AbstractThe present study summarizes results of cathodo-luminescence (CL) microscopy and spectroscopy appliedto minerals and materials. CL can be used both in a purelydescriptive way to detect and distinguish different miner-als or mineral generations by their variable CL colours oras an effective method for spatially resolved analysis ofpoint defects in solids by spectral CL measurements. Thecathodoluminescence emission is in all cases either re-lated to lattice defects (e.g. electron defects on brokenbonds, vacancies or radiation induced defects) and/or totrace activator ions such as REE2+/3+, Fe3+, Cr3+, Al3+,Mn2+, Pb2+, Cu2+, Sn2+or uranyl groups. CL spectroscopyis an outstanding method to characterize the degree of pu-rity of materials or to detect trace elements in natural andsynthetic minerals. In this way, alterations, diffusion oftrace elements or formation of new phases are success-fully detectable even in the case of materials with hetero-geneous texture and high contents of non-crystallinephases.Keywords Cathodoluminescence · Microscopy · Spectroscopy · Minerals · MaterialsIntroductionLuminescence (luminescence glow) is a common phe-nomenon in inorganic and organic substances resultingfrom an emission transition of anions, molecules, or acrystal from an excited electronic state to a ground orother state with lesser energy [1]. According to the meth-ods of excitation several types of luminescence can bedistinguished such as photo-, cathodo-, thermo- or X-rayluminescence. Because of the wide range of individual lu-minescence behaviour of mineral species, luminescencetechniques are used for the investigation and interpreta-tion of the composition and structure of minerals and ma-terials. The detection of luminescence spectra in combina-tion with other spectral measurements (e.g. electron para-magnetic resonance, absorption spectrometry) especiallyallows the determination of impurity ions, molecules andother centres in solids as well as the valence of the ions,their coordination and their local symmetry. Because ofthese advantages luminescence techniques have devel-oped into standard analytical techniques in different fieldsof science and industry, and luminescence properties ofminerals have found many applications. The aim of thepresent work is to demonstrate the advantages of cathodo-luminescence (CL) microscopy and spectroscopy in theinvestigation of minerals and materials.Jens GötzePotential of cathodoluminescence (CL) microscopy and spectroscopy for the analysis of minerals and materialsAnal Bioanal Chem (2002) 374:703–708DOI 10.1007/s00216-002-1461-1Received: 25 February 2002 / Revised: 11 June 2002 / Accepted: 24 June 2002 / Published online: 3 September 2002SPECIAL ISSUE PAPERJ. Götze (✉)TU Bergakademie Freiberg, Institute of Mineralogy, Brennhausgasse 14, 09596 Freiberg, Germanye-mail: [email protected]© Springer-Verlag 2002Fig.1a–c Process of charge transfer and luminescence productionin insulator crystals: a excitation of several energy levels by ab-sorption of photons and resulting radiative transitions (lumines-cence emission); b excitation of an electron by high-energy parti-cles or photons from the valence band to the conduction band andrecombination with an activator resulting in luminescence emis-sion (1) or trapping of the electron (2); c thermal or optical stimu-lation of a trapped electron to the conduction band and recombina-tion with an activator (e.g. thermoluminescence)Principles of luminescenceLuminescence processes can be described based on ascheme of the energy levels in a crystal. In insulators andsemiconductors, a band gap (forbidden band) exists be-tween the valence and conduction bands. A preconditionfor luminescence is the existence of activators (impurityions, lattice defects), which occupy discrete energy levelsin this forbidden zone between the valence and conduc-tion bands (Fig.1). These luminescence centres in miner-als are defect centres which may be intrinsic (e.g. elec-tron-hole centres) or impurity-related extrinsic ones, whichare classified according to electronic structure: (1) transi-tion metal ions (e.g. Mn2+, Cr3+, Fe3+), (2) rare earth ele-ments (REE), (3) actinides (especially uranyl UO22+), (4) heavy metals (e.g. Pb2+), (5) electron-hole centres(molecular ions S2–, O2–, F-centres) and (6) crystallophos-phors of the ZnS type (sphalerite, cinnabar, realgar) [1].More extended defects such as dislocations and clustersmay also take part in the luminescence production process[1, 2]. The occurrence of luminescence can be related to threeelementary processes: excitation (absorption), emissionand radiationless transitions. When exciting the crystalwith various kinds of energy, the ions with unfilled shellspass from the ground state to the excited state, which is at-tended by the appearance of an absorption band in the op-tical spectrum (excitation/absorption). The ions can returnfrom the excited to the ground state by emissive transi-tions or through radiationless transitions (absorption oremission of lattice vibrations = phonons). In the case ofemissive transition, the wavelength of the emitted light(photon energy) depends on the energy difference be-tween excited and ground state.Due to the interaction of the activator ion with the sur-rounding crystal field, some of the excitation energy istransferred to the crystal lattice resulting in a shift (Stokesshift) of the emission band in relation to the correspond-ing absorption band towards longer wavelengths. The ac-tivator–ligand distances in the different states and theslope of the energy levels depend on the intensity of thecrystal field (expressed as crystal field splitting ∆=10Dq).The stronger the interaction of the activator ion with thelattice, the greater the Stokes shift and the width of theemission line. Factors influencing values of ∆ or 10Dq arethe type of cation, the type of ligand, the interatomic dis-tance, pressure, temperature and the symmetry of the lig-and environment [3].Natural samples commonly contain a great variety ofdifferent centres which result in complex emission spectrathat are often difficult to interpret. An interaction betweentwo or more activator ions present in a crystal can takeplace resulting in changes in their luminescence spectra704Fig.2 SEM micrographs ofSE, BSE, and CL images of azircon grain (ZrSiO4). Internalstructures are only visible withBSE and CL. Note that BSEand CL intensities show a re-versed
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