Different route of hydroxide incorporation and thermal stability of new type of water clathrate: X-ray single crystal and Raman investigation

Chlormayenite Ca12Al14O32[♦4Cl2] (♦-vacancy) is partially hydrated micro porouss mineral with hydroxide groups situated at various crystallographic sites. There are few mechanisms describing its hydration. The first one assumes Cl− substitution by OH− at the center of the structural cages (W-site). The second one determines the converting a T1O4 tetrahedron to a T1O3(OH)3 octahedron due to the replacement of oxygen at the O2 site by three OH-groups according to the scheme: (O2O2− + WCl−) → 3 × O2aOH. The third mechanism, not considered so far in the case of zeolite-like minerals, includes the hydroxide incorporation in form of hydrogarnet defect due to the arrangement of tetrahedral (OH)4 in vacant cages. This yields a strong hydrated phase containing even up to 35% of water more than in any currently known mineral applicable to Portland cement. Moreover, water molecules present in different structural cages are stable up to 355 K while dehydroxylation linked to the gradual loss of only 8% of OH− groups according to 3O2aOH− → O2O2− +WOH− + gH2O occurs at temperature range from 355 K to 598 K.

derivatives of C 12 A 7 have shown that 2 F − or 2 Cl − ions substitute the extra-framework, O 2− occupying the center of the structural cage 25 .
The key issue in order to characterize the synthetic mayenite and their natural counterparts is the analysis of hydration process. It is crucial in the development of new type of cement as well as in solving various geological problems. The process of water incorporation was previously reported due to the scheme: Ca 12 2 , where two OH − groups occupying the W-site are stable up to 1500 K 26,27 . However, temperatures associated with dehydration or dehydroxylation processes for these mineral phases have been not determined yet. Another mechanism of hydroxide incorporation is connected with the presence of the "unusual H 2 O molecule" which occupies the empty structural cages according to the scheme: Ca 12 4 (F,Cl) 2 ] 3, 5 . It has been shown that the molecular water is completely released from the mineral structure at a temperature range between 700-850 K.
This paper presents detailed studies of chlormayenite from Eifel with a simplified formula Ca 12 Al 13.5 Fe 3+ 0.5 O 31.3 (OH) 2.1 [♦ 4.7 Cl 1.3 ] as well as describes a various route of hydroxide incorporation into its crystal structure. The single crystal X-ray diffraction technique and Raman spectroscopy were applied to develop different models of hydroxylation. Structure stability under various temperatures were analyzed in detail based on ex-situ heated grains at 573 K, 773 K and 1073 K as well as in-situ one at the temperature range between 293 K and 873 K. Hence, the paper will provide a wide knowledge of the characterization of the mechanisms of hydroxide incorporation, H-bonding scheme and thermal stability of chlormayenite.

X-ray diffraction and Raman spectroscopy data of natural chlormayenite
The chemical composition of a natural chlormayenite is expressed by x1  end-member, the T2-site is four coordinated with the site symmetry −4, while the T1 with site symmetry 3 is situated at the threefold axis. T1-aluminum tetrahedra shares only three bonds with O1 while the fourth apex is formed by O2 oxygen (Fig. 1). The crystal framework is terminated by O2 which is not shared with an adjacent tetrahedron. Here, only 1/3 of the cages are occupied by chlorine and this member may be subdivided into 21% of Cl − containing at W-site and 42% of vacant W cages. The low values of atomic displacement for all tetrahedral framework sites indicates their very strong similarity independent of whether W-sites are occupied or vacant (data not shown) 2 . In turns, the Ca 12 Al 14 O 30 (OH) 6 end-member is characterized by octahedral T1 with O2 which may partially be replaced by three OH − groups (O2a). After such structural change, three of the O2a ligands form an equilateral triangle (model 1 in Fig. 1a). Moreover, the short distance of 1.76 Å between O2a and W requires that the W-site must be vacant. All structural cages are filled by a pair of X = Ca atoms with site symmetry 2 coordinated by four O1 and two O2 sites. Two X-sites enclose a central cage of about 5 Å in a diameter. Bond lengths for chlormayenite summarized in Table 1. The different cages considered here have following characteristics (1) 4T1 and 4T2 tetrahedra with vacant W in the center (C1); (2) 4T1 and 4T2 tetrahedra with chlorine in the center (C2); (3) 3T1 tetrahedra, T1 octahedron, and 4T2 tetrahedra with vacant W-site (C3); (4) 4T1 and 4T2 tetrahedra with hydroxide in the center in which the adjacent X-sites are shifted towards the center of the cage due to size and charge of the anion (C4) (Fig. 2).
The potential acceptors of hydrogen bonds in C3 (O2a donor) are the adjacent O2a of the AlO 3 OH 3 octahedral with 2.64 Å bond distance (Fig. 1a). The two O1 sites defining the triangular face O2a-O1-O1 of the AlO 3 OH 3 octahedron are the second closest potential H-bond acceptors with a donor-acceptor (D-A) distances of 2.74 Å and 2.78 Å (Fig. 1a). Further potential acceptors are determined for O1 of adjacent T2O 4 tetrahedra with D-A distances of 2.82 Å and another O1 across the cage with a D-A distance of 2.92 Å (model 1 in Figs 1b and 3a). It turns, X-ray structural refinement of C4 indicated D-A distance between W OH donor and O1 or O2 acceptors, respectively as 3.26 Å and 3.29 Å (model 2 in Fig. 3b). Furthermore, an indicator of hydrogen bonds seems also to be the bond valence calculations for the potential acceptor atoms 28,29 . Here, the calculate valence deficit of around . What is more, the O1 acceptor due to shorter d OH…O distance may form a stronger H-bond than oxygen in O2 site. However, the real molecular structure may differ from the model structure and bond valence calculations have to be treated with considerable caution. Hence, relying only on such approach, it is difficult to predict real configuration of the hydroxyl group orientation and the most favorable proton arrangement in particular O…O direction. Therefore, to solve the problem of hydroxide groups in chlormayenite crystal structure the theoretical and experimental Raman studies have been performed. Theoretical approach developed first by Nakamoto and corrected further by Libowitzky has a form of the equation which describes the dependence between the hydroxyl Raman band position and the O…O bond distance (d O…O ) 30,31 . Nakamoto diagram predicts an occurrence of Raman bands assigned to the OH modes in the region of 3660-2070 cm −1 when O…O distances varied in the range of 2.50-3.20 Å 31 . On the other hand, Libowitzky relation reports that the d O…O = 2.44-3.50 Å are linked to Raman bands in wide wavenumbers range from 3582 to 1758 cm −1 30 .
The fitting procedures performed for the hydroxyl region on the chlormayenite grains reveals a different type of vibrations which might be separated into three spectral regions of (1) 3750-3580 cm −1 , (2) 3580-3500 cm −1 and (3) 3500-3300 cm −1 (Fig. 3c). In the region (1) the bands appear at 3686, 3675, 3645 and 3610 cm −1 , however, their nature is not quite clear. According to Nakamoto diagram, these bands might correspond to the d O…O distance equal to 3.20, 3.10 and 3.00 Å suggesting the OH group presence within cage C4. However, Libowitzky equation showed, the d O…O distance is not fully conclusive and points to an atypical hydroxyl group arrangements. Moreover, the presence of O2a oxygen as a potential proton acceptor should lead to the formation of structural defects, which are statically or dynamically disordered in the mayenite structure. This issue is much more complicated and will be discussed in detail further on. The bands of the region (2) arise probably from the vibration of hydroxyl group occupied W-site of cage C4. It is worth to note that Nakamoto diagram does not work here well while Libowitzky formula predicts d O…O bond distance close to 3.10 Å and 3.00 Å. It corresponds to bands with the maximum at 3581 and 3538 cm −1 (model 2 in Fig. 3b) and correlates to interpretations for synthetic mayenite 32,33 . In turns, bands with relatively low intensity observed in the region (3) might originate from an O2a-O1 bond distance with length equal to 2.92 Å, and 2.82 Å as can be predicted on the base of theoretical XRD data for cage C3.

X-ray diffraction and Raman spectroscopy data of heat treated chlormayenite crystals
The structural studies and electron microbe analysis (EMPA) were carried out on individual grains and gave the opportunity to obtain their general crystal chemical formulas: ] can be explained by the different chlormayenite grains which were taken into consideration through the studies. However, the chemical composition of various grains is slightly different from each other and does not affect the further analysis. Noticeable differences are appearing during the annealing process up to 573 K, 773 K and 1073 K. Annealing at 573 K decreases the number of OH groups at O2a from originally 10% 2 to about 8% wherein the occupancy at the W-site slightly increases (Table S1 in supplementary file). Those changes may be described to the scheme: O2a 3OH − + ♦ → O2 O 2− + W OH − + g H 2 O and the data are in agreement with the model. In addition, the refined crystal structure of sample annealed at 573 K points to the splitting of Ca which results in occupancies of 0.973(2) for Ca1 and 0.027(2) for Ca2. However, the small electron density of OH − at W-site is overlapping with the one of Cl − and cannot be refined separately (Table S1 in supplementary file). After crystal annealing at 773 K and 1073 K, OH groups originally located at the O2a site are no longer detected (Table S1 in supplementary file) and T1 converts to tetrahedral coordination. In addition, the refined structure of crystal annealed at 773 K shows an increase of the electron density at W-site and 0.2 Cl − at W-site converges to ca. 0.14 additional OH − . This central OH attracts adjacent Ca, and leads to split the calcium positions into Ca1 and Ca2 (due to OH). Since a considerable amount of OH − moving to W-site at 773 K, the occupancy factor of OH − has been constrained to the occupancy of displaced Ca2 (0.147 (6)) while the occupancy of Cl − at W-site (0.199 (3)) were refined as detached from OH − (Table S1 in supplementary file). The data set of the measurement at 1073 K has been refined identically to the measurement carried out at 773 K. For all refinements, the anisotropic displacement parameters for OH − and Cl − at W-site have been set equally (Tables S2-S3 in supplementary file). It is worth to add that the bond distances between Ca1-O are practically unaffected by temperature (Table 1) while for the average Ca2-O bond distance, only an individual Ca2-O1 value increases to around 2.84 Å after annealing at 773 K and 1073 K (Table 1). Additionally, similar like for unheated crystal, the presence of hydroxyl groups at W-site at all of the annealed samples points to the potential H-bond acceptors at O1 (d OH…O ≈ 3.26 Å) and at O2 (d OH…O ≈ 3.29 Å). It should as well be noticed that the shift of Ca1 to Ca2 results in a higher bond valence deficit for the O1 (0.230 vu) sites than for O2 (0.226 vu) at the OH-populated cage wall. In consequence, the proton-acceptor oxygen in O1 site seems to be formed a stronger H-bond than oxygen in O2 site.
On the Raman spectrum of unheated crystal, weak bands at 3581 cm −1 and 3538 cm −1 resulted from the presence of single hydroxyl group occupied the central part of the C4 cage. Further, bands at 3474 and 3415 cm −1 reflect a process of partial hydration in which a small number of T1 aluminum changes its coordination from tetrahedral to octahedral as a result of further protonated oxygen by the scheme: O2 O 2-+ W Cl(F) − → O2a 3OH − . Moreover, to determine the dehydroxylation temperature and analyze the thermal stability of O2а ОН chlormayenite unit, the temperature-dependent in-situ Raman measurements on one single crystal were performed (Fig. 4). The thermal experiment has shown the change of the integral intensity values in two spectral hydroxyl regions which were associated with a reorganization of OH within C3. The intensities of the Raman bands in regions (1) 3750-3580 cm −1 and (3) 3500-3300 cm −1 decreased while for bands in the region (2) 3580-3500 cm −1 increased with rising temperature. Moreover, the disappearance of the band centered at 3686 cm −1 suggests vanishing of the absorbed water from a mineral surface while reorganization of OH groups starts above 355 K and ends at about 598 K (Fig. 4a,b).

Discussion
Recently published papers highlighted the existence of two routes of hydroxide incorporation into the chlormayenite crystal structure. The most energetically favorable mechanisms assumed replacement of one oxygen positioned at the O2 site by three OH − groups and converting of a T1O 4 tetrahedron to a T1O 3 (OH) 3 octahedron (model 1 in Figs 1 and 3a) as well as substituting of chlorine atom by one OH − ion into a central W-site (model 2 in Fig. 3b). The presence of O2a ОН groups distinguishes two O-O distances: O2a-O1 = 2.92 Å, O2a-O1 = 2.82 Å which due to Libowitzky correlation ought to be associated with medium strength hydrogen bonds 30 . In turns, the hydroxyl units occupied the W-site and give rise to the formation of a weak H-bonds by the location of hydrogen acceptors at O1 (d O…O ≈ 3.26 Å) and at O2 (d O…O ≈ 3.29 Å) with a bond-valence sum close to 1.73 vu for O1 and O2 oxygen. The theoretical models are referring to Raman spectrum in the 3600-3200 cm −1 range whereas the nature of bands with strong intensity above 3600 cm −1 is still not resolved and remains a subject of intense discussion.
In order to solve the origin of undefined chlormayenite bands on Raman spectrum, it is necessary to consider two concepts: (1) the process of hydroxide incorporation into the structure of garnets [34][35][36] and (2) electrostatic interaction between a proton and cations (including other protons) due to the formation of hydrogarnet defects (Fig. 5a). Hydrogrossular 37-39 , andradite 40 and other garnets belong to minerals forming at anomaly low pressures (~ few bars) and temperature of 1000-1350 K while the process of water incorporating into their structure provides formation of hydrogarnet defects in which four O atoms around a silicon vacancy are terminated by hydroxyl groups 34,35,38 (Fig. 5b). A similar condition is crucial to form synthetic Sr-hydrogarnet with a structure closely related to mayenite 36 . However, the degree of hydration strictly depends on the number of OH − ions, the length of shared or unshared polyhedral edges 41 . The high temperature and low pressure conditions are linked to the formation, structurally and chemically similar to garnets, porous mayenite-type phases with the possibility of storing inside the cage, haloids (e.g. chlorine, fluorine), or oxygen according to general pattern Ca 12 Al 14 O 28 O 4 [♦ 4 (F,Cl) 2 ] (Fig. 5c). In such minerals, the chlorine or fluorine ions distributed in the vacancies, form so-called mayenite defects 42 . Similar conditions of a mayenite-type mineral formation, relative to garnets might favor the emergence of hydrogarnet defects (Fig. 5b and c). However, their appearance will strictly depend on insufficiently high haloid activity, and high abundance of water in the environment. In addition, such groups will not only be linked to the surface but will be distributed in the whole structure (Fig. 5d). Therefore, the water capacity of the mayenite-type structure becomes even a few times higher than previously described in literature 2 .
To shed more light on chlormayenite spectrum in region (1), the obligatory is to look more precisely into the data for garnets. It is due to the correlation between position of hydroxide within polyhedral sites with bands centered between 3700-3500 cm −1 . Literature reports that the tetrahedral hydrogarnet substitution in hydrogrossular gives rise to overlapping bands centered around 3660 and 3600 cm −1 wherein the hydroxide content linearly correlates with the intensity of hydroxyl bands 43 . More detailed studies revealed that band around 3660 cm −1 originates from a presence of very weak hydrogen bond scheme within the (OH) 4 tetrahedron 34 . Here, one can look more closely on the electrostatic interaction between atoms to explain the origin of the band. The size of this subunit is smaller in relation to typical tetrahedron implying appearance of an electrostatic interaction. As a result, the repulsive force between proton and cations including other protons of OH groups starts to have a stronger influence than attraction force typically considered in a crystalline structure. As a result, such Raman band is shifted towards higher wavenumber. Similar band assignment was also found for a natural and synthetic hydroandradite. However, in the case of these minerals, a shift of hydroxyl bands towards lower wavenumber (3610 and 3560 cm −1 ) might be assigned to the greater average of tetrahedral cation-cation distance within (OH) 4 group, what implies a decrease of repulsive force and increase the share of attraction ones 44 . Finally, detailed studies for non-cubic garnet such as uvarovite-grossular highlighted that Raman bands observed at 3652, 3640 and 3602 cm −1 originate from vacancies on octahedral or dodecahedral cation sites 45 . Thus, the great variability and complexity of the chlormayenite spectrum prove that hydrogarnet substitution probably plays an essential role during water incorporation (model 3 in Fig. 6). Moreover, it may be a key process in clarifying a route of a hydration and help in a description of hydrogen storage mechanism in nominally anhydrous or low hydrous mayenite-type structure minerals. Hence, description of water incorporation will be the first step to expand physicochemical properties of mayenite-type phases, provides unique information about their formation, and finally ScIentIfIc RepORts | 7: 9046 | DOI:10.1038/s41598-017-08152-1 helps to develop the hypothesis of formation a strongly hydrated garnet-type structure mineral, katoite. This phase is typical for mayenite cement 46 and gives an opportunity to improve features of building materials, especially sulfur corrosion resistance 47 . It is worth to note here that the hydrogarnet defect leads to the appearance of repulsive force between protons within the (OH) 4 unit or proton and calcium acting the only way to explain the problem of Raman bands in the 3800 -3600 cm −1 region.
Raman experiment and X-ray diffraction data shed a new light on the temperature stability of O2a OH, and (OH) 4 with relation to W OH groups. The temperature-dependent in-situ Raman experiment shows that position of bands from regions (1) and (2) are slightly red shifted (Fig. 4a) while the intensity of bands from region (1) is decreasing with temperature rise. The band near 3682 cm −1 disappears at 355 K, supports its assignment to the OH − group vibration of the surface water (Fig. 4b). Moreover, in-situ Raman analysis shows that two bands of region (3) arising from OH vibration typically occurs in the octahedral aluminum coordination which disappears at around 355 K as an effect of removal of O2a OH moieties. These results confirm a very high instability of AlO 3 (OH) 3 units and provide proof of the transformation of structural subunits from octahedral to tetrahedral coordination (Fig. 4). More detailed ex-situ XRD and in-situ Raman experiments imply that aluminum tetrahedra above 355 K stayed intact without any trace of structural transformation. At the temperature of ~430 K, the most intense bands associated with hydrogarnet substitution are shifted towards higher wavenumber due to hydroxide mobility and weakening of the hydrogen interaction within the (OH) 4 groups. Similar outcomes were previously reported for hydrogrossular or Sr-hydrogarnet where hydroxyl group was stable up to 573 K 48,49 or 673 K 36 . It turned out that a strong diffusivity of hydroxyl moieties might be responsible for a complete disappearance of characteristic chlormayenite bands at temperatures above 598 K. At the same time, temperature-dependent Raman data were correlated with an increase of band intensity from the region (2)

Methods
The chemical composition was measured using a CAMECA SX100 electron microprobe operating in wavelength-dispersive spectrometry mode at 15 kV, 10-20 nA and using natural standards (Full description of the methodology is summarized in ref. 2). The single-crystal X-ray data were obtained using SuperNova Dual diffractometer with a mirror monochromator (MoKα, 0.71073 Å) and Atlas CCD detector. The structure was solved by direct methods, with subsequent analyses of difference-Fourier maps, and refined with neutral atom scattering factors using SHELX97 50 . Experimental details for untreated and annealed crystals summarized in supplementary file in Table S3. The Raman experiment was performed using WITec confocal CRM Alpha 300 Raman microscope at excitation laser line λ = 488 nm and CCD detector. The spectra were collected in the range between 4000-120 cm −1 with the spectral resolution of 3 cm −1 and integration time, 0.3 s for a single spectrum.