The crystal structure of the monohydrate R2Mo6O21·H2O (R=Pr, Nd, Sm, and Eu): a layer structure containing disordered [Mo2O7]2− groups

Although R2O3:MoO3=1:6 (R=rare earth) compounds are known in the R2O3–MoO3 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 R2Mo6O21·H2O (R=Pr, Nd, Sm, and Eu) by thermal decomposition of [R2(H2O)12Mo8O27]·nH2O at around 685–715 °C for 2 h, and determined their crystal structures. The simulated XRD patterns of R2Mo6O21·H2O were consistent with those of previously reported R2O3:MoO3=1:6 compounds. All R2Mo6O21·H2O compounds crystallize isostructurally in tetragonal, image (No. 130), image, 8.9689(6), 8.9207(4), and 8.875(2) Å; image, 26.519(2), 26.304(2), and 26.15(1) Å; image; image, 0.024, 0.024, and 0.021, for R=Pr, Nd, Sm, and Eu, respectively. The crystal structure of R2Mo6O21·H2O consists of two [Mo2O7]2−-containing layers (A and B layers) and two interstitial R(1)3+ and R(2)3+ cations. Each [Mo2O7]2− group is composed of two corner-sharing [MoO4] tetrahedra. The [Mo2O7]2− in the B layer exhibits a disorder to form a pseudo-[Mo4O9] group, in which four Mo and four O sites are half occupied. R(1)3+ achieves 8-fold coordination by O2− to form a [R(1)O8] square antiprism, while R(2)3+ achieves 9-fold coordination by O2− and H2O to form a [R(2)(H2O)O8] monocapped square antiprism. The disorder of the [Mo2O7]2− group in the B layer induces a large displacement of the O atoms in another [Mo2O7]2− group (in the A layer) and in the [R(1)O8] and [R(2)(H2O)O8] polyhedra. A remarkable broadening of the photoluminescence spectrum of Eu2Mo6O21·H2O supported the large displacement of O ligands coordinating Eu(1) and Eu(2).