Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal

Jun 3, 2015 - A basis set containing Ba(6s,5d,4f), Sr(5s,4d), Ni(4s,4p,3d), Cu(4s,4p,3d), and P(3s,3p) orbitals was employed for a self-consistent cal...
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Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal Structures of Unconventional Clathrates, AM2P4 (A = Sr, Ba; M = Cu, Ni) Juli-Anna Dolyniuk,† Jian Wang,† Kathleen Lee, and Kirill Kovnir* Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California 95616, United States S Supporting Information *

ABSTRACT: A new strontium nickel polyphosphide, SrNi2P4, was synthesized from elements and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the orthorhombic space group Fddd (No. 70), with Z = 8. The crystal structure is that of a clathrate type, composed of Ni8P16, 14-faced polyhedral cages that encapsulate Sr atoms. Together with the previously reported but unrecognized clathrate VII, BaNi2P4, and another previously reported clathrate, BaCu2P4, which is isostructural to SrNi2P4, a family of transition metal−phosphorus clathrates is represented. The crystal structures of each of the discussed transition metal-based clathrates are composed of unique polyhedra containing square faces. These structural fragments were predicted to be unstable for the conventional clathrates based on Si, Ge, and Sn. In this work, we report the crystal and electronic structures, chemical bonding, as well as the thermoelectric properties of this novel class of unconventional clathrates.

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INTRODUCTION Gas hydrates, also known as clathrates, are a fascinating class of compounds with a crystal structure composed of a tetrahedral water framework encapsulating guest atoms and molecules inside large polyhedral cages.1 Inorganic clathrates have similar crystal structures, comprised of covalently bound tetrahedral frameworks of heavier elements.2−4 The frameworks of the majority of inorganic clathrates are formed from group 14 elements, Si, Ge, or Sn, which are known as tetrels. Guest cations or anions can be encapsulated inside those cages (Figure 1 top), providing atomic vibrations that are essentially independent of the surrounding framework, making clathrates promising thermoelectric materials. Inorganic clathrates crystallize into six structure types, which can be described as a tiling of space using several types of polyhedra. For known tetrel clathrates, the polyhedra are composed of pentagonal and hexagonal faces (Figure 1 top).2−4 The crystal structure of the most common type I clathrate is composed of pentagonal dodecahedra and tetrakaidecahedra (Figures 1a, b). The stability of known and hypothetical tetrel clathrates was scrutinized by means of theoretical calculations, which predicted the lowest stability for clathrate structure types composed mainly of tetrel atoms with local coordinations that deviate furthest from a regular tetrahedral coordination.6 The highest distortion of this local coordination occurs for tetrel elements occupying vertices of the square faces of polyhedral cages. It was proposed that clathrates formed by polyhedra with square faces should be unstable, and indeed, no such tetrel clathrates have been reported.2−4,6 The energy landscape and stability of clathrates can be significantly modified by diverging from tetrel elements. Several groups have recently shown that type I clathrate frameworks © XXXX American Chemical Society

can be formed from a combination of late transition metals, Zn, Cd, Cu, and Au, along with a pnicogen, a group 15 element: P, As, or Sb.7−11 In this work, we discuss the synthesis, crystal structures, and properties of nickel and copper phosphide clathrates whose crystal structures feature polyhedra containing square faces. Experimental Section. Samples of AM2P4 (A = Ba, Sr; M = Ni, Cu) were prepared by solid-state reactions. All manipulations with the initial materials were performed inside an argon-filled glovebox (p(O2) ≤ 1 ppm). The starting materials, metallic barium (Sigma-Aldrich, 99.9%), metallic strontium (Sigma-Aldrich, 99.9%), nickel powder (Alfa Aesar, 99.996%), copper powder (Sigma-Aldrich, 99.9%), red phosphorus (Alfa Aesar, 99%), NiCl2 (Alfa Aesar, 99%), and BaBr2 (Alfa Aesar, 99%) were used as received. Various attempts to synthesize ANi2P4 samples using either NiCl2 as a source of Ni and/or BaBr2 as a reaction flux were not successful. The best method of synthesis was a reaction of pure elements. Samples of SrNi2P4 and BaNi2P4 were synthesized by stoichiometric solid-state reactions of the elements in glassy carbon crucibles sealed in evacuated silica ampules. These samples were heated over 17 h to 1123 K and annealed at this temperature for 140 h. Subsequently, these samples were opened in an Ar glovebox, finely ground, resealed, and reannealed in a similar manner as before. This was repeated 2−3 times until nearly pure phase samples were obtained. Samples of BaCu2P4 were synthesized using the same heating rate and annealing time as ANi2P4, but at a temperature of 1100 K. As with the Ni samples, after annealing, the samples were Received: April 29, 2015 Revised: June 3, 2015

A

DOI: 10.1021/acs.chemmater.5b01592 Chem. Mater. XXXX, XXX, XXX−XXX

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Figure 1. Convex polyhedra present in conventional clathrates (top) and transition metal phosphorus-based clathrates (bottom). Pictograms of the number and types of polyhedral faces are presented below their respective polyhedra. For e and f, Schlegel diagrams are also shown. In the literature, the (e) truncated octahedron is referred to as a “Kelvin cell” and the (f) polyhedron is referred to as a “twisted Kelvin cell”.5

Systematic extinctions clearly indicated Fddd (No. 70) as the only possible space group. An analytical absorption correction was applied using face-indexing of the crystal. The solution and refinement of the crystal structure were carried out using the SHELX suite of programs.12 The structure was solved in the Fddd space group and the final refinement was performed using anisotropic atomic displacement parameters for all atoms. A summary of pertinent information relating to unit cell parameters, data collection, and refinement is provided in Table 1 and the atomic parameters and interatomic distances are provided in Tables 2 and 3.

cooled down, opened in an Ar glovebox, ground, then resealed and reannealed under the same conditions as their initial annealing. Three such annealings produced black, uniform powders of BaCu2P4. X-ray Powder Diffraction and Elemental Analysis. The samples were characterized by powder X-ray diffraction (XRD) using a Rigaku Miniflex 600 diffractometer employing Cu-Kα radiation. A few low intensity diffraction peaks corresponding to binary nickel and copper phosphides were present. The intensities of these peaks decreased after SPS sample treatment (Figure S1 in the Supporting Information). Elemental analysis of selected single crystals was carried out on a Hitachi S4100T scanning electron microscope (SEM) with energy-dispersive Xray (EDX) microanalysis (Oxford INCA Energy) (Figure S3 in the Supporting Information). EDX analysis of single crystals of AM2P4 confirmed the presence of only A, M, and P in the samples, A = Sr or Ba and M = Ni or Cu. Differential Scanning Calorimetry. The thermal behavior of AM2P4 was characterized using a Netzsch 409 TG/DSC Differential Scanning Calorimeter. DSC measurements were performed using small evacuated and sealed silica ampules with enough sample to cover the base of the ampule (approximately 30−50 mg). Samples of ANi2P4 were ramped to 673 K at a rate of 10 K/min, and then heating was slowed to 5 K/min from 673 to 1273 K. The reverse was used for the cooling curve with an initial cooling rate of 5 K/min from 1273 to 673 K, and a faster cooling rate of 10 K/min back to room temperature. For BaCu2P4 samples, a uniform heating and cooling rate of 5 K/ min was used. Powder X-ray diffraction was subsequently used on all samples to determine the products of DSC thermal treatment. The powder X-ray diffraction pattern of DSC treated BaCu2P4 sample is presented in Figure S2 in the Supporting Information. Single-Crystal X-ray Diffraction. Data for SrNi2P4 was collected at 90 K using a Bruker AXS SMART diffractometer employing Mo-Kα radiation with an APEX-II CCD detector. The data set was recorded as ω-scans with a 0.3° step width and integrated with the Bruker SAINT software package. The data set was indexed in an orthorhombic face-centered unit cell.

Table 1. Data Collection and Structure Refinement Parameters for SrNi2P4a space group T (K) a (Å) b (Å) c (Å) V (Å3) Z λ (Å)

Fddd (No. 70) 90(2) 5.1928(2) 9.5598(4) 18.9575(8) 941.09(7) 8 0.71073

ρ (g cm−3) μ (mm−1] θ (deg) data/params R1 wR2 goodness-of-fit diff. peak and hole (e/Å3)

4.64 20.385 4.30 < θ < 30.49 363/19 0.010 0.026 1.22 0.40 and −0.28

a

Further details of the crystal structure determination may be obtained from Fachinformations-zentrum Karlsruhe, D-76344 EggensteinLeopoldshafen, Germany, by quoting the depository number CSD429359.

Table 2. Atomic Coordinates and Equivalent Displacement Parameters for SrNi2P4 atom Sr Ni P

x/a

site 8b 16g 32h

1

1

1

/8 /8 0.06351(6)

z/c

Ueqa

/8 0.00541(2) 0.06867(2)

0.00460(8) 0.00332(8) 0.00349(8)

y/b

1

/8 /8 0.31813(3)

5

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. B

DOI: 10.1021/acs.chemmater.5b01592 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Table 3. Selected Interatomic Distances and Angles for SrNi2P4 atoms

distance (Å)

atoms

distance (Å)

atoms

angle (deg)

Ni−P Ni−P Ni−Ni P−P P−P

2.2100(4) × 2 2.2245(3) × 2 2.7275(1) × 2 2.2293(7) 2.2394(7)

Sr−Ni Sr−P Sr−P

3.4468(2) × 4 3.1200(3) × 4 3.1382(3) × 4

∠ P−Ni−Pmin* ∠ P−Ni−Pave* ∠Ni−Ni−Ni ∠ P−P−P

101.09(2) 109.41(3) 171.37(2) 104.50(1)

*

Minimum and average angles are represented by min and ave, respectively.

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Sample Densification. The polycrystalline samples of AM2P4 were carefully ground into a fine powders in an Ar glovebox, after which they were packed into graphite dies and sintered at 873 K through spark plasma sintering (SPS, Dr. Sinter Lab Jr. SPS-211Lx, Sumitomo Coal Mining Co, Ltd.). Samples were compressed with an initial uniaxial pressure of 50 MPa, gradually heated to 773 K over 7 min, and then finally heated to 873 K over 3 min. At the maximum temperature, a uniaxial pressure of 160 MPa was applied and held for 10 min to form pellets with dimensions of ∼Ø 5 mm × 1 mm. The geometrical densities of the pellets were 92% for BaCu2P4, 94% for BaNi2P4, and 95% for SrNi2P4. The graphite foil and any possible surface contaminations were removed by polishing. The samples’ purities after SPS were checked by PXRD and no sample decompositions or phase transitions were detected (Figure S1 in the Supporting Information). Physical Properties. The physical properties of all samples were measured in the temperature range of 2−300 K using a commercial multipurpose Physical Properties Measurement System (PPMS, Quantum Design). The electrical resistivities were measured by a standard four-point alternating-current technique to exclude the resistance of the leads. The Seebeck thermopowers and thermal conductivities were measured using the Thermal Transport Option. Quantum-Chemical Calculations. Density functional band structure calculations using a full potential all-electron local orbital code FPLO (version fplo7.00−28) within the local density approximation (LDA) were performed.13,14 The Perdew−Wang parametrization of the exchange-correlation potentials was employed. The density of states (DOS) and band structures were calculated after a convergence of the total energies on a dense k-mesh with 32 × 32 × 32 points, with 4505 irreducible k-points (SrNi2P4 and BaCu2P4) and with 40 × 40 × 40 points, with 4531 (tetragonal BaNi2P4) and 8631 (orthorhombic BaNi2P4) irreducible k-points. Additionally, electronic structure calculations and bonding analyses were carried out using the tight binding−linear muffin tin orbitals− atomic sphere approximation (TB−LMTO−ASA) program package.15 The Barth−Hedin exchange potential was employed for LDA calculations. The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. A basis set containing Ba(6s,5d,4f), Sr(5s,4d), Ni(4s,4p,3d), Cu(4s,4p,3d), and P(3s,3p) orbitals was employed for a self-consistent calculation, with downfolded Ba(6p), Sr(5p,4f), and P(3d) functions. For each bonding analysis, the energy contributions of all electronic states for selected atom pairs were evaluated with crystal orbital Hamilton population (COHP) analysis.16 Integration over all filled states yielded −ICOHP values as measures of relative overlap populations. The electron localization function (ELF, η)17−20 was evaluated with the modules implemented within the TB−LMTO−ASA program package, and the ParaView program was used for visualization of ELF isosurfaces.21,22

RESULTS AND DISCUSSION Synthesis and Thermal Stability. A systematic search for new clathrates based on Cu/P and Ni/P frameworks resulted in the discovery of SrNi2P4. Initially, SrNi2P4 was found as a product of a solid-state reaction of elements in a 1:2:4 ratio at 1123 K in a carbonized, evacuated silica ampule. SrNi2P4 was present in the products together with the impurity phases SrP3 and SrNi10P6. We also attempted much shorter single annealing reactions utilizing NiCl2 and excess Ba to produce highly reactive metallic Ni in situ. Samples containing significant amounts of SrNi2P4 were also synthesized at 1073 K using NiCl2 precursor as a source of Ni and BaBr2 as a flux, but the target phase was always contaminated with substantial amounts of another ternary phase, SrNi10P6. Differential scanning calorimetry (DSC) was used to study the thermal stability of SrNi2P4 (Figure S4 in the Supporting Information). It revealed the melting and/or decomposition of SrNi2P4 occurs at 1139(5) K. Two crystallization peaks were detected upon cooling of the melt at 1126(5) and 1038(5) K. Powder XRD of the melted sample shows that a complete conversion of SrNi2P4 into a mixture of SrP3 and SrNi10P6 occurs as a result of DSC heat treatment. For the incongruently melted SrNi2P4 phase, the best method of synthesis appears to be several annealings of small batches of stoichiometric mixtures of elements at 1123 K with intermediate sample regrinding and reannealing. To compare the thermoelectric properties of the new phase to two other related compounds, we also synthesized samples of the previously reported phases BaNi2P423 and BaCu2P4.24 BaNi2P4 samples were produced by starting from a stoichiometric mixture of elements using the same temperature as was used for SrNi2P4 and a sequence of three annealings, with sample grinding in between subsequent annealings. The thermal stability of BaNi2P4 was also investigated. According to the DSC data coupled with subsequent powder XRD of the measured sample, BaNi2P4 melts at 1144(5) K (Figure S4 in the Supporting Information). The crystallized melt contains mostly Ba3P14 and BaNi10P6 together with BaNi2P4. The synthesis of BaCu2P4 was hampered by the existence of the type I clathrate compound of a similar composition, Ba8Cu16P30 = BaCu2P3.75.8,9 Slow crystallization of the melt or annealing of a sample with the nominal composition BaCu2P4 resulted primarily in the formation of BaCu2P3.75. A detailed DSC investigation revealed that BaCu2P4 is a low-temperature phase, which transforms to the clathrate-I, BaCu2P3.75, at high temperatures (Figure 2). A polycrystalline sample of BaCu2P4 was obtained by annealing the stoichiometric mixture of elements at 1100 K three times (Figure S1 in the Supporting Information). Upon heating, two endothermic peaks at 1107(5) and 1139(5) K were observed. The second endothermic peak temperature is close to the melting temperature of the BaCu2P3.75 clathrate-I, 1134 K.9 We hypothesize that the first peak, which has a significantly lower C

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Crystal Structure. SrNi2P4 has a clathrate-like crystal structure (Figure 3), which has no analog in gas hydrate chemistry.1 The crystal structure can be described as a tiling of space with Sr@Ni8P16 24-vertex polyhedra. This differentiates SrNi2P4 from tetrel clathrates, as it does not contain the pentagonal dodecahedra, which are a key component in the crystal structures of all tetrel clathrate types.2−4 In the crystal structure of SrNi2P4, each Ni atom is surrounded by four phosphorus atoms, forming a distorted tetrahedron. The NiP4 tetrahedra share common edges, forming infinite ∞1[NiP2] chains. The Ni−Ni distances between the centers of the tetrahedra, 2.73 Å, are similar to those in other ternary Sr−Ni phosphides, like SrNi10P625 and SrNi2P226 (2.50−2.72 Å). Each phosphorus atom in the NiP2 chain is connected to two other phosphorus atoms at distances of 2.23−2.24 Å, which are typical for single covalent P−P bonds.27−31 The phosphorus atoms form infinite one-dimensional chains, ∞1(P1−) (Figure 3b), similar to those found in alkali metal monophosphides, as well as in ZnP2 and CdP2.27,28,32 The Ni−P distances in SrNi10P625 and SrNi2P226 range from 2.17 to 2.55 Å, which is similar to the Ni−P distances in SrNi2P4, 2.21−2.22 Å. Note that the distances in the Ni−P clathrate framework are much shorter than those in traditional Si clathrates with Si−Si distances above 2.3 Å.2−4 In the crystal structure of SrNi2P4, four Sr@Ni8P16 polyhedra are fused together around the infinite phosphorus chains (Figure 3c). Assuming that Sr has a +2 oxidation state, the synthesized compound can be described as (Sr2+)(Ni1+)2(P1−)4 with Ni in a formal d9 state. SrNi2P4 is isostructural to BaCu2P418 where Cu1+ has a completely filled d-electron shell. In both structures there are relatively short metal−metal distances (2.73 Å for Ni−Ni and 2.90 Å for Cu−Cu). To understand the chemical bonding, we applied quantumchemical calculations (vide infra). In SrNi2P4, Ni8P16 polyhedra are formed from 4 square, 4 pentagonal, and 6 hexagonal faces (Figure 1f), [44.54.66] polyhedra by standard polyhedral notation. According to

Figure 2. Differential scanning calorimetry plot of BaCu2P4 is shown with the temperature stability regions for BaCu2P4 and the type I clathrate, BaCu2P3.75, highlighted in blue and pink, respectively.

intensity, corresponds to the phase transformation BaCu2P4 → BaCu2P3.75 + 0.25P. Several annealing experiments were carried out to verify this hypothesis (Figure S2 in the Supporting Information). Two phase-pure samples of BaCu2P4 were annealed at 1223 K for 10 h, i.e., above the melting temperature, one was quenched in water, and another was slowly cooled down to room temperature in the furnace. Powder X-ray diffraction indicates that the main phase in the quenched sample was BaCu2P3.75 with a small admixture of BaCu2P4. In turn, the slow-cooled sample was an equimolar mixture of both phases as shown in Figure S2 in the Supporting Information. These results demonstrate that the type I clathrate phase, BaCu2P3.75, is the high temperature phase that is stable over a small temperature interval, whereas BaCu2P4 is stable at lower temperatures. This is additionally supported by a previous report that the BaCu2P3.75 clathrate cannot be synthesized from elements by annealing at 1073 K, with the formation of BaCu2P4 observed instead.8 We have confirmed these findings; the cleanest samples of BaCu2P4 were obtained by annealing the elements at 1100 K.

Figure 3. Crystal structures of SrNi2P4 and BaNi2P4 are shown on the left and right, respectively: (a, d) general views; (b) a phosphorus chain and (c) polyhedra around the chain in the crystal structure of SrNi2P4; as well as (e) a Ni chain and (f) polyhedra around the chain in the crystal structure of BaNi2P4. Ba and Sr, blue; Ni, black; P, yellow. D

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from 109.5°. Tetrel elements prefer to be tetrahedrally coordinated which explains why clathrates VI and VII are considerably less stable than other clathrate types. Unlike tetrels, late transition metals and phosphorus are more structurally flexible and are known to form a variety of fragments with local coordinations that are quite different from the regular tetrahedral coordination. Thus, phosphorus is able to form stable polyphosphides with square and triangular fragments where ∠P−P−P angles are 90 and 60°, correspondingly.27,28,36−38 Because of this flexibility, greater structural diversity is expected for transition metal−phosphorus clathrates. Among the heavier transition metals of groups 10 and 11, only Pd was reported to form a clathrate VII structure isostructural to BaNi2P4.39 No AM2P4 phases are reported for Ag and Pt, but we recently reported a BaAu2P4 compound.32 Similar to SrNi2P4 and BaCu2P4, BaAu2P4 contains infinite 1 1− ∞ (P ) chains. However, Au prefers to be linearly coordinated by only two phosphorus atoms, resulting in the formation of a layered crystal structure that is different from clathrate-like structures. Electronic Structure. The band structure calculation results obtained from two different methods were similar. The results of FPLO are shown in Figure 4. According to calculations, all AM2P4 phases exhibit metallic properties with

O’Keeffe and Delgado−Friedrichs, this is one out of 10 possible 14-faced polyhedra capable of tiling space isohedrally; the polyhedra tiling code is ISX.5 They systematically addressed the question formulated by Lord Kelvin in 1887 concerning the, “division of space with minimum partitional area”.33 The crystal structures of SrNi2P4 and BaCu2P4 represent the experimental implication of one type of predicted space partition. BaCu2P4 can be transformed upon heating into the conventional type I clathrate, BaCu2P3.75. This is supported by a theoretical analysis of the possible topological paths for the transformation of the 24-vertex tetrakaidecahedron (Figure 1b) present in type I clathrates into the [44.54.66] polyhedron.34 The type I clathrate is another example of space partitioning with two different types of polyhedra, pentagonal dodecahedra and tetrakaidecahedra (Figure 1a, b). The third member of this family of unconventional clathrates, BaNi2P4, crystallizes in a distorted sodalite structure type (Figure 3d).23 The sodalite polyhedron is a truncated octahedron, [46.68], which is the simplest solution to Lord Kelvin’s question (Figure 1e). It is expected that more ways of space tiling are to be realized in the crystal structures of unconventional clathrates that are based on pnicogens and transition metals. BaNi2P4 crystallizes into two modifications, one tetragonal and the other orthorhombic.23 Both are distorted variants of the cubic sodalite framework. In the crystal structures of both modifications, ∞1[NiP2] chains are present, which are similar to the chains found in SrNi2P4. In contrast to SrNi2P4, in the crystal structure of BaNi2P4, the phosphorus atoms form isolated (P4)4− squares which connect Ni chains to form a 3D framework (Figure 3e). In the tetragonal modification, there is only one Ni−Ni distance of 2.88 Å, whereas in the orthorhombic modification there are two alternating Ni−Ni distances of 2.77 and 3.02 Å. The Ni−Ni bonding in both modifications of BaNi2P4 as well as possible driving forces for the tetragonal-to-orthorhombic transition were addressed by Hoffmann et al.35 Transition metal-based clathrates exhibit high flexibility in their three-dimensional frameworks. For example, the Ni−P framework adapts to accommodate guest cations of different sizes. Larger Ba cations are encapsulated inside larger truncated octahedral cages, V = 126 Å3, in the structure of BaNi2P4 (Figure 1e), whereas smaller Sr cations are situated in the smaller twisted Kelvin cells, V = 124 Å3, in the structure of SrNi2P4 (Figure 1f). Another way to tune the framework topology is by changing the framework size: substitution of larger Cu atoms [d(Cu−P) = 2.33−2.37 Å] for smaller Ni atoms [d(Ni−P) = 2.21−2.22 Å] results in an increase of the average volume of the 24-vertex cages. Cu8P16 truncated octahedra appeared to be too large for the Ba cations, and in the crystal structure of BaCu2P4, the Ba atoms are encapsulated in twisted Kelvin cells, V = 139 Å3. Cage volumes were calculated from single-crystal data for all three compounds collected at 90 K. According to gas hydrate classifications, the sodalite-type framework corresponds to clathrate VII.1 No known tetrelbased clathrate crystallizes in the clathrate VII type. Moreover, a detailed theoretical analysis of the stability of tetrel clathrates with basic clathrate hydrate structures predicts the lowest stability for the type VI and VII clathrates, which are the only clathrate types formed by polyhedra with square faces.6 Atoms situated at the corners of the square faces exhibit the highest deviations from ideal tetrahedral coordination with angles far

Figure 4. Density of states (DOS) diagrams for AM2P4. The contributions from A, M(3d), M(4s), P(3p), and P(3s) are shown in green, red, dashed magenta, blue, and dashed purple lines, respectively. E

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atoms. This suggests the absence of strong metal−metal bonding in the investigated compounds. Similar results were obtained for LaM2Ge2 (M = Ni, Cu) intermetallic compounds.40 To further characterize metal−metal bonding, we applied COHP analysis. As expected, strong bonding was revealed for all M−P and P−P interactions with integrated values in the range of 1.8−3.3 eV/bond (Figure S5 in the Supporting Information). M−P and P−P bonds are essentially optimized with nonbonding or weakly antibonding states at the Fermi level. The COHP diagrams for the M−M bonding are shown in Figure 6. All M−M interactions are significantly weaker than

nonzero DOS at the Fermi level. Ba and Sr orbitals contribute mainly to the states in the conduction band at energies > +1 eV. For Ni-containing phases, the states in the vicinity of the Fermi level are mainly comprised of Ni(3d) and P(3p) orbitals with small contributions from P(3s) and Ni(4s) orbitals. This is in line with the formal description of Ni1+ with a 3d9 electron configuration, since Ni(3d) orbitals are not completely filled. High electrical conductivity and low Seebeck thermopower are expected for the Ni-containing compounds. The DOS for BaCu2P4 is different. The Fermi level is situated in a pseudogap. Cu(3d) orbitals contribute in a smaller extent to the states near the Fermi level, which is in agreement with a formal Cu0 3d10 configuration. Therefore, a higher resistivity and Seebeck thermopower are expected for BaCu2P4 when compared to the Ni-containing compounds. In the AM2P4 crystal structures, there are relatively short (