✍ Scribed by Haruo Naruke; Toshihiro Yamase
No coin nor oath required. For personal study only.
The crystal structure of R 2 Mo 6 O 21 Á H 2 O consists of two [Mo 2 O 7 ] 2À -containing layers (A and B layers) and two interstitial R(1) 3+ and R(2) 3+ cations. Each [Mo 2 O 7 ] 2À group is composed of two corner-sharing [MoO 4 ] tetrahedra. The [Mo 2 O 7 ] 2À in the B layer exhibits a disorder to form a pseudo-[Mo 4 O 9 ] group, in which four Mo and four O sites are half occupied. R(1) 3+ achieves 8-fold coordination by O 2À to form a [R(1)O 8 ] square antiprism, while R(2) 3+ achieves 9-fold coordination by O 2À and H 2 O to form a [R(2)(H 2 O)O 8 ] monocapped square antiprism. The disorder of the [Mo 2 O 7 ] 2À group in the B layer induces a large displacement of the O atoms in another [Mo 2 O 7 ] 2À group (in the A layer) and in the [R(1)O 8 ] and [R(2)(H 2 O)O 8 ] polyhedra. A remarkable broadening of the photoluminescence spectrum of Eu 2 Mo 6 O 21 Á H 2 O supported the large displacement of O ligands coordinating Eu(1) and Eu(2).
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