RE³⁺ (RE = Pr, Sm, Tb, Er, Dy)-activated Y₄Si₂O₇N₂ samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. The absorption band located at about 250 nm is attributed to the host absorption. The 5d bands of Pr³⁺ and Tb³⁺ are at rather low energy in Y₄Si₂O₇N₂ compared to oxide. The direct Pr³⁺ 4f² → 4f¹5d¹ excitation at 275 nm leads to typical 4f² → 4f² line emissions (450–700 nm) and strong 4f¹5d¹ → 4f² broad band emission (300–450 nm), respectively. The charge transfer (N³⁻ → Sm³⁺) band of Sm³⁺ was observed at a somewhat lower energy of 4.68 eV compared to oxide, and Sm³⁺-activated sample shows a bright red emission originating from ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2) transitions. For Tb³⁺-doped sample, the direct Tb³⁺ 4f⁸ → 4f⁷5d¹ excitation leads to ⁵D₃ → ⁷F_J (J = 6, 5, 4, 3) (blue) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) (green) line emissions, the cross-relaxation depended on Tb concentration has happened. The incorporation of Er³⁺ (or Dy³⁺) into Y₄Si₂O₇N₂ resulted in a typical Er³⁺ (or Dy³⁺) f-f line absorptions and emissions. Moreover, the energy transfer from the host lattice to the luminescent activators (Pr³⁺, Tb³⁺ and Sm³⁺) is observed. The energy level diagram containing the position of 4f and 5d energy levels of all Ln²⁺ and Ln³⁺ ions relative to the valence and conduction band of Y₄Si₂O₇N₂ has been established and studied based on the data presented in this work, and further provides a platform for studying the photoluminescence properties as well as the valence stability of the lanthanide ions.

Since the global positioning system (GPS) is not applicable underwater, source localization using wireless sensor networks (WSNs) is gaining popularity in oceanographic applications. Unlike terrestrial WSNs (TWSNs) which uses electromagnetic signaling, underwater WSNs (UWSNs) require underwater acoustic (UWA) signaling. Received signal strength (RSS)-based source localization is considered in this paper due to its practical simplicity and the constraint of low-cost sensor devices, but this area received little attention so far because of the complicated UWA transmission loss (TL) phenomena. In this paper, we address this issue and propose two novel semidefinite programming (SDP) approaches which can be solved more efficiently. The numerical results validate our proposed SDP solvers in underwater environments, and indicate that the placement of the anchor nodes influences the RSS-based localization accuracy similarly as in the terrestrial counterpart. We also highlight that adopting traditional terrestrial RSS-based localization methods will fail in underwater scenarios.