The crystal structure of the monohydrate R2Mo 6O 21·H 2O ( R=Pr, Nd, Sm, and Eu): a layer structure containing disordered [Mo 2O 7] 2− groups

Abstract

Although R 2O 3:MoO 3=1:6 ( R=rare earth) compounds are known in the R 2O 3–MoO 3 phase diagrams since a long time, no structural characterization has been achieved because a conventional solid-state reaction yields powder samples. We obtained single crystals of R 2Mo 6O 21·H 2O ( R=Pr, Nd, Sm, and Eu) by thermal decomposition of [ R 2(H 2O) 12Mo 8O 27]· nH 2O at around 685–715 °C for 2 h, and determined their crystal structures. The simulated XRD patterns of R 2Mo 6O 21·H 2O were consistent with those of previously reported R 2O 3:MoO 3=1:6 compounds. All R 2Mo 6O 21·H 2O compounds crystallize isostructurally in tetragonal, P 4 / ncc (No. 130), a = 8.9962 ( 5 ) , 8.9689(6), 8.9207(4), and 8.875(2) Å; c = 26.521 ( 2 ) , 26.519(2), 26.304(2), and 26.15(1) Å; Z = 4 ; R 1 = 0.026 , 0.024, 0.024, and 0.021, for R=Pr, Nd, Sm, and Eu, respectively. The crystal structure of R 2Mo 6O 21·H 2O consists of two [Mo 2O 7] 2−-containing layers ( A and B layers) and two interstitial R(1) 3+ and R(2) 3+ cations. Each [Mo 2O 7] 2− group is composed of two corner-sharing [MoO 4] tetrahedra. The [Mo 2O 7] 2− in the B layer exhibits a disorder to form a pseudo-[Mo 4O 9] group, in which four Mo and four O sites are half occupied. R(1) 3+ achieves 8-fold coordination by O 2− to form a [ R(1)O 8] square antiprism, while R(2) 3+ achieves 9-fold coordination by O 2− and H 2O to form a [ R(2)(H 2O)O 8] monocapped square antiprism. The disorder of the [Mo 2O 7] 2− group in the B layer induces a large displacement of the O atoms in another [Mo 2O 7] 2− group (in the A layer) and in the [ R(1)O 8] and [ R(2)(H 2O)O 8] polyhedra. A remarkable broadening of the photoluminescence spectrum of Eu 2Mo 6O 21·H 2O supported the large displacement of O ligands coordinating Eu(1) and Eu(2).