Syntheses, Structures, Physical Properties, and Theoretical Studies of CeM_xOS (M = Cu, Ag; x ≈ 0.8) and CeAgOS

Abstract

Black single crystals of the two nonstoichiometric cerium coinage-metal oxysulfide compounds CeCu(x)OS and CeAg(x)OS (x approximately 0.8) have been prepared by the reactions of Ce2S3 and CuO or Ag2O at 1223 or 1173 K, respectively. A black powder sample of CeAgOS has been prepared by the stoichiometric reaction of Ce2S3, CeO2, Ag2S, and Ag at 1073 K. These isostructural materials crystallize in the ZrSiCuAs structure type with two formula units in the tetragonal space group P4/nmm. Refined crystal structure results and chemical analyses provide evidence that the previously known anomalously small unit-cell volume of LnCuOS for Ln = Ce (Ln = rare-earth metal) is the result of Cu vacancies and the concomitant presence of both Ce3+ and Ce4+. Both CeCu(0.8)OS and CeAgOS are paramagnetic with mu(eff) values of 2.13(6) and 2.10(1) mu(B), respectively. CeCu(0.8)OS is a p-type semiconductor with a thermal activation energy Ea = 0.22 eV, sigma(electrical) = 9.8(1) 10(-3) S/cm at 298 K, and an optical band gap Eg < 0.73 eV. CeAgOS has conductivity sigma(conductivity) = 0.16(4) S/cm and an optical band gap Eg = 0.71 eV at 298 K. Theoretical calculations with an on-site Coulomb repulsion parameter indicate that the Ce 4f states are fully spin-polarized and are not localized in CeCuOS, CeCu(0.75)OS, or CeAgOS. Calculated band gaps for CeCu(0.75)OS and CeAgOS are 0.6 and 0.8 eV, respectively.

Structural chemistry of superconducting pnictides and pnictide oxides with layered structures

“…At room temperature this sulfide oxide is tetragonal, space group I4/mmm and shows a 75% random copper occupancy within the tetrahedral layer, similar to several CeCu 1Àx SO samples [94,95]. Upon cooling the sample below 240 K, long-range ordering of filled (75%) and vacant (25%) copper sites occurs (Fig.…”

Preparation and crystal structures of novel LaOAgS-type copper and silver fluoride chalcogenides

“…A more general comparison of the a cell parameters of the AFTCh compounds (A = Eu, Sr, Ba) with the corresponding minimal values (termination points) for the LnOTCh series (CuS: 3.874Å, CuSe: 3.865Å [8]; CuTe: 4.040Å [22]; AgS: 3.926Å [21]; AgSe: 4.028Å, AgTe: 4.233Å [14]) clearly indicates that deformations of the [T 2 Ch 2 ] 2− layers in the AFTCh are much smaller, so existence and relatively easy preparation of the fluoride chalcogenides is not surprising. The only exception yet not clearly understood is SrFAgS where the reason probably lies in low reactivity of both SrS and Ag 2 S. Note that CeOAg 1−x S had also been prepared with difficulty and a rather low yield [21], and a ternary compound BaAg 2 S 2 can be obtained only via an indirect way [23].…”

Syntheses, crystal and electronic structure, and some optical and transport properties of LnCuOTe (Ln=La, Ce, Nd)

“…This was previously ascribed to nonstoichiometry [24] or to short Ce-S distances [25]. A recent single-crystal study demonstrated that the compound is nonstoichiometric [26]. For this reason a refinement of the structure of CeCuOTe was carried out with the occupancy of the Ce site as an additional variable.…”

“…The Cu/Q materials mostly contain anti-PbO-like [Cu 2 Q 2 ] layers separated by highly ionic atoms or motifs, such as Ba 2+ or the PbO-like [Ln 2 O 2 2+ ] layers. The lanthanum copper oxychalcogenides LaCuOQ have been studied extensively owing to their interesting structure and optical and electrical properties [17][18][19][20][21][22][23][24][25][26]. The important structural feature of this type of compound is the alternation of stacks of [La 2 O 2 ] and [Cu 2 Q 2 ] layers.…”