Structural changes and relaxations monitored by luminescence

Abstract

Luminescence data have often been used to study imperfections and to characterize lattice distortions because the signals are sensitive to changes of structure and composition. Previous studies have included intentionally added probe ions such as rare earth ions to sense distortions in local crystal fields caused by modified structural environments. An under-exploited extension of this approach was to use luminescence to monitor crystalline phase changes. A current overview of this new and powerful technique shows that continuous scanning of the sample temperatures immediately offered at least three types of signatures for phase transitions. Because of high sensitivity, luminescence signals were equally responsive to structural changes from inclusions and nanoparticles. These coupled to the host material via long-range interactions and modified the host signals. Two frequently observed examples that are normally overlooked are from nanoparticle inclusions of water and CO2. Examples also indicated that phase transitions were detected in more diverse materials such as superconductors and fullerenes. Finally, luminescence studies have shown that in some crystalline examples, high dose ion implantation of surface layers could induce relaxations and/or structural changes of the entire underlying bulk material. This was an unexpected result and therefore such a possibility has not previously been explored. However, the implications for ion implication are significant and could be far more general than the examples mentioned here.

Unexploited Information from Luminescence Spectra

“…The Sussex spectral system has contributed to detection of phase transitions for many materials, as summarised and referenced in our reviews [19][20][21], not just for insulators, but also for fullerenes and high temperature superconductors. In some cases the signals originate via contaminants rather than the target system.…”

“…For CO2 there is a well documented sublimation temperature, ice has a cubic to hexagonal structural change, and the gases all go through the usual shifts from solid to liquid to gas. Numerous such examples have been detected and reviewed [19][20][21]. Note however the phase transitions of nanoparticles may occur at temperatures which vary with particle size.…”

Substrate lattice relaxations, spectral distortions, and nanoparticle inclusions of ion implanted zinc oxide

“…The effects are clear since the phase change provides a discontinuity in the pressure conditions. In many examples, there are wavelength displacements of the spectra, and or major discontinuities in intensity, as reviewed, 15,38 for MgO 39 or for Nd:YAG where both spectral and lattice parameter changes were reported from CO 2 sublimation within nanoparticles, and ice effects were seen. 40 Such features are quite obvious from spectral changes of the host lattice that occur when the host undergoes a phase transition.…”

Temperature dependence of bulk luminescence from ZnO

“…The high sensitivity for luminescence spectral resolution offers discrimination between the variants that are linked to the presence of impurities, and/or structural defects. Less familiar is that the signals are responsive to embedded impurity nanoparticles …”