09 An Introduction to Solid Solutions and Disordered Systems.pdf - An introduction to solid solutions and disordered systems MSSC2016 Torino Ph.D\u2019Arco

09 An Introduction to Solid Solutions and Disordered Systems.pdf

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An introduction to solid solutions and disordered systems MSSC2016 Torino Ph.D’Arco 1 1 Institut des Sciences de la Terre de Paris (UMR 7193) UPMC - Sorbonne Universit´ es, France September 6, 2016 this talk is dedicated to Roberto Orlando Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Goal of these presentations give general idea to numerically study disordered systems present unfamiliar application of the symmetry introduce the techniques implemented in CRYSTAL show few examples CONFCNT CONFRAND/RUNCONFS Comparing computation and experiment is not straightforward The same ideas or techniques are valid to study defects. Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Non ideal crystalline systems Using space group supposes that : translational symmetry is strictly obeyed! Some crystalline systems cannot be strictly periodic at the ”atomic” scale ! Disordered materials Solid solutions Magnetic spin distribution Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Disordered systems Different atoms occupy positions that are equivalent at some temperature Ex : Gehlenite Ca 2 Al 2 SiO 7 Space group : P ¯ 42 1 m , a=7.6850 ˚ A, c=5.0636 ˚ A atom x y z occ Ca .3389 .1611 .5104 AlT1 0 0 0 AlT2 .1434 .3556 .9540 .50 SiT2 .1434 .3556 .9540 .50 O1 .5 0 .1765 O2 .1427 .3573 .2835 O3 .0876 .1678 .8078 XRD cannot resolve atomic distribution over different positions (sites) Atoms are very similar (Si, Al) Statistical disorder Dynamical disorder Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Textbook example of Disordered system Normal to inverse spinel A IV B VI 2 O 4 : AB 2 O 4 B (A,B) O 4 Symmetry remains cubic statistical occupation of the octahedral site! In general disordered over A and B sites : A 1 - x B x A x B 2 - x O 4 . Non-converging disorder! Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Solid solutions Crystalline materials displaying large chemical variations without apparent discontinuity. Isostructural materials : Alloys Simple oxides : Al 2 O 3 -Cr 2 O 3 Complex silicates : NaAlSi 3 O 8 – KAlSi 3 O 8 Chemical variations are described on the basis of end-members that are considered as pure compounds. End-members may be stable or not : ZnS (Zincblende) -FeS (NiAs). The solution can be either continuous or partial between end-members. Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Macroscopic properties depend on composition/disorder Melilite Gehlenite Ackermanite Ca 2 Al 2 SiO 7 Ca 2 MgSi 2 O7 300 301 302 303 304 305 306 0 20 40 60 80 100 Volume/formula (A 3 ) Mol. per cent Ca 2 MgSi 2 O 7 Ge Ak 11 12 13 14 15 0 20 40 60 80 100 Δ H sol /formula (kcal) Mol. per cent Ca 2 MgSi 2 O 7 Ge Ak Charlu et al, 1981, geochim. Cosmochim. Acta, 45, p1609-1617 Ph.D’Arco 1 An introduction to solid solutions and disordered systems MSS
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Macroscopic properties depend on composition/order ZnS-FeS system Very large exces volume.
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