Abstract
Titanite (CaTiSiO5) is a naturally occurring silicate phase, recently recognised as a potential material for immobilization of nuclear wastes, high-end ceramic lining and optical device development. The silicate undergoes a pressure-induced structural transition (P21/c to C2/c) at ~3.5 GPa, which we confirm from density function perturbation theory. Using first principles calculations pressure-dependent structural parameters of both the phases are presented. This article features the mechanical property of C2/c titanite characterized by a negative component (C36 = -16.41 GPa) of the elastic constant tensor (Cij), an enigmatic phenomenon seen not only in low symmetry (monoclinic), but also in the high symmetry system (cubic). We propose rotational kinematics of bonds, controlled by the valence charge distributions as a crucial atomic scale mechanism for structural collapse of the lattice under strain resulting in negative C36. This kinematic model allows us to predict the necessary and sufficient condition for the pressure dependent softening of their shear elastic constants (C44 and C55). The present study also sheds a new light upon the electronic properties of titanite, accounting for the intraband and interband transitions that influence the optical activity. Its anisotropic optical properties of titanite evaluated in the range 0–60 eV are marked by an attractive optical behaviour of this phase. It is transparent in the visible spectra, but showing excellent absorption and reflectivity in the UV region. We thus project titanite as an industrially potential UV shield material. Our theoretical estimate yields the highest value of anisotropic refractive index, 2.21 in [001].
Highlights
Titanite is a multifunctional nesosilicate phase [1], well known as a versatile host for rare earth elements (REE) [2,3,4,5], and widely used as U-Pb geochronometer for dating geological events [6]
The phonon dispersion at 0 GPa suggests that the P21/c phase is dynamically stable, but it develops a negative acoustic branch along G–A at 5 GPa, implying that the P21/c phase becomes dynamically unstable at 5 GPa
Our analysis reveals that the negative elastic constant does not affect the mechanical stability of the phase, even at high pressures
Summary
Titanite is a multifunctional nesosilicate phase [1], well known as a versatile host for rare earth elements (REE) [2,3,4,5], and widely used as U-Pb geochronometer for dating geological events [6] This crystalline phase, rich in TiO2 content, is a demanding material owing to its applications for developing strategic matrices for nuclear waste disposal, which is currently a challenging and intriguing field of research [2,4]. The reports on the negative elastic constants are scanty, experimental investigations performed on high symmetry cubic phases confirm the existence of this exceptional mechanical behaviour of solids [28,29,30,31]. Our findings provide a new insight into the applicability of titanite as UV-shield materials
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