2Clathrate Semiconductors Not in the Texts! A research interest for me for about the last 12 years!“New” crystalline phases of theGroup IV Elements:Si, Ge, Sn (not C yet).Few pure elemental phases yet. Mostly compounds, usually with Groups I & II elements (Na, K, Cs, Ba).Interesting properties (possible applications are for use as a thermoelectric material).Clathrate Crystal Structureswill be discussed briefly now & contrasted to the diamond structure. More properties as class proceeds.
3[n = 2, C; n = 3, Si; n = 4, Ge; n = 5, Sn] Group IV ElementsThe valence electron configurations of the free atoms are:ns2 np2[n = 2, C; n = 3, Si; n = 4, Ge; n = 5, Sn]
4Group IV Crystals Diamond Structure Si, Ge, Sn: Their ground state crystal structure is theDiamond StructureEach atom tetrahedrally (4-fold) coordinated (4 nearest-neighbors) with sp3 covalent bondingBond angles: Perfect, tetrahedral = 109.5ºSi, Ge: SemiconductorsSn: (α-tin or gray tin) - Semimetal
5Carbon Crystals C: Graphite & Diamond Structures Diamond An insulator or a widebandgap semiconductorGraphite A planar structuresp2 bonding a 2d metal (in plane)The Ground State (lowest energy configuration) isgraphite at zero temperature & atmospheric pressure.The graphite-diamond energy difference is VERY small!
6Other Group IV Crystal Structures (Higher Energy) C: “Buckyballs” (C60) “Buckytubes” (nanotubes),other fullerenesGraphene
7Si, Ge, Sn: The Clathrates. Sn: (β-tin or white tin) - body centered tetragonal lattice, 2 atoms per unit cell.Metallic.Si, Ge, Sn: The Clathrates.
8ClathratesCrystalline Phases of Group IV elements: Si, Ge, Sn (not C yet!) “New” materials, but known (for Si) since 1965!J. Kasper, P. Hagenmuller, M. Pouchard, C. Cros, Science 150, 1713 (1965)As in the diamond structure, all Group IV atoms are 4-fold coordinated in sp3 bonding configurations.Bond angles: Distorted tetrahedra Distribution of angles instead of the perfect tetrahedral 109.5ºLattice contains hexagonal & pentagonal rings, fused together with sp3 bonds to form large “cages”.
9Pure materials: Metastable, expanded volume phases of Si, Ge, Sn Few pure elemental phases yet. Compounds with Group I & II atoms (Na, K, Cs, Ba).Potential applications: ThermoelectricsOpen, cage-like structures, with large “cages” of Si, Ge, or Sn atoms.“Buckyball-like” cages of 20, 24, & 28 atoms.Many varieties. The two most common varieties are:Type I (X46) & Type II (X136)X = Si, Ge, or Sn
10Meaning of “Clathrate” ? From Wikipedia, the free encyclopedia:“A clathrate or clathrate compound or cage compound is achemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule. The word comes from the Latin clathratus meaning furnished with a lattice.”“For example, a clathrate-hydrate involves a special type of gashydrate consisting of water molecules enclosing a trapped gas.A clathrate thus is a material which is a weak composite, withmolecules of suitable size captured in spaces which are left bythe other compounds. They are also called host-guestcomplexes, inclusion compounds, and adducts.”
11Type I clathrate-hydrate crystal structure X8(H2O)46: Group IV clathrates have the same crystal structure as clathrate-hydrates (ice).Type I clathrate-hydrate crystal structure X8(H2O)46:
12Si46, Ge46, Sn46: ( Type I Clathrates) 20 atom (dodecahedron) cages & 24 atom (tetrakaidecahedron) cages,fused together through 5 atomrings. Crystal structure =Simple Cubic, 46 atoms per cubic unit cell.Si136, Ge136, Sn136: ( Type II Clathrates)28 atom (hexakaidecahedron) cages,Face Centered Cubic, 136 atoms per cubic unit cell.
13Clathrate Building Blocks 24 atom cage:Type I ClathrateSi46, Ge46, Sn46(C46?)Simple Cubic20 atom cage:Type II ClathrateSi136, Ge136, Sn136(C136?)Face Centered Cubic28 atom cage:
14Clathrate Lattices Type I Clathrate Si46, Ge46, Sn46 simple cubic directionType II Clathrate Si136, Ge136, Sn136face centeredcubicdirection
15Synthesis: NaxSi46 (A theorists view!) Group IV ClathratesNot found in nature. Synthesized in the lab.Not normally in pure form, but with impurities (“guests”) encapsulated inside the cages.Guests “Rattlers”Guests: Group I (alkali) atoms (Li, Na, K, Cs, Rb) or Group II (alkaline earth) atoms (Be, Mg, Ca, Sr, Ba)Synthesis: NaxSi46 (A theorists view!)Start with a Zintl phase NaSi compound.An ionic compound containing Na+ and (Si4)-4 ionsHeat to thermally decompose. Some Na vacuum.Si atoms reform into a clathrate framework around Na.Cages contain Na guests
16Type I Clathrate (with guest “rattlers”) 20 atom cagewith a guest atom direction+24 atom cagewith a guest atom direction
17Guest Modes Rattler Modes Pure Materials: Semiconductors.Guest-containing materials:Some are superconducting materials (Ba8Si46) from sp3 bonded, Group IV atoms!Guests are weakly bonded in cages: A minimal effect on electronic transportHost valence electrons taken up in sp3 bondsGuest valence electrons go to conduction band of host ( heavy doping density).Guests vibrate with low frequency (“rattler”) modes A strong effect on vibrational properties =Guest Modes Rattler Modes
18Good thermoelectrics should have Clathrates of Interest: Possible use as thermoelectric materials.Good thermoelectrics should havelow thermal conductivity!Guest Modes Rattler Modes:A focus of recent experiments.Heat transport theory says: The low frequency rattlermodes can scatter efficiently with the acoustic modes of the host.The guest vibrations lower the thermal conductivity A good thermoelectric!Clathrates of Interest:Sn (mainly Type I). Si & Ge, (mainly Type II).Recently, “Alloys” of Ge & Si (Type I ).