Temperature-induced P21/c to C2/c phase transition in partially amorphous (METAMICT) titanite revealed by Raman spectroscopy

In situ temperature-dependent Raman spectroscopic measurements of two partially amorphous (metamict) titanite samples with accumulated radiation doses ~1.2 × 1018 α-events/g (E2335) and ~2.2 ×1018 α-events/g (M28696) and amorphous fractions of 0.24 and 0.5, respectively, provide evidence for a thermally induced transformation analogous to the phase transition P21/c ↔ C2/c, which is characteristic of endmember titanite (CaTiSiO5). Quantitative analysis of the temperature evolution of the wavenumbers and widths of Raman-active vibrations in both partially amorphous titanites reveals an anomaly near 500 K which is consistent with the P21/c ↔ C2/c phase-transition temperature of titanite close to its endmember composition. The observed spectral changes are not due to structural recovery of the radiation-damaged structure because synchrotron X-ray diffraction experiments of M28696 titanite show that relevant recrystallization occurs at annealing temperatures above 600 K. The structural transformation near 500 K is observed by Raman spectroscopy even in heavily radiation-damaged and chemically inhomogeneous titanite (M28696). The microstructure of the samples consists of coexisting crystalline and amorphous domains, with the P21/c to C2/c phase-transition occurring in the latter

Chemical mixing and hard mode spectroscopy in ferroelastic lead phosphate arsenate: local symmetry splitting and multiscaling behaviour

The phase transition in ferroelastic Pb 3 ( PO 4 ) 2 –Pb 3 ( AsO 4 ) 2 mixed crystals shows typical multiscaling behaviour with two relevant length and timescales. One length scale is macroscopic and shows uniform, weakly first-order phase transitions between a rhombohedral paraphase (R ¯ 3m) and a monoclinic ferroelastic phase (C2 / c). The second length scale is on the level of tetrahedral complexes which display monoclinic distortions at temperatures well above the macroscopic transition point. For instance, in Pb 3 ( P 0 . 43 As 0 . 57 O 4 ) 2 the AsO 4 polyhedra show static deformation up to ∼ 60 K above the deformation of the PO 4 tetrahedra. The two timescales are either short compared with the time of observation, namely the dynamic reorientation of the PO 4 tetrahedral distortion, or very long. The long timescale refers then to the (quasi)static distortion of the AsO 4 tetrahedra which persists at T > T c . These distortions appear to be uncorrelated or only weakly correlated and their random field leads to an order/disorder aspect of the phase transition which remains, on a phonon timescale, essentially displacive in character

Fracture toughness of radiation-damaged zircon studied by nanoindentation pillar-splitting

Nanoindentation micro-pillar splitting was employed to measure the fracture toughness (KC) of growth-zones in radiation-damaged zircon with varying degrees of disorder (∼45%-80% amorphous fraction). The radiation-induced amorphization is caused by α-decay events from incorporated U and Th (∼0.22-0.43 wt. % UO2 and ∼0.02-0.08 wt. % ThO2). KC has been found to increase with the increase in the amorphous fraction (∼2.39 to 3.15 MPa*m1/2). There is a good correlation with the modulus/hardness (E/H) ratio evolution over the investigated zones. As zircon has been proposed as a nuclear waste form for the incorporation and disposal of Pu, a deeper knowledge of KC as a function of radiation damage is important, as radiation-induced cracking provides diffusion paths for the release of incorporated actinides. Zoned zircon provides a model for the development of multilayer coatings and complex ceramics that can be designed to be resistant to crack propagation

Recrystallization of Metamict Titanite

Allanite is a common accessory mineral in igneous rocks. Allanite becomes metamict over geological time-scales as a result of the alpha-decay of radioactive elements in the crystal structure. This study focuses on the recrystallization of metamict allanite from Savvushka, Russia. The structural recovery produced by annealing was investigated by X-ray powder diffraction, single-crystal synchrotron X-ray diffraction and infrared spectroscopy. A kinetic analysis is presented that shows that the recrystallization process proceeds by at least two different mechanisms