Solid State Chemistry

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4th week

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Some Important Structure Types

• Rock salt (NaCl): O occupied, T+ and T- empty

• Zinc blende or sphalerite (ZnS): T+ (or T-) occupied; O, T- (or T+) empty

• Antifluorite (Na2O): T+ and T- occupied, O empty

* All have ccp/fcc anions and differ only in the positions of the cations.

* No rule as to which sites should be labelled T+ and T-

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Rock Salt Structure

* ccp/fcc anions with octahedral sites fully occupied by cations and tetrahedral sites empty

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Rock Salt Structure

* very common (ionics, covalents, intermetallics)

* most alkali halides (CsCl, CsBr, CsI excepted)

* most oxides/ chacogenides of alkaline earths

* many nitrides, carbides, hydrides (e.g. ZrN, TiC, NaH)

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Abundance of elements in the Universe

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Abundance of elements in Earth’s crust

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Carbide

In chemistry, a carbide is a compound composed of carbon and a less

electronegative element. Carbides can be generally classified by chemical

bonding type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC2), silicon carbide (SiC), tungsten carbide (WC) (often

called simply carbide when referring to machine tooling), and cementite (Fe3C),

[1] each used in key industrial applications. The naming of ionic carbides is not systematic.

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Nitride

In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications.^[1]Like carbides, nitrides are often refractory

materials owing to their high lattice energy which reflects the strong attraction of "N³⁻" for the metal cation. Thus, titanium nitride and silicon nitride are used as cutting materials and hard coatings. Hexagonal boron nitride, which adopts a layered structure, is a useful high-temperature lubricant akin to molybdenum disulfide. Nitride compounds often have large band gaps, thus nitrides are usually insulators or wide bandgap semiconductors, examples include boron nitride and silicon nitride. The wide band gap material gallium nitride is prized for emitting blue light in LEDs.^[2][3] Like some oxides, nitrides can absorb

hydrogen and have been discussed in the context of hydrogen storage, e.g.

lithium nitride.

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Oxide

An oxide /ˈɒksaɪd/ is a chemical compound that contains at least one oxygen atom and one other element^[1] in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon

oxides: carbon monoxide and carbon dioxide. Even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 (called a passivation layer) that protects the foil from further corrosion.^[2] Different oxides of the same element are distinguished by Roman numerals denoting their oxidation number, e.g. iron(II) oxide versus iron(III) oxide

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Oxide

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SiO

2

too !

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Chalcogenide

Chalcos (ore) + Gen (formation) —> Chalcogen (ore formation)

The chalcogens (/ˈkælk%dʒᵻnz/) are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. It consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the

radioactive element polonium (Po).

A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16

elements of the periodic table are defined as chalcogens, the term chalcogenide is more

commonly reserved for sulfides, selenides, and tellurides, rather than oxides.

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Zinc Blende (sphalerite) Structure

* ccp/fcc anions with cations in one set of tetrahedral sites, either T+ or T-

*The bonding is less ionic than in

compounds with the rock salt structure

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Zinc Blende (sphalerite) Structure

* common for Be, Zn, Cd, Hg chacogenides (i.e., ZmS, ZnSe, ZeTe)

* common for III-V compounds (B, Al, Ga, In with N, P, As, Sb)

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Antifluorite/fluorite Structure

=

* ccp/fcc anions with cations in all tetrahedral sites, both T+ and T-

*antifluorite: an anion array with tetrahedral cations

*fluorite: a cation array with tetrahedral anions

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Antifluorite/fluorite Structure

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Zinc Blende (sphalerite) Structure

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Interstitial Sites in an HCP array

T+

T-

O

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Wurtzite/ NiAs

*This structures have in common an hcp arrangement of anions and differ only in the positions of the cations

* wurtzite: T+ (or T-) sites occupied; T- (or T+), O empty

* nickel arsenide: O sites occupied; T- and T+ empty

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Wurtzite

T+

T-

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Wurtzite

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Wurtzite

c = 1.633 a

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NiAs

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NiAs

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NiAs

Each As is surrounded by

6 Ni in a trigonal prism (0.707a) 12 As in a hcp arrangement (a)

Each Ni is surrounded by

6 As octahedrally (0.707a)

2 Ni linearly (0.816a = c/2)

6 Ni in a hcp arrangement (a)

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Crystal Structures (Ionic)

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Wurtzite vs. Zinc Blende

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* Projections perpendicular to close-packed planes

* Vertex-lined tetrahedra only, but layers skrewed in Wurtzite and not in Zinc Blende

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Cesium Chloride (CsCl)

* primitive cube (not body centered cubic)

* The CsCl structure is not closed packed

* They fall into two groups, (1) halides of large monovalent elements and (2) a variety of inter metallic compounds

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AX Structure

* five main AX structure types : - rock salt (NaCl)

- CsCl - NiAs

- sphalerite - wurtzite

* Some are distorted variants of one of the main structure.

- see textbook (chapter 1.17.5; p.48)

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AX

2

Structure

* five main AX2 structure types : - fluorite or altifluorite

- rutile (TiO2)

- cadmium iodide (CdI2) - cadmium chloride (CdCl2)

* Two main groups of compounds exhibit the rutile structure - oxides of tetravalent metals

- fluorides of divalent metals

- In both cases, the metals are too small to have eight coordination and form the fluorite structure. The rutile structure may be regarded as essentially ionic.

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Rutile (TiO

2

) Structure

* coordination of Ti

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Rutile (TiO

2

) Structure

* rotation about c by 90° / c/2 out of step with each other

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Rutile (TiO

2

) Structure

c = 0 c = 1/2

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Rutile (TiO

2

) Structure

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Li batteries

* Intercalation and Deintercalation in Li ion Batteries

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CdI

2

Structure

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Crystal Structures (Ionic)

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Perovskite Structure (SrTiO

3

)

* ABX3 structure

- primitive cubic unit cell - Ti : cube corners (0 0 0)

- Sr : body center (1/2 1/2 1/2)

- O : edge centers (1/2 0 0), (0 1/2 0), (0 0 1/2)

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Perovskite Structure (SrTiO

3

)

=

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3

)

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3

)

* Perovskite does not contain closed packed oxide ions as such but O and Sr, considered together, do form a ccp array with the layers parallel to the {111}

FCC

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3

)

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3

)

* Several hundred oxides and halides form the perovskite structure

- The oxides contain two cations whose combined oxidation state is six - +I, +V (e.g. KNbO3)

- +II, +IV (e.g. CaTiO3) - +III, +III (e.g. LaGaO3)

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Perovskite Structure (SrTiO

3

)

* A variety of distorted, non-cubic structures often form on cooling the high- temperature cubic structure depending on the size requirements of the 12- coordinate A and 6-coordinate B sites and whether adjustments to the

structure are required to accommodate different-sized cations

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3

: ABO

3

)

* Tolerance factor

- A and B atoms are not exactly the right size to fit the sites generated by the remainder of the structure

- In the ideal, cubic perovskite structure, the bond lengths a = 2^1/2rA-O = 2rB-O

- tolerance factor, t : t = (2^1/2rA-O) / (2rB-O)

A

B

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Perovskite Structure (BaTiO

3

)

- tolerance factor, t = 1.06

- Ti is slightly small for its octahedral site - Ti displaces by about 6% of the Ti-O

distance towards one of the corner O

- Ti atoms in adjacent unit cells undergo a similar displacement in the same

direction and the resulting structure has a large dipole moment due to the

separation of positive and negative charge centers

- high polarizability, high permittivity - ferroelectricity

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Spinel Structure (AB

2

O

4

; MgAl

2

O

4

)

* Several commercially important magnetic oxides have the spinel structure.

— It has ccp oxide ions with Mg²⁺, Al³⁺ in tetrahedral and octahedral interstices, respectively

* Many oxides, sulfides and halides have the spinel structure and different cation charge combinations are possible

* Extremely flexible structure, adopted by 100s of compounds

2, 3 as in MgAl2O4 ZnAl2S4

2, 4 as in Mg2TiO4 Cu2SnS4

1, 3, 4 as in LiAlTiO4

1, 3 as in Li0.5Al2.5O4

1, 2, 5 as in LiNiVO4

1, 6 as in Na2WO4