Chapter 6 - Structures and energetics of metallic and ionic solids Flashcards
In close-packing of spheres, how much of the space is occupied by spheres?
74%
Hexagonal close-packing arrangement
ABABAB
Cubic close-packing arrangement
ABCABC
In hexagonal and cubic close packing, each sphere is surrounded by _____ nearest neighbors.
12 (coordination number of 12)
Unit cell
The smallest repeating unit of a structure which carries all the information needed to construct an infinite lattice.
Cubic close-packing is also called
face-centered cubic packing
Simple cubic coordination number
6
Body centered cubic coordination number
8
The packing of spheres model is applicable to
group 18 elements, metals, and H2 and F2
Noble gases (except He and Rn) have _____ crystalline structures.
ccp
Most metals crystallize with ___, ___, or ___ lattices
ccp, hcp, or bcc
Bcc packing efficiency
68%
Polymorph
The existence of a solid material in more than one form or crystal structure.
Metallic radius
Half of the distance between the nearest-neighbor atoms in a solid state metal lattice, and is dependent upon coordination number.
Alloy
An intimate mixture, or in some cases, a compound of two or more metals, or non-metals.
In a substitutional alloy,
atoms of the solute occupy sites in the lattice of the solvent metal (both atoms must be similar in size and tolerate the same coordination environment).
In interstitial alloys,
an atom of radius 0.41 times that of the atoms in the close-packed array can occupy an octahedral whole, while significantly smaller atoms may be accommodated in tetrahedral holes.
In intermetallic alloys,
when metal mixtures solidify, the alloy formed may possess a definite structure type that is different from those of the pure metals.
The electrical conductivity of a metal decreases with
an increase in temperature.
The electrical conductivity of a semiconductor decreases with
a decrease in temperature.
Electrical resistance increases as temperature increases because
thermal vibrations of the nuclei increase and don’t allow a straight path for the electrons to flow through.
A band is
a group of MOs, the energy differences between which are so small that the system behaves as if a continuous, non-quantized variation of energy within the band is possible.
Valence band
the band of electron orbitals that electrons can jump out of, moving into the conduction band when excited.
Conduction band
the band of electron orbitals that electrons can jump up into from the valence band when excited.
Conductors have ________ between their valence and conducting bands.
overlap
Insulators have _______ between their valence and conducting bands.
a gap
A band gap occurs when
there is a significant energy difference between two bands.
Fermi level
The highest energy level that an electron can occupy at the absolute zero temperature.
Fermions
Particles with half-integer spin such as electrons.
Intrinsic semiconductor
A material that behaves as a semiconductor without the addition of dopants.
A charge carrier in a semiconductor is either
a positive hole or an electron that is able to conduct electricity.
Dopant
An impurity introduced into a semiconductor in minute amounts to enhance its electrical conductivity.
P-type semiconductor
An electron deficient site is introduced (like exchanging group 13 for group 14). This creates a positive hole in which it can be thought that the positive hole moves in the opposite direction of the current.
N-type semiconductor
An electron rich site is introduced (like exchanging group 15 for group 14). These extra electrons are in a donor level below the conduction band and can populate the conduction band and move freely.
In salts of formula MX, the coordination numbers of M and X must be
equal.
In salts of formula MX2, the coordination number of X must be
half that of M.
Rock salt structure
MX
Coordination number = 6
Fluorite structure
MX2
Coordination number = 8 (M) and 4 (X)
Zinc blende structure
MX
Coordination number = 4
Cesium chloride structure
MX
Coordination number = 8
Antifluorite structure
M2X
Coordination number = 4 (M) and 8 (X)
Rutile structure
MX2
Coordination number = 6 (M) and 3 (X)
Lattice energy
The change in internal energy that accompanies the formation of one mole of the solid from its constituent gas-phase ions at 0 K.
The Madelung constant is used in determining
the electrostatic potential of a single ion in a crystal by approximating the ions by point charges.
Born forces
Electron-electron and nucleus-nucleus repulsions due to the finite size of ions in an ionic lattice.
Madelung constants for MX2 structures are approximately ____ % higher than those for MX lattices
50
A species disproportionates if
it undergoes simultaneous oxidation and reduction.
Intrinsic defects occur in
lattices of pure compounds.
Extrinsic defects result from
the addition of dopants.
A Schottky defect arises from
vacant lattice sites.
Frenkel defect
In ionic lattices in which there is a significant difference in size between the cation and anion, the smaller ion may occupy a site that is vacant in the ideal lattice.
A tetrahedral hole can accommodate a sphere with a radius
<= 0.23 times that of the close-packed spheres.
An octahedral hole can accommodate a sphere with a radius
<= 0.41 times that of the close-packed spheres.
Metals with coordination numbers less than ____ are among those that are the most volatile.
8
