important structure types - eth zn.ethz.ch/~nielssi/download/4. semester/ac ii... · 4. zns- blende...

Important Structure Types

1 5/23/2013 L.Viciu| ACII| Imprtant structure types

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


• 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.

T • Oh sites in f.c.c. arrangement of anions (fcc unit cell) •4 Oh sites in total • location:

O

41124

1)(

)(

centreedge

Voids in f.c.c. structure


A-1. Rock salt: NaCl (halite), Sp. Group, Fm-3m

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

95.0

81.1

Na

Cl

r

r

4 NaCl per unit cell

Edge shared Oh

Ionic structure

4

dcoordinateOhNa

r

r

Cl

Na

52.0

Red balls are Cl-

Purple balls are Na+

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

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.


A-2. CaF2-fluorite/Na2O antiflorite (Fm-3m)

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

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)

In the Anti-Fluorite (Na2O) structure, Cation and Anion positions are reversed!

Ionic compound

6

Edge shared FCa4 Td

Corner shared CaF8 cubes


• Fluorite: Halides: SrF2, SrCl2, BaF2, BaCl2, CdF2, HgF2

Oxides: PbO2, CeO2, PrO2,ThO2

Compounds with CaF2 (fluorite) and Na2O (antifluorite) structure:


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

• Antifluorite: Oxides: Li2O, Na2O, K2O, Rb2O Chalcogenides: Li2S, Li2Se, Na2S, Na2Se, Na2Te, K2S, K2Se, K2Te

ZrO2 stabilized with CaO or Y2O3: conduction through O2-

8

Batteries = energy conversion + energy storage Solid oxide fuel cells = energy conversion

http://www.gepower.com/research/seca/sofc_research.htm

Fluorite type compounds: Fast Ionic Conductors

High mobility of anion vacancies gives rise to fast ionic (anionic) conduction in fluorite type structure.

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

are empty)

C: 8 x 1/8+6 x ½ = 4

C: 4 x 1 = 4

C.N.: 4

The most stable covalent structure


0,1

0,1 ½

½

¼

¼

¾

¾

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) 10 5/23/2013 L.Viciu| ACII| Imprtant structure types

Lattice constant (Å)

Melting

Point (ºC)

Conductor? 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

Group 4 of elements: Si, Ge and -Sn

radius

Compounds with diamond like structure

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)

All have the cubic structures (space group: Fd-3m)


Changing the motif in diamond structure

5/23/2013 L.Viciu| ACII| Imprtant structure types 12

diamond Zinc Blende

A-4. ZnS- Zinc Blende (Sphalerite) Similar with diamond structure

Layers of ZnS4 Td stacked

..ABCABC..

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.

13

•S2- 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

Red spheres – S2- Green spheres – Zn2+


Corner shared ZnS4 Td

A

• CuF, CuCl, -CuBr, -CuI, -AgI

• -MnS red, -MnSe, BeS, , ZnS,

• -SiC, BN, BP

• III-V compounds: GaP, GaAs, GaSb, InP, InAs, InSb

Compounds with Zinc Blende- type structure

Note: Crystals containing tetrahedral groups are often piezoelectric (a Td symmetry doesn’t have an inversion center).

14

Unstressed ZnS4 Td Stressed ZnS4 Td

i.e. Zinc blende is piezoelectric


This small cation structure is found for small metallic elements, which tend to form strong sp3 covalent bonds.

Most semiconductors of commercial importance are isomorphous with diamond and zinc blende

Structure – electronic properties relations important for evaluating:

Band gap

Mobility


16

Band Gap (Eg)

Band gap generally increases with ionicity

Eg increases with increasing the

electronegativity difference between

constituent ions.

kTEge /~ -conductivity -mobility Eg-Band gap T- temperature K-Boltzman constant


Band gap increases with ionicity

Covalent semiconductors have narrow Eg

Generally, band gap and transparency are interconnected

kTEge /~

Ele

ctro

ne

gati

vity

dif

fere

nce

Mobility () for rock salt and zinc blende type materials

2. Mobility as the electronegativity difference btw ions (polarization effect

of mobile electrons or holes on the surrounding atoms) 17

In materials free of defects, the mobility is determined by the effective mass interaction with lattice vibration

Compounds with ionic bonding have low electron mobility

L.Viciu| ACII| Imprtant structure types

1. Mobility as the molecular weight

(heavy mass gives low scattering)

5/23/2013

GaAs

ZnS (Zinc Blende) Structure

4 Ga atoms at (0,0,0)+ FCC translations

4 As atoms at (¼,¼,¼)+FCC translations

Bonding: covalent, partially ionic

Silicon

Diamond Cubic Structure

4 atoms at (0,0,0)+ FCC translations

4 atoms at (¼,¼,¼)+FCC translations

Bonding: covalent

Typical Semiconductors


19

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.


• Why semiconductors have diamond or ZnS –blende structure?


• 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).


Structural Changing

• 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”

22

Graphite : C.N.= 3; dC-C = 1.415Å; =2.26g/cm3

Diamond: C.N. = 4 dC-C = 1.54Å; = 3.51g/cm3

typeNaClCdSeCdSInAstypeblendeZinc

DiamondGraphite

pressure

pressure

,,:


U. Müller-Inorganic Structural Chemistry

• 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.

T • Oh sites in f.c.c. arrangement of anions (fcc unit cell) •4 Oh sites in total • location:

O

41124

1)(

)(

centreedge

Voids in f.c.c. structure


24

all Td

½ Td

ZnS CaF2

all Oh

NaCl

all Td and all Oh

Li3Bi

c.c.p.

Filling voids in c.c.p. structures


½ Td


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 formulas form the cubic closest-packing”

Ulrich Müller: “Inorganic structural chemistry”

all Td sites filled

½ of the Td sites filled

¼ of the Td sites filled

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 4. MgAlO4 - Spinel 26


Voids in h.c.p. structure

Td void

The voids are identical to the ones found in FCC

Octahedral voids occur in 1 orientation, tetrahedral voids occur in 2 orientations 27

The spacing of the close packed layers: d = √8r/√3 = 1.633r c=2x1.633r=2x1.633xa/2=1.633a c/a=1.633 A

B

A

(0,0,5/8), (⅔,⅓,7/8)

(⅔, ⅓,1/8),

(0,0,3/8)

Oh void (1/3, 2/3, ¼)

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

A

B

)2

1,

3

1,

3

2(


B-1. Wurtzite (ZnS) (P63mc)

•Layers of ZnS4 Td stacked ..ABAB… •Alternate layers are rotated by 180ᵒ about c axis relative to each other.

28

•S2- form the h.c.p. array (c/a=1.633)

•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)

A

B

S2--yellow spheres Zn2+-green spheres

A


Zn neighbors in Wurtzite structure


nearest neighbors: 4 S ions Next nearest neighbors: 12 Zn ions

1

(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)

S2--yellow spheres Zn2+-green spheres

Two unit cells of the Wurtzite structure

30

(1/3, 2/3, 3/8)

(1/3, 2/3 ,0)

(0,0,0)

(0,0, 5/8)


31

•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)

Different view of the Wurtzite structure


• ZnO, ZnS, ZnSe, ZnTe

• BeO

• CdS, CdSe, MnS, MnSe

• AgI

• AlN, GaN, InN, TlN,

• SiC

Compounds with wurtzite type structure

32

The highlighted blue compounds are piezoelectrics

The symmetry of the wurtzite type structure allows for a distortion along the c axis distorted Td


33

Zinc Blende vs. Wurtzite

Zn is Td coordinated corner

shared Td

…ABCABC…

Zn is Td coordinated corner shared Td but the

layers are rotated by 180ᵒ relative to each other

…ABAB…

•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


= 4.11g/cm3 = 3.98g/cm3

Zn next nearest neighbors in zinc blende structure


Zinc Blende vs. Wurtzite


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

(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)

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

Covalent compounds with tendency towards lattice instability as ionicity increases

37

B2. NiAs- Nickel Arsenide(P63/mmc)

• 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)

•NiAs6 Oh share opposite faces chains of face sharing Oh along c •Chains of edge shared Oh in the ab plane

5, 7 and 8 are arsenic ions common to two Oh; 3 and 7 are arsenic ions common to two Oh

•Edge sharing AsNi6 trigonal prisms


5/23/2013 38 L.Viciu| ACII| Imprtant structure types

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)

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!

1/4

3/4

0, 1/2

As

Ni

Ni

Ni

As

As

1/2

1/4, 3/4

0,1

NiAs – alternative views

I. II.

II. As’s form the hexagonal close packed sub-lattice, which is interpenetrated by a primitive hexagonal sublattice of the metal (Ni) atoms.

Compounds with NiAs type structure

39

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


c/a < 1.633 due to metallic bonding on c direction

Overlap of 3d orbitals gives rise to metallic bonding.

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.

Most NiAs type materials are metallic.

A.West: page 249

NiAs vs. NaCl

A

A

B

B


A

B

C

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

B3: CdI2: Cadmium Iodide (P-3m1)

42

• I form the h.c.p. array

• Cd2+ fills ½ of Oh voids

• Hexagonal lattice

•1CdI2 in the unit cell

I-

Cd2+

A

B

A

B

• one Cd at (0,0,0);

• Two I:

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

C.N.: Cd - 6 (Octahedral) : I - 3 (base pyramid) 5/23/2013 L.Viciu| ACII| Imprtant structure types

43

Alternative views

C.N.: Cd 6 (Octahedral) I 3 (base pyramid)

6:3 Cd ion in the highlighted sulfur unit cell


¾

0,1

¼

Compounds with CdI2 structure

44

•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


van der Waals attraction between neighboring iodine layers

The structure is stabilized by highly covalent interactions and large, polarizable

anions

Anisotropic properties due to the layered structure

NiAs vs. CdI2

45

CdI2 view on the c axis (top view)

NiAs view on c axis (top view)


Ni

As

As

Ni

Ni

Ni Ni Ni

Ni Ni Ni

Ni Ni Ni

Cd

I

I

Cd

Cd Cd

Cd Cd

Cd Cd

¼

¾ 0, ½ , 1

¼

¾ 0, 1

CdI2 vs. CdCl2 (R-3m)

46

•2D hexagonal structures with different stacking in the 3rd direction •Layers made of CdX6 octahedra •Between layers only van der Waals interactions

A

B

C

A

B

C

A

B

A

B

A

B

A

B


Cubic close packed anions hexagonal close packed anions

Compounds with CdCl2 structure

47

•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


Hexagonal structure with c.c.p. anion arrangement therefore not

h.c.p. derived!

Anisotropic properties due to the layered structure


Filling voids in h.c.p. structures

h.c.p. array

½ Oh filled

all Oh filled

½ Td filled

all Td filled?

No!

CdI2

NiAs

ZnS

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


D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4. MgAlO4 - Spinel 49 5/23/2013 L.Viciu| ACII| Imprtant structure types

50

C1: CsCl- Cesium Chloride (Pm-3m)

½

•Cl- ions form a primitive array Cubic lattice

• 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.


•C.N.: Cs - 8 (cubic) : Cl - 8 (cubic)

C2: MoS2 – Molybdenite (P63/mmc)

51

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) L.Viciu| ACII| Imprtant structure types

1/8, 3/8, ¾

¼ , 5/8, 7/8 Layers of edge shared MoS6 trigonal prisms

5/23/2013

MoS2 vs. CdI2

MoS2

A

B

A

B

A

B

CdI2

Staggered stacks of prisms Eclipsed stacks of octahedra

A

A

B

B

A

A



Compounds with MoS2 structure

Compounds of type: TX2

T = transition metal of group IVB, VB or VIB X= S, Se, Te

where

Anisotropic electronic properties due to the layered structure

Ion intercalation gives mixed valence materials with interesting physics

Li-intercalation in MoS2 changes the coordination of Mo from trigonal prismatic to Oh

MoS2, ZrS2, and HfS2 when intercalated with alkali metals become superconducting

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


D. Metal oxide structures 1. TiO2- rutile 2. ReO3 – rhenium trioxide 3. CaTiO3 – perovskite 4. MgAlO4 - Spinel 54 5/23/2013 L.Viciu| ACII| Imprtant structure types

D1: Rutile, TiO2(P42/mnm)

•O2- ions form a distorted h.c.p. array or a

tetragonal structure

•Ti4+ fills ½ of the Oh voids

• 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)

• 2TiO2 per unit cell (Ti2O4) 55

Chains of edge shared TiO6 Oh on

c direction

Edge-shared chains are linked by corners

Blue spheres Ti4+

Red spheres O2-


½

½

½ 0,1

0,1 0, 1

C.N.: Ti - 6 (Oh) : O - 3 (trigonal planar)

Two unit cells on top of each other are shown

56

h.c.p.

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

TiO2 (Rutil): tetragonal structure resulted from h.c.p. distortion

tetragonal


TiO2 – is a 3 D structure!!!

distortion

Strong M-O bonds

CdI2 vs. TiO2

h.c.p. array of I- with Cd2+ in ½ Oh voids

The Oh voids in one layered empty

Layered structure

h.c.p. array of O2- with Ti4+ in ½ Oh voids

The Oh voids are alternating in a layer

3D structure 5/22/2013 57 L.Viciu| ACII| Imprtant structure types

Examples of TiO2 –type structure adoption

58

Oxides: MO2 (e.g. Ti, Nb, Cr, Mo, Ge, Pb, Sn)

Fluorides: MF2 (e.g. Mn, Fe, Co, Ni, Cu, Zn, Pd)

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)


Rutile-type oxides with one or more d electrons often display remarkable electronic and magnetic properties.

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

Ti4+ (d0)are equidistant

Mo4+ (d2)

TiO2

MoO2


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

Structure -properties relationship in the rutile compounds

TiO2-rutile VO2-rutile type VO2-monoclinic

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

structure-insulator

Brookite Anatase Rutile

60

TiO2 polymorphs

C915 C750

Tetragonal *Eg = 1.78eV

Tetragonal *Eg=2.04eV

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

D2: Rhenium Trioxide, ReO3 (bronzes)(Pm-3m)

•Defective f.c.c. array : one oxygen site on the face

missing; Cubic lattice

• Re at (0, 0, 0);

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

•1ReO3 per unit cell

61

Black spheres Re6+

Red spheres O2- Corner shared ReO6 Oh


C.N.: Re - 6 (Oh) : O - 2 (linear)

0, ½ , 0

0,1

0,1


Compounds with ReO3 structure

Oxides: WO3 , UO3, Fluorides: AlF3, ScF3 , FeF3 , CoF3, MoF3 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

ternary structures derived from this 3D octahedral network are among the most

important in oxide chemistry

important structure types - eth zn.ethz.ch/~nielssi/download/4. semester/ac ii... · 4. zns- blende...

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