Synthesis and Characterization of Nacd 0.92 Sn 1.08 , Na(Cd 0.28 Sn 0.72 ) 2 and Na 2 CdSn 5 with Three-Dimensional Cd-Sn Frameworks

: The crystal structures of three new ternary compounds, NaCd 0.92 Sn 1.08 (I), Na(Cd 0.28 Sn 0.72 ) 2 (II), and Na 2 CdSn 5 (III) synthesized in a sodium-cadmium-tin system were determined by single-crystal X-ray analysis to be the following: (I) LiGeZn-type structure (hexagonal, a = 4.9326(1) Å, c = 10.8508(3) Å, space group P -6 m 2); (II) CaIn 2 -type structure (hexagonal, a = 4.8458(2) Å, c = 7.7569(3) Å, P 6 3 / mmc ); and (III) isotype with tI -Na 2 ZnSn 5 (tetragonal, a = 6.4248(1) Å, c = 22.7993(5) Å, I -42 d ). Each compound has a three-dimensional framework structure mainly composed of four-fold coordinated Cd and Sn atoms with Na atoms located in the framework space. Elucidation of the electrical properties of the polycrystalline samples indicated that compounds (I) and (II) are polar intermetallics with metallic conductivity, and compound (III) is a semiconducting Zintl compound. These properties were consistent with the electronic structures calculated using the ordered structure models of the compounds.


Introduction
Intermetallic compounds composed of alkali or alkali earth metals (A and AE, respectively) and early p-block elements (groups 13-15; B) are classified as polar intermetallics and Zintl phases [1][2][3][4]. These intermetallic compounds are known to present a variety of crystal structures comprising various polyanions, such as clusters and networks formed by highly electronegative B atoms that receive electrons from the A and AE atoms with low electronegativity. These intermetallic compounds have ionic and covalent bonds and exhibit electrical properties ranging from metallic to semi-metallic, semiconducting, and even superconducting behaviors [5][6][7][8]. The potential applications of these intermetallic compounds for thermoelectric conversion, photovoltaic power generation, and catalytic reactions have been investigated [8][9][10][11][12]. Furthermore, first-principles calculations have predicted that many intermetallic compounds [13][14][15][16] are topological materials that display unique electronic properties. Therefore, the synthesis of new polar intermetallics and Zintl phases and the characterization of their crystal structures and electronic properties have attracted interest in recent years.
Recently, Fässler et al. studied ternary alkali metal compounds containing Sn (group 14 element) and Zn (group 12 late-transition metal element), and synthesized nine intermetallic compounds in the Na-Zn-Sn system [17][18][19][20]. The crystal structures of the synthesized compounds were determined by single-crystal X-ray diffraction (XRD), and their electrical properties were evaluated by density functional theory (DFT) calculations. Seven out of the nine compounds contain clusters of deltahedra or icosahedra composed of Sn and Zn atoms to which electrons are donated from the Na atoms. The icosahedral clusters are characteristic of the compounds of group 13 elements. While, two polymorphs (body-centered tetragonal lattice (tI) and hexagonal primitive lattice (hP) phases) have

Synthesis of Na-Cd-Sn Compounds
Na (99.95 % purity, Nippon Soda Co. Ltd., Tokyo, Japan), Cd (99.9 % purity, Atom Shield Co. Ltd., Saitama, Japan), and Sn (99.999 % purity, Mitsuwa Chemicals Co. Ltd., Osaka, Japan) metals were used to synthesize the Na-Cd-Sn compounds. The Na metal was weighed in an Ar-filled glovebox (MBRAUN, Garching, Germany, O 2 , H 2 O < 1 ppm) to avoid reaction with oxygen and moisture in air, and the Cd and Sn metals were weighed in air at the prescribed molar ratios. The source metals (total mass of approximately 1.0 g) were put together in the Ar-filled glovebox and placed into a polycrystalline sintered BN crucible (purity, 99.5 %; inner diameter, 6.5 mm; depth, 18 mm; Showa Denko K.K., Tokyo, Japan) and sealed in a stainless steel (SUS 316) container (inner diameter of 10.7 mm, depth of 80 mm). The container was heated at 773 K for 2 h in an electric furnace to produce a melt-solidified bulk of the source metals. The obtained solid was pulverized using an agate mortar and pestle, and the powder was pressed into rectangular compacts (14 × 3 × 3 mm 3 ) by uniaxial die-pressing. The compacts were heated in a BN crucible in an Ar atmosphere at 593 K (for Na 2 CdSn 5 ) and 673 K (for NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 ) for 36 h. The pulverizing-molding-heating process was repeated twice to prepare a polycrystalline sintered sample, which was used to identify the crystalline phases in the samples and evaluate the electrical properties. Single crystals used for crystal structure analysis were obtained by heating the source metals at 773 K for 2 h, and then cooling to 533 K at the rate of −4.0 Kh −1 .

Characterization
The polycrystalline sintered samples were pulverized and placed in a cell with a Mylar film window in an Ar atmosphere, and the powder XRD patterns were measured using a powder diffractometer (D2 PHASER, Bruker AXS, Karlsruhe, Germany) and Cu-Kα radiation. Single crystals were sealed in a glass capillary under an Ar atmosphere, and the XRD data were collected using a single-crystal diffractometer (D8 QUEST, Bruker AXS Karlsruhe, Germany) with Mo Kα radiation. The APEX3 software package (version 2018.1-9, Bruker AXS, Madison, Wisconsin, USA) [27] was used to collect data, refine the lattice constants, and correct the X-ray absorption effect. The SHELXL-2018 program (version 2018/3) [28] and WinGX software (version 2018.3) [29] were used to analyze the crystal structures of the compounds. The VESTA software (version 3.5.3) [30] was used to visualize the crystal structures. COD 3000292, 3000293, and 3000294 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.crystallography.net/search.html (accessed on 8 February 2021).
Electrical resistivity (ρ) and Seebeck coefficient (S) of the sintered polycrystalline samples were measured by the direct current four-terminal and thermoelectric-power temperature-difference methods under an Ar atmosphere in the temperature range of 295-400 K. The electronic structures of the Na-Cd-Sn ternary compounds found in this study were calculated on the basis of the DFT calculations using the "Advance/PHASE" software package (Advance Soft Corp., Tokyo, Japan).

Crystal Structure
The crystallographic and refinement details of the single-crystal XRD structu ysis for NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2, and tI-Na2CdSn5 are presented in Tabl atomic coordinates, equivalent isotropic atomic displacement parameters (ADPs) teratomic distances are summarized in Tables 2 and 3, and the anisotropic ADPs a in Table S1.

Crystal Structure
The crystallographic and refinement details of the single-crystal XRD structure analysis for NaCd 0.92 Sn 1.08 , Na(Cd 0.28 Sn 0.72 ) 2 , and tI-Na 2 CdSn 5 are presented in Table 1. The atomic coordinates, equivalent isotropic atomic displacement parameters (ADPs), and interatomic distances are summarized in Tables 2 and 3, and the anisotropic ADPs are listed in Table S1.   The crystal structure of NaCd 0.92 Sn 1.08 was initially refined using a structural model of LiZnGe. Li (1d and 2h sites), Zn (1e, 2g), and Ge (1a, 2i) were replaced with Na, Cd, and Sn, respectively. The reliability factors, R1 and wR2, of the refinement for all data were 1.78 and 4.20%, respectively. In general, it is difficult to distinguish between atoms with similar X-ray scattering powers, such as Cd and Sn, by XRD. Indeed, the occupancies of Cd and Sn atoms could not be refined by placing both Cd and Sn atoms at the Zn(1e, 2g) and Ge (1a, 2i) sites. The other three ordered configurations of Cd and Sn atoms and Cd/Sn equimolar occupation at the four sites of Zn (1e, 2g) and Ge (1a, 2i) resulted in R1 values of 2.06-2.96% larger than that of the initial model (1.78%) (Table S2). When 8% of the Cd atoms were replaced with Sn at the 1e and 2g sites in accordance with the composition of the polycrystalline sample (NaCd 0.92 Sn 1.08 ), the R1 value was the same (1.78%), but wR2 decreased slightly from 4.20% to 4.08%. Thus, the final refinement was performed with a composition of NaCd 0.92 Sn 1.08 .
The crystal structure of NaCd 0.92 Sn 1.08 (hexagonal, a = 4.9326(1) Å, c = 10.8508(3) Å, P-6m2) is shown in Figure 2. Cd and Sn atoms form a three-dimensional framework, and Na atoms are arranged in the channels. The interatomic distances between Na and Cd or Na and Sn ranged between 3.198(3) Å and 3.550(4) Å, whereas the distances between the Cd and Sn atoms were between 2.84784(6) Å and 3.2492(17) Å (see Table 3 and Figure 2b,c). The Cd-Sn framework comprises a flat honeycomb sheet of Cd/Sn1 and Sn1 (A) and two puckered honeycomb layers of Cd/Sn2 and Sn2 (B and B'). The sheet and layers are stacked in the sequence ABB' in the c-axis direction. Focusing on the Cd/Sn2-centered distorted tetrahedra of Sn1 and Sn2, the tetrahedra form double layers on the ab plane, sandwiching the Cd/Sn1-Sn1 honeycomb sheet by sharing the vertex Sn1 atoms. The interatomic distance between the Cd/Sn1 and Sn1 atoms in the flat honeycomb sheet was 2.84784(6) Å. The distances between the central Cd/Sn2 and vertex Sn atoms of the tetrahedra were 3.2492(17) Å (d Cd/Sn2-Sn1 ) and 2.9243(5) Å (d Cd/Sn2-Sn2 ), and the distance between the Sn2 atoms in the tetrahedral layer was 3.0235(18) Å. hedra were 3.2492(17) Å (dCd/Sn2-Sn1) and 2.9243(5) Å (dCd/Sn2-Sn2), and the distance between the Sn2 atoms in the tetrahedral layer was 3.0235(18) Å.
The previously reported Na-Cd-Sn compound, Na2CdSn, (hexagonal, P63mmc, a = 4.990 Å, and c = 10.111 Å) [22] has a Li2CuAs-type crystal structure in which a Na atom layer and a flat Cd-Sn honeycomb lattice sheet similar to that of the NaCd0.92Sn1.08 stack alternately. The interatomic distance between the Cd and Sn atoms of the honeycomb sheet in Na2CdSn is 2.881 Å, which is comparable to that in NaCd0.92Sn1.08 (2.84784(6) Å). The crystal structure of Na(Cd0.28Sn0.72)2 was derived to be of CaIn2-type with hexagonal lattice parameters of a = 4.8458(2) Å, c = 7.7569(3) Å, and space group P63/mmc (No. 194). The refinement gave the R1 and wR2 values of 1.64 and 2.76%, respectively, for all data ( Table 1). The structure of Na(Cd0.28Sn0.72)2 consists of two crystallographically independent atomic positions: the Na1 (12d) and Cd/Sn1 (6d) sites. The occupancies of Cd and Sn in the Cd/Sn1 site were fixed to be 0.28/0.72 in accordance with the source composition from which the single-phase sample was obtained. As illustrated in Figure 3, the atoms at the Cd/Sn1 site are tetrahedrally coordinated with each other and form a three-dimensional framework. The arrangement of the Cd/Sn1 atoms is similar to that of the C atoms in lonsdaleite (hexagonal diamond). The layer of puckered Cd/Sn six-membered rings in the ab plane were stacked in an AB order in the direction of the c-axis. The Cd/Sn interatomic distance in the c-axis direction (3.0382(5) Å) was longer than the interatomic distance between the other three equivalent Cd/Sn atoms (2.92117(18) Å). Na atoms are located between the Cd/Sn layers and surrounded by 12 Cd/Sn1 atoms with Na-Cd/Sn distances of 3.18354 (16)   The previously reported Na-Cd-Sn compound, Na 2 CdSn, (hexagonal, P6 3 mmc, a = 4.990 Å, and c = 10.111 Å) [22] has a Li 2 CuAs-type crystal structure in which a Na atom layer and a flat Cd-Sn honeycomb lattice sheet similar to that of the NaCd 0.92 Sn 1.08 stack alternately. The interatomic distance between the Cd and Sn atoms of the honeycomb sheet in Na 2 CdSn is 2.881 Å, which is comparable to that in NaCd 0.92 Sn 1.08 (2.84784(6) Å).  Table 1). The structure of Na(Cd 0.28 Sn 0.72 ) 2 consists of two crystallographically independent atomic positions: the Na1 (12d) and Cd/Sn1 (6d) sites. The occupancies of Cd and Sn in the Cd/Sn1 site were fixed to be 0.28/0.72 in accordance with the source composition from which the single-phase sample was obtained. As illustrated in Figure 3, the atoms at the Cd/Sn1 site are tetrahedrally coordinated with each other and form a threedimensional framework. The arrangement of the Cd/Sn1 atoms is similar to that of the C atoms in lonsdaleite (hexagonal diamond). The layer of puckered Cd/Sn six-membered rings in the ab plane were stacked in an AB order in the direction of the c-axis. The Cd/Sn interatomic distance in the c-axis direction (3.0382(5) Å) was longer than the interatomic distance between the other three equivalent Cd/Sn atoms (2.92117(18) Å). Na atoms are located between the Cd/Sn layers and surrounded by 12 Cd/Sn1 atoms with Na-Cd/Sn distances of 3.18354 (16)

Na2CdSn5
Na2CdSn5 crystallizes in a tetragonal cell with the lattice parameters of a = 6.42700(10) Å and c = 22.8086(5) Å, space group I-42d. It is isostructural with tI-Na2ZnSn5 (tetragonal, a = 6.3410(5) Å, c = 22.3947(18) Å, I-42d) [21], which is one of the two polymorphs of Na2ZnSn5 [20]. When the crystal structure of tI-Na2ZnSn5 was used as a starting model for the refinement, the reliability factors R1 and wR2 were 2.28 and 3.34%, respectively, for all data (Table 1). In this model, Cd and Sn atoms are arranged in an orderly manner, as shown in Figure 4. A refinement using a model in which Cd and Sn atoms are statistically located at the atomic sites of the framework with an occupancy of 1/6 Cd and 5/6 Sn gave R1 and wR2 values of 2.48% and 4.49%, respectively, which are larger than those of the ordered model.
The atomic sites of Na2CdSn5 are Cd1 (4a), Sn1 (16e), Sn2 (4b), and Na1 (16e). The atoms at the Cd1, Sn1, and Sn2 sites are tetrahedrally coordinated with each other and form a three-dimensional framework, in which spiral tunnel-like spaces extend in the aand b-axis directions. The Na atoms are statistically situated at the split site of Na1 (16e) with an occupancy of 0.5 in the spaces. The layers of the unit, which are composed of Cd1and Sn2-centered Sn14 tetrahedra (Cd1-Sn14 and Sn2-Sn14) and Na1 on the ab plane, stack in an ABCD sequence in the c-axis direction ( Figure 4). The Cd-Sn distances of the Cd1centered and Sn2-centered Sn1 tetrahedra are 2.8699(3) × 4 and 2.8401(3) Å × 4, respectively, and the Sn1-Sn1 distances between the Cd1-and Sn2-centered Sn1 tetrahedra are 2.9115(4) Å in the ab plane and 2.8852(9) Å in the c-axis direction. The lattice volume (942.14 Å 3 ) of Na2CdSn5 is 4.6% greater than that of tI-Na2ZnSn5 (900.45 Å 3 ) [21]. The Cd-Sn distance in the Cd1-Sn14 tetrahedra is 4.3 % larger than the Zn-Sn distance (2.7509 Å) of the Zn1-Sn14 tetrahedra of the tI-Na2ZnSn5 (Table S3). These differences are consistent with the difference between the covalent radii of Cd (1.40 Å) and Zn (1.20 Å) [33].

Na 2 CdSn 5
Na 2 CdSn 5 crystallizes in a tetragonal cell with the lattice parameters of a = 6.42700(10) Å and c = 22.8086(5) Å, space group I-42d. It is isostructural with tI-Na 2 ZnSn 5 (tetragonal, a = 6.3410(5) Å, c = 22.3947(18) Å, I-42d) [21], which is one of the two polymorphs of Na 2 ZnSn 5 [20]. When the crystal structure of tI-Na 2 ZnSn 5 was used as a starting model for the refinement, the reliability factors R1 and wR2 were 2.28 and 3.34%, respectively, for all data (Table 1). In this model, Cd and Sn atoms are arranged in an orderly manner, as shown in Figure 4. A refinement using a model in which Cd and Sn atoms are statistically located at the atomic sites of the framework with an occupancy of 1/6 Cd and 5/6 Sn gave R1 and wR2 values of 2.48% and 4.49%, respectively, which are larger than those of the ordered model.

Electrical Properties
The electrical resistivity (ρ) of the polycrystalline samples of NaCd 0.92 Sn 1.08 , Na(Cd 0.28 Sn 0.72 ) 2 , and Na 2 CdSn 5 in the temperature range of 295-400 K are shown in Figure 5. The relative densities of the samples were 87, 74, and 74 % of the theoretical densities of NaCd 0.92 Sn 1.08 , Na(Cd 0.28 Sn 0.72 ) 2 , and Na 2 CdSn 5 , respectively. The ρ values of the NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 samples at 300 K were 0.14 and 1.21 mΩ cm, respectively. The values increased slightly with increasing temperature and reached 0.17 mΩ cm (NaCd 0.92 Sn 1.08 , 399 K) and 1.28 mΩ cm (Na(Cd 0.28 Sn 0.72 ) 2 , 401 K) at the highest measurement temperatures. Whereas, the ρ value for the sample of Na 2 CdSn 5 at 300 K was 1.36 Ω cm, which was three to four orders of magnitude larger than those of the NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 samples. The ρ value of the Na 2 CdSn 5 sample decreased significantly with increasing temperature and reached a value of 0.30 Ω cm at 395 K. The Seebeck coefficients (S) of NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 were small positive values of +5.7 and +9.0 µV K −1 , respectively, at 300 K, while the S value of Na 2 CdSn 5 had a large positive value of +568 µV K −1 . The S values and temperature dependences of ρ measured for the sintered samples indicate that the electronic structures of NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 are metallic, while that of Na 2 CdSn 5 is semiconducting.

Electrical Properties
The electrical resistivity (ρ) of the polycrystalline samples of NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2, and Na2CdSn5 in the temperature range of 295-400 K are shown in Figure  5. The relative densities of the samples were 87, 74, and 74 % of the theoretical densities of NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2, and Na2CdSn5, respectively. The ρ values of the NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 samples at 300 K were 0.14 and 1.21 mΩ cm, respectively. The values increased slightly with increasing temperature and reached 0.17 mΩ cm (NaCd0.92Sn1.08, 399 K) and 1.28 mΩ cm (Na(Cd0.28Sn0.72)2, 401 K) at the highest measurement temperatures. Whereas, the ρ value for the sample of Na2CdSn5 at 300 K was 1.36 Ω cm, which was three to four orders of magnitude larger than those of the NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 samples. The ρ value of the Na2CdSn5 sample decreased significantly with increasing temperature and reached a value of 0.30 Ω cm at 395 K. The Seebeck coefficients (S) of NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 were small positive values of +5.7 and +9.0 μV K −1 , respectively, at 300 K, while the S value of Na2CdSn5 had a large positive value of +568 μV K −1 . The S values and temperature dependences of ρ measured for the sintered samples indicate that the electronic structures of NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 are metallic, while that of Na2CdSn5 is semiconducting. To elucidate the electrical properties of the compounds found in this study, the electronic band structures were examined. As NaCd 0.92 Sn 1.08 and Na(Cd 0.28 Sn 0.72 ) 2 have mixed Cd/Sn sites, and the Na atoms in Na 2 CdSn 5 statistically occupy the Na1 site with an occupancy of 0.5 in their crystal structures, the total and partial electronic densities of states (DOS) ( Figure 6) were calculated using the ordered models of NaCdSn, Na(Cd 1/4 Sn 3/4 ) 2 , and Na 2 CdSn 5 shown in Figure S1. For NaCdSn and Na(Cd 1/4 Sn 3/4 ) 2  To elucidate the electrical properties of the compounds found in this study, the electronic band structures were examined. As NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 have mixed Cd/Sn sites, and the Na atoms in Na2CdSn5 statistically occupy the Na1 site with an occupancy of 0.5 in their crystal structures, the total and partial electronic densities of states (DOS) ( Figure 6) were calculated using the ordered models of NaCdSn, Na(Cd1/4Sn3/4)2, and Na2CdSn5 shown in Figure S1. For NaCdSn and Na(Cd1/4Sn3/4)2, the electronic states near the Fermi level have a finite DOS, and there is a deep dip at approximately 0.4-0.6 eV above the Fermi level. Na2CdSn5 presented a band gap of approximately 0.3 eV and the DOSs at the top of the valence band and the bottom of the conduction band are both large, leading to a large S value. The results of this calculation provide a good description of the metallic behavior in terms of ρ and the low S values of NaCd0.92Sn1.08 and Na(Cd0.28Sn0.72)2 and the semiconducting behavior in terms of ρ and the large S value of Na2CdSn5. The activation energy calculated from the Arrhenius plot of the electrical conductivity of the Na2CdSn5 sample is approximately 0.15 eV, which is consistent with the band gap of 0.3 eV presented by the DOS calculation. It has been reported that a polycrystalline bulk sample of tI-Na2ZnSn5 (relative density: 94%), which is isostructural with Na2CdSn5, exhibited semiconducting properties, and the ρ and S values at 295 K were 362 Ω cm and −455 μV K −1 , respectively. The lattice thermal conductivity of tI-Na2ZnSn5 was estimated to be 0.61 W m −1 K −1 , which may be caused by the large thermal vibration (rattling) of the Na atoms [21]. The ρ and S values measured for the Na2CdSn5 sample with a relative density of 74% are similar to those obtained for tI-Na2ZnSn5, although the sign of the Seebeck coefficient is different. Moreover, the distribution of the calculated DOS near the Fermi level is similar [20]. Therefore, Na2CdSn5 could be employed as a thermoelectric material by adjusting the carrier density with an appropriate amount of dopant.

Conclusions
Three new Na-Cd-Sn compounds, NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2, and Na2CdSn5, were synthesized, and their crystal structures were determined by single-crystal XRD. These compounds have three-dimensional frameworks formed by the Cd and Sn atoms, in which the Na atoms are incorporated. In the structure of NaCd0.92Sn1.08, four-coordinated tetrahedra of Cd and Sn atoms sandwich a Cd-Sn honeycomb sheet and Na atom layers. The structure of Na(Cd0.28Sn0.72)2 was characterized as a framework comprising four-coordinated Cd and Sn atoms similar to the arrangement of C atoms in lonsdaleite; Na atoms were included in the voids of the framework. These compounds are polar intermetallics with metallic properties. Na2CdSn5 is a typical Zintl compound described by the formulation [Na + ]2[Cd 2− ][Sn 0 ]5. It is a semiconductor with a small band gap and has a framework structure composed of Cd and Sn-centered Sn4 tetrahedra. Na atoms with large ADPs are situated in the tunnel-like spaces within the framework.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: Ordered structure models for DFT calculations, Table S1: Anisotropic displacement parameters of It has been reported that a polycrystalline bulk sample of tI-Na 2 ZnSn 5 (relative density: 94%), which is isostructural with Na 2 CdSn 5 , exhibited semiconducting properties, and the ρ and S values at 295 K were 362 Ω cm and −455 µV K −1 , respectively. The lattice thermal conductivity of tI-Na 2 ZnSn 5 was estimated to be 0.61 W m −1 K −1 , which may be caused by the large thermal vibration (rattling) of the Na atoms [21]. The ρ and S values measured for the Na 2 CdSn 5 sample with a relative density of 74% are similar to those obtained for tI-Na 2 ZnSn 5 , although the sign of the Seebeck coefficient is different. Moreover, the distribution of the calculated DOS near the Fermi level is similar [20]. Therefore, Na 2 CdSn 5 could be employed as a thermoelectric material by adjusting the carrier density with an appropriate amount of dopant.

Conclusions
Three new Na-Cd-Sn compounds, NaCd 0.92 Sn 1.08 , Na(Cd 0.28 Sn 0.72 ) 2 , and Na 2 CdSn 5 , were synthesized, and their crystal structures were determined by single-crystal XRD. These compounds have three-dimensional frameworks formed by the Cd and Sn atoms, in which the Na atoms are incorporated. In the structure of NaCd 0.92 Sn 1.08 , four-coordinated tetrahedra of Cd and Sn atoms sandwich a Cd-Sn honeycomb sheet and Na atom layers. The structure of Na(Cd 0.28 Sn 0.72 ) 2 was characterized as a framework comprising fourcoordinated Cd and Sn atoms similar to the arrangement of C atoms in lonsdaleite; Na atoms were included in the voids of the framework. These compounds are polar intermetallics with metallic properties. Na 2 CdSn 5 is a typical Zintl compound described by the formulation [Na + ] 2 [Cd 2− ][Sn 0 ] 5 . It is a semiconductor with a small band gap and has a framework structure composed of Cd and Sn-centered Sn 4 tetrahedra. Na atoms with large ADPs are situated in the tunnel-like spaces within the framework.
Supplementary Materials: The following are available online at https://www.mdpi.com/2304-6 740/9/3/19/s1, Figure S1: Ordered structure models for DFT calculations, Table S1: Anisotropic displacement parameters of three Na-Cd-Sn compounds found in this study, Table S2: Structure refinement results of NaCdSn based on the crystal structure of LiZnGe, Table S3: Selected interatomic distances for Na 2 CdSn 5 and tI-Na 2 ZnSn 5 , A combined CIF and checkCIF for all discussed crystal structures.
Author Contributions: Y.A. and T.Y. conducted the sample preparation and phase identification, while the crystal structure analysis was performed by T.Y. and H.Y.; T.Y. accomplished the electronic structure calculation, independently designed and supervised the project. The manuscript was written with input from all authors (Y.A., T.Y., and H.Y.), who approved the final version of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research was financially supported by JSPS KAKENHI Grant (JP20H02820).