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 • T Td sites in the f.c.c . arrangement anions (fcc unit cell) of anions •4 Oh sites in total •8 Td sites in total 1 •Location: on the body diagonals – two • location: 12 1(centre)  4 on each body diagonal at ¼ of the 4 (edge) distance from each end. 5/23/2013 L.Viciu| ACII| Imprtant structure types 3 A-1. Rock salt: NaCl (halite), Sp. Group, Fm-3m Ionic structure

r  1.81 Cl

r   0.95 Na

r  Na  0.52  r  Cl Red balls are Cl- + Purple balls are Na Edge shared Oh Na Oh coordinated 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  4 NaCl per unit cell

5/23/2013 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)

2+ 1 Ca : 8 x /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

- 1 F : 8 x /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

2- ZrO2 stabilized with CaO or Y2O3: conduction through O

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 0,1 are empty) ¾ ¼ 1 0,1 ½ C: 8 x /8+6 x ½ = 4 ¼ 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 Lattice Melting Conductor? Eg(eV) constant (Å) Point (ºC) Carbon - 3.56 3550 Insulator 5.4 diamond radius Silicon 5.43 1410 Semiconductor 1.1 Germanium 5.66 940 Semiconductor 0.7 -Tin 6.49 230 Zero gap 0 semiconductor 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 Zinc Blende

5/23/2013 L.Viciu| ACII| Imprtant structure types 12 A-4. ZnS- Zinc Blende (Sphalerite) Similar with diamond structure A

Corner shared ZnS Td Red spheres – S2- 4 Layers of ZnS4 Td stacked Green spheres – Zn2+ ..ABCABC.. •S2- form the c.c.p. array •Zn2+ fills ½ of the Td voids (Oh voids are empty) • 1 S: 8 x /8+6 x ½ = 4 The crystal may be thought of as two interpenetrating •Zn: 4 x 1 = 4 fcc lattices, one for sulfur the other for zinc, with their •C.N.: 4 origins displaced by one quarter of a body diagonal. 5/23/2013 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 Stressed ZnS4 Td 5/23/2013 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) -conductivity Eg / kT -mobility  ~ e 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 16 5/23/2013 L.Viciu| ACII| Imprtant structure types Mobility () for rock salt and zinc blende type materials

Eg / kT

 ~ e  In materials free of defects, the mobility is

determined by the effective mass interaction difference difference with lattice vibration

1. Mobility  as the molecular weight 

(heavy mass gives low scattering) Electronegativity

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) 17 5/23/2013 L.Viciu| ACII| Imprtant structure types 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

Graphite pressureDiamond Zinc blende type : InAs,CdS,CdSe pressure NaCl type

3 Graphite : C.N.= 3; dC-C = 1.415Å; =2.26g/cm 3 Diamond: C.N. = 4 dC-C = 1.54Å;  = 3.51g/cm 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 • T Td sites in the f.c.c . arrangement anions (fcc unit cell) of anions •4 Oh sites in total •8 Td sites in total 1 •Location: on the body diagonals – two • location: 12 1(centre)  4 on each body diagonal at ¼ of the 4 (edge) distance from each end. 5/23/2013 L.Viciu| ACII| Imprtant structure types 23 Filling voids in c.c.p. structures CaF ZnS 2

all Td ½ Td

Li3Bi NaCl ½ Td c.c.p.

all Td and all Oh all Oh L.Viciu| ACII| Imprtant structure types 24 5/23/2013 Ulrich  all Td sites filled Müller: “Inorganic structural chemistry”

 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 the formulas5/23/2013 form the cubic closest-packing”L.Viciu| ACII| Imprtant structure types 25 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. 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 26 5/23/20134. MgAlO 4 - Spinel L.Viciu| ACII| Imprtant structure types Voids in h.c.p. structure

A The spacing of the close packed

2 1 1 layers: ( , , ) B 3 3 2 d = √8r/√3 = 1.633r

c=2x1.633r=2x1.633xa/2=1.633a A c/a=1.633 The voids are identical to the ones found in FCC

1 2 3 ( /3, /3, /4)

5 7 (0,0, /8), (⅔,⅓, /8) B 3 (0,0, /8) A 1 1 2 (⅔, ⅓, /8), ( /3, /3, ¼) Oh void 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) •Zn2+ fills ½ of Td voids (T or T ) •Layers of ZnS4 Td stacked ..ABAB… + - •Alternate layers are rotated by 180ᵒ 2 1 •S: at (0,0,0) and ( /3, /3, ½) about c axis relative to each other. - 2 1 1 5 •Zn : at ( /3, /3, /8) and (0,0, /8) •c/a = 1.636 (the ideal c/a=1.633) 28 5/23/2013 L.Viciu| ACII| Imprtant structure types 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 2 3 ( /3, /3, /8)

5 (0,0, /8)

1 2 ( /3, /3 ,0)

5/23/2013 L.Viciu| ACII| Imprtant structure types 30 Different view of the Wurtzite structure

- 1 2 2 1 •Zn : 2 x ½ + 1=2 per cell at ( /3, /3 ,0) + h.c.p translation ( /3, /3, ½) 1 2 3 2 1 7 •S: 2 x 1 = 2 per unit cell at ( /3, /3, /8) + h.c.p translation ( /3, /3, /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 Zn is Td coordinated corner shared Td but the shared Td layers are rotated by 180ᵒ relative to each other …ABCABC… …ABAB… 3  = 4.11g/cm 3  = 3.98g/cm 33 5/23/2013 L.Viciu| ACII| Imprtant structure types 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 (A = constant in the lattice energy formula which depends on the crystal geometry. It is the sum of a i.e. Awurtzite = 1.641 series of numbers representing the number of nearest neighbors and their relative distance from a Azinc blende = 1.638 given ion)

zinc blende wurtzite  m.p. sublimes at t=1185C 1850C Eg 3.68eV 3.911eV

5/23/2013 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  3 and 7 are arsenic ions common to two Oh chains of face sharing Oh along c •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)

1 2 1 •2As at (0,0,0) and ( /3, /3, /2 2 1 1 2 1 3 •2Ni at ( /3, /3, /4) and ( /3, /3, /4) •Edge sharing AsNi trigonal prisms •C.N.: Ni 6 (octahedral) : As 6 (trigonal prismatic) 6 37 5/23/2013 L.Viciu| ACII| Imprtant structure types NiAs – alternative views

I. II. As Ni 1 3 3 /4, /4 /4 1/ 1/ Ni 2 4 0,1 As 1 0, /2 As Ni

I. Ni at the corners of the hexagonal cell. II. As’s form the hexagonal close packed sub- One As is in the center of a hexagonal prism lattice, which is interpenetrated by a primitive formed by six Ni atoms. hexagonal sublattice of the metal (Ni) atoms. The result is doubling of the repeat unit in the c- direction. 2NiAs per unit cell (Z=2)

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 41 L.Viciu| ACII| Imprtant structure types B3: CdI2: Cadmium Iodide (P-3m1) B

A

B

Cd2+ I- A • I form the h.c.p. array • one Cd at (0,0,0); • 2+ Cd fills ½ of Oh voids • Two I:

• Hexagonal lattice 2 1 1 1 2 3 ( /3, /3, /4); ( /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.: Cd  6 (Octahedral)  6:3 Cd ion in the highlighted I  3 (base pyramid) sulfur unit cell

5/23/2013 43 L.Viciu| ACII| Imprtant structure types 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 vs. CdI2

Ni Ni Ni Ni Cd Cd NiAs view on c Cd Cd As axis (top view) I Ni Ni Ni Ni

As I Ni Ni Cd Cd Ni Ni Cd Cd 0, 1 0, ½ , 1 ¾ ¾ ¼ ¼

CdI2 view on the c axis (top view)

5/23/2013 45 L.Viciu| ACII| Imprtant structure types CdI2 vs. CdCl2 (R-3m) B B A 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 46 5/23/2013 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

CdI 2 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. 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

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 5/23/2013 L.Viciu| ACII| Imprtant structure types 49 4. MgAlO4 - Spinel C1: CsCl- Cesium Chloride (Pm-3m)

½

•Cl- ions form a primitive array  Cubic lattice • One Cl atom at (0,0,0); •C.N.: Cs - 8 (cubic) : Cl - 8 (cubic)

1 1 1 •One Cs at ( /2, /2, /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.

50 5/23/2013 L.Viciu| ACII| Imprtant structure types C2: MoS2 – Molybdenite (P63/mmc)

5 7 ¼ , /8, /8 Layers of edge shared MoS6 trigonal prisms 1 3 /8, /8, ¾ Hexagonal layers of S are not close-packed in 3D Hexagonal lattice

2 1 3 1 2 1 •2Mo at ( /3, /3, /4) and ( /3, /3, /4) 2 1 1 2 1 3 1 2 5 1 2 7 • 4I at ( /3, /3, /8), ( /3, /3, /8), ( /3, /3, /8) & ( /3, /3, /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 Eclipsed stacks of octahedra

5/23/2013 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. 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

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 5/23/2013 L.Viciu| ACII| Imprtant structure types 54 4. MgAlO4 - Spinel 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- Two unit cells on top of each other are shown •O2- ions form a distorted h.c.p. array or a C.N.: Ti - 6 (Oh) : O - 3 (trigonal planar) tetragonal structure •Ti4+ fills ½ of the Oh voids 0, 1 ½ 0,1 4+ 1 1 1 • two Ti ions at (0, 0, 0) and ( /2, / 2, /2) ½ 2- 1 0,1 ½ • four O at ±(0.3, 0.3, 0) and (0.8, 0.2, /2)

• 2TiO2 per unit cell (Ti2O4) 55 5/23/2013 L.Viciu| ACII| Imprtant structure types TiO2 (Rutil): tetragonal structure resulted from h.c.p. distortion TiO – is a 3 D structure!!! 2 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  3D structure

5/22/2013 L.Viciu| ACII| Imprtant structure types 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.

4+ 0 TiO2 Ti (d )are equidistant One type of M-M bonds (2.96Å) (in Ti metal, Ti-Ti bond is 2.92Å)

4+ 2 Alternating short (2.51Å vs 2.725Å in Mo metal) MoO2 Mo (d ) 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

TiO -rutile VO2-rutile type 2 VO2-monoclinic

V, d1 ion -metal V, d1 ion in a distorted Ti, d0 ion -insulator 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 Rutile Brookite

  750 C 915C 

Tetragonal Tetragonal Orthorhombic *Eg=2.04eV *Eg = 1.78eV *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+ Corner shared ReO Oh Red spheres O2- 6 •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 • Re at (0, 0, 0); 0,1

1 1 1 0,1 •3O at ( /2, 0, 0), (0, /2, 0), (0, 0, /2)

•1ReO3 per unit cell 61 5/22/2013 L.Viciu| ACII| Imprtant structure types Compounds with ReO3 structure

ternary structures derived from this 3D octahedral Oxides: WO3 , UO3, network are among the most Fluorides: AlF3, ScF3 , FeF3 , CoF3, MoF3 important in oxide chemistry Others: Sc(OH)3, TaO2F, Cu3N, 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