Density Functional Theory on potassium silicate crystals. Application to mechanical properties and Vickers microhardness calculus in glasses

Linares Fuentes T.C.; Garrido Schaeffer C.J.A.; More W.; Cornejo N.F.; Tamayo A.; Rubio J. · Boletin de la Sociedad Espanola de Ceramica y Vidrio (2022) · DOI: 10.1016/j.bsecv.2020.08.001

Resumen

In this work, a study is presented in which the density functional theory (DFT) is applied, using the CASTEP software, for crystals of silicate (K2SiO3, K2Si2O5 and K2Si4O9) and of silicon oxide (quartz). The Density of State (DOS) of these silicates shows that the two types of oxygen atoms, bridging (BO) and non-bridging (NBO), have a significantly different electron distribution in the 2p and 2s orbitals, due to a different and close chemical environment. This leads to a displacement to lower energies (around 2 eV) of the s and p valence bands of the partial density state (PDOS) of the NBO regarding the PDOS of the BO. In the crystal with the lowest K2O concentration (K2Si4O9), the covalent Si-O bonds are increased due to the electronic charge transfer of the 4s orbital from the potassium atom to the NBO, and from this NBO to the silicon atom and, finally, to all the atoms of the tetrahedron, which allows to improve the bond strength of the whole system. This fact results in the final material having a higher mechanical resistance. On the other hand, in K2Si2O5 (which has one NBO per tetrahedron) the Si-O bond is weaker compared to K2Si4O9, a tendency that is highlighted in K2SiO3 (which has two NBOs per tetrahedron). This same trend follows the theoretically calculated Vickers hardness. This study has been applied to experimentally obtained potassium silicate glasses and several mechanical properties have been compared with the theoretical properties of the respective crystals of similar chemical composition. The NBO of both crystals and glasses have been quantified from the Qn structural units. It has been observed that the glass mechanical properties cannot be only explained by the contributions of the free tetrahedral joined by the vertice and characterized by their BO and NBO as a function of the concentration in K2O, that is by Qn. This is because CASTEP's DFT theory for crystalline systems considers every order of interaction. Therefore, it is necessary to use intermediate-range structures called rigid structural units (RSUs). The contribution of free tetrahedral has been calculated using the Vickers hardness of the crystal and a screening factor for the loss of medium and long range interactions. This work explains the behaviour of the Vickers hardness and Young's modulus of SiO2-K2O glasses in a wide range of molar compositions.

Instituciones: Universidad Nacional Mayor de San Marcos