From mystery to modulation: the structural story of Rb 2 [Si 2 O 5 ]

The crystal structure of Rb 2 [Si 2 O 5 ], a phyllosilicate with previously questionable structural details, has been solved and refined as an incommensurately modulated phase in (3 + 1)-dimensional superspace. This paper describes the monoclinic structure in superspace group C 2/ c (0β0) s 0 with unit-cell parameters a = 9.8662 (6), b = 8.3986 (5), c = 14.7641 (9) Å, β = 90.114 (5)°, V = 1223.4 (1) Å 3 and with modulation wavevector q = 0.377 (1) b *, refined to an R 1 value of 0.0419 for 2603 reflections with intensities greater 3σ. All [SiO 4 ] tetrahedra within the structure adopt two distinct orientations, and the positions of their constituent silicon and oxygen atoms are modeled using a combination of crenel and positional modulation functions. For the rubidium atoms, harmonic modulation waves were used for the atomic coordinates and the anisotropic displacement parameters. Rubidium cations are predominantly coordinated by six oxygen atoms, forming linkages between adjacent layers built of [SiO 4 ] tetrahedra. However, the ionic radius of Rb in comparison to the mesh-size of the silicate layer leads to deformation and modulation of the layers. The modulation results in more balanced bond valence sums for rubidium, acceptable Si–O distances and anisotropic displacement parameters, which is not the case for a three-dimensionally periodic structure model with split atom positions. A comparison with related phyllosilicates featuring the same 4.8 2 net layer topology, but with larger interlayer cations or higher cationic content, reveals that these structures exhibit more relaxed silicate layers. This highlights the role of cation size and content in influencing the structural modulation and strain within the crystal structure of phyllosilicates.