Local ligand distortion broadens the near-infrared emission of Kramers neodymium ions by partially replacing La with Gd in NaLa(WO4)2 crystals

Abstract

A series of x at% Gd3+: NaLa0.975-xNd0.025Gdx(WO4)2 mixed crystals were grown by means of the Czochralski method. The mixed crystals are formed by replacing La3+ ions in a part of NaLa(WO4)2 host with Gd3+ ions, and the first principle proves that doping Gd3+ improves the disorder and activity of NaLa(WO4)2 crystals. For NaLa0.915Nd0.025Gd0.06(WO4)2 mixed crystals, the physical and thermal properties were characterized by using structural refinement, infrared spectrum and thermal expansion coefficient. The results show that the grown crystal belongs to the space group Ι41/a with high phase purity, and has excellent thermal stability and anisotropy. Compared with NaLa0.975Nd0.025(WO4)2 single crystal, with the introduction of Gd3+, the local distortion electric field is generated, the original degenerate state of Nd3+ is changed, and the intensity of Nd3+ emission peak at 1062 nm and 1332 nm is increased by 2.34 and 1.89 times, respectively. The spectral bands of absorption and emission spectra are broadened by about 4 nm. The broadening of spectral band is discussed in detail by using the theory of uniform and non-uniform broadening. For uniform band broadening, the density functional theory is used to calculate the phonon state density and vibration mode of the mixed crystal, and the Raman spectrum proves that the spectral band broadening is caused by the introduction of Gd3+, which generates a new vibrational level and increases the electron-phonon coupling, thus widening the band. For the non-uniform broadening of the spectral band, combined with the variation of the dipole moment caused by the electric field distortion, the spectral band is proved. Finally, the J-O theoretical parameters, absorption and emission cross-sections of mixed crystals are calculated. The experimental results show that NaLa0.915Nd0.025Gd0.06(WO4)2 mixed crystals are excellent laser crystal materials, which can improve the utilization of laser pump light and improve the efficiency of laser.