The microporous titanosilicate sitinakite, KNa₂Ti₄(SiO₄)₂O₅(OH)·4H₂O, was first discovered in the Khibiny alkaline massif. This material is also known as IONSIV IE-911 and is considered as one of the most effective sorbents for Cs⁺ and Sr²⁺ from water solutions. We investigate a mechanism of cooperative crystal chemical adaptation caused by the incorporation of La³⁺ ions into sitinakite structure by the combination of theoretical (geometrical–topological analysis, Voronoi migration map calculation, structural complexity calculation) and empirical methods (PXRD, SCXRD, Raman spectroscopy, scanning electron microscopy). The natural crystals of sitinakite (a = 7.8159(2), c = 12.0167(3) Å) were kept in a 1M solution of La(NO₃)₃ for 24 h. The ordering of La³⁺ cations in the channels of the ion-exchanged form La³⁺Ti₄(SiO₄)₂O₅(OH)·4H₂O (a = 11.0339(10), b = 11.0598(8), c = 11.8430(7) Å), results in the symmetry breaking according to the group–subgroup relation P4₂/mcm → Cmmm.

A simple method for the direct transformation of Sr-exchanged titanosilicate with the sitinakite structure (IONSIV) into ceramic material through cold pressing and subsequent sintering at 1100 °C for 4 h is presented. The temperature transformation of Sr-exchanged sitinakite showed the stages of recrystallization of the material with the formation of Sr-Ti phases matsubaraite (Sr₄Ti₅[Si₂O₇]₂O₈), jeppeite (SrTi₆O₁₃), tausonite (SrTiO₃), and rutile. Leaching experiments showed the efficiency of fixation of Sr cations in a ceramic matrix; extraction into water does not exceed 0.01% and desorption in 1 M HNO₃ solution is only 0.19% within three days. The leaching rates of immobilized Sr demonstrate the structural integrity of the formed phases in the ceramic (2.8 × 10⁻⁵–1.0 × 10⁻⁵ g/(m²·day). The decrease in the crystallinity of the initial Na-sitinakite, which is achieved by reducing the synthesis temperature from 250 to 210 °C, does not affect the sorption capacity and the fixation of cations in the ceramic matrix. The obtained results confirm the prospect of using inexpensive precursors, titanium ore enrichment waste, for the synthesis of sorption materials.

Titanosilicates with the structure of sitinakite and ivanyukite were synthesized with the use of enrichment wastes of quartz-leucoxene concentrate (Yarega deposit, Komi Republic) by autoclave method. It was found that using a mixture of mineralizer KOH and NaOH at a concentration of 0.15 and 1 mol/l, K-Na sitinakite was formed. Increasing the concentration of KOH to 0.45 mol/L and decreasing NaOH to 0.7 mol/L led to crystallization of ivanyukite. The obtained titanosilicates are characterized by high sorption activity towards Sr2+ and Cs+ cations. Sorption capacity of K-Na sitinakite and ivanyukite for Sr2+ was 95–110 and 102–114 mg/g, respectively. The Cs+ sorption capacity for K-Na sitinakite is 240 mg/g, the sorption capacity for ivanyukite is much higher — 370–380 mg/g. It was shown that the differences in the sorption capacity of K-Na sitinakite and ivanyukite are caused by the features of the crystal structure.