High-Pressure Synthesis of the Iodide Carbonate Na 5 (CO 3 ) 2 I

: Here, we present the synthesis of a novel quaternary compound, iodide carbonate Na 5 (CO 3 ) 2 I, at 18(1) and 25.1(5) GPa in laser-heated diamond anvil cells. Single-crystal synchrotron X-ray diffraction provides accurate structural data for Na 5 (CO 3 ) 2 I and shows that the structure of the material can be described as built of INa 8 square prisms, distorted NaO 6 octahedra, and trigonal planar CO 32 − units. Decompression experiments show that the novel iodide carbonate is recoverable in the N 2 atmosphere to ambient conditions. Our ab initio calculations agree well with the experimental structural data, provide the equation of state, and shed light on the chemical bonding and electronic properties of the new compound.


Introduction
Inorganic carbonates and alkali halides are two common classes of compounds that are ubiquitous in nature.Their high-pressure (HP) behavior has been intensively studied for decades.The HP synthesis of new carbonates enriched the list of C-O anions, including now not only the planar trigonal [CO 3 ] 2− group with the sp 2 -hybridized carbon, common for numerous carbonates known at ambient conditions, but also "the sp 3 carbonates" [1] and other anions (ex.[CO 4 ] 4− and [C 2 O 5 ] 2− ) [2][3][4].Recent HP studies on alkali halides (such as the synthesis of a series of novel compounds including NaCl 3 , Na 2 Cl 3 , and Na 4 Cl 5 [5,6]) provide examples of how the chemistry of materials dramatically changes under HP [7,8].
The successful synthesis of alkali metal halide carbonate compounds has rarely been reported, and, so far, only two alkali metals fluoride carbonates (K 3 CO 3 F and Rb 3 CO 3 F) can be found in the Inorganic Crystal Structure Database (ICSD) [9].The characterization of complex (quaternary) compounds in a laser-heated diamond anvil cell (LHDAC) has been extremely difficult.However, recent developments in the methodology of experiments on the single-crystal X-ray diffraction (SCXRD) from multiphase microcrystalline samples, as well as in the data analysis, especially using the DAFi program [10] for data processing, have made possible the structural characterization of individual phases in complex mixtures in DACs.
In this study, we synthesized a novel iodide carbonate Na 5 (CO 3 ) 2 I through a reaction of sodium carbonate Na 2 CO 3 with iodine I 2 at 18(1) GPa and a reaction of sodium iodide containing hygroscopic water (NaI + H 2 O) with carbon tetraiodide CI 4 at 25.1(5) GPa after heating the mixtures to ~2000 K in DACs.The crystal structure of Na 5 (CO 3 ) 2 I was solved and refined.Our ab initio calculations support experimental observations and reveal the electronic properties and chemical bonding of the novel compound.

Sample Preparation
Two BX90-type screw-driven diamond anvil cells (DACs) [11] equipped with 250 µm culet diamond anvils were used.The sample chambers were formed by the pre-indenting of steel (DAC #1) and rhenium (DAC #2) gaskets to ~25 µm thickness and laser-drilling a hole of 120 µm.DAC #1 was loaded with a plate (~5 µm thick, filling the whole sample chamber) of well-dried sodium carbonate Na 2 CO 3 together with a piece (~10 µm in diameter and ~5 µm thick) of solid iodine.DAC #2 was loaded with one stack of a plate of NaI (2-5 µm thick, filling the whole sample chamber) and a piece of CI 4 .Although Na 2 CO 3 and NaI powders were dried on a heating table at 220 • C for 48 h before loading, NaI still contained water, being very hygroscopic.The pressure was measured in situ using the first-order Raman mode of the stressed diamond anvils [12].Double-sided laser-heating of the samples up to ~2000 K was performed in the home laboratory at the Bayerisches Geoinstitut [13]; iodine and CI 4 served as laser light absorbers.

X-ray Diffraction
Synchrotron X-ray diffraction (XRD) measurements of the compressed samples were performed at ID15b (λ = 0.4100 Å, beam size ~1.5 × 1.5 µm 2 ) of the EBS-ESRF in Grenoble, France.In order to determine the sample position for single-crystal X-ray diffraction data acquisition, full X-ray diffraction mapping of the pressure chamber was performed.The sample positions displaying the greatest number of single-crystal reflections belonging to the phases of interest were chosen, and step-scans of 0.5 • from −36 • to +36 • ω were performed.CrysAlis Pro software (version 43.67a) [14] was utilized for single-crystal data analysis.To calibrate the instrumental model in the CrysAlis Pro software, i.e., the sample-todetector distance, detector's origin, offsets of the goniometer angles, and inclination of both the X-ray beam and detector surface with respect to the instrument axis, we used a single crystal of orthoenstatite [(Mg 1.93 Fe 0.06 )(Si 1.93 Al 0.06 )O 6 , Pbca space group, a = 8.8117(2) Å, b = 5.1832(10) Å, and c = 18.2391(3)Å].The DAFi program [10] was used for the search of reflections' groups belonging to individual single-crystal domains.The crystal structures were then solved and refined using OLEX2 (version 1.5) [15] software.The crystallite sizes were estimated from X-ray maps using XDI software (version 1.0.0.210) [16].The crystallographic information is available in below Table 1 of Section 3.1.Powder XRD patterns during the decompression experiments were obtained using in-house XRD with a high-brilliance Rigaku diffractometer (Ag Kα radiation) (λ = 0.5609 Å) equipped with Osmic focusing X-ray optics and a Bruker Apex-II CCD detector.

Density Functional Theory Calculations
First-principles calculations were performed using the framework of density functional theory (DFT), as implemented in the Vienna Ab initio Simulation Package (VASP.5.4.4) [17].The Projector-Augmented-Wave (PAW) method [18,19] was used to expand the electronic wave functions in a plane wave basis.The Generalized Gradient Approximation (GGA) functional was used for calculating the exchange-correlation energies, as proposed by Perdew-Burke-Ernzerhof (PBE) [20].The PAW potentials adapted from the VASP library with the following valence configurations of 3s for Na, 5s5p for I, 2s2p for C, and 2s2p for O were used with the Na, I, C, and O POTCARs.The plane-wave kinetic energy cutoff was set to 800 eV.We performed variable cell relaxations including lattice parameters and atomic positions on the synthesized experimental structure to optimize the atomic coordinates and the cell vectors until the total forces were smaller than 10 −3 eV Å −1 per atom using the conjugate-gradient (CG) algorithm.In geometry optimization, we used a Gamma-centered k-mesh of 7x7x3.To increase the accuracy of ground-state electron density and density of states (eDOS), a denser Gamma-centered k-mesh of 11x11x11 and the tetrahedron smearing method with Blöchl corrections (ISMEAR = −5) were used.The crystal structure and the ELF visualization were made with VESTA software (version 3.5.7)[21].The finite displacement method, as implemented in PHONOPY [22], was used to calculate harmonic phonon frequencies and phonon band structures.A supercell size of 2 × 2 × 2 was used with a k-mesh size of 2 × 2 × 2 for the harmonic phonon calculations at 0 K.

Crystal Structure of the Iodide Carbonate Na 5 (CO 3 ) 2 I
The structure of iodide carbonate Na 5 (CO 3 ) 2 I synthesized at 18(1) and 25.1(5) GPa has the tetragonal space group I4 1 /amd (#141) (Figure 1).This structure type has not been known before.Full crystallographic data and refinement details are provided in Table 1, and the CIFs are deposited at CSD 2348295 and 2348296.The lattice parameters are a = 6.4543(17)Å and c = 14.638(6)Å at 18(1) GPa.In the structure of Na 5 (CO 3 ) 2 I, iodine atoms are in the 4b Wyckoff site, Na1 and Na2 atoms are in the 4a and 16f sites, O1 and O2 atoms are in the 8e and 16h sites, and C atoms are in the 8e site.Each I atom is surrounded by eight Na2 atoms forming an INa 8 square prism with a height of ~3.66 Å and a basis length of ~3.26 Å at 18(1) GPa.(Figure 1).The prisms are linked through common edges forming the 3D framework, with rectangular channels running along the a and b directions (Figure 1a).Sodium Na1 atoms and CO 3 groups are located in the channels (Figure 1b).The Na1 atoms are coordinated by six O atoms (two O1 and four O2) forming a distorted octahedron.These distorted octahedra are connected to each other through oxygen atoms of planar trigonal CO 3 2− units (Figure 1c).At 18(1) GPa, the Na-I distance in the INa 8 square prism is 2.9436( 16) Å, which is compatible with the distances in the known TlI-type sodium iodide NaI (~2.82-2.95Å) at 31 GPa [23]).As experimental structural data of Na 2 CO 3 at high pressures are absent, one could only compare the average Na-O distances in the NaO 6 octahedra of Na 5 (CO 3 ) 2 I (2.150(8) Å at 18(1) GPa) with the γ-Na 2 CO 3 at ambient (~2.35 Å) [24] or the P6 3 /mcm-Na 2 CO 3 at 20 GPa (~2.24 Å) from theoretical calculations [25].5) GPa, respectively).Considering this distortion, the planar trigonal CO 3 2− unit in Na 5 (CO 3 ) 2 I does not possess the equally delocalized π-bond between C and three O atoms (unlike the CO 3 2− unit in γ-Na 2 CO 3 , whose distortion at 1 bar is insignificant; average C-O1 vs. C-O2 are 1.281(3) vs. 1.283(2)Å, giving ~0.002 Å difference [24]).
Table 1.Crystal structure, data collection, and refinement details of Na 5 (CO 3 ) 2 I at 18(1) and 25.1 (5) GPa in comparison to the corresponding DFT-relaxed structure.

Stability and Decompression Behavior of Na 5 (CO 3 ) 2 I
The DFT calculations reproduced the crystal structure of the Na 5 (CO 3 ) 2 I compound in good agreement with the experiment (Table 1).The formation enthalpy (∆H) was calculated via the synthesis route of NaI + 2Na 2 CO 3 → Na 5 (CO 3 ) 2 I.The values of ∆H are equal to −0.217 eV/f.u. at 20 GPa (B1-type NaI [23] and the theoretically predicted P6 3 /mcm-Na 2 CO 3 [25] were adopted) and 0.130 eV/f.u. at 1 bar (average structure of the γ-Na 2 CO 3 [24] and B1-type NaI were adopted).The latter value corresponds to 26 meV/atom, which is well below the "standard" 70 meV/atom limit for metastability [26].Calcu-lated phonon modes of Na 5 (CO 3 ) 2 I revealed that the compound is dynamically stable both at the experimental volume of ~597 Å 3 (corresponding to theoretical pressure of ~21.5 GPa) (Figure 2a) and at 1 bar (Figure 2b).These results indicate that Na 5 (CO 3 ) 2 I, though metastable at 1 bar, is expected to be quenchable to ambient conditions.N2 atmosphere.Then, it was closed, and the pressure after closing was 4(3) GPa.A powder XRD pattern from the recovered sample was measured in-house (see Materials and Methods).It shows that the Na5(CO3)2I was still preserved after the decompression (Figure 3a,b).The pressure dependence of the volume per atom for Na5(CO3)2I (based on the pressure-volume relations from our DFT calculations) is shown in Figure 3c along with the experimental data points.Note that the experimental data point obtained at 4(3) GPa (black circle in Figure 3c) was a result of the Le Bail fit of the powder pattern.The measured experimental volumes from SCXRD (red dots in Figure 3c) matched with the theoretical calculations, and the bulk modulus of K0 = 43.6(3)GPa (V0 = 13.793(9)Å 3 /atom, K' = 5.10( 2)) was determined from the third-order Birch-Murnaghan equation of state (3BM EOS).DAC #1 was fully decompressed from 18(1) GPa and opened in the glove bag in an N 2 atmosphere.Then, it was closed, and the pressure after closing was 4(3) GPa.A powder XRD pattern from the recovered sample was measured in-house (see Materials and Methods).It shows that the Na 5 (CO 3 ) 2 I was still preserved after the decompression (Figure 3a,b).The pressure dependence of the volume per atom for Na 5 (CO 3 ) 2 I (based on the pressure-volume relations from our DFT calculations) is shown in Figure 3c along with the experimental data points.Note that the experimental data point obtained at 4(3) GPa (black circle in Figure 3c) was a result of the Le Bail fit of the powder pattern.The measured experimental volumes from SCXRD (red dots in Figure 3c) matched with the theoretical calculations, and the bulk modulus of K 0 = 43.6(3)GPa (V 0 = 13.793(9)Å 3 /atom, K' = 5.10(2)) was determined from the third-order Birch-Murnaghan equation of state (3BM EOS).

Electronic Properties of Na5(CO3)2I
In order to analyze the electronic properties and chemical bonding of the Na5(CO3)2I compound, we performed DFT calculations of the electron localization function (ELF), band structure, and total and projected electron densities of states (TDOS and PDOS) (Figure 4).In order to analyze the electronic properties and chemical bonding of the Na 5 (CO 3 ) 2 I compound, we performed DFT calculations of the electron localization function (ELF), band structure, and total and projected electron densities of states (TDOS and PDOS) (Figure 4).

Conclusions
To summarize, we synthesized a novel iodide carbonate Na5(CO3)2I at 18(1) and 25.1(5) GPa.The structures were solved and refined in situ using HP synchrotron SCXRD in LHDACs.On the basis of the structural data and the results of ab initio calculations, we revealed the chemical bonding, dynamical stability, equation of state, and electronic properties of the new compound.The ELF calculated in the (100) plane of the Na 5 (CO 3 ) 2 I compound confirms that the I-Na2 and Na1-O interactions (Figure 1c) are closed-shell interactions (ionic), and the C-O form covalent bonds by sharing electrons (Figure 4a).The calculated band structure using the PBE GGA functional shows that Na 5 (CO 3 ) 2 I has a wide bandgap of 5.447 eV at 21.5 GPa (corresponding to the experimental volume of ~597 Å 3 ) (Figure 4b), which means the insulator behavior of Na 5 (CO 3 ) 2 I.The electronic states are highly localized and the valence band maximum (VBM) occurs between M and S high symmetric points, and the conducting band minimum (CBM) is located between S 0 and Γ (Figure 4b).In the eDOS of the Na 5 (CO 3 ) 2 I compound, the I-5p and O-2p orbitals contribute to the highest energy of VBM, while O-2p and C-2p contribute to the lowest energy of CBM (Figure 4c), which is similar to the case of γ-Na 2 CO 3 [27] (O-2p contribute to the highest energy of VBM and O-2p and C-2p contribute to the lowest energy of CBM).The bandgap of Na 5 (CO 3 ) 2 I is relatively large if one compares it with the bandgap of γ-Na 2 CO 3 (3.69eV using PBE GGA [28], and 3.94 eV using PW91 GGA [27]), or the B1-NaI (3.9 eV using PBE GGA [29]) at 1 bar.

Conclusions
To summarize, we synthesized a novel iodide carbonate Na 5 (CO 3 ) 2 I at 18(1) and 25.1(5) GPa.The structures were solved and refined in situ using HP synchrotron SCXRD in LHDACs.On the basis of the structural data and the results of ab initio calculations, we revealed the chemical bonding, dynamical stability, equation of state, and electronic properties of the new compound.

Figure 1 .
Figure 1.The structure of the novel iodide carbonate Na 5 (CO 3 ) 2 I compound synthesized in this work: (a) 3D framework of the INa 8 cubes (carbon and oxygen atoms are not shown); (b) unit cell (I, Na, O, and C atoms are shown in purple, yellow, red, and brown, respectively); (c) building blocks of the structure: INa 8 and Na1O 6 polyhedra and planar CO 3 groups.The interatomic distances (in Å) in the trigonal CO 3 2− at 18(1) (black numbers) and 25.1(5) GPa (red numbers) are notated.The C-O distances in the triangular CO 3 2− unit are different: at 18(1) GPa, the C-O1 distance is 1.287(14) Å, whereas the two C-O2 bonds are 1.256(9) Å by, that is by ~0.03 Å shorter (black numbers in Figure 1c).At 25.1(5) Gpa, the C-O1 and C-O2 are 1.318(19) and 1.254(12) Å, respectively (red numbers in Figure 1c), giving ~0.06 Å difference due to the shortening of the C-O1 bonds oriented along the c direction.The length of the C-O2 bond is practically unaffected.Bond angles in the CO 3 2− unit are slightly off from the ideal sp 2 hybridization angle of 120 degrees (O1-C-O2 vs. O2-C-O2 are 118.4(6) vs. 123.2(11)and

Figure 2 .
Figure 2. Phonon dispersion curves calculated at (a) experimental volume of ~597 Å 3 , which corresponds to theoretical pressure at ~21.5 GPa, and (b) 1 bar along high-symmetry directions in the Brillouin zone.

Figure 2 .
Figure 2. Phonon dispersion curves calculated at (a) experimental volume of ~597 Å 3 , which corresponds to theoretical pressure at ~21.5 GPa, and (b) 1 bar along high-symmetry directions in the Brillouin zone.

Figure 3 .
Figure 3.Some experimental and calculation data for Na 5 (CO 3 ) 2 I. (a) Powder XRD pattern collected from the recovered sample of Na 5 (CO 3 ) 2 I at 4(3) GPa compared with that at the previous pressure point of 18(1) Gpa.Miller indices for Na 5 (CO 3 ) 2 I are notated with black numbers; "G" refers to the gasket material.(b) Le Bail fit of the powder XRD pattern of Na 5 (CO 3 ) 2 I at 4(3) GPa; vertical ticks correspond to Bragg peaks of Na 5 (CO 3 ) 2 I. X-ray wavelength λ = 0.5609Å.(c) The pressure dependence of the volume per atom based on the pressure-volume relations from our DFT calculations.The dashed line represents DFT-calculated volume for given pressures fitted by the 3BM EOS (V 0 = 13.793(9)Å 3 /atom, K 0 = 43.6(3)GPa, K' = 5.10(2)).Black circles (powder XRD) and red dots (SCXRD) represent our experimental data points.

Figure 4 .
Figure 4. Calculated properties of Na 5 (CO 3 ) 2 I at 21.5 GPa (experimental volume of ~597 Å 3 ).(a) ELF calculated in the (100) plane, (b) band structure, and (c) TDOS and PDOS curves of Na 5 (CO 3 ) 2 I. I, Na, O, and C atoms are shown in purple, yellow, red, and brown, respectively.The horizontal (in (b)) and vertical (in (c)) dashed black lines indicate the Fermi energy.