Elastic softening of metamict titanite CaTiSiO5: Radiation damage and annealing

Abstract

We have measured the elastic response of radiation-damaged titanite, CaTiSiO 5 , as a function of thermal annealing. We estimate the bulk modulus of the damaged samples (∼24% amorphous) to be 85 GPa, which is much softer than for undamaged crystalline titanite [131.4 GPa;Angel et al. (1999)]. Conversely, the lowest shear modulus of the radiation-damaged material is 52-58 GPa, which is harder that of the undamaged titanite, 46-52 GPa. The bulk and shear moduli of the radiation-damaged materials are close to those of thermal titanite glass, B glass ≈ 75 GPa and G glass ≈ 47 GPa, and are much smaller than expected based on other radiation-damaged materials such as zircon (ZrSiO 4 ). Surprisingly, annealing of the damaged titanite in the range 600 < T < 1000 K leads to additional massive softening of the shear moduli. During annealing the shear modulus of titanite sample 1 softened from 58 to 29 GPa, and sample 2 softened from 52 to 19 GPa. The temperature range for the softening coincides with that found for crystallization of the amorphous regions, as measured previously by diffraction and spectroscopic methods. In contrast to the huge softening of the ultrasonically measured shear modulus, the calorimetrically measured Debye temperature θ D increases by ∼5%, suggesting a small intrinsic hardening of the acoustic shear modes. Additional heating to 1473 K leads, in one titanite sample, to a steep increase of the shear modulus to values much larger than that of the initial, radiation-damaged material. Theoretical models are discussed to rationalize the massive softening due to both radiation damage and subsequent anneal.

Ferroelectric precursor behavior in PbSc0.5Ta0.5O3detected by fi

“…Coupling between the ferroelectric order parameter P and the strains e P and e flip includes linear-quadratic terms which can greatly influence the stability of the individual phases. 67 If there was any possibility of bilinear coupling between a symmetry-breaking shear strain and P , precursor elastic softening, which we observe experimentally, would reflect a pseudoproper transition mechanism 53 in addition to other softening mechanisms. However, as discussed below, this does not appear to be the case.…”

Partially amorphous (metamict) titanite: a silicate mineral in a natural radiation induced non-equilibrium phase

“…self-destruction of the long-range-ordered crystal lattice introducing an almost amorphous state referred to as metamict [16,34,35]. During the metamictization process the material structure becomes partially destroyed leading to the coexistence of defect-rich crystalline and amorphous regions [16,35,36].…”

Interface phenomena

“…The mineral titanite (CaTiSiO 5 ) is an attractive object of study because of its radiation-induced structural damage resulting from the radioactive decay of incorporated radionucleides over geological times [6]. After long exposure to radioactive irradiation the material becomes metamict, i.e.…”

“…2). Between the damaged and undamaged material occur complex interfaces with mismatch features and features of porous media [6].…”

“…dielectric studies [4]. Also between amorpheous and crystalline material like in metamict minerals [6] occur particular interfaces as well as in biomaterials like mollusc shells [7].…”