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Synthesis, Crystal Structure, and Properties of Three La-ZnP Compounds with Different Dimensionality of Zn-P Framework Jian Wang, Philip Yox, Jackson Voyles, and Kirill Kovnir Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00445 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Crystal Growth & Design

Synthesis, Crystal Structure, and Properties of Three La-Zn-P Compounds with Different Dimensionality of Zn-P Framework

Jian Wang,a,b,* Philip Yox,a Jackson Voyles,a Kirill Kovnira,b

a

Department of Chemistry, Iowa State University, Ames, IA 50011, United States

b

Ames Laboratory, U.S. Department of Energy, Ames, IA 50011, United States

This paper is dedicated to Professor Xin-Tao Wu in honor of his 55th anniversary of independent career

ACS Paragon Plus Environment

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Abstract Two novel ternary compounds in La-Zn-P system, La2Zn11P9 and La7Zn2P11, were synthesized via high-temperature transport reactions. The crystal structures for both compounds were established by means of single crystal X-ray diffraction. Complex three-dimensional (3D) crystal structure of metal-rich La2Zn11P9 is composed of a Zn-P framework with large channels accommodating four atomic columns of La atoms. The isolated columns of La atoms alternating with Zn-P tetrahedral chains and disordered P3 chains, resembling polyacene fragments, build up the crystal structure of phosphorus-rich La7Zn2P11. The previously reported La3Zn2−xP4 compound with intermediate phosphorus content has a two-dimensional (2D) structural motif composed of Zn2P2 and La3P2 layers. A structural dimensionality reduction from 3D-La2Zn11P9 to 2D-La3Zn2-xP4 to 1D-La7Zn2P11 is due to both the flexibility of Zn-P framework with ZnP4 tetrahedra and ZnP3 planar building units and ability of phosphorus to form homonuclear bonds and polyatomic phosphorus chains. A polycrystalline sample La3Zn1.75P4 was purified by high temperature solid-state method. The electron counting rules and computations predict the n-type metallic nature of La3Zn1.75P4. The transport properties tests performed on a sintered pellet of La3Zn1.75P4 confirm its metallic behavior with negative thermopower indicating that the major carriers are electrons. La3Zn1.75P4 exhibits moderate thermal conductivity, 4.5 Wm−1K−1 at 300 K, where lattice thermal conductivity has the dominating contributions.


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Introduction Phosphides of the two different types of metals, rare-earth and transition metals, exhibit rich structural chemistry and diverse portfolio of properties implying application in the fields of superconductivity,[1-3] magnetocalorics,

[16-18]

semiconductors,[4-6]

thermoelectrics,[7-14]

electrocatalysts,[15]

and energy storage materials.[19,20] Our research is focused on new

materials based on complex phosphides of low-cost and non-toxic metals, including Zn and Cu, and electropositive cations of alkaline-earth or rare-earth elements

[21-23]

. We discovered two

novel ternary compounds in unexplored La-Cu-P system, which exhibits low thermal conductivity.[21] Our investigations of the La-Zn-P system reveal the flexibility of Zn-P frameworks, which are capable of hosting both isolated rare-earth cations[24] and onedimensional chains of X@R4 tetrahedra, X = Cl, Br; R = La, Ce.[25] Prior to our research efforts, only two ternary La-Zn-P compounds, La5Zn2−xP6 and La3Zn2−xP4, were reported.[26, 27] Those two compounds show high structural similarities because their layered crystal structure can be described as alternating Zn2P2 layers of edge-shared ZnP4 tetrahedra and La-P layers composed of fused P@La6 octahedra. We have recently reported another compound with three-dimensional Zn-P framework, La4Zn7P10.[24] During the exploration of the La-Zn-P system, two novel ternary compounds with three- and one-dimensional Zn-P frameworks, La2Zn11P9 and La7Zn2P11 were discovered. The synthesis details and crystal structure for La2Zn11P9 and La7Zn2P11 are reported in this work together with electronic structure and thermoelectric properties for previously reported two-dimensional La3Zn1.75P4.



Experiments Synthesis. To avoid possible contamination of foreign elements or oxidization, the preparation and handling of all samples were carried out in a glovebox (argon atmosphere) with the O2 level not exceeding 0.5 ppm. All starting materials are commercial grade without further purification: ZnCl2 powder (Sigma Aldrich, anhydrous, 99+%), ZnBr2 powder (Sigma Aldrich, anhydrous, 99+%), red P powder (Alfa Aesar, 99%), La filings (Alfa Aesar, 99.9%), and Zn shot (Alfa Aesar, 99.8%). La2Zn11P9: A La2Zn11P9 single crystal was first discovered in a transport reaction attempting to synthesize La3Zn4P6Br [25]. This synthesis utilized ZnBr2 as both a transport agent and a source of zinc. The starting materials ratio was 1:5:6:0.15 for La:Zn:P:ZnBr2. These reagents were mixed together and placed and sealed in an evacuated silica ampoule. The ampoule was heated quickly at a rate of 100 K/h from 300 K to 773 K and annealed for 24 h. Following this annealing the ampoule was heated to 1073 K within 10 hours and annealed for another 144 h. Then the ampoule was cooled down to 773 K at a rate of 12.5K/h and held at this temperature for 96 h and afterwards the furnace was shut off. The main products came out as amorphous powder and binary La and Zn phosphides with a large amount of visible black needle-like crystals found as side products. Single crystal X-ray diffraction on selected crystals revealed La2Zn11P9 composition without presence of Br. The elemental ratios were further verified by energydispersive X-ray (EDX) microanalysis as La/Zn/P=2/10.9(1)/8.2(2) when normalized to 2 La atoms (Table S1). The high discrepancy in the content of light elements, like P, is common for this method.[24, 25] No incorporation of Br was found in the crystals. Some other transport agents such as I2 and ZnI2 were also found useful for La2Zn11P9 crystals growth. An iodine-assisted reaction was performed using loading ratio 2:11:9:0.15 for La:Zn:P:I2 and the aforementioned


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temperature profile. Similar larger black needle-like crystals of La2Zn11P9 were collected in the reaction with a significantly improved yield. Various conditions attempting to synthesize single phase of polycrystalline La2Zn11P9 sample were unsuccessful. La3Zn2P4: La3Zn2-xP4 was reported to have two distinct compositions, La3Zn1.88P4 and La3Zn1.75P4.[26] We only succeeded in synthesis of single-phase sample of La3Zn1.75P4. Conventional high-temperature solid state reaction of elements resulted in a single-phase polycrystalline sample of La3Zn1.75P4. The La/Zn/P elements were loaded in 3:1.75:4 ratio into carbonized silica ampoules, which were subsequently evacuated and sealed. The ampoules were heated from 300 K to 1073 K at a rate of 47 K/h and then held at this temperature for 144 hours and afterwards the furnace was shut off. After the furnace cooled to room temperature, the samples were ground in a mortar and reloaded into new ampoules in the glovebox, resealed, and reheated using the same temperature profile as the first annealing. The same reaction procedure was repeated for a third time, and after the third cycle the fine polycrystalline powder of La3Zn1.75P4 was shown to be almost single-phase with small amounts of unidentified admixture through powder X-ray diffraction (Figures S1). All synthetic attempts aimed at La3Zn2−xP4 samples with x = 0.12, 0.5, 0.6, and 0.75 using similar or different synthetic schemes resulted in significant amounts of admixtures. La7Zn2P11: Single crystals of La7Zn2P11 were first discovered in a flux reaction initially aimed at synthesis of “LaZnP3” via loading La:ZnCl2:P in a 3:2:6: ratio, using ZnCl2 as a flux and source of zinc. This mixture was loaded in a carbonized silica ampoule, evacuated, and flamesealed. The ampoule was heated from 300 K to 1073 K within 20h, and then annealed for 144 h at 1073 K. Then the ampoule was quickly cooled down to room temperature at a rate of 20 K/h. Many tiny black needle-like crystals were found as main products coming out together with



binary Zn phosphides, amorphous powder, and LaCl3. The La7Zn2P11 composition without presence of Cl was revealed by applying single crystal X-ray diffraction on selected crystals. The chemical ratios in the sample were confirmed by the energy-dispersive X-ray (EDX) microanalysis as La/Zn/P=6.8(1)/2/9.0(1) when normalized to 2 Zn atoms (Table S2). No chlorine was detected in the crystals. The La7Zn2P11 crystals were also detected several times upon synthesis of La4Zn7P10

[24]

using a transport agent. In such cases, La7Zn2P11 was a minor

phase as compared to La4Zn7P10. A single-phase polycrystalline synthesis of La7Zn2P11 was not successful via conventional solid-state reaction of elements with various conditions. X-Ray Powder Diffraction. A Rigaku Miniflex 600 powder diffractometer employing Cu-Kα radiation with a Ni-Kβ filter was applied to characterize the phase purity of the samples. Elemental Analysis. A Hitachi S4100T scanning electron microscope (SEM) with energydispersive X-ray (EDX) microanalysis (Oxford INCA Energy) was applied to characterize elemental composition of selected crystals.

The EDX analysis serves to validate the

incorporation or absence of flux elements and silicon into the samples. The EDX measurements confirmed the presence of only La, Zn, and P without incorporation of any foreign elements in the crystals of La2Zn11P9 and La7Zn2P11 (Table S1 and S2). Single Crystal X-ray Diffraction. Selected single crystals were mounted on a Bruker AXS SMART diffractometer with an APEX-II CCD detector with Mo-Kα radiation to collect single crystal diffraction at 90 K. The data sets were collected as ω-scans with a 0.4° step width and integrated with the Bruker SAINT software package.[28] Multi-scan absorption corrections were applied.[28] The SHELX suite of programs was used to refine and solve the crystals structures.[29] For all but the disordered atomic positions, anisotropic atomic displacement parameters were


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used for the final refinements. (Table S3). A summary of data collection, selected crystal data, and refinements is provided in Table 1 and the coordination parameters and selected interatomic distances are listed in Tables S2 and S3. Further details of the crystal structure determination may be obtained from Fachinformationszentrum Karlsruhe, Germany, by quoting the depository number CSD-434304 (La2Zn11P9) and CSD-434305 (La7Zn2P11). During the refinement of La2Zn11P9, a large non-negligible residual electron density peak was observed at the distance of 0.65(4) Å from La1 atomic position. This peak was refined as partially occupied La11 position with constraints of equivalent atomic displacement parameters (ADPs) and the total occupancy of La1 and La11 to be 100%. The residual electron density peaks were also found in the proximity of Zn9 and Zn12 atomic positions. Each such position was refined as split to three partially occupied Zn sites, (Zn9A, Zn9B, Zn9C) and (Zn12, Zn13, Zn14) with constrained ADPs and a total occupancy of 100%. An abnormally high ADPs of P3 was observed. The refining site occupancy for P3 site reduces ADP back to normal range and resulted in an occupancy of 95.4(5)%. Without such handling of the disorder significantly higher R-values and difference electron density peaks were observed. To ensure that this is not due to low crystal quality we run several experiments on crystals collected from different synthetic batches. All tested crystals were prone to disorder. The refinement of the best crystal is reported in Tables 1, S3, and S4. The refinement of La7Zn2P11 resulted in all atoms fully occupied with reasonable ADPs except P1. The refining site occupancy of P1 revealed its half-occupied nature, occupancy of 0.51(2). The details of phosphorus atoms substructure in La7Zn2P11 are discussed in results and discussion part. Table 1. Selected crystal data and structure refinement parameters for La2Zn11P8.95 and La7Zn2P11.



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Empirical formula

La2Zn11P8.95

La7Zn2P11

ICSD-number CCDC-number Formula weight Temperature Radiation, wavelength Crystal system Space group, Z Unit cell

434304 1831895 1274.07g/mol

434305 1831896 1443.78g/mol

90(2)K Mo-Kα, 0.71073 Å Monoclinic P21/m (No. 11), 2 C2/m (No.12), 2 a = 9.838(2) Å a = 29.790(2) Å b = 4.021(1) Å b = 4.135(3) Å c = 21.643(5) Å c = 7.775(5) Å β = 98.676(3)° β = 103.11(1)°

Unit cell volume Density (calc.) /g cm-3

846.3(4) Å3 5.0

932.7(11) Å3 5.1

Absorption coefficient, mm-1 Goodness-of-fit Data/param. Largest diff. peak and hole Final R indicesa [I > 2σ(I)]

21.1

19.1

1.08 2327/142 4.8/ -4.0 e- Å3 R1 = 0.059 wR2 = 0.141 R1 = 0.074 wR2 = 0.151

1.02 1084/67 2.2/-2.4 e- Å3 R1 = 0.041 wR2 = 0.076 R1 = 0.060 wR2 = 0.085

Final R indicesa [all data]

R1 = ∑||Fo| – |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 – Fc2)2]/∑[w(Fo2)2]]1/2, and w = 1/[σ2Fo2 + (A·P)2 + B·P], P = (Fo2 + 2Fc2)/3; A and B are weight coefficients. Sample Densification. A nearly-single phase sample of La3Zn1.75P4 was ground into fine powder in the glovebox via mortar and pestle. The fine powder was placed between WC plungers inside a WC die with graphite foil slices between the plungers and powder. The sample was sintered at 873 K for 10 min with a uniaxial pressure of 156 MPa to form a pellet with dimension of ∅5 mm and thickness of ~2 mm. The experiment was carried on SPS 1050: Sumitomo Coal Mining Co, Ltd. The density of the pellet was calculated through geometrical methods, which was about 91% of the theoretical value. Possible surface contaminations and


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graphite were removed by carefully polishing pellet in the glovebox. The sample purity after SPS was confirmed by powder X-ray diffraction. No sample decomposition was detected (Figure S1). DSC Measurements. A Netzsch 114 scanning calorimeter was used to run differential scanning calorimetry (DSC) measurement. 11.6 mg of polycrystalline sample of La3Zn1.75P4 was sealed inside an evacuated silica ampoule and heated to 1123 K with a heating/cooling rate of 10 K/min. No decomposition or melting DSC peaks were found during the heating and cooling process, which indicate that La3Zn1.75P4 is stable up to the highest used temperature of 1123 K (Figure S2). Based on this information, the temperature used for the synthesis of La3Zn1.75P4 was 1073 K and the SPS (Spark Plasma Sintering) process was carried out at a much lower temperature of 873 K. Calculations. The electronic structure of disorder free La3Zn2P4 was studied through using the tight binding-linear muffin tin orbitals-atomic sphere approximation (TB-LMTO-ASA) program.[30] LDA calculations utilized the Barth Hedin exchange potential. [31] By solving the radial scalar relativistic Dirac equation the partial waves were obtained. The basis set used contained P(3s,3p), Zn(4s,4p,3d), and La(6s,5d,4f) orbitals with downfolded P(3d) and La(6p) functions. The density of states and band structures were calculated after converging the total energy on a dense k-mesh of 24×24×24 points with 1313 irreducible k-points. Physical Properties. The thermal and electrical transport properties in the temperature range of 10-300 K were studied using the commercial multipurpose Physical Properties Measurement System (PPMS, Quantum Design). Measurements of thermal conductivity and Seebeck thermopower were performed using the Thermal Transport Option (TTO). These measurements were carried along uniaxial pressure direction, namely cross-plane direction. A standard four-



point alternating-current technique, which is to exclude the resistance of leads, was applied to measure the electrical resistivity of La3Zn1.75P4. This measurement was carried out across the uniaxial pressure direction. Results and Discussion Synthesis Both La2Zn11P9 and La7Zn2P11 crystals had been encountered several times as side products with small yields when we explored the quaternary La-Zn-P-X (X = Cl, Br, I) and ternary La-ZnP systems. The La2Zn11P9 crystals were discovered in the reaction of crystal growth of La3Zn4P6Br using ZnBr2 as a transport agent. The La7Zn2P11 crystal were found as side products during the exploration of La4Zn7P10 using ZnCl2 as flux. The yields of La2Zn11P9 and La7Zn2P11 crystals were significantly improved by running stoichiometric ratio of elements with suitable transport agents as discussed in experimental parts.

The morphologies of La2Zn11P9 and

La7Zn2P11 crystals are shown in Figure 1. La2Zn11P9 grown as bar crystals (Figure 1 left) while La7Zn2P11 formed long needle crystals. (Figure 1 right). No sizeable crystals were found in La3Zn2P4, which was finally found as uniform powder (Figure 1 middle). Applying three-stages annealing and grinding to improve homogeneity without any transport agent or flux may be the reason for the uniform powder form of La3Zn2P4. The original discovery of La3Zn2P4 crystals was also grown with I2 transport agent.[26]


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Figure 1. Optical photographs of the La2Zn11P9, La3Zn2P4, and La7Zn2P11 samples. All images are taken with optical microscope at similar magnifications. Background of the right figure is mm-paper, 1 square = 1 mm2. Crystal Structure La2Zn11P9 (Space group: P21/m; Pearson symbol: mP44; idealized Wyckoff sequence: e22). Several single crystal experiments were performed for crystals of La2Zn11P9 from different batches. All crystals exhibit significant degree of disorder. The provided structural description should be used as a model. In the following description all split positions were replaced with one average position for clarity.



Figure 2. Crystal structure of La2Zn11P9: A) general view along [010]; B) block of edge- and corner-shared ZnP4 tetrahedra and ZnP3 triangles; C) Zn2P2 fragment connecting the blocks. La: green, Zn: blue; P: orange, ZnP4: blue. Unit cell is shown in black lines. The La2Zn11P9 crystal structure is composed of three-dimensional anionic Zn-P framework with channels running along [010] where La cations are located (Figure 2). Each channel accommodates four atomic columns of La atoms. The Zn-P framework is composed of two parts fused together, large 2D Zn-P blocks consisting of edge- and apex-sharing ZnP4 tetrahedra and ZnP3 planar units (Figure 2B). Such blocks are condensed into a 3D framework by means of Zn2P2 fragments (Figure 2C) which are common building block for the framework of Zn phosphides.[24,25] There are no P-P homoatomic bonds in La2Zn11P9. The interatomic Zn-P distances fall into a broad range, 2.19(4)-2.850(5) Å. In ordered compounds the Zn-P interatomic distances are typically longer than 2.3 Å: La4Zn7P10 (2.39-2.51 Å),[24] La3Zn2-xP4 (2.48-2.52 Å),[26] La5Zn2-xP6 (2.46-2.54 Å),[27] and La3Zn4P6Cl (2.36-2.56 Å).[25] The longest Zn-P distance in La2Zn11P9, 2.850(5) Å, is significantly longer than typical Zn-P distances 2.3-2.6 Å, but such long distances can be found among ternary Zn phosphides such as LiZnP (2.889 Å).[32] La7Zn2P11 (Space group: C2/m; Pearson symbol: mS40; idealized Wyckoff sequence: i10a). Crystal structure of La7Zn2P11 is composed of two types of one-dimensional anionic fragments running along [100], [ZnP4]8– and [P3]5–, separated by La cations (Figure 3). Zn-P chains are composed of the ZnP4 tetrahedra sharing two corners. The apical P atom each tetrahedra is engaged into P-P bond (Figure 3B). Terminal P atoms are surrounded by five La atoms. Such coordination of the terminal main group element is also found in Ba3Sb2Q7 (Q = S, Se).[33]


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Figure 3. Crystal structure of La7Zn2P11: A) general view along [100]; B) a chain of cornershared ZnP4 tetrahedra with additional P atoms; C) a disordered [P3]5– phosphorus chain with all atoms shown and unreasonable P1-P1 distance of 1.57 Å shown as dashed line; D) and E) different ordered possibilities of the real fragments of phosphorus chains. La: green, Zn: blue; P: orange, ZnP4: blue. Unit cell is shown in black lines. Phosphorus chain with all atoms plotted resembles polyacene (Figure 3C). However, the occupancy for P1 atoms forming shared P-P bond is 50% (Table S3) and the P1-P1 bond distance is only 1.57 Å which is physically impossible. The P=P double bond in diphosphenes is ~2.03 Å[34] and the shortest P=P double bond reported in crystalline compound is 1.917 Å.[35] The P≡P triple bond in P2 diphosphorus molecule is 1.89 Å,[36] which is still 0.3 Å longer than the observed P1-P1 separation. The unrealistically short P1-P1 distance implies that in the P1-P1 dumbbell one phosphorus atom is always absent. Two possibilities for such arrangements are shown in Figures 3D and 3E. Zn-P distances, 2.379(5) Å to 2.498(5) Å, are similar to those in other La-Zn-P phases.[24-27] The P-P distance of P2 dimers capping the ZnP4 tetrahedra, 2.207(7) Å, is common for homoatomic P-P bonds taking into account a covalent radius for P (1.1 Å) and reported P-P bonds in various phosphides, 2.19-2.33 Å.[12,25,37-39] In turn, the P-P distances in the P3 chains are significantly longer, 2.403(5) Å. Such long covalent P-P bonds were reported for Ba8Cu16P30 (2.46 Å),[40] Ba8Au16P30 (2.12-2.72 Å),[7] Ba6.4La1.6Cu16P30 (2.20-2.38 Å),[13] NiP



(2.43 Å),[41] CeP2 (2.40-2.45 Å),[42] and LaCuP2 (2.20-2.47 Å).[21] According to Zintl electron count and assuming full charge transfer from La and Zn to P, we can assign following oxidation states: La3+, Zn2+, isolated P3–, dumbbell P24–, and phosphorus coordinated to two phosphorus atoms P1–. The formula of the compound can be written as (La3+)7([Zn2P4]8–)2[P3]5–. Based on electron count, semiconducting properties are predicted. La3Zn2-xP4. La3Zn2–xP4 (x = 0.12, 0.25) compounds were first synthesized and structurally characterized by Lincke et al.[26] This compound can be viewed as a layered crystal structure composed from two different types of layers stacked along [001] direction (Figure 4). A Zn2P2 layer consists of ZnP4 tetrahedra sharing three edges with neighboring tetrahedra. La3P2 fragment is a double layer composed of the edge-shared P@La6 octahedra. The absence of certain Zn atoms in La3Zn2P4 leads to the vacancy-containing composition of La3Zn2–xP4. Crystal structures were reported for two compositions, La3Zn1.75P4 and La3Zn1.88P4.[26] Another known compound, La5Zn2-xP6, adopts a similar crystal structure.[27] Inserting additional La2P2 layers between the [ZnP] slabs in La3Zn2-xP4 results in La5Zn2-xP6. La5Zn2-xP6 can also be viewed as alternate stacking of [Zn2P2] layers and [La5P4] slabs along [001] direction. The La5P4 slabs are composed of four layers of fused P@La6 octahedra.


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Figure 4. Crystal structure of La3Zn2–xP4. La: green, Zn: blue; P: orange, ZnP4: blue; P@La6: green. Edge- and corner-shared fragments based on the ZnP4 tetrahedra are common building blocks for all reported La-Zn-P ternary compounds. For the three considered compounds here, a structural dimensionality reduction is observed from La2Zn11P9 (3D) to La3Zn2P4 (2D) to La7Zn2P11 (1D). Dimensionality reduction is accompanied with the increase of P/(La+Zn) ratio. Indeed, no P-P homoatomic bonds were found in La2Zn11P9 and La3Zn2P4 while P-P homoatomic bonds are abundant in the crystal structure of La7Zn2P11. Dimensionality reduction is an important concept to rationalize the composition-structure relationship of inorganic compounds, which is widely applied in chalcogenide chemistry,[43,44] especially in the diverse family of rareearth containing chalcogenides.[45-48] The dimensionality reduction of La-Zn-P ternary system may affect physical properties of the reported compounds. Among all reported phases, we were able to produce single-phase samples for only one composition, namely La3Zn1.75P4. Electronic structure of La3Zn2–xP4



By applying Zintl electron counting rule, the electron balance for the La3Zn1.75P4 can be written as [La3+]3[Zn2+]1.75[P3–]4[e–]0.5, which implies the metallic behavior with electrons as major charge carriers. Our multiple efforts to synthesize the hypothetical electron-balanced and electron-poor compositions with x = 0.5 and 0.75, [La3+]3[Zn2+]1.5[P3–]4 and [La3+]3[Zn2+]1.25[P3– ]4[h+]0.5, were demonstrated to be unsuccessful. The density of state (DOS) and electronic band structure for hypothetical vacancy-free La3Zn2P4 are shown in Figure 5. To simplify the electronic structure calculations, 100% occupancy of the Zn position was applied. Absence of the bands crossing the Fermi level in the

Γ-Ζ direction are typical for 2D compounds. The change of composition from the stoichiometric one with x = 0 to the experimental composition with x = 0.25 (red line in Figure 5) lowers the position of the Fermi level to −0.3 eV. In this case there is only one band which crosses the Fermi level with predominant La5d contribution. The gap is observed at the energies of −1.0 eV which corresponds to the predicted by Zintl count electron-balanced composition with x = 0.5, La3Zn1.5P4 (magenta line in Figure 5). La, Zn, and P orbitals have comparable contributions to the states in the vicinity of the Fermi level, while the states below −1.0 eV are mainly contributed by La5d orbitals and P3p orbitals. The strong contribution of the most electropositive rare-earth element to the states below bandgap or pseudogap have been reported for other rareearth rich intermetallics, such as Eu9Cd4Sb9 and Eu7Cd4Sb7As1.[49,50] In La3Zn2P4 the presence of NaCl-like slabs composed of P@La6 octahedra might be responsible for strong La-P interactions. The calculated position of the Fermi level (red line) for the experimental composition La3Zn1.75P4 is in the conduction band implying the n-type metallic behavior of La3Zn2P4, which agrees with the electron count and observed physical properties (vide infra).


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Figure 5. Band structure (top) and density of states with atomic contributions for La3Zn2–xP4 (bottom). The positions of the Fermi level for the calculated hypothetical composition with x = 0, experimental composition with x = 0.25, and hypothetical electron-balanced composition with x = 0.5 are shown with dashed black, red, and magenta lines, correspondingly. Transport Properties Transport properties of a dense pellet of La3Zn1.75P4 are shown in Figure 6. As predicted by calculations and electron count, La3Zn1.75P4 exhibits metal-like behavior emphasized in the



electrical resistivity increase with temperature throughout the whole measured range, 10-300 K,

ρ 300 K/ρ 10 K = 2.05 (Figure 6 top). The resistivity of La3Zn1.75P4 at 300 K (16.2 µΩ⋅m) is much smaller than that for semiconducting La-Zn-P compounds such as La4Zn7P10 (3790 µΩ⋅m),[24] La3Zn4P6Cl (1.2 Ω⋅m),[25] and La3Zn4P6.6Br0.8 (0.13 Ω⋅m).[25] The thermopower of La3Zn1.75P4 was negative throughout the temperature range which points out electrons as the major charge carriers. The absolute values of the Seebeck coefficients of La3Zn1.75P4 increases with temperature reaching a value of −8.5 µVK−1 at 300 K, which is reasonable for a metallic material (Figure 6, middle). The thermal conductivity of La3Zn1.75P4 rapidly increases in the 10-50 K temperature range and reaches 3.4 Wm−1K−1 at 50 K. The thermal conductivity then gradually increases from 3.4 Wm−1K−1 at 50 K to 4.4 Wm−1K−1 at 300 K. The total thermal conductivity has two contributions, the lattice part due to phonons and electronic part due to charge carriers. The latter one should not be neglected for metallic systems. The electronic contribution to thermal conductivity, κe, can be estimated based on the electrical resistivity of the material, ρ, as κe = L⋅T/ρ, where L is the Lorenz number and T is the absolute temperature. For metallic phases with low resistivity and low absolute values of the thermopower, the Lorenz number L can be approximated as a temperature-independent constant of 2.44⋅10−8 WΩK−2.[51] The lattice and electronic thermal conductivity contributions are both shown in Figure 6, bottom. The lattice thermal conductivity plays the predominant role of thermal conductivity in La3Zn1.75P4. At 300 K the charge carrier contribution is 0.46 Wm−1K−1 (10%) and lattice thermal part is 3.94 Wm−1K−1 (90%). The thermal conductivity of La3Zn1.75P4 is small but higher than the thermal conductivities, 0.6-1.4 Wm−1K−1 at 300 K, of other reported La-Zn-P compounds, La4Zn7P10 and


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La3Zn4P6X (X = Cl, Br),[24-25] which exhibit higher levels of structural complexity and/or presence of vacancies and mixed occupied atomic sites.

Figure 6. Electrical and thermal transport properties for La3Zn1.75P4: electrical resistivity (top); thermopower (middle); and thermal conductivity (bottom) together with electronic (blue stars) and lattice contributions (orange triangles).



A good thermoelectric material should combine high thermopower, high electrical conductivity and low thermal conductivity. The metallic nature of La3Zn1.75P4 resulted in low absolute thermopower values, which can be enhanced by tuning the charge carrier concentration via aliovalent substitution in either Zn- or La-sublattices[13,52]. An aliovalent substitution in a metallic system was demonstrated to be capable to enhance thermoelectric performance on the example of Sb/Cr substitution in CuCr2S4.[53] The optimization of thermoelectric properties of La3Zn1.75P4 is undergoing. Conclusions Three new ternary compounds, La2Zn11P9, La3Zn1.75P4, and La7Zn2P11 are synthesized and characterized. Two of them, La2Zn11P9 and La7Zn2P11, are novel compounds with unique crystal structures. A structural dimensionality reduction is observed upon increasing the phosphorus content going from 3D La2Zn11P9 to 2D La3Zn2–xP4 to 1D La7Zn2P11. The newly discovered compounds demonstrate the structural flexibility of La-Zn-P ternary system. A single composition, La3Zn1.75P4, was synthesized via solid-state reactions of elements. Characterization of the electronic structure and physical properties revealed metallic-type behavior with electrons as main charge carriers. La3Zn1.75P4 shows moderate thermal conductivity, 4.4 Wm−1K−1 at 300 K, which is mainly contributed by lattice thermal conductivity. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. EDS analyses of selected single crystals of La2Zn11P9 and La7Zn2P11; Refined atomic coordinates and isotropic displacement parameters for La2Zn11P9 and La7Zn2P11; Selected interatomic


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distances in La2Zn11P9 and La7Zn2P11; Powder X-ray diffraction patterns of the as-synthesized and after SPS treatment samples of La3Zn1.75P4; Differential scanning calorimetry plot of La3Zn1.75P4. Accession Codes CCDC 1831895-1831896 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author Email: [email protected] ORCID: Jian Wang: 0000-0003-1326-4470 Kirill Kovnir: 0000-0003-1152-1912 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. DEDICATION Dedicated to the 80th birthday for Prof. Academician Xin-Tao Wu. ACKNOWLEDGMENTS The authors thank Prof. Susan M. Kauzlarich at UC Davis for access to DSC and Prof. Julia V.



Zaikina at Iowa State University for access to SPS. This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0008931.

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For Table of Contents Use Only

Synthesis, Crystal Structure, and Properties of Three La-Zn-P Compounds with Different Dimensionality of Zn-P Framework

Jian Wang,a,b,* Philip Yox,a Jackson Voyles,a Kirill Kovnira,b

Table of Contents

A structural dimensionality reduction from 3D La2Zn11P9 to 2D La3Zn2-xP4 to 1D La7Zn2P11 is due to both the flexibility of Zn-P framework with ZnP4 tetrahedra and ZnP3 planar building units and ability of phosphorus to form homonuclear bonds and polyatomic phosphorus chains. Electronic band and thermoelectric properties of 2D La3Zn2-xP4 are also investigated.


Synthesis, Crystal Structure, and Properties of ... - ACS Publications

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