RESUMEN
Recently, halide perovskites have attracted a substantial attention. Although the focus was mostly on hybrid ones with organic polyatomic cations and with inadequate stability, there is a sizable inorganic halide space that is not well explored and may be more stable than hybrid perovskites. In this work, a robust automated framework is used to calculate the essential properties of the highly stable phases of 168 inorganic halide perovskites. The considered space of ABX3 compounds consists of A = Li, Na, K, Rb, Cs, Tl, B = Be, Mg, Ca, Ge, Sr, Sn, Pb, and X = F, Cl, Br, I. The targeted properties are the structure, the formation energy to assess stability, and the energy gap for potential applicability. The calculations are carried out using the density functional theory (DFT) integrated with the precision library of Standard Solid-State Pseudopotentials (SSSP) for structure relaxation and PseudoDojo for energy gap calculation. Furthermore, we adopted a very sufficient and robust random sampling to identify the highly stable phases. The results illustrated that only 118 of the possible 168 compounds are formidable and have reliable results. The remaining 50 compounds are either not formidable or suffer from computational inconsistencies.
RESUMEN
The fraction of edge- and corner-sharing tetrahedra in liquid ZnCl2 is quantified as a function of temperature using Raman spectroscopy and ab initio molecular dynamic simulations. Two distinct regimes are found in the temperature dependence of the change in these structural units. This behavior is consistent with the existence of a fragile-to-strong transition in liquid ZnCl2 as suggested by calorimetric and viscosity measurements. The structural origin of this transition is rationalized in terms of a constraint counting formalism. It is suggested that the ratio of edge- to corner-sharing tetrahedra controls the configurational entropy and in turn the viscosity of the melt. The temperature dependence of this ratio above the melting point is also found to be qualitatively consistent with neutron diffraction data. The observation of a similar fragile-to-strong transition in the isostructural GeSe2 melt indicates that it may be a common feature of tetrahedral liquids.
RESUMEN
The structure of molten ZnCl2 is investigated using a combination of computer simulation and experimental methods. Ab initio molecular dynamics (AIMD) is used to model the structure of ZnCl2 at 600 K. The structure factors and pair distribution functions derived from AIMD show a good match with those previously measured by neutron diffraction (ND). In addition, Raman spectroscopy is used to investigate the structure of liquid ZnCl2 and identify the relative fractions of constituent structural units. To ascertain the assignment of each Raman mode, a series of ZnCl2 crystalline prototypes are modeled and the corresponding Raman modes are derived by first-principles calculations. Curve fitting of experimental Raman spectra using these mode assignments shows excellent agreement with both AIMD and ND. These results confirm the presence of significant fractions of edge-sharing tetrahedra in liquid ZnCl2. The presence of these structural motifs has significant impact on the fragility of this tetrahedral glass-forming liquid. The assignment of Raman bands present in molten ZnCl2 is revised and discussed in view of these results.