ALBERT

Description: The low-frequency mechanical properties of pure and Ca-doped lead orthophosphate, (Pb1–xCax)3(PO4)2, have been studied using simultaneous dynamical mechanical analysis, X-ray diffraction (XRD), and optical video microscopy in the vicinity of the first-order ferroelastic phase transition. Both samples show mechanical softening at T 〉 Tc, which is attributed to the presence of dynamic short-range order and microdomains. Stress-induced nucleation of the low-temperature ferroelastic phase within the hightemperature paraelastic phase was observed directly via optical microscopy at T ≈ Tc. Phase coexistence is associated with rapid mechanical softening and a peak in attenuation, P1, that varies systematically with heating rate and measuring frequency. A second peak, P2, occurs ≈3–5°C below Tc, accompanied by a rapid drop in the rate of mechanical softening. This is attributed to the change in mode of anelastic response from the displacement of the paraelastic/ferroelastic phase interface to the displacement of domain walls within the ferroelastic phase. Both the advancement/retraction of needles (W walls) and wall translation/rotation (W′ walls) modes of anelastic response were identified by optical microscopy and XRD. A third peak, P3, occurring ≈ 15°C below Tc, is attributed to the freezing-out of local flip disorder within the coarse ferroelastic domains. A fourth peak, P4, occurs at a temperature determined by the amplitude of the dynamic force. This peak is attributed to the crossover between the saturation (high temperature) and the superelastic(low temperature) regimes. Both samples display large superelastic softening due to domain wall sliding in the ferroelastic phase. Softening factors of 20 and 5 are observed in the pure and doped samples, respectively, suggesting that there is a significant increase in the intrinsic elastic constants (and hence the restoring force on a displaced domain wall) with increasing Ca content. No evidence for domain freezing was observed down to −150°C in either sample, although a pronounced peak in attenuation, P5, at T ≈ −100°C is tentatively attributed to the interaction between domain walls and lattice defects.Both samples show similar high values of attenuation within the domain-wall sliding regime. It is concluded that the magnitude of attenuation for ferroelastic materials in this regime is determined by the intrinsic energy dissipation caused by the wall-phonon interaction, and not by the presence of lattice defects. This will have a large impact on attempts to predict the effect of domain walls on seismic properties of mantle minerals at high temperature and pressure.

Description: Phase relations along the join bornite (Cu5FeS4)-digenite (Cu8.52Fe0.12S4.88) have been redefined using a combination of in situ high-resolution neutron diffraction and differential scanning calorimetry (DSC). Time-of-flight neutron diffraction patterns were collected on a synthetic sample of bn90 at 16 temperatures between 35 and 350°C. This data is compared with data from a natural end-member bornite sample obtained in an earlier study under identical conditions. Phase relations along the bornite-digenite join are inferred from the temperature evolution of the lattice parameters and the intensity of subcell and supercell reflections of coexisting phases.The DSC scans over the temperature range 50–300°C were performed on a natural digenite sample and samples synthesized at 5 mol.% intervals along the join Cu5FeS4-Cu9S5. The thermal anomalies are correlated with structural phase transitions in componen phases and the solvus temperature for each bulk composition. A phase diagram topology is defined, which was consistent with both diffraction and calorimetric data, but in marked contrast to previous diagrams, shows a consolute point at X = Cu5FeS4 and T = 265°C. This temperature corresponds to that of the tricritical intermediate-high transition in bornite. Isothermal annealing experiments carried out on synthetic starting materials for up to 7 months showed coarsening behaviour consistent with the revised phase diagram topology.

Description: The systematics of cation ordering in binary spinel solid solutions have been investigated using an interatomic potential model combined with Monte Carlo simulations. The formalism to describe a system containing three cation species ordering over two non-equivalent sub-lattices is developed and the method applied to the MgAl2O4-FeAl2O4 binary solid solution. Our results compare favourably with experimental measurements of site-occupancy data, although the experiments display a slightly larger degree of non-ideality than the simulations. A possible kinetic origin of the non-ideal behaviour was examined by performing simulations in which only exchange of Mg and Fe2+ between tetrahedral and octahedral sites was permitted below the Al-blocking temperature of 1160 K. This approach improves the agreement with the experimental site occupancies, and suggests that the blocking temperature for moving Mg and Fe2+ between tetrahedral and octahedral sites is significantly lower than for moving Al