A. Ciccioli

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 thermodynamics of MgB2 and MgB4

The vaporization behavior of MgB2 and MgB4 under thermodynamic conditions has been studied by the Knudsen effusion-mass spectrometry technique. In the temperature range explored (883–1154 K), magnesium borides are observed to decompose by loss of gaseous Mg only. The equilibrium pressures of Mg(g) have been measured during high-temperature decompositions involving MgB2/MgB4 and MgB4/MgB7 two-phase mixtures and the corresponding standard reaction enthalpies were determined. The decomposition temperatures for MgB2 and MgB4 were also inferred by the relevant Van’t Hoff plots.

Thermodynamic stabilities of intermediate phases in the Ca–Si system

Abstract Vaporization thermodynamics in the binary system calcium–silicon has been studied by Knudsen effusion-mass spectrometry and vacuum microbalance techniques. The equilibrium partial pressure of Ca(g) over the two-phase regions in the composition range 20–75at.% Si has been measured and the standard enthalpy changes for the appropriate vaporization reactions were determined from the temperature dependence of the measured vapor pressures. The standard reaction enthalpy changes were also evaluated by the third-law method using the pressure data in conjunction with estimated Gibbs energy functions. Standard enthalpies of formation of the calcium silicides were derived from the standard reaction enthalpy values at room temperature. The results obtained for ΔfH°298 were the following: Ca2Si=−56.1±3.1, Ca5Si3=−55.3±3.5, CaSi=−49.6±2.2, Ca3Si4=−40.6±1.5, Ca14Si19=−44.4±2.3, CaSi2=−37.8±1.6 all in kJ/mol atoms. The results for Ca2Si, CaSi and CaSi2 may be compared with previous measurements, all other results are first determinations.

A mass spectrometric and density functional study of the intermetallic molecules AuBe, AuMg, and AuCa

The intermetallic molecules AuBe and AuCa were identified by means of the Knudsen-Effusion Mass Spectrometry technique in the high-temperature vapors produced by vaporizing Au-Be-Ca alloys of proper composition. The gaseous equilibria AuBe(g)+Au(g)=Au(2)(g)+Be(g) and AuCa(g)+Au(g)=Au(2)(g)+Ca(g) were studied in the temperature ranges 1720-1841 K and 1669-1841 K, respectively, by monitoring the partial pressures of all the species involved. The equilibrium data were analyzed by the third-law method, obtaining for the first time the dissociation energy D(0) ( composite function) of the two intermetallic species: D(0) ( composite function)(AuBe)=234.0+/-4.0 kJ/mol; D(0) ( composite function)(AuCa)=246.7+/-4.0 kJ/mol. These values are significantly higher than the recently published D(0) ( composite function) of the species AuMg (175.4+/-2.7 kJ/mol). Furthermore, the ionization energies (IE) of AuBe, AuMg, and AuCa were obtained by measuring the electron impact ionization efficiency curves, IE(AuBe)=7.5+/-0.3 eV, IE(AuMg)=6.7+/-0.3 eV, and IE(AuCa)=5.5+/-0.3 eV. Theoretical calculations were also carried out for these species by density functional theory methods (PW91 and BP86) used in conjunction with Stuttgart relativistic effective core potentials. Both functionals were found to perform very well in reproducing experimental D(0) ( composite function), IE, and molecular parameters.

Thermodynamics of the YAl–YAl2 system

Abstract The emf method using solid-state galvanic cells based on a CaF 2 single crystal as electrolyte has been used to investigate the thermodynamics of the intermetallic couple YAl–YAl 2 . These represent two out of the five intermetallic compounds shown in the Y–Al phase diagram. Two kinds of cells were studied: − Pt/Y,YF 3 /CaF 2 s.c./YAl,YAl 2 ,YF 3 /Pt + and − Pt/YAl,YAl 2 ,CaAlF 5 ,CaF 2 /CaF 2 s.c./Fe Al, a + ,CaAlF 5 ;CaF 2 /Pt + The cell reactions expected are, respectively, 2YAl (s)=YAl 2 (s)+Y (s) and YAl (s)+Al (l, s)=YAl 2 (s). The standard thermodynamic quantities Δ G °, Δ H ° and Δ S ° have been measured as a function of temperature for both the reactions. The Δ f H ° 298 of YAl has also been derived by selecting from the literature the best value of Δ f H ° 298 for YAl 2 . The value to assign to the formation of YAl have been found equal to: Δ f H ° 298 =−107±6 kJ/mol. Moreover, Δ f S ° 298 =2±5 J/K.mol and Δ f G° T =(−100±5)+(1.8±0.5)10 −2 T kJ/mol have been estimated in the temperature range from 792 to 1007 K.

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.

Thermodynamic study of intermetallic phases in the HfAl system

Abstract The vaporization thermodynamics of the aluminum-rich portion of the HfAl system have been investigated by Knudsen cell-mass spectrometry in the temperature range 1280–1680 K. The aluminum vapor pressures were measured in the two-phase regions in the composition range 50%–75% at. Al, and the enthalpy changes of the decomposition reactions were determined by second-law and third-law methods. Hence, the enthalpies of formation of the intermetallic compounds were derived: HfAl3, −44.7 kJ g−1 atom−1; Hf2Al3, −40.8 kJ g−1 atom−1. The results of the vaporization experiments which yielded a residue not assignable to a definite phase were tentatively submitted to thermodynamic analysis, by introducing the formation of HfAl or Hf5Al4, and their enthalpies of formation were derived.

Thermodynamics of the Ni-Yb system

The experimental description of the Ni-rare earth systems is incomplete, with many gaps and uncertainties remaining in both the phase diagram and the thermochemical information. No thermodynamic information is available for the Ni-Yb system, except for an estimation of the enthalpy of formation of the Ni5Yb compound. In this work, we investigated the thermochemistry of the Ni-Yb intermediate phases by means of tensimetric measurements and, on a few compositions, calorimetric measurements. The equilibrium vapor pressures over two solidphase regions were measured by Knudsen effusion-mass spectrometry and Knudsen effusion-weight loss, and the data were analyzed by the second- and the third-law methods in order to derive the enthalpy changes for the HT decomposition reactions. Furthermore, the enthalpy of formation of the NiYb compound was preliminarily determined by direct reaction calorimetry (DRC). The heats of formation of the five intermediate phases was finally obtained by a combined analysis of all the collected data. The values are as follows: Ni17Yb2, −13.9±0.3 kJ/mol atoms; Ni5Yb, −20.9±4.4 kJ/mol atoms; Ni3Yb, −26.0±4.8 kJ/mol atoms; Ni2Yb, −32.0±4.6 kJ/mol atoms; and NiYb, −28.0 ± 2 kJ/mol atoms. The results are compared with those estimated by the Miedema’s model.

Thermochemistry of ytterbium silicides

Abstract The results of the investigation of the high temperature decomposition reactions in vacuum under equilibrium conditions of ytterbium silicides in the whole composition range are reported. By means of the Knudsen Effusion–Mass Spectrometry (KE–MS) and the Knudsen Effusion–Weight Loss (KE–WL) techniques, the Yb(g) vapour pressures in equilibrium over the various high temperature and low temperature biphasic regions were measured in the temperature range 781–1395 K and the reaction enthalpies for the respective decompositions were derived. From this set of experimental data we derived for the first time the heats of formation of all the six known Si–Yb intermediate phases. The following values ΔfH°298 are recommended: Si3Yb5=−48.3±3.6, Si4Yb5=−53.2±4.6, SiYb=−51.1±5.1, Si4Yb3=−48.0±3.1, Si5Yb3=−41.3±2.6, Si1.74Yb=−37.4±0.9, all in kJ/mol atoms.

A study on the nature of the thermal decomposition of methylammonium lead iodide perovskite, CH3NH3PbI3: an attempt to rationalise contradictory experimental results

The nature of the gas phase product released during the thermal decomposition of CH3NH3PbI3 (methylammonium lead iodide) to PbI2 (lead diiodide) under vacuum is discussed on the basis of thermodynamic predictions, recently published experimental results, and new experiments presented here. From the limited data currently available, the nature of the main decomposition path is not clear because, both, the process releasing HI(g) + CH3NH2(g) (1) and that leading to NH3(g) + CH3I(g) (2) were observed under different conditions. Our thermodynamic analysis showed that process (2) is largely favoured for all the CH3NH3PbX3 (X = Cl, Br, I) compounds. However, Knudsen effusion mass spectrometry experiments (temperature range 140–240 °C) showed that HI(g) and CH3NH2(g) were the predominant species in the vapor, with process (2) occurring to a much smaller extent than suggested by the thermodynamic driving force, thus being of minor importance under effusion conditions. We also found that this process was comparatively enhanced by high temperatures and low effusion rates (high impedance orifice). Our experimental evidence suggested that the thermodynamically favoured process (2) was affected by a significant kinetic hindrance. Overall, the prevailing decomposition path is likely to markedly depend on the actual operative conditions.