EPR Evidence of Liquid Water in Ice: An Intrinsic Property of Water or a Self-Confinement Effect?

Liquid water (LW) existence in pure ice below 273 K has been a controversial aspect primarily because of the lack of experimental evidence. Recently, electron paramagnetic resonance (EPR) has been used to study deeply supercooled water in a rapidly frozen polycrystalline ice. The same technique can also be used to probe the presence of LW in polycrystalline ice that has formed through a more conventional, slow cooling one. In this context, the present study aims to emphasize that in case of an external probe involving techniques such as EPR, the results are influenced by the binary phase (BP) diagram of the probe-water system, which also predicts the existence of LW domains in ice, up to the eutectic point. Here we report the results of our such EPR spin-probe studies on water, which demonstrate that smaller the concentration of the probe stronger is the EPR evidence of liquid domains in polycrystalline ice. We used computer simulations based on stochastic Liouville theory to analyze the lineshapes of the EPR spectra. We show that the presence of the spin probe modifies the BP diagram of water, at very low concentrations of the spin probe. The spin probe thus acts, not like a passive reporter of the behavior of the solvent and its environment, but as an active impurity to influence the solvent. We show that there exists a lower critical concentration, below which BP diagram needs to be modified, by incorporating the effect of confinement of the spin probe. With this approach, we demonstrate that the observed EPR evidence of LW domains in ice can be accounted for by the modified BP diagram of the probe–water system. The present work highlights the importance of taking cognizance of the possibility of spin probes affecting the host systems, when interpreting the EPR (or any other probe based spectroscopic) results of phase transitions of host, as its ignorance may lead to serious misinterpretations

Exploring Defect-Induced Emission in ZnAl₂O₄: An Exceptional Color-Tunable Phosphor Material with Diverse Lifetimes

Activator-free zinc aluminate (ZA) nanophosphor was synthesized through a sol–gel combustion route, which can be used both as a blue-emitting phosphor material and a white-emitting phosphor material, depending on the annealing temperature during synthesis. The material also has the potential to be used in optical thermometry. These fascinating color-tunable emission characteristics can be linked with the various defect centers present inside the matrix and their changes upon thermal annealing. Various defect centers, such as anionic vacancy, cationic vacancy, antisite defect, etc., create different electronic states inside the band gap, which are responsible for the multicolor emission. The color components are isolated from the complex emission spectra using time-resolved emission spectroscopy (TRES) study. Interestingly, the lifetime values of the various defect centers were found to change significantly from milliseconds to microseconds upon thermal annealing, which makes the phosphors more diverse (i.e., either long-persistent blue-emitting phosphors or short-persistent white-emitting phosphors). Fourier transform infrared (FTIR) and diffuse reflectance spectroscopy (DRS) confirmed the presence of antisite defect centers such as Al_Zn⁺ or Zn_Al^– in the matrix. X-ray absorption fine structure (EXAFS) study showed that the spinel structure was more disordered in nature for low-temperature-annealed compounds. Electron paramagnetic resonance (EPR) and positron annihilation lifetime spectroscopy (PALS) studies were also carried out in order to characterize various anionic and cationic vacancies and their clusters present in the compounds. Antisite defect centers such as Al_Zn⁺ or Zn_Al^–, which act as an electron or hole trap, were found to be responsible for the diverse lifetime behavior. To gain insight about the electronic states inside the band gap, density functional theory (DFT)-based calculations were performed for both pure and various vacancy-introduced spinel structures. Finally, based on the theoretical and experimental results, for the first time, a detailed investigation of various defect-induced emission behavior in ZA is presented, which also explains the mechanism of color tunability and dynamic lifetimes