One-dimensional fibrous low dimensional caesium silicate, LDS-1, was synthesized by performing solid-state reactions with dry gels of silica containing tetrabutylammonium and caesium cations. We report the detailed crystal structure of LDS-1 comprising alternating alignments of zigzagged frameworks with four-membered rings and strong hydrogen bonds between adjacent terminal silanols; this structure is very similar to that of CsHSi 2 O 5 , whereas their crystal symmetries are different. The chain-like frameworks with a Q 3 ((SiO) 3 SiOH) local structure form an elliptical topology with pseudo-micropores by strong hydrogen bonding. The hydrogen bonding was clearly observed in the electron density distributions analyzed by the maximum entropy method using high-resolution synchrotron powder diffraction data. Solid-state MAS NMR spectroscopy also exhibits strong hydrogen bonding between adjacent oxygen atoms, as indicated by an atomic distance of d(O-O) E 2.45 A ˚. The elliptical spaces hold large caesium cations. Experimental SynthesisWe initially discovered the LDS series while examining the effect of alkali-metal cations on the solid-state synthesis 25 of

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