Kevin L. Bray

High Pressure Probes of Electronic Structure and Luminescence Properties of Transition Metal and Lanthanide Systems

Recent work has demonstrated that unique insight into the electronic structure and optical properties of solid state transition metal and lanthanide systems is achievable through high pressure studies. In this review, we present selected examples that illustrate the advances in understanding possible from high pressure luminescence experiments. The ability of pressure to continuously vary energy level structure and coordination environment is emphasized and related to variations in luminescence properties. Pressure is shown to influence luminescence properties through crystal field, covalency, and spin-orbit coupling effects. The effect of pressure on luminescence energy, intensity, and decay properties will be considered. Additional phenomena to be discussed include electronic crossovers, vibrational coupling, admixing of electronic states, energy transfer and site selective spectroscopy.High pressure luminescence properties of transition metal and lanthanide ions in crystalline solids, metal complexes, and glasses will be reviewed.

Phosphors help switch on xenon

A phosphor material that emits almost twice as many photons as it absorbs has been discovered by researchers at Utrecht University in the Netherlands (R Wegh et al. 1999 Science 283 663). The discovery may lead to a new generation of highly efficient fluorescent lights and plasma displays that are also environmentally safe.

Luminescence properties of nanocrystalline CdS and CdS:Mn2+ doped silica-type glasses

In the present work, we report our results for the sol-gel preparation and optical properties of CdS nanometer sized particles and Mn 2+ -doped CdS particles in silica-type glass matrices. Gels containing 4.5 wt.% CdO in 95.5SiO 2 and 4.5Na 2 O-18B 2 O 3 -73.5SiO 2 matrices were prepared through a hydrolysis of tetraethoxysilane, cadmium acetate and, in the sodium borosilicate composition, boron ethoxide and sodium acetate. We designed several heat treatments in H 2 S to convert CdO to CdS in the gels. The influence of the heat treatment conditions on the optical properties of the nanoparticle semiconductor materials was studied using transmission electron microscopy, absorption, and temperature-dependent photoluminescence. A model of the photoluminescence processes was proposed to explain the observed emission bands.

Laser spectroscopy and electron paramagnetic resonance of Cr3+ doped silicate glasses

Abstract Gels and glasses in the SiO 2 –Al 2 O 3 –ZnO–Cr 2 O 3 and SiO 2 –Al 2 O 3 –MgO–Cr 2 O 3 systems were synthesized by the sol–gel method. A description of the change of the Cr 3+ environment during the xerogel–glass–glass ceramic transformation is presented using optical properties of Cr 3+ ions. Absorption and emission spectra, electron paramagnetic resonance measurements, and site-selective laser spectroscopy were performed to characterize the Cr 3+ fluorescent centers. Absorption and fluorescence are interpreted by structural considerations showing the variation of Cr 3+ environment during heat treatment. By heating glasses, the glass-ceramics containing chromium-doped ZnAl 2 O 4 (gahnite) are formed. The results indicate the presence of chromium ions in a gahnite crystalline phase and emission from the 2 E level is observed. Also a Cr 3+ fluorescence centered at 770 nm results from 4 T 2 level of Cr 3+ sites at low ligand field in glassy phase.

The effect of pressure on vibrational modes in Li3PO4

We have studied the effect of pressure on the stretching and bending modes of (PO4)3- molecular groups in undoped Li3PO4 and (MnO4)3- groups in Mn5+-doped Li3PO4 using Raman spectroscopy and luminescence. The high-pressure Raman spectroscopy study confirmed an irreversible phase transition from the high-temperature phase to the low-temperature phase, observed in our previous high-pressure luminescence study (Riedener T, Shen Y R, Smith R J and Bray K L 2000 Chem. Phys. Lett. 321 445) and further characterized the rate and irreversibility of the phase transition. We observed and analysed vibronic transitions occurring in the 1E emission of Mn5+ in both phases. A stronger vibronic transition associated with the bending mode is interpreted in terms of an E⊗e Jahn-Teller coupling. Bulk and local compressibilities were predicted from variations of the energies of the (PO4)3- and (MnO4)3- stretching modes with pressure.

Luminescence measurements on Sm2+-doped sol–gel glasses

Sol-gel glass samples containing nominally 1, 2, 5, and 10 wt% Sm 2 O 3 doped into the molar compositions 5Na 2 O-10Al 2 O 3 -85SiO 2 and 10Al 2 O 3 -90SiO 2 were prepared from metal alkoxide solution. After heat treatment in air at 500 °C, the samples were heated at 750 °C under flowing H 2 or at 800 °C under a flowing H 2 /N 2 to reduce Sm 3+ to Sm 2+ , Samarium ions in the divalent and trivalent states were identified by luminescence and lifetime measurements. Samples incorporated with Sm 2+ showed emission with peaks at 683, 700, 725 and 763 nm due to transitions 5 D 0 → 7 F 0.12 3 , respectively, of Sm 2+ ions. The luminescence properties of Sm 2+ ions are discussed in relation to the concentration of Sm 2 O 3 and the glass matrix composition. The relative proportion of Sm 2+ decreases with increasing Sm 2 O 3 doping concentration. Preliminary high pressure luminescence results for Sm 2+ in the glasses show a complete quenching of Sm 2 luminescence and an irreversible local modification in the glass network.

A spectroscopic investigation of temperature effects on solution complexation in the Eu3+-acetate system

Abstract Europium–acetate complexation was studied using luminescence spectroscopy as a function of temperature between 8 and 59°C. The 5 D 0 → 7 F 0 luminescence transition of Eu 3+ was used to identify the presence of 1:1, 1:2, and 1:3 complexes with acetate in NaClO 4 media. The results indicate an increase in complexation for all three species with increasing temperature. An estimate of the stability quotient β 1 for Eu(Ac) 2+ at 25°C was obtained by observing changes in the intensity of its 5 D 0 → 7 F 0 transition as the acetate concentration was varied. A value of log β 1 =1.94±0.13 was obtained, consistent with reported stability quotients measured by other techniques. Although overall complexation increases with temperature, little change was observed in the distributions of the 1:1, 1:2, and 1:3 complexes at constant ligand-to-metal ratios.

Pressure induced phase transition and spectroscopy of Mn5+-doped Li3PO4

Abstract A pressure induced phase transition from the high temperature to the low temperature form of Li3PO4 was observed and confirmed using Mn5+ luminescence spectroscopy and Raman scattering. The larger 1 E splitting and shorter lifetime observed in the low temperature phase are interpreted in terms of a larger distortion of the phosphate group of the low temperature phase from regular tetrahedral symmetry. The results illustrate that a competition between cubic crystal field and non-tetrahedral distortions is crucial to the understanding of the emission properties of d2 systems.

High Pressure Spectroscopy of Ti Doped Al2O3 and YAIO3 Host Crystals

We report high-pressure luminescence spectroscopy studies of Ti3☎ in Al2O3 and YAlO3. High-pressure luminescence spectra were measured for Al2O3:Ti3☎ up to 90kbar and for YAlO3:Ti3☎ up to 181 kbar. In both cases, a blue shift of the luminescence peak with pressure was observed and is attributed to an increase in the octahedral crystal-field strength (10 Dq) with pressure. In the case of YAlO3;Ti3☎, an additional luminescence peak was observed at 181 kbar and below ∼ 200 K, and is attributed to a metastable state. The metastability of the 2E excited state was induced by pressure that removes the equivalency of three energy minima of the 2E state coupled to the ϵ mode.

High pressure spectroscopy of chromium doped LiTaO3 crystals

Abstract The effect of pressure on the luminescence spectrum of the weak field system Cr3+ : LiTaO3 is presented. By using pressure to induce a low to high crystal field strength transition, we demonstrate the ability of pressure to identify distinct dopant bonding environments in luminescent materials.