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Ce3+/Eu2+, Tb3+ and Mn2+ co-doping in single-phase hosts is a common strategy to achieve white-light phosphors via energy transfer, which provides a high color rendering index (CRI) value and good color stability. However, not all hosts are suitable for white-light phosphors due to inefficient energy transfer. In this study, the site-sensitive energy transfer from different crystallographic sites of Ce3+ to Tb3+/Mn2+ in Ca3Al2O6 has been investigated in detail. The energy transfer from purplish-blue Ce3+ to Tb3+ is an electric dipole-dipole mode, and the calculated critical distance (Rc) suggests the existence of purplish-blue Ce3+-Tb3+ clusters. No energy transfer is observed from purplish-blue Ce3+ to Mn2+. In co-doped phosphors based on greenish-blue Ce3+, however, the radiative mode dominates the energy transfer from Ce3+ to Tb3+, and an electric dipole-quadrupole interaction is responsible for the energy transfer from Ce3+ to Mn2+. A detailed discussion on the site-sensitive energy transfer modes might provide a new aspect to discuss and understand the possibilities and mechanisms of energy transfer, according to certain crystallographic sites in a complex host with different cation sites, as well as provide a possible approach in searching for single-phase white-light-emitting phosphors. [ABSTRACT FROM AUTHOR]

This paper investigates the station-keeping control of autonomous and remotely-operated vehicles (ARVs) for free-floating manipulation under model uncertainties and external disturbances. A modified adaptive generalized super-twisting algorithm (AGSTA) enhanced by adaptive tracking differentiator (ATD) and reduced-order extended state observer (RESO) is proposed. The ATD is used to obtain the smooth reference signal and its derivative. The RESO is used to estimate and compensate for the model uncertainties and external disturbances in real-time, which enhances the robustness of the controller. The modified AGSTA ensures the fast convergence of the system states and maintains them in a predefined neighborhood of origin without overestimating control gains. Besides, the proposed new variable gain strategy completely avoids the control gains vibrating near the set minimum value. Thanks to the RESO, the proposed controller is model-free and can be easily implemented in practice. The stability of the closed-loop system is analyzed based on Lyapunov’s direct method in the time domain. Finally, the proposed control scheme is applied to the station-keeping control of Haidou-1 ARV, and the simulation results confirm the superiority of the proposed control scheme over the original AGSTA.