Synthesis, physical and optical properties, and electronic structure of the rare-earth oxysul®des Ln 2 O 2 S (Ln  Sm, Eu) J. Llanos a,* , V. Sa Ânchez a , C. Mujica a , A. Buljan b a Departamento de Quõ Âmica, Fac. de Ciencias, Universidad Cato Âlica del Norte, Casilla 1280, Antofagasta, Chile b Grupo de Quõ Âmica Teo Ârica y Computacional, Fac. de Ciencias Quõ Âmicas, Universidad de Concepcio Ân, Casilla 160-C, Concepcio Ân, Chile (Refereed) Received 5 May 2002; accepted 23 August 2002 Abstract ThecompoundsLn 2 O 2 S(Ln  Sm,Eu)werepreparedfromtherespectiveoxidesbypartial sulfurization under ¯owing CS 2 /Ar/O 2 atmosphere. The main feature of the structure is the packing of S 2 and O 2 anions with Ln 3 located in the center of a capped octahedron. Electrical conductivity measurements on pressed pellets showed that both phases are highly resistivewithconductivitieslowerthan10 8 Scm 1 atroomtemperature.Theopticalbandgaps were assessed at 4.2 and 4.4 eV for Sm 2 O 2 S and Eu 2 O 2 S, respectively. Electronic structure calculationswereperformedbymeansofextendedHu Èckeltight-bindingmethod.Theinsulating properties are in good agreement with the results predicted by theoretical calculations. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic materials; B. Chemical synthesis; C. Impedance spectroscopy; D. Electrical properties; D. Optical properties 1. Introduction Incontrasttothelargenumberofoxidesandsul®desthatappearintheliterature,a very few oxysul®des have been reported. Only two naturally occurring oxysul®des areknown,Sb 2 OS 2 , kermesite [1] and CaSb 10 O 10 S 6 , sarabauite [2].Themajorityof Materials Research Bulletin 37 (2002) 2285±2291 0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0025-5408(02)00936-4 * Corresponding author. Tel.: 56-55-355615; fax: 56-55-355632. E-mail address: jllanos@ucn.cl (J. Llanos).