La6Pd2+xSb15 (x = 0.28): A rare-earth palladium intermetallic compound with extended pnictogen ribbons

Article abstract of DOI:10.1016/j.jssc.2020.121578

A new intermetallic ternary compound La₆Pd_2.28Sb₁₅ was synthesized from the reaction of lanthanum and palladium metals in a molten antimony flux. The compound crystallizes in the orthorhombic space group Immm with a ?= ?4.3082(9) ?, b ?= ?15.399(3) ? and c ?= ?19.689(4) ?. The crystal structure contains a three-dimensional framework of Sb squares and ribbons that extend along the a axis, including complex Sb–Sb bonding. La₆Pd_2.28Sb₁₅ is diamagnetic, with a magnetic susceptibility weakly dependent on temperature (T). The resistivity (ρ) decreases when lowering the temperature, indicating metallic behavior, and at low temperatures ρ depends quadratically on T. Interestingly, both the Hall resistivity and magnetoresistance present a nonlinear dependence on the applied magnetic field, suggesting a multiband behavior. This is supported by density-functional electronic structure calculations which show a complex Fermi surface originated in the antimonide substructures and containing both electron and hole pockets as well as open sheets.

Full text of DOI:10.1016/j.jssc.2020.121578

Journal Pre-proof
La Pd
Sb (x = 0.28): A rare-earth palladium intermetallic compound with
6
2+x 15
extended pnictogen ribbons
Mihai I. Sturza, M. Lourdes Amigó, Jorge I. Facio, Federico Caglieris, Saicharan
Aswartham, Silvia Seiro, Christian Hess, Jeroen van den Brink, Sabine Wurmehl,
Bernd Büchner
PII:
S0022-4596(20)30408-4
DOI:
https://doi.org/10.1016/j.jssc.2020.121578
YJSSC 121578
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 18 May 2020
Revised Date: 8 July 2020
Accepted Date: 9 July 2020
Please cite this article as: M.I. Sturza, M.L. Amigó, J.I. Facio, F. Caglieris, S. Aswartham, S. Seiro, C.
Hess, J. van den Brink, S. Wurmehl, B. Büchner, La Pd
intermetallic compound with extended pnictogen ribbons, Journal of Solid State Chemistry (2020), doi:
https://doi.org/10.1016/j.jssc.2020.121578.
Sb (x = 0.28): A rare-earth palladium
6
2+x 15
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CRediT Author Statement
Mihai I. Sturza: Conceptualization, Data curation, Investigation, Formal analysis, Methodology,
Software, Visualization, Writing - original draft.
M. Lourdes Amigó: Data curation, Investigation, Formal analysis, Writing - original draft.
Jorge I. Facio: Software, Data Curation, Writing - original draft.
Federico Caglieris: Investigation, Formal analysis, Writing - Review & Editing
Saicharan Aswartham: Supervision, Writing - Review & Editing, Validation.
Silvia Seiro: Supervision, Writing - Review & Editing, Validation.
Christian Hess: Supervision, Validation.
Jeroen van den Brink: Supervision, Validation.
Sabine Wurmehl: Supervision, Writing - Review & Editing, Validation.
Bernd Büchner: Resources, Validation.

La6Pd2+xSb15 (x = 0.28): A Rare-earth Palladium Intermetallic Compound with
Extended Pnictogen Ribbons
Mihai I. Sturza,1, * M. Lourdes Amigó,1 Jorge I. Facio,1 Federico Caglieris1, Saicharan
Aswartham1, Silvia Seiro1, Christian Hess1, Jeroen van den Brink1, Sabine Wurmehl1,2, Bernd
Büchner1,2
1 Leibniz Institute for Solid State and Materials Research, IFW Dresden, Helmholtzstr. 20, 01069
Dresden, Germany,
2 Institute of Solid State and Materials Physics, TU Dresden, 01062 Dresden, Germany
* E-mail: m.i.sturza@ifw-dresden.de
GRAPHICAL ABSTRACT
A new intermetallic ternary compound La6Pd2.28Sb15 was synthesized from the reaction of
lanthanum and palladium metals in a molten antimony flux. The compound crystallizes in the
orthorhombic space group Immm with a= 4.3082(9) Å, b=15.399(3) Å and c=19.689(4) Å.

La6Pd2+xSb15 (x = 0.28): A Rare-earth Palladium Intermetallic Compound with
Extended Pnictogen Ribbons
Mihai I. Sturza,1, * M. Lourdes Amigó,1 Jorge I. Facio,1 Federico Caglieris1, Saicharan
Aswartham1, Silvia Seiro1, Christian Hess1, Jeroen van den Brink1, Sabine Wurmehl1,2, Bernd
Büchner1,2
1 Leibniz Institute for Solid State and Materials Research, IFW Dresden, Helmholtzstr. 20, 01069
Dresden, Germany,
2 Institute of Solid State and Materials Physics, TU Dresden, 01062 Dresden, Germany
* E-mail: m.i.sturza@ifw-dresden.de
ABSTRACT
A new intermetallic ternary compound La6Pd2.28Sb15 was synthesized from the reaction of
lanthanum and palladium metals in a molten antimony flux. The compound crystallizes in the
orthorhombic space group Immm with a= 4.3082(9) Å, b=15.399(3) Å and c=19.689(4) Å. The
crystal structure contains a three-dimensional framework of Sb squares and ribbons that extend
along the a axis, including complex Sb‒Sb bonding. La6Pd2.28Sb15 is diamagnetic, with a
magnetic susceptibility weakly dependent on temperature (T). The resistivity (ρ) decreases when
lowering the temperature, indicating metallic behavior, and at low temperatures ρ depends
quadratically on T. Interestingly, both the Hall resistivity and magnetoresistance present a
nonlinear dependence on the applied magnetic field, suggesting a multiband behavior. This is
supported by density-functional electronic structure calculations which show a complex Fermi
surface originated in the antimonide substructures and containing both electron and hole pockets
as well as open sheets.
Keywords: intermetallic, single crystal X-ray diffraction, electronic structure calculations,
magnetoresistance, p-type metallic, Hall resistivity
1

1 INTRODUCTION
New materials are the gateway to discover new phenomena, to a better understanding of
known phenomena and to enhance performance and efficiency within a device. The past decade
or so has seen an increasing appreciation of the importance for exploring novel materials with
interesting electronic states1-3, nematic4-6 charge7, orbital8 and magnetic9 orders. In particular,
strongly correlated electron materials can show interesting physical properties such as special
forms of magnetism10-12 and unconventional superconductivity13-15 driven by thermal and
quantum fluctuations. As one candidate class of materials, intermetallic compounds exhibiting
strong electron−electron or electron−lattice interactions16-18 may host such fascinating and
unusual phenomena.
Among those, ternary rare-earth pnictogen compounds RE–M–Pn, where RE = rare-earth
element, M = transition metal or post-transition metal and Pn = P, As, Sb, Bi continue to attract
high interest because of their rich structural chemistry and their possible useful physical
properties (thermoelectricity, superconductivity).19-23 Ternary rare-earth antimonides RE–M–Sb
are of importance, particularly because of their attractive variety of polyanionic Sb-based
networks assembled from low-dimensional units such as clusters, chains, nets, and rings, with a
wide range of Sb-Sb bonding interaction which are both predicted and experimentally verified to
be linked with fascinating physical properties.19, 23-27 The interesting magnetic properties such as
29
Kondo lattice behavior (e.g., Yb14−xLaxMnSb1128, α-CeNiSb3 , CeNiSb30), anisotropic
29
magnetism (β-CeNiSb3 ) and colossal magnetoresistance (e.g., Eu14MnSb1131) have been
explained by the interplay of localized f-electrons from the RE element contrasting with the
delocalized d-electrons from the transition metal component. As one example, the La6MnSb15-
structure type common to the RE6MSb15 (M=Mn, Zn, Cu, In)32-37 antimonide family depicts the
structural complexity consisting of linked antimony sheets and three-atom-wide ribbons which
are connected through transition metal sites. The transition metal atoms accommodate interstitial
sites of distorted tetrahedral geometry within the anionic framework and the square-pyramidal
sites between the Sb ribbons and SbRE6 prisms. A closely related structure type is
38
(U0.5Ho0.5)3Sb7, adopted by the binary bismuthides Nd3Bi7 and Sm3Bi7 with hypervalent Bi–Bi
interactions. The bonding present in the complex Sb-substructures have been the subject of
comprehensive theoretical study in which a “retrotheoretical” approach was applied.35, 39, 40
The ternary rare earth phosphides and arsenides of palladium are more numerous
compared to the corresponding antimonides. Several compounds containing lanthanides,
2

palladium and antimony (RE–Pd–Sb) with various crystal structures type are reported in the
literature: REPdSb41-43 with TiNiSi, CaIn2 or MgAgAs-structure type, LaPd2Sb244 superconductor
45
with CaBe2Ge2 type structure, LaPdSb3 with structure type average between the CeCrSb3 and
46
47
CeNiSb3, REPdSb2 with ZrCuSiAs-CuHfSi2 structure type, RE3Pd6Sb5 adopting an ordered
defect structure that derives from the CaBe2Ge2 type.
With regard to our interest on the exploratory synthesis of layered compounds with
homoatomic square lattices (namely Sb nets) co-existing with and without magnetic slabs, we
began our investigation in an effort to re-examine the crystal growth of LaPdSb3. To the best of
our knowledge there is only one report in the literature about this palladium layered intermetallic
compound.45 From the efforts to grow larger crystals of LaPdSb3 for physical properties
measurements by using Sb as flux, we have discovered a new ternary antimonide,
La6Pd2+xSb15 (x = 0.28), with p-type metallic behavior as discussed in the work at hand. In a
nutshell, we report the synthesis, structure, and properties of this ternary rare-earth palladium
intermetallic phase, which can be considered a stuffed variant of the orthorhombic La6MnSb15
structure type, in which square pyramidal and tetrahedral sites are partially occupied by Pd
atoms.
2 METHODS
2.1 Reagents: The following reagents were used as received: lanthanum flake (Chempur,
99.9%), palladium powder, -60 mesh, (Alfa Aesar, 99.9%) and antimony lumps (Puratronic,
99.999%). Antimony lumps (Puratronic, 99.999%) were ground to a coarse powder prior to use.
Initial manipulations were carried out in glovebox under an inert Ar atmosphere (< 0.1 ppm H2O
and O2).
2.2 Crystal Growth of La6Pd2+xSb15 (x = 0.28). The reaction mixtures consisted of rare-
earth element La (0.4197g, 3.02 mmol), Pd (0.2411g, 2.26 mmol), and Sb ( 2g, 16.4 mmol)
were loaded into 2 mL Canfield Crucible Set (CCS – frit-disc alumina crucibles set).48 Next, the
alumina crucibles were placed in fused-silica tubes and flame-sealed under a vacuum of < 10-5
mbar. In a Nabertherm programmable chamber furnace, the tubes were heated to 1125°C in 14 h,
held at 1125°C for 12 h, cooled to 700°C with rate of 3°C/h. After dwelling at 700 °C for a few
hours, the tubes were quickly centrifuged to filter the crystals formed from excess liquid Sb
metal. Remaining Sb flux was tried to be removed from the sample by dissolution in a 1:1:1
mixture of HCl, HNO3, and H2O or in a dilute solution of HCl, followed by gravity filtration and
3

rinsing with H2O and acetone to isolate the crystals of La6Pd2+xSb15 (x = 0.28). Despite these
efforts, some of the Sb flux remains on the surface of the crystals. Crystals formed as shiny black
rectangular needle crystals. Other small percentage spurious phases present in the crystalline
product, as identified by single crystal X-ray diffraction and energy dispersive x-ray (EDX),
included several binary phases LaSb2, LaSb and PdSb2.
2.3 Scanning Electron Microscopy. Microstructure and composition were analyzed by
4
scanning electron microscopy on cleaned single crystals on a Zeiss EVOMA15 with AzTec
software equipped with an electron microprobe analyzer for semi-quantitative elemental analysis
using the energy dispersive x-ray (EDX) mode. The acceleration voltage applied was 30 kV.
2.4 Single Crystal X-ray Diffraction. A single crystal of La6Pd2+xSb15 was selected and
mounted on the tip of quartz fiber for X-ray diffraction. Intensity data were collected at 298 K
using ω scans on a STOE IPDS-II imaging plate diffraction system using graphite-
monochromatized Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and 40 mA with a 34 cm
diameter imaging plate. Individual frames were collected with a 4 min exposure time and a 0.8°
ω rotation and the crystal-to-detector distance was 100 mm. X-AREA, X-RED, and X-SHAPE
software packages49 were used for data collection, integration, and analytical absorption
corrections, respectively. SHELXL50 and JANA200651 software packages were used to solve and
refine the structure. Each final refinement included a secondary extinction correction. The
parameters for data collection and details of the structure refinements are given in Table 1.
Atomic coordinates, thermal displacement parameters (Ueq) and occupancies of all atoms are
given in Table 2, anisotropic thermal displacement parameters (ADPs) are given in Table 3,
selected bond lengths and angles are given in Table 4.
2.5 Powder X-ray Diffraction. Powder X-ray diffraction (XRD) was measured on
powderized single crystals using a STOE STADI diffractometer in transmission geometry with
Cu-Kα1 radiation equipped with a Germanium monochromator and a DECTRIS MYTHEN 1 K
detector. The data were analyzed by the Rietveld method using the FULLPROF SUITE
program.52 The peak shape was assumed to be a pseudo-Voigt function and the refinement
included the following features: (a) the background, which was extrapolated from 30 background
parameters; (b) the scale factors; (c) the global instrumental parameters (zero-point 2 θ shift and
systematic shifts, determined by transparency and off-centering of the sample); (d) the lattice
parameters; and (e) the profile parameters (Caglioti half-width parameters of the pseudo-Voigt
4

function). The March-Dollase model53 for preferred orientation was used in all of the
refinements.
2.6 Band Structure Calculations. We performed fully relativistic calculations based on
Density Functional Theory using the Perdew-Burke-Ernzerhof implementation54 of the
Generalized Gradient Approximation, as implemented in the FPLO code.55 We used a linear
tetrahedron method with 14×14×14 subdivisions in the Brillouin zone. We used an ordered model
(i.e. without deficiency of Pd) as a crystal structure corresponding to La6Pd4Sb15. To describe the
low-energy electronic structure, this approximation is justified because the Pd atoms have only a
marginal weight in the band structure close to the Fermi energy.
2.7 Magnetization measurements. For the magnetization measurements, we used a
Vibrating Sample Magnetometer (VSM) from Quantum Design. Measurements were carried out
as a function of temperature or magnetic field in the temperature range 1.8 K to 300 K and up to
7 T. The samples were glued with Duosan to the sample holder. Due to the small signal of the
samples a careful subtraction of the background was needed. For this, we have measured
separately the quartz sample holder with the glue (without the sample), and then we subtracted
this background from the measurements of the quartz holder, the glue and the sample.
2.8 Charge Transport. Resistivity, magnetoresistivity and Hall resistivity measurements
were performed on single crystals. The resistivity as a function of temperature was measured in a
dip-stick sample holder immersed in a Dewar with liquid He while the magnetoresistivity and the
Hall resistivity measurements were carried out in an OXFORD cryostat with a superconducting
magnet under magnetic fields up to µ0H=15 T. A standard four probe method was used in all
cases. Specifically, we employed gold wires of a diameter ~25 µm and silver paste to make the
contacts. The current was applied along the needle direction in all measurements. In the case of
the Hall resistivity and of the magnetoresistivity, the magnetic field was applied perpendicular to
the current, i.e. in the ab-plane. To correct a possible misalignment of the contacts, we obtained
the Hall resistivity as ρxy(H)=[ρxy(meas)(H)- ρxy(meas)(-H)]/2, where ρxy(meas)(H) and ρxy(meas)(-H) are
the measured transverse resistivities for a positive or negative magnetic field, respectively.
Similarly, we obtained the field dependence of the resistivity as ρ(H)=[ρ(meas)(H)+ρ(meas)(-H)]/2,
where ρ(meas)(H) and ρ(meas)(-H) are the correspondingly measured longitudinal resistivities for a
positive or negative magnetic field.
5

3 RESULTS AND DISCUSSION
3.1 Synthesis. La6Pd2+xSb15 (x = 0.28) was first discovered during exploratory synthesis
using molten Sb metal flux containing La and Pd in a 20:1:1 molar ratio. In order to grow larger
crystals of LaPdSb3 we have used the Sb flux method with a different temperature profile
compared to the one reported in ref.45. The results were quite unexpected, the discovery of a new
ternary lanthanum – palladium – antimony compound, La6Pd2.28Sb15. After the structure and the
composition of the new phase were established from single crystal XRD and EDX analysis, an
optimized reaction of the stoichiometric mixture of high purity metals La, Pd and Sb in excess Sb
flux was performed at 1125 °C. Micrometer-sized needle crystals of La6Pd2.28Sb15 (inset Figure 1)
were obtained.
Powder XRD was performed on crushed crystals of La6Pd2+xSb15 (x = 0.28) to confirm
the purity of the phase. The single-phase material was validated by the good match between the
calculated and the experimental powder XRD patterns using Rietveld refinements. Figure 1
depicts the results of refinement, agreement factors, and refined lattice constants of powder XRD
patterns (Cu-Kα1 radiation) of the sample studied in this work. The results are in good agreement
(Bragg R-factor: 7.15 %; Rf-factor= 6.03) with the structure parameters determined by single
crystal XRD. Since removal of Sb remnants from flux was unsuccessful, Sb metal was found in
the powder of La6Pd2.28Sb15 crushed crystals as secondary phase in amount of 11%.
A SEM image of a representative La6Pd2.28Sb15 crystal is shown in the inset of the
Figure 2. The microprobe analyses by EDX analysis taken at several spots on the crystal shows
an elemental distribution La5.984(7)Pd2.230(8)Sb15.212(4). The given error values correspond to the
standard deviation obtained with 10 points measurements. Hence, the EDX result is consistent
with the expected La:Pd:Sb stoichiometry of 6:2+x:15 (x = 0.28) within the error bars of the
EDX method ( 1-2 at%), which is in a good agreement with the composition obtained from the
single crystal X-ray structure refinement. A typical EDX spectrum is displayed on Figure 2.
3.2 Crystal Chemistry. From single-crystal XRD data, we have determined that the
La6Pd2.28Sb15 compound crystallizes in the orthorhombic La6MnSb15 – structure type (space
group Immm; a= 4.3082(9) Å, b=15.399(3) Å, c=19.689(4) Å; Z = 2). The structure of
La6Pd2.28Sb15 is isostructural with La6MSb15 (M = Mn, Zn)32, 33, 56 and Pr6InSb1537, they exhibit
the same substructures of Sb and the same coordination environment for the rare earth elements.
6

The crystal structure is a three-dimensional framework (Figure 3(a)) composed of various squares
and ribbons that extend along the a axis, including complex Sb‒Sb bonding. There are ten
crystallographic sites in the structure, with two La atom positions, two Pd atom positions, and six
Sb atom positions. The site symmetries are m for La(1), Sb(1), and Sb(2), mm2 for La(2), Pd(1),
and Sb(3), m2m for Pd(2), Sb(4), Sb(6), and mmm for Sb(6).
The average La6MnSb15-type structure has previously been explained in detail34, 35, so we
will only provide a few succinct details here. In the La6Pd2.28Sb15 investigated compound, the Sb
polyanionic network is built up of four-atom-wide (Sb(2)–Sb(1)– Sb(1)– Sb(2)) and three-atom-
wide (Sb(4)–Sb(5)–Sb(4)) ribbons extending down the a direction (Figure 3). The four-atom-
wide ribbon that forms the antimony kinked sheets are linked by Sb(6)–Sb(6) pairs to create a
widespread network edging large channels. These large channels are separated by three-atom-
wide Sb ribbons into two smaller triangular channels, which contain face-sharing columns of
trigonal prisms with La atoms at the vertices and single Sb(3) atoms at the centers. The Pd atoms
partially fill two types of interstitial sites: Pd(1) (4j Wyckoff site) partially occupied at 0.349(4) is
five-fold coordinated with distorted square pyramidal geometry and Pd(2) (4h Wyckoff site) with
an occupancy of 0.785(4), is six-fold coordinated with a bi-capped tetrahedron geometry. The
coordination environment of the Pd(1) depicted in Figure 4(a) is similar to the Mn and Zn atoms
found in the corrugated anionic layers of the Rb4Zn7As7 and Rb7Mn12Sb12-type structures.57 Pd(1)
has five Sb neighbors in a square pyramidal fashion, with two sets of Pd(1)–Sb(4) and Pd(1)–
Sb(5) bonds with a distance of 2.752(1) Å and 2.498(7) Å, and the Pd(1)–Sb(3) bond which
measures 2.685(8) Å. The Sb(5) sites presents an elongated anisotropic thermal displacement
parameter along the a crystallographic axis indicative of atomic positional disorder in the
37
antimony network similarly to the Ce6MSb15 (M=Mn, Zn)32 and Pr6InSb15 systems. As
suggested from previous studies, the disorder was modeled as a split position between two
different crystallographic sites (Figure 4(b)): 2c Wyckoff site (½, ½, 0) for Sb(5a) with an
occupancy of 0.5(7) and 4f Wyckoff site (x, ½, 0) for Sb(5b) with an occupancy of 0.2(4). The
local ordering in La6Pd2.28Sb15 can be explained as follows: if the Sb(5a) are occupied, the Sb4
squares within the ribbon are equivalent (Sb(4)–Sb(5a), 3.258(7) Å) and none of the Pd(1) sites
are occupied. Because the Sb–Sb–Sb angles are not exactly 90º, these are really Sb4 rhombi, but
are considered to be squares for clarity. In another scenario, if the Sb(5b) sites are occupied, the
situation is similar to what has been suggested for the Bi and Sb ribbons in Ce6ZnBi14 and
Pr6InSb1537, with alternately small (Sb(5b)–Sb(4), 3.067(5) Å) or large (Sb(5b)–Sb(4), 3.465(2)
7

Å) Sb4 squares spreading along the a direction. Exclusively the larger Sb4 squares are capped by
Pd(1) atoms, giving Pd(1)–Sb(5a) and Pd(1)–Sb(5b) distances of 2.498(7) – 2.762(8) Å.
Nevertheless, capping on both sides of the large Sb4 square by Pd(1) atoms is now inhibited
because the resulting Pd(1)–Pd(1) distances of 2.53(8) Å are too short compared with those
founded in elemental Pd (2.79(9) Å).58 As expected, the orientation of the Pd(1)Sb5 square
pyramids need to be distributed “all up” or “all down”, as in Figure 4(b), along the length of the
Sb ribbon. The second palladium site, Pd(2) is placed among the kink in the antimony sheets and
the antimony ribbon subunit, it is also partially occupied at 0.785(4). The coordination
environment is drawn in Figure 4(c). Such a strange coordination geometry, a distorted
tetrahedron with two additional capping Sb atoms has been reported previously in RE6MSb15
(M=Mn, Zn, Cu, In) compounds.32-37 This site is six-fold coordinated and the corresponding Pd–
Sb bond distances are in the range of 2.564(8) to 2.871(4) Å.
We investigated also the possibility of lower symmetry space groups (Imm2 and its
permutations, I222, and I212121), previously reported by Sologub et al.33 Unfortunately,
refinements did not support an ordered model in which the three atom wide Sb ribbons
(containing the problematic Sb5 sites) are distorted exclusively in one direction, as occurs in
La6ZnSb15.
Figure 5(a) presents the coordination sphere of the La(1) atoms. Around La(1), the
coordination surroundings of the Sb atoms can be depicted as a monocapped square antiprism
(with Sb(1) – Sb(1) – Sb(1) – Sb(2) squares) or a tricapped trigonal prism (with eclipsed Sb(1) –
Sb(3) – Sb(4) triangles). The distances of La(1) – Sb measure between 3.300(9) – 3.746(4) Å, in
good accord with other reported palladium ternary antimonides (e.g., 3.307(9) – 3.936(8) Å in
46
LaPdSb2 or 3.277(5) – 3.602(2) Å in LaPdSb345). The local coordination for the La(2) atom
depicted in Figure 5(b) is CN 10. Some of the La(2) – Sb distances are quite long (3.791(8) Å)
for La(2) – Sb(6)) but are comparable with 3.814(5) Å La—Sb bond distances found in the
ternary compound LaPd2Sb2.44
A comprehensive description of the bonding has been investigated through extended
Hückel calculations in the La6MnSb1535, isostructural with the new ternary rare-earth palladium
intermetallic phase La6Pd2.28Sb15 discussed in the present work. A valuable conclusion of the
previous study has been the identification of the antimonide substructures as distinguishing
building blocks that can be treated individually in defining the overall electronic structure.38, 39
Surprisingly, the Zintl-Klemm path approaching electron counting, adjusted for the hypervalent
8

Sb – Sb bonding in the linear and square-planar Sb environments, provides an equitable
description for the bonding. The accurate description of the bonding in La6Pd2.28Sb15 is not
obvious to develop, but it has been done in correlation with the bonding of the isostructural
compounds RE6MSb15 (M = Mn, Zn)32, 34, 56, Ce6ZnBi14 and Pr6InSb15.37 The lanthanum atoms
are trivalent (similarly to other rare-earth atoms, e.g., Ce3+ or Pr3+), and the Pd2+ interstitial atoms
(similarly to Mn2+, Zn2+, In3+) probably have filled the tetrahedral and square pyramidal sites.
Three types of antimonide substructures are present in La6Pd2.28Sb15: four-atom-wide ribbon,
linked to similar ribbons by Sb(6)–Sb(6) pairs, three-atom-wide ribbon, and single Sb(3) atoms
centered within La6 trigonal prisms (see Figure 3). For a four-atom Sb ribbon, simple electron
counting rules propose a formal charge of Sb-1, where an octet is completed by extra lone pairs on
each atom and the Sb–Sb long distances are treated to be "one-electron" bonds.38 Still, the
existence of diverse intermediate Sb–Sb interactions makes such an assignment questionable. In
this manner, the four-bonded Sb(1), Sb(2) and Sb(5) are allowed to be -1, the two-bonded Sb(4)
atoms are -2, and the unique Sb(3) atoms are -3. The Sb(6) atoms achieve less negative charge
and may assigned to be neutral, because they participate in a greater number of hypervalent Sb–
Sb contacts.34 The total formal negative charge for all the Sb atoms is –19 per formula unit, four
electrons are missing to compensate charge balance if we consider the La and Pd to be 3+ and 2+.
Another method to estimate non-integral formal charges on the Sb atoms is bond valence
sum, first reported by Jeitschko et al.59 The equation νij= exp[(Rij – dij)/0.37] was used to
calculate the bond order of Sb – Sb interactions, where Rij is the bond-valence parameter, chosen
to be 2.80 Å for Sb–Sb interactions and dij is the distance in Å.60 Using this method, the formal
charges in La6Pd2.28Sb15 have been found to be –0.78 for Sb(1), –1.03 on Sb(2), –3 for Sb(3), –
2.42 on Sb(4), –1.83 for Sb(5) and –1.02 on Sb(6). The overall charge for all Sb atoms is –22
which is in good accord with the positive compensated charges from the six La3+ and around two
Pd2+ per formula unit. Future spectroscopic methods may shed light on the actual oxidation states
of the different Sb positions.
3.3 Electronic Structure. We performed density-functional calculations allowing for a spin-
polarization which nevertheless vanishes in the self-consistent solution. Figure 6 (a) shows the
density of states (DOS) as a function of energy. We present the total DOS as well as its projection
on atomic orbitals associated with different atoms. The overall shape of the DOS is similar to that
of the ternary antimonide superconductor La6ZnSb15.56 In particular, the metallicity, as measured
by the DOS at the Fermi energy (9.9 states/eV per formula unit), is mainly accounted by the
9

antimonide substructures (specifically, atoms Sb(1), Sb(2) and Sb(6) forming squares and ribbons
in Figure 3). Figure 6 (b) shows the energy dispersion along high symmetry lines of the Brillouin
zone for calculations done with or without the inclusion of the spin-orbit coupling (SOC) and
Figure 6 (d-g) present the Fermi surface. The effects of the SOC on the electronic structure are
moderate. Since the material is inversion symmetric and the solution presents time-reversal
symmetry, the associated Kramer's degeneracy reflects in that all the bands are at least double
degenerate and, therefore, the main effect of the SOC is to avoid accidental band crossings. This
slightly affects the size and connectivity of some Fermi surface sheets, most notably along ΓY.
Close to the Fermi energy, the electronic structure hosts states that exhibit an energy dispersion
significantly smaller within the plane ΓYS than along the perpendicular direction ΓX. These
quasi-one-dimensional states contribute to the existence of open Fermi surface sheets
[Figure 6 (e)]. Altogether, the electronic structure is signaled by multiple bands at low energy
which lead to a Fermi surface composed of different sheets including both electron and hole
pockets as well as sheets that are open in planes parallel to ΓYS.
3.4 Magnetic Properties. Figure 7 presents the magnetic susceptibility (χ) as a function of
the temperature (T) for La6Pd2+xSb15 (x = 0.28) and the inset shows the magnetization as a
function of field for several temperatures. In both cases, we have subtracted the core diamagnetic
contribution that can be estimated to be -4x10-4emu.mol-1.61 Above 120K, the material is
diamagnetic with only a weak dependence on temperature (χ evolves from ~-4.3x10-5emu.mol-1
at 200K to ~-5.3x10-5emu.mol-1 at 300K). As the temperature is reduced, a paramagnetic
response develops and becomes dominating below ~50K. This Curie-Weiss type positive
contribution corresponds to an effective moment of 0.23µB per formula unit. The smallness of
this value, together with the nonlinear M vs H dependence at low temperatures, indicates that it
may originate from additional impurities present in the crystal, as for example Fe, which is a
common impurity in La metal used as reactant. Regarding the remaining temperature-
independent diamagnetic response, we attribute it to the intrinsic response of La6Pd2.28Sb15 and,
in smaller amount, also to traces of Sb coming from the flux. This is present in samples in a small
percentage and is diamagnetic.62 The Pauli paramagnetism can be estimated from the band
2
structure calculation as χP=µB g(εF), where g(εF) is the density of states at the Fermi energy. We
obtain χP=3.2x10-4emu.mol-1, a value an order of magnitude larger than the measured
diamagnetic response at, e.g., 160K. This suggests that the band-structure contributes with the
10

same order of magnitude to the Pauli paramagnetism and to the Landau diamagnetism in this
material.
Last, in contrast to the related compound La6ZnSb1556, we have not observed
superconductivity down to 1.8K. It is possible that this difference follows from the significantly
heavier mass of Pd as compared to Zn, which could act to reduce the low-energy phonon
frequencies and, in turn, the superconducting critical temperature.
3.5 Transport Properties. Figure 8 presents the temperature dependence of the resistivity (ρ)
for a single crystal of La6Pd2.28Sb15. The resistivity decreases upon lowering the temperature,
showing a metallic behavior. The residual resistance ratio (RRR) defined as ρ(300Κ)/ρ(10Κ) is
around 1.8. This low value can be related to the disorder introduced by the deficiency of Pd, the
positional disorder in the Sb site, and by impurities present in the crystal structure. Similar RRR
values can be found in the related compounds La6ZnSb15 (RRR~2.5)63 and Pr6InSb15
(RRR~1.2)63. At low temperature, ρ exhibits a quadratic dependence on T, consistent with
electron-electron scattering dominating the transport properties, as expected for a Fermi-liquid.63
This can be more clearly seen in the inset of Figure 8, where we present ρ as a function of T2. A
fit of the equation ρ=ρ0+ρ1T2 below ~40K results in ρ0=6.74(5)mΩ.cm and ρ1=3.4x10-4(5)
mΩ.cm.K-2. Around 50K, ρ presents a change of curvature with a decrease of its growing rate.
We find that the deviation from the T2 behaviour as T increases is not well described by the
addition of the electron-phonon scattering, as described by the Bloch-Gruneisen model with a T5
power law and a Debye model for the phonons (not shown). A similar failure has been reported
56
in the related compound La6ZnSb15 , which could suggest that this family of compounds host a
more complex phonon spectrum in this relevant low energy scale. However, given the complex
multiband structure of La6Pd2.28Sb15 near the Fermi level, other more subtle causes for a more
involved temperature evolution as, e.g., anisotropies in the relaxation time over the Fermi surface
or band-structure changes with temperature, are hard to exclude. Last, consistently with the
magnetization measurements, we have not observed superconductivity down to 2.3K.
Figure 9(a) and 9(b) show the Hall resistivity (ρxy) and the magnetoresistance
(∆ρ/ρ(H=0)=(ρ(H)-ρ(H=0))/ρ(H=0)), respectively, as a function of the magnetic field for several
temperatures. Interestingly, ρxy exhibits a strong departure from the conventional linear
dependence, which could be suggestive of a multiband character64, 65. This is again consistent
with the band structure calculation, which shows several bands crossing the Fermi level. The
11

inset of Figure 9(a) presents the Hall coefficient (RH), computed as the slope of ρxy in the linear
regime, i.e. µ0H→0T. The positive values indicate that the transport is slightly dominated by
hole-type carriers. A change of regime is clearly visible around 50K, consistent with the one
observed in the resistivity. It is worth stressing that ∆ρ/ρ is remarkably large (up to 200% at low
temperature), which together with its quadratic behavior at low fields followed by a saturation at
higher fields further supports a multiband scenario66. The inset of Figure 9(b) presents the
magnetoresistance as a function of temperature for a magnetic field of 15T, where a faster
increase of ∆ρ/ρ(H=0) is observed at low temperatures.
4 CONCLUDING REMARKS
The new lanthanum palladium antimonide compound, La6Pd2+xSb15 (x = 0.28), forms
from the reaction of La and Pd metal in molten Sb flux. It crystallizes in the complex La6MnSb15
structure type (orthorhombic space group Immm) with strong bonding extended in three
dimensions. The Sb polyanionic network is built up of four-atom-wide (Sb(2)–Sb(1)– Sb(1)–
Sb(2)) and three-atom-wide (Sb(4)–Sb(5)–Sb(4)) ribbons extended down the a direction. The
structure contains La trigonal prisms centered by isolated Sb atoms, and two types of Sb ribbons.
The four-atom-wide Sb ribbons are connected by Sb–Sb pairs. The Pd atoms partially fill two
type of interstitial sites with either square pyramidal or tetrahedral geometry. The palladium is
partially occupied on certain positions as observed in single crystal XRD. The electronic structure
calculation shows that the metallicity is mainly accounted by the antimonide squares and ribbons,
with a resulting complex Fermi surface including both electron- and hole-like pockets as well as
open sheets. Magnetization measurements indicate that the intrinsic magnetic behavior of
La6Pd2+xSb15 (x = 0.28) is diamagnetic, which we associate with Landau diamagnetism slightly
larger than Pauli paramagnetism in this compound. The resistivity decreases upon lowering the
temperature, exhibiting metallic behavior, while the Hall resistivity and the magnetoresistivity
present features which support a multiband scenario. Particularly interesting is the large
magnetoresistivity (up to 200% at low temperature). The La6Pd2.28Sb15 is a new example of the
compounds where atoms with d10 configuration can be inserted into the interstitial sites within the
Sb-anionic framework, as Pr6InSb1537 andLa6MnSb15.33
ACKNOWLEDGMENTS
12

The authors thank S. Müller-Litvanyi, R. Wachtel, L. Giebeler and S. Gass (all IFW Dresden) for
support. The work was supported by the Deutsche Forschungsgemeinschaft (DFG) through Grant
No. STU 695/1-1 (M.S.). and Grant No. DFG AS 523/4-1 (S.A.). MLA and JIF acknowledge
support from the Alexander von Humboldt Foundation through the Georg Forster program. JIF
thanks Ulrike Nitzsche for technical assistance.
13

TABLES AND FIGURES
Table 1. Structure Refinement Details from Single-Crystal X-ray Diffraction for La6Pd2.28Sb15 at
room temperature.
Empirical formula
Formula weight
Temperature
La6 Pd2.28 Sb15
2908.8
293(2) K
Wavelength
0.71073 Å
orthorhombic
Immm
Crystal system
Space group
a = 4.3082(9) Å°
b = 15.399(3) Å°
c = 19.689(4) Å
Unit cell dimensions
Volume
Z
1306.2(5) Å3
2
Density (calculated)
Absorption coefficient
F(000)
7.3856 g/cm3
26.301 mm-1
2426
Crystal size
0.075 x 0.048 x 0.021 mm3
θ range for data collection
Index ranges
1.68 to 29.13°
-5<=h<=5, -21<=k<=21, -26<=l<=26
6391
Reflections collected
Independent reflections
Completeness to θ = 29.13
Refinement method
Data / restraints / parameters
Goodness-of-fit
1053 [Rint = 0.0294]
100%
Full-matrix least-squares on F2
1053 / 0 / 52
1.46
Final R indices [I > 2σ(I)]a
R indices [all data]a
Extinction coefficient
Largest diff. peak and hole
Robs = 0.0196, wRobs = 0.0428
Rall = 0.0241, wRall = 0.0436
0.00743(8)
1.836 and -2.184 e·Å-3
a R =
where P=(Fo2+2Fc2)/3
Σ
||Fo|-|Fc|| /
Σ
|Fo|, wR = {
Σ
[w(|Fo|2 - |Fc|2)2] /
Σ
[w(|Fo|4)]}1/2 and w=1/[σ2(Fo2)+(0.0436P)2+1.7365P]
14

Table 2. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x103)
of La6Pd2.28Sb15 at room temperature.
a
Wyckoff
Label
La(1)
La(2)
Pd(1)
Pd(2)
Sb(1)
Sb(2)
Sb(3)
Sb(4)
Sb(5a)
Sb(5b)
Sb(6)
x
y
1388(1)
0
z
3652(1)
1755(1)
4356(1)
5000
2020(1)
3860(1)
2993(1)
0
Occupancy
Ueq
7(1)
8l
4i
0
0
1
1
9(1)
4j
5000
0
0.351(4)
15(1)
11(1)
9(1)
4h
8l
0
2755(1)
2152(1)
3564(1)
0
0.790(4)
0
1
1
1
8l
0
14(1)
9(1)
4j
5000
0
4g
2c
4f
3413(1)
5000
1
12(1)
20(3)b
20(3)b
0.5(6)
5000
4400(5)
0
0
0.2(4)
5000
0
4g
1013(1)
0
1
14(1)
a Ueq is defined as one third of the trace of the orthogonalized Uij tensor.
b Constrained with isotropic displacement parameters of equal magnitude.
Table 3. Anisotropic Displacement Parameters (Å2 x 103) for La6Pd2.28Sb15 at room temperature.a
Label
La(1)
La(2)
Pd(1)
Pd(2)
Sb(1)
Sb(2)
Sb(3)
Sb(4)
Sb(5a)
Sb(5b)
Sb(6)
U11
U22
U33
8(1)
U12
0
U13
0
U23
0(1)
0
8(1)
7(1)
9(1)
7(1)
10(1)
13(2)
8(1)
0
0
20(2)
12(1)
9(1)
14(1)
12(1)
9(1)
0
0
0
0
0
0
9(1)
0
0
1(1)
-2(1)
0
10(1)
7(1)
10(1)
9(1)
22(1)
9(1)
0
0
0
0
12(1)
20(8)
20(8)
12(1)
15(1)
10(1)
10(1)
18(1)
10(1)
33(1)
33(1)
13(1)
0
0
0
0
0
0
0
0
0
0
0
0
aThe anisotropic displacement factor exponent takes the form: -2π2[h2a*2U11 + ... + 2hka*b*U12].
15

Table 4. Representative bond lengths (Å) and bond angles (o) in the structure of La6Pd2.28Sb15 at
room temperature. Estimated standard deviations are given in parentheses.
atom−atom
bond length atom−atom−atom
bond angle
81.485(8)
99.76(2)
47.69(3)
133.820(9)
80.41(2)
125.836(19)
82.025(7)
100.214(19)
177.98(3)
52.266(12)
82.025(16)
133.820(2)
177.98(3)
100.214(19)
52.266(12)
79.097(17)
135.260(12)
94.15(3)
La(1)-Sb(3) (x2)
La(1)-Pd(1) (x2)
La(1)-Sb(2) (x1)
La(1)-Sb(1) (x2)
La(1)-Pd(2) (x1)
La(1)-Sb(5a) (x1)
La(1)-Sb(5b) (x2)
La(1)-Sb(1) (x1)
La(1)-Sb(4) (x2)
La(2)-Sb(3) (x2)
La(2)-Sb(2) (x4)
La(2)-Sb(1) (x2)
Pd(1)-Sb(5a) (x2)
Pd(1)-Sb(5b) (x2)
Pd(1)-Sb(5b) (x2)
Pd(1)-Sb(3) (x1)
Pd(1)-Sb(4) (x2)
Pd(2)-Sb(2) (x2)
Pd(2)-Sb(4) (x2)
Pd(2)-Sb(6) (x2)
Pd(1)-Pd(1) (x1)
3.3005(5)
3.3369(9)
3.3750(9)
3.3830(6)
3.3876(8)
3.4087(1)
3.4214(4)
3.4201(8)
3.4323(6)
3.2537(7)
3.3160(6)
3.3543(8)
2.495(13)
2.315(4)
Sb(3)-La(1)-Sb(3)
Sb(3)-La(1)-Pd(1)
Sb(3)-La(1)-Pd(1)
Sb(3)-La(1)-Sb(2)
Pd(1)-La(1)-Pd(1)
Pd(1)-La(1)-Sb(2)
Sb(3)-La(1)-Sb(1)
Pd(1)-La(1)-Sb(1)
Pd(1)-La(1)-Sb(1)
Sb(2)-La(1)-Sb(1)
Sb(3)-La(1)-Sb(1)
Sb(3)-La(1)-Sb(1)
Pd(1)-La(1)-Sb(1)
Pd(1)-La(1)-Sb(1)
Sb(2)-La(1)-Sb(1)
Sb(1)-La(1)-Sb(1)
Sb(3)-La(1)-Pd(2)
Pd(1)-La(1)-Pd(2)
Sb(2)-La(1)- Pd(2)
2.735(4)
2.683(2)
2.7533(9)
2.5666(1)
2.8063(1)
44.609(16)
43.48(3)
48.36(8)
2.8705(9) Pd(1)-La(1)-Sb(5a)
2.536(3)
Pd(1)-La(1)-Sb(5b)
16

Figure 1. Rietveld refinement plot showing calculated (black line), observed (red open circle)
and difference (blue line) powder XRD patterns of La6Pd2+xSb15 (λ = 1.5406 Å, Bragg R-factor:
7.15 %; Rf-factor= 6.03). Sb metal from the flux is present as secondary phase (see text for
details). The inset depicts typical crystals of La6Pd2.28Sb15.
17

Figure 2. Typical EDX spectrum of a point measurement on a La6Pd2.28Sb15 crystal. The minor
bump at 7.5 keV corresponds to Ni and it is a spurious contribution coming from the sample
holder. The inset shows the SEM image in BSE mode of a typical La6Pd2.28Sb15 crystal.
18

Figure 3. Ball-and-stick representations of three-dimensional structure of La6Pd2+xSb15 (x = 0.28)
viewed down the crystallographic a-axis. The red dashed lines outline the La6 trigonal prisms. La
atoms are gray, Pd atoms are blue, Sb(1) atoms are violet, Sb(2) atoms are bright green, Sb(3)
atoms are red, Sb(4) atoms are orange, Sb(5) atoms are turquoise (for the sake of clarity, the split
position of Sb(5) is not visualized here) and Sb(6) atoms are green.
19

Figure 4. (a) Coordination environment around Pd(1) atom in the average ordered structure of
La6Pd2.28Sb15. The thermal ellipsoids were set up at 50%. (b) Random distribution of the local
positional disorder (partially occupied Pd(1), Sb(5a), and Sb(5b) sites) along the length of the
three-atom-wide Sb ribbons in La6Pd2.28Sb15. (c) Local coordination environment of Pd(2). Pd
atoms are blue, Sb(2) atoms are bright green, Sb(3) atoms are red, Sb(4) atoms are orange, Sb(5a)
and Sb(5b) atoms are turquoise and Sb(6) atoms are green.
20

Figure 5. Local coordination environment of the (a) La(1) and (b) La(2) atoms in La6Pd2.28Sb15.
La atoms are gray, Sb(1) atoms are violet, Sb(2) atoms are bright green, Sb(3) atoms are red,
Sb(4) atoms are orange, Sb(5) atoms are turquoise and Sb(6) atoms are green.
21

Figure 6. Electronic structure La6Pd2.28Sb15. (a) Density of states as a function of energy. The
inset shows a low-energy zoom. (b) Energy dispersion along high symmetry lines (depicted in c)
including or not the spin-orbit coupling in the calculation. (d-g) Fermi surface sheets. Red and
blue correspond to the outer and inner parts of the Fermi surface, respectively, so that the pockets
in f are hole-type while those in g are electron-type.
22

Figure 7. Temperature dependence of the susceptibility for zero field cooling (ZFC) and field
cooling (FC) procedures. Inset: Field dependence of the magnetization for several temperatures.
In all cases, the background of the glue and the sample holder was subtracted, as well as, the core
diamagnetic contribution.
23

Figure 8. Temperature dependence of the resistivity. The arrow indicates a temperature at which
ρ(T) presents a change of curvature. Inset: Resistivity as a function of T 2. The dotted line is a fit
to the equation ρ=ρ0+ρ1T 2.
24

Figure 9. (a) Hall resistivity as a function of applied magnetic field. Inset: Hall coefficient (RH=
ρxy/dH|µ H→0) as a function of the temperature. (b) Magnetoresistance as a function of the
d
0
applied magnetic field for different temperatures. Inset: Temperature dependence of the
magnetoresistance for µ0H=15T.
25

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