Introduction The development of laser technology is related to laser materials. In particular, high-power solid-state lasers require the laser material to have the superior performance (i.e., lower pumping thresholds, large absorption and emission cross sections). Compared with glass and single crystal, transparent ceramics have many potential advantages that glass and single crystal are incomparable. In recent years, the breakthrough of ceramic preparation technology has gradually brought ceramics into the field of vision of the majority of scientific researchers. From the perspective of luminescence of doped rare-earth element ions, materials with a low symmetry system have more potential advantages. Strontium fluorophosphate has a hexagonal structure as a typical asymmetric system material. It can provide superior doping sites for rare-earth element ions, having broad potential application prospects in luminescence. Also, neodymium ion (Nd3+) is a commonly used active ion, its intense emission wavelength is mainly approximately 1 μm, which is widely used in atmospheric detection and infrared imaging and other fields. However, neodymium ion has a problem of concentration quenching, which will cause the reduction of luminous efficiency. To solve this problem, the commonly used solution is to co-dope inert regulatory ions to break the cluster phenomenon of neodymium ions in the matrix material. Therefore, Nd-doped strontium fluorophosphates transparent ceramics were prepared with S-FAP as a matrix material and Nd3+ as an activator ion. In addition, the effect of inert regulatory ions as a sensitizer on the microstructure and spectral properties of Nd:S-FAP was also investigated.Materials and method According to the stoichiometric ratio of (Nd0.015Re0.02Sr0.965)5(PO4)3F1.175, raw materials were weighed and cation was dropped into the anionic solution at a fixed titration rate, and then the suspension was separated in a centrifuge at 11 000 r/min for 4 times. Finally, the separated products were dried in an oven for 24 h. The dried powder was ground in a mortar. The dried powder of 3 g was put into a graphite mold, placed in a vacuum hot press furnace, and sintered at 900 ℃ for 2 h to obtain (Nd, Re):S-FAP transparent ceramics.The phase composition of the ceramic was analyzed by a model D8-Advance X-ray diffractometer. The Nd3+ and Re3+ concentrations of (Nd, Re): S-FAP were determined in argon at 0.3 MPa by a model Prodigy 7 inductively coupled plasma emission spectrometer. The transmittance of ceramics was measured by a model Lambda 750S spectrophotometer. The microstructure and pore distribution of ceramics were characterized by a model S4800 scanning electron microscope, and the emission spectrum and fluorescence lifetime of ceramic samples were measured by a model FLS920 spectrometer.Results and discussion After doping different kinds of inert regulated ions, each sample has no impurity and a high crystallinity. After high-temperature sintering, the ceramic still maintains a hexagonal structure with a pure phase S-FAP. Moreover, the position of the ceramic emission peak shifts to the right, indicating that the lattice constant gradually decreases The ceramic samples have a good optical transmittance, showing the absorption peaks of the 4I9/2 ground state level transition to the excited state level. For the samples doped with different kinds of inert regulated ions, there are differences in their microstructures. This is because the distortion caused by the doped S-FAP lattice is also different due to the different radii of inert ions. Y ion has the smallest radius and the greater lattice distortion degree. In sintering, the lattice distortion will provide part of the sintering energy, increase the diffusion rate of the grain boundary, eventually resulting in the larger grain size. The main reason affecting the overall transmittance of the sample is a difference in density, and the higher the density of the sample is, the higher the transmittance will be. The emission intensity of transparent ceramics is enhanced after co-doping inert regulated ions. The addition of inert regulated ions breaks the clusters of neodymium ions and increases the distance between different neodymium ions, thereby avoiding the phenomenon of cross relaxation between neodymium ions to a certain extent and thus improving the emission intensity. In addition, the emission peaks of the samples shift to the right to a certain extent after the addition of inert ions, indicating that the crystal field environment of neodymium ions can be changed by co-doped inert ions. The fluorescence lifetimes of ceramic samples doped with different kinds of inert rare-earth ions are 328.44, 331.82 and 355.68 μs, respectively. Compared with 1.5% Nd:S-FAP transparent ceramics, the fluorescence lifetime of ceramic samples doped with inert rare-earth ions increases. The neodymium ion clusters are broken, resulting in an increase in fluorescence intensity/lifetime. In addition, from doping of different kinds of inert rare-earth ions, the fluorescence life of Y ions is the longest after incorporation, reflecting that Y ions are more efficient in breaking clusters in S-FAP matrix materials, and are the most suitable inert regulatory ions for doping.Conclusions Inert rare-earth ions doped (Nd, Re):S-FAP transparent ceramics were prepared by a hot pressing sintering method. The lattice constant of rare-earth ion doping decreased, and the main factor affecting the change of lattice constant was rare-earth ion radius. The emission intensity and fluorescence lifetime of ceramic doped inert ions were increased, compared with that of ceramic undoped ions, and the regulation effect of Y ions was dominant. This indicated that the clusters between neodymium ions were broken and the luminous efficiency was improved.