A new type of orthoborates: ASr4La3(BO3)6 (A = Li, Na)

Abstract Two new rare earth containing orthoborate crystals ASr 4 La 3 (BO 3 ) 6 (A = Li, Na) have been obtained by spontaneous nucleation from high-temperature melts of A 2 O–SrO–La 2 O 3 –B 2 O 3 –AF. X-ray diffraction analyses show that they both crystallize in the rhombohedral space group R -3 with cell parameters of a  = 12.309(7) A, c  = 9.316(7) A and a  = 12.4049(13) A, c  = 9.348(2) A for the Li and Na compounds respectively. Similar to the large A′ 6 MM′(BO 3 ) 6 family, these compounds are all related to the structure of Sr 3 Y(BO 3 ) 3 with La and Sr statistically occupy the Sr site, and the alkaline elements and remaining Sr enter the ordered Y1 and Y2 sites, which can be approximately represented as (La 2.91 Sr 3.09) (La 0.09 Sr 0.91 )Li[B 6 O 18 ] and (La 2.85 Sr 3.15 )(La 0.15 Sr 0.85 )Na[B 6 O 18 ]. The characteristic of the structure is that the La/Sr and isolated BO 3 groups form a network with tunnels along the c -axis where the alkaline A and Sr ions alternatively reside. The optical transmission spectrum shows that the ultraviolet absorption edge of NaSr 4 La 3 (BO 3 ) 6 crystal is about 193 nm and Raman spectra reveal that both crystals possess sharp peaks at 930 cm −1 .