Crystal Chemistry of the Microporous Zirconosilicate Na 6 Zr 3 [Si 9 O 27 ], a Product of High-Temperature Transformation of Catapleiite, and Its Ag-Exchanged Form

: The crystal structure of the Ag-exchanged form of the zirconosilicate with the simplified formula (Na 6-2 x 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 P 6 3 cm . Ag + cations preferably occupy the sites that are close to the Na sites in Na 6 Zr 3 [Si 9 O 27 ].


Introduction
The search for new crystalline microporous materials with technologically important properties (ion-exchange, sorption, radionuclide immobilization, catalytic properties, etc.) is one of the actual problems of modern crystallography and materials science. Among such materials, much attention is attracted to minerals, their synthetic analogues and related materials with heteropolyhedral (usually, octahedral-tetrahedral) frameworks. In particular, zeolite-like microporous zirconosilicates with the general formula of heteropolyhedral frameworks [ZrmSinO3m+2n] −2m are considered as perspective materials due to the prospects of their wide application in chemical technologies [1]. The review of crystal chemical features of natural zirconosilicates with heteropolyhedral frameworks is given in [2].
The crystal structure of catapleiite was first studied in 1936 [3]. Later different structural varieties of catapleiite, as well as the related mineral calciocatapleiite CaZrSi3O9·2H2O and K-exchanged form of catapleiite obtained in the laboratory, were studied. Crystallographic data for all known varieties of catapleiite, its K-exchanged form, and calciocatapleiite are listed, with corresponding references, in Table 1. All these zirconosilicates retain the catapleiite-type heteropolyhedral framework but differ from each other in the arrangement of extra-framework cations and H2O molecules that result in different symmetry.
Recently, thermal behavior of catapleiite with Ca admixture from Mt. Aikuaiventchorr in the Khibiny alkaline complex (Kola peninsula, Russia) has been investigated by means of X-ray diffraction, electroconductivity measurements (using impedance spectroscopy), DSC, TG, and IR spectroscopy [4]. It was revealed that two-step exothermal transformation preceded by dehydration results in the formation of the compound with the simplified formula (Na6-2xCax x)Zr3[Si9O27] with x < 1 (the idealized formula can be written as Na6Zr3[Si9O27]) which remains stable down to room temperature. This compound drastically differs in the structure from catapleiite: its structure is based on the heteropolyhedral framework which is built of nine-membered rings [Si9O27] formed by SiO4 tetrahedra which are connected with isolated [ZrO6] octahedra. The tetrahedral rings are located under each other. Two crystallographically non-equivalent Zr-centered octahedra play different role in the structure: Zr(2)O6 octahedra are located inside tetrahedral rings and share all vertices with tetrahedra thus forming heteropolyhedral columns which are linked via Zr(1)O6 octahedra [each Zr(1)O6 octahedron is linked to three columns]. Extra-framework Na (with admixed Ca) cations are located in wide channels of the framework and between the rings of SiO4 tetrahedra [4]. The crystal structures of catapleiite and Na6Zr3[Si9O27] are shown in Figure 1a,b. The topology of the heteropolyhedral framework of the latter and dimensions of zeolite-like channels allowed us to assume for it ion-exchange properties that were confirmed in [5].  Here we report data for the Ag-exchanged form of Na6Zr3[Si9O27]. The crystal data of both initial Na6Zr3[Si9O27] and its Ag-exchanged form are reported in Table 1.

Materials and Methods
The zirconosilicate Na6Zr3[Si9O27] obtained by annealing of catapleiite at 1000 °C was used for ion-exchange experiments carried out in titanium alloy (VT-8) autoclaves that had about 10-mL capacity. A 0.05 g sample of Na6Zr3[Si9O27] was placed in 5 mL of 1 M AgNO3 aqueous solution and heated for 30 days at 250 °C. The solid products were carefully washed free from entrained salts by deionized water and dried at 60 °C before further investigations.
Chemical data for the Ag-exchanged form of Na6Zr3[Si9O27] were obtained by means of a JEOL JXA-8230 instrument (WDS mode) at the Laboratory of Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University. Standard operating conditions included an accelerating voltage of 20 kV and beam current of 20 nA. The following standards were used: Ca and Si-CaMgSi2O6, Zr-ZrSiO4, Ag-Ag. Contents of other constituents, including Na, are below detection limits.
In order to obtain IR absorption spectra, powdered samples were mixed with anhydrous KBr, pelletized, and analyzed using an ALPHA FTIR spectrometer (Bruker Optics, Karlsruhe, Germany) at a resolution of 4 cm −1 . Then, 16 scans were collected for each spectrum. The IR spectrum of an analogous pellet of pure KBr was used as a reference.
The Ag-exchanged form of Na6Zr3[Si9O27] was studied using single-crystal X-ray diffraction (XRD) technique. The measured intensities were corrected for Lorentz, background, polarization, and absorption effects. Data reduction was performed using CrysAlisPro Version 1.171.39.46 [13]. The obtained silver zirconosilicate is hexagonal with doubled a parameter [a = 23.3462(3), c = 10.10640(10) Å, V = 4770.45(13) Å 3 , space group P63cm] as compared to those of Na6Zr3[Si9O27]: a = 11.5901(9), c = 9.9546(9) Å, V = 1158.05(16) Å 3 , space group P63/mcm [4]. The crystal structure of the Ag-exchanged form of Na6Zr3[Si9O27] was obtained by direct methods and refined using the SHELX software package [14]. Despite the final R value is rather low [0.0677 for 4089 unique reflections with I > 2σ(I)], relatively poor quality of the crystal which was firstly effected by thermal transformation and then involved in cation-exchange experiments prevented from the refinement of all sites in the anisotropic approximation. The crystal data and the experimental details are presented in Table 2. * This formula is written taking into account electron microprobe data whereas Ca was not included in the structure refinement. The refined numbers of electrons in Ag sites formally correspond to 5.35 Ag atoms per formula unit (apfu). ** Calculated based on structural data. *** Largest diff. peak is 0.64 Å from Zr4 site (the site with the smallest multiplicity) and largest hole is 1.28 Å from Ag4a site.

Chemical Composition
The chemical composition of the Ag-exchanged form of Na6Zr3 Representative chemical data are given in Table 3.

Infrared Specroscopy
The IR spectra of Na6Zr3[Si9O27] and its Ag-exchanged form (Figure 2) are similar and are typical for cyclosilicates. Estimation according to the correlation t = (1827 − k) (0.6428 k − 337.8) −1 where k ≈ 960 cm −1 is the weighted average frequency of Si-O stretching vibrations (in the range 800-1200 cm −1 ) , and t is the atomic ratio O: Si in the tetrahedral part of the crystal structure [15] results in t ≈ 3.05 and t ≈ 3.1 for Na6Zr3[Si9O27] and its Ag-exchanged form, respectively, which is close to the ideal value of 3. The distinct bands at 707 and 693 cm −1 in the IR spectra of Na6Zr3[Si9O27] and its Ag-exchanged form correspond to mixed vibrations of the tetrahedral ring (so-called "ring bands" [16]).
The Ag + cation has lower force characteristics (i.e., lower force constants of the metal-oxygen bonds) and is much heavier than Na + . As a result, most absorption bands in the IR spectrum of the Ag-exchanged form of Na6Zr3[Si9O27] are shifted towards lower frequencies as compared to the initial Na6Zr3[Si9O27]. In addition, the substitution of Na + for Ag + results in some enhancement of the mean refraction index and enhanced IR radiation scattering above 2000 cm −1 in the case of the Ag-exchanged form of Na6Zr3[Si9O27].
The absence of absorption bands in the range 1500-1700 cm −1 indicates the absence of H2O molecules in both compounds. The Ag-exchanged form of Na6Zr3[Si9O27] differs from the initial Na6Zr3[Si9O27] in the absence of absorption bands in the range 3300-3500 cm −1 which correspond to silanol groups Si-OH [4]. Consequently, the substitution Si-OH +δ → Si-O + Ag + takes place along with the main ion-exchange scheme Na + → Ag + .
Atom coordinates and displacement parameters for the Ag-exchanged form of Na6Zr3[Si9O27] are given in Table 4 and interatomic distances in Table 5. The crystal structure of the Ag-exchanged form of Na6Zr3[Si9O27] in comparison with those of catapleiite and Na6Zr3[Si9O27] is shown in Figure  1c. For better clarity, only the main Ag sites (see Table 4) are depicted. Figure 3 shows all extra-framework sites in the structure of the Ag-exchanged form of Na6Zr3[Si9O27].

Discussion
The studied compound Na6Zr3[Si9O27] contains nine-membered tetrahedral Si-O rings that is very rare for natural silicates. Among minerals, nine-membered rings of SiO4 tetrahedra are known only in members of the eudialyte group. It is noteworthy that eudialyte-group minerals are also microporous zirconosilicates. However, these minerals are only remotely related to the phase discussed in the present paper. Eudialyte-group minerals contain, besides nine-membered rings  [17,18]. In nature, eudialyte-group members demonstrate zeolitic properties [19].
Common structural fragment consisting of nine-membered rings of SiO4 tetrahedra connected via SiO6 octahedra was reported in the synthetic high-pressure compound Na6 VI Si3[ IV Si9O27] [20].
The microporous zirconosilicate Na6Zr3[Si9O27] demonstrates high exchange capacity to Ag+. Silver cations replace sodium; however, calcium cations remain practically unreplaced. As well as in the recently reported results of Ag exchange in another zirconosilicate elpidite, ideally Na2ZrSi6O15⋅3H2O [21], the incorporation of Ag+ in the structure of Na6Zr3[Si9O27] causes a significant distortion of the heteropolyhedral framework resulting in the doubling of a parameter of the hexagonal unit cell and the change of the space group from centrosymmetric P63/mcm (Na6Zr3[Si9O27]) to acentric P63cm (its Ag-exchanged form); in elpidite, the cation exchange accompanied by a significant distortion of the framework results in the doubling of the parameters of the orthorhombic unit cell and the change in symmetry from space groups Pma2 or Pbcm (characteristic to the initial elpidite samples from different localities) to Cmce. Funding: This work was supported by the Russian Foundation for Basic Research, grants nos. 18-29-12007-mk. A part of this work (IR spectroscopy) has been carried out in accordance with the state task, state registration number 0089-2019-0013.

Conflicts of Interest:
The authors declare no conflict of interest.