Crystal growth, characterization and physical properties of PrNiSb3, NdNiSb3 and SmNiSb3

The crystal structures of three new intermetallic ternary compounds in the Ln NiSb 3 ( Ln =Pr, Nd and Sm) family have been characterized by single crystal X-ray diffraction. PrNiSb 3 , NdNiSb 3 and SmNiSb 3 all crystallize in an orthorhombic space group, Pbcm (No. 57), Z ¼ 12 ; with a ¼ 12 : 5700 ð 2 Þ ( A ; b ¼ 6 : 2010 ð 4 Þ ( A ; c ¼ 18 : 670 ð 6 Þ ( A ; and V ¼ 1431 : 64 ð 11 Þ ( A 3 ; a ¼ 12 : 5090 ð 2 Þ ( A ; b ¼ 6 : 1940 ð 3 Þ ( A ; c ¼ 18 : 3350 ð 6 Þ ( A ; and V ¼ 1420 : 61 ð 9 Þ ( A 3 ; and a ¼ 12 : 3900 ð 1 Þ ( A ; b ¼ 6 : 1760 ð 3 Þ ( A ; c ¼ 18 : 2650 ð 6 Þ ( A ; and V ¼ 1397 : 65 ð 8 Þ ( A 3 ; for Ln =Pr, Nd and Sm, respectively. These compounds consist of rare-earth atoms located above and below layers of nearly square, buckled Sb nets, along with layers of highly distorted edge-and face-sharing NiSb 6 octahedra. Resistivity data indicate metallic behavior for all three compounds. Magnetization measurements show antiferromagnetic behavior with T N ¼ 4 : 5K (PrNiSb 3 ), 4.6 K (NdNiSb 3 ), and 2.9K (SmNiSb 3 ). Effective moments of 3.62 m B , 3.90 m B and 0.80 m B are found for PrNiSb 3 , NdNiSb 3 and SmNiSb 3 , respectively, and are consistent with Pr 3+ ( f 2 ), Nd 3+ ( f 3 ), and Sm 3+ ( f 4 ).

Another interesting class of Sb-containing compounds is LaTSb 3 (T=V, Cr) [5,6].The V and Cr analogs of LaTSb 3 are isostructural and consist of planes of nearly square Sb nets, with Ln atoms positioned both above and below these Sb net planes.Also included are layers of TSb 6 octahedra which are face sharing along the c-axis and edge sharing along the b-axis [7].The LnVSb 3 (Ln=La-Nd, Sm) phases show no magnetic ordering for the transition metal and therefore no 3d moment is present [8].However, the Cr analog shows two magnetic transitions-one for the ordering of Cr at high temperatures (T Cr $132 K) [6,9,10] and a second for the coupled Ln-Cr ordering found at low temperatures (T Ln-Cr $10 K) [6,[8][9][10][11][12][13]. NdCrSb 3 shows negative magnetoresistance up to À13% at 5 K with an applied field of 4 T [13].
Closely related to the structure of LaCrSb 3 is CeNiSb 3 , for which the synthesis and structure have been recently reported [14].The structure of CeNiSb 3 also consists of distorted, nearly square Sb nets capped by Ce atoms, and Ni octahedra layers.However, the NiSb 6 octahedra are face-and edge sharing along the c-axis.Resistivity measurements for CeNiSb 3 suggest Kondo lattice behavior [14].
We report here the synthesis, structure, and measurements of electrical and magnetic properties of three new phases, LnNiSb 3 (Ln=Pr, Nd and Sm).The systematic substitution of the lanthanide atom allows for the opportunity to investigate the effects of the structure, particularly of the distorted Sb sheets and NiSb 6 octahedra, on the physical properties of these antimonides.

Synthesis
Single crystals of LnNiSb 3 (Ln=Pr, Nd and Sm) were prepared with excess Sb.These phases were synthesized by placing ingots of Pr (99.9% purity, Alfa Aesar), Nd (99.9% purity, Alfa Aesar), or Sm (99.9% purity, Alfa Aesar), along with Ni powder (99.999% purity, Alfa Aesar), and Sb shot (99.9999% purity, Alfa Aesar), into alumina crucibles in a 1:2:20 (Ln:Ni:Sb) molar ratio.Each crucible was sealed into an evacuated silica tube.The samples were heated to 1150 1C and cooled at a rate of 5 1C h À1 to 670 1C, at which point, excess flux was removed by centrifugation.Once the samples were cooled to room temperature, the metallic, plate-shaped crystals with dimensions up to 1 Â 2 Â 2.5 mm 3 , were mechanically extracted.The crystals possessed clean surfaces with no evidence of flux contamination and appeared to be moisture sensitive after a week of exposure to air (apparent by the grayish color change).

Physical property measurements
Magnetic moments were measured using a Quantum Design SQUID magnetometer.The temperature-dependent susceptibility data were taken with the applied field of 1000 G up to room temperature after being cooled to 1.8 K under zero magnetic field.Magnetic field-dependent magnetization data were also checked from zero field to 5.5 T at 2 K.The resistivity data have been measured using a standard four-probe method down to 0.35 K with Quantum Design Physical Property Measurement System at ambient pressure.

Single crystal X-ray diffraction
Plate-shaped crystals with dimensions of approximately 0.040 Â 0.100 Â 0.080 mm 3 (PrNiSb 3 ), 0.050 Â 0.025 Â 0.125 mm 3 (NdNiSb 3 ), and 0.075 Â 0.050 Â 0.100 mm 3 (SmNiSb 3 ) were mounted onto a glass fiber of a goniometer and placed on a Nonius Kappa CCD Xray diffractometer (MoKa=0.71073A ˚). Data were collected at 298 K. Data collection parameters and crystallographic data are provided in Table 1.The lattice parameters were determined from images taken

ARTICLE IN PRESS
from a scan in 10 1j.The structures were solved using the structure of CeNiSb 3 [14] as an initial structural model and were refined using SHELXL97 [15].After the refinement of atomic positions, the data were corrected for absorption and displacement parameters were refined as anisotropic.Extinction and weighting schemes were also applied to the refinement.The R½F 2 42sðF 2 Þ was 4.32%, 4.06% and 3.60%, with largest features in the difference map of 3.831/À2.698e A ˚À3 , 4.119/ À2.467 e A ˚À3 and 2.954/À3.014e A ˚À3 for Ln=Pr, Nd and Sm, respectively.Atomic coordinates and anisotropic displacement parameters are provided in Table 2. Table 3 provides selected interatomic distances for CeNiSb 3 [14], PrNiSb 3 , NdNiSb 3 and SmNiSb 3 for comparison.Detailed data collection parameters and crystallographic data are provided as Supporting Information.

Structure
LnNiSb 3 (Ln=Pr, Nd and Sm) crystallize in the CeNiSb 3 structure type [14].The lattice parameters are provided in Table 1 and the structure of PrNiSb 3 is shown in Fig. 1.The structure of LnNiSb 3 (Ln=Ce-Nd and Sm) has been described as being built up of Ln atoms between layers of highly distorted, Ni-centered octahedra, and layers of buckled, nearly square Sb nets [14].
The distorted Sb nets found in LnNiSb 3 (Ln=Pr, Nd, Sm) are composed of 4-coordinate Sb1 and Sb3 atoms.For Ln=Pr, the Sb1-Sb1 and Sb3-Sb3 bonds with distances of 3.1018(2) A ˚regularly repeat along the b-direction, but Sb1-Sb3 and Sb1-Sb1 bonds with  [7] and LnIn 1Àx Sb 2 [24].Substituting smaller lanthanide atoms bears only a slight influence on the Sb-Sb distances and Sb-Sb-Sb angles in the square nets of LnNiSb 3 (Ln=Ce-Nd, Sm).The Ln (Pr, Nd, Sm) atoms are located above and below the Sb square net in a checkered fashion.Consequently, one can view the Ln layer as being interleaved between the Ni octahedra and Sb square nets [14].The local monocapped-square antiprismatic Ln environment is shown as dashed lines in Fig. 1.This geometry is similar to the rare-earth capped antimony nets seen in LnSb 2 [25], CeNiSb 2 [26], and LnCrSb 3 [7].

ARTICLE IN PRESS
The Ln-Sb interatomic distances and thus, the lattice parameters, decrease with ionic radii, as expected due to lanthanide contraction.As Ln progresses from Ce to Sm in LnNiSb 3 , a decreases by $2% and both the b-and cparameters decrease by $1%.The contraction of the aparameter may induce bond strain between the Ln and Sb atoms and further distort the Ni octahedra, rendering the latter lanthanide analogs (Ln=Gd-Tm) unstable.Crystal growth of other analogs (Gd-Tm) were attempted; however, we obtained crystals of LnNiSb 2 for Gd-Er.When Ln=Tm, TmSb was the primary phase present.Similar results were found for the LnCrSb 3 (Ln=La-Nd, Sm, Gd-Dy) compounds, although a different synthetic route was employed for the synthesis of Ln=(Gd-Dy).Hence, it may be possible to prepare LnNiSb 3 (Ln=late lanthanides) with a different synthetic route.
In LnNiSb 3 , there are Ni1 and Ni2 octahedra which run along the b-direction.These octahedra are edge sharing in the b-direction whereas they are face sharing with every third octahedron sharing edges in the cdirection.Table 4 provides the bond distances and angles for the Ni1 and Ni2 octahedra in LnNiSb 3 (Ln=Ce-Nd, Sm).The distances between Ni and Sb also contract with increasing Ln size, and are all in good agreement with the Ni-Sb distances of 2.6082 A ˚found in NiSb [27].

Physical properties
Fig. 2 (a-c) shows the temperature-dependent susceptibility under applied magnetic field of 1000 G along the crystalline bc1-and bc2-planes and the a-axis.Cusps in the data indicate antiferromagnetic transitions in these systems and the corresponding Ne´el temperature is around 4.5, 4.6, and 2.9 K for PrNiSb 3 , NdNiSb 3 , and SmNiSb 3 , respectively.The anisotropy in the susceptibility data is more obvious from the applied field along the crystalline a-axis than the field in the bc-plane.The inverse susceptibilities, shown in Fig. 2 insets, follow linear dependencies with temperature, suggesting Curie-Weiss behavior above the transition temperatures.Anisotropies at high temperature are small and the averaged effective moments from the high-temperature Fig. 2. Zero field-cooled magnetic susceptibility (M/H) as a function of temperature (T) in a magnetic field of H=1000 G for single crystals of (a) PrNiSb 3 with T N =4.5 K, (b) NdNiSb 3 with T N =4.6 K, and (c) SmNiSb 3 with T N =2.9 K. Inset: inverse susceptibility with respect to temperature.The constant paramagnetic offset has been subtracted for SmNiSb 3 .
Curie-Weiss fitting are 3.62 m B (PrNiSb 3 ), 3.90 m B (NdNiSb 3 ), and 0.80 m B (SmNiSb 3 ) , which are in good agreement with those of Pr 3+ (3.58), Nd 3+ (3.62), and Sm 3+ (0.84) as shown in Table 5.This along with the fact that LaNiSb 3 does not order magnetically down to 2 K indicates that the magnetic moments only come from the rare-earth atoms (Pr, Nd and Sm) and not Ni.
Negative Weiss temperatures, y w , of $À5 K for PrNiSb 3 , À10 K for NdNiSb 3 , and À10$À40 K for SmNiSb 3 suggest antiferromagnetic correlations.Fig. 3 (a-c) shows magnetization data with respect to applied magnetic field.In PrNiSb 3 (Fig. 3a), the magnetization with the applied magnetic field oriented along the crystalline a-axis depends linearly on the field  up to about 1 T. Above 1 T, it deviates from the linearity and increases rapidly up to 2 T. The slope is reduced again above 2 T. Similar behavior appears also in NdNiSb 3 (Fig. 3b) over a broader magnetic field range.This is most likely due to a spin-flop type of transition by an alternation of the spin arrangement, where antiferromagnetic alignment is broken partially under applied magnetic field.No magnetic-field-induced transition appears until 5.5 T in SmNiSb 3 (Fig. 3c), which shows relatively stronger antiferromagnetic correlation than the Pr and Nd cases, that is consistent with larger magnitude of the Weiss temperature.

ARTICLE IN PRESS
The temperature dependence of the electrical resistivity of PrNiSb 3 , NdNiSb 3 and SmNiSb 3 along the bcplane is shown in Fig. 4.These compounds show metallic behavior with kinks at their Ne´el temperatures, indicating magnetic transitions.There are dramatic drops of the resistivity below the Ne´el temperatures due to antiferromagnetic ordering in PrNiSb 3 and SmNiSb 3 .In NdNiSb 3 , on the other hand, resistivity starts increasing suddenly below T N .Similar features have been reported in Cr and URu 2 Si 2 [28][29][30], where the enhancement of resistivity is attributed to partial gapping of the Fermi surface by the magnetic order.

Summary
The compounds, PrNiSb 3 , NdNiSb 3 , and SmNiSb 3 have been characterized by single crystal X-ray diffraction experiments.All three compounds are isostructural to CeNiSb 3 and are built up of Ln 3+ atoms interleaved between layers of highly distorted, Ni-centered octahedra, and layers of buckled, nearly square Sb nets.The Sb sheets and Ni-centered octahedra in SmNiSb 3 contain the highest degree of distortion.The Ln-Sb distances are shortened for Ln=Ce-Sm, as expected.These Ln-Sb distances may play an important role in affecting the property measurements and RKKY interactions.PrNiSb 3 , NdNiSb 3 , and SmNiSb 3 are antiferromagnets with effective moments close to what is expected for Ln 3+ .It is notable that all three phases have antiferromagnetically ordered ground states with low Ne´el temperatures, while a ferromagnetic state appears in the Ce analog.Moreover, the magnetic properties in Pr and Nd are similar in that they have nearly the same Ne´el temperature and exhibit instability in their antiferromagnetic behavior under magnetic field above 1 T. The antiferromagnetic state in SmNiSb 3 is more stable in applied field than Pr and Nd.

ARTICLE IN PRESS
It is worthwhile to note that CeNiSb 3 (T C $6 K) [31] is least distorted and has the highest ordering temperature of all the Ln analogs in this structure type.Examining the Pr-, Nd-and Sm-analogs, we have found that the octahedra in SmNiSb 3 are most distorted.This distortion may be significant in not only the stability of the structure type, but also the exchange interactions that may influence the ordering temperatures.

Fig. 1 .
Fig. 1.The crystal structure of PrNiSb 3 viewed down the b-axis with the unit cell shown as a solid line.The shaded circles are Pr atoms, the solid circles are Ni atoms and the open circles are Sb atoms.The mono-capped square antiprism environments of the Pr1 and Pr2 atoms are shown as dashed lines.

Table 5
Magnetic properties of LnNiSb 3 (Ln=Ce-Nd, Sm) compounds a Abbreviations: FM, ferromagnetic; AFM, antiferromagnetic; m exp , experimental effective moment; m Calc , calculated effective moment for Ln 3+ ; T C , Curie temperature; T N , Ne´el temperature. a