MORPHOTROPY OF LnSc3(BO3)4 SERIES.

G.Kuz'micheva¹, V.Rybakov², O.Kuz'min³, S.Kutovoi³

¹M.V.Lomonosov State Academy of Fine Chemical Technology, Moscow 117571,Russia
E-mail: kuz7micheva@glasnet.ru
²M.V.Lomonosov State University, Moscow 119899, Russia
³Scientific & Production Association "Firn", Krasnodar 350000, Russia

Keywords: huntite family, crystal structure, structure transition

The single crystals of LnSc₃(BO₃)₄ (Ln=La, Ce, Pr, Nd) have been grown by the Czochralski technique. Full X-ray diffraction studies of the crystal structures of the new compounds have been investigated. LnSc₃(BO₃)₄ and CeSc₃(BO₃)₄ belong to the space group C2/c with lattice parameters a_mon=7.727(1), b_mon=9.840(1), c_mon=12.046(3)A, b =105.42(2)^o for LnSc₃(BO₃). The PrSc₃(BO₃)₄ is attributed to the noncentrosymmetric space group C2 with subcell of space group C2/c and lattice parameters a_mon=7.7138(6), b_mon=9.8347(5), c_mon=12.032(2)A, b=105.387(7). The NdSc₃(BO₃)₄ has a trigonal superstructure (sp. gr. P321 or P3) with a_s/hex=19.526(3), c_s/hex=15.838(2)A. A relation between the crystal structures of these compounds and huntite structure (a_hex, c_hex; sp. gr. R32) has been established:

a'_mon=0.666 (c²_hex+0.75a²_hex)^1/2, b'_mon=b_hex,, c'_mon=0.6664(c²_hex+0.75a²_hexcos230^o)^1/2

a_s/hex=2a_hex, c_s/hex=2c_hex

The compound of TbSc₃(BO₃)₄ starting composition crystallizes in the trigonal superstructure (sp. gr. R-3; a=4.773(5), c=15.48(1)A) inferred from ScBO₃ structure (sp. gr. R-3c). The change of symmetry is due to the partial ordering of Tb and Sc in octahedron sites of ScBO₃ structure.

The morphotropy of LnSc₃(BO₃)₄ series is explained in terms of the change in symmetry of BO₃ coordination polyhedra as a result of the composition variation.