Preparation method and application of sodium barium fluoroborate birefringent crystal

A method for preparing sodium barium fluoborate birefringent crystal, characterized in that said crystal is grown by melt method including the top seeding method, the Kyropoulos method or the Czochralski method, according to the specific steps as follows:

• a. mixing and grinding a sodium-containing compound, barium-containing compound, boron-containing compound and fluorine-containing compound at a molar ratio of sodium:barium:boron:fluorine=3:2:6:1 to provide a mixture, charging the mixture into a platinum crucible, heating the mixture to a temperature higher than 820° C. to obtain a high-temperature mixed melt, and maintaining this temperature for 1-100 hours;
• b. growing the crystal on the surface of or inside the mixed melt:
• decreasing the temperature of the mixed melt of Step (a) to 820-825° C., fixing a seed crystal to a seed crystal rod and preheating the seed crystal above the mixed melt for 1-100 minutes, then decreasing the temperature of the mixed melt to 810-820° C., descending the seed crystal onto the surface of or into the mixed melt while rotating the seed crystal or the crucible at a rotation speed of greater than 0 to 100 r/min, increasing or decreasing the temperature of the mixed melt at a rate of greater than 0 to 5° C./d, and meanwhile drawing the crystal at a drawing speed of greater than 0 to 15 mm/h;
• c. when growing the crystal to a desired size, increasing the drawing speed so as to draw the crystal out of the surface of the mixed melt, decreasing the temperature to room temperature at a rate of 1-100° C./h, and then taking the crystal out of the crucible to obtain a Na3Ba2(B3O6)2F birefringent crystal.