New Antimony Lanthanide Disulfide Dibromides LnSbS_2Br_2 (Ln = La, Ce): crystal and Electronic Structures and Optical Properties

摘要

CeSbS_2Br_2 (I), Ce_(1/2)La_(1/2)SbS_2Br_2 (II), and LaSbS_2Br_2 (III) have been synthesized at 700 deg C from a mixture of LnBr_3, Ln_2S_3, Sb, and S and characterized by single-crystal X-ray diffraction. The three phases are isostructural (space group P2_1/c, z = 4) and crystallize in a novel ,dense, bidimensional structure with cell parameters a = 8.709(3) A, b = 9.187(2) A, c = 17.397(5) A, #beta# = 104.26(3) deg for I,a = 8.739(7) A, b = 9.219(7) A, c = 17.41(2) A, #beta# = 104.3(1) deg for II, and a = 8.785(1) A, b = 9.236(2) A, c = 17.372(3) A, #beta# = 104.09(2) deg for III. In these compounds, [LnS_5Br_4] and [LnS_3Br_6] (Ln = Ce, La) distorted tricapped trigonal prisms define infinite _#propor.to#~2[LnS_2Br_2] layers counterbalanced and capped 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 absorption threshold of 2.42(1), 2.55(1), and 2.72(1) eV for I,II, and III, respectively. In the two cerium containing bromothioantimonates I and II, the origin of the color is assigned to a Ce-4f->Ce-5d electronic transition, which shifts to higher energy from I to II due either to a matrix effect (increase of the mean Ln-S distances under the substitution of Ce for La) or to an atomic ordering between Ce and La cations on the Ln(1) and Ln(2) crystallographic sites. In contrast, the electronic transition at play in III involves a charge transfer from the bromine and sulfur ions to the antimony ions, the latter contributing substantially to the lowermost levels of the conduction band.