The single crystalline growth of lanthanide based magnetic intermetallics has garnered much interest in the last two decades, primarily due to their unusual magnetic and electrical properties, such as valence fluctuations and magnetically mediated superconductivity. 1,2 As part of our efforts to understand the structural stability of the Yb 3 Rh 4 Sn 13 structure type, 3 we have determined that early lanthanides, such as Ce 3 Co 4 Sn 13 , 4 can be stabilized for the stannides, while germanides can be prepared only with latter lanthanides such as Lu 3 Co 4 Ge 13 . 5,6 While exploring the Ln-Co-Sn (Ln = lanthanides) phase space with substructural units of targeted intermetallics, we have focused on the orthorhombic La 3 Co 2 Sn 7 structure type for our study. La 3 Co 2 Sn 7 crystallizes in the space group Cmmm with cell dimensions of a ∼ 4.59 Å × b ∼ 27.6 Å × c ∼ 4.60 Å. 7 The structure type is composed of one lanthanide in a cuboctahedral coordination environment (2d site) and a second (4i site) in a trigonal prismatic environment forming an intergrowth of AuCu 3 and CeNiSi 2 type subunits. The structure also consists of one transition metal site (4j) and four sites occupied by a Group 14 element.
The magnetic and electrical properties of several Ni analogues have been reported, including Ce 3 Ni 2 Sn 7 , [8][9][10] and detailed studies on oriented crystals show that magnetic transitions are highly dependent on Ni concentration and magnetic fields. Ln 3 Ni 2-x Sn 7 (Ln = La, Ce, Pr, and Nd) are highly anisotropic, and metamagnetic transitions were found for the Ce 3 Ni 1.69 Sn 7 and Pr 3 Ni 1.56 Sn 7 analogues. Another related structure is the Ce 3 Ni 2+x Si 8-x (x ∼ 1) phase, which not only has Ni on the 4j site but also has Ni mixed on one of the Si sites (4i) in the asymmetric unit. 11 However, relatively few Co analogues have been reported. More recently, polycrystalline Ce 3 Co 2 Sn 7 was shown to behave similarly to the Ni compound, where the Ce located at the 4i site has a well-localized magnetic moment and increasing magnetic fields lead to lowering of the antiferromagnetic ordering temperature 12 along with a second Ce site being nonmagnetic or valence fluctuating. 13 In U 3 Co 2 Ge 7 , two magnetic transitions were observed near 40 and 20 K that have been attributed to a ferromagnetic state and a spin reorientation, respectively, and the field-dependent magnetization shows a stepwise increase in the magnetization below 2 K. To study the structural stability of Co containing compounds adopting the La 3 Co 2 Sn 7 structure type, we present the crystal growth, structural characterization, and magnetic properties of Ln 3 -Co 2+x Ge 8-y Sn y (Ln = Pr, Nd, Sm). In this paper, we highlight the Pr analogues Pr 3 Co 2.3376(5) Ge 7.056 (7) and Pr 3 Co 2.514(5) Ge 6.66(7) -Sn 0.360(2) .
2.1. Synthesis. Samples of Ln 3 Co 2+x Ge 7-y Sn y (Ln = Pr, Nd, Sm) were synthesized by arc-melting stoichiometric amounts of Ln, Co, and Ge pieces (3:2:7, all ≥99.9 wt % purity, metal basis) in an Ar atmosphere. Each button was arc-melted and flipped four times to ensure homogeneity. The buttons were then ground into a fine powder and combined with Sn in a 1:20 (Ln:Sn) ratio in a Canfield crucible set. 14 The reaction was then backfilled with Ar (∼0.2 atm) and sealed in a fused-silica tube. The sealed reaction tube was then heated to 1175 °C at 100 °C/h, dwelled for 24 h, cooled to 815 °C at 3 °C/h, and centrifuged to remove excess Sn. The phases were etched with 3 M HCl, and single crystals of Ln 3 Co 2+x Ge 7-y Sn y up to ∼1 mm in length were mechanically extracted.
Single crystals of Pr 3 Co 2.3376(5) Ge 7.056 (7) were grown successfully with In flux (instead of Sn). Similar to the synthesis with Sn flux, the reactions with In flux were heated to 1175 °C at a rate of 100 °C/h, then slow cooled to 815 °C at a rate of 3 °C/h before centrifuging the excess In flux. The silver, metallic plate-shaped crystals were up to ∼1 mm in length. For completeness, a polycrystalline ingot of Pr 3 Co 2 Sn 7 was synthesized via arc-melting as previously described above, followed a U eq is defined as one-third of the trace of the orthogonalized U ij tensor.
with annealing at 850 °C for 7 days. Homogeneity of the polycrystalline ingot was determined using powder X-ray diffraction.
Attempts to synthesize La analogues, such as La 3 Co 2 Ge 7 or La 3 Co 2 Ge 7-x Sn x , using either Bi, In, or Sn flux, consistently led to single crystalline LaCoGe 3 . 15 Bi flux was also attempted for the growth of Pr 3 Co 2 Ge 7 , but was unsuccessful and led to the synthesis of Pr 2 Co 3 Ge 5 . While the single crystalline growth of germanides has been known to be successful using excess In flux, 16 attempts to grow single crystals of Ln 3 Co 2+x Ge 7 (Ln = Nd and Sm) led to the stable formation of LnCoGe 2 (Ln = Nd, Sm). Growths were attempted for Gd 3 Co 2 X 7 (X = Ge, Sn), but the resulting products were Gd 3 Co 4 Ge 13 and GdCoSn 2 , respectively. We speculate that Gd or smaller lanthanides are not stable in this structure type; however, the stability of competing phases makes it difficult to isolate Ln 3 Co 2 Ge 7 via flux-growth methods. For this reason, we did not attempt any syntheses with lanthanide elements beyond Gd.
2.2. Structure Determination. Powder X-ray diffraction was used to determine the homogeneity and phase purity of samples and was performed using a Bruker D8 Advance powder X-ray diffractometer operating at 40 kV/30 mA with a Cu Kα radiation (λ = 1.54184 Å). Single crystal X-ray diffraction data were collected on a Bruker D8 Quest Kappa single crystal X-ray diffractometer operating at 50 kV and 1 mA equipped with an IμS microfocus source (Mo Kα, λ = 0.71073 Å), a HELIOS optics monochromator, and a CMOS detector. The Bruker SAINT program was used to integrate the diffraction data while the absorption correction was performed using the Bruker program SADABS 2016/2 (multiscan method). 17 The crystal structures of Ln 3 Co 2+x Ge 7-y Sn y (Ln = Pr, Nd, Sm) and Pr 3 Co 2+x Ge 7 were solved using intrinsic phasing methods in SHELXT, 18 and anisotropically refined using SHELXL2014. 19 The data collection and refinement parameters are presented in Table 1, and the atomic positions for Pr 3 Co 2+x Ge 7 and Pr 3 Co 2+x Ge 7-y Sn y are presented in Table 2. The atomic positions for the Nd and Sm analogues are provided in the Supporting Information. For clarity throughout the paper, we will denote the composition of Pr 3 Co 2.3376(5) Ge 7.056 (7) as Pr 3 Co 2+x Ge 7 and Pr 3 Co 2.514(5) Ge 6.66 (7) 2.3. Physical Properties. Magnetic susceptibility data were collected between 1.8 and 290 K and in magnetic fields up to 9 T, using a Quantum Design Physical Property Measurement System (PPMS). Magnetic susceptibility was also measured in a PPMS under a constant magnetic field of 1000 Oe. Single crystals were zero-fieldcooled (ZFC) to 2 K, then magnetic field was applied and susceptibility was measured up to 300 K. The change in magnetization under field, up to 7 T, was measured at a constant temperature of 3 K. The specific heat was measured in the PPMS using a time-relaxation method between 2 and 20 K at zero applied field. The heat capacity of the addenda (background from sample platform and mounting grease) was subtracted from the data as shown in the Supporting Information.
3.1. Structures of Pr 3 Co 2+x Ge 7 and Pr 3 Co 2+x Ge 7-y Sn y . Pr 3 Co 2+x Ge 7 and Pr 3 Co 2+x Ge 7-y Sn y adopt the La 3 Co 2 Sn 7 structure type 7 and are best modeled in the Cmmm space group with cell dimensions of a ∼ 4.23 Å, b ∼ 25.9 Å, c ∼ 4.27 Å, and V ∼ 468 Å. 3 As shown in Figure 1 (left), Pr 3 Co 2+x Ge 7 can be described as an intergrowth of the "PrGe 3 " (AuCu 3 -type) with alternating "PrCoGe 2 " of the CeNiSi 2 type along the b-direction. The compound consists of distinctive subunits: Pr1@Ge 12 facesharing cuboctahedra (AuCu 3 -type) capped by two Co1 atoms along the axial position and Ge4@Pr 6 trigonal prisms. The Pr2 environment can be described as the part of the "PrCoGe 2 " structural unit adopting the CeNiSi 2 structure type. The Pr1@ Ge 12 cuboctahedron consists of three atomic sites with Co occupying a 4j site (labeled as Co1C), with the Pr1-Co1C distance being 3.294(5) Å in Pr 3 Co 2+x Ge 7 and Pr-Ge distance of 3.1-3.2 Å (Table 3), which is in excellent agreement with contacts found in PrGe 3.36 as prepared by high pressure. 20 The rectangular antiprism has Co1-Ge2, Co1-Ge3, and Co1-Ge4A contacts of 2.344(5) Å, 2.3479(9) Å, and 2.3407(19) Å, respectively, which are also in excellent agreement with the Co- Ge contacts found in isostructural U 3 Co 2 Ge 7 , ranging from 2.295(4) to 2.315(2) Å. 21 We also note a slight occupational/ positional disorder in the Ge4 site, which is split into two sites Ge4A and Ge4B (occupancy percent ratio of ∼99%:∼1%). Although the occupancy of the Ge4B is small for Pr 3 Co 2+x Ge 7 , the occupation of Ge4B systematically increases as a function of lanthanide for the Ln 3 Co 2+x Ge 7-y Sn y series where the Sm analogue has the highest occupancy (∼10%). This also correlates well with the occupational trend observed with the systematic decrease in the Co1 occupation (i.e., Sm analogue has Co1 occupancy ∼ 91%). Furthermore, the modeling of the split Ge4 sites into two sites (Ge4A/Ge4B) is similar to the modeling used in the splitting of a Sn site in the related Eu 2 Ni 1.49(1) Sn 5 compound. 22 While the occupancy of Co1 (4j) in Ln 3 Co 2+x Ge 7 is close to 90%, the Ni-containing analogues are typically transition metal deficient with a Ni occupancy of 70% in Pr 3 Ni 2-x Sn 7 . The occupancies of Co in both the Co1 and Co1C in the germanide analogues also decrease slightly with decreasing Ln covalent radii, though not as much as observed in Ln 3 Ni 2-x Sn 7 (Ln = La-Nd).
3.2. Structural Disorder in the Cuboctahedron. Unlike the cuboctahedra found in La 3 Co 2 Sn 7 , 7 the Ge1 site in Pr 3 Co 2.38 Ge 7.06 , or Pr 3 Co 2+x Ge 7 for clarity, exhibits a prolated thermal ellipsoid which is more appropriately modeled as three partially occupied positionally disordered sites to compensate for residual electron density. Furthermore, the distance of 1.347 Å from Ge1A to Co1C is too short to be a reasonable chemically bond, with the Ge1B being approximately halfway in between these two atomic sites. Thus, in Pr 3 Co 2+x Ge 7 , the Ge1 site is modeled as being split into three crystallographic sites, including the Ge1A (2a, mmm), Ge1B (4i, m2m), and the Co1C (4i, m2m) Wyckoff sites. The partially occupied split sites were allowed to refine with only the constraints of the sum of the occupancies being equal to unity (necessary for stability of the refinement), and the Co1C-Co1C bond distance of 2.69(2) Å was modeled as a dimer. The resulting occupancies are 54%, 26%, and 20% for Ge1A, Ge1B, and Co1C-Co1C, respectively. It is chemically reasonable for Co to occupy the 4i site given the excellent agreement of the Co1C-Ge2 distance of 2.344(5) Å and Co1C-Ge3 distances of 2.366(5) Å with the Co1-Ge2 and Co1-Ge3 distances of 2.344(5) Å (×2) and 2.3479(9) Å (×2) of the cobalt rectangular antiprism. Allowing Co1C to occupy the atomic site as shown in Figures 2 and3 leads to a more reasonable Co-Co bond distance, given that the Ge-Ge distances are typically at least ∼3.0 Å in a polyhedral environment. Figure 2 shows all the possible occupancy configurations: When Ge1A is occupied, Ge1B and Co1C will not be occupied given the short distances. When Ge1B is occupied, only one of the symmetry related sites will be occupied. When Co1C is occupied, both of the symmetry related sites are occupied and have a contact of 2.69(2) Å, which is within the range of Co-Co distances in germanides found in Ln 6 Co 5 Ge 1+x Al 3-x (Ln = Pr, Nd). 23 The occupation of a transition metal on the 4i site was also observed in the related compound Ce 3 Ni 2+x Si 8-x (x ∼ 1), where an elongation of the thermal ellipsoids along the b-direction led to the disorder. 11 The result of the Co incorporation forming the cuboctahedron leads to higher transition metal concentration in "Pr 3 Co 2+x Ge 7 ". Co1C-Co1C, Co1C-Ge2, and Co1C-Ge3 of 2.856(10) Å, 2.3791(8) Å, and 2.3566(8) Å, respectively. This observation is also consistent throughout the Nd and Sm analogues. On the basis of the crystallographic data, it is most likely that rare earth Ge polyhedra are stabilized by Sn incorporation, thereby, expanding the polyhedral units. The Co1C at the 4i site leads to subsequent higher transition metal occupancy than that observed in Ln 3 Ni 2 Sn 7 (Ln = La-Nd). 12) Å, the 5.3 K transition can be attributed to the Pr1-Pr1 contacts, which is in the same chemical environment as the PrCo 0.85 Ge 2 of the CeNiSi 2 slab. The higher ordering transition found in the Pr 3 Co 2+x Ge 7 at 9.3 K can be attributed to the Pr2-Pr2 contact with a distance of 4.024 Å, which makes up the trigonal prisms, as highlighted in Figure 1 (right). Similar to the RE 6 Co 5 Ge 1+x Al 3-x , 24 magnetic moments are also higher than the spin-only moments for the Pr analogues. While Co can contribute to the magnetic moment, the small occupancy might not lead to a long-range ordering. For example, CaCo 2 Ge 2 and the corresponding BaCo 2 Ge 2 of the ThCr 2 Si 2 structure type show a ferromagnetic transition below 70 K. 25 The results from the measurement of the specific heat capacity plotted as C/T vs T are shown in the Supporting Information and provide additional evidence of magnetic ordering in Pr 3 Co 2+x Ge 7 , and verifies the presence of two magnetic transitions, in agreement with the magnetic susceptibility data.
The field (H)-dependent magnetization at temperatures below the Neél temperature T N for Pr 3 Co 2 Sn 7 , Pr 3 Co 2+x Ge 7-y -Sn y , and Pr 3 Co 2+x Ge 7 is shown in Figure 4. Pr 3 Co 2+x Ge 7 exhibits three metamagnetic transitions at 0.46, 0.80, and 2.1 T. When Sn is substituted into the structure, the metamagnetic transitions seem to be suppressed and the remaining transitions occur at lower fields. Both Pr 3 Co 2+x Ge 7-y Sn y (0.04 and 0.46 T) and Pr 3 Co 2 Sn 7 (0.03 and 0.41 T) have two low field transitions, compared to three metamagnetic transitions in Pr 3 Co 2+x Ge 7 .
As the temperature increases to 8 K, the metamagnetic features are suppressed, as shown in Figure 4c. Figure 5 shows the magnetic susceptibility and field-dependent magnetization for Nd 3 Co 2+x Ge 7-y Sn y . With decreasing temperature, the magnetization of Nd 3 Co 2+x Ge 7-y Sn y shows a transition near 28 K, followed by another antiferromagnetic like transition below 3.8 K, and is presumably due to two rare earth magnetic sublattices. Above 100 K, a Curie-Weiss fit led to an effective moment of μ eff = 3.7 μ B /mol Nd with Weiss constant of θ = -18.7(3) K, in excellent agreement with the Nd 3+ spin only moment of 3.62 μ B . We note the small magnetization for the Sm analogue, and the compound is most likely a temperatureindependent paramagnet.
To study the Ge counterpart of La 3 Co 2 Sn 7, we have grown single crystals of Pr 3 Co 2+x Ge 7 and Sn-substituted compounds Ln 3 -Co 2+x Ge 7-y Sn y (Ln = Pr, Nd, Sm) via In and Sn flux. Structural investigation reveals that the size of the lanthanide has an impact on the structural disorder of the cuboctahedra and Co concentration. Despite the disorder of transition metal and some of the Ge sites, Sn-substitution leads to higher transition metal occupancy by expanding the Pr-Ge polyhedra. We have identified magnetic and multiple metamagnetic transitions similar to those of previously studied Sn compounds, but with transition temperatures higher than the Sn counterparts. Here, all but the Sm compound show Curie-Weiss behavior above the transition temperature, and estimation of the free moment indicates the magnetism, is due to the rare earth element, and we would expect Ln 3 Co 2 Ge 7 to likely host equally complex and rich magnetic properties to those of Ln 3 Co 2 Sn 7 .