Crystal growth, structure and phase transition of the nonlinear optical crystal BaCaBO3F

• BaCaBO3F (BCBF) crystal can be grown in one step with an BaF2 overdose.
• BCBF crystallizes in a large unit cell with space group R3 at room temperature (RT).
• BCBF undergoes a order–disorder phase transition at 242 ºC.
• The high temperature phase belongs to the P-62m structure previously reported for RT.
• The R3 unit cell is tripling in every dimension related to that of P-62m.

The nonlinear optical (NLO) crystal BaCaBO3F (BCBF) was reported about 20 years ago and was shown to have favorable lasing and self frequency doubling properties when doped with ytterbium. Detailed studies on its structure and thermal physical properties are still lacking. We grew BCBF crystals with dimensions up to 32×33×14 mm3 in an optimized solution below 1000 °C with NaF as an additive. A reversible phase transition at 242 °C was revealed by specific heat, differential thermal analysis, thermal expansion and variable temperature powder X-ray diffraction measurements. It is found that BCBF crystallizes in the reported hexagonal space group P-62m at high temperature (e.g. at 400 °C, a=9.09470(9) Å, c=4.37147(4) Å and Z=3). It undergoes a phase transition when the temperature drops below 242 °C to a trigonal space group R3 with a large unit cell of a=27.1478(5) Å, c=12.9835(3) Å and Z=81 at room temperature (RT), which is related to the high temperature structure by tripling in every three dimensions. The main difference of the two structures concerns one third of the BO3 groups in the unit cell: in the RT phase they (27 of 81 in total) are long range ordered whereas above the phase transition temperature they lose long range correlation and collapse to only one BO3 group in a disordered position at the corner of the small hexagonal unit cell. The newly found structures of BCBF may help to understand its mechanisms for laser and NLO performance.