140 research outputs found
Effects of pressure on the local atomic structure of CaWO4 and YLiF4: Mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions
The pressure response of the scheelite phase of CaWO4 (YLiF4) and the
occurrence of the pressure induced scheelite-to-wolframite (M-fergusonite)
transition are reviewed and discussed. It is shown that the change of the axial
parameters under compression is related with the different pressure dependence
of the W-O (Li-F) and Ca-O (Y-F) interatomic bonds. Phase transition mechanisms
for both compounds are proposed. Furthermore, a systematic study of the phase
transition in 16 different scheelite ABX4 compounds indicates that the
transition pressure increases as the packing ratio of the anionic BX4 units
around the A cations increases.Comment: 38 pages, 10 figures (Figure 5 corrected), accepted for publication
in Journal of Solid State Chemistr
Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia
The high-temperature cubic-tetragonal phase transition of pure stoichiometric
zirconia is studied by molecular dynamics (MD) simulations and within the
framework of the Landau theory of phase transformations. The interatomic forces
are calculated using an empirical, self-consistent, orthogonal tight-binding
(SC-TB) model, which includes atomic polarizabilities up to the quadrupolar
level. A first set of standard MD calculations shows that, on increasing
temperature, one particular vibrational frequency softens. The temperature
evolution of the free energy surfaces around the phase transition is then
studied with a second set of calculations. These combine the thermodynamic
integration technique with constrained MD simulations. The results seem to
support the thesis of a second-order phase transition but with unusual, very
anharmonic behaviour above the transition temperature
Microscopy in forensic science
This chapter examines the use of electron microscopy, atomic force microscopy and other analytical techniques in forensic investigation and research. These tools can be used to enhance examination of human remains and trace evidence to improve understanding of cause of death, victim identification or post mortem interval.A police-designed scenario is used to highlight trace evidence such as glass, gun shot residue and paint. The validity of forensic techniques is discussed, with reference to international standards, repeatability, and false convictions. Ballistic evidence is used to highlight the complexities in evidence interpretation, including manufacturing variability, environmental effects and likelihood ratios.The use of scanning electron microscopy (SEM), atomic force microscopy (AFM) and other techniques in the development of forensic research is showcased, with particular examples from the field of fingerprints. Examples include improvements in the development of fingermarks from difficult surfaces, interaction of evidence types, and added intelligence from the crime scene, such as forensic timeline or gender of perpetrator
The Mechanism of Gunshot Residue Deposition And Its Probing Characteristics to Reconstruct Shootings
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