Crystalline State
Basic Structural Chemistry
Structure Types
Lattice Energy
Pauling Rules
1
Degree of Crystallinity
Crystalline – 3D long range order
Single-crystalline
Polycrystalline - many crystallites of different sizes and
orientations (random, oriented)
Paracrystalline - short and medium range order,
lacking long range order
Amorphous – no order, random
2
 Single Crystalline
 Polycrystalline
 Semicrystalline
 Amorphous
Degree of Crystallinity
Grain boundaries
3
A crystalline solid: HRTEM
image of strontium titanate.
Brighter atoms are Sr and
darker are Ti.
Degree of Crystallinity
A TEM image of amorphous
interlayer at the Ti/(001)Si
interface in an as-deposited
sample.
4
The building blocks of these two are
identical, but different crystal faces
are developed
Crystal Structure
Cleaving a crystal of rocksalt
Conchoidal fracture in chalcedony
5
Single crystal X-ray diffraction
structure analysis
a four circle X-ray diffractometer 6
X-ray structure analysis with single crystals
7
CAD4 (Kappa Axis Diffractometer)
8
IPDS (Imaging Plate Diffraction System)
9
Crystals
• Crystal consist of a periodic arrangement of structural motifs =
building blocks
• Building block is called a basis: an atom, a molecule, or a group
of atoms or molecules
• Such a periodic arrangement must have translational symmetry
such that if you move a building block by a distance:
then it falls on another identical building block with the same
orientation.
• If we remove the building blocks and replace them with points,
then we have a point lattice or Bravais lattice.
10
Planar Lattice 2D
LATTICE
A lattice is the geometrical pattern
formed by points representing the
locations of these basis or motifs.
BASIS OR MOTIFS
Basis are the positions of the
atoms inside the unit cell.
11
Five Planar Lattices
12
graphene
Ten Planar Point Groups
13
17 Plane Space Groups
14
15
Unit Cell: An „imaginary“ parallel sided region of a structure from which
the entire crystal can be constructed by purely translational
displacements. It contains one unit of the translationally repeating
pattern. Content of a unit cell represents its chemical composition.
The unit cells that are commonly formed by joining neighbouring lattice
points by straight lines, are called primitive unit cells.
Space Lattice: A pattern that is formed by the lattice points that have
identical environment.
Coordination Number (CN): Number of direct neighbours of a given
atom (first coordination sphere)
16
Crystal = Periodic Arrays of Atoms
Lattice point
(Atom, molecule, group of molecules,…)
Translation Vectors
Primitive Cell:
• Smallest building block for
the crystal lattice.
• Repetition of the primitive cell
gives a crystal lattice
a
c
ba, b , c
17
Seven Crystal Systems
All angles 90 18
Seven Crystal Systems
19
Fourteen Bravais Lattices
Seven Crystal Systems
+ Centering
20
3D Lattices and Space Groups
21
Add one atom at the
center of the cube
Body-Centered Cubic (BCC)
a
c
b
a = b = c
a  b  c
Simple Cubic (SC)
Add one atom
at the center of
each face
Face-Centered Cubic (FCC)
Conventional Cell = Primitive Cell
Conventional Unit Cell  Primitive Cell 22
Primitive Cell
A primitive cell of the lattice = volume of space translated through all
the vectors in a lattice that just fills all of space without overlapping or
leaving voids.
A primitive cell contains just one Bravais lattice point.
The primitive cell is the smallest cell that can be translated throughout
space to completely recreate the entire lattice.
There is not one unique shape of a primitive cell, many possible shapes.
The primitive cell for the simple cubic lattice is equal to the simple cubic
unit cell (they are identical).
(magenta)
23
Primitive Cell
A primitive cell of the lattice may be constructed in 2 ways:
 The primitive cell may have the lattice point confined at its
CENTER = the WIGNER-SEITZ cell
 The primitive cell may be formed by constructing lines
BETWEEN lattice points, the lattice points lie at the
VERTICES of the cell
24
Primitive Cell
25
Nonprimitive Unit Cell vs. Primitive Cell
The primitive cell is smaller or equal in size to the unit cell.
The unit cells possesses the highest symmetry present in the
lattice (for example Cubic). 26
Nonprimitive Unit Cell vs. Primitive Cell
The primitive cell is smaller or equal in size to the unit cell.
The unit cells possesses the highest symmetry present in the
lattice (for example Cubic). 27
1) Coordinates within a unit cell
2) Express the coordinates u v w as fractions of unit cell
vectors (lattice parameters) a, b, and c
3) Entire lattice can be referenced by one unit cell
Index System for Points
28
Central point coordinates?
Index System for Directions
(Miller Indices)
1) Determine coordinates of two points in
direction of interest (simplified – origin):
u1 v1 w1 and u2 v2 w2
2) Subtract coordinates of the second point
from those of the first point:
u’ = u1 - u2, v’ = v1 - v2, w’ = w1 - w2
3) Clear fractions from the differences to give
indices in lowest integer values.
4) Write indices in [ ] brackets - [uvw]
5) Negative = a bar over the integer.
A = [100]
B = [111]
C = [1¯2¯2]
29
Index System for Directions
(Miller Indices)
In the cubic system directions having the same
indices regardless of order or sign are equivalent
For cubic crystals, the directions are all
equivalent by symmetry:
[1 0 0], [ 1¯ 0 0], [0 1 0], [0 1¯ 0], [0 0 1], [0 0 1¯ ]
Families of crystallographic directions
e.g. <1 0 0>
Angled brackets denote a family of
crystallographic directions.
30
1. If the plane passes through the origin, select an equivalent plane or
move the origin
2. Find the intercepts on the axes in terms of the lattice constants a, b,
c. The axes may be those of a primitive or nonprimitive unit cell.
3. Take the reciprocals of these numbers and then reduce to three
integers having the same ratio, usually the smallest three integers.
4. (1/∞ = 0)
5. The result enclosed in parenthesis (hkl), is called the index of the
plane.
Index System for Crystal Planes
(Miller Indices)
31
Index System for Crystal Planes
(Miller Indices)
32
Index System for Crystal Planes
(Miller Indices)
Cubic system - planes having the same indices regardless of order or
sign are equivalent
(111), (11¯1), (111¯) ….
belong to {111} family
(100), (1¯00), (010), and (001) …..
belong to {100} family
33
Index System for Crystal Planes
(Miller Indices)
The Miller indices (hkl) is the same vector as the normal to the plane [hkl]
34
Index System for Crystal Planes
(Miller Indices)
35
2 nm
Quasiperiodic Crystals
36
2 nm
Quasiperiodic crystal = a structure that is ordered but not periodic
continuously fills all available space, but it lacks translational symmetry
Penrose - a plane filled in a nonperiodic
fashion using two different types of tiles
Five-fold symmetry
Only 2, 3, 4, 6fold
symmetry
allowed to fill 2D
plane
completely
Crystals and Crystal Bonding
• metallic (Cu, Fe, Au, Ba, alloys )
metallic bonding, electron delocalization
• ionic (NaCl, CsCl, CaF2, ... )
ionic bonds, cations and anions, electrostatic
interactions, ions pack into extremely regular crystalline
structures, in an arrangement that minimizes the lattice
energy (maximizing attractions and minimizing
repulsions). The lattice energy is the summation of the
interaction of all sites with all other sites.
• covalent network solid (diamond, graphite, SiO2, AlN,... )
atoms, covalent bonding, a chemical compound (or
element) in which the atoms are bonded by covalent
bonds in a continuous network extending throughout the
material, there are no individual molecules, the entire
crystal or amorphous solid may be considered a
macromolecule
• molecular (Ar, C60, HF, H2O, organics, proteins )
molecules, van der Waals and hydrogen bonding
37
Covalent network solids
38
Three Cubic Cells
SC or Primitive (P) BCC (I) FCC (F)
39
Three Cubic Cells
40
41
a
a
a
d
D
a = edge
d = face diagonl
(d2 = a2 + a2 = 2a2)
D = body diagonal
(D2 = d2 + a2 = 2a2 + a2 = 3a2)
Cube
42
CN 6
Simple Cubic SC = Polonium
Space filling 52%
Z = 1
43
Space filling 68%
CN 8
BCC = W, Tungsten
a
d
D
r
Fe, Cr, V, Li-Cs, Ba
Z = 2
44
45
BCC
46
Space filling 74%
CN 12
FCC = Copper, Cu = CCP
d
r
Z = 4
47
Close Packing in Plane 2D
48
B and C holes cannot be occupied at the same time
49
Close Packing in Space 3D
Cubic
CCP
Hexagonal
HCP 50
hexagonal
cubic
51
cubichexagonal
52
hexagonal cubic
53
cubic
hexagonal
Mg, Be, Zn, Ni, Li, Be, Os, He,
Sc, Ti, Co, Y, Ru
Cu, Ca, Sr, Ag, Au, Ni, Rh, solid
Ne-Xe, F2, C60, opal (300 nm)
54
Structures with Larger Motifs
55
Structures with Larger Motifs
TEM images of
superlattices composed of
11.3 nm Ni nanoparticles
56
Structures with Larger Motifs
57
Structures with Larger Motifs
58
C60 - FCC = CCP
SEM - Opal – 300 nm SiO2 FCC = CCP
59
60
Coordination Polyhedrons
61
Coordination Polyhedrons
62
Space Filling
a = lattice
parameter
Atom Radius,
r
Number of
Atoms (lattice
points), Z
Space filling
SC a/2 1 52%
BCC 3a/4 2 68%
FCC 2a/4 4 74%
Diamond 3a/8 8 34%
63
64
CCP = FCC
(ABC)
Close packed layers of CCP are oriented perpendicularly to the
body diagonal of the cubic cell of FCC
CCP FCC
65
66
Two Types of Voids (Holes)
67
Tetrahedral Holes TN
cp atoms in lattice cell
N Octahedral Holes
2N Tetrahedral Holes
Tetrahedral Holes T+ Octahedral Holes
68
Two Types of Voids (Holes)
69
Two Types of Voids (Holes)
Tetrahedral HolesOctahedral Holes
70
Z = 4
number of atoms in the
cell (N)
N = 8
number of tetrahedral
holes (2N)
Tetrahedral Holes (2N)
71
Octahedral Holes (N)
Z = 4
number of atoms in
the cell (N)
N = 4
number of octahedral
holes (N)
72
Two Types of Voids (Holes)
N cp atoms in lattice cell
N Octahedral Holes
2N Tetrahedral Holes
73
Tetrahedral Holes (2N)
74
Characteristic Structures of Solids = Structure
Types
Rock salt NaCl LiCl, KBr, AgCl, MgO, TiO, FeO, SnAs, UC, TiN, ...
Fluorite CaF2 BaCl2, K2O, PbO2 ...
Lithium bismutide Li3Bi
Sphalerite (zinc blende) ZnS CuCl, HgS, GaAs ...
Nickel arsenide NiAs FeS, PtSn, CoS ...
Wurtzite ZnS ZnO, MnS, SiC
Rhenium diboride ReB2
ICSD
3555 NaCl
3438 MgAl2O4
2628 GdFeO3
75
Structure Types Derived from CCP = FCC
76
Structure Types Derived from CCP = FCC
77
Structure Types Derived from CCP = FCC
Anions/cell (= 4) Oct. (Max 4) Tet. (Max 8) Stoichiometry Compound
4 100% = 4 0 M4X4 = MX NaCl
(6:6 coord.)
4 0 100% = 8 M8X4 = M2X Li2O
(4:8 coord.)
4 0 50% = 4 M4X4 = MX ZnS, sfalerite
(4:4 coord.)
4 50% = 2 0 M2X4 = MX2 CdCl2
4 100% = 4 100% = 8 M12X4 = M3X Li3Bi
4 50% = 2 12.5% = 1 M3X4 MgAl2O4,
spinel
78
o/t fcc(ccp) hcp
all oct. NaCl NiAs
all tetr. CaF2 ReB2
o/t (all) Li3Bi (Na3As) (!) problem
½ t sphalerite (ZnS) wurtzite (ZnS)
½ o CdCl2 CdI2
Comparison between structures
with filled octahedral and tetrahedral holes
79
Fluorite CaF2 and antifluorite Li2O
Fluorite structure = a face-centered cubic array (FCC)
of cations = cubic close packing (CCP) of cations with
all tetrahedral holes filled by anions = a simple cubic
(SC) array of anions.
Antifluorite structure = a face-centred cubic (FCC)
array of anions = cubic close packing (CCP) of anions,
with cations in all of the tetrahedral holes (the reverse
of the fluorite structure).
80
K2[PtCl6], Cs2[SiF6], [Fe(NH3)6][TaF6]2
Fluorite (CaF2, antifluorite Li2O)
F / Li
81
Fluorite structures (CaF2, antifluorite Li2O)
Oxides: Na2O, K2O, UO2,
ZrO2, ThO2
alkali metal sulfides,
selenides and tellurides
K2[PtCl6], (NH4)2[PtCl6],
Cs2[SiF6],
[Fe(NH3)6][TaF6]2.
CaF2, SrF2, SrCl2, BaF2, BaCl2, CdF2, HgF2, EuF2, -PbF2, PbO2
Li2O, Li2S, Li2Se, Li2Te, Na2O, Na2S, Na2Se, Na2Te, K2O, K2S82
Fluorite structures (CaF2, antifluorite Li2O)
83
Sphalerite (zincblende, ZnS)
Cubic close packing of anions
with 1/2 tetrahedral holes
filled by cations
84
Sphalerite (zincblende, ZnS)
85
Sphalerite (zincblende, ZnS)
13-15 compounds: BP, BAs, AlP, AlAs, GaAs, GaP, GaSb, AlSb, InP, InAs, InSb
12-16 compounds: BeS, BeSe, BeTe, -MnS (red), -MnSe, -CdS, CdSe, CdTe, HgS,
HgSe, HgTe, ZnSe, ZnTe
Halogenides: AgI, CuF, CuCl, CuBr, CuI, NH4F
Borides: PB, AsB
Carbides: -SiC
Nitrides: BN
86
Diamond
87
cubic hexagonal
SiO2 cristobalite Lonsdaleite
SiO2 tridymite
ice
Diamond
88
Cubic Diamond
89
Diamond Structure
C, Si, Ge, grey-Sn
• Add 4 atoms to a FCC
• Tetrahedral bond arrangement
• Each atom has 4 nearest neighbors and 12 next nearest neighbors
90
Elements of the 14th Group
91
Cuprite Cu2O Cubic Lattice
92
Wurzite, ZnS
Hexagonal close packing of anions
with 1/2 tetrahedral holes filled by
cations
93
Wurzite, ZnS
ZnO, ZnS, ZnSe, ZnTe, BeO, CdS, CdSe, MnS, AgI, AlN
94
Semiconductors of 13-15 and 12-16 type
95
Rock Salt, NaCl
Cubic close packing of anions with all
octahedral holes filled by cations
96
Rock Salt, NaCl
97
Anion and cation sublattices
Rock Salt, NaCl
98
99
Rock salt structures (NaCl)
Hydrides: LiH, NaH, KH,
NH4BH4 – H2 storage material
Pd(H)
Borides: ZrB, HfB
Carbides: TiC, ZrC, VC, UC
Nitrides: ScN, TiN, UN, CrN, VN, ZrN
Oxides: MgO, CaO, SrO, BaO, TiO, VO, MnO, FeO,
CoO, NiO
Chalcogenides: MgS, CaS, SrS, BaS, -MnS, MgSe,
CaSe, SrSe, BaSe, CaTe
Halides: LiF, LiCl, LiBr, LiI, NaF, NaBr, NaI, KF,
KCl, KBr, KI, RbF, RbCl, RbBr, AgCl, AgF, AgBr
Intermetallics: SnAs
Other
FeS2 (pyrite), CaC2, NaO2
100
Rock salt structures (NaCl)
101
Rock salt structures (NaCl)
FeS2 (pyrite), CaC2, NaO2
SiO2 (pyrite - high pressure polymorph,
Uranus and Neptune core)
102
NiAs - type
Hexagonal close packing of
anions with all octahedral holes
filled by cations
NiS, NiAs, NiSb, NiSe, NiSn, NiTe, FeS,
FeSe, FeTe, FeSb, PtSn, CoS, CoSe,
CoTe, CoSb, CrSe, CrTe, CoSb,
PtB (anti-NiAs structure)
103
NiAs - type
Hexagonal close packing of anions with all octahedral holes filled by cations
104
ReB2 - type
Hexagonal close packing of
anions with all tetrahedral holes
filled by cations
105
[Cr(NH3)6]Cl3, K3[Fe(CN)6]
bcc
Li3Bi - type (anti BiF3)
106
Li3Bi - type (anti BiF3)
Fe3Al
[Cr(NH3)6]Cl3
K3[Fe(CN)6]
Cubic close packing of anions
with all tetrahedral and
octahedral holes filled by
cations
107
Li3Bi - type (anti BiF3)
M3C60
Cubic close packing of C60
3
anions with all tetrahedral
and octahedral holes filled by
cations
108
CsCl Primitive cubic packing of
anions with all cubic holes
filled by cations
Primitive cubic packing of
CsCl8 cubes sharing all faces
109
CsCl
CsCl is not BCC
CsBr, CsI, CsCN, NH4Cl, NH4Br, TlCl, TlBr, TlI, CuZn, CuPd, LiHg
110
NaTl
Niggli – 230 space groups – restrictions on arrangement of atoms:
There are only 4 possible AB cubic structures: NaCl, ZnS-sfalerite, CsCl, and NaTl
Both sublattices form independent diamond
structures.
The atoms sit on the sites of a bcc lattice with
abcc = ½ a.
111
SC of ReO6 octahedra
ReO3
NaCl structure with 3/4 of cations removed and 1/4 of
anions removed
Cubic-WO3, UO3, MoF3, NbF3, TaF3, Cu3N 112
What type of unit cell?
sc, bcc, fcc
WO3
113
MoO3
114
Perovskite, CaTiO3
Two equvivalent views of the unit cell of perovskite
Ti
CaO
Ti
O
Ca
Cubic "close packing" of Ca and O with 1/4 octahedral holes filled by Ti cations115
Perovskite, CaTiO3
Two equvivalent views of the unit cell of perovskite
Cubic "close packing" of Ca and O with 1/4 octahedral holes filled by Ti cations116
TiO6 – octahedra
CaO12 – cuboctahedra
(Ca2+ and O2- form a cubic close packing)
preferred structure of piezoelectric,
ferroelectric and superconducting
materials
Perovskite structure CaTiO3
Goldschmidt’s tolerance factor
117
Similarity to CsCl
Perovskite, CaTiO3
Cubic "close packing" of A and X with 1/4 octahedral holes filled by B cations
118
Perovskite, CaTiO3
MgSiO3, CaSiO3
KNbO3, KTaO3, KIO3,
NaNbO3, NaWO3, LaCoO3,
LaCrO3, LaFeO3, LaGaO3,
LaVO3, SrTiO3, SrZrO3,
SrFeO3
ThTaN3, BaTaO2N
119
Perovskite, BaTiO3
120
Perovskite - ferroelectric BaTiO3
121
Perovskite - ferroelectric BaTiO3
122
Perovskite structure of YBCO
123
Perovskite structure of CH3NH3PbI3
124
Rutile, TiO2
CN – stoichiometry Rule
AxBy
CN(A) / CN(B) = y / x
Distorted hexagonal close packing of anions with 1/2
octahedral holes filled by cations (giving a tetragonal
lattice)
125
Rutile, TiO2
GeO2, CrO2, IrO2, MoO2, NbO2, -MnO2, OsO2, VO2
(>340K), RuO2, CoF2, FeF2, MgF2, MnF2
126
TiO6 – octahedra
OTi3 – trigonal planar
(alternative to CaF2 for highly
charged smaller cations)
The rutile structure: TiO2
127
Three polymorphs of TiO2
anatase (a), rutile (b) and brookite (c)
128
fcc array of O2- ions, A2+
occupies 1/8 of the tetrahedral
and B3+ 1/2 of the octahedral
holes
 normal spinel:
AB2O4
 inverse spinel:
B[AB]O4
Fe3O4 = Fe3+[Fe2+Fe3+]O4
 basis structure for several
magnetic materials
The spinel structure: MgAl2O4
129
130
 MAGNETITE (Fe3O4)
 Cubic inverse spinel
 O2- atoms are arranged in
close-packed FCC lattice
 Fe2+ occupy ½ of OCT sites
 Fe3+ are split evenly across the
remaining OCT and TET sites
 MAGHEMITE (γ-Fe2O3)
 Fully oxidized form of magnetite
 Inverse spinel with cation
deficiency
 One of every six octahedral sites
in magnetite is vacant in
maghemite structure
 Stoichiometry Fetet(Fe5/3□1/3)octO4
Spinel
AB2X4 Spinel normal: Cubic close packing of anions with 1/2 octahedral
holes filled by B cations and 1/8 tetrahedral holes by A cations
MgAl2O4, CoAl2O4, MgTi2O4, Fe2GeO4, NiAl2O4, MnCr2O4
AB2X4 Spinel inverse: As for spinel but A cations and 1/2 of B cations
interchanged
MgFe2O4, NiFe2O4, MgIn2O4, MgIn2S4, Mg2TiO4, Zn2TiO4, Zn2SnO4,
FeCo2O4.
δ = the inversion parameter
(AδB1-δ)A[A1-δB1+δ]BO4
Values from δ = 1 (normal) to δ = 0 (inverse)
May depend on synthesis conditions
131
Corundum
Al2O3 consists of hcp O2− ions
Al3+ fill ……. of all octahedral holes
The Al centres are surrounded by ……. oxides
Oxide ligands are ……………coordinated by Al
132
Corundum
AlO6 octahedral units are linked in both facesharing
and edge-sharing orientations as parallel
and perpendicular to the c-axis, respectively. The
relative orientation of the metal centres causes a
pseudo Peierls distortion, resulting in
neighbouring metal centres that are rotated at an
angle of 64.3° away from each other. Elongation in
pairs of the surrounding oxide ligands results in a
pentagonal bi-pyramidal geometry belonging to
the space group R-3c. The material is largely ionic
in nature with a wide band gap of 9.25 eV.
133
Garnets
Naturally occuring garnets A3B2Si3O12 = A3B2(SiO4)3
A3 = divalent cation (Mg, Fe, Mn or Ca) dodecahedral
B2 = trivalent (Al, Fe3+, Ti, or Cr) octahedral
Si3 = tetravalent, tetrahedral
Since Ca is much larger in radius than the other divalent cations, there
are two series of garnets: one with calcium and one without:
pyralspite contain Al (pyrope, almandine, spessartine)
ugrandite contain Ca (uvarovite, grossular, andradite)
Synthetic garnets A3B5O12
A3 = trivalent cations, large size (Y, La,…)
B5 = trivalent (Al, Fe3+, Ti, or Cr) 2B octahedral, 3B tetrahedral
Y3Al5O12
Y3Fe5O12
134
Garnets
Garnet Y3Al5O12
Y3 = red - dodecahedral
trivalent cations, large size
Al5 = blue
2 octahedral
3 tetrahedral
O12
135
Fullerides
M1C60 all the octahedral (O) sites (dark blue) are occupied (NaCl)
M2C60 all the tetrahedral (T) sites (light blue) are occupied (CaF2)
M3C60 both the O and the T sites are occupied (BiF3)
M4C60 rearranged to a body-centered tetragonal (bct) cell and both the O and
the T sites of the bct lattice are occupied
M6C60 a bcc lattice and all its T sites are occupied
136
Fullerides
BCC unit cell of Rb6C60 and Cs6C60
137
Layered Structures
CdI2 Hexagonal close packing of anions with 1/2 octahedral holes filled
by cations
CoI2, FeI2, MgI2, MnI2, PbI2, ThI2, TiI2, TmI2, VI2, YbI2, ZnI2, VBr2,
TiBr2, MnBr2, FeBr2, CoBr2, TiCl2, TiS2., TaS2.
CdCl2 Cubic close packing of anions with 1/2 octahedral holes filled by
cations
CdCl2, CdBr2, CoCl2, FeCl2, MgCl2, MnCl2, NiCl2, NiI2, ZnBr2, ZnI2,
Cs2O* (anti-CdCl2 structure)
138
CdI2 Hexagonal Close Packing
139
CdCl2 Cubic Close Packing
140
CdCl2 Cubic Close Packing
141
Strukturbericht Symbols
A partly systematic method for specifying the structure of a crystal
A - monatomic (elements), B - diatomic with equal numbers of atoms of each
type (AB), C - a 2-1 abundance ratio (AB2), D0 - 3-1, etc.
Structure type Struktur
bericht
Space group
(S.G. No.)
Lattice
Cu A1 Fm-3m (225) fcc
W, Fe A2 Im-3m (229) bcc
Mg A3 P63/mmc (194) hcp
C - diamond A4 Fd-3m (227) diamond
NaCl B1 Fm-3m (225)
CsCl B2 Pm-3m (221)
ZnS B3 F43m (216) Zincblende
ZnS B4 P63/mc (186) Wurtzite
CaF2 C1 Fm-3m (225) Fluorite
142
Pearson Symbols
Indicate the crystal symmetry and the number of atoms in the unit cell
e.g.: NaCl - a face-centered (F) cubic (c) structure with 8 atoms in the unit cell = cF8
monoclinic (m), hexagonal (h), orthorhombic (o), asymmetric (a), primitive (P)
the Pearson symbol does not necessarily specify a unique structure (see cF8)
Structure type Pearson
Symbol
Struktur
bericht
Space group
(S.G. No.)
Cu cF4 A1 Fm-3m (225)
W, Fe cI2 A2 Im-3m (229)
Mg hP2 A3 P63/mmc (194)
C - diamond cF8 A4 Fd-3m (227)
NaCl cF8 B1 Fm-3m (225)
CsCl cP2 B2 Pm-3m (221)
ZnS (zb) cF8 B3 F43m (216)
ZnS (w) hP4 B4 P63/mc (186)
CaF2 cF12 C1 Fm-3m (225)
143
Space Group Symbols
primitive (P), face-centered (F), body-centered (I), base-centered (A, B, C),
rhombohedral (R)
S. G. Class Centering Symbol syntax (examples)
Triclinic P P1, P-1
Monoclinic P, C, B Paxis, Pplane, Paxis/plane (P21, Cm, P21/c)
Orthorhombic P, F, I, C, A Paxisaxisaxis, Pplaneplaneplane (Pmmm, Cmc21)
Tetragonal P, I P4, P4axisaxisaxis, P4planeplaneplane (I4/m, P4mm)
Trigonal P, R P3axis, P3plane (R-3m)
Hexagonal P P6, P6axisplane (P63/mmc)
Cubic P, F, I Paxis3plane, Pplane3plane (Pm-3m, Fm-3m)
144
Bonding models for covalent and ionic
compounds
Organic vs inorganic bonding
G. N. Lewis 1923
Electron pair sharing
Orbital overlap
Chemical bond
Number of bonds = atomic valence
Born, Lande, Magelung, Meyer
1918
Electrostatic attraction (Coulomb)
Repulsion
1911145
The lattice enthalpy change, L, is the standard molar enthalpy change
for the process:
M+
(gas) + X
(gas)  MX(solid)
The formation of a solid from ions in the gas phase is always exothermic
Lattice enthalpies are usually negative
The most stable crystal structure of a given compound is the one with the
highest (most negative) lattice enthalpy.
(entropy considerations neglected)
Lattice Enthalpy, L
(L)
146
Lattice Enthalpy, L, kJ/mol
147
∆Hform
o = - 411 kJ mol1
∆Hsubl
o = 108 kJ mol1
½ D= 122 kJ mol1
EA = - 355 kJ mol1
IE = 502 kJ mol1
L = ?Na(s) + 1/2 Cl2 (g)
Na(g) + 1/2 Cl2 (g)
Na(g) + Cl (g)
Na+
(g) + Cl (g)
Na+
(g) + Cl-
(g)
NaCl (s)
0 = ∆Hform
o + ∆Hsubl
o + 1/2 D + IE + EA+ L
0 = 411 + 108 +122 + 502 + (-355) + L
L =  788 kJ mol1
Born-Haber cycle
all enthalpies: kJ mol-1 for normal conditions  standard enthalpies
148
Lattice Enthalpy
L = Ecoul + Erep
One ion pair
Ecoul = (1/40) zA zB / d
(calculated exactly)
Erep = B / dn
(modelled empirically)
n = Born exponent
(experimental measurement of
compressibilty)
B = a constant
149
Lattice Enthalpy
1 mol of ions
Ecoul = NA A (e2 / 4  0) (zA zB / d)
A = Madelung constant - a single ion interacts with all other ions
Erep = NA B / dn
L = Ecoul + Erep
Find minimum dL/d(d) = 0
150
Coulombic contributions to lattice enthalpies
Calculation of Lattice Enthalpies
Coulomb potential of
an ion pair
ECoul : Coulomb potential (electrostatic potential)
A : Madelung constant (depends on structure type)
NA: Avogadro constant
z : charge number
e : elementary charge
o: dielectric constant (vacuum permittivity)
d : shortest distance between cation and anion
151
Madelung Constant
Ecoul = (e2 / 4  0)*(zA zB / d)*[+2(1/1)  2(1/2) + 2(1/3)  2(1/4) + ....]
Ecoul = (e2 / 4  0)*(zA zB / d)*(2 ln 2)
Count all interactions in the crystal lattice
Madelung constant A = 1.3863…
for an infinite linear chain of ions
= sum of convergent series
The simplest example : 1D lattice
152
Madelung Constant
Count all interactions in the crystal lattice
of one ion with all others
Madelung constant A = 1.3863…
for an infinite linear chain of ions
= sum of convergent series
The simplest example : 1D lattice
153
Madelung Constant for NaCl
Ecoul = (e2 / 4  0) * (zA zB / d) * [6(1/1) - 12(1/2) + 8(1/3) - 6(1/4) + 24(1/5) ....]
Ecoul = (e2 / 4  0) * (zA zB / d) * A
convergent series
3D ionic solids:
Coulomb attraction and
repulsion
a single ion interacts with all
other ions
154
Madelung Constants for other Structural Types
Structural Type A
NaCl 1.74756
CsCl 1.76267
CaF2 2.519
ZnS Sfalerite 1.63805
ZnS Wurtzite 1.64132
Linear Lattice 1.38629
Ion Pair ?
155
Born repulsion Erep
Because the electron density of atoms decreases
exponentially towards zero at large distances
from the nucleus the Born repulsion shows the
same behavior
approximation:
B and n are constants for a given atom type; n can be
derived from compressibility measurements (~8)
r
r0
Repulsion arising from
overlap of electron clouds
156
Total lattice enthalpy from Coulomb interaction
and Born repulsion
(set first derivative of the sum to zero)
Measured (calculated) lattice enthalpies (kJ mol-1):
NaCl: –772 (–757)
CsCl: –652 (–623)
(measured from Born Haber cycle)
157
The Kapustinskii equation
Kapustinskii found that if the Madelung constant for a given structure is
divided by the number of ions in one formula unit () the resulting values
are almost constant:
Structure Madelung constant (A) A/ Coordination
CsCl 1.763 0.88 8:8
NaCl 1.748 0.87 6:6
CaF2 2.519 0.84 8:4
-Al2O3 4.172 0.83 6:4
 general lattice energy equation that can be applied to
any crystal regardless of the crystal structure
K, G = constants
158
structure M CN stoichm M / v
CsCl 1.763 (8,8) AB 0.882
NaCl 1.748 (6,6) AB 0.874
ZnS sfalerite 1.638 (4,4) AB 0.819
ZnS wurtzite 1.641 (4,4) AB 0.821
CaF2 fluorite 2.519 (8,4) AB2 0.840
TiO2 rutile 2.408 (6,3) AB2 0.803
CdI2 2.355 (6,3) AB2 0.785
Al2O3 4.172 (6,4) A2B3 0.834
v = the number of ions in one formula unit
Kapustinski
159
Most important advantage of the Kapustinski equation
 it is possible to apply the equation for lattice calculations of crystals with
polyatomic ions (e.g. KNO3, (NH4)2SO4 ...).
 a set of „thermochemical radii“ was derived for further calculations of lattice
enthalpies
160
Lattice Enthalpy
El. config. n Example
He-He 5 LiH
Ne-Ne 7 NaF, MgO
Ar-Ar 9 KCl, CaS, CuCl, Zn2+,
Ga3+
Kr-Kr 10 RbBr, AgBr, Cd2+, In3+
Xe-Xe 12 CsI, Au+, Tl3+
Born–Mayer
d* = 0.345 Å
Born–Lande
For compounds of mixed ion types, use
the average value (e.g., for NaCl, n = 8).
161
Lattice Enthalpy of NaCl
Born–Lande calculation L =  765 kJ mol1
Only ionic contribution
Experimental Born–Haber cycle L =  788 kJ mol1
Lattice Enthalpy consists of ionic and covalent contribution
162
Applications of lattice enthalpy calculations:
 thermal stabilities of ionic solids
 stabilities of oxidation states of cations
 solubility of salts in water
 calculations of electron affinity data
 lattice enthalpies and stabilities of „non existent“ compounds
163
Five principles which could be used to determine the
structures of complex ionic/covalent crystals
Pauling’s Rule no. 1 Coordination Polyhedra
A coordinated polyhedron of anions is formed about each cation.
Cation-Anion distance is determined by sums of ionic radii.
Cation coordination environment is determined by radius ratio.
Pauling’s Rules
164
Coordination Polyhedra
165
Different Types of Radii
166
P – Pauling radius
G – Goldschmidt radius
S – Shannon radius
Variation of the electron density
along the Li – F axis in LiF
167
Variation of ionic radii with coordination number
The radius of one ion was fixed to a
reasonable value
(r(O2-) = 140 pm) (Linus Pauling)
That value is then used to compile a
set of self consistent values for all
other ions. 168
Variation of atomic radii
through the Periodic table
169
R.D. Shannon and C.T. Prewitt, Acta Cryst. B25, 925-945 (1969)
R.D. Shannon, Acta Cryst. A32, 751-767 (1976)
Ionic Radii
As the coordination number (CN) increases, the Ionic Radius increases
Sr 2+
CN Radius, Å
6 1.32
8 1.40
9 1.45
10 1.50
12 1.58
As the oxidation state increases, cations get smaller
(6-fold coordination, in Å)
Mn2+ 0.810
Mn3+ 0.785
Mn4+ 0.670
Ti2+ 1.000
Ti3+ 0.810
Ti4+ 0.745 170
Ionic Radii
The radius increases down a group in the periodic table.
The exception - 4d/5d series in the transition metals - the lanthanide contraction
(6-fold coordination, in Å)
Al3+ 0.675
Ga3+ 0.760
In3+ 0.940
Tl3+ 1.025
Ti4+ 0.745
Zr4+ 0.86
Hf4+ 0.85
Right to left across the periodic table the radius decreases.
(6 coordinate radii, in Å)
La3+ 1.172
Nd3+ 1.123
Gd3+ 1.078
Lu 3+ 1.001
171
1. Ionic radii increase down a group.
(Lanthanide contraction restricts the increase of heavy ions)
2. Radii of equal charge ions decrease across a period
3. Ionic radii increase with increasing coordination number
the higher the CN the bigger the ion
4. The ionic radius of a given atom decreases with increasing
charge (r(Fe2+) > r(Fe3+))
5. Cations are usually the smaller ions in a cation/anion
combination (exceptions: r(Cs+) > r(F-))
6. Frequently used for rationalization of structures:
„radius ratio“ r(cation)/r(anion) (< 1)
General trends for ionic radii
172
Cation/anion Radius Ratio
CN r/R
12 – hcp/ccp 1.00 (substitution)
8 – cubic 0.732 – 1.00
6 – octahedral 0.414 – 0.732
4 – tetrahedral 0.225 – 0.414
optimal radius
ratio for
given CN
ions are in touch
173
174
175
Structure Map
Structural map as function of
radius ratios for AB
compounds.
Structural map as function of
radius ratios for A2BO4
compounds.
Dependence of the structure type on parameters, such as ionic radii, ionicity,
electronegativity etc.
176
Structure Map
Dependence of the structure type (coordination number) on the
electronegativity difference and the average principal quantum
number (size and polarizability)
AB compounds
177
Pauling’s Rule no. 2 Bond Strength
The strength of an electrostatic bond = valence / CN
The bond valence sum of each ion equals its oxidation state.
The valence of an ion (Vi, equal to the oxidation state of the ion) is equal to a sum of
the valences of its bonds (sij).
In a stable ionic structure the charge on an ion is balanced by the sum of electrostatic
bond strengths (sij) to the ions in its coordination polyhedron.
TiO2 (Rutile) Ti - oxidation state of +4, coordinated to 6 oxygens.
VTi = +4 = 6 (sij) sij = +2/3
The bond valence of oxygen, coordinated by 3 Ti atoms
Vo = 3 (sij) = 3 (-2/3) = 2
Each bond has a valence of sij with respect to the cation
and sij with respect to the anion.
Pauling’s Rules
178
Bond Strength
Brown, Shannon, Donnay, Allmann:
Correlation of the valence of a bond sij with the (experimental) bond distance dij.
Rij = standard single bond length - determined empirically from (many) structures
where bond distances and ideal valences are accurately known.
Tables of Rij values for given bonding pairs (i.e. Nb-O, Cr-N, Mg-F, etc.) have
been calculated, just as tables of ionic radii are available.
A constant b = 0.37
R = d s = e0 = 1
R  d s = e1 < 1 a bond longer than R is weaker than 1
R  d s = e1 > 1 a bond shorter than R is stronger than 1
179
Bond Strength
Correlation of the valence of a bond sij with the (experimental) bond distance dij.
Use of the bond valence concept
A) To check experimentally determined structures for correctness, or bonding
instabilities
B) To predict new structures
C) To locate light atoms such as hydrogen or Li ion, which are hard to find
experimentally
D) To determine ordering of ions which are hard to differentiate experimentally, such
as Al3+ and Si4+, or O2- and FE)
To check/confirm oxidation states of atoms (Co2+ /Co3+ , Fe2+ / Fe3+ ) 180
Bond Strength
Correlation of the valence of a bond sij with the (experimental) bond distance dij.
181
FeTiO3 (mineral Ilmenite) possesses the corundum structure – an hcp array of
oxides with cations filling 2/3 of octahedral holes.
Decide which oxidation states are present: Fe(II) Ti(IV) or Fe(III) Ti(III)
Bond Distances (dexp, Å) Tabulated Rij values Constants
FeO = 3×2.07 and 3×2.20 R0(FeO) = 1.795 Å b = 0.30
TiO = 3×1.88 and 3×2.09 R0(TiO) = 1.815 Å b = 0.37
Oxygen valence and coordination number O?
Each oxygen is bound to Fe and Ti with both bond distances.
Pauling’s Rule no. 3 Polyhedral Linking
The presence of shared edges, and particularly shared faces decreases the
stability of a structure. This is particularly true for cations with large
valences and small coordination number.
Avoid shared polyhedral edges and/or faces.
Pauling’s Rules
182
Polyhedral Linking
183
Polyhedral Linking
184
The Coulombic interactions - maximize the cation-anion interactions
(attractive), and minimize the anion-anion and cation-cation interactions
(repulsive).
The cation-anion interactions are maximized by increasing the coordination
number and decreasing the cation-anion distance. If ions too close - electronelectron
repulsions.
The cation-cation distances as a function of the cation-anion distance (M-X)
Polyhedron/Sharing
Corner Edge Face
2 Tetrahedra 2 M-X 1.16 MX 0.67 MX
2 Octahedra 2 M-X 1.41 MX 1.16 MX
The cation-cation distance decreases, (the Coulomb repulsion increases) as
the
•degree of sharing increases (corner < edge < face)
•CN decreases (cubic < octahedral < tetrahedral)
•cation oxidation state increases (this leads to a stronger Coulomb repulsion)
185
Pauling’s Rule no. 4 Cation Evasion
In a crystal containing different cations those with large valence and
small coord. number tend not to share polyhedral elements (anions).
Pauling’s Rules
Perovskite, CaTiO3
CaII 12-coordinate CaO12 cuboctahedra share FACES
TiIV 6-coordinate TiO6 octahedra share only VERTICES
186
Pauling’s Rule no. 5 Environmental Homogeneity
the rule of parsimony
The number of chemically different coordination environments for a given
ion tends to be small.
Once the optimal chemical environment for an ion is found, if possible all
ions of that type should have the same environment.
Pauling’s Rules
187
High Pressure Transformations
•high pressure phases
•higher density
•higher coodination number
•higher symmetry
•transition to from nonmetal to metal
•band mixing
•longer bonds
Pressure/Coordination Number Rule: increasing pressure – higher CN
Pressure/Distance Paradox: increasing pressure – longer bonds
188