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Article

Structure and Chemical Bonding of the Li-Doped Polar Intermetallic RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13, 0.28, 0.43, 0.53) System

by
Junsu Lee
,
Jieun Jeon
and
Tae-Soo You
*
Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 28644, Korea
*
Author to whom correspondence should be addressed.
Materials 2018, 11(4), 495; https://doi.org/10.3390/ma11040495
Submission received: 12 February 2018 / Revised: 11 March 2018 / Accepted: 20 March 2018 / Published: 26 March 2018
(This article belongs to the Special Issue Advances in Zintl Phases)
Browse Figures
Versions Notes

Abstract

:
Four polar intermetallic compounds belonging to the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system have been synthesized by the traditional solid-state reaction method, and their crystal structures have been characterized by single-crystal X-ray diffraction (SXRD) analyses. The isotypic crystal structures of four title compounds adopt the Mo2FeB2-type structure having the tetragonal space group P4/mbm (Z = 2, Pearson code tP40) with three crystallographically independent atomic sites and can be simply described as a pile of the identical 2-dimensioanl (2D) RE2In1-xLixGe2 slabs stacked along the c-axis direction. The substituting Li atom shows a particular site preference for replacing In at the Wyckoff 2a site rather than Ge at the Wyckoff 4g in this crystal structure. As the size of a used rare-earth metal decreases from La3+ to Gd3+ throughout the title system, the Ge-Ge and Ge-In/Li bond distances, both of which consist of the 2D anionic Ge2(In/Li) layer, gradually decrease resulting in the reduction of a unit cell volume. A series of theoretical investigations has been performed using a hypothetical structure model Gd2In0.5Li0.5Ge2 by tight-binding linear muffin-tin orbital (TB-LMTO) method. The resultant densities of states (DOS) value at the Fermi level (EF) suggests a metallic conductivity for this particular composition, and this calculation result is in a good agreement with the formal charge distribution assigning two extra valence electrons for a metal-metal bond in the conduction band. The thorough analyses of six crystal orbital Hamilton population (COHP) curves representing various interatomic interactions and an electron localization function (ELF) diagram indicating the locations of paired-electron densities are also provided in this article.

Graphical Abstract

1. Introduction

A family of intermetallic compounds belonging to the RE5Tt4 (RE = rare-earth metals, Tt = tetrels) series [1,2,3] have drawn worldwide attention during the last decade due to their extraordinary chemical and physical characteristics, such as giant magnetoresistance [4,5] and colossal magnetorestriction [6,7,8,9]. Among its members, a ternary compound Gd5Si2Ge2 [10] proves that the giant magnetocaloric effect (MCE) can possibly occur at near room temperature through the first-order magnetic phase transition between the ferromagnetic and paramagnetic phases as a function of temperature. Given that this interesting magnetic phase transition has been proven to be triggered by the crystal structure transformation between the orthorhombic and tetragonal crystal systems, numerous research activities have focused on understanding the correlation among composition-structure-property of the particular ternary solid-solution Gd5Si2−xGe2+x system [10,11], as well as various other RE5Tt4 series [12,13,14]. My research group has recently published an article about the isotypic RE4LiGe4 (RE = La, Ce, Pr, Sm) system [15], where a monovalent Li specifically substituted RE at the particular site among three available candidate sites. As a result of our thorough study, we claimed that a significant size difference between RE and Li generated a sufficient enough chemical pressure causing the observed structural transformation, and this transformation eventually resulted in modifying magnetic characteristics of these compounds.
During our systematic investigations, we serendipitously obtained four quaternary polar intermetallics belonging to the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system, and all four title compounds crystallized in the Mo2FeB2-type structure [16]. Interestingly, this type of crystal structure is isotypic to the structural moiety of the 2-dimensioanl (2D) RE2LiGe2 slabs consisting of the parental RE4LiGe4 (RE = La, Ce, Pr, Sm) system [15]. As already mentioned in our previous article, the imaginary RE2LiGe2 compound adopting the Mo2FeB2-type structure could be derived from the parental RE4LiGe4 system by simply removing the interlayer Ge2 dimers bridging two neighboring RE2LiGe2 slabs [15]. In fact, large numbers of the Mo2FeB2-type analogues belonging to the RE2MTt2 (RE = rare-earth metals; M = Li, Mg, Al, Sc, Y, In; Tt = Si, Ge) family have already been reported due to their intriguing magnetic and electrical properties, and their crystal structures [17,18,19,20,21,22,23,24,25,26,27]. For instance, the magnetic property of Gd2MgGe2 [17] is successfully shifted by the non-magnetic Mg substitution for the magnetic Ni in Gd2NiGe2. In addition, the crystal structure of Rh2LiSi2 [28] contains the 3D anionic open-framework structure rather than the typical 2D anionic Mo2FeB2-type layered structure due to the size ratio between cationic and anionic elements. Although there are numerous reports about the ternary RE2MTt2 family, there is little report about quaternary derivatives belonging to this family. Therefore, we decided to conduct in-depth investigations into the title quaternary system in terms of experimental and theoretical perspectives.
In this article, we report the comprehensive research results of our systematic investigation about the monovalent Li substituted RE2In2−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system. Our experimental studies include the high-temperature synthesis, crystal structure characterization by single-crystal X-ray diffraction (SXRD) analyses, and thorough structural discussion. Theoretical investigations performed by tight-binding linear muffin-tin orbital (TB-LMTO) using a hypothetical Gd2In0.5Li0.5Ge2 model provide the basis for detailed discussions about an overall electronic structure including densities of states (DOS), crystal orbital Hamilton populations (COHP) curves and an electron localization function (ELF) contour map.

2. Materials and Methods

2.1. Synthesis

All of the sample preparation processes were performed inside a N2-filled glovebox with O2 and H2O contents below 0.1 ppm or inside an arc-melting furnace under Ar atmosphere. The reactant elements were purchased from Alfa Aesar or Aldrich: La (pieces, 99.9%), Nd (ingot, 99.9%), Sm (pieces, 99.9%), Gd (chip, 99.9%), In (shot, 99.99%), Li (wire, 99.8%) and Ge (ingot, 99.9999%). Rare-earth metals and Li were cleaned by scraping off the lightly tanned surfaces using a scalpel or a metal brush inside a glovebox before loading in a Nb ampoule. Originally, the four title compounds in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system were serendipitously produced as side products during our investigations of the RE4LiGe4 (RE = La, Ce, Pr, Sm) system [15]. Once the crystal structures of the four products were determined as the Mo2FeB2-type structure based on the SXRD analysis, we attempted to reproduce these compounds by loading the refined stoichiometric compositions. In addition, we also tried to obtain the phase-pure products by changing the loaded compositions. However, all these reactions also produced some other secondary phases, such as Nd3Ge5, besides the title RE2In1−xLixGe2 phase.
Each reactant was cut into small pieces and loaded in one end-sealed Nb ampoule inside a glovebox, and the other end of the ampoule was sealed by arc welding under an Ar atmosphere. Then, the Nb ampoule was sealed again in a secondary container of a fused-silica jacket under vacuum to prevent a Nb ampoule from the oxidation during the high-temperature reaction process. The reactant mixtures were heated to 1353 K at a rate of 473 K/h, kept there for 5 h, and then cooled to 1023 K at a rate of 10 K/h, after which these products were annealed for two days. After that, the reactions were naturally cooled down to room temperature by turning off the furnace [15].

2.2. X-ray Diffraction Experiments

Four title compounds in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system were characterized by SXRD analyses. Data were collected at room temperature using a Bruker SMART APEX2 CCD-based diffractometer equipped with Mo Kα1 radiation (λ = 0.71073 Å). Initially, several silvery lustrous needle-shaped single crystals were chosen from each batch of products. After the quality check, the best single crystal was carefully selected, and a full data collection was conducted using Bruker’s APEX2 program [29]. Data reduction, integration, and unit cell parameter refinements were executed using the SAINT program [30]. SADABS [31] was used to perform semiempirical absorption corrections based on equivalents. The entire sets of reflections of the four title compounds were in good agreements with the tetragonal crystal system, and the space group P4/mbm (No. 127) was chosen for the four isotypic polar intermetallics. The detailed crystal structures were solved by direct methods and refined to convergence by full matrix least-squares methods on F2. The refined parameters of these Mo2FeB2-type phases include the scale factor, atomic positions including anisotropic displacement parameters (ADPs), extinction coefficients, and occupancy factors of the In/Li-mixed sites. During the last stage of this structure refinement cycle, each atomic position was standardized using STRUCTURE TIDY [32]. Important crystallographic data, atomic positions with ADPs and several selected interatomic distances are provided in Table 1, Table 2 and Table 3. Further details regarding each crystal structure can be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (49) 7247-808-666; e-mail: [email protected])—under depository numbers CSD-433825 for La2In0.72(1)Li0.28Ge2, CSD-433826 for Nd2In0.87(1)Li0.13Ge2, CSD-433828 for Sm2In0.57(1)Li0.43Ge2 and CSD-433828 for Gd2In0.47(1)Li0.53Ge2.

2.3. Electronic Structure Calculations

To understand the electronic structures of the four title compounds, including individual chemical bonding and locations of paired electrons, a series of theoretical calculations were performed using a hypothetical structure model Gd2In0.5Li0.5Ge2 by the Stuttgart TB-LMTO47 program [33,34,35,36,37] with atomic sphere approximation (ASA). Since the experimentally refined composition of the Gd compound is the closest to a practically accessible composition for calculations, we decided to exploit the Gd compound as a calculation model. To apply an idealized composition of Gd2In0.5Li0.5Ge2, we designed a 2 × 2 × 2 superstructure involving an alternating atomic arrangement of In and Li along the ab-plane as well as the c-axis directions in the expanded unit cell. For this superstructure model, the orthorhombic space group Pmma (No. 51) was exploited, rather than the crystallographically refined tetragonal space group P4/mbm (No. 127). Lattice parameters and atomic coordinates were estimated from the SXRD data of Gd2In0.47(1)Li0.53Ge2. Further details about this structure model are provided in Supporting Information (Table S1). In the ASA method, space is filled with overlapping Wigner-Seitz (WS) atomic spheres [33,34,35,36,37]. All relativistic effects, except spin-orbit coupling, were taken into account using a scalar relativistic approximation. The symmetry of the potential inside each WS sphere was considered spherical, and a combined correction was used to take into account the overlapping part. The radii of the WS spheres were obtained by requiring the overlapping potential be the best possible approximation of the full potential and were determined by an automatic procedure [33,34,35,36,37]. This overlap should not be too large, because the error in kinetic energy introduced by the combined correction is proportional to the fourth power of the relative sphere overlap. The used WS radii are listed as follows: Gd = 1.831 Å, In = 1.771 Å, Li = 1.711 Å and Ge = 1.343 Å for Gd2In0.5Li0.5Ge2. The basis sets included 6s, 6p, 5d, and 4f orbitals for Gd; 2s, 2p, and 3d orbitals for Li; 5s, 5p, 5d and 4f orbitals for In; and 4s, 4p and 4d orbitals for Ge. The Gd 6p, In 5d and 4f, Li 2p and 3d, and Ge 4d orbitals were treated by the Löwdin downfolding technique [38]. The k-space integration was conducted by the tetrahedron method [39], and the self-consistent charge density was obtained using 343 irreducible k-points in the Brillouin zone.

3. Results and Discussion

3.1. Crystal Structure Analysis

Four polar intermetallic compounds belonging to the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system were synthesized by the high-temperature reaction method, and their crystal structures were characterized by SXRD analyses. All four title compounds crystallized in the tetragonal space group P4/mbm (Pearson code tP20, Z = 2) and adopted the Mo2FeB2-type structure [16] (Table 1), which is a ternary version of the U3Si2-type structure [40]. There exist three crystallographically independent asymmetric atomic sites, each including one rare-earth metal site, one Ge site and one In/Li-mixed site (Table 2).
The overall isotypic crystal structures of the four title compounds can be simply described as a pile of identical 2D RE2In1−xLixGe2 slabs propagating along the ab-plane direction (Figure 1a). Each of these 2D slabs can be further considered as the 1:1 intergrowth of the REGe2 and RE(In/Li) moieties adopting the AlB2-type and CsCl-type structures, respectively. A Ge atom occupies the central site of a trigonal-prismatic polyhedron formed by six RE atoms (Figure 1b), whereas the In/Li-mixed site occupies the central site of a cubic polyhedron shaped by eight RE atoms (Figure 1c). Interestingly, the substituting Li atom shows a particular site preference for the In site (Wyckoff 2a), rather than the Ge site (Wyckoff 4g), as it produces this quaternary system. This type of site preference of Li resembles that of Li in the previously reported RE4LiGe4 (RE = La, Ce, Pr, and Sm) system [15].
The overall crystal structure of the RE4LiGe4 system can be considered as an assembly of two basic substructures: (1) the Mo2FeB2-type 2D infinite RE2LiGe2 slabs, and (2) the dumbbell-shaped Ge2 dimers bridging two of those neighboring 2D RE2LiGe2 slabs. Interestingly, the local structural geometry of the 2D RE2LiGe2 slab in the RE4LiGe4 system is nearly identical to that of the 2D RE2In1−xLixGe2 slab in our title compounds, which can be regarded as a building block establishing the overall crystal structure. One noticeable difference between these two structure types is the additional bridging Ge2 dimers in-between two neighboring 2D RE2LiGe2 slabs in the RE4LiGe4 system as illustrated in Figure 2.
Two Ge-Ge and Ge-In/Li bond distances in the 2D Ge2(In/Li) anionic layer and the local coordination geometries of structural moieties in the title RE2In1−xLixGe2 system can be compared with those observed in the known isotypic or homeotypic compounds, such as the RE2InGe2 (RE = La − Ho, Yb) [18,19] and RE4LiGe4 (RE = La, Ce, Pr, Sm) systems [15]. First, the Ge-Ge bond distance of the Ge2 dimer along the ab-plane direction in our title compounds ranges from 2.4998(12) to 2.5384(8) Å (Table 3), and this range of bond distances is very similar to that found in the isotypic RE2InGe2 (RE = La − Ho, Yb) system, ranging between 2.504 and 2.526 Å. This implies that the Li substitution for In nearby Ge hardly influences the given coordination environment of Ge in this system. In particular, these Ge-Ge distances in two of our title compounds La2Li0.28(1)In0.72Ge2 (2.5384(8) Å) and Sm2Li0.43(1)In0.57Ge2 (2.5252(14) Å) are also comparable to those in La4LiGe4 (2.549(1) Å) and Sm3.98Li1.02Ge4 (2.532(1) Å) [15]. Second, the Ge-In/Li distances along the ab-plane in four compounds varying from 3.0448(3) to 2.8985(9) Å are also comparable to the Ge-In distances ranging from 2.855 to 3.009 Å in the RE2InGe2 (RE = La − Ho, Yb) system [18,19]. However, four Ge-Li bond distances along the ac-plane direction in each of La4LiGe4 and Sm3.98Li1.02Ge4, which are the counterparts of the Ge-In/Li distance in this work, display substantial differences from one another between 3.808 and 2.864 Å and between 3.658 and 2.742 Å, respectively [15]. This type of rather large deviation of the Ge-Li distance could be attributed to the significantly distorted cubic RELi polyhedra (CsCl-type) in La4LiGe4 and Sm3.98Li1.02Ge4, and therefore the four Ge-Li bonds connecting this RELi moiety to the four neighboring REGe2 moieties can hardly be the same. On the other hand, the cubic RE(In/Li) polyhedra in the title compounds are relatively more symmetric and contain only one type of Ge-In/Li distance in each compound, as compared in Figure 3.
Finally, the bond distances of Ge-Ge and Ge-In/Li consisting of the 2D anionic Ge2(In/Li) layer gradually decrease from 2.5384(8) to 2.500(1) Å and from 3.044(3) to 2.8985(9) Å, respectively, as the size of a cation decreases from La3+ to Gd3+ (r(La3+) = 1.27 Å for 10 coordinates; r(Gd3+) = 1.11 Å for 9 coordinates) [41] throughout the title system (Table 3). In addition, these bond shortenings influenced by the cationic-size reduction eventually result in the contraction of a unit cell volume (Table 1). This kind of intimate correlation among the bond distance, the cationic size, and the unit cell volume should be attributed to the slightly distorted pentagonal-prismatic coordination environment surrounding a cationic RE, which is formed by a total of ten anionic elements, including six Ge and four In/Li-mixed sites (see Figure 4). Table 3 also shows the declining trend of these RE-RE and RE-In/Li distances along the four title compounds.

3.2. Electronic Structure and Chemical Bonding

To understand the overall electronic structure of the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system, including interatomic interactions and locations of paired electron densities, we conducted a series of theoretical calculations by the TB-LMTO-ASA method [33,34,35,36,37] and analyzed DOS and COHP curves, as well as an ELF contour map. For practical reasons, we should use a structural model that employs an idealized composition of Gd2In0.5Li0.5Ge2 with an alternating In and Li arrangement along the ab-plane and the c-axis directions in a unit cell. Therefore, the √2 × 2 × √2 superstructure with a space group of Pmma was designed, and lattice parameters and atomic positions extracted from SXRD data of Gd2Li0.53(1)In0.47Ge2 were applied to this model. Further details about this structure model are provided in Supporting Information Table S1.
Figure 5a displays the total and partial DOS (TDOS and PDOS) curves of Gd2In0.5Li0.5Ge2. The overall features of these DOS curves below EF can be divided into three sectors. The lowest region, between ca. −10.5 and −9 eV, shows a large peak that can be mostly attributed to Ge 4s states forming the σs bonding interaction within the Ge2 dimers, with slight additions of Gd 6s and Li 2s states also being observed. The region between −7.7 and −7 eV is also mostly contributed by Ge 4s states of the Ge2 dimers, but forming the σs* antibonding interaction. The relatively wider energy region between −5.5 and 0 eV displays a mixed contribution of various orbital states including Gd 4f, 5d, In 5p, Li 2s, and Ge 4p states. The conduction band region beyond EF is dominated by Gd 4f and 5d states with some contributions from three other anions. A large DOS value at EF implies the metallic conductivity of this compound. Overall, no particularly strong orbital mixing of the s and p states of any individual atom was observed throughout the entire energy window.
The chemical formula of this Li-doped model can be re-written as [(Gd3+)2(In3+)0.5(Li+)0.5][(Ge3−)2](e−)2 on the basis of the Zintl-Klemm formalism. Several previously reported isotypic compounds, such as (Gd3+)2(Mg2+)2(Ge3−)2(e−)2 and (Yb2+)2(In3+)(Ge3−)2](e−), already followed the same formalism [17]. In our Gd-containing title compound, Gd, In and Li are considered cationic elements donating eight electrons, whereas each Ge forming one covalent bond with the neighboring Ge requires only three more electrons to satisfy the Zintl-Klemm formalism. Therefore, the Ge2 dimer should be treated as an isoelectronic to a halogen dimer with a formal charge of 6-. As a result, two extra valence electrons per formula unit remain for the metal-metal bonding in the conduction band. Although the Zintl-Klemm formalism can successfully provide an insight to understand the formal charge of each atom in Gd2In0.5Li0.5Ge2, one should keep in mind that this formalism also supplies a rather simplified understanding about the crystal structure since there still exist unneglectable orbital contributions from Gd, In and Li forming the Gd-Ge, In-Ge and Li-Ge interactions in the valence band region as shown in Figure 5b. Figure 5b,c illustrate six COHP curves representing the bonding and antibonding interactions between two neighboring atoms. For clarity, the Gd f-orbitals are not included in these COHP curves. First, Figure 5b shows three interactions, including Gd and three anionic elements. Although the magnitude of these COHP curves is doubled for easier comparison with the three other COHP curves shown in Figure 5c, it is clear that rather large contribution from Gd orbitals in the valence band region creates unneglectable interatomic interactions with surrounding anionic elements. In particular, the Gd-Ge COHP curve shows the strongest bonding character among these three COHP curves, which continues up to +1.5 eV beyond EF. This additional bonding character around EF can be used to compensate for the antibonding character of the Ge-Ge COHP curve just below EF. Second, Figure 5c displays three interatomic interactions among three anionic elements. As one can expect from the selected interatomic distances provided in Table 3, the Ge-Ge COHP curve shows the largest bonding character among three COHP curves in the valence band. In addition, the integrated COHP (ICOHP) values of these three bond, which represent the bonding energy of a particular chemical bond, also prove that the homonuclear Ge-Ge bonding (0.180 eV/bond) is relatively stronger than two other heterogeneous Ge-In (0.098 eV/bond) and Ge-Li (0.027 eV/bond) bonding in the 2D anionic Ge2(In/Li) layer, as previously reported (Table 4) [42].
Interestingly, this type of the stronger open-shell covalent bond character of the Ge2 dimer on the 2D anionic Ge2(In/Li) layer can be clearly visualized by an ELF contour map. ELF evaluation is known to visually represent the paired-electron densities observed in bonding pairs as well as lone pairs of electrons [43]. For the current example of Gd2In0.5Li0.5Ge2, we particularly chose the (0 0 0) sliced-plane for the ELF mapping analysis since the 2D Ge2(In/Li) layer involving the Ge2 dimers as well as Li and In atoms were precisely located on the z = 0 plane. In Figure 6, the stronger Ge-Ge bonding character, which is previously proven by the COHP curve and the ICOHP value, is represented by the relatively higher ELF value in-between the two Ge atoms. On the other hand, two other relatively weaker bonding characters between Ge-In and Ge-Li bonding are depicted by the lower ELF values.

4. Conclusions

We successfully synthesized four quaternary polar intermetallic compounds belonging to the RE2In1−xLixGe2 (RE = La, Nd, Sm, x = 0.13(1), 0.28(1), 0.43(1), 0.53(1) by the conventional high-temperature reactions, and their crystal structures were characterized by SXRD analyses. The isotypic crystal structure of the four title compounds adopts the Mo2FeB2-type structure, and the 2D RE2In1-xLixGe2 layers can be alternately viewed as the 1:1 intergrowth of two types of hypothetical polyhedral of the REGe2 (AlB2-type) and RE(In/Li) (CsCl-type) moieties. Interestingly, we revealed that the particular bond distances on the 2D Ge2(In/Li) layer gradually decrease as the size of a cation decreases from La3+ to Gd3+ throughout our title system, and this kind of bond shortening caused by the cationic size reduction should be attributed to the pentagonal-prismatic coordination environment formed by ten anions surrounding the central cation. Further investigations using the late rare-earth metals are in process to see this type of size reduction trend remains. According to the Zintl-Klemm formalism, two extra valence electrons remain on the metal-metal bond in the conduction. In addition, the non-zero DOS value at EF implies metallic behavior of the title compounds. Thus, in order to verify the metallic conductivity of the title system, the phase pure product should be obtained first, and the electrical conductivity measurement also need to be performed.

Supplementary Materials

The following are available online at https://www.mdpi.com/1996-1944/11/4/495/s1, Table S1: Detailed information of a hypothetical structural model Gd2In0.5Li0.5Ge2.

Acknowledgments

This research is supported by Basic Science Research Program through NRF funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A1A1A05027845).

Author Contributions

Tae-Soo You conceived and designed the experiments. Jieun Jeon synthesized compounds, and Junsu Lee designed a model structure and performed a series of calculations. Tae-Soo You wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

  1. Mozharivskyj, Y.; Pecharsky, A.O.; Pecharsky, V.K.; Miller, G.J. On the High-Temperature Phase Transition of Gd5Si2Ge2. J. Am. Chem. Soc. 2005, 127, 317–324. [Google Scholar] [CrossRef] [PubMed]
  2. Wang, H.; Wang, F.; Jones, K.; Miller, G.J. Chemical Pressure and Rare-Earth Orbital Contributions in Mixed Rare-Earth Silicides La5–xYxSi4 (0 ≤ x ≤ 5). Inorg. Chem. 2011, 50, 12714–12723. [Google Scholar] [CrossRef] [PubMed]
  3. Wu, L.-M.; Kim, S.-H.; Seo, D.-K. Electron-Precise/Deficient La5−xCaxGe4 (3.4 ≤ x ≤ 3.8) and Ce5−xCaxGe4 (3.0 ≤ x ≤ 3.3):  Probing Low-Valence Electron Concentrations in Metal-Rich Gd5Si4-type Germanides. J. Am. Chem. Soc. 2005, 127, 15682–15683. [Google Scholar] [CrossRef] [PubMed]
  4. Morellon, L.; Blasco, J.; Algarabel, P.A.; Ibarra, M.R. Nature of the first-order antiferromagnetic-ferromagnetic transition in the Ge-rich magnetocaloric compounds Gd5(SixGe1−x)4. Phys. Rev. B 2000, 62, 1022–1026. [Google Scholar] [CrossRef]
  5. Levin, E.M.; Pecharsky, V.K.; Gschneidner, K.A., Jr. Spontaneous generation of voltage in Gd5(SixGe4−x) during a first-order phase transition induced by temperature or magnetic field. Phys. Rev. B 2001, 63, 174110–1474117. [Google Scholar] [CrossRef]
  6. Morellon, L.; Algarabel, P.A.; Ibara, M.R.; Blasco, J.; Garcia-Landa, B. Magnetic-field-induced structural phase transition in Gd5(Si1.8Ge2.2). Phys. Rev. B 1998, 58, R14721–R14724. [Google Scholar] [CrossRef]
  7. Morellon, L.; Stankiewicz, J.; Garcia-Landa, B.; Algarabel, P.A.; Ibarra, M.R. Giant magnetoresistance near the magnetostructural transition in Gd5(Si1.8Ge2.2). Appl. Phys. Lett. 1998, 73, 3462–3464. [Google Scholar] [CrossRef]
  8. Levin, E.M.; Pecharsky, V.K.; Gschneidner, K.A., Jr. Magnetic-field and temperature dependencies of the electrical resistance near the magnetic and crystallographic first-order phase transition of Gd5(Si2Ge2). Phys. Rev. B 1999, 60, 7993–7997. [Google Scholar] [CrossRef]
  9. Levin, E.M.; Pecharsky, V.K.; Gschneidner, K.A., Jr.; Tomlinson, P. Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): A study of the electrical resistance behavior. J. Magn. Magn. Mater. 2000, 210, 181–188. [Google Scholar] [CrossRef]
  10. Percharsky, V.K.; Gschneidner, K.A., Jr. Giant Magnetocaloric Effect in Gd5(Si2Ge2). Phys. Rev. Lett. 1997, 78, 4494–4497. [Google Scholar] [CrossRef]
  11. Pecharsky, V.K.; Gschneidner, K.A., Jr. Tunable magnetic regenerator alloys with a giant magnetocaloric effect for magnetic refrigeration from ~20 to ~290 K. Appl. Phys. Lett. 1997, 70, 3299–3301. [Google Scholar] [CrossRef]
  12. Wang, H.; Misra, S.; Wang, F.; Miller, G.J. Structural and Magnetic Characteristics of Gd5GaxSi4−x. Inorg. Chem. 2010, 49, 4586–4593. [Google Scholar] [CrossRef] [PubMed]
  13. Misra, S.; Poweleit, E.T.; Miller, G.J. On the Crystal Structure, Metal Atom Site Preferences and Magnetic Properties of Nd5–xErxTt4 (Tt = Si or Ge). Z. Anorg. Allg. Chem. 2009, 635, 889–897. [Google Scholar] [CrossRef]
  14. Miller, G.J. Complex rare-earth tetrelides, RE5(SixGe1−x)4: New materials for magnetic refrigeration and a superb playground for solid state chemistry. Chem. Soc. Rev. 2006, 35, 799–813. [Google Scholar] [CrossRef] [PubMed]
  15. Nam, G.; Jeon, J.; Kim, Y.; Kang, S.; Ahn, K.; You, T.-S. Combined effect of chemical pressure and valence electron concentration through the electron-deficient Li substitution on the RE4LiGe4 (RE = La, Ce, Pr, and Sm) system. J. Solid State Chem. 2013, 205, 10–20. [Google Scholar] [CrossRef]
  16. Rieger, W.; Nowotny, H.; Benesovsky, F. Die Kristallstruktur von Mo2FeB2. Monatsh. Chem. 1964, 95, 1502–1503. [Google Scholar] [CrossRef]
  17. Choe, W.; Miller, G.J.; Levin, E.M. Crystal structure and magnetism of Gd2MgGe2. J. Alloy. Compd. 2001, 329, 121–130. [Google Scholar] [CrossRef]
  18. Zaremba, V.I.; Kaczorowski, D.; Nychyporuk, G.P.; Rodewald, U.C.; Pöttgen, R. Structure and physical properties of RE2Ge2In (RE = La, Ce, Pr, Nd). Solid State Sci. 2004, 6, 1301–1306. [Google Scholar] [CrossRef]
  19. Tobash, P.H.; Lins, D.; Bobev, S.; Lima, A.; Hundley, M.F.; Thompson, J.D.; Sarrao, J.L. Crystal Growth, Structural, and Property Studies on a Family of Ternary Rare-Earth Phases RE2InGe2 (RE = Sm, Gd, Tb, Dy, Ho, Yb). Chem. Mater. 2005, 17, 5567–5573. [Google Scholar] [CrossRef]
  20. Zaremba, V.I.; Tyvanchuk, Y.B.; Stepien-Damm, J. Crystal structure of diytterbium digermanium indide, Yb2Ge2In. Z. Kristallogr. New Cryst. Struct. 1997, 212, 291. [Google Scholar] [CrossRef]
  21. Zaremba, V.I.; Stepien-Damm, J.; Nichiporuk, G.P.; Tyvanchuk, Y.B.; Kalychak, Y.M. Structure of R2Ge2In crystals, where R is a rare-earth metal. Crystallogr. Rep. 1998, 43, 8–11. [Google Scholar]
  22. Steinberg, G.; Schuster, H.-U. Ternary Silieides of Lithium with Yttrium or Neodymium in a Modified U3Si2-Type Structure. Z. Naturforsch. 1979, 34, 1237–1239. [Google Scholar] [CrossRef]
  23. Dhar, S.K.; Manfrinetti, P.; Palenzona, A. Magnetic ordering in CeMg2Si2 and Ce2MgSi2. J. Alloys Compd. 1997, 252, 24–27. [Google Scholar] [CrossRef]
  24. Kranenberg, C.; Mewis, A. Darstellung und Kristallstrukturen von Ln2Al3Si2 and Ln2AlSi2 (Ln: Y, Tb–Lu). Z. Anorg. Allg. Chem. 2000, 626, 1448–1453. [Google Scholar] [CrossRef]
  25. Dhar, S.K.; Manfrinetti, P.; Palenzona, A.; Kimura, Y.; Kozaki, M.; Onuki, Y.; Takeuchi, T. Magnetic, transport and thermal behaviour of R3Si2, R2YSi2 (R = La and Ce), Ce2ScSi2 and Ce2Sc3Si4. Physics B 1999, 271, 150–157. [Google Scholar] [CrossRef]
  26. Kraft, R.; Pöttgen, R. Syntheses and Crystal Structure of the Ternary Silicides RE2Si2Mg (RE = Y, La–Nd, Sm, Gd–Lu) and Structure Refinement of Dy5Si3. Monatsh. Chem. 2005, 136, 1707–1713. [Google Scholar] [CrossRef]
  27. Kraft, R.; Pöttgen, R. Ternary Germanides RE2Ge2Mg (RE = Y, La–Nd, Sm, Gd, Tb). Monatsh. Chem. 2004, 135, 1327–1334. [Google Scholar] [CrossRef]
  28. Dinges, T.; Rodewald, U.C.; Matar, S.F.; Eckert, H.; Pöttgen, R. New Ternary Silicide LiRh2Si2—Structure and Bonding Peculiarities. Z. Anorg. Allog. Chem. 2009, 635, 1894–1903. [Google Scholar] [CrossRef]
  29. Bruker. APEX2; Bruker AXS Inc.: Madison, WI, USA, 2007. [Google Scholar]
  30. Bruker. SAINT; Bruker AXS Inc.: Madison, WI, USA, 2002. [Google Scholar]
  31. Sheldrick, G.M. SADABS; University of Göttingen: Göttingen, Germany, 2003. [Google Scholar]
  32. Gelato, L.M.; Parthé, E. STRUCTURE TIDY—A computer program to standardize crystal structure data. J. Appl. Crystallogr. 1987, 20, 139–143. [Google Scholar] [CrossRef]
  33. Andersen, O.K. Linear methods in band theory. Phys. Rev. B. 1975, 12, 3060–3083. [Google Scholar] [CrossRef] [Green Version]
  34. Andersen, O.K.; Jepsen, O. Explicit, first-principles tight-binding theory. Phys. Rev. Lett. 1984, 53, 2571–2574. [Google Scholar] [CrossRef]
  35. Andersen, O.K. Minimal basis sets in the linear muffin-tin orbital method: Application to the diamond-structure crystals C, Si, and Ge. Phys. Rev. B. 1986, 34, 2439–2449. [Google Scholar] [CrossRef]
  36. Jepsen, O.; Burkhardt, A.; Andersen, O.K. The TB-LMTO-ASA Program, Version 4.7; Max-Plank-Institut fur Festkorperforschung: Shuttgart, Germany, 1999. [Google Scholar]
  37. Andersen, O.K.; Jepsen, O.; Glötzel, D. Highlights of Condensed Matter Theory; Bassani, F., Fumi, F., Tosi, M., Eds.; Elsevier Science: North Holland, NY, USA, 1985. [Google Scholar]
  38. Jepsen, O.; Andersen, O.K. Calculated electronic structure of the sandwich d1 metals LaI2 and CeI2: Application of new LMTO techniques. Z. Phys. B Condens. Matter 1995, 97, 35–47. [Google Scholar] [CrossRef]
  39. Blöchl, P.E.; Jepsen, O.; Andersen, O.K. Improved tetrahedron method for Brillouin-zone integrations. Phys. Rev. B 1994, 49, 16223–16233. [Google Scholar] [CrossRef]
  40. Zachariasen, W.H. Crystal chemical studies of the 5f-series of elements. VIII. Crystal structure studies of uranium silicides and of CeSi2, NpSi2, and PuSi2. Acta Crysallogr. 1949, 2, 94–99. [Google Scholar] [CrossRef]
  41. Shannon, R.D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. 1976, A32, 751–767. [Google Scholar] [CrossRef]
  42. Dronskowski, R.; Blöchl, P.E. Crystal orbital Hamilton populations (COHP): Energy-resolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 1993, 97, 8617–8624. [Google Scholar] [CrossRef]
  43. Becke, A.D.; Edgecombe, K.E. Classification of chemical bonds based on topological analysis of electron localization functions. Nature 1994, 371, 683–686. [Google Scholar] [CrossRef]
Materials 11 00495 g001 550
Figure 1. (a) The 2D infinite layered structure consisting of the overall crystal structure of the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system. The crystal structure is represented by the combination of ball-and-stick and polyhedra representations, and viewed down the c-axis direction. The Ge2 dimers are highlighted in orange color. The local coordination geometry of (b) the REGe2 moiety (AlB2-type) and (c) the RE(In/Li) moiety (CsCl-type) are also illustrated. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
Figure 1. (a) The 2D infinite layered structure consisting of the overall crystal structure of the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system. The crystal structure is represented by the combination of ball-and-stick and polyhedra representations, and viewed down the c-axis direction. The Ge2 dimers are highlighted in orange color. The local coordination geometry of (b) the REGe2 moiety (AlB2-type) and (c) the RE(In/Li) moiety (CsCl-type) are also illustrated. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
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Materials 11 00495 g002 550
Figure 2. Schematic illustration showing the structural correlation between (a) the RE4LiGe4 (RE = La, Ce, Pr, Sm) system and (b) the RE2In1−xLixGe (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system. A unit cell of each structure type is outlined in red, and the both of the interlayer and intralayer Ge2 dimers are highlighted in orange-color. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
Figure 2. Schematic illustration showing the structural correlation between (a) the RE4LiGe4 (RE = La, Ce, Pr, Sm) system and (b) the RE2In1−xLixGe (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system. A unit cell of each structure type is outlined in red, and the both of the interlayer and intralayer Ge2 dimers are highlighted in orange-color. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
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Materials 11 00495 g003 550
Figure 3. Local coordination geometries of the CsCl-type cubic-shaped (a) LaLi moiety in La4LiGe4 and (b) La(In/Li) moiety in La2In0.72(1)Li0.28Ge2. Selected interatomic distances are also displayed. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
Figure 3. Local coordination geometries of the CsCl-type cubic-shaped (a) LaLi moiety in La4LiGe4 and (b) La(In/Li) moiety in La2In0.72(1)Li0.28Ge2. Selected interatomic distances are also displayed. Color codes: RE, gray; Ge, orange; In/Li mixed-site, blue.
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Figure 4. Distorted pentagonal-prismatic coordination environment surrounding a cationic RE site in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Figure 4. Distorted pentagonal-prismatic coordination environment surrounding a cationic RE site in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
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Materials 11 00495 g005 550
Figure 5. DOS and COHP curves of Gd2In0.5Li0.5Ge2. (a) TDOS (solid-black line), Gd PDOS (gray region), In PDOS (light-blue region), Ge PDOS (orange region), and Li PDOS (blue region). Six COHP curves represent interatomic interactions (b) for the distorted-pentagonal prismatic coordinate environment surrounding the central Gd, and (c) among three anionic elements. EF (dashed vertical line) is the energy reference (0 eV).
Figure 5. DOS and COHP curves of Gd2In0.5Li0.5Ge2. (a) TDOS (solid-black line), Gd PDOS (gray region), In PDOS (light-blue region), Ge PDOS (orange region), and Li PDOS (blue region). Six COHP curves represent interatomic interactions (b) for the distorted-pentagonal prismatic coordinate environment surrounding the central Gd, and (c) among three anionic elements. EF (dashed vertical line) is the energy reference (0 eV).
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Figure 6. The ELF contour map of Gd2In0.5Li0.5Ge2. The (0 0 0) sliced-plane is along the ab-plane direction with z = 0. The overall diagram is depicted as a filled-and-line contour map, and the 2D Ge2(In/Li) layered structure is overlapped on the ELF diagram. The valence electron density color scheme ranges from blue to red (0.0–0.8 e−/Å3), and values of ELF higher than 0.5 represent the area exceeding free-electron ELF value. Unit cell is outlined in white-color.
Figure 6. The ELF contour map of Gd2In0.5Li0.5Ge2. The (0 0 0) sliced-plane is along the ab-plane direction with z = 0. The overall diagram is depicted as a filled-and-line contour map, and the 2D Ge2(In/Li) layered structure is overlapped on the ELF diagram. The valence electron density color scheme ranges from blue to red (0.0–0.8 e−/Å3), and values of ELF higher than 0.5 represent the area exceeding free-electron ELF value. Unit cell is outlined in white-color.
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Table
Table 1. SXRD data and structure refinement results for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Table 1. SXRD data and structure refinement results for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Refined CompositionLa2In0.72(1)Li0.28Ge2Nd2In0.87(1)Li0.13Ge2Sm2In0.57(1)Li0.43Ge2Gd2In0.47(1)Li0.53Ge2
Formula weight (g/mol)923.06917.41940.93989.65
Space group; ZP4/mbm; 2
Lattice parameters (Å)a7.6140(3)7.4368(2)7.3834(3)7.2885(3)
c4.4496(2)4.3263(10)4.2704(2)4.2416(2)
Volume (Å3)257.957239.121232.799225.323
dcalcd (g/cm3)11.8812.7413.4214.59
Independent reflections594 (Rint = 0.0406)251 (Rint = 0.0460)120 (Rint = 0.0355)153 (Rint = 0.0529)
Data/restraints/parameters594/0/13251/0/13120/0/13153/0/13
Ra indices (I > 2σI)R10.02010.01460.01610.0188
wR20.03750.03090.03490.0397
GOF on F21.0961.0981.2491.161
Largest differences of peak/hole (e/Å3)1.373/−1.2762.320/−2.6930.768/−0.7052.436/−0.846
a R1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = (Σ(w(Fo2 − Fc2)/Σ(w(Fo2)2))1/2, where w = 1/(σ2Fo2 + (A − P)2 + B – P), and P = (Fo2 + 2Fc2)/3; A and B—weight coefficients.
Table
Table 2. Atomic coordinates and equivalent isotropic displacement parameters (Ueq a) from the SXRD refinements for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Table 2. Atomic coordinates and equivalent isotropic displacement parameters (Ueq a) from the SXRD refinements for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
AtomWyckoff SiteOccupationxyzUeq (Å2)
La2In0.72(1)Li0.28Ge2
La4h10.1796(1)0.6796(1)1/20.009(1)
Ge4g10.6179(1)0.1179(1)00.011(1)
In/Li2a0.72(1)/0.280000.018(1)
Nd2In0.87(1)Li0.13Ge2
Nd4h10.1791(1)0.6791(1)1/20.007(1)
Ge4g10.6197(1)0.1197(1)00.008(1)
In/Li2a0.87(1)/0.130000.014(1)
Sm2In0.57(1)Li0.43Ge2
Sm4h10.1785(1)0.6785(1)1/20.008(1)
Ge4g10.6209(1)0.1209(1)00.009(1)
In/Li2a0.57(1)/0.430000.015(1)
Gd2In0.47(1)Li0.53Ge2
Gd4h10.1792(1)0.6792(1)1/20.008(1)
Ge4g10.6213(1)0.1213(1)00.010(1)
In/Li2a0.47(1)/0.530000.010(1)
a Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.
Table
Table 3. Selected interatomic distances (Å) for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Table 3. Selected interatomic distances (Å) for the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Atomic Pair Bond Distance (Å)
La2In0.72(1)Li0.28Ge2Nd2In0.87(1)Li0.13Ge2Sm2In0.57(1)Li0.43Ge2Gd2In0.47(1)Li0.53Ge2
RE-RE (shorter)3.8670(2)3.7681(3)3.7273(6)3.6942(5)
RE-RE (longer)3.9552(2)3.8649(3)3.8398(6)3.7876(5)
RE-Ge (shorter)3.1157(3)3.0246(4)2.9911(7)2.9543(7)
RE-Ge (longer)3.2092(2)3.1319(4)3.1026(8)3.0776(7)
RE-In/Li3.5737(1)3.4845(2)3.4541(4)3.4362(3)
Ge-Ge2.5384(8)2.5185(7)2.5252(14)2.4998(12)
Ge-In/Li3.0448(3)2.9649(5)2.9378(10)2.8985(9)
Table
Table 4. ICOHP values (ev/bond) of the six selected interatomic interactions in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Table 4. ICOHP values (ev/bond) of the six selected interatomic interactions in the RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13(1), 0.28(1), 0.43(1), 0.53(1)) system.
Atomic PairGd-GeGd-InGd-LiGe-GeGe-InGe-Li
ICOHP0.0880.060.0090.1800.0980.027

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Lee, J.; Jeon, J.; You, T.-S. Structure and Chemical Bonding of the Li-Doped Polar Intermetallic RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13, 0.28, 0.43, 0.53) System. Materials 2018, 11, 495. https://doi.org/10.3390/ma11040495

AMA Style

Lee J, Jeon J, You T-S. Structure and Chemical Bonding of the Li-Doped Polar Intermetallic RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13, 0.28, 0.43, 0.53) System. Materials. 2018; 11(4):495. https://doi.org/10.3390/ma11040495

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Lee, Junsu, Jieun Jeon, and Tae-Soo You. 2018. "Structure and Chemical Bonding of the Li-Doped Polar Intermetallic RE2In1−xLixGe2 (RE = La, Nd, Sm, Gd; x = 0.13, 0.28, 0.43, 0.53) System" Materials 11, no. 4: 495. https://doi.org/10.3390/ma11040495

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