Solid State Chemistry Chem 331

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Solid State ChemistryChem 331

• Dr. Bailey
• Stratton 219
• x3286
• cbailey_at_wells.edu

Koloman Moser
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Wall decorations in the Sala del Reposo, Alhambra.
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Mauritis C. Escher
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Introduction to the Solid State

• There are 20,000,000 known chemical substances.
• 95 are molecular (predominantly containing C).
• 4 are inorganic molecular.
• 1 are non-molecular extended structures (e.g.
  solid salts and most elements).

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Single Element Packing
Square Lattice
first layer
second layer (directly above first)
Simple Cubic Lattice Packing used by e.g. Po.
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Single Element Packing
Square Lattice
first layer
second layer (sitting in indentations)
BCC Lattice Packing used by e.g. Li, K, V, Fe,
W, etc.
third layer (directly above first)
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Single Element Packing
first layer
Closest Packed Lattice
A A A
second layer (over any indentation)
B B B

• Two types of indentations
• Directly over first layer.
• Over first layer indentation.
• So, two different places to start third layer!

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Single Element Packing
first layer
Closest Packed Lattice
A A A
second layer (over any indentation)

• Two types of indentations
• Directly over first layer.
• Over first layer indentation.
• So, two different places to start third layer!

IF directly over A, get ABAB packing, also known
as hexagonal closest packed (hcp). Used by Mg,
Ca, Co, Zn, etc.
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Single Element Packing
first layer
Closest Packed Lattice
second layer (over any indentation)

• Two types of indentations
• Directly over first layer.
• Over first layer indentation.
• So, two different places to start third layer!

IF third layer directly over B, get ABCABC
packing, also known as face centered cubic
(fcc). Used by Al, Cu, Ni, Ag, Au etc.
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Lattice Packing

• Elemental Cu and Ni each uses fcc packing and
  both have very similar lattice parameters (e.g.
  internuclear distances).
• If we heat the two elements to melting and then
  mix together and cool slowly, the fcc packing is
  retained, but with a random placement of the two
  elements.
• Known as a solid solution alloy.

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Lattice Packing

• Elemental Cu and Au each uses fcc packing but
  have very different lattice parameters (Au gtgt
  Cu).
• Upon reaction (melt and cool) yields a
  specifically ordered arrangement an
  intermetallic compound, which may not conform to
  oxidation state rules.

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Binary Compounds (MX)

• Which elements do we need to be concerned about?
• ignore noble gases no known extended structures.
• ignore radioactive elements.
• this leaves about 80 elements of possible
  interest.
• mathematically, this results in 3,160 possible
  binary elemental combinations (not taking into
  account various stoichiometries, AB, AB2, A2.3B3,
  etcetera).
• 90 of known binary compounds have simple
  stoichiometries MX, MX2, MX3, M3X5, etcetera.
• For MX there are 20 common structure types (well
  look at 3).
• For MX2 there are 26 common structure types
  (well look at 2).
• Each of these structural types can be thought of
  as starting from single element packing lattices.

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Most Common MX Structures
NaCl structure Na in fcc lattice Cl- in Oh
holes
CsCl structure Cl- in simple cubic lattice Cs
in cubic hole
Zinc blende (ZnS) structure S-2 in fcc
lattice Zn2 in alternating Td holes
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Most Common MX2 Structures
Fluorite (CaF2) structure Ca2 in fcc
lattice F- in all Td holes
Rutile (TiO2) structure Ti2 in body centered
cubic lattice Oxygens in lower symmetry array.
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Ternary Compounds (ABC)

• 82,160 potential combinations 20,000 known.
• 700 structural types known (so far and growing
  fast) very few examples of each type.
• Synthesis grind together and heat Li2O and MoO3,
  for example. Reacts before melting.
• Forms one of three compounds, depending on ratio
  on mixing
• 1 Li2O 1 MoO3 ? Li2MoO4 100yield
• 1 Li2O 4 MoO3 ? Li2Mo4O13 100yield
• 2 Li2O 5 MoO3 ? Li4Mo5O17 100yield
• If react with other ratios, get mixtures of these
  3 plus startting material.
• If use Na2O instead of Li2O, get entirely
  different compounds.

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Pseudoternary Compounds.

• Both sodium chloride and silver chloride utilize
  NaCl structure.
• If react (melt and re-cool) non-stoichiometric
  amounts, get solid solution of NaCl structure
  type, but with random occupation of Na/Ag sites.
• Called pseudoternary because it contains 3 types
  of elements, but still adopts binary-type
  structure.

(1-x) NaCl xAgCl ? Na1-xAgxCl
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Future? Why Do We Care?

• Consider superconductors (a metal that, when
  cooled to a low enough temperature, Tc, will
  carry a charge with no resistance).
• best single element superconductor is Nb, Tc 9
  K.
• best binary superconductor is Nb3Ge, Tc 20 K.
• best ternary superconductor is La2CuO4, Tc 40
  K.
• best quarternary superconductor is Ba2YCu3O4, Tc
  92 K above N2(l).
• best superconductor is Tl2Ba2Ga2Cu3O10, Tc 135
  K.
• to extrapolate out to room temperature, would
  need 8 elements, which means 2.90 x 1010
  possible combinations before stoichiometry!
• what are the most complicated compounds in
  nature (max cations in different
  crystallographic environments)?
• Asbecaite Cu3TiAs6Be2Si2O30
• Mordite LaSrNa3ZnSi6O17

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Synthesis of Non-Molecular Solids

• Typically start with powdered reactants, mix
  together, press together, heat, and then let cool
  (aka shake and bake).
• See West Ch 9 for specifics of many methods.
• By convention, high temp 800oC low temp
  200-600oC.
• Dont want material to react with container, so
  common to use fused quartz (up to 1200oC), but do
  use other materials.
• Often start with oxides that are stable in air.
• 3CuO 2BaO2 Y(OH)3
• mix well and press into a pellet heat in
  aluminum oxide container 920oC for 24 hours.
• yields YBa2Cu3O6, which when reacted with 3/2H2O
  5/2O2 and annealed below 500oC in O2 produces
  YBa2Cu3O4 (superconductor).

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Nucleation and Diffusion

• These solid state reactions occur in two steps
  first nucleation, where product forms within a
  few nm of where the reactants contact one
  another, then product growth through diffusion.
• The reason the material is pressed is to get
  points of contact (on the molecular scale) as the
  reaction only occurs when contact occurs.
• Very little is known about nucleation, but
  diffusion is reasonably well understood.
• Different atoms diffuse at different rates, but
  typically D 10-10 to 10-12 cm2/sec at 2/3Tm (in
  K).
• Therefore, it would take 320 years to move 1cm
  in a solid. But, they ARE moving! Atomic scale on
  order of ?.
• Atoms can also diffuse (migrate) in crystalline
  solids because of defects (more later).

d v D t
time
diffusion distance
diffusion constant
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Melting Points

• Compounds may melt congruently (Tm) , with a
  single melting point. Changes from solid to
  liquid of same composition.
• e.g. H2O(s) ? H2O(l) all elements melt
  congruently.
• Some compounds melt incongruently (Tin),
  decomposing on heating to components with
  different composition.
• e.g. solid ? solid and liquid of different
  composition.
• e.g. YBa2Cu3O4 melts incongruently.