The admixture of CeO 2 ,C e, CeCl 3 ,a nd MoO 3 with an excess of LiCl as flux in evacuated silica ampules leads to large black single crystalsa sw ell as ab lack microcrystalline powdero fC e 3 Cl 3 [MoO 6 ]a fter tempering at 850 8Cf or three days. The title compound crystallizes in the hexagonal space group P6 3 /m (a = 934.93(4), c = 538.86(2) pm) with two formula units per unit cell. The crystals tructure consists of rather unusualt rigonal-prismatic [MoO 6 ] 6À units besides Ce 3 + ions in at etra-capped trigonal-prismaticc oordination, formedb yf our Cl À and six O 2À ions. The black color is related to an opticalb and gap of 1.35(2) eV,w hich was determinedb yd iffuse reflectance spectroscopy andc onfirmed by theoretical calculations. The low band gap between the 4f 1 state of cerium (HOMO) and the 5d 0 state of molybdenum (LUMO) gave rise to the idea of electronice xcitation betweent heset wo states by IR irradiation, creating ad rop in the resistivity of the material, which was detected by appropriate measurements.
Results and DiscussionCrystal structure Ce 3 Cl 3 [MoO 6 ]c rystallizes isotypically to the Ln 3 Cl 3 [MoO 6 ]r epresentatives for Ln = La, Pr,a nd Nd [1] as well as to the Ln 3 Cl 3 [WO 6 ]d erivatives with Ln = La-Nd, Sm-Tb [3] in the hexagonal space group P6 3 m (a = 934.93(4), c = 538.86(2)pm) with two formula units per unit cell. The Ce 3 + ions are surrounded by four chloridea nd six oxide ions in the shape of tetracapped trigonalp risms (Figure 1, top left). The correspondingi nteratomicd istances (d(Ce 3 + ÀCl À ) = 294-305pm; d(Ce 3 + ÀO 2À ) = 242-264pm, see Table 1) are well within the range of other compounds bearing Ce 3 + cations and Cl À and O 2À anions, respectively (e.g.,A -type Ce 2 NCl 3 : d(Ce 3 + ÀCl À ) = 287-339pm; [5] A-typeC e 2 O 3 = 234-269 pm [6] ). The Mo 6 + ions represent the [a] Dr.