Abstract
Silicates are among the most abundant and important inorganic materials, not only in the Earth’s crust, but also in the interstellar medium in the form of micro/nanoparticles or embedded in the matrices of comets, meteorites, and other asteroidal bodies. Although the crystalline phases of silicates are indeed present in nature, amorphous forms are also highly abundant. Here, we report a theoretical investigation of the structural, dielectric, and vibrational properties of the amorphous bulk for forsterite (Mg2SiO4) as a silicate test case by a combined approach of classical molecular dynamics (MD) simulations for structure evolution and periodic quantum mechanical Density Functional Theory (DFT) calculations for electronic structure analysis. Using classical MD based on an empirical partial charge rigid ionic model within a melt-quenching scheme at different temperatures performed with the GULP 4.0 code, amorphous bulk structures for Mg2SiO4 were generated using the crystalline phase as the initial guess. This has been done for bulk structures with three different unit cell sizes, adopting a super-cell approach; that is, 1 × 1 × 2, 2 × 1 × 2, and 2 × 2 × 2. The radial distribution functions indicated a good degree of amorphization of the structures. Periodic B3LYP-geometry optimizations performed with the CRYSTAL14 code on the generated amorphous systems were used to analyze their structure; to calculate their high-frequency dielectric constants (ε∞); and to simulate their IR, Raman, and reflectance spectra, which were compared with the experimental and theoretical crystalline Mg2SiO4. The most significant changes of the physicochemical properties of the amorphous systems compared to the crystalline ones are presented and discussed (e.g., larger deviations in the bond distances and angles, broadening of the IR bands, etc.), which are consistent with their disordered nature. It is also shown that by increasing the unit cell size, the bulk structures present a larger degree of amorphization.
Highlights
Amorphous materials are disordered solid-state systems that present a lack of long-range order, and from a thermodynamic standpoint, they are less stable than the corresponding crystalline structures
All models were generated by means of classical molecular dynamics simulations using the atomistic Born model of solids [80] implemented in the General Utility Lattice program (GULP) [81]
The first part addresses the generation of an amorphous bulk structure of forsterite (Mg2 SiO4 ) by means of classical molecular dynamics (MD) simulations and subsequent optimization at a quantum chemical level
Summary
Amorphous materials are disordered solid-state systems that present a lack of long-range order, and from a thermodynamic standpoint, they are less stable than the corresponding crystalline structures. The understanding and the prediction of the physicochemical properties of noncrystalline solids rely on the proper description of their structure at the atomic level Experimental techniques with this purpose are X-ray and neutron diffraction, which are directly sensitive to the structure, and vibrational-related spectroscopies such as infrared (IR) and Raman, in which detailed structural information can be achieved. Classical force field MD simulations are computationally less expensive, allowing larger timescales of up to several hundred picoseconds By contrast, these classical MD simulations require accurate and reliable interatomic potentials, since the results are biased by the employed force fields. These classical MD simulations require accurate and reliable interatomic potentials, since the results are biased by the employed force fields Proceeding this way, first-principles calculations onto the generated structural models are subsequently mandatory if the electronic structure and spectroscopic properties are desired
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