Solid State Structure Edward A. Mottel Department of Chemistry Rose-Hulman Institute of Technology

Ionic Solid State Structures  Many ionic structures are based on the larger ion packing very efficiently (cubic or hexagonal closest pack). the smaller ion occupying holes in the structure.  Not every hole is occupied by the smaller ion, and sometimes whole sheets of hole locations are vacant.

6/12/2015 Layered Structures rock saltCdCl 2 alternate sheets of cations are missing

6/12/2015 CdCl 2 layered structure rock salt

Ionic Solid State Structures Imperfections and Disorder  Above absolute zero, crystals will not be perfectly packed. Schottky defects F center Frenkel defect

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Ionic Solid State Structures Imperfections and Disorder Schottky defects

Ionic Solid State Structures Imperfections and Disorder F centerFrenkel defect Cl - Na + -

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Ionic Solid State Structures Conductivity  Solid ionic compounds are poor electrical conductors.  Mobile charges (ions or electrons) are needed for electrical conductivity.

Ionic Solid State Structures Imperfections and Disorder controlled valency

Ionic Solid State Structures Mixed Valence Conductivity mixed valency

Solid State Structures Ionic and Covalent Character polarization of the cation ionic harder lower Z* covalent softer higher Z* polarization of the anion CdCl 2 CdI 2 HgCl 2 SiO 2 TiO 2 CaF 2 EN Ca: 1.00 Cd: 1.69 Hg: 2.00 more covalent bonding

6/12/2015 Solid State Bonding  Ionic bonding favors small, hard ions. is isotropic (independent of angle). follows the radius ratio rule.  Covalent bonding becomes important as atoms become softer (more polarizable). is directional (uses specific orbitals for overlap. may result in a lower coordination number.

6/12/2015 Solid State Bonding  Small differences in electronegativity and smaller atoms form more covalent bonds.

6/12/2015 Solid State Bonding  Small differences in electronegativity and smaller atoms form more covalent bonds. less overlap

6/12/2015 Bonding Band Theory directional localized bonds Si4 Si atoms Si silicon (diamond structure)

6/12/2015 Bonding Band Theory band gap valence band conduction band for silicon the valence band is full the conduction band is empty directional localized bonds Si4 Si atoms

6/12/2015 Band Gap diamondsilicongermaniumtin Smaller gap for heavier elements

6/12/2015 Band Gap Smaller gap for heavier elements C Si Ge Sn diamond - insulator semimetal, semiconductor grey tin - metallic, conductor graphite has a different structure than diamond and is a conductor

6/12/2015 Bonding Covalent and Metallic Bonding directional localized bonds C4 C atoms no band gap valence band conduction band InsulatorMetal

6/12/2015 Density of States valence band conduction band InsulatorMetalSemiconductor overlapping mo’s in extended structure levels are not uniformly spaced

6/12/2015 Conductivity metal semiconductor insulator TT  e-e-

Band Gap Energy  number of electrons average T 2 > T 1 free electrons or holes move charge higher temperature puts more e - in conduction band

6/12/2015 Elements in Semiconductors N P As Sb O S Se Te B Al Ga In C Si Ge Sn Zn Cd Alloys: GaP, GaAs, ZnS, CdS, CdSe, SiC Intrinsic: Si, Ge, Fe 3 O 4 Cu Ag F Cl Br I

6/12/2015 Doped Semiconductors valence band conduction band n-type semiconductor 1% As in Ge excess mobile electrons p-type semiconductor 1% Ga in Ge excess mobile holes

6/12/2015 Diode a combination of an n-type semiconductor and a p-type semiconductor that allows current flow in a preferred direction n-typep-type

6/12/2015 Diode e - flow can occur with e - moving to more stable energy levels - + Both conduct because there are mobile electrons or holes and locations to move to. n-typep-type e-e- e-e- Battery provides e - on one side and drain on the other side.

6/12/2015 Diode - + n-typep-type e-e- e-e- Current flow in the reverse direction requires e - move to higher energy levels, and occurs only with large applied potentials (breakdown voltage). The semiconductors are charge neutral, and additional charge will build up in the valence band preventing significant current flow.

Diode Current   Reverse BiasForward Bias  Applied Voltage

Light Emitting Diodes = E h = c h = 6.62 x J·s·molecule -1 c = 3.00 x 10 8 m·s -1 band gap wavelength (color)

Solid State Photoreactions Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + h

Solid State Photoreactions Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br Ag Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag + Br - Ag +

6/12/2015 AgBr (s)  Ag (s) + ½ Br 2 (l) Ag(s) + Br 2 ( l ) Ag( l ) Ag( g ) ½ Br 2 ( g ) AgBr( s ) Ag + ( g ) Br - ( g ) 325 kJ·mol kJ·mol -1 major energy requirement is reverse of EA of Br -

6/12/2015 AgBr (s)  Ag (s) + ½ Br 2 (l) E = 325,000 J·mol -1 = E h = c = hc E = 3.68 x m = 3680 Å h = 6.62 x J·s·molecule -1 c = 3.00 x 10 8 m·s -1 mol = 6.02 x molecules (near uv)

1-2-3 Superconductor YBa 2 Cu 3 O 7 barium yttrium copper oxygen

Resistivity resistivity temperature resistivity temperature metal superconductor TcTc

Superconductivity

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Types of Bonds Formed N N P P P P O O S S S S S S S S Lighter members of a family often form multiple bonds Heavier members favor additional single bonds.

6/12/2015 Heavier Members of a Family tend to form single bonds C O O carbon dioxide Si O O O O O O O O O silicon dioxide quartz, glass, sand

6/12/2015 Si silicon

Ionic Solid State Structures Imperfections and Disorder controlled valency

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