Flux Growth And Properties Of NdPO₄Crystal

Laser diode pumped all solid-state lasers have been greatly developed due to their compactness, high efficiency and longevity. They have been extensively utilized in scientific research, military, industry and medical treatment. In recent decades, one of their development directions towards compact and miniature products has been receiving people’s interests.Laser material is a key factor in the design of all solid-state lasers and its properties play an important role on the performance and application of lasers. Among the laser media, Nd-doped laser crystals have been widely used in lasers. In order to absorb enough pumping power in the short cavity, the Nd concentration in the laser crystal should be as high as possible for keeping single longitudinal mode output. However, it is difficult to increase the Nd concentration by doping because of concentration quenching and energy transfer between the Nd3+ions, etc. Therefore, self-activated Nd-stoichiometric laser crystals with high Nd-concentration and low concentration quenching have attracted people’s attention as potential microchip laser media.In our work, the growth and characterization of NdPO4crystals were studied. NdPO4crystals can only be grown in flux because of the high melting point. Firstly, through some experiments on the exploring proper flux systems, we obtain NdPO4crystals by using Li2CO3-2MoO3and Li2CO3-2WO3as flux, respectively. The phase identification and structure of the as-grown crystals was performed using the X-ray powder diffraction technique. Centimeter sized neodymium phosphate single crystals (NdPO4) have been grown for the first time using a top-seeded solution growth (TSSG) method. The physical and chemical properties of NdPO4crystals are studied, and the main work and results are as follows:1. Growth of NdPO4crystals in flux method:Two different flux systems (Li2CO3-2MoO3and Li2CO3-2WO3) can be used to grow NdPO4crystals in a spontaneous nucleation method. In order to obtain large size NdPO4crystals, the top-seeded solution growth (TSSG) method is also used with the seed-crystals and the above flux systems. As a result, NdPO4crystal with size of20x10x5mm3has been grown with Li2CO3-2WO3as a flux. The proper concentration of the solution is18wt%and the rotation speed of the seed crystal is30rpm. The factors, such as the temperature field, the cooling rate and the seed crystal, that influence the quality of the crystal in the growth process are discussed.2. The structure and defects of NdPO4crystal:The crystal structure was determined using a four-circle diffractometer at room temperature. The results show the crystal belongs to the monoclinic crystal system with a centrosymmetric space group P21/n and the unit cell parameters are a=6.7417(2) A, b=6.9570(2) A, c=6.4084(2) A,(3=103.6838(13),ρtheory=5.441g·cm-3. The structure was also determined by X-ray powder diffraction method at room temperature and the unit cell parameters were calculated by using the FULLPROF program and the DICVOL method. The results are a=6.7453A, b=6.9584A, c=6.4068A, β=103.703°, which are similar to the data of single crystal structure. The defects in NdPO4crystals were observed by optical microscope. The main defects in these crystals are cracks, growth stripe and inclusion. The formation mechanism of growth defects was discussed in order to decrease these defects.3. The basic physical and chemical properties of NdPO4crystals:such as density, hardness, thermal stability, specific heat, thermal expansion, absorption spectrum and fluorescence spectrum. The density of NdPO4crystal was measured to be5.429g·cm-3by using buoyancy method at20℃. The Vickers hardness value of NdPO4crystal is HV635and the Mohs hardness value is about5.8. The crystal was stable over the temperature range from26℃to1500℃. The specific heat of NdPO4crystal at room temperature was0.41J/g-K. High temperature X-ray diffraction was conducted to investigate the thermal expansion behavior of NdPO4crystals in the temperature range from25℃to900℃. Abnormal diffraction peaks shifts were observed along the (200) and (020) direction. The results indicate that as temperature increases, the crystal in the (200) and (020) direction contracted (from 25℃to700℃) at first, and then expanded (from700℃to900℃). The mechanism responsible for the abnormal thermal expansion in NdPO4was attributed to the existence of rigid unit modes (RUM). The absorption spectrum and fluorescence spectrum of NdPO4crystal were also measured at room temperature. There are several strong absorption bands around523,578,677,745,800and867nm, which can be assigned to different spin-and electric-dipole-allowed transitions from the ground state to other energy levels, respectively. There are two emission transitions bands from850nm to1500nm, which centered at the wavelengths of1060nm and1339nm.4. The study of NdPO4crystal with La3+-doped and Yb3+-doped:La0.5Nd0.5PO4and Yb:NdPO4crystals have been grown by a spontaneous nucleation method and the optical properties of them have also been studied. The phase identification of the La0.5Nd0.5PO4and Yb:NdPO4crystals were performed using the X-ray powder diffraction technique. The elementary mass percentage of the La0.5Nd0.5PO4and Yb: NdPO4crystals were measured by using X-ray fluorescence (XRF) spectrometer and an energy-dispersive X-ray spectroscopy (EDS). The infrared and absorption spectra of La0.5Nd0.5PO4crystal were measured and compared with the spectra of NdPO4crystal, respectively. From the fluorescence spectrum of La0.5Nd0.5PO4crystal upon excitation at355nm, the strong emission peak locating at1059nm corresponds to the Nd3+ions emission transition from4F3/2to4I11/2. The transmittance spectra of the Yb:NdPO4crystal and NdPO4crystal were compared. The fluorescence spectrum of Yb:NdPO4crystal was also measured pumped at332nm at room temperature. The strongest fluorescence-emission peak locates at995nm, which can be attributed to the2F5/2�'7/2emission band of Yb3+. The peak locating at1059nm corresponds to the4F3/2�'4I11/2emission band of Nd3+. The results indicate there should be energy transfer from Nd3+ions to Yb3+ions at room temperature.