Thorium copper phosphides: More diverse metal-phosphorus and phosphorus-phosphorus interactions than U analogues

Article abstract of DOI:10.1039/c7dt02145c

To explore the chemical analogy between thorium and heavier actinides in soft anionic environments, three new thorium phosphides (ThCuP₂, β-ThCu₂P₂, and ThCu₅P₃) have been prepared through solid-state reactions using CuI as a reaction promoter. The structure of ThCuP₂ can be described as a filled UTe₂-type with both dimeric P₂^4- and monomeric P^3- anions, in which Th is coordinated by eight P atoms in a bicapped trigonal prismatic arrangement and Cu is tetrahedrally coordinated by four P atoms. β-ThCu₂P₂ contains only P^3- anions and is isostructural with BaCu₂S₂. In this structure, Th is coordinated by seven P atoms in monocapped trigonal prismatic geometry and Cu is tetrahedrally coordinated by four P atoms. ThCu₅P₃ adopts the YCo₅P₃-type structure consisting of P^3- anions. This structure contains Th atoms coordinated by six P atoms in a trigonal prismatic arrangement and Cu atoms that are either tetrahedrally coordinated by four P atoms or square pyramidally coordinated by five P atoms. Electric resistivity measurements and electronic structure calculations on β-ThCu₂P₂ indicate a metal. These new compounds may be charge-balanced and formulated as Th⁴⁺Cu⁺(P₂^4-)_1/2P^3-, Th⁴⁺(Cu⁺)₂(P^3-)₂, and Th⁴⁺(Cu⁺)₅(P^3-)₃, respectively. The structural, bonding, and property relationships between these Th compounds and related actinide and rare-earth phases are discussed. Titled compounds display more diverse ion-ion interactions and different electronic structures from those in UCuP₂ and UCu₂P₂ that were synthesized under similar experimental conditions, suggesting divergence of thorium-phosphide chemistry from uranium-phosphide chemistry.

Full text of DOI:10.1039/c7dt02145c

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Thorium copper phosphides: more diverse
metal–phosphorus and phosphorus–phosphorus
interactions than U analogues†
Cite this: DOI: 10.1039/c7dt02145c
Geng Bang Jin, *a Christos D. Malliakas a,b and Jian Lin‡a
To explore the chemical analogy between thorium and heavier actinides in soft anionic environments,
three new thorium phosphides (ThCuP2, β-ThCu2P2, and ThCu5P3) have been prepared through solid-
state reactions using CuI as a reaction promoter. The structure of ThCuP2 can be described as a filled
UTe2-type with both dimeric P2 and monomeric P3− anions, in which Th is coordinated by eight
4−
P atoms in a bicapped trigonal prismatic arrangement and Cu is tetrahedrally coordinated by four P atoms.
β-ThCu2P2 contains only P3− anions and is isostructural with BaCu2S2. In this structure, Th is coordinated
by seven P atoms in monocapped trigonal prismatic geometry and Cu is tetrahedrally coordinated by four
P atoms. ThCu5P3 adopts the YCo5P3-type structure consisting of P3− anions. This structure contains Th
atoms coordinated by six P atoms in a trigonal prismatic arrangement and Cu atoms that are either tetra-
hedrally coordinated by four P atoms or square pyramidally coordinated by five P atoms. Electric resistivity
measurements and electronic structure calculations on β-ThCu2P2 indicate
a
metal. These new
P
, Th4+(Cu+)2(P3−)2, and
1/2
compounds may be charge-balanced and formulated as Th4+Cu+(P2
)
4−
3−
Th4+(Cu+)5(P3−)3, respectively. The structural, bonding, and property relationships between these Th
compounds and related actinide and rare-earth phases are discussed. Titled compounds display more
diverse ion–ion interactions and different electronic structures from those in UCuP2 and UCu2P2 that
were synthesized under similar experimental conditions, suggesting divergence of thorium-phosphide
chemistry from uranium-phosphide chemistry.
Received 12th June 2017,
Accepted 26th July 2017
DOI: 10.1039/c7dt02145c
rsc.li/dalton
ing on the actinide and its chemical environment, will modify
the participation of the 5f states in bonding and change the
Introduction
Actinide (An) chalcogenides (Q = S, Se, Te) and pnictides (Pn = crystal radii and formal charges of the actinide elements. One
P, As, Sb) exhibit extremely rich structural diversity and elec- of the fundamental issues in the field of actinide materials is
tronic properties, mainly owing to the flexible bonding and the itinerancy of 5f electrons. Softer chalcogens and pnicto-
redox capabilities of both metal and anions.1–4 Partially filled gens, compared with oxygen and halogens, exhibit a greater
actinide 5f orbitals can have varying degrees of localization tendency to engage in covalent bonding with actinides.7 In
intermediate between itinerant 3d and localized 4f orbitals.5,6 fact, covalency in the f element–chalcogen/pnictogen bond in
Small variations in the itinerancy of the 5f electrons, depend- molecular complexes has been extensively studied recently to
provide a chemical bonding rationale for separating lantha-
nides and actinides during nuclear fuel reprocessing.8–10 In
aChemical Sciences and Engineering Division, Argonne National Laboratory,
Argonne, Illinois 60439, USA. E-mail: gjin@anl.gov; Tel: +1 630 252 3658
bDepartment of Chemistry, Northwestern University, Evanston, Illinois, 60208-3113,
USA
addition, chalcogens and pnictogens have a propensity to
form a variety of homoatomic interactions, including classic
two-electron bonds and ‘nonclassical’ bonds involving frac-
tional bond orders.2,11 Depending on the redox potential of
the metal cation (M), the charge transfer between M and Q or
Pn substructures varies and, consequently, so do the structures
and electronic properties of anions.2,12 As a result, flexible
chalcogenides and pnictides can capture gradual transitions of
actinide 5f electronic states in comparable anionic environ-
ments, as demonstrated by recent studies of An2Q5 (An = Th,
U, Np; Q = S, Se, Te).13,14 Therefore, studying the metal/anion
†Electronic supplementary information (ESI) available: Powder X-ray diffraction
patterns, synthesized precession images, picture of products, and atomic coordi-
nates. The crystallographic files in CIF format for ThCuP2, β-ThCu2P2, and
ThCu5P3 have been deposited with FIZ Karlsruhe as CSD numbers 431441,
431442, and 431443, respectively. These may be obtained free of charge by con-
tacting FIZ Karlsruhe at +49 7247 808-666 (fax) or crysdata@fiz-karlsruhe.de
(e-mail). See DOI: 10.1039/c7dt02145c
‡Present address: Shanghai Institute of Applied Physics, Chinese Academy of
Sciences, Shanghai 201800, China.
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substructures of these materials provides a great platform to
elucidate chemical periodicity across the actinide series.
Experimental
Thorium, with an unoccupied 5f band, has properties Caution! 232Th is an α-emitting radioisotope and as such is
similar to both light actinides and tetravalent transition considered a health risk. Its use requires appropriate infra-
metals, such as titanium, zirconium, and hafnium.15 As such, structure and personnel trained in the handling of radioactive
Th is often used as an analogue for the redox-active tetravalent materials.
actinides with partially filled 5f orbitals.16–18 However, because
it is larger and chemically softer than any of the tetravalent
Syntheses
actinides or transition metals, the extent to which the chemi- Cu (Aldrich, 99.5%), P (Aldrich, 99.99%), CuI (Sigma-Aldrich,
cal analogy holds in chalcogenide and pnictide systems is 99.999%), LiCl (Alfa Aesar, 99.9%), and KCl (Alfa Aesar,
unclear, especially for the less studied pnictides.13,19,20 99.95%) were used as received. Soft 232Th metal bars were cut
Compared with the chalcogens, the electronegativities of acti- into small fragments and used as provided (ANL stock).
nides and pnictogens are closer, as is the energy of An 5f and A eutectic LiCl–KCl mixture (eutectic point: 628 K) was
Pn p orbitals. As such, mixing of actinide 5f and pnictogen p prepared by grinding LiCl and KCl powders together in a
states may increase compared with chalcogens of similar ionic molar ratio of 0.595 : 0.405 in an Ar-filled glove box.33
radii, resulting in greater participation of the 5f states in
For all the syntheses, the reactants were loaded into fused-
bonding.5 The majority of actinide pnictide studies in the lit- silica ampoules in an Ar-filled glove box and then flame-sealed
erature describe uranium-based phases,1,3 although there are under vacuum. The reaction mixtures were placed in a tube
some data available for Th compounds, which have been furnace with a programmable controller and heated according
largely studied as nonmagnetic reference materials for the U to a specific temperature profile. After completion, the reaction
systems.21 Transuranic pnictides are limited to only a few mixtures were removed from the inner ampoules and placed
examples of simple binary phases. From these limited data, under oil. All of the crystals found in these reactions were
substantial deviation of Th-pnictide chemistry from the other manually extracted from the product mixture and character-
actinides is observed.20 In the case of binary actinide phos- ized by single-crystal X-ray diffraction. Bulk solid products
phide systems, attempts to prepare the Th analogue of AnP2 were characterized by powder X-ray diffraction (PXRD).
(An = Pa, U) have been unsuccessful.19 Instead, two unique
ThCuP2
polyphosphides, Th2P11 and ThP7, which feature extensive P
networks, were synthesized.22,23 Polyphosphides with a P : M This compound was prepared through a reaction of Th
ratio >2 have not been reported for other actinides, but are (0.022 g, 0.095 mmol), Cu (0.006 g, 0.094 mmol), P (0.006 g,
common for lower-valent metals, such as MP15 (M = Li–Rb, 0.19 mmol), and CuI (0.003 g, 0.016 mmol). The reaction
Ag24) and MP5 (M = rare earth).11
mixture was heated to 923 K in 30 hours, held at 923 K for
To further explore the chemical and structural analogy 7 days, cooled to 573 K in 6 days, and then cooled to 298 K in
between Th and heavier actinides in soft phosphide environ- 8 hours. The reaction products included large clusters of black
ments, we have examined the ternary Th/Cu/P system for com- prismatic crystals of ThCuP2, a few black polyhedral crystals of
parison with the well-studied U compounds, including UCuP2 CuP2,34 black microcrystalline solids, and white needles of
and UCu2P2.25,26 Compared with the parent binary actinide CuI. No noticeable iodine incorporation into the Th products
phosphides, the presence of the covalent Cu–P bond results in was observed. CuI was readily separated from the products
variable interactions between the An and P. In the U system, after the reaction without further processing. The microcrystal-
intercalating extra layers of nonmagnetic Cu+ (d10) between line solids are mainly composed of ThCuP2 (∼80%) and CuP2,
uranium phosphide networks not only changes the bonding as suggested by powder X-ray diffraction experiments (Fig. S1a
environments of the actinide and phosphide but also alters in the ESI†).
the electronic interplay between neighboring U cations, such
β-ThCu2P2 and ThCu5P3
as the magnetic coupling.27–30 For example, U is nine-coordi-
nate by P atoms in UP2,31 while it is eight-coordinate in These two compounds were prepared through the reaction of
25
UCuP2 and six-coordinate in UCu2P2.26 UP2 undergoes an Th (0.045 g, 0.19 mmol), Cu (0.025 g, 0.39 mmol), P (0.012 g,
antiferromagnetic ordering below 203 K,31 whereas UCuP2 and 0.39 mmol), and CuI (0.006 g, 0.032 mmol). The reaction
UCu2P2 are ferromagnetic with Tc = 75 K (ref. 28) and 216 K,27 mixture was heated to 923 K in 48 hours, held at 923 K for
respectively. To date, only one related Th-based compound, 8 days, cooled to 573 K in 8 days, and then cooled to 298 K in
ThCu2P2, which is isostructural with UCu2P2, has been 8 hours. The reaction products included black crystals of
reported.32 Herein, we report the syntheses and characteriz- β-ThCu2P2, ThCu5P3, and CuP2 in prismatic, acicular, and
ation studies of three new thorium copper phosphides with polyhedral forms, respectively, and black microcrystalline
more diverse An–P and P–P interactions than those in U-based solids, and white needles of CuI. The black solids are mostly
compounds with similar compositions. The formation of these composed of β-ThCu2P2 (∼50%), ThCu5P3 (∼30%), and CuP2
phases underscores the further divergence of thorium-phos- (Fig. S1b in the ESI†). A similar reaction with a molar ratio of
phide chemistry from uranium-phosphide chemistry, as pre- Th : Cu : P at 1 : 5 : 3 at 1073 K using CuI as a reaction promoter
viously observed.
produced ThCu5P3 (∼80%), CuP2, and a significant amount of
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black plates of ThCuOP35 (Fig. S1c in the ESI†). Oxide impuri- Based on these results, P(1) atoms do not have long-range
ties were probably from the etching of the silica ampoules.
order in the crystal. Therefore, the structure of ThCuP2 is
reported in the space group Immm with disordered
positions.
P
Structure determination
Single-crystal X-ray diffraction data for ThCuP2, β-ThCu2P2,
The structural refinements for ThCu2P2 were straight-
and ThCu5P3 were collected using graphite-monochromatized forward without any disordering.
MoKα radiation (λ = 0.71073 Å) at 100 K on a Bruker APEX2
For ThCu5P3, the unit cell parameters and the intensity
diffractometer.36 The crystal-to-detector distance was 5.000 cm. symmetry were consistent with the YCo5P3-type40 structure in
Data were collected by a scan of 0.3° in ω in groups of 600 the orthorhombic centrosymmetric space group Pnma.
frames at φ settings of 0°, 90°, 180°, and 270°. The exposure Beginning with the model for YCo5P3, the refinement of
time was 10 s per frame for ThCuP2, 20 s per frame for “ThCu5P3” proceeded smoothly to an R1 index of 0.015. As
ThCu2P2, and 15 s per frame for ThCu5P3. Collection of inten- commonly observed in the other YCo5P3-type structures, the
sity data as well as cell refinement and data reduction were displacement ellipsoids of the five-coordinate Cu(5) position
carried out using the program APEX2.36 Absorption corrections (4c, .m.) were highly oblate.40 This “rattling” behavior of the
as well as incident beam and decay corrections were per- atom Cu(5) is likely due to its larger coordination environment
formed using the program SADABS.37 The structures were compared with four-coordinate Cu positions.41 A disorder
solved using the direct-methods program SHELXS and refined model splitting the Cu(5) atom in two close 4c positions with
using the least-squares program SHELXL.38 The assignment of short Cu(5A)–Cu(5B) distances (∼0.3 Å) was attempted to
Th, Cu, and P atoms was straightforward based on their elec- improve the refinement result. An anisotropic refinement
tron densities and interatomic distances. The final refinement without constraint led to occupancies of 0.53(3) and 0.41(3) at
included anisotropic displacement parameters for all atoms.
the Cu(5A) and Cu(5B) positions, respectively. Both Cu(5) posi-
The structure of ThCuP2 was solved in the orthorhombic tions exhibit much larger thermal parameters than other Cu
centrosymmetric space group Immm with one crystallographi- sites. Attempts to further model the disorder of the Cu(5) atom
cally unique Th, one Cu, and two P positions. Closer inspec- into the Cu(5A), the Cu(5B) and the nearest Q peaks failed.
tion of atomic connectivities revealed that the distance The constrained refinement with unity occupancy for two posi-
between the two P(1) positions (8n, ..m) related by a mirror tions did not change the refinement statistics significantly. As
plane perpendicular to the a axis is ∼0.5 Å. Therefore, P(1) no supercell reflections were observed (Fig. S3 in the ESI†), a
sites cannot be fully occupied. The occupancy of the P(1) atom potential long-range order of Cu(5A) and Cu(5B) atoms in
was refined freely, giving a value of 0.50(1). The refinement reduced symmetry groups was investigated in the same unit
statistics, including an R1 index of 0.0164, did not change cell. Continued disorder of the Cu(5) atoms was observed in all
when the occupancy at the P(1) position was constrained to subgroups of Pnma. Similar to ThCuP2, twinning was also
0.50. No supercell reflections were observed as demonstrated investigated as a plausible reason for the observed disorder in
by the synthesized precession images (Fig. S2 in the ESI†), a the structure of ThCu5P3 and none of the twinning tests gave
potential long-range order of P(1) atoms in reduced symmetry better agreement factors. To conclude, the distribution of Cu
groups was investigated in the same unit cell. The mirror (5) atoms within the structure of ThCu5P3 is random, as
plane perpendicular to the a axis in the space group Immm was expected.
removed, which resulted in an ordered P(1) position (4d, ..m) in
The program STRUCTURE TIDY42 was used to standardize
the non-standard non-centrosymmetric subgroup I2mm. Two the positional parameters. Additional experimental details are
equivalent twin components were included in the final refine- given in Table 1 and in the ESI.†
ments in subgroup I2mm to fit the centrosymmetric intensi-
ties, which gave an R1 index of 0.017. The displacement ellip-
soids for atom P(1) were highly prolate along the a axis indicat-
Powder X-ray diffraction measurements
ing disorder, while the displacement ellipsoid for atom P(2) Powder X-ray diffraction patterns were collected with the use of
(4c, .m.) became nonpositive definite. Overall, no improvement Cu Kα radiation (λ = 1.5418 Å) on a Scintag X1 diffractometer
in the refinement results was obtained for the reduced sym- in a 2θ range of 2–120°.
metry subgroup I2mm, compared with those in the space
group Immm. All other subgroups were also examined.
Continued disorder of the corresponding P(1) atoms in the
Electrical resistivity measurements
space group Immm was observed during the refinements in Standard four-probe electrical resistivity measurements were
these subgroups and less satisfactory agreement factors were performed on a crystal of β-ThCu2P2 (∼0.6 mm × 0.2 mm ×
obtained. For monoclinic subgroups such as I2/m, potential 0.15 mm) using an MMR Variable Temperature Resistivity
pseudomerohedral twinning was examined using the program Measuring System from 100 to 340 K. A small fragment of the
PLATON,39 to improve the refinement; however, no twinning crystal was cut off and examined by X-ray diffraction measure-
law was detected. Other lower symmetry space groups were ments to confirm the identity. Numerous attempts to grow
tested, including P1, in combination with various twin laws. suitable single-crystal samples of ThCuP2 and ThCu5P3 for
None of the twinning tests gave better agreement factors. resistivity measurements failed.
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a
Table 1 Crystal data and structure refinements for ThCuP2, β-ThCu2P2, and ThCu5P3
ThCuP2
β-ThCu2P2
ThCu5P3
Fw
357.52
Black prism
Immm
421.06
Black prism
Pnma
642.65
Black needle
Pnma
Color and habit
Space group
a, Å
b, Å
4.026(1)
5.598(1)
8.9882(5)
3.8726(2)
12.7255(4)
3.8766(1)
c, Å
13.863(3)
312.4(1)
7.60
55.05
9.8376(6)
342.42(3)
8.17
56.28
10.8422(4)
534.86(3)
7.98
47.96
V, Å3
ρc, g cm−3
μ, mm−1
F(000)
596
712
1120
Crystal size, mm3
Theta range for data collection, °
Index ranges
0.058 × 0.082 × 0.130
2.939 to 32.796
−6 ≤ h ≤ 5, −8 ≤ k ≤ 8,
−20 ≤ l ≤ 20
2675
360 [Rint = 0.0271]
360/0/20
0.012 × 0.032 × 0.220
3.070 to 32.941
−13 ≤ h ≤ 13, −5 ≤ k ≤ 5,
−14 ≤ l ≤ 14
5351
689 [Rint = 0.0166]
677/0/31
0.008 × 0.012 × 0.080
2.468 to 27.486
−16 ≤ h ≤ 16, −5 ≤ k ≤ 5,
−14 ≤ l ≤ 14
6612
700 [Rint = 0.0204]
694/0/64
Total reflections collected
Independent reflections
Data [I > 2σ(I)]/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
1.354
1.271
1.230
R1 = 0.0164, wR2 = 0.0458
R1 = 0.0164, wR2 = 0.0458
0.0013(2)
R1 = 0.0144, wR2 = 0.0350
R1 = 0.0147, wR2 = 0.0351
n/a
R1 = 0.0113, wR2 = 0.0290
R1 = 0.0116, wR2 = 0.0292
0.00127(8)
Extinction coefficient
Largest diff. peak and hole, e Å−3
1.516 and −3.549
2.001 and −2.084
0.989 and −0.987
a For all structures, orthorhombic system, Z = 4, T = 100(2) K, λ = 0.71073 Å, completeness to theta = 25.242°: 100%, refinement method: full-
matrix least-squares on F2.
Theoretical calculations
binary phosphide phases, which could include Th3P4,50 ThP,51
CuP2, and Cu3P,52 was observed. In sharp contrast, in the
absence of CuI, targeted syntheses of these ternary pnictides
under similar conditions resulted in mostly poorly crystalline
binary compounds. For example, a reaction of Th, Cu, and P in
a molar ratio of 1 : 2 : 2 at 923 K for 8 d only resulted in Th3P4,
ThP, CuP2, and Cu3P (Fig. S1d, S1e, and S4 in the ESI†). Single
crystals of ThCuP2 and ThCu5P3 can be prepared using a eutec-
tic LiCl–KCl flux as well; however, a much larger quantity
(100 mg vs. 3 mg for CuI) of flux and relatively higher tempera-
tures (1023–1123 K) were required for their syntheses. The for-
mation of binary impurities was also observed in the reactions
with LiCl–KCl.
Based on these results, CuI and LiCl–KCl appear to facili-
tate the solid-state reactions of Th/Cu/P and help to grow
ternary Th-phosphide crystals. Compared to existing methods
to prepare actinide phosphide crystals including using a Sn-
flux or an I2-transport agent,53,54 the mineralizing agent CuI
allows access to some new thorium phosphides that do not
form under similar experimental conditions using other
methods. Furthermore, only a small amount of CuI was
Electronic structure calculations for β-ThCu2P2 were performed
using the self-consistent full-potential linearized augmented
plane wave method (LAPW)43 within the density functional
theory (DFT) plus U method,44,45 and the generalized gradient
approximation (GGA) of Perdew, Burke, and Ernzerhof46 for
the exchange and correlation potential. The values of the
atomic radii were taken as 2.5 a.u. for Th atoms, 2.3 a.u. for
Cu atoms, and 2.0 a.u. for P atoms, respectively, where a.u. is
the atomic unit (0.529 Å). The on-site U corrections were
applied to the rare-earth f orbitals with 3.5 eV for Th.
Convergence of the self-consistent iterations was performed
for 170 k points inside the irreducible Brillouin zone to within
0.0001 Ry with a cutoff of −6.0 Ry between the valence and the
core states. The calculations were performed using the
WIEN2k program using the experimental cell parameters and
atomic coordinates obtained by single crystal diffraction.47
Results and discussion
Small quantities of metal halides were used as mineralizing required for the syntheses and the flux was readily separated
agents to promote the solid-state reactions of transition metal from the products after the reaction without further proces-
phosphides.48,49 Following this concept, three new com- sing. This is advantageous when working with small quantities
pounds, ThCuP2, β-ThCu2P2, and ThCu5P3, have been syn- of actinide compounds particularly for transuranic samples.
thesized with CuI (m.p. 878 K and b.p. 1563 K) as a reaction To test the effectiveness of CuI for other actinide pnictide
promoter. Product compositions appear to be highly affected systems, a series of U/Cu/P reactions with CuI were conducted
by the molar ratio of Th : Cu : P in the starting materials. under similar conditions to the Th/Cu/P system, successfully
Increasing the amount of Cu seems to promote the formation producing UP2,31 UCuP2,25 and UCu2P2.26 For example, UP2,
of products with a higher Cu content, as exemplified by the UCuP2, and UCu2P2 crystals were obtained from reactions with
described syntheses in the experimental section. Co-formation stoichiometric ratios of elements and 5–10 wt% CuI at 1123 K,
of these ternary products with small amounts of undesired 923 K, and 1073 K, respectively. It is noted that numerous
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Table 2 Selected interatomic distances (Å) for ThCuP2
attempts to synthesize the Th analogue of UP2 under compar-
able reaction conditions failed. Instead, Th3P4 and ThP and a
new Th polyphosphide were identified that eluded full struc-
tural characterization owing to severe disordering of P atoms
(cell parameters: a = 8.1096(6) Å, b = 20.520(1) Å, c = 5.7089(4)
Å, β = 134.5777(8) °, V = 676.69(8) Å3, monoclinic). Overall,
solid-state reactions using CuI as a reaction mineralizer could
be a valuable addition to the small arsenal of existing synthetic
approaches for actinide pnictides.
Th(1)–P(1) × 2
Th(1)–P(1) × 2
Th(1)–P(2) × 2
Th(1)–P(2) × 2
Cu(1)–P(1) × 2
Cu(1)–P(2) × 2
2.904(2)
3.249(3)
2.886(2)
2.9048(9)
2.377(3)
2.335(2)
P(1)–P(1)
P(1)–P(1)
Th(1)–Th(1) × 2
Th(1)–Cu(1)
Cu(1)–Cu(1)
2.190(7)
2.252(7)
4.026(1)
3.252(2)
3.272(3)
The structure of ThCuP2 contains one crystallographically are 2.335(2) and 2.377(3) Å within the CuP4 tetrahedra, typical
unique Th, one Cu, and two P positions with the site sym- for four-coordinated Cu+ ions.35 The closest Th–Th, Th–Cu,
metries of mm2 (Th), mm2 (Cu), ..m (P(1)), and mm2 (P(2)) and Cu–Cu distances are 4.026(1), 3.252(2), and 3.272(3) Å,
(Fig. 1a). The Th atom is surrounded by four P(1) and four P(2) respectively. These values are much longer than the sum of
atoms in a bicapped trigonal prismatic arrangement. The Cu metallic radii of the corresponding elements, which are 3.590,
atom is tetrahedrally coordinated by two P(1) and two P(2) 3.071, and 2.552 Å, respectively.56
4−
atoms. The disordered P(1) atoms form P2 dimeric anions
As shown in Fig. 2a and b, the structure of ThCuP2 is highly
with P(1)–P(1) distances of 2.190(7) or 2.252(7) Å, which are related to that of UTe2,57 a rare structure type for metal chalco-
close to the single bond distances of 2.224(2) and 2.244(2) Å in genides and pnictides.20 The former structure can be obtained
the layers of the black phosphorus.55 P(2) is an isolated P3− by inserting Cu atoms into the tetrahedral tubes within the
anion with the closest P–P distances of 3.409(7) Å, too long to networks of UTe8 bicapped trigonal prisms in UTe2 and break-
have any significant interactions. As a result, ThCuP2 can be ing the linear Te chains into dimers. In comparison, other acti-
formulated as Th4+Cu+(P2
)
P
.
nide or lanthanide analogues with similar chemical formulae
4−
3−
1/2
Each ThP8 bicapped trigonal prism either shares one of MCuPn2 adopt either a filled UGeTe-type or ZrSiS-type
square face, one trigonal face, or one edge with each of five structure.25,58–61 Both structure types have been commonly
identical neighbors to form one-dimensional double chains observed among the actinide- and lanthanide transition-metal
propagating along the a axis (Fig. 1b). These double chains pnictides, and their close relationships have been detailed in
further share edges with four other units to construct a three- the literature.25 For example, the structure of UCuP2, derived
dimensional structure (Fig. 1a). The space within the network from the UGeTe-type structure, consists of layers of UP8 square
of ThP8 polyhedra is filled by isolated double chains of CuP4 antiprisms separated by tetrahedrally coordinated Cu atoms
tetrahedra running along the a axis, where each tetrahedron (Fig. 2c);25 similarly, the structure of GdCuP2.20, derived from
either shares one edge or one corner with each of three identi- the ZrSiS-type structure, is composed of alternating layers of
cal neighbors.
GdP8 square antiprisms and tetrahedrally coordinated
Selected interatomic distances for ThCuP2 are listed in Cu atoms (Fig. 2d).59 It is interesting to note that half of the
Table 2. Th–P distances range from 2.886(2) to 3.249(3) Å P atoms in the UCuP2 structure form ideal two-dimensional
within the ThP8 polyhedra, which are comparable to the values square nets, whereas the corresponding P layers in the
for eight-coordinate Th4+ in Th3P4 (2.98 Å).50 Cu–P distances GdCuP2.20 structure form P chains, which are further con-
nected by an additional partially filled P site (P(6) in Fig. 2d)
forming puckered sheets. Overall, the geometry and dimen-
sionality of ThP8 polyhedra in the structure of ThCuP2 are
different from those in UCuP2 and GdCuP2.20. In addition, the
extent of P–P interactions in the anion substructure increases
in the order: ThCuP2 < UCuP2 < GdCuP2.20. Considering
the similar crystal radii for eight-coordinate ThIV (1.19 Å),
UIV (1.14 Å), and GdIII (1.193 Å) ions,62 the difference in P–P
interactions among three compounds may be largely attributed
to electronic origins, such as the effective charge of metal
cations. Both the extent of Pn–Pn bonding and the suscepti-
bility to the Peierls distortion depend on the electron count in
the anion entities, which is determined by the redox chemistry
between the metal and pnictides (M–Pn interactions).2,11 The
relative MIV standard reduction potentials (E°) in aqueous
solutions under acidic conditions decrease in the order:
Fig. 1 (a) Crystal structure of ThCuP2 viewed down the a axis; (b) a
double chain of face-sharing ThP8 bicapped trigonal prisms propagating
along the a axis; (c) a double chain of corner- and edge-sharing CuP4
Th (−3.8 V) < U (−0.553(4) V) < Ln (≥+1.72 V) as a result of
localization of the f electrons.63,64 Whereas these potentials
are not expected to be reproduced in the solid-state pnictides
discussed here, their trend suggests that among the MIII
tetrahedra propagating along the a axis. P–P bonds are highlighted in
green. Only half of the disordered P(1) positions are shown for clarity.
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59
4−
Fig. 2 Structural comparisons between ThCuP2, UTe2,57 UCuP2,25 and GdCuP2.20
.
(a) ThCuP2 consisting of P2 dimers: a three-dimensional
network of ThP8 bicapped trigonal prisms with the voids filled by Cu atoms; (b) UTe2 featuring linear Te chains: a three-dimensional network of UTe8
bicapped trigonal prisms with open voids; (c) UCuP2 including undistorted square nets of P: two dimensional layers of UP8 square antiprisms separ-
ated by layers of Cu atoms; (d) GdCuP2.20 featuring puckered nets of P: two dimensional layers of GdP8.8 polyhedra separated by layers of Cu atoms.
P–P and Te–Te bonds are highlighted in green. Cu–P bonds are omitted for clarity.
cations, ThIII is the most susceptible toward the loss of elec- tances of 3.633(1) Å. β-ThCu2P2 may be charge-balanced and
trons whereas LnIII is the least susceptible when surrounded formulated as Th4+(Cu+)2(P3−)2.
by the same pnictide in a similar coordination environment.
As shown in Fig. 3b, each ThP7 polyhedron shares one
Therefore, the number of electrons provided by the metal to trigonal face with each of two identical neighbors to form one-
reduce the pnictide network should decrease across the series dimensional chains propagating along the b axis. These
from ThCuP2, UCuP2, to GdCuP2.20, which is consistent with chains further share edges or corners with four adjacent units
the trend of increasing P–P interactions in the anion substruc- to construct a three-dimensional network (Fig. 4a). Similarly,
ture across the series.
each Cu(1)P4 or Cu(2)P4 tetrahedron shares one edge or one
65
β-ThCu2P2 is isostructural with BaCu2S2 and crystallizes corner with each of four identical neighbors to form zigzag
in the space group Pnma of the orthorhombic system. The
structure of β-ThCuP2 includes one crystallographically unique
Th, two Cu, and two P positions with the site symmetry
of .m. (Fig. 3a). The Th atom is coordinated by seven P atoms
in a mono-capped trigonal prism (Fig. 3b). Both Cu atoms are
connected to four P atoms in a tetrahedral geometry (Fig. 3c).
Both P atoms are isolated P3− anions with the closest P–P dis-
Fig. 4 A structural comparison of β-ThCu2P2 and α-MCu2P2 (An = Th,
U, Hf, Zr).26,32,69 (a) Two interpenetrating three-dimensional networks of
Fig. 3 (a) β-ThCu2P2 unit cell viewed down the b axis; (b) a single chain
of face-sharing ThP7 mono-capped trigonal prisms propagating along
the b axis; (c) single zigzag chains of corner- and edge-sharing CuP4
tetrahedra propagating along the b axis.
ThP7 mono-capped trigonal prisms and CuP4 tetrahedra in the structure
of β-ThCu2P2; (b) alternating layers of edge-sharing MP6 octahedra and
layers of edge- and corner-sharing CuP4 tetrahedra on the ab plane in
the structure of α-MCu2P2.
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Table 3 Selected interatomic distances (Å) for β-ThCu2P2
have two close neighbors with Ni–Ni distances of 2.565(2) Å
and Cu–Cu distances of 2.4885(6) Å. In comparison, the corres-
ponding Cu–Cu distances in BaCu2S2 are 2.714(5) Å, indicating
no significant metallic bonding.65 These variances could be
largely due to the different d electron counts of the transition
metals, where a higher electron count (Ru < Ni < Cu) leads to
weaker bonding interactions with neighboring atoms.67
The structure of β-ThCu2P2 is radically different from those
of the isostructural series MCu2P2 (An = Th, U, Hf, Zr),
denoted the α phase in the current discussion.26,32,69 As shown
in Fig. 4b, the structure of α-ThCu2P2 adopts a hexagonal
Th(1)–P(1) × 2
Th(1)–P(1)
2.9184(7)
2.9175(9)
3.0868(8)
3.1378(8)
2.3210(6)
2.399(1)
2.305(1)
2.300(1)
Cu(2)–P(2) × 2
Cu(2)–P(2)
Th(1)–Th(1) × 2
Th(1)–Cu(1)
2.3785(6)
2.414(1)
3.8726(2)
3.2774(5)
3.1777(5)
2.8846(7)
2.4885(6)
Th(1)–P(2) × 2
Th(1)–P(2) × 2
Cu(1)–P(1) × 2
Cu(1)–P(1)
Th(1)–Cu(2)
Cu(1)–Cu(1) × 2
Cu(2)–Cu(2) × 2
Cu(1)–P(2)
Cu(2)–P(1)
single chains running along the b axis (Fig. 3c), which is a very Ce2OP2-type atomic arrangement, which consists of alternating
common type of building unit observed in other reported layers of ThP6 octahedra and CuP4 tetrahedra along the
copper phosphide compounds.25,66 These CuP4 chains are c axis.32 Within the thorium phosphide layers, each ThP6 octa-
further connected to four other units by corner-sharing to hedron shares one edge with each of six other identical neigh-
construct a network of CuP4 tetrahedra (Fig. 4a). The resulting bors. The copper phosphide layers are constructed from zigzag
ThP7 and CuP4 networks are interconnected with each other single chains of CuP4 tetrahedra that are similar to those in
by sharing edges between these two types of polyhedra to form the β phase, which further connect via edge- and corner-
the overall complex three-dimensional structure.
sharing in the ab plane in the α phase. Overall, the structure of
Selected interatomic distances for β-ThCu2P2 are presented β-ThCu2P2 contains lower symmetric networks of ThP7 and
in Table 3. The average Th–P distances within the ThP7 mono- CuP4 polyhedra that are more closely packed in higher dimen-
capped trigonal prisms (3.029 Å) are shorter than those of sionalities than those in the α phase. For example, the closest
ThP8 polyhedra (3.086 Å) in α-ThNi2P2 with the same BaCu2S2- Cu–Cu distances (2.4885(6) Å) in the β structure are much
type structure (vide infra),67 as expected, but are a bit longer shorter than those in the α structure (2.924(4) Å), and the cal-
than those of ThP8 polyhedra (2.986 Å) in ThCuP2, despite culated density for the former compound (8.167 g cm−3) is
their smaller coordination numbers. This decrease in Th–P higher than that of the latter phase (7.692 g cm−3). It is noted
bonding in the BaCu2S2-type structure is compensated by that α-ThCu2P2 was prepared at temperatures above 1273 K,
more interactions between metal centers than those in the which is higher than the synthesis temperature for β-ThCu2P2
structure of ThCuP2. For example, the closest Th–Th and Th– (923 K). These results are consistent with the general trend
Cu(2) distances are 3.8726(2) Å and 3.1777(5) Å, which are observed in the literature regarding the characters of poly-
much shorter than those in ThCuP2. It is noted that the Th–Cu(2) morphs obtained at different temperatures, where, for a given
distances in β-ThCu2P2 are close to the sum of metallic radii of composition, the low-temperature form usually has a lower
elemental Th and Cu (3.071 Å).56 Following the same reason- symmetry and higher density than those of the high-tempera-
ing, the average Cu–P distances within Cu(1)P4 tetrahedra ture form.19,67,70
(2.336 Å) are shorter than those of Cu(2)P4 tetrahedra
ThCu5P3 adopts an orthorhombic YCo5P3-type40 structure.
(2.368 Å), whereas the interatomic distances between Cu and This new compound belongs to a large category of metal rich
the closest neighboring metal atoms are considerably longer systems with metal-to-nonmetal ratios equal to or close to
for Cu(1) than for Cu(2). For example, the Cu–Cu distances 2 : 1.53,66 Their structures have been often described from an
between two edge-sharing CuP4 tetrahedra are 2.8846(7) Å for anion centric perspective. In the current discussion, the struc-
Cu(1) and 2.4885(6) Å for Cu(2). The latter distances are ture of ThCu5P3 is described in a similar manner to the other
shorter than those found in elemental Cu (2.552 Å).56
two titled structures for the purpose of direct structural
The structure of β-ThCu2P2 is similar to, but not identical comparison. Its structure comprises one crystallographically
to those of intermetallic ThRu2P2 and α-ThNi2P2, which adopt unique Th, five Cu, and three P positions with the site sym-
distorted BaCu2S2-type structures.67,68 The overall atomic metry of .m. (Fig. 5a). The Th atom is bound to six P atoms in
coordination numbers decrease in the order: ThRu2P2
>
a trigonal prismatic arrangement (Fig. 5b). Each Cu(1–4) atom
α-ThNi2P2 > β-ThCu2P2 (BaCu2S2). For example, the Th atom is is coordinated by four P atoms in a tetrahedral geometry,
eight-, eight-, and seven-coordinate and the transition metal is whereas the atom Cu(5) is surrounded by five P atoms in a
five/four-, four-, and four-coordinate by P atoms in the struc- square pyramidal geometry (Fig. 5c). All P atoms are isolated
ture of ThRu2P2, α-ThNi2P2, and β-ThCu2P2, respectively. In P3− anions with the closest P–P distances of 3.702(2) Å.
addition, there are more P–P and metal–metal interactions in ThCu5P3 may be formally charge-balanced as Th4+(Cu+)5(P3−)3.
the Ru compound. P(2) atoms in ThRu2P2 form infinite zigzag Each Cu(1–4)P4 tetrahedron is connected in a similar fashion
chains with P–P distances of 2.587(9) Å, but no significant P–P to those in the structure of β-ThCu2P2 to form single zigzag
interactions exist in the Ni and Cu compounds. In ThRu2P2, chains along the b axis (Fig. 5c). These chains further share
Ru(2) atoms have four close Ru neighbors with Ru–Ru dis- edges or corners with six other units to construct a three-
tances of 2.844(3) Å and 2.969(3) Å, whereas the Ni(2) and dimensional network with triangular spaces along the b axis,
Cu(2) atoms in α-ThNi2P2 and β-ThCu2P2, respectively, only which are filled by isolated chains of face-sharing ThP6
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and REMnCu4P3 (Re = Gd–Ho).66 It is interesting to note that
ThCu5P3 represents the first example of a ternary Cu-contain-
ing compound that adopts the YCo5P3-type structure. No
ternary U or rare-earth metal analogues have been reported,
despite extensive studies in U/RE–Cu–P systems. This may
imply a different underlying chemistry for Th versus U/RE
systems. Compared with the structure of ThFe5P3,41 ThCu5P3
has shorter Th–P distances within the ThP6 trigonal prisms
(average 2.927 Å vs. 2.933 Å for Fe) but longer Th–Th distances
between two face-sharing prisms (3.8766(1) Å vs. 3.767 Å for
Fe). This result indicates stronger metal–metal interactions in
the Fe-based compound.
The strong influence of Cu on the structures and properties
Fig. 5 (a) ThCu5P3 unit cell viewed down the b axis; (b) a single chain of
face-sharing ThP6 trigonal prisms propagating along the b axis; (c) single of the UCunP2 (n = 0, 1, 2) system not only changes the environ-
zigzag chains of corner- and edge-sharing Cu(1–4)P4 tetrahedra and
single chains of edge-sharing Cu(5)P5 square pyramids propagating
along the b axis.
ments surrounding U cations, but also impacts the electronic
interplay between the U cations.27–30,73 Unlike the U–Cu–P
system, the existence of analogous “ThP2” has not been con-
firmed.19 However, “ThP2” was expected to be isostructural
with α-ThAs2, which consists of nine-coordinate Th by As
atoms and infinite zigzag polyarsenide (As−) chains.19 Using
Table 4 Selected interatomic distances (Å) for ThCu5P3
α-ThAs2 as a surrogate for the hypothetical “ThP2”, structural
Th(1)–P(1) × 2
Th(1)–P(2) × 2
Th(1)–P(3) × 2
Cu(1)–P(1) × 2
Cu(1)–P(1)
Cu(1)–P(2)
Cu(2)–P(1)
Cu(2)–P(2) × 2
Cu(2)–P(3)
Cu(3)–P(1) × 2
Cu(3)–P(2)
2.964(1)
2.920(1)
2.896 (1)
2.4322(9)
2.501(1)
2.423(1)
2.587(2)
2.3625(8)
2.414(1)
2.4714(9)
2.450(1)
2.370(1)
Cu(4)–P(2)
Cu(4)–P(3) × 2
Cu(4)–P(3)
2.364(1)
2.4394(8)
2.344(1)
2.361(3)
3.023(4)
2.564(3)
2.354(9)
2.82(2)
comparisons of α-ThAs2, ThCuP2, β-ThCu2P2, α-ThCu2P2, and
ThCu5P3 are presented in Fig. 6. As the Cu content increases
in the chemical formula, the coordination number of Th by P
decreases from nine in “ThP2”, eight in ThCuP2, seven in
β-ThCu2P2, to six in α-ThCu2P2 and ThCu5P3. Additionally, the
Th/P network gradually breaks down from three-dimensional
lattices in “ThP2”, ThCuP2, and β-ThCu2P2 to two-dimensional
layers in α-ThCu2P2 to one-dimensional chains in ThCu5P3. At
the same time, the valence of P anions and the connectivities
of anion substructures are reduced from P− chains in “ThP2”,
Cu(5A)–P(1)
Cu(5A)–P(2) × 2
Cu(5A)–P(3) × 2
Cu(5B)–P(1)
Cu(5B)–P(2) × 2
Cu(5B)–P(3) × 2
Th(1)–Th(1) × 2
Th(1)–Cu(2)
2.74(2)
3.8766(1)
3.1324(6)
2.635(4)
Cu(3)–P(3)
Cu(4)–Cu(5A)
4−
P2 dimers in ThCuP2, to solely monomeric P3− anions in
trigonal prisms (Fig. 5b) and isolated chains of edge-sharing
Cu(5)P4 square pyramids (Fig. 5c).
β-ThCu2P2, α-ThCu2P2, and ThCu5P3. In contrast, introducing
more Cu increases the connectivity between Cu/P polyhedra
such that there are more neighboring Cu atoms and closer
Cu–Cu distances. From a bonding perspective, increasing Cu
content in these compounds disrupts the Th–P and P–P inter-
actions while introducing more covalent Cu–P and metallic
Cu–Cu and Th–Cu bonding.
Temperature dependent electrical resistivity for a crystal of
β-ThCu2P2 was measured between 100 and 340 K. The resis-
tivity increases monotonically with temperature, which
suggests a metallic behavior (Fig. 7). The resistivity value of
β-ThCu2P2 at 300 K is approximately 0.05 mΩ cm. Previous
reports of resistivity data for thorium phosphides are very scarce
in the literature. Temperature dependent resistivity measure-
ments conducted on Th2Fe12P7 crystals showed a very similar
metallic trend and a resistivity value of about 0.25 mΩ cm at
300 K.21 In contrast, the transport properties of many of the
uranium and rare earth phosphides have been reported, most of
which show metallic or semimetallic conductivities. For example,
ferromagnetic UCu2P2 and UCuP2 show a steep increase of resis-
tivity up to the Curie temperature followed by either a shallower
increase27 or a slight decrease above this temperature,28 respect-
ively. The value of resistivity at room temperature is about 1 and
0.2 mΩ cm for UCu2P2 and UCuP2, respectively.
Selected interatomic distances for ThCu5P3 are shown in
Table 4. Th–P distances within the ThP6 trigonal prisms
(average 2.927 Å) are shorter than those of ThP7 and ThP8 poly-
hedra found in β-ThCu2P2 and ThCuP2 (see above). The closest
Th–Th distances are 3.8766(1) Å, comparable to those in
β-ThCu2P2. Cu–P distances in Cu(5)P5 square pyramids
(average 2.700 Å) are longer than those in Cu(1–4)P4 tetrahedra
(average 2.429 Å), as expected for a larger coordination
number. The latter Cu–P distances are much longer than the
values for CuP4 tetrahedra in β-ThCu2P2 (average 2.352 Å) and
ThCuP2 (average 2.356 Å). In contrast to the weaker Cu–P
bonding, there are more extensive interactions between Cu
and neighboring metal atoms in the structure of ThCu5P3 than
in the structures of β-ThCu2P2 and ThCuP2. For example, each
Cu(1–4) atom has three or four Cu with Cu–Cu distances
ranging from 2.635(4) Å to 3.037(5) Å. In comparison, in
β-ThCu2P2 each Cu atom has two Cu with Cu–Cu distances of
2.8846(7) Å and 2.4885(6) Å, whereas in ThCuP2 each Cu atom
has two Cu with Cu–Cu distances of 3.272(3) Å.
Versatile YCo5P3-type structures have been commonly
observed for metal-rich phosphides, including ThFe5P3,41
UMn5P3,71 REM5P3 (RE = rare-earth metal, M = Co, Fe),40,72
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Fig. 6 Structural comparisons of α-ThAs2,19 ThCuP2, β-ThCu2P2, α-ThCu2P2,32 and ThCu5P3. (a) α-ThAs2 featuring infinite As− chains: a three-
4−
dimensional close-packed network of ThAs9 tricapped trigonal antiprisms; (b) ThCuP2 consisting of P2 dimers: a three-dimensional network of
ThP8 bicapped trigonal prisms with open voids filled by Cu atoms; (c) β-ThCu2P2 with only P3− monomeric anions: a three-dimensional network of
ThP7 monocapped trigonal prisms with open voids filled by Cu atoms; (d) α-ThCu2P2 with only P3− monomeric anions: two-dimensional layers of
ThP6 octahedra separated by Cu atoms; (e) ThCu5P3 with only P3− monomeric anions: one-dimensional chains of ThP6 trigonal prisms separated by
Cu atoms. Pn–Pn bonds are highlighted in green. Cu–P bonds are omitted for clarity.
Fig. 8 (a) Density of states plot and (b) electronic band structure of
β-ThCu2P2 near the Fermi level.
Fig. 7 Temperature-dependent resistivity of
β-ThCu2P2 showing metallic behavior between 100 and 340 K.
a
single crystal of
formula of Th4+(Cu+)2(P3−)2. In comparison, EF in the band
structures of reported ternary uranium copper phosphides
including UCu2P2 is located within the U 5f bands, which is
strongly hybridized with P 3p states.29,74 This different behav-
Electronic band structure calculations for β-ThCu2P2 were ior of 5f band structures and altered atomic arrangements for
performed to understand the transport properties. The density β-ThCu2P2 and UCu2P2 may explain the discrepancies between
of states (PDOS) plot of β-ThCu2P2 is shown in Fig. 8a, and the their conducting properties.
band structure near the Fermi level (EF) is plotted in Fig. 8b.
The reaction outcomes of the Th/Cu/P system drastically
The calculation indicates that the β-ThCu2P2 system is metallic deviate from those of the U/Cu/P under similar experimental
with two bands crossing the Fermi level that is consistent with conditions, where all three titled Th phosphides display
the results from transport measurements. The total DOS different ion–ion interactions and electronic properties from
decreases around the Fermi level with the formation of a the U analogues reported. Similar divergence exists between
pseudo-gap, Fig. 8a. The filled states near EF mostly originate binary Th phosphides and U phosphides as well. Considering
from the Cu 3d and P 3p orbitals, whereas Th states are mostly that U phosphides have been more explored than Th phos-
found empty above EF. This is consistent with the simple phides, these observations are most likely not incidental. This
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is different from the chalcogenide chemistry, where Th(IV)
phases are mostly a subset of U compounds with mostly U(IV).
Conflicts of interest
For example, all Th sulfide binaries have isostructural U ana- There are no conflicts of interest to declare.
logues.20 The large deviation between the chemistry of ThIV
(f0) phosphides and U (f2 assuming UIV) phosphides can be
attributed to stronger hybridization between U 5f and P 3p
orbitals as suggested by electronic structure calculations,29,74
rather than different crystal radii, which could lead to more
significant covalent character in U–P bonds.75,76 This is con-
sistent with the higher symmetries of actinide coordination
environments and structures observed in ternary An/Cu/P
systems (square antiprismatic UP8 polyhedra in the tetragonal
structures of UCuP2 and octahedral UP6 polyhedra in the hexa-
gonal structure of UCu2P2) compared with those in Th com-
pounds (trigonal prismatic ThP6−8 polyhedra in the ortho-
rhombic structures of ThCuP2, β-ThCu2P2, and ThCu5P3). The
hybridization between actinide 5f and chalcogen p orbitals is
probably less significant, resulting in less participation of 5f
states in bonding in the actinide chalcogenides compared with
actinide pnictides. For this reason, less deviation between Th
and U chalcogenide chemistry is expected.
Acknowledgements
This work was performed at the Argonne National Laboratory
operated by UChicago Argonne LLC for the United States
Department of Energy under contract number DE-AC02-
06CH11357 and was supported by the DOE Office of Basic
Energy Sciences, Chemical Sciences, Heavy Elements
Chemistry. C. D. M. was supported by the IMSERC at
Northwestern University, which has received support from the
Soft and Hybrid Nanotechnology Experimental (SHyNE)
Resource (NSF NNCI-1542205), the State of Illinois and the
International Institute for Nanotechnology (IIN).
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Conclusions
Flexible pnictides provide a great system to study the systema-
tic behaviour of actinides; however, with the exception of U
pnictides, actinide pnictides chemistry is still in its infancy,
despite decades of research. In the current study, CuI-
mediated solid-state syntheses provide access to three new
thorium phosphides. This synthesis method could be a prom-
ising alternative route to prepare other heavier actinide pnic-
tides in the future. In comparison to binary thorium phos-
phides, these new compounds exhibit diverse ion–ion inter-
actions as a function of Cu content. As Cu content increases in
these compounds, covalent Cu–P and metallic Cu–Cu and Th–
Cu bonding also increases, resulting in the gradual dissolution
of Th–P and P–P interactions and networks. For example, Th–P
coordination numbers decrease from greater than eight in
binary polyphosphides to six–eight in ternary polyphosphides.
Increasing Cu content also causes disruption of the extensive
anion substructure into dimeric and monomeric anions. The
three compounds reported here for the softest member of the
tetravalent metal cations, ThIV, show very little structural and
electronic property similarity with other tetravalent metal pnic-
tides, including the U pnictides. This is attributed to the
8 J. L. Brown, S. Fortier, R. A. Lewis, G. Wu and T. W. Hayton,
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9 B. M. Gardner, G. Balázs, M. Scheer, F. Tuna,
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