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While the overwhelming number of papers on multi-principal-element alloys (MPEAs) focus on the mechanical and microstructural properties, there has been growing interest in these alloys as solid-state hydrogen stores. We report here the synthesis optimization, the physicochemical and the hydrogen sorption properties of Ti0.325V0.275Zr0.125Nb0.275. This alloy was prepared by two methods, high temperature arc melting and ball milling under Ar, and crystallizes into a single-phase bcc structure. This MPEA shows a single transition from the initial bcc phase to a final bct dihydride and a maximum uptake of 1.7 H/M (2.5 wt%). Interestingly, the bct dihydride phase can be directly obtained by reactive ball milling under hydrogen pressure. The hydrogen desorption properties of the hydrides obtained by hydrogenation of the alloy prepared by arc melting or ball milling and by reactive ball milling have been compared. The best hydrogen sorption properties are shown by the material prepared by reactive ball milling. Despite a fading of the capacity for the first cycles, the reversible capacity of the latter material stabilizes around 2 wt%. To complement the experimental approach, a theoretical investigation combining a random distribution technique and first principle calculation was done to estimate the stability of the hydride.

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We have employed femtosecond laser writing in order to induce refractive-index changes and waveguides in Ti3+-doped sapphire. Doping the sapphire crystal with an appropriate ion significantly reduces the threshold for creating structural changes, thus enabling the writing of waveguide structures. Passive and active buried channel waveguiding is demonstrated and images of the guided modes, propagation-loss values, fluorescence spectra, and output efficiencies are presented. The guiding area is located around the laser-damaged region, indicating that the guiding effect is stress induced. Refractive-index changes are measured by digital holography. Proper active doping should enable femtosecond processing and waveguide writing in various crystalline materials.

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Two major fiber crystal growth methods have been applied for the synthesis and spectroscopic characterizations of Yb3+-doped Y3Al5O12 (Yb:YAG) to show the feasibility of a flexible single crystal fiber laser. One is the micropulling-down (μ-PD) method and the other is the laser heated pedestal growth (LHPG) method. 500 mm length fiber with no Yb segregation has been grown by the μ-PD method. A detailed analysis of Yb:YAG spectroscopy is made to contribute to the discussion on the determination of energy levels. On the other hand, a combinatorial chemistry approach has been applied on concentration gradient crystal fibers grown by the LHPG method allowing the measurement of the intrinsic radiative lifetime and the analysis of concentration quenching processes of Yb3+ ions in YAG.

A new quinary multi-principal element alloy Ti0.30V0.25Zr0.10Nb0.25Ta0.10 was prepared by high temperature melting technique and the physicochemical as well as hydrogen sorption properties have been determined. The as-cast alloy crystallizes into a single-phase bcc lattice and can very quickly absorb hydrogen at room temperature forming a fcc hydride phase with capacity of 2 H/M (2.5 wt.%). The absorption/desorption of hydrogen is reversible and occurs within one step, as proven by in situ neutron diffraction for the desorption reaction. The capacity is slightly fading during the first 10 cycles and then stabilizes at around 2.2 wt.% for the next 10 cycles.