Abstract
Titanite (CaTiSiO5) is a naturally occurring silicate, recently recognised as a potential material for immobilization of nuclear wastes, high-end ceramic lining and optical device development. However, a detailed analysis of its mechanical and electronic properties is still lacking. This article shows that the mechanical properties of titanite is characterized by negative elasticity, an enigmatic mechanical behaviour seen not only in low symmetry (monoclinic), but also in the high symmetry system (cubic). Using first principles calculations a new microscopic basis is developed to explain the negative elasticity of titanite. Rotational bond kinematics, controlled by the valence charge distributions is proposed as a crucial atomic scale mechanism for structural collapse of the lattice under strain, leading to the negative elastic constant. Our bond dynamic model provides a novel approach to characterise materials with unique strain–energy behaviour, which allows us to predict the necessary and sufficient condition for the pressure dependent softening of its shear elastic constants. This article also sheds a new light upon the electronic properties of titanite, accounting for the intraband and interband transition that influence the optical activity. We report the anisotropic optical properties of titanite in the 0–60 eV range for the first time, featuring an attractive optical behaviour of this phase. It is transparent in the visible spectra, but shows 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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