New antimony lanthanide disulfide dibromides LnSbS(2)Br(2) (Ln = La, Ce): Crystal and electronic structures and optical properties

CeSbS2Br2 (I), Ce1/2La1/2SbS2Br2 (II), and LaSbS2Br2 (III) have been synthe sized at 700 degreesC from a mixture of LnBr(3), Ln(2)S(3), Sb, and S and c haracterized by single-crystal X-ray diffraction, The three phases are isos tructural (space group P2(1)/c, Z = 4) and crystallize in a novel, dense, b idimensional structure with cell parameters a = 8.709(3) Angstrom, b = 9.18 7(2) Angstrom, c = 17.397(5) Angstrom, beta = 104.26(3)degrees for I, a = 8 .709(3) Angstrom, b = 9.219(7) Angstrom, c = 17.41(2) Angstrom, beta = 104. 3(1)degrees for II, and a = 8.785(1) Angstrom, b = 9.236(2) Angstrom, c = 1 7.372(3) Angstrom, beta = 104.09(2)degrees for III. In these compounds, [Ln S(5)Br(4)] and [LnS(3)Br(6)] (Ln = Ce, La) distorted tricapped trigonal pri sms define infinite (2)(infinity)[LnS(2)Br(2)] layers counterbalanced and c apped by antimony cations, In good accordance with the structural features, the charge balance in these materials is to be written Ln(III)Sb(III)S(2)( -II)Br(2)(-I). These compounds exhibit a yellow hue with a measured absorpt ion threshold of 2.42(1), 2.55(1), and 2.72(1) eV for I, II, and III, respe ctively, In the two cerium containing bromothioantimonates I and II, the or igin of the color is assigned to a Ce-4f --> Ce-5d electronic transition, w hich shifts to higher energy from I to II due either to a matrix effect (in crease of the mean Ln-S distances under the substitution of Ce for La) or t o an atomic ordering between Ce and La cations on the Ln(l) and Ln(2) cryst allographic sites, In contrast, the electronic transition at play in III in volves a charge transfer from the bromine and sulfur ions to the antimony i ons, the latter contributing substantially to the lowermost levels of the c onduction band. (C) 2001 Academic Press.