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Important Structure Types 5/23/2013 L.Viciu| ACII| Imprtant structure types 1 A. Structures derived from cubic close packed 1. NaCl- rock salt 2. CaF2 – fluorite/Na2O- antifluorite 3. diamond 4. ZnS- blende B. Structures derived from hexagonal close packed 1. NiAs – nickel arsenide 2. ZnS – wurtzite 3. CdI2 – cadmium iodide 4. CdCl2 – cadmium chloride C. Non close packed structures 1. CsCl – cesium chloride 2. MoS2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4. MgAlO4 - Spinel 5/23/2013 L.Viciu| ACII| Imprtant structure types 2 Voids in f.c.c. structure • O Oh sites in f.c.c. arrangement of anions (fcc unit cell) •4 Oh sites in total • location: 5/23/2013 1 12 1( centre) 4 4 ( edge) • T Td sites in the f.c.c . arrangement of anions •8 Td sites in total •Location: on the body diagonals – two on each body diagonal at ¼ of the distance from each end. L.Viciu| ACII| Imprtant structure types 3 A-1. Rock salt: NaCl (halite), Sp. Group, Fm-3m Ionic structure rCl 1.81 rNa 0.95 rNa rCl Cl- Red balls are Purple balls are Na+ Edge shared Oh 0.52 Na Oh coordinate d Cl- form the c.c.p. array Na+ fills all the Oh holes while the Td holes are empty Na+: 8x1/8+6x ½= 4 Cl-: 12x ¼ +1=4 5/23/2013 4 NaCl per unit cell L.Viciu| ACII| Imprtant structure types 4 Compounds with NaCl-rock salt structure • Halides: LiX, NaX, KX, RbX, AgX –except AgI • Oxides: MgO, CaO, SrO, BaO, TiO, MnO, FeO, CoO • Chalcogenides: MgS, CaS, MnS, MgSe, CaSe, CaTe, At room temperature, they are electrical insulators and transparent in the visible spectral region. At elevated temperatures, they could become ionic conductors, with the major contribution to charge transport from positive ion vacancy motion. 5/23/2013 L.Viciu| ACII| Imprtant structure types 5 A-2. CaF2-fluorite/Na2O antiflorite (Fm-3m) Ionic compound I. Ca2+ ions form the c.c.p. array F- fills all Td voids (Oh voids are empty) Ca2+: 8 x 1/8 + 6 x ½ = 4 F-: 8 x 1=8 Edge shared FCa4 Td II. F- ions form a simple cubic array Ca2+ – in the ½ of the cubic sites F-: 8 x 1/8 +12 x ¼ + 6x ½ +1= 8 Ca2+: 4 x 1 = 4 4 CaF2 in the unit cell C.N.: Ca-8(cubic): F-4(Td) Corner shared CaF8 cubes In the Anti-Fluorite (Na2O) structure, Cation and Anion positions are reversed! 5/23/2013 L.Viciu| ACII| Imprtant structure types 6 Compounds with CaF2 (fluorite) and Na2O (antifluorite) structure: • Fluorite: Halides: SrF2, SrCl2, BaF2, BaCl2, CdF2, HgF2 Oxides: PbO2, CeO2, PrO2,ThO2 • Antifluorite: Oxides: Li2O, Na2O, K2O, Rb2O Chalcogenides: Li2S, Li2Se, Na2S, Na2Se, Na2Te, K2S, K2Se, K2Te Compounds with fluorite structure are ionic conductors: the charge is carried by anions The fluorite structure favors anion motion because the anions have less charge and are closer together than the cations 5/23/2013 L.Viciu| ACII| Imprtant structure types 7 Fluorite type compounds: Fast Ionic Conductors ZrO2 stabilized with CaO or Y2O3: conduction through O2- High mobility of anion vacancies gives rise to fast ionic (anionic) conduction in fluorite type structure. Batteries = energy conversion + energy storage Solid oxide fuel cells = energy conversion http://www.gepower.com/research/seca/sofc_research.htm 8 A-3. Diamond Structure Covalent structure: the directionality of the covalent bonds dictates the crystal structure. C- hybridized sp3 ½ of the C form the c.c.p. array ½ of C fills ½ of the Td voids (Oh voids ½ ¾ are empty) ½ C: 8 x 1/8+6 x ½ = 4 ¼ 0,1 ¼ 0,1 ¾ C: 4 x 1 = 4 C.N.: 4 The most stable covalent structure 5/23/2013 L.Viciu| ACII| Imprtant structure types 9 Properties of diamond •High pressure allotrope of C (graphite diamond @80kbars) •Insulator (Eg = 5.4 eV) and transparent; color in diamonds originates from impurities i.e. colored diamond: • good thermal conductivity i.e. used in semiconductors industry to prevent them from overheating (thermal sink) • high refractive index and high optical dispersion(shine) 5/23/2013 L.Viciu| ACII| Imprtant structure types 10 Compounds with diamond like structure Group 4 of elements: Si, Ge and -Sn radius Lattice constant (Å) Melting Conductor? Point (ºC) Eg(eV) Carbon diamond 3.56 3550 Insulator 5.4 Silicon 5.43 1410 Semiconductor 1.1 Germanium 5.66 940 Semiconductor 0.7 -Tin 6.49 230 Zero gap semiconductor 0 All have the cubic structures (space group: Fd-3m) Eg is inverse proportional with the bond lengths Longer bonds are weaker and the electrons are easily liberated small band gaps -Tin is the largest in the group weakest bonds (larger unit cell) 5/23/2013 L.Viciu| ACII| Imprtant structure types 11 Changing the motif in diamond structure diamond 5/23/2013 Zinc Blende L.Viciu| ACII| Imprtant structure types 12 A-4. ZnS- Zinc Blende (Sphalerite) Similar with diamond structure A Red spheres – S2Green spheres – Zn2+ •S2- Corner shared ZnS4 Td Layers of ZnS4 Td stacked ..ABCABC.. form the c.c.p. array •Zn2+ fills ½ of the Td voids (Oh voids are empty) •S: 8 x 1/8+6 x ½ = 4 •Zn: 4 x 1 = 4 •C.N.: 4 5/23/2013 The crystal may be thought of as two interpenetrating fcc lattices, one for sulfur the other for zinc, with their origins displaced by one quarter of a body diagonal. L.Viciu| ACII| Imprtant structure types 13 Compounds with Zinc Blende- type structure • • • • CuF, CuCl, -CuBr, -CuI, -AgI This small cation structure is found for small metallic elements, which tend to form -MnS red, -MnSe, BeS, , ZnS, strong sp3 covalent bonds. -SiC, BN, BP III-V compounds: GaP, GaAs, GaSb, InP, InAs, InSb Note: Crystals containing tetrahedral groups are often piezoelectric (a Td symmetry doesn’t have an inversion center). i.e. Zinc blende is piezoelectric Unstressed ZnS4 Td 5/23/2013 Stressed ZnS4 Td L.Viciu| ACII| Imprtant structure types 14 Most semiconductors of commercial importance are isomorphous with diamond and zinc blende Structure – electronic properties relations important for evaluating: Band gap Mobility 5/23/2013 L.Viciu| ACII| Imprtant structure types 15 Band Gap (Eg) ~ e Eg / kT -conductivity -mobility Eg-Band gap T- temperature K-Boltzman constant Eg increases with increasing the electronegativity difference between constituent ions. Generally, band gap and transparency are interconnected Band gap generally increases with ionicity Band gap increases with ionicity Covalent semiconductors have narrow Eg 5/23/2013 L.Viciu| ACII| Imprtant structure types 16 Mobility () for rock salt and zinc blende type materials Electronegativity difference ~ e Eg / kT In materials free of defects, the mobility is determined by the effective mass interaction with lattice vibration 1. Mobility as the molecular weight (heavy mass gives low scattering) Compounds with ionic bonding have low electron mobility 2. Mobility as the electronegativity difference btw ions (polarization effect of mobile electrons or holes on the surrounding atoms) 5/23/2013 L.Viciu| ACII| Imprtant structure types 17 Typical Semiconductors Silicon GaAs Diamond Cubic Structure ZnS (Zinc Blende) Structure 4 atoms at (0,0,0)+ FCC translations 4 Ga atoms at (0,0,0)+ FCC translations 4 atoms at (¼,¼,¼)+FCC translations 4 As atoms at (¼,¼,¼)+FCC translations Bonding: covalent Bonding: covalent, partially ionic 5/23/2013 L.Viciu| ACII| Imprtant structure types 18 Properties GaAs Si Crystal structure zinc blende diamond Lattice constant 5.6532 5.43095 Band gap (eV) at 300 K 1.424 (direct) 1.12 (indirect) Mobility (cm2/V.s) 8500 1500 Intrinsic carrier conc. (cm-3) 1.79x106 1.45x1010 Difficulty in growing stoichiometric GaAs crystals due to the loss of arsenic evaporation (>600ᵒ); also the crystals are very brittle crystal perfection and purity in silicon has reached levels never achieved with any other synthetic materials. 5/23/2013 L.Viciu| ACII| Imprtant structure types 19 • Why semiconductors have diamond or ZnS –blende structure? 5/23/2013 L.Viciu| ACII| Imprtant structure types 20 • Why semiconductors have diamond or ZnS –blende structure? Due to the covalent character of its bonding interaction (the lattice is always composed of those elements with the smallest difference in electronegativity). 5/23/2013 L.Viciu| ACII| Imprtant structure types 21 Structural Changing pressure Graphite Diamond pressure Zinc blende type : InAs , CdS , CdSe NaCl type Graphite : C.N.= 3; dC-C = 1.415Å; =2.26g/cm3 Diamond: C.N. = 4 dC-C = 1.54Å; = 3.51g/cm3 U. Müller-Inorganic Structural Chemistry • Pressure –coordination rule: “with increasing pressure an increase of the coordination number takes place” • Pressure-distance paradox: “when the coordination number increases according to the previous rule, the interatomic distances also increases” 5/23/2013 L.Viciu| ACII| Imprtant structure types 22 Voids in f.c.c. structure • O Oh sites in f.c.c. arrangement of anions (fcc unit cell) •4 Oh sites in total • location: 5/23/2013 1 12 1( centre) 4 4 ( edge) • T Td sites in the f.c.c . arrangement of anions •8 Td sites in total •Location: on the body diagonals – two on each body diagonal at ¼ of the distance from each end. L.Viciu| ACII| Imprtant structure types 23 Filling voids in c.c.p. structures CaF2 ZnS all Td ½ Td NaCl Li3Bi ½ Td c.c.p. 5/23/2013 all Oh L.Viciu| ACII| Imprtant structure types all Td and all Oh 24 Ulrich Müller: “Inorganic structural chemistry” all Td sites filled ½ of the Td sites filled ¼ of the Td sites filled Fig. 128/pag203 “Relationships among the structures of CaF2, PbO, PtS, ZnS, HgI2, SiS2, and α-ZnCl2. In the top row all tetrahedral interstices (= centers of the octants of the cube) are occupied. Every arrow designates a step in which the number of =occupied tetrahedral interstices is halved; this includes a doubling of the unit cells in the bottom row. Light hatching = metal atoms, dark hatching = non-metal atoms. The atoms given first in 5/23/2013 L.Viciu| ACII| Imprtant structure types 25 the formulas form the cubic closest-packing” A. Structures derived from cubic close packed 1. 2. 3. 4. NaCl- rock salt CaF2 – fluorite/Na2O- antifluorite diamond ZnS- blende B. Structures derived from hexagonal close packed 1. ZnS – wurtzite 2. NiAs – nickel arsenide 3. CdI2 – cadmium iodide C. Non close packed structures 1. CsCl – cesium chloride 2. MoS2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4. MgAlO4 - Spinel 5/23/2013 L.Viciu| ACII| Imprtant structure types 26 Voids in h.c.p. structure A The spacing of the close packed layers: d = √8r/√3 = 1.633r 2 1 1 ( , , ) 3 3 2 B c=2x1.633r=2x1.633xa/2=1.633a c/a=1.633 A The voids are identical to the ones found in FCC (1/3, 2/3, 3/4) (0,0,5/8), (⅔,⅓,7/8) B (0,0,3/8) A (1/ 2 3, /3, ¼) Oh void (⅔, ⅓,1/8), Td void Octahedral voids occur in 1 orientation, tetrahedral voids occur in 2 orientations 5/23/2013 L.Viciu| ACII| Imprtant structure types 27 B-1. Wurtzite (ZnS) (P63mc) A B A S2--yellow spheres Zn2+-green spheres •S2- form the h.c.p. array (c/a=1.633) •Layers of ZnS4 Td stacked ..ABAB… •Alternate layers are rotated by 180ᵒ about c axis relative to each other. •Zn2+ fills ½ of Td voids (T+ or T-) •S: at (0,0,0) and (2/3, 1/3, ½) •Zn-: at (2/3, 1/3, 1/8) and (0,0, 5/8) •c/a = 1.636 (the ideal c/a=1.633) 5/23/2013 L.Viciu| ACII| Imprtant structure types 28 Zn neighbors in Wurtzite structure 1 S2--yellow spheres Zn2+-green spheres nearest neighbors: 4 S ions Next nearest neighbors: 12 Zn ions (ex: the ion 1 has 6 Zn ions at distance a in the same plane with it and three Zn in the plane below and then three in the plane above it –the next cell) 5/23/2013 L.Viciu| ACII| Imprtant structure types 29 Two unit cells of the Wurtzite structure (0,0,0) (1/3, 2/3, 3/8) (0,0, 5/8) (1/3, 2/3 ,0) 5/23/2013 L.Viciu| ACII| Imprtant structure types 30 Different view of the Wurtzite structure •Zn-: 2 x ½ + 1=2 per cell at (1/3, 2/3 ,0) + h.c.p translation (2/3, 1/3, ½) •S: 2 x 1 = 2 per unit cell at (1/3, 2/3, 3/8) + h.c.p translation (2/3, 1/3, 7/8) •2 ZnS per unit cell •C.N.: 4:4 (Td) 5/23/2013 L.Viciu| ACII| Imprtant structure types 31 Compounds with wurtzite type structure • • • • • • ZnO, ZnS, ZnSe, ZnTe BeO CdS, CdSe, MnS, MnSe AgI AlN, GaN, InN, TlN, SiC The highlighted blue compounds are piezoelectrics The symmetry of the wurtzite type structure allows for a distortion along the c axis distorted Td 5/23/2013 L.Viciu| ACII| Imprtant structure types 32 Zinc Blende vs. Wurtzite •Different electrostatic interaction between an atom and its third neighbors (…ABCABC… VS …ABAB…); •Covalent compounds with tendency towards lattice instability as ionicity increases A B A Zn is Td coordinated corner shared Td Zn is Td coordinated corner shared Td but the layers are rotated by 180ᵒ relative to each other …ABCABC… …ABAB… = 4.11g/cm3 5/23/2013 = 3.98g/cm3 L.Viciu| ACII| Imprtant structure types 33 5/23/2013 L.Viciu| ACII| Imprtant structure types 34 Zn next nearest neighbors in zinc blende structure 5/23/2013 L.Viciu| ACII| Imprtant structure types 35 Zinc Blende vs. Wurtzite Covalent compounds with tendency towards lattice instability as ionicity increases Wurtzite structure is more open Wurtzite is more ionic than Zinc Blende: the lattice energy of wurtzite is larger than that of zinc blende i.e. Awurtzite = 1.641 Azinc blende = 1.638 m.p. Eg 5/23/2013 (A = constant in the lattice energy formula which depends on the crystal geometry. It is the sum of a series of numbers representing the number of nearest neighbors and their relative distance from a given ion) zinc blende sublimes at t=1185C 3.68eV wurtzite 1850C 3.911eV L.Viciu| ACII| Imprtant structure types 36 B2. NiAs- Nickel Arsenide(P63/mmc) 5, 7 and 8 are arsenic ions common to two Oh; •NiAs6 Oh share opposite faces chains of face sharing Oh along c 3 and 7 are arsenic ions common to two Oh •Chains of edge shared Oh in the ab plane • As form the h.c.p. array (c/a=1.391) • Ni fills all Oh voids (all Td voids empty) •2As at (0,0,0) and (1/3,2/3,1/2 •2Ni at (2/3,1/3,1/4) and (2/3,1/3,3/4) •C.N.: Ni 6 (octahedral) : As 6 (trigonal prismatic) 5/23/2013 •Edge sharing AsNi6 trigonal prisms L.Viciu| ACII| Imprtant structure types 37 NiAs – alternative views I. II. As Ni 1/ As 3/ 4 Ni 4 0, 1/2 1/ 1/ 3/ 4, 4 2 0,1 As Ni I. Ni at the corners of the hexagonal cell. One As is in the center of a hexagonal prism formed by six Ni atoms. The result is doubling of the repeat unit in the c- direction. 2NiAs per unit cell (Z=2) II. As’s form the hexagonal close packed sublattice, which is interpenetrated by a primitive hexagonal sublattice of the metal (Ni) atoms. Hexagonal layers of nickel alternating with hexagonal layers of arsenic. Note: this is not a layered structure ; it is a tightly connected three dimensional array! 5/23/2013 L.Viciu| ACII| Imprtant structure types 38 Compounds with NiAs type structure The NiAs structure is a common structure in metallic compounds of (a) transition metals with (b) heavy p-block elements (As, Sb, Bi, S, Se). •Intermetallic compounds: NiSb, NiSn, FeSb, PtSn, MnAs, MnBi, PtBi •Transition metals chalcogenides: NiS, NiSe, NiTe, FeS, FeSe, FeTe, CoS, CoSe, CoTe, CrSe, CrTe, MnTe Overlap of 3d orbitals gives rise to metallic bonding. c/a < 1.633 due to metallic bonding on c direction 5/23/2013 L.Viciu| ACII| Imprtant structure types 39 Most NiAs type materials are metallic. Bond distance, dNi-Ni, in NiAs is 2.55Å Typical dNi-Ni is the the range 2.7-2.9 Å Change at the Fermi surface with change in the bond distance change in c/a ratio as changing the electron count. A.West: page 249 5/23/2013 L.Viciu| ACII| Imprtant structure types 40 NiAs vs. NaCl B A A C B B A A Both structures have all the octahedral voids filled AB compounds: appreciable metallic bond adopt NiAs structure type appreciable ionic bond adopt NaCl structure type 5/23/2013 L.Viciu| ACII| Imprtant structure types 41 B3: CdI2: Cadmium Iodide (P-3m1) B A B A • I form the h.c.p. array • one Cd at (0,0,0); • Cd2+ fills ½ of Oh voids • Two I: • Hexagonal lattice Cd2+ I- (2/3,1/3,1/4); (1/3,2/3,3/4) •1CdI2 in the unit cell C.N.: Cd - 6 (Octahedral) : I - 3 (base pyramid) 5/23/2013 L.Viciu| ACII| Imprtant structure types 42 Alternative views ¾ ¼ 0,1 C.N.: 5/23/2013 Cd 6 (Octahedral) 6:3 I 3 (base pyramid) Cd ion in the highlighted sulfur unit cell L.Viciu| ACII| Imprtant structure types 43 Compounds with CdI2 structure van der Waals attraction between neighboring iodine layers The structure is stabilized by highly covalent interactions and large, polarizable anions •Iodides of moderately polarizing cations; bromides and chlorides of strongly polarizing cations; e.g. PbI2, FeBr2, VCl2 •Hydroxides of many divalent cations e.g. (Mg,Ni)(OH)2 •Di-chalcogenides of many quadrivalent cations e.g. TiS2, ZrSe2, CoTe2 Anisotropic properties due to the layered structure 5/23/2013 L.Viciu| ACII| Imprtant structure types 44 NiAs Ni Ni Ni Ni Ni As Ni Ni As Ni Ni ¾ vs. Ni NiAs view on c axis (top view) CdI2 Cd Cd Cd Cd I Ni Ni Cd I Cd Cd Cd 0, 1 0, ½ , 1 ¾ ¼ ¼ CdI2 view on the c axis (top view) 5/23/2013 L.Viciu| ACII| Imprtant structure types 45 CdI2 B A vs. CdCl2 (R-3m) B A C B B A A B C A B A hexagonal close packed anions Cubic close packed anions •2D hexagonal structures with different stacking in the 3rd direction •Layers made of CdX6 octahedra •Between layers only van der Waals interactions L.Viciu| ACII| Imprtant structure types 5/23/2013 46 Compounds with CdCl2 structure Hexagonal structure with c.c.p. anion arrangement therefore not h.c.p. derived! •Chlorides of moderately polarizing cations e.g. MgCl2, MnCl2 •Di-sulfides of quadrivalent cations e.g. TaS2, NbS2 •Cs2O has the anti-cadmium chloride structure Anisotropic properties due to the layered structure 5/23/2013 L.Viciu| ACII| Imprtant structure types 47 Filling voids in h.c.p. structures ½ Oh filled ½ Td filled h.c.p. array CdI2 ZnS all Oh filled all Td filled? NiAs 5/23/2013 L.Viciu| ACII| Imprtant structure types No! 48 A. Structures derived from cubic close packed: 1. 2. 3. 4. NaCl- rock salt CaF2 – fluorite/Na2O- antifluorite diamond ZnS- blende B. Structures derived from hexagonal close packed 1. NiAs – nickel arsenide 2. ZnS – wurtzite 3. CdI2 – cadmium iodide C. Non close packed structures 1. CsCl – cesium chloride 2. MoS2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite L.Viciu| ACII| Imprtant structure types 4.5/23/2013 MgAlO4 - Spinel 49 C1: CsCl- Cesium Chloride (Pm-3m) ½ •Cl- ions form a primitive array Cubic lattice •C.N.: Cs - 8 (cubic) : Cl - 8 (cubic) • One Cl atom at (0,0,0); •One Cs at (1/2,1/2,1/2) •1CsCl unit in the cell Adopted by chlorides, bromides and iodides of large cations: Cs+, Tl+, NH4+ Adopted by intermetallic compounds: CuZn, CuPd, TiX with X=Fe, Co, Ni; etc. 5/23/2013 L.Viciu| ACII| Imprtant structure types 50 C2: MoS2 – Molybdenite (P63/mmc) ¼ , 5/8, 7/8 1/ 8, 3/ 8, Layers of edge shared MoS6 trigonal prisms ¾ Hexagonal layers of S are not close-packed in 3D Hexagonal lattice •2Mo at (2/3,1/3,3/4) and (1/3,2/3,1/4) • 4I at (2/3,1/3,1/8), (2/3,1/3,3/8), (1/3,2/3,5/8) & (1/3,2/3,7/8) •2MoS2 in unit cell •C.N.: Mo - 6 (Trigonal Prismatic) : S 3 (base pyramid) 5/23/2013 L.Viciu| ACII| Imprtant structure types 51 MoS2 vs. CdI2 B A A A B B B A A B A A MoS2 CdI2 Staggered stacks of prisms 5/23/2013 Eclipsed stacks of octahedra L.Viciu| ACII| Imprtant structure types 52 Compounds with MoS2 structure Compounds of type: TX2 where T = transition metal of group IVB, VB or VIB X= S, Se, Te Anisotropic electronic properties due to the layered structure Ion intercalation gives mixed valence materials with interesting physics MoS2, ZrS2, and HfS2 when intercalated with alkali metals become superconducting Li-intercalation in MoS2 changes the coordination of Mo from trigonal prismatic to Oh 5/23/2013 L.Viciu| ACII| Imprtant structure types 53 A. Structures derived from cubic close packed 1. 2. 3. 4. NaCl- rock salt CaF2 – fluorite/Na2O- antifluorite diamond ZnS- blende B. Structures derived from hexagonal close packed 1. NiAs – nickel arsenide 2. ZnS – wurtzite 3. CdI2 – cadmium iodide C. Non close packed structures 1. CsCl – cesium chloride 2. MoS2 - molybdenite D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4.5/23/2013 MgAlO4 - Spinel L.Viciu| ACII| Imprtant structure types 54 D1: Rutile, TiO2(P42/mnm) Chains of edge shared TiO6 Oh on c direction Edge-shared chains are linked by corners Blue spheres Ti4+ Red spheres O2- •O2- ions form a distorted h.c.p. array or a tetragonal structure Two unit cells on top of each other are shown C.N.: Ti - 6 (Oh) : O - 3 (trigonal planar) •Ti4+ fills ½ of the Oh voids ½ • two Ti4+ ions at (0, 0, 0) and (1/2, 1 / 2, 1 /2) • four O2- at ±(0.3, 0.3, 0) and (0.8, 0.2, 1 /2) 0, 1 0,1 ½ 0,1 ½ • 2TiO2 per unit cell (Ti2O4) 5/23/2013 L.Viciu| ACII| Imprtant structure types 55 TiO2 (Rutil): tetragonal structure resulted from h.c.p. distortion TiO2 – is a 3 D structure!!! Strong M-O bonds distortion h.c.p. tetragonal network of corner sharing Oh in a h.c.p. array made of O2- ions with Ti4+ filling ½ of Oh sites in an alternant manner: one full then one empty 5/23/2013 L.Viciu| ACII| Imprtant structure types 56 CdI2 vs. TiO2 h.c.p. array of I- with Cd2+ in ½ Oh voids h.c.p. array of O2- with Ti4+ in ½ Oh voids The Oh voids in one layered empty The Oh voids are alternating in a layer Layered structure 5/22/2013 L.Viciu| ACII| Imprtant structure types 3D structure 57 Examples of TiO2 –type structure adoption Oxides: MO2 (e.g. Ti, Nb, Cr, Mo, Ge, Pb, Sn) Fluorides: MF2 (e.g. Mn, Fe, Co, Ni, Cu, Zn, Pd) Rutile-type oxides with one or more d electrons often display remarkable electronic and magnetic properties. TiO2 Ti4+ (d0)are equidistant MoO2 Mo4+ (d2) One type of M-M bonds (2.96Å) (in Ti metal, Ti-Ti bond is 2.92Å) Alternating short (2.51Å vs 2.725Å in Mo metal) and long M-M bonds TiO2-x anisotropic conductor (extensive overlap of the d-orbitals along c axis and no orbital overlap on the perpendicular direction the conductivity in the ab plane is 3 order of magnitude smaller than on the c axis) 5/22/2013 L.Viciu| ACII| Imprtant structure types 58 Structure -properties relationship in the rutile compounds TiO2-rutile Ti, d0 ion -insulator VO2-rutile type V, d1 ion -metal VO2-monoclinic V, d1 ion in a distorted structure-insulator 2 Ti t2g orbitals overlap with O p orbitals metal-oxygen π band 1 Ti t2g orbital (along the tetragonal c axis) forms nonbonding cation sublattice band (the conduction band, ) (a) empty (b) partially filled by the 2 e- of V (c) split into localized bonding and antibonding levels 5/22/2013 L.Viciu| ACII| Imprtant structure types 59 TiO2 polymorphs Anatase 750 C 915 C Tetragonal *Eg=2.04eV Brookite Rutile Tetragonal *Eg = 1.78eV Orthorhombic *Eg = 2.20eV * Calculated values High refractive index; Excellent optical transmittance in the VIS and NIR region; High dielectric constant; All have been studied for their photocatalytic and photoelectrochemical applications. 5/22/2013 L.Viciu| ACII| Imprtant structure types 60 D2: Rhenium Trioxide, ReO3 (bronzes)(Pm-3m) Black spheres Re6+ Red spheres O2- Corner shared ReO6 Oh •Defective f.c.c. array : one oxygen site on the face C.N.: Re - 6 (Oh) : O - 2 (linear) missing; Cubic lattice 0, ½ , 0 0,1 • Re at (0, 0, 0); 0,1 •3O at (1/2, 0, 0), (0, 1/2, 0), (0, 0, 1/2) •1ReO3 per unit cell 5/22/2013 L.Viciu| ACII| Imprtant structure types 61 Compounds with ReO3 structure Oxides: WO3 , UO3, Fluorides: AlF3, ScF3 , FeF3 , CoF3, MoF3 Others: Sc(OH)3, TaO2F, Cu3N, ternary structures derived from this 3D octahedral network are among the most important in oxide chemistry Re6+ is d1 system and metallic conductivity is expected Ion intercalation/substitution led to mixed oxidation state magnetic and electronic properties Ex: WO3 is a band insulator with a band gap of 2.6 eV WO3-xFx – superconducts at 0.4K (x up to 0.45 Li doped WO3 is metallic Na doped WO3 shows superconductivity (NaxWO3 (0.2 < x < 0.4), 0.7 K < Tc < 3 K 5/22/2013 L.Viciu| ACII| Imprtant structure types 62