Fluorescence of Manganese in mixed sulphate glasses and Crystalline medium.

Abstract

The observed fluorescence decay of Mn²? in potassium sulphate–zinc sulphate glass is of single exponential at low (3 mole %) and high concentrations (12 mole %) of manganese. At higher concentrations of manganese, concentration quenching by ion–ion interaction becomes important. This interaction is reflected in the decrease in the decay time of manganese at higher concentrations. This result is consistent with reported results by others [7]. The measured decay time of Mn²? decreases as K? is replaced with Na? or Li?. This important fact that emerges from this work is in agreement with earlier studies [162], in that, as the composition is changed, increasing the M? radius (monovalent ion) in the sequence Li? ? Na? ? K?, the cross?section increases resulting in a decrease of decay time. The effect of varying the manganese concentration is examined in ternary sulphate system. The effect of concentration quenching at higher concentration of manganese resulted in the decrease in the decay time as in double sulphate system.

VI.3. A DECAY TIME MEASUREMENT OF Mn²? ION IN POTASSIUM ZINC SULPHATE CRYSTAL Potassium manganese sulphate, K?Mn?(SO?)? (KMS), belongs to the family of compounds having the general formula (M?)?(M²?)?(SO?)?, where M? is a monovalent ion and M²? a divalent ion. These double sulphate salts are classified as langbeinite?type family which is named after the mineral langbeinite [147, 148]. Many crystals of this family undergo a phase transition at low temperature and some of them show ferro?electric and ferro?elastic phases while others do not [149]. In this family of crystals the true order parameter is not the spontaneous polarisation unlike in the ferro?electric crystals KH?PO? and BaTiO?. Dvorak [150], using Landau theory of phase transitions, showed that a langbeinite crystal could transform to one of the four space groups, P1, P2?2?2? and P3. Most of the crystals of this family belong to the cubic P2?3 symmetry at room temperature with four formula units (Z = 4) per primitive cell. The structure of the mineral langbeinite K?Mg?(SO?)? was solved by Zemann et al. [147] and Galtow and Zemann [148]. Their study shows that the SO?²? tetrahedra are located in general position, and Mg²? ions are on the three?fold axis. The coordination polyhedra around Mg²? are octahedra and the coordination around K? is irregular. The replacement of K? and Mg²? ions by different monovalent and divalent ions leads to other langbeinite structures. Hikita et al. [152] observed that the compound composed of the combinations of smaller monovalent and larger divalent ions have a tendency to undergo phase transitions. It has been suggested that (NH?)?Mn?(SO?)? and (NH?)?Mg?(SO?)? may show ferro?electric transition. So far, this has not been confirmed. Yamada et al. [153], classified the crystals of this family into three groups according to their phase transitions. Crystals belonging to the first group undergo transitions from the cubic P2?3 to the orthorhombic P2?2?2? phase. In the second group the crystals show a sequence of transitions, wherein the monoclinic, triclinic intermediate phases are observed. The crystals in the third group do not show any phase transitions. They also observed that the appearance of ferro?electricity was restricted to the range of r?/r²? between 0.45 Å and 0.51 Å, where r? and r²? are the ionic radii of the monovalent and divalent cation, respectively. Table 4 shows various crystals classified into the above three groups and their phase?transition schemes. Studies on dielectric properties, thermal expansion [154], laser Raman [155] and infrared absorption spectra [156] on various langbeinites have been extensively studied and reported in the literature. A study was therefore undertaken to see if the phase transition influences the fluorescence decay above and below the phase transition temperature [157] in the potassium manganese sulphate crystal. Potassium manganese sulphate crystal belongs to the langbeinite family. Potassium manganese sulphate crystal is not ferro?electric but undergoes a structural phase transition from the space group P2?3 at room temperature to another lower symmetry group. By X?ray diffraction and the optical observation of ferro?electric domains, Hikita et al. [158] determined the space groups of potassium manganese sulphate crystal in the high and low temperature phases. In the high?temperature phase Mn²? ions are on the three?fold axes. At low temperature they lose their symmetry and are situated in a general position. Raman spectroscopic studies of this phase transition have been made in this laboratory [159]. Potassium manganese sulphate crystals were grown by a slow evaporation of the aqueous solution at 85 °C. The crystals obtained were pale pink in colour. The optical absorption spectra in the wavelength 300–700 nm range were taken at room temperature. They showed features similar to those of Mn²? ions in MnF? and RbMnF? crystals [160, 161]. The potassium manganese sulphate crystal was kept in a low?temperature optical cryostat, whose temperature could be varied from liquid nitrogen temperature to room temperature. The crystal was excited with the pulsed 337.1 nm line of the fabricated nitrogen laser. The detection system was similar to that used before. The fluorescence spectrum of potassium manganese sulphate crystal at liquid?nitrogen temperature showed a wide band extending from the red (623 nm) into the green region (500 nm). The fluorescence was considerably weaker at room temperature. Similar red fluorescence is reported by others in MnF? and RbMnF? crystals [160, 161]. The fluorescence decay time of Mn²?, originating from ?T?(G) and terminating on ?A?, with the wavelength 630 nm has been measured from liquid?nitrogen temperature to room temperature. The decay of the red fluorescence recorded at room temperature and at liquid?nitrogen temperature is shown in Figure 20. The decay curve appears as positive exponential as the cathode of the photomultiplier tube is biased with a negative voltage. It is clear from the figure that the fluorescence decay times are widely different at liquid?nitrogen temperature and at room temperature. Assuming a single exponential decay, the decay times are found to be: LT = 30 ms RT = 0.8 ms The fluorescence decay times at various temperatures from liquid?nitrogen to room temperature is shown in Figure 21. VI.3.B. CONCLUSION The decay time of Mn²? ion in potassium manganese sulphate crystal is found to vary remarkably from low temperature (100 K) to room temperature (300 K). The temperature dependence of decay time of Mn²? ion, 630 nm band, is due to the strong coupling of the impurity ion with the lattice vibrations, that is, the electronic transitions taking place in the Mn²? ion are affected by the electric fields of the surrounding ions. As the temperature of the crystal is raised, the lattice vibrational energy increases and this leads to a greater modulation of the crystal field resulting in a decrease in the decay time. As the temperature is reduced, the intensity of the vibration decreases and absorption centres in a crystal find themselves in electric fields of identical symmetry which no longer change with time leading to increase in decay time. It is found that the decay time increases with lowering of temperature (30 ms / 0.8 ms at 300 K / 100 K).