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Lecture: Solid State Chemistry(Festkörperchemie)

Part 2

(Further spectroscopical methods, 15.7.04)

H.J. Deiseroth, SS 2004

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Orders of magnitude in size microscopic techniques

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Orders of magnitude in energy spectroscopic techniques

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Orders of magnitude in electrical conductivity

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Impedance spectroscopy (basic aspects)

Purpose: Exploring the electrical behavior of a microcrystalline solid sample as function of an alternating current (ac) with a variable frequency.

(note: difference between ac-/dc- and ionic/electronic conduction !!!)

three basically different regions for the exchange interactions between current and sample:

a) inside the grains („bulk“)

b) at grain boundaries

c) surface of the electrodes

the electrical behavior is simulated by a suitable combination of RC circuits: R = resistivity, C = capacity

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Impedance spectroscopy (electrical quantities)

The direct current (dc) resistance (R) is defined as:

I

UR U: applied voltage (V), I: curent (A)

R: resistance ()

The alternative quantity for an alternating current (ac) is the impedance (Z*) which, however, is a more complex quantity than R because there is a phase shift between U and I in general.

ac-voltage: U(t) = U0sint (t: time, = 2 with : frequency) I(t) = I0 sin(t+) (: phase angle)

If only capacities are present the phase shift between voltage and current amounts to /2 = 900.

further details in a separate talk by Holger Mikus this afternoon

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ESCA: Photoemission or Photoelectron spectroscopy

Basic equation: Eout = h - Ebind.

e-UV or X-Ray

solid

UHV

(Eout)

h

Ebind.

Eout

Int.

Ebind.

h

- the higher the binding energy (Ebind.) the lower the Eout !

- ESCA is in particular a surface sensitive method (UHV !)

ESCA = Electron Spectroscopy for Chemical Analysis

strong weak

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ESCA: Photoemission or Photoelectron Spectroscopy

Exciting Radiation Outcoming Radiation

UV (~ 20 eV) UPS electrons from occupied

valence states

X-Ray (~ 10 keV) XPS electrons from (occupied) core states

-commercial laboratory based spectrometers (UHV-technique) are available but relatively expensive and of limited versatility !

- more promising for the future is the use of synchroton radiation: continuous spectrum of exciting radiation (UV X-Ray)

- intensity of synchroton radiation is orders of magnitudes higher ! (e.g. angle resolved detection of outcoming radiation is possible - detection of „orbital shapes“)

- polarization of synchroton radiation allows spin polarized experiments

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Synchroton Storage Ring

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Synchroton Radiation

Source of radiation

Magnitude of wavelength

Type of radiation

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ESCA: Photoemission or Photoelectron spectroscopy

- analysis of the energy levels of electrons in molecules („chemical shift“)- band structure of solids

Eout

Ebind.

S2O32-

KCr3O8

Eout

Ebind.

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ESCA: Photoemission or Photoelectron spectroscopy

e-UV or X-Ray

solid

UHV

UPS spectrum

DOS (below EF, occupied states only)

Band structure

(chemical bonding in different directions of a crystalline solid)

The „energy spectrum“ of the emitted electrons is analyzed !

- energy- momentum- spin energy

ESCA = Electron Spectroscopy for Chemical Analysis

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Moessbauer Spectroscopy

the nucleus of the specific isotope of an atom embdedded in a solid (e.g. 57Fe) is excited by -rays emitted by an instable isotope of a neighbor element (e.g. 57Co)

source: e.g. 57Co(tunable)

absorber: e.g. 57Fe frequently applied for

57Fe, 119Sn, 127J ...

chemical surrounding (symmetry, coordination number, oxidation state, magnetism) of atoms with these nuclei in a solid can be probed in a highly sensitive way (~10-8 eV)

„Chemical shift“ and „Hyperfine splitting“

Doppler effect

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Moessbauer Spectroscopy

Two major informations from Moessbauer spectra:

a) „Chemical Shift“(not to be confused with the same term in NMR and ESCA) oxidation state

b) Hyperfine Splitting magnetic interactions

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Spectroscopical methods associated with specific physical effects at/near characteristical X-ray absorption edges:

EXAFS: Extended X-Ray Absorption Fine StructureXANES: X-Ray Absorption Near Edge Structure

- tunable synchroton radiation in the X-Ray region necessary

EXAFS and XANES

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Thermal Analysis

DTA: Differential Thermal Analysis

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TG: Thermogravimetry(mass change during heating or cooling combined with DTA)

Hysteresis

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Thermal Analysis

DSC: Differential Scanning Calorimetry- Quantitative measurement of enthalpy changes

TMA: Thermo Mechanical Analysis („dilatometry“)- Mechanical changes that occur upon temperature changes

(see lab courses in inorganic chemistry and construction materials chemistry)