Exploring Potential Beryllium-free, Deep-Ultraviolet Optical Crystals in the Rare Earth Fluoride Carbonate-Water System

摘要

Rare earth fluoride carbonates have been proven to be potential deep-ultraviolet optical materials, but lacked the systematic study on phase stability domains to guide synthesis. In this work, detailed investigations of the Na2CO3-GdF3-H2O system have been done in subcritical hydrothermal conditions. A series of sodium-rare earth carbonates crystals NaGdCO3F2 (1), Na2GdCO3F3 (2), Na3Gd(CO3)(3) (3), and Na3Gd(CO3)(3)center dot 6H(2)O (4) have been synthesized. NaGdCO3F2 (1) is composed of a three-dimensional (3-D) [GdCO3F2](-) framework with Na+ as the charge balance cation. Na2GdCO3F3 (2) showed two-dimensional (2-D) [GdCO3F3](2-) layers along the c-axis and further bridged by Na+ to form a 3-D framework. Na3Gd(CO3)(3) (3) is made up of alternately stacked layers [Na(CO3)(2)](infinity) along the a-axis and [Na2Gd(CO3)(2)](infinity) in the be plane, forming a 3-D framework. Na3Gd(CO3)(3)center dot 6H(2)O (4) exhibits an intricate 3-D structure composed of a [Gd(CO3)(3)(H2O)(3)](3-) complex and bridging Na+ ions. The results from the UV-vis diffuse reflectance spectroscopy study indicated that the short-wavelength absorption edge of all four compounds were below 200 nm, suggesting that they are potential deep-ultraviolet optical materials. Powder second harmonic generation measurements indicated that the compounds 3 and 4 are phase-matchable (type I), and they exhibit relatively strong second harmonic generation responses of approximately 3 and 2 times that of KH2PO4 (KDP).