Lecture: Solid State Chemistry (Festkörperchemie) Part 2 (Further spectroscopical methods, 15.7.04) H.J. Deiseroth, SS 2004 1 Orders of magnitude in size microscopic techniques 2 Orders of magnitude in energy spectroscopic techniques 3 Orders of magnitude in electrical conductivity 4 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 5 Impedance spectroscopy (electrical quantities) The direct current (dc) resistance (R) is defined as: U R I 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 I(t) = I0 sin(t+) (t: time, = 2 with : frequency) (: phase angle) further details in a separate talk by Holger Mikus this afternoon If only capacities are present the phase shift between voltage and 6 current amounts to /2 = 900. ESCA: Photoemission or Photoelectron spectroscopy ESCA = Electron Spectroscopy for Chemical Analysis Basic equation: Eout = h - Ebind. UV or X-Ray h Int. e- (Eout) UHV solid Ebind. strong h Eout weak Ebind. - the higher the binding energy (Ebind.) the lower the Eout ! - ESCA is in particular a surface sensitive method (UHV !) 7 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 experiments8 Synchroton Storage Ring 9 Synchroton Radiation Source of radiation Magnitude of wavelength Type of radiation 10 ESCA: Photoemission or Photoelectron spectroscopy - analysis of the energy levels of electrons in molecules („chemical shift“) - band structure of solids S2O32- KCr3O8 Eout Ebind. Eout Ebind. 11 ESCA: Photoemission or Photoelectron spectroscopy ESCA = Electron Spectroscopy for Chemical Analysis UV or X-Ray e- UPS spectrum UHV DOS (below EF, occupied states only) solid The „energy spectrum“ of the emitted electrons is analyzed ! - energy - momentum - spin Band structure (chemical bonding in different directions of a crystalline solid) energy 12 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) „Chemical shift“ and „Hyperfine splitting“ source: 57Co e.g. (tunable) Doppler effect 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) 13 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 14 EXAFS and XANES 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 15 Thermal Analysis DTA: Differential Thermal Analysis 16 TG: Thermogravimetry (mass change during heating or cooling combined with DTA) Hysteresis 17 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) 18