Philippe Smet

Cs7Nd11(SeO3)12Cl16: First Noncentrosymmetric Structure among Alkaline-Metal Lanthanide Selenite Halides

Cs7Nd11(SeO3)12Cl16, the complex selenite chloride of cesium and neodymium, was synthesized in the NdOCl-SeO2-CsCl system. The compound has been characterized using single-crystal X-ray diffraction, electron diffraction, transmission electron microscopy, luminescence spectroscopy, and second-harmonic-generation techniques. Cs7Nd11(SeO3)12Cl16 crystallizes in an orthorhombic unit cell with a = 15.911(1) Å, b = 15.951(1) Å, and c = 25.860(1) Å and a noncentrosymmetric space group Pna2(1) (No. 33). The crystal structure of Cs7Nd11(SeO3)12Cl16 can be represented as a stacking of Nd11(SeO3)12 lamellas and CsCl-like layers. Because of the layered nature of the Cs7Nd11(SeO3)12Cl16 structure, it features numerous planar defects originating from occasionally missing the CsCl-like layer and violating the perfect stacking of the Nd11(SeO3)12 lamellas. Cs7Nd11(SeO3)12Cl16 represents the first example of a noncentrosymmetric structure among alkaline-metal lanthanide selenite halides. Cs7Nd11(SeO3)12Cl16 demonstrates luminescence emission in the near-IR region with reduced efficiency due to a high concentration of Nd(3+) ions causing nonradiative cross-relaxation.

Incommensurate modulated structures and luminescence in scheelites

Scheelite (CaWO4) related compounds (A,A)n[(B,B)O4]m with B, B=W and/or Mo are promising new materials for red phosphors in pc-WLEDs (phosphor-converted white-light-emitting-diode) and solid-state lasers. Scheelites can be prepared with a large concentration of vacancies in the A sublattice, giving compositions characterized by a (A+A):(BO4+BO4) ratio different from 1:1. The creation of cation vacancies in the scheelite-type framework and the ordering of A cations and vacancies are a new factor in controlling the scheelite-type structure and properties. Very often the substitution of Ca2+ by M+ and R3+ (R3+ = rare earth elements) in the scheelite-type structure leads to switching the structure from 3D to (3+n)D (n = 1,2) regime. The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor controlling the scheelite-type structure and luminescent properties of CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y (0x1, 0y1) solid solutions. Within this series all complex molybdenum oxides have (3+2)D incommensurately modulated structures with superspace group I41/a(,,0)00(-,,0)00, while the structures of all tungstates are (3+1)D incommensurately modulated with superspace group I2/b(0)00. In both cases the modulation arises due to cation-vacancy ordering at the A site. The replacement of the smaller Gd3+ by the larger Eu3+ at the A-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo6+ by W6+ switches the modulation from (3+2)D to (3+1)D regime. Acknowledgement. This research was supported by FWO (project G039211N, Flanders Research Foundation) and Russian Foundation for Basic Research (Grants 11-03-01164, and 12-03-00124).

CCDC 798441: Experimental Crystal Structure Determination

Related Article: Ya-guang Sun, Bing Jiang, Tian-fang Cui, Gang Xiong, P.F.Smet, Fu Ding, En-jun Gao, Tian-yi Lv, K.Van den Eeckhout, D.Poelman, F.Verpoort|2011|Dalton Trans.|40|11581|doi:10.1039/c1dt10156k