Crystal Chemistry of the Microporous Zirconosilicate Na6Zr3[Si9O27], a Product of High-Temperature Transformation of Catapleiite, and Its Ag-Exchanged Form

Abstract

D.A.K.); igorpekov@mail.ru (I.V.P.); art4anya@ya.ru (A.A.A.); Abstract: The crystal structure of the Ag-exchanged form of the zirconosilicate with the simplified formula (Na 6-2x Ca x x )Zr 3 [Si 9 O 27 ] with x < 1 (the idealized formula Na 6 Zr 3 [Si 9 O 27 ]), a product of thermal transformation of catapleiite, ideally Na 2 Zr[Si 3 O 9 ]·2H 2 O, was studied using single crystal X-ray diffraction data. The crystal structure of Na 6 Zr 3 [Si 9 O 27 ] is based on a heteropolyhedral framework built by nine-membered tetrahedral rings [Si 9 O 27 ] and isolated [ZrO 6 ] octahedra. This zirconosilicate demonstrates high exchange capacity to Ag (experiment with 1 M AgNO 3 aqueous solution, 250 • C, 30 days). Its Ag-exchanged form with the simplified formula (Ag 5 Ca 0.5 )Zr 3 [Si 9 O 27 ] is characterized by a significant distortion of the heteropolyhedral framework and strongly disordered arrangement of extra-framework cations (Ag) which results in the doubling of a parameter of the hexagonal unit cell [a = 23.3462(3), c = 10.10640(10) Å, V = 4770.45(13) Å 3 ] and space group P6 3 cm. Ag + cations preferably occupy the sites that are close to the Na sites in Na 6 Zr 3 [Si 9 O 27 ].

The role of local heteropolyhedral substitutions in the stoichiometry, topological characteristics and ion-migration paths in the eudialyte-related structures: a quantitative analysis

“…titano-and niobosilicates) that frequently contain subunits of corner-or edge-sharing M-centered polyhedra, Zrcentered polyhedra in zirconosilicates are usually isolated from each other. Most microporous zirconosilicate structures possess large cages and channels, resulting in interesting ionexchange properties of respective materials (Pekov et al, 2010;Zubkova et al, 2007Zubkova et al, , 2020Zubkova et al, , 2019Fewox et al, 2011;Coleman et al, 2010;Ferreira et al, 2010;Mesto et al, 2014;Chukanov et al, 2010;Aksenov et al, 2016;Zubkova & Pushcharovsky, 2008). Despite the presence of large N(1)-N(5) cages, ionexchange properties of the EGMs have never been experimentally confirmed (Chukanov et al, 2004;.…”

“…The frameworks may contain different types of anionic ligands: apical (p I -type), 'bridged' (p II -type), and triplebonded (p III -type) O atoms. Moreover, the crystal structure is characterized by unique fragments, such as the ninemembered rings of tetrahedra, which were found in EGMs only and in the compounds Na 6 {Zr 3 [Si 9 O 27 ]} (Zubkova et al, 2020) (a product of high-temperature transformation of catapleiite) and Na 6 { VI Si 3 [ IV Si 9 O 27 ]}-HP (Fleet, 1996). In both structures, the micro-region at the center of the ninemembered ring is filled by the MO 6 octahedra [M = Zr (Zubkova et al, 2020) or Si (Fleet, 1996)], while, in the EGMs, this micro-region can be filled by additional T* and M* cations or remain vacant.…”

“…Moreover, the crystal structure is characterized by unique fragments, such as the ninemembered rings of tetrahedra, which were found in EGMs only and in the compounds Na 6 {Zr 3 [Si 9 O 27 ]} (Zubkova et al, 2020) (a product of high-temperature transformation of catapleiite) and Na 6 { VI Si 3 [ IV Si 9 O 27 ]}-HP (Fleet, 1996). In both structures, the micro-region at the center of the ninemembered ring is filled by the MO 6 octahedra [M = Zr (Zubkova et al, 2020) or Si (Fleet, 1996)], while, in the EGMs, this micro-region can be filled by additional T* and M* cations or remain vacant. In the last case, the terminal p I -type O atoms of three inner SiO 4 tetrahedra of the Si 9 O 27 ring should be protonated and form silanol Si-OH groups because of BVS requirements.…”