Structural Instability in Single-Crystal Rare-Earth Scandium Borates RESc3(BO3)4

An influence of the type of rare-earth (RE) cation and composition of initial charge on the symmetry, structural features, and real composition of the single-crystal huntite family rare-earth scandium borates RESc3(BO3)(4) (RE = Ce, Pr, Nd), grown by the Czochralski method, has been studied by single crystal and powder X-ray diffraction. A crystallization of scandium borates in the space groups C2/c (RE = Ce) and P321 (RE = Pr, Nd) has been found. Disordering in the structures with the space group P321, which has been first determined for the huntite family compounds, is due to the RE and Sc redistribution over two trigonal-prismatic sites to maintain the stability of crystal structure. The crystals grown from the initial charges NdSc3(BO3)(4) and Nd1.25Sc2.75(BO3)(4) are characterized by the greatest disordering, and they are isotypic, rather than isostructural, to the crystals obtained from the charges Pr1.1Sc2.9(BO3)(4) and Pr1.25Sc2.75(BO3)(4). A change in the unit cell parameters and interatomic distances depending on the RE radius and composition of the initial charge is found and explained. Analysis of our and literary data for RESc3(BO3)(4) with the RE = La, Ce, Pr, Nd, Sm, Eu allowed to reveal a morphotropic series for scandium borates based on the size factor (the RE3+ radius), suggest methods for evaluating the specific crystal symmetry depending on the type of RE cation, and propose factors affecting its realization. The comparison of huntite family REM3(BO3)(4) (M = Al, Ga, Sc) single crystals allowed to establish basic differences between their crystal structures, mainly due to the different sizes of RE and M ions.