Recently, BaAl 4 was identified as topologically nontrivial, sparking interest in isostructural compounds and derivative structures such as ThCr 2 Si 2 and noncentrosymmetric BaNiSn 3 structure types. 1 Electronic structure calculations also identified CeCoGe 3 , a system to study the interplay of superconductivity and antiferromagnetism, as topologically nontrivial. 2,3 Additionally, calculations have made connections between the superconducting and topological states through Coulombic interactions between the Ce-4f orbitals. 4 The recently discovered structurally complex homologous series A n+1 B n X 3n+1 (A = lanthanide; B = transition metal; X = tetrel) is a new family of intergrowth structures 5 with several of the members containing BaNiSn 3 subunits. While we were optimizing the reaction conditions to grow and maximize the yield of members of the homologous series, we grew large single crystals (as shown in Figure 1) of Ln 2 Co 3 Ge 5 (Ln = Pr, Nd, and Sm) in high yield.
Ln 2 M 3 Ge 5 (Ln = lanthanide element; M = transition metal) is rich in intriguing physical phenomena, with La 2 Pt 3 Ge 5 showing superconductivity and Pr 2 Pt 3 Ge 5 showing superconductivity and antiferromagnetism. [6][7][8] Another highlight is Sm 2 Ru 3 Ge 5 , which adopts two different polymorphs: one in the Sc 2 Fe 3 Si 5 structure type (P4/mnc) and the other in the U 2 Co 3 Si 5 structure type (Ibam). Each polymorph shows charge density wave order: one at 175 °C and the other at 240 °C, respectively. 9,10 There are six different structure types reported for intermetallic tetrels with the formula Ln 2 M 3 X 5 : 11 two of which are the orthorhombic U 2 Co 3 Si 5 12 and the monoclinic Lu 2 Co 3 Si 5 -type structures, 13 both distorted supercells of the BaAl 4 prototype. 14 Ce 2 Co 3 Ge 5 has been identified in the U 2 Co 3 Si 5 structure type, and its transport and magnetic properties are well reported in powder samples, with Ce understood to be valence instable. 15,16 The Pr 15,17 and Nd 18 analogues, however, have been previously identified in both the orthorhombic U 2 Co 3 Si 5 and monoclinic Lu 2 Co 3 Si 5 (C2/c) 15,19 The Sm analogue adopts the monoclinic structure. 20 Furthermore, only the properties of the Pr 2 Co 3 Ge 5 orthorhombic analogue have been reported, where it was determined to be paramagnetic. 15 The close structural relationship to other BaAl 4 derivatives and the polymorphism in Pr 2 Co 3 Ge 5 and Nd 2 Co 3 Ge 5 motivated us to further investigate the physical properties and crystal chemistry of Ln 2 Co 3 Ge 5 (Ln = Pr, Nd, and Sm). Herein, we present the crystal growth, magnetic susceptibility, and resistivity of Ln 2 Co 3 Ge 5 in relation to LnCo 2 Ge 2 and LnCoGe 3 .
2.1. Synthesis. Ln 2 Co 3 Ge 5 (Ln = Pr, Nd, Sm) was prepared using the flux growth method with tin as a metallic nonreactive flux. The elements were combined in a ratio of 3 Ln: 2 Co: 7 Ge: 52 Sn for Ln = Nd and Sm and 3 Ln: 2 Co: 7 Ge: 50 Sn for Ln = Pr, with the total mass of each batch as ∼1.5 g for Pr and Nd analogues and ∼3 g for the Sm analogue. The elements were each transferred to an alumina Canfield crucible 21 and then sealed in fused silica tubes under ∼1/3 atm of Ar gas. The ampules were placed in a furnace at 300 °C and heated to 1175 °C at a rate of 100 °C/h. The samples were heated at 1175 °C for 24 h and cooled to 815 °C at a rate of 3 °C/h. The reaction ampules were removed, inverted, and centrifuged at 815 °C to remove the excess flux. The residual flux on the crystal surfaces were removed by etching the crystals in diluted HCl. The yield ranges from 50-80%. Additionally, two samples of Pr 2 Co 3 Ge 5 were synthesized by arc-melting the Pr, Co, and Ge pieces in the stoichiometric ratio with a total mass of approximately 0.3 g. Each button was flipped and arc-melted multiple times before being placed in an alumina crucible and sealed in fused silica tubes under ∼1/3 atm of Ar gas. The weight loss during arc-melting was negligible. From there, samples were annealed at 797 and 1000 °C for 500 h, respectively, in attempts to obtain the U 2 Co 3 Si 5 structure type. Elemental analysis was also performed and details can be found in the Supporting Information.
2.2. Structure Determination. Powder X-ray diffraction was performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54184 Å). The powder diffraction data confirm the results of the single crystal refinement for all reported compounds as provided in Figure SI1, SI2, and SI3. Single crystal X-ray diffraction data were obtained at room temperature using a Bruker D8 Quest Kappa single crystal X-ray diffractometer, equipped with an IμS microfocus source (Mo Kα, λ = 0.71073 Å), a HELIOS optics monochromator, and a PHOTON II CPAD detector. The Bruker SAINT program was used to integrate the diffraction data, and the intensities were corrected for absorption via a multiscan method implemented in SADABS 2016/2. 22 The structures were solved with the intrinsic phasing methods in SHELXT 23 and subsequently anisotropically refined (full-matrix least-squares on F 2 ) using SHELXL2018. 24 The crystal structure of Ln 2 Co 3 Ge 5 adopts the monoclinic Lu 2 Co 3 Si 5 structure type and not the related U 2 Co 3 Ge 5 orthorhombic structure type with the space group Ibam. Additional pseudosymmetries were found for the Pr and Nd analogues; however, the symmetry elements do not warrant a change in space group choice. During the visual inspection of the precession images of the Sm and Nd analogues, nonmerohedral twinning along the b-directions was observed. The data were integrated and processed with CELL_-NOW 25 to identify the twinned domains. It was determined that the second domain of the Sm 2 Co 3 Ge 5 crystal, corresponding to 18.6%, is rotated with respect to the first domain by 179.9°around the reciprocal axis (-0.002 1.000 1.000) and the real axis (0.963 1.000 0.999). For Nd 2 Co 3 Ge 5 , the second domain of 43.87% is rotated by 179.8°around the reciprocal axis (-0.978 -0.001 1.000) and real axis (0.002 0.000 1.000) compared to the first domain. The determined unit cell was revised using the data generated from CELL_NOW. The data were then reduced with the SAINT software. A multiscan absorption correction was applied with TWINABS 26 implemented in the Bruker APEX3 software package. An initial structure model was developed with the intrinsic phase model, and the structure refinement was performed with the least-squares refinement method using SHELXL2018 for both twinned domains. The lattice parameters and single crystal refinement statistics can be found in Table 1, while the fractional atomic coordinates and anisotropic displacement parameters are provided in Table SI1.
The largest crystals of each batch were selected for physical property measurements. Temperature-dependent DC magnetization data were collected using a Quantum Design MPMS system. Samples were zero-field cooled (ZFC) and then measured on warming from 2-300 K in an external magnetic field of 0.02 T for Pr and Nd analogues and 0.2 T for Sm analogue. The magnetization versus applied field was measured at 5 K in fields between ±7 T. The electrical resistance of the polyhedral single crystals was measured using the AC transport option in a Quantum Design PPMS system. A standard 4-probe method was used, where electrical contacts to the sample consisted of 2-mil diameter Pt wire attached with Epotek H20E conductive epoxy.
3.1. Structure. Ln 2 Co 3 Ge 5 (Ln = Pr, Nd, and Sm) crystallizes in the monoclinic Lu 2 Co 3 Si 5 -type structure 13 with the C2/c space group (Figure 2). The Lu 2 Co 3 Si 5 type structure is a distorted variant of the orthorhombic U 2 Co 3 Si 5 -type 12 consisting of seven crystallographic sites. The structural relationship between U 2 Co 3 Si 5 and Lu 2 Co 3 Si 5 becomes evident when the Lu 2 Co 3 Si 5 -type structure is described in the nonstandard space group I2/c (Figure SI4). 13 For the Lu 2 Co 3 Si 5 -type structure in I2/c, the angle β, which is exactly
90°in the orthorhombic structure, deviates slightly from 90°a nd can be written as 90°+ δ. In the Ln 2 Co 3 Ge 5 series, δ increases with decreasing Ln-radius, which correlates with the orthorhombic variant for Ce 2 Co 3 Ge 5 and the monoclinic variant for Sm 2 Co 3 Ge 5 and beyond. 19 Pr 2 Co 3 Ge 5 and Nd 2 Co 3 Ge 5 have been reported in both the Lu 2 Co 3 Si 5 and U 2 Co 3 Si 5 structure types based on powder diffraction. 17,19,27 The similarity of powder diffraction patterns between the orthorhombic and monoclinic variant could be the reason for the difference in the previous reports. Structure determination from single crystal X-ray diffraction of our flux grown crystals confirm that the Pr and Nd analogues adopt the monoclinic crystal structure at room temperature.
The arc-melting and annealing procedures developed by Fedyna et al. and Layek et al. 15,17 A zigzag chain of the Ge1, Ge2, and Ge4 atoms extends along the c-axis. The Zintl-Klemm concept was applied to a similar compound, La 2 Pd 3 Ge 5 , but it was determined not to be applicable. 28 Further connections of the Lu 2 Co 3 Si 5 -and the U 2 Co 3 Si 5 -type structures to the other variants of the BaAl 4 -type structure become apparent by comparison to the ThCr 2 Si 2 -and the BaNiSn 3 -type structures. These structural relations will be discussed in detail later. Next, we present the physical properties of the Ln 2 Co 3 Ge 5 analogues.
3.2. Physical Properties. The temperature-dependent magnetic susceptibility χ = M/H of Pr 2 Co 3 Ge 5 presented in Figure 3a (inset) shows a magnetic transition at 33 K. A spin reorientation occurs at 27 K, as evident by a plateau in the susceptibility, followed by a further increase of the susceptibility, χ, before an antiferromagnetic-like cusp at 6 K. A linear fit corresponding to the Curie-Weiss law is applied to the paramagnetic region of the inverse susceptibility 1/χ between 60-300 K (Figure SI5). The effective moment obtained from the fit is μ eff = 3.52 μ B /Pr atom is in good agreement with the theoretical expected moment for Pr 3+ of 3.57 μ B suggesting the moment is solely from the lanthanide. This contrasts with prior reports of orthorhombic Pr 2 Co 3 Ge 5 measured at H = 1 T, which was paramagnetic down to 2 K with μ eff = 3.76 μ B /Pr atom and a Weiss constant of Θ = 5.48 K. The authors noted a deviation from the Curie-Weiss fit below 50 K, but this was attributed to crystal field effects. 15 At 5 K, the field-dependent magnetization shows a rapid increase upon application of a small external magnetic field H up to 0.025 T (see Figure 3b and Figure SI6). Beyond H = 0.025 T, the magnetization M increases more slowly, but does not saturate up to 7 T. Similarly, the field-dependent magnetization at 25 K increases sharply, then increases steadily, reminiscent of the metamagnetic transitions found in PrCo 2 Ge 2 . 29,30 The temperature-dependent magnetization of Nd 2 Co 3 Ge 5 is shown in the inset of Figure 4a. The susceptibility increases below 29 K and plateaus between 15 and 25 K. Upon further cooling, a shoulder is observed at 6 K and the magnetization increases followed by a downturn below 3 K. We note that the inverse magnetic susceptibility shows a slight deviation from a linear behavior in the temperature range of 130-165 K (Figure SI5b), though not intrinsic to the sample as no anomaly was observed in the resistivity. The Weiss constant Θ is -8.7 K, signaling predominantly antiferromagnetic interactions. Surprisingly, the effective moment μ eff is 4 μ B /Nd atom, larger than the expected moment for Nd 3+ of 3.62 μ B (Figure SI5). The field-dependent magnetization M of Nd 2 Co 3 Ge 5 is similar to the Pr-analogue at both 5 and 25 K, respectively, and does not saturate up to 7 T. As shown in Figure 4b and Figure SI6, a jump in magnetization is also seen for the Nd-analogue at H = 0.025 T, and the magnetization does not reach saturation.
The temperature-dependent susceptibility χ of Sm 2 Co 3 Ge 5 is presented in Figure 5a. A magnetic transition can be seen at 26 K followed by a small upturn of the susceptibility below 6 K that could be attributed to a spin reorientation. The inverse susceptibility 1/χ (see Figure SI5 and Figure SI7) follows a modified Curie-Weiss law (1) with a large diamagnetic contribution of χ 0 = -1.4(1) × 10 -3 emu/mol.
The Weiss constant Θ for Sm 2 Co 3 Ge 5 is 22.8 K, and the experimentally obtained effective moment is 1.03 μ B /Sm atom, slightly higher than the theoretical moment of 0.85 μ B /Sm 3+ . As shown in Figure 5b, the magnetization M increases linearly with increasing strength of the external applied field H, consistent with the antiferromagnetic behavior at 5 K.
The normalized temperature dependent resistivities of Ln 2 Co 3 Ge 5 (Ln = Pr, Nd, and Sm) are presented in Figure 6. The resistivity R(T) has been normalized by R 0 (T = 250 K).
The compounds are metallic but deviate from a linear behavior, potentially indicating Kondo screening. A change of slope occurs in the range of 29-35 K which coincides well with the ordering temperatures observed in the temperaturedependent magnetization data. However, the observed spin reorientations are not reflected in the temperature-dependent resistivity. The residual resistivity ratios (RRR) of these compounds are in the range of 4-10. Next, we compare the properties of the Ln 2 Co 3 Ge 5 analogues to those of LnCoGe 3 and LnCo 2 Ge 2 to establish their structural relationship and correlate their physical properties.
The U 2 Co 3 Si 5 12 and Lu 2 Co 3 Si 5 13 structure types are both compositionally and structurally intermediate to the ThCr 2 Si 2 and BaNiSn 3 structures. While the CaBe 2 Ge 2 structure type is more fitting to describe the U 2 Co 3 Si 5 structure, since it is not stabilized in the Ln-CoGe system, the ThCr 2 Si 2 was selected instead. 34 In the present case, the ThCr 2). This is consistent with a stepwise substitution of the smaller Co in the square nets of LnCo 2 Ge 2 with the larger Ge square nets of Ln 2 Co 3 Ge 5 and LnCoGe 3 . In contrast, the average interlayer distance does not increase monotonically, but it is the shortest for Ln 2 Co 3 Ge 5 . The monoclinic distortion is accompanied by a tilt of the Co-Ge dimer formed by the apical atoms of adjacent [Co 3 Ge 5 ] slabs as emphasized in Figure 7a. A similar dimer is observed in LnCoGe 3 ; however, the corresponding bond distance in Ln 2 Co 3 Ge 5 is significantly longer than the bond distance in LnCoGe 3 . Additionally, LnCoGe 3 in the BaNiSn 3 -type structure has been observed experimentally only for the early rare-earth elements (Ln = La -Eu), whereas the Ln 2 Co 3 Ge 5 is observed for Ln = La -Er. The decreasing radius of the rare-earth atoms forces a decreasing dimer bond distance, resulting in very short Co-Ge bonds in the apical dimer of the BaNiSn 3 -type structure, which limits their stability under ambient pressure to the larger rare-earth elements. The tilting of the Co-Ge dimer in the monoclinic Lu 2 Co 3 Si 5 -type structure alleviates the chemical pressure and allows the formation of Ln 2 Co 3 Ge 5 compounds with rare-earth elements smaller than Eu. The three crystal structures are very intertwined and are distinct by small structural differences. Next, we analyze the effects of these small differences on their physical properties. The magnetic susceptibility of Sm 2 Co 3 Ge 5 above the magnetic ordering temperature is remarkably similar to SmCo 2 Ge 2 and does not obey Curie-Weiss Law, possibly due to thermal population of Hund's rule excited states. 29 The effective magnetic moment of Sm 2 Co 3 Ge 5 obtained from the modified Curie-Weiss fit is greater than the expected Sm 3+ moment, possibly due to Co contribution. Similarities in the magnetic and transport properties between Ln 2 Co 3 Ge 5 , LnCoGe 3 , and LnCo 2 Ge 2 are present for the Pr and Nd analogues as well, most notably in the ordering temperatures and in the field strength H where metamagnetic transitions occur as seen in Table 2. There is another metamagnetic transition at 10 T previously reported from experimental data for PrCo 2 Ge 2 , which is not listed in Table 2. 30 The magnetic moments of the Pr analogues all agree with a Pr 3+ state; however, Nd 2 Co 3 Ge 5 shows an elevated magnetic moment compared to what is expected for Nd 3+ , which is unlike NdCoGe 3 and NdCo 2 Ge 2 . Another key difference between the properties of these compounds is seen in the resistivity. In Ln 2 Co 3 Ge 5 , the temperature-dependent resistivity does not increase linearly, as in both LnCoGe 3 and LnCo 2 Ge 2 . This curvature could be indicative of Kondo scattering, or it could be due to crystal field effects in Pr 2 Co 3 Ge 5 as previously suggested. 15 Since Ln 2 Co 3 Ge 5 is structurally and compositionally intermediate to LnCoGe 3 and LnCo 2 Ge 2 , these structures can be used as a system to study the suppression of Kondo scattering in lanthanide cobalt germanides. Finally, the differences in magnetic behavior we see across the structure types could be a result of subtle changes in the local environment of the Ln atom impacting their crystal field splitting. Although the field dependent magnetization of Ln 2 Co 3 Ge 5 (Ln = Pr, Nd) are reminiscent of the reported PrCoGe 3 and NdCoGe 3 , the transition temperatures of the Ln 2 Co 3 Ge 5 are higher and metamagnetic transitions occur at lower fields. CEF effects can be attributed to the ground state behavior for both structure types, and the distortion about the lanthanides is significant to the magnetic phase transitions. 33
In this paper, we presented the magnetic susceptibility, resistivity, flux growth synthesis, and crystal structures of Pr 2 Co 3 Ge 5 , Nd 2 Co 3 Ge 5 , and Sm 2 Co 3 Ge 5 , which crystallize in the monoclinic Lu 3 Co 3 Si 5 -type structure. The structure is a derivative of the BaAl 4 -type structure and, as such, contains distorted square nets. The square net motif presents itself in many topological materials, such as TaAs 35 and LaAlGe, 36 and has been correlated with the presence of topological band structures. 37,38 The potential for rare earth germanide ternary compounds to host magnetic topological states, as seen in LnAlGe (Ln = Ce, Pr), 39 is exciting and merits further investigation of the BaAl 4 -related phases.
https://doi.org/10.1021/acs.inorgchem.1c01978Inorg. Chem. 2021, 60, 15343-15350