistry of 3D and 2D Rare Earth Oxychlorides (REOCls).

The thermodynamic stability of rare earth (RE) materials plays a key role in the design of separation and recycling processes for RE elements. Thermodynamic stability is fundamentally influenced by the lanthanide contraction, as observed in the systematic reduction of unit cell volumes with increasing atomic number. RE materials are found in the form of solids having primary bonds in three dimensions (3D materials) as well as ones with primary bonds in two dimensions (2D materials) whose layers are held together by weak van der Waals (vdW) forces. While studies of synthesis, structure, and physical properties of 2D RE materials are numerous, no systematic research has compared their thermodynamic stability to that of 3D materials. In the present work, RE oxychlorides (REOCls), which display a structural transition from a 3D-polyhedral network (PbFCl-type) to a vdW-bonded layered one (SmSI-type) as the RE size decreases, were all synthesized by the flux method. High-temperature oxide melt solution calorimetry was used to determine their formation enthalpies to enable Born-Haber cycles to calculate lattice energies. Our results indicate that REOCl compounds are thermodynamically stable when compared to their binary oxides and chlorides. The lattice energies of 3D REOCls increase with decreasing RE size yet are insensitive to unit cell volumes for 2D REOCls. This is caused by interatomic interactions parallel and perpendicular to layers in the SmSI-type REOCls, causing a different structure response to the lanthanide contraction than 3D RE materials.

Structural and thermodynamic limits of layer thickness in 2D halide perovskites.

In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A')2(A) n-1M n X3n+1 [where A = Cs+, CH3NH3+, HC(NH2)2+; A' = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl-, Br-, I-] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure-function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for "(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28" ("n = 9"). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9-12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.

Insight on the Stability of Thick Layers in 2D Ruddlesden-Popper and Dion-Jacobson Lead Iodide Perovskites.

Two-dimensional (2D) hybrid organic-inorganic halide perovskites are a preeminent class of low-cost semiconductors whose inherent structural tunability and attractive photophysical properties have led to the successful fabrication of solar cells with high power conversion efficiencies. Despite the observed superior stability of 2D lead iodide perovskites over their 3D parent structures, an understanding of their thermochemical profile is missing. Herein, the calorimetric studies reveal that the Ruddlesden-Popper (RP) series, incorporating the monovalent-monoammonium spacer cations of pentylammonium (PA) and hexylammonium (HA): (PA)2(MA)n-1PbnI3n+1 (n = 2-6) and (HA)2(MA)n-1PbnI3n+1 (n = 2-4) have a negative enthalpy of formation, relative to their binary iodides. In contrast, the enthalpy of formation for the Dion-Jacobson (DJ) series, incorporating the divalent and cyclic diammonium cations of 3- and 4-(aminomethyl)piperidinium (3AMP and 4AMP respectively): (3AMP)(MA)n-1PbnI3n+1 (n = 2-5) and (4AMP)(MA)n-1PbnI3n+1 (n = 2-4) have a positive enthalpy of formation. In addition, for the (PA)2(MA)n-1PbnI3n+1 family of materials, we report the phase-pure synthesis and single crystal structure of the next member of the series (PA)2(MA)5Pb6I19 (n = 6), and its optical properties, marking this the second n = 6, bulk member published to date. Particularly, (PA)2(MA)5Pb6I19 (n = 6) has negative enthalpy of formation as well. Additionally, the analysis of the structural parameters and optical properties between the examined RP and DJ series offers guiding principles for the targeted design and synthesis of 2D perovskites for efficient solar cell fabrication. Although the distortions of the Pb-I-Pb equatorial angles are larger in the DJ series, the significantly smaller I···I interlayer distances lead to overall smaller band gap values, in comparison with the RP series. Our film stability studies on the RP and DJ perovskites series reveal consistent observations with the thermochemical findings, pointing out to the lower extrinsic stability of the DJ materials in ambient air.

Effect of Annealing on Structural and Thermodynamic Properties of ThSiO4-ErPO4 Xenotime Solid Solution.

The effect of annealing on structural and thermochemical properties of a thorite-xenotime solid solution Th1-xErx(SiO4)1-x(PO4)x was assessed. The samples synthesized at low temperatures and stored at room temperature for 2 years retained their tetragonal structures. This structure was also maintained after heating to 1100 °C. During annealing, the structure lost water and exsolved some thorianite phases. The thermodynamic parameters did not change much after annealing, suggesting that xenotime was not a low-temperature metastable phase but rather a stable structure able to withstand elevated temperatures regardless of the thorium content. The solid solution exhibited subregular behavior with the Margules function W(x) = (73.1 ± 20.1) - (125.7 ± 49.8)·x.

Fe(II) Induced Reduction of Incorporated U(VI) to U(V) in Goethite.

Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)oxides such as goethite and adsorbed Fe(II). These interactions can also induce varying degrees of iron (oxyhydr)oxide recrystallization (<4% to >90%). Here, the fate of U(VI)-incorporated goethite during exposure to Fe(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U(VI) was reduced to U(V) as the reaction with Fe(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)oxides acting as a secondary barrier to radionuclide migration in the environment.

Linker Substituents Control the Thermodynamic Stability in Metal-Organic Frameworks.

We report the first systematic experimental and theoretical study of the relationship between the linker functionalization and the thermodynamic stability of metal-organic frameworks (MOFs) using a model set of eight isostructural zeolitic imidazolate frameworks (ZIFs) based on 2-substituted imidazolate linkers. The frameworks exhibit a significant (30 kJ·mol-1) variation in the enthalpy of formation depending on the choice of substituent, which is accompanied by only a small change in molar volume. These energetics were readily reproduced by density functional theory (DFT) calculations. We show that these variations in the enthalpy of MOF formation are in linear correlation to the readily accessible properties of the linker substituent, such as the Hammett σ-constant or electrostatic surface potential. These results provide the first quantifiable relationship between the MOF thermodynamics and the linker structure, suggesting a route to design and tune MOF stability.

Thermodynamic Evidence of Structural Transformations in CO2-Loaded Metal-Organic Framework Zn(MeIm)2 from Heat Capacity Measurements.

Metal-organic frameworks are a class of porous compounds with potential applications in molecular sieving, gas sequestration, and catalysis. One family of MOFs, zeolitic imidizolate frameworks (ZIFs), is of particular interest for carbon dioxide sequestration. We have previously reported the heat capacity of the sodalite topology of the zinc 2-methylimidazolate framework (ZIF-8), and in this Article we present the first low-temperature heat capacity measurements of ZIF-8 with various amounts of sorbed CO2. Molar heat capacities from 1.8 to 300 K are presented for samples containing up to 0.99 mol of CO2 per mol of ZIF-8. Samples with at least 0.56 mol of CO2 per mol of ZIF-8 display a large, broad anomaly from 70 to 220 K with a shoulder on the low-temperature side, suggesting sorption-induced structural transitions. We attribute the broad anomaly partially to a gate-opening transition, with the remainder resulting from CO2 rearrangement and/or lattice expansion. The measurements also reveal a subtle anomaly from 0 to 70 K in all samples that does not exist in the sorbate-free material, which likely reflects new vibrational modes resulting from sorbate/ZIF-8 interactions. These results provide the first thermodynamic evidence of structural transitions induced by CO2 sorption in the ZIF-8 framework.

In Situ High-Temperature Synchrotron Diffraction Studies of (Fe,Cr,Al)3O4 Spinels.

The modeling of a loss-of-coolant-accident scenario involving nuclear fuels with FeCrAl cladding materials in consideration to replace a Zircaloy requires knowledge of the thermodynamics of oxidized structures. At temperatures higher than 1500 °C, oxidation of FeCrAl alloys forms (Fe,Cr,Al)3O4 spinels. In situ high-energy X-ray diffraction in a conical nozzle levitator installed at beamline 6-ID-D of the APS was used to study the structural evolution of the oxides as a function of the temperature. Single-phase (spinel) and multiphase (spinel-corundum-FeAlO3) regions are mapped as a function of the temperature for three different compositions of FeCrAl oxidation products. The thermal expansion coefficients and cation distribution in the spinel structure have been refined. The temperature at which complete melting of the fuel cladding is expected has been determined by the liquidus temperatures of the oxidized products to be between 1657 and 1834 °C in a 20% O2/Ar atmosphere using the cooling trace method. The liquidus temperature increases with increasing Al and Cr content in the spinel phase.

Hydration structure and water exchange kinetics at xenotime-water interfaces: implications for rare earth minerals separation.

Hydration of surface ions gives rise to structural heterogeneity and variable exchange kinetics of water at complex mineral-water interfaces. Here, we employ ab initio molecular dynamics (AIMD) simulations and water adsorption calorimetry to examine the aqueous interfaces of xenotime, a phosphate mineral that contains predominantly Y3+ and heavy rare earth elements. Consistent with natural crystal morphology, xenotime is predicted to have a tetragonal prismatic shape, dominated by the {100} surface. Hydration of this surface induces multilayer interfacial water structures with distinct OH orientations, which agrees with recent crystal truncation rod measurements. The exchange kinetics between two adjacent water layers exhibits a wide range of underlying timescales (5-180 picoseconds), dictated by ion-water electrostatics. Adsorption of a bidentate hydroxamate ligand reveals that {100} xenotime surface can only accommodate monodentate coordination with water exchange kinetics strongly depending on specific ligand orientation, prompting us to reconsider traditional strategies for selective separation of rare-earth minerals.

Energetics of CO2 and H2O adsorption on alkaline earth metal doped TiO2.

The process of CO2 and H2O adsorption on the surface of nano-oxide semiconductors is important in the overall performance of artificial photosynthesis and other applications. In this study, we explored the thermodynamics of CO2 and H2O adsorption on TiO2 as a function of surface chemistry. We applied gas adsorption calorimetry to investigate the energetics of adsorption of those molecules on the surface of anatase nanoparticles. In an attempt to increase TiO2 surface affinity to CO2 and H2O, TiO2 was doped with alkaline earth metals (MgO, CaO, SrO, and BaO) by manipulating the chemical synthesis. Adsorption studies using diffuse reflectance infrared spectroscopy at different temperatures indicate that due to the segregation of alkaline earth metals on the surface of TiO2 nanoparticles, both CO2 and subsequent H2O adsorption amounts could be increased. CO2 adsorbs in two different manners, forming carbonates which can be removed at temperatures lower than 700 °C, and a more stable linear adsorption that remains even at 700 °C. Additionally to the surface energetic effects, doping also increased specific surface area, resulting in further improvement in net gas adsorption.

Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane.

Manganese oxides with layer and tunnel structures occur widely in nature and inspire technological applications. Having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the their thermodynamic stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn3O4, Mn2O3, and MnO2), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing average manganese oxidation state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aqueous solution is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodynamics and the energetics of the aqueous phase drive cryptomelane formation from birnessite, perhaps aided by oxidation-state differences. Transformation among birnessite phases of increasing surface area favors compositions with lower surface energy. These quantitative thermodynamic findings explain and support qualitative observations of phase-transformation patterns gathered from natural and synthetic manganese oxides.