Chiharu Hidaka

A novel monoclinic phase of impurity-doped CaGa2S4 as a phosphor with high emission intensity

In the solid-state synthesis of impurity-doped CaGa2S4, calcium tetrathiodigallate(III), a novel phosphor material (denominated as the X-phase), with monoclinic symmetry in the space group P21/a, has been discovered. Its emission intensity is higher than that of the known orthorhombic polymorph of CaGa2S4 crystallizing in the space group Fddd. The asymmetric unit of the monoclinic phase consists of two Ca, four Ga and eight S sites. Each of the Ca and Ga atoms is surrounded by seven and four sulfide ions, respectively, thereby sharing each of the sulfur sites with the nearest neighbours. In contrast, the corresponding sites in the orthorhombic phase are surrounded by eight and four S atoms, respectively. The photoluminescence peaks from Mn2+ and Ce3+ in the doped X-phase, both of which are supposed to replace Ca2+ ions, have been observed to shift towards the high energy side in comparison with those in the orthorhombic phase. This suggests that the crystal field around the Mn2+ and Ce3+ ions in the X-phase is weaker than that in the orthorhombic phase.

Light figure spectroscopy of optically active anisotropic materials

Abstract Low order interference curves in optic-axis light figures of the optically active AgGaS 2 and TeO 2 are investigated in the visible spectral range at room temperature. Light figure patterns of AgGaS 2 are found not to be affected by optical activity within the incidence angles of 10° for which, according to our symmetry considerations, the normal component of gyration tensor is small and evolution of light figures with wavelength is overwhelmingly determined by optical anisotropy. The absolute value of this anisotropy is falling down to null when passing through the isotropic point, with the result of a remarkable change of the parameters of light figure patterns. Unlike the case of AgGaS 2 and consistently with our symmetry considerations, light figures of TeO 2 are influenced by optical activity especially strongly in low orders of interference. The obtained results are compared with similar results for optically inactive CaCO 3 and discussed in the frameworks of our recent analyses.

Effect of Rare Earth Element Codoping on the Mn Red Emission in Ca and Sr Thiogallates: Focusing on Its Mechanism through Electron Spin Resonance Study of Single Crystals

The weak Mn2+ involved red emission in CaGa2S4 can be greatly enhanced by codoping with a suitable rare earth element (REE). To clarify the involved physical mechanism, the actual Mn sites in the host crystal are investigated through electron spin resonance (ESR) measurements of single crystals. Three different patterns of ESR signals with different angular dependences are observed by rotating the magnetic field around the three crystal axes. As a result, three independent sites occupied by the Mn2+ ions are confirmed. Based on the experimental finding that the hyper fine constants (HFS) for the three sites are nearly the same, the Mn2+ ion sites may be ascribed to the Ca or Ga sites. To finally solve this ascription issue, ESR measurements of Eu2+ in CaGa2S4 are carried out, where the Eu2+ ions have been shown to exclusively substitute at Ca sites. From the similar angular variation of the ESR signals of Eu2+ and Mn2+, the Mn substitutional sites are finally determined to be the three Ca ones. A broad ESR signal appearing in the vicinity of the Lande g factor (g = 2.0) suggests the possible presence of Mn clusters. We have to investigate to see whether these may be the actual emission centers.

Determination of a pseudo-binary SrSe–Ga2Se3 phase diagram and single crystal growth of SrGa2Se4 compounds

Abstract We have investigated chemical reaction processes and thermal characteristics of IIa–III 2 –VI 4 compounds in order to grow their single bulk crystals. Up to now, single crystals of Ca and Sr thiogallates have been successfully grown by the melt growth method based on their pseudo-binary phase diagrams. Here, a similar diagram of the SrSe–Ga 2 Se 3 system has been constructed for the first time, where a eutectic reaction is found in the range of excess Ga 2 Se 3 concentration, and it is shown that the SrGa 2 Se 4 compound has a congruent melting point (1110 °C) suitable for the melt growth. A single crystal is grown from the melt by the horizontal Bridgman method. A trial is also made to grow a high-quality single crystal of CaGa 2 S 4 already known as being grown easily.

ESR study of Ce3+ ions doped in the CdIn2S4 lattice

Abstract Rare earth elements doped IIa–III 2 –VI 4 compounds are expected as host materials for fluorescent devices. In particular, since blue emission was observed in alkaline earth thiogallates doped with Ce 3+ ions, much attention has been paid to develop full color displays made of thin films of the compounds above. Especially Ce 3+ ions are thought as promising candidates for dopants realizing blue light emission. Normally doped Ce 3+ ions are considered to substitute the sites which are occupied with divalent elements such as Ca or Cd. However, the substituting mechanism is not fully understood, since in view of the number of valence, it is natural for a Ce atom to substitute a trivalent one. In this report, we have grown single crystals of CdIn 2 S 4 as host materials for Ce doping and investigated the site substitution of the Ce atom in this crystal lattice by means of ESR. Two types of ESR centers are found. One consists of a Ce 3+ ion substituting a Cd site and a sulfur vacancy, and the other consists of a similar Ce 3+ ion with 4 tetrahedrally surrounding sulfur atoms.

Analysis of the Enhancing Effect of Rare Earth Ion Co-Doping on the Red Emission of CaGa2S4:Mn2+ in View of Its Decay Time

When certain trivalent rare earth elements (REEs) are co-doped with Mn2+ in CaGa2S4, the Mn2+ red emission of the compounds is enhanced. To clarify its mechanism, its decay curves of the Mn2+ red emission from CaGa2S4:Mn2+, REE3+ are investigated. For these analyses, we present a tentative transition model including the trap levels due to REE3+ together with a process enabling electron supply from REE3+ to the excited 3d state of Mn2+. This model can explain the enhancing effect on the Mn2+ red emission.

Effect of Ho Co-Doping on the Long Lasting Emission of CaGa2S4:Eu

To investigate the long-lasting green afterglow from Ho added CaGa2S4:Eu compounds, photoluminescence (PL) and photoluminescence excitation (PLE) spectra are measured as a function of Ho concentration by keeping Eu one at 0.1 and 0.5 mol %. As Ho concentration increases, PL intensity of Eu decreases. PL from Ho at 660 nm has an additional broad excitation band ranging from 400 to 500 nm, emerging only if Eu2+ ion is co-doped. Since this new band just coincides with the Eu2+ emission in energy, electrons excited to the 5d states of Eu2+ are considered to transfer to the Ho excited states, which are actually confirmed by the weakened Eu emission observed. In the decay curve of Eu emission in a co-doped sample, two long-lasting components are observed. Their lifetimes, however, steeply decrease when Ho concentration increases more than 1 mol %. In this paper, energy transfer between the two dopants is discussed.