1 Lecture: Solid State Chemistry (Festkörperchemie) Part 2 (Further spectroscopical methods, ) H.J. Deiseroth, SS 2004

2 Orders of magnitude in size  microscopic techniques

3 Orders of magnitude in energy  spectroscopic techniques

4 Orders of magnitude in electrical conductivity

5 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

6 Impedance spectroscopy (electrical quantities) The direct current (dc) resistance (R) is defined as: 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) = U 0  sin  t (t: time,  = 2  with : frequency) I(t) = I 0  sin(  t+  ) (  : phase angle) If only capacities are present the phase shift between voltage and current amounts to  /2 = further details in a separate talk by Holger Mikus this afternoon

7 ESCA: Photoemission or Photoelectron spectroscopy Basic equation: E out = h - E bind. e-e- UV or X-Ray solid UHV (E out ) h E bind. E out Int. E bind. h - the higher the binding energy (E bind. ) the lower the E out ! - ESCA is in particular a surface sensitive method (UHV !) ESCA = Electron Spectroscopy for Chemical Analysis strong weak

8 ESCA: Photoemission or Photoelectron Spectroscopy Exciting RadiationOutcoming 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

9 Synchroton Storage Ring

10 Synchroton Radiation Source of radiation Magnitude of wavelength Type of radiation

11 ESCA: Photoemission or Photoelectron spectroscopy - analysis of the energy levels of electrons in molecules („chemical shift“) - band structure of solids E out E bind. S 2 O 3 2- KCr 3 O 8 E out E bind.

12 ESCA: Photoemission or Photoelectron spectroscopy e-e- UV or X-Ray solid UHV UPS spectrum DOS (below E F, 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

13 Moessbauer Spectroscopy  the nucleus of the specific isotope of an atom embdedded in a solid (e.g. 57 Fe) is excited by  -rays emitted by an instable isotope of a neighbor element (e.g. 57 Co) source: e.g. 57 Co (tunable) absorber: e.g. 57 Fe frequently applied for  57 Fe, 119 Sn, 127 J...  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

14 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

15 Spectroscopical methods associated with specific physical effects at/near characteristical X-ray absorption edges: EXAFS: Extended X-Ray Absorption Fine Structure XANES: X-Ray Absorption Near Edge Structure - tunable synchroton radiation in the X-Ray region necessary EXAFS and XANES

16 Thermal Analysis DTA: Differential Thermal Analysis

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

18 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)