Cu3As: Uncommon Crystallographic Features, Low-Temperature Phase Transitions, Thermodynamic and Physical Properties.

The formation and crystal structure of the binary Cu3As phase have been re-investigated. Some physical properties were then measured on both single crystal and polycrystalline bulk. Cu3As melts congruently at 835 °C. At room temperature (RT), this compound has been found to crystallize in the hexagonal Cu3P prototype (hP24, P63cm) with lattice parameters: a = 7.1393(1) Å and c = 7.3113(1) Å, rather than in the anti HoH3-type (hP24, P-3c1) as indicated in literature. A small compositional range of 74.0-75.5 at.% Cu (26.0-24.5 at.% As) was found for samples synthesized at 300 and 400 °C; a corresponding slight understoichiometry is found in one out of the four Cu atomic sites, leading to the final refined composition Cu2.882(1)As. The present results disprove a change in the crystal structure above RT actually reported in the phase diagram (from γ' to γ on heating). Instead, below RT, at T = 243 K (-30 °C), a first-order structural transition to a trigonal low-temperature superstructure, LT-Cu3-xAs (hP72, P-3c1) has been found. The LT polymorph is metrically related to the RT one, having the c lattice parameter three times larger: a = 7.110(2) Å and c = 21.879(4) Å. Both the high- and low-temperature polymorphs are characterized by the presence of a tridimensional (3D) uncommon and rigid Cu sublattice of the lonsdaleite type (Cu atoms tetrahedrally bonded), which remains almost unaffected by the structural change(s), and characteristic layers of triangular 'Cu3As'-units (each hosting one As atom at the center, interconnected each other by sharing the three vertices). The first-order transition is then followed by an additional structural change when lowering the temperature, which induces doubling of also the lattice parameter a. Differential scanning calorimetry nicely detects the first low-temperature structural change occurring at T = 243 K, with an associated enthalpy difference, ΔH(TR), of approximately 2 J/g (0.53 kJ/mol). Low-temperature electrical resistivity shows a typical metallic behavior; clear anomalies are detected in correspondence to the solid-state transformations. The Seebeck coefficient, measured as a function of temperature, highlights a conduction of n-type. The temperature dependence of the magnetic susceptibility displays an overall constant diamagnetic response.

Synthesis, Structure, and Characterization of 4,4'-(Anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) Bismuth Iodide (C30H22N2)3Bi4I18, an Air, Water, and Thermally Stable 0D Hybrid Perovskite with High Photoluminescence Efficiency.

4,4'-(Anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) bismuth iodide (C30H22N2)3Bi4I18 (AEPyBiI) was obtained as a black powder by a very simple route by mixing an acetone solution of BiI3 and an aqueous solution of C30H22N2I2. This novel perovskite is air and water stable and displays a remarkable thermal stability up to nearly 300 °C. The highly conjugated cation C30H22N2 2+ is hydrolytically stable, being nitrogen atoms quaternarized, and this accounts for the insensitivity of the perovskite toward water and atmospheric oxygen under ambient conditions. The cation in aqueous solution is highly fluorescent under UV irradiation (emitting yellow-orange light). AEPyBiI as well is intensely luminescent, its photoluminescence emission being more than 1 order of magnitude greater than that of high-quality InP epilayers. The crystal structure of AEPyBiI was determined using synchrotron radiation single-crystal X-ray diffraction. AEPyBiI was extensively characterized using a wide range of techniques, such as X-ray powder diffraction, diffuse reflectance UV-vis spectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopies, thermogravimetry-differential thermal analysis (TG-DTA), elemental analysis, electrospray ionization mass spectroscopy (ESI-MS), and photoluminescence spectroscopy. AEPyBiI displays a zero-dimensional (0D) perovskite structure in which the inorganic part is constituted by binuclear units consisting of two face-sharing BiI6 octahedra (Bi2I9 3- units). The C30H22N2 2+ cations are stacked along the a-axis direction in a complex motif. Considering its noteworthy light-emitting properties coupled with an easy synthesis and environmental stability, and its composition that does not contain toxic lead or easily oxidable Sn(II), AEPyBiI is a promising candidate for environmentally friendly light-emitting devices.

Theoretical-Computational Modeling of Gas-State Thermodynamics in Flexible Molecular Systems: Ionic Liquids in the Gas Phase as a Case Study.

A theoretical-computational procedure based on the quasi-Gaussian entropy (QGE) theory and molecular dynamics (MD) simulations is proposed for the calculation of thermodynamic properties for molecular and supra-molecular species in the gas phase. The peculiarity of the methodology reported in this study is its ability to construct an analytical model of all the most relevant thermodynamic properties, even within a wide temperature range, based on a practically automatic sampling of the entire conformational repertoire of highly flexible systems, thereby bypassing the need for an explicit search for all possible conformers/rotamers deemed relevant. In this respect, the reliability of the presented method mainly depends on the quality of the force field used in the MD simulations and on the ability to discriminate in a physically coherent way between semi-classical and quantum degrees of freedom. The method was tested on six model systems (n-butane, n-butane, n-octanol, octadecane, 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic pairs), which, being experimentally characterized and already addressed by other theoretical-computational methods, were considered as particularly suitable to allow us to evaluate the method's accuracy and efficiency, bringing out advantages and possible drawbacks. The results demonstrate that such a physically coherent yet relatively simple method can represent a further valid computational tool that is alternative and complementary to other extremely efficient computational methods, as it is particularly suited for addressing the thermodynamics of gaseous systems with a high conformational complexity over a large range of temperature.

The AuSc, AuTi, and AuFe molecules: Determination of the bond energies by Knudsen effusion mass spectrometry experiments combined with ab initio calculations.

The AuTi gaseous molecule was for the first time identified in vapors produced at high temperature from a gold-titanium alloy. The homogeneous equilibria AuTi(g) = Au(g) + Ti(g) (direct dissociation) and AuTi(g) + Au(g) = Au2(g) + Ti(g) (isomolecular exchange) were studied by Knudsen effusion mass spectrometry in the temperature range 2111-2229 K. The so determined equilibrium constants were treated by the "third-law method" of thermodynamic analysis, integrated with theoretical calculations, and the dissociation energy at 0 K was derived as D0K ° (AuTi) = 241.0 ± 5.2 kJ/mol. A similar investigation was carried out for the AuSc and AuFe species, whose dissociation energies were previously reported with large uncertainties. The direct dissociation and the isomolecular exchange with the Au2 dimer were studied in the 1969-2274 and 1842-2092 K ranges for AuSc and AuFe, respectively, and the dissociation energies derived as D0K ° (AuSc) = 240.4 ± 6.0 and D0K ° (AuFe) = 186.2 ± 4.2 kJ/mol. The experimental bond energies are compared with those calculated here by coupled cluster with single, double, and perturbative triple excitations with the correlation-consistent basis sets cc-pVXZ(-PP) and cc-pwCVXZ(-PP) (with X = T, Q, 5), also in the limit of complete basis set, and with those from complete active space self-consistent field-multi-reference configuration interaction calculations, recently available in the literature. The stronger bond of AuTi compared to AuFe parallels the trend observed in monochlorides. This analogy is shown to be more generally observed in the AuM and MCl diatomic series (with M = first row transition metal), in accordance with a picture of "pseudo-halogen" bonding behavior of gold.

Thermodynamic Study of Formamidinium Lead Iodide (CH5N2PbI3) from 5 to 357 K.

In the present study, the molar heat capacity of solid formamidinium lead iodide (CH5N2PbI3) was measured over the temperature range from 5 to 357 K using a precise automated adiabatic calorimeter. In the above temperature interval, three distinct phase transitions were found in ranges from 49 to 56 K, from 110 to 178 K, and from 264 to 277 K. The standard thermodynamic functions of the studied perovskite, namely the heat capacity C°p(T), enthalpy [H0(T) - H0(0)], entropy S0(T), and [G°(T) - H°(0)]/T, were calculated for the temperature range from 0 to 345 K based on the experimental data. Herein, the results are discussed and compared with those available in the literature as measured by nonclassical methods.

Sublimation Study of Six 5-Substituted-1,10-Phenanthrolines by Knudsen Effusion Mass Loss and Solution Calorimetry.

The vapor pressures of six solid 5-X-1,10-phenanthrolines (where X = Cl, CH3, CN, OCH3, NH2, NO2) were determined in suitable temperature ranges by Knudsen Effusion Mass Loss (KEML). From the temperature dependencies of vapor pressure, the molar sublimation enthalpies, ΔcrgHm0(⟨T⟩), were calculated at the corresponding average ⟨T⟩ of the explored temperature ranges. Since to the best of our knowledge no thermochemical data seem to be available in the literature regarding these compounds, the ΔcrgHm0(⟨T⟩) values obtained by KEML experiments were adjusted to 298.15 K using a well known empirical procedure reported in the literature. The standard (p0 = 0.1 MPa) molar sublimation enthalpies, ΔcrgHm0(298.15 K), were compared with those determined using a recently proposed solution calorimetry approach, which was validated using a remarkable amount of thermochemical data of molecular compounds. For this purpose, solution enthalpies at infinite dilution of the studied 5-chloro and 5-methylphenantrolines in benzene were measured at 298.15 K. Good agreement was found between the values derived by the two different approaches, and final mean values of ΔcrgHm0(298.15 K) were recommended. Finally, the standard molar entropies and Gibbs energies of sublimation were also derived at T = 298.15 K. The volatilities of the six compounds were found to vary over a range of three orders of magnitude in the explored temperature range. The large difference in volatility was analyzed in the light of enthalpies and entropies of sublimation. The latter was tentatively put in relation to the rotational contribution of the substituent group on the phenanthroline unit.

Thermodynamic and Kinetic Aspects of Formamidinium Lead Iodide Thermal Decomposition.

We report the results of a multi-technique study on the thermodynamics and kinetics of formamidinium lead iodide (FAPI) thermal decomposition. Thermodynamics was investigated by means of Knudsen effusion techniques. Kinetics was studied either by temperature-controlled powder X-ray diffraction or by two isoconversional treatments of differential scanning calorimetry data. FAPI appears to be much more thermally stable compared to methylammonium lead iodide, as predictable from the lower acidity of the formamidinium cation compared to methylammonium. The chemical processes responsible for its thermal degradation appear to be quite complex as highlighted by the composition of the gaseous phase evolved during the process. The apparent activation energy values of the decomposition obtained from X-ray diffraction (XRD) (112 ± 9 kJ/mol) and differential scanning calorimetry (DSC) measurements (205 ± 20 and 410 ± 20 kJ/mol, respectively, for the first and second decomposition steps identified by the deconvolution procedure) reflect the different steps of the process observed by the two techniques. The thermodynamic properties of the more important decomposition channels and the enthalpy of formation of FAPI were estimated by combining the results of Knudsen effusion measurements.

Stabilizing lead halide perovskites with quaternary ammonium cations: the case of tetramethylammonium lead iodide.

Organoammonium lead halide perovskites, especially methylammonium lead iodide CH3NH3PbI3, are promising photovoltaic materials, but they are far from commercial applications due in particular to their thermal instability and moisture sensitivity. Here, we present a multitechnique study aimed at investigating the kinetic and thermodynamic stability of the simplest quaternary ammonium lead iodide, tetramethylammonium lead iodide N(CH3)4PbI3. The kinetics of thermal decomposition was studied by X-ray powder diffraction of samples treated in air at different temperatures combined with Rietveld quantitative phase analysis, and by the isoconversional analysis of differential thermal analysis measurements. Evidence for first order kinetics was obtained, with an activation energy of 280-290 kJ mol-1, suggesting that the breaking of the C-N bond is the rate determining step. The composition of the gas phase released under heating was investigated by Knudsen Effusion Mass Spectrometry, giving evidence for the occurrence of the process N(CH3)4PbI3(s) = PbI2(s) + N(CH3)3(g) + CH3I(g), consistent with the kinetic results. Decomposition pressures and thermodynamic properties were derived by Knudsen effusion mass loss experiments, obtaining values of 391.5 ± 2.0 kJ mol-1 and -577.4 ± 4.0 kJ mol-1 for the decomposition and formation enthalpies at 298 K, respectively. The reactivity towards water of N(CH3)4PbI3 was checked by XRD after total and prolonged immersion in water at room temperature. Overall, N(CH3)4PbI3 was found to be thermally much more stable than CH3NH3PbI3, both kinetically and thermodynamically, and much less prone to water-induced degradation, suggesting that the use of a quaternary ammonium cation may be an effective strategy in order to produce more stable materials.

Thermodynamics and the Intrinsic Stability of Lead Halide Perovskites CH3NH3PbX3.

The role of thermodynamics in assessing the intrinsic instability of the CH3NH3PbX3 perovskites (X = Cl,Br,I) is outlined on the basis of the available experimental information. Possible decomposition/degradation pathways driven by the inherent instability of the material are considered. The decomposition to precursors CH3NH3X(s) and PbX2( s) is first analyzed, pointing out the importance of both the enthalpic and the entropic factor, the latter playing a stabilizing role making the stability higher than often asserted. For CH3NH3PbI3, the disagreement between the available calorimetric results makes the stability prediction uncertain. Subsequently, the gas-releasing decomposition paths are discussed, with emphasis on the discrepant results presently available, probably reflecting the predominance of thermodynamic or kinetic control. The competition between the formation of NH3(g) + CH3X(g), CH3NH2(g) + HX(g) or CH3NH3X(g) is analyzed, in comparison with the thermal decomposition of methylammonium halides. In view of the scarce and inconclusive thermodynamic studies to-date available, the need for further experimental data is emphasized.

Corrigendum: On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites.

This corrects the article DOI: 10.1038/srep31896.

Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2.

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas-phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source.

On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites.

The interest of the scientific community on methylammonium lead halide perovskites (MAPbX3, X = Cl, Br, I) for hybrid organic-inorganic solar cells has grown exponentially since the first report in 2009. This fact is clearly justified by the very high efficiencies attainable (reaching 20% in lab scale devices) at a fraction of the cost of conventional photovoltaics. However, many problems must be solved before a market introduction of these devices can be envisaged. Perhaps the most important to be addressed is the lack of information regarding the thermal and thermodynamic stability of the materials towards decomposition, which are intrinsic properties of them and which can seriously limit or even exclude their use in real devices. In this work we present and discuss the results we obtained using non-ambient X-ray diffraction, Knudsen effusion-mass spectrometry (KEMS) and Knudsen effusion mass loss (KEML) techniques on MAPbCl3, MAPbBr3 and MAPbI3. The measurements demonstrate that all the materials decompose to the corresponding solid lead (II) halide and gaseous methylamine and hydrogen halide, and the decomposition is well detectable even at moderate temperatures (~60 °C). Our results suggest that these materials may be problematic for long term operation of solar devices.

Vaporization of the prototypical ionic liquid BMImNTf2 under equilibrium conditions: a multitechnique study.

The vaporization behaviour and thermodynamics of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2) were studied by combining the Knudsen Effusion Mass Loss (KEML) and Knudsen Effusion Mass Spectrometry (KEMS) techniques. KEML studies were carried out in a large temperature range (398-567) K by using effusion orifices with 0.3, 1, and 3 mm diameters. The vapor pressures so measured revealed no kinetically hindered vaporization effects and provided second-law vaporization enthalpies at the mean experimental temperatures in close agreement with literature. By exploiting the large temperature range covered, the heat capacity change associated with vaporization was estimated, resulting in a value of -66.8 J K(-1) mol(-1), much lower than that predicted from calorimetric measurements on the liquid phase and theoretical calculations on the gas phase. The conversion of the high temperature vaporization enthalpy to 298 K was discussed and the value Δ(l)(g)H(m)(298 K) = (128.6 ± 1.3) kJ mol(-1) assessed on the basis of data from literature and present work. Vapor pressure data were also processed by the third-law procedure using different estimations for the auxiliary thermal functions, and a Δ(l)(g)H(m)(298 K) consistent with the assessed value was obtained, although the overall agreement is sensitive to the accuracy of heat capacity data. KEMS measurements were carried out in the lower temperature range (393-467) K and showed that the largely prevailing ion species is BMIm(+), supporting the common view of BMImNTf2 vaporizing as individual, neutral ion pairs also under equilibrium conditions. By monitoring the mass spectrometric signal of this ion as a function of temperature, a second-law Δ(l)(g)H(m)(298 K) of 129.4 ± 7.3 kJ mol(-1) was obtained, well consistent with KEML and literature results. Finally, by combining KEML and KEMS measurements, the electron impact ionization cross section of BMIm(+) was estimated.

The Si-Sn chemical bond: an integrated thermochemical and quantum mechanical study of the SiSn diatomic molecule and small Si-Sn clusters.

The silicon-tin chemical bond has been investigated by a study of the SiSn diatomic molecule and a number of new polyatomic Si(x)Sn(y) molecules. These species, formed in the vapor produced from silicon-tin mixtures at high temperature, were experimentally studied by using a Knudsen effusion mass spectrometric technique. The heteronuclear diatomic SiSn, together with the triatomic Si(2)Sn and SiSn(2) and tetratomic Si(3)Sn, Si(2)Sn(2), and SiSn(3) species, were identified in the vapor and studied in the overall temperature range 1474-1944 K. The atomization energy of all the above molecules was determined for the first time (values in kJ mol(-1)): 233.0+/-7.8 (SiSn), 625.6+/-11.6 (Si(2)Sn), 550.2+/-10.7 (SiSn(2)), 1046.1+/-19.9 (Si(3)Sn), 955.2+/-26.8 (Si(2)Sn(2)), and 860.2+/-19.0 (SiSn(3)). In addition, a computational study of the ground and low-lying excited electronic states of the newly identified molecules has been made. These electronic-structure calculations were performed at the DFT-B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels, and allowed the estimation of reliable molecular parameters and hence the thermal functions of the species under study. Computed atomization energies were also derived by taking into account spin-orbit corrections and extrapolation to the complete basis-set limit. A comparison between experimental and theoretical results is presented. Revised values of (716.5+/-16) kJ mol(-1) (Si(3)) and (440+/-20) kJ mol(-1) (Sn(3)) are also proposed for the atomization energies of the Si(3) and Sn(3) molecules.