Article pubs.acs.org/IC
Synthesis, Crystal Structure, and Properties of La4Zn7P10 and La4Mg1.5Zn8.5P12 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: Two new zinc phosphides, La4Zn7P10 and La4Mg1.5Zn8.5P12, were synthesized via transport reactions, and their crystal structures were determined by single crystal X-ray diffraction. La4Zn7P10 and La4Mg1.5Zn8.5P12 are built from three-dimensional Zn−P and Zn−Mg−P anionic frameworks that encapsulate lanthanum atoms. The anionic framework of La4Zn7P10 is constructed from one-dimensional Zn4P6, Zn2P4, and ZnP4 chains. The Zn4P6 chains are also the main building units in La4Mg1.5Zn8.5P12. In La4Zn7P10, the displacement of a zinc atom from the origin of the unit cell causes the Zn4 position to split into two equivalent atomic sites, each with 50% occupancy. The splitting of the atomic position substantially modifies the electronic properties, as suggested by theoretical calculations. The necessity of splitting can be overcome by replacement of zinc with magnesium in La4Mg1.5Zn8.5P12. Investigation of the transport properties of a densified polycrystalline sample of La4Zn7P10 demonstrates that it is an n-type semiconductor with a small bandgap of ∼0.04 eV at 300 K. La4Zn7P10 also exhibits low thermal conductivity, 1.3 Wm−1 K−1 at 300 K, which mainly originates from the lattice thermal conductivity. La4Zn7P10 is stable in a sealed evacuated ampule up to 1123 K as revealed by differential scanning calorimetry.
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INTRODUCTION Phosphorus is capable of forming binary phosphides with almost every metal in the periodic system with two exceptions, Hg and Bi.1−4 The addition of electropositive cations result in high structural diversity and interesting physical properties in polar ternary phosphides, including superconductivity, unconventional magnetism, ultralow thermal conductivity, and magnetoresistance.5−13 Thus, it is not surprising that phosphides have numerous applications as semiconductors,14−16 thermoelectrics,10,17−20 magnetocalorics,21−24 and battery materials.25,26 The focus of our work is phosphides of cheap and abundant late transition metals, Zn and Cu. Both metals exhibit flexibility in their local coordination by phosphorus atoms27−31 and are capable of forming 3D metalpnictide (P, As, and Sb) frameworks that encapsulate large alkali or alkaline-earth cations.30,32−34 While there are numerous compounds containing large alkali or alkaline-earth cations, less information is available for compounds containing smaller La3+ cations. We recently explored the La−Cu−P system and found two new ternary compounds.27 Our studies of the La−Zn−P system using chloride and bromide fluxes resulted in the discovery of quaternary compounds formed by the incorporation of one-dimensional La-chloride or Labromide tetrahedral chains into channels of a Zn4P6 framework.35 In this work, we turn our attention to the effect of removing the halogen atoms from the structure. This results in the formation of chains comprised exclusively of La atoms, which is accompanied by significant rearrangements in the Zn− © XXXX American Chemical Society
P framework in the crystal structure of La4Zn7P10. This compound crystallizes in a new structure type and the crystal structure exhibits disorder due to the displacement of Zn atoms from the center of the ZnP6 octahedra. The disorder can be partially eliminated by means of substitution of Mg for Zn, resulting in a new quaternary compound La4Mg1.5Zn8.5P12. Crystal and electronic structures, chemical bonding, charge, and thermal transport properties of La4Zn7P10 are discussed herein.
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EXPERIMENTAL SECTION
Synthesis. All preparation and handling of samples were performed in an argon-filled glovebox with the O2 level below 1 ppm. All starting materials are commercial grade and were used as received: La filings (Alfa Aesar, 99.9%), Zn shot (Alfa Aesar, 99.8%), red P (Alfa Aesar, 99%), Mg turnings (Alfa Aesar, 99.98%), and ZnI2 (Sigma-Aldrich, anhydrous, 99+%). La4Zn7P10. Single crystals of La4Zn7P10 were first synthesized in a transport reaction upon loading La:Zn:P:ZnI2 in a 3:3.675:7:0.325 ratio, using ZnI2 as source of zinc and iodine. The latter may serve as a transport agent. This mixture was placed in a silica ampule, evacuated, and flame-sealed. The ampule was heated from room temperature to 773 K at a rate of 50 K/h, and then annealed at this temperature for 24 h. The ampule was then heated up to 1073 K over 10 h and annealed at this temperature for 168 h. Finally, the ampule was cooled to 773 K over 24 h and annealed for 96 h at this temperature, and then the furnace was turned off. Black needle-like crystals were found as the main product together with binary La- and Zn-phosphides. A Received: September 13, 2016
A
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry polycrystalline sample of La4Zn7P10 was obtained via solid-state reaction of elements without iodine. The elements in a stoichiometric ratio (4:7:10) were loaded into a carbonized silica ampule, evacuated, and flame-sealed. The reaction was heated from room temperature to 1073 K over 17 h, annealed at this temperature for 144 h, and cooled in the furnace. The sample was ground and reloaded into a new ampule in the glovebox, resealed, and reheated using the same temperature profile as the first annealing. This procedure was repeated a third time. After three annealing cycles, fine polycrystalline black powder of La4Zn7P10 was obtained as indicated by powder X-ray diffraction (Figure S1). An admixture of LaZn3P3 was found in the samples.36 All our attempts to produce a single-phase sample were unsuccessful. La4Mg1.5Zn8.5P12. A large amount of needle-like crystals were discovered in a transport reaction with ZnI2. La:Mg:Zn:P:ZnI2 in a 4:2:4.45:10:0.55 ratio were combined and sealed inside an evacuated carbonized silica ampule and heated with the same temperature profile as the transport reaction of La4Zn7P10. All samples of La4Mg1.5Zn8.5P12 contain substantial amounts of poorly crystalline powders and Zn3P2,37 thus preventing properties characterization. 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 with a Ni Kβ filter. Elemental analysis of selected crystals was carried out on a Hitachi S4100T scanning electron microscope (SEM) with energy-dispersive X-ray (EDX) microanalysis (Oxford INCA Energy) to check for possible incorporation of iodine or silicon into the samples. The analyses confirmed the presence of only La, Zn, and P in crystals of La4Zn7P10 and La, Mg, Zn, and P in crystals of La4Mg1.5Zn8.5P12 (Table S1). Single Crystal X-ray Diffraction. Single crystal X-ray diffraction experiments were collected at 90 K using a Bruker AXS SMART diffractometer with an APEX-II CCD detector with Mo-Kα radiation. The data sets were recorded as ω-scans with a 0.4° step width and integrated with the Bruker SAINT software package.38 Multiscan absorption corrections were applied.38 The solutions and refinements of the crystal structures were carried out using the SHELX suite of programs.39 Upon refinement of the crystal structure of La4Zn7P10, three split positions, each with 50% occupancy, were found: Zn4, P2, and P3 (Tables S2 and S3, Figure S4). A similar splitting of the P1 position was found in the crystal structure of La4Mg1.5Zn8.5P12 (Figure S4). Final refinements were performed using anisotropic atomic displacement parameters for all atoms with the exception of the split P2 and P3 atoms in the crystal structure of La4Zn7P10. A summary of pertinent information relating to unit cell parameters, data collections, and refinements is provided in Table 1 and the atomic parameters and interatomic distances are provided in Tables S2 and S3. Further details of the crystal structure determinations may be obtained from Fachinformationszentrum Karlsruhe, Germany, by quoting the depository numbers CSD-431949 (La4Zn7P10) and CSD-431950 (La4Mg1.5Zn8.5P12). Sample Densification. A polycrystalline sample of La4Zn7P10 was carefully ground into fine powder in the glovebox, loaded into a graphite die with WC plungers, and sintered at 873 K through spark plasma sintering (SPS 1050: Sumitomo Coal Mining Co, Ltd.) for 10 min with a uniaxial pressure of 156 MPa to form a pellet with dimension of 5 mm ⌀ and thickness of ∼2.5 mm. The geometrical density of the pellet was about 91% of the theoretical value. Graphite and possible surface contaminations were removed by polishing in the glovebox. The sample purity after SPS was confirmed by powder X-ray diffraction and no sample decomposition was detected (Figure S3). Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was conducted using a using a Netzsch Thermal Analysis STA 409 calorimeter. A powder sample of La4Zn7P10 (mass: 11.6 mg) was sealed inside an evacuated silica ampule and heated to 1123 K with a heating/cooling rate of 10 K/min. Electronic Structure Calculations. Electronic structure calculations and bonding analyses were carried out using the tight bindinglinear muffin tin orbitals-atomic sphere approximation (TB-LMTOASA) program.40−44 The Barth-Hedin exchange potential was
Table 1. Selected Crystal Data and Structure Refinement Parameters for La4Zn7P10 and La4Mg1.5Zn8.5P12 empirical formula CSD-number formula weight temperature radiation, wavelength crystal system space group unit cell dimensions
unit cell volume Z density (calc.) absorption coefficient Rint goodnees-of-fit final R indicesa [I > 2σ(I)] final R indicesa [all data] largest peak and hole/e− Å3
La4Zn7P10
La4Mg1.5Zn8.5P12
431949 1322.9 g/mol
431950 1518.6 g/mol 90(2) K Mo Kα, 0.71073 Å monoclinic C2/m (No. 12) a = 15.579(7) Å a = 15.447(2) Å b = 4.131(2) Å b = 4.1050(6) Å c = 14.409(6) Å c = 11.059(3) Å β = 111.627(5)° β = 131.494(1)° 862.1(6) Å3 525.3(2) Å3 2 1 5.097 g/cm3 4.801 g/cm3 20.12 mm−1 18.37 mm−1 0.079 0.051 1.02 1.07 R1 = 0.043 R1 = 0.028 wR2 = 0.100 wR2 = 0.058 R1 = 0.060 R1 = 0.033 wR2 = 0.109 wR2 = 0.060 2.11/−1.93 1.25/−1.05
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. a
employed for the LDA calculations.41 The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. The basis set used contained La(6s,5d,4f), Zn(4s,4p,3d), and P(3s,3p) orbitals with downfolded La(6p) and P(4d) functions. Two models were calculated for La4Zn7P10: one model with C2 symmetry where the Zn4 position was split into two positions with only one position occupied to simulate the 50% occupancy of this position and another model with C2/m symmetry, which is the idealized scenario with the Zn4 position at (0,0,0). In both models idealized positions for the P2 and P3 atoms were used. The density of states and band structures were calculated after converging the total energy on a dense k-mesh of 20 × 20 × 12 points with 2420 irreducible k-points. The electron localization function (ELF) was also evaluated using the modules within the TB-LMTO-ASA package and visualized with the ParaView program.45,46 Crystal orbital Hamilton population (COHP) calculations were computed using the LMTO package. −ICOHP values were calculated by integrating over all filled states to give relative overlap populations.47 Physical Properties. Transport properties in the temperature range of 2−400 K were studied using the commercial multipurpose physical properties measurement system (PPMS, Quantum Design). The Seebeck thermopower and thermal conductivity were measured using the thermal transport option. The electrical resistivity was measured by a standard four-point alternating-current technique to exclude the resistance of the leads.
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RESULTS AND DISCUSSION Crystal Structure of La4Zn7P10. Prior to our work, only two compounds were structurally characterized in the La−Zn− P system: La3Zn2−xP428 and La5Zn2−xP6.29 Both compounds exhibit three-dimensional crystal structures composed of [ZnP] slabs built from edge-shared ZnP4 tetrahedra and [LaP] slabs composed of LaP6 octahedra that run along the [001] direction. The addition of extra La2P2 slabs between the [ZnP] slabs in La3Zn2−xP4 results in La5Zn2−xP6. The existence of a LaZn3P3 compound with the ScAl3C3-struture type has been suggested by Nientiedt and Jeitschko based on the similarities between B
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. General view of the crystal structure of La4Zn7P10 viewed along [010] (A) together with three main constituent fragments of the Zn−P framework (B−D). Note that the Zn atoms in the ZnP4 fragment have 50% occupancy. La: green; Zn: blue; P: orange. Unit cell is shown in red lines.
the longest (3.22−3.41 Å) La−P distances in the structure. The latter distances are slightly longer than those found in La3Zn2−xP4 (2.94−3.06)28 and La5Zn2−xP6 (2.95−3.05 Å).29 The crystal structure of La4Zn7P10 is a new structure type with the Pearson symbol mS42, space group C2/m, and idealized Wyckoff sequence i10a1. Structural frameworks made of Mg and Ge atoms, such as the one in the crystal structure of La4Mg7Ge6, exhibit Mg2Ge2 building blocks that are similar to the Zn2P2 blocks observed in La4Zn7P10.48 All of the zinc atoms are tetrahedrally coordinated by four phosphorus atoms in La3Zn2−xP4,28 La5Zn2−xP6,29 and La4Zn7P10. In general, Zn atoms exhibit high structural flexibility as evidenced by the structurally similar Zn2P2 units found in frameworks of various Zn pnictides.28,29,35,49 In the crystal structure of La3Zn4P6Cl a portion of Zn atoms have trigonal planar coordination resulting in tubular Zn2P2 units (Figure 2, top). Replacement of chlorine with bromine results in rearrangements in the framework and a 3 + 1 coordination for Zn by P atoms (Figure 2, middle).35 Finally, in the crystal structure of La4Zn7P10 all Zn atoms in the Zn2P2 unit are tetrahedrally coordinated (Figure 2, bottom). Electronic Structure and Chemical Bonding Analysis. The band structures and densities of states (DOS) were calculated for two models of La4Zn7P10. The two models were constructed to determine the effect of the Zn4 displacement. As shown in Figure 3, the compound with C2 symmetry with the displacement of Zn4 from the origin of the unit cell has a narrow indirect bandgap of 0.07 eV between the Γ and A points. The smallest direct bandgap was calculated to be 0.2 eV at the Γ point. When Zn4 is placed in the ideal position at the origin of the unit cell, as in the model with C2/m symmetry, the gap between the valence and conduction bands at the Γ and A points becomes much smaller. Moreover, the gap closes at the Z point. Qualitatively, the band structures are similar in shape and result in similar DOS plots, with the main contribution to the valence band from P atoms and the main contribution to the conduction band from La atoms in both models. The primary difference between the two models is that, when Zn4 is displaced from the origin, a bandgap opens. This indicates that the displacement of the Zn atom has a significant impact on the electronic structure. ELF analysis was conducted on the C2 model to elucidate the bonding environment of the Zn4 atom. The expected covalent bonding was present for the Zn−P bonds in the Zn2P4
the powder diffraction patterns of the La-containing sample and the structurally characterized PrZn3P3 sample.36 In the crystal structure of RZn3P3 the two-dimensional Zn3P3 layers are separated by layers of rare-earth (R) cations. The crystal structure of La4Zn7P10 reported in this work has a different structural motif (Figure 1A). La4Zn7P10 crystallizes in the monoclinic space group C2/m (No. 12). La4Zn7P10 is built from a 3D anionic framework containing large open channels hosting four chains of La atoms. The framework is composed of three main structural chain-like units running along [010]: Zn4P6, Zn2P4, and ZnP4. The Zn4P6 chains are composed of edge-sharing Zn2P2 squares with additional bridging P atoms (Figure 1B). The Zn2P4 chains are also composed of Zn2P2 squares, which are connected to each other solely via additional P atoms connecting Zn atoms of different squares (Figure 1B). The Zn4P6 and Zn2P4 chains are connected to each other by means of P−P bonds forming two-dimensional layers. These layers are connected to each other via additional Zn atoms, Zn4 (Figure 1D). Zn4 atoms are situated inside distorted octahedra of P atoms near the origin of the unit cell. If the Zn4 atom was located exactly at the origin of the unit cell it would be linearly coordinated to two apical P atoms at distances of 2.36 Å and have four additional equatorial P atoms at distances of 3.03 Å. 2 + 4 coordination is unfavorable, and as such, the Zn4 atoms are displaced from the origin of the unit cell along [010] by 0.45 Å. This shift results in two alternative positions for Zn4 atoms with 50% occupancy, both of which are shown in Figure 1 for the ZnP4 fragment (Figure 1D). The coordination of the Zn atom approaches that of tetrahedral coordination with two apical P atoms at distances of 2.40 Å, two equatorial P atoms at distances of 2.75 Å, with the remaining two equatorial P atoms at nonbonding distances of 3.35 Å. The occupancy of this Zn position was refined to be 52(1)% and was fixed at the final stage of the refinement to be 50%. With the exception of the Zn atom in the center of the distorted octahedron, the zinc−phosphorus bonding distances fall into the range of 2.39−2.51 Å, which is comparable to the distances in La3Zn2−xP4 (2.48−2.52 Å)28 and La5Zn2−xP6 (2.46−2.54 Å).29 The splitting of P2 and P3 phosphorus positions do not greatly affect the Zn−P and P−P distances (Figure S4). The most affected distances are the La−P distances (Table S3). The distances between La and the split P2 and P3 positions are both the shortest (2.94−2.99 Å) and C
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (Top) Band structures and (bottom) density of states of two models of La4Zn7P10. (Left) Model with C2 symmetry has the Zn4 atom displaced from the origin of the cell. (Right) Model with C2/m symmetry has the Zn4 atom occupying the origin of the unit cell.
Figure 2. Different topologies of Zn2P2 units in the crystal structures of (top) La3Zn4P6Cl,35 (middle) La3Zn4P6.6Br0.8,35 and (bottom) La4Zn7P10. Zn: blue; P: orange.
and Zn4P6 fragments (Figure S5). The expected covalent interaction is seen for the Zn4−P1 interactions, but the localization of the electrons between Zn4 and P5 is less clear (Figure 4). The ELF distribution around P5 is slightly distorted toward the Zn4−P5 interactions of 2.75 Å indicating weak Zn− P interactions (Figure 4B,C). To further elucidate the bonding between Zn4 and P5, COHP calculations were performed on both the C2 and C2/m models to determine the effect of the Zn4 displacement. The Zn4−P1 interactions in both models have mainly bonding character in the valence band and antibonding character in the conduction band (Figure S6). The integrated COHP (−ICOHP) values for the Zn4−P1 bonds for both models are similar, 2.38 and 2.39 eV/bond for the C2 and C2/m models, respectively (Figure S6). The main difference can be seen in the COHP for the Zn4−P5 interactions. When Zn4 is displaced from the unit cell origin (C2 model), the shorter Zn4−P5 bond, 2.75 Å, has primarily bonding character below the Fermi level and antibonding character above the Fermi level (Figure 5). The longer Zn4−P5 interaction in the C2 model (3.35 Å) is longer than the Zn4−P5 distance in the ideal case when Zn4 is in the ideal position at the origin of the unit cell (C2/m, 3.03 Å). The COHP for Zn4−P5 interactions, which are longer than 3 Å, have antibonding and nonbonding character below the Fermi level (Figure 5). In accordance with
Figure 4. Electron localization diagrams of the Zn4 fragment. (A) Isosurface of the ZnP4 chain (η = 0.76); (B) slice of the Zn4−P5 bonds of 2.75 Å; and (C) isosurface of the ZnP4 tetrahedron (η = 0.76).
the length of the Zn4−P5 bond, the −ICOHP values for the C2 model and the C2/m model are different, 0.03 and 0.26 eV/ bond, respectively. COHP analysis shows that, while there is a non-negligible interaction between the Zn4 and equatorial P atoms when Zn4 is in the ideal position, the strongest interaction occurs in the distorted octahedra for the short Zn4−P5 bond (2.75 Å) with a negligible interaction between Zn4 and the other two equatorial positions. Crystal Structure of La4Mg1.5Zn8.5P12. As quantum mechanical calculations suggested, the displacement of Zn4 D
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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1B). In the structure of La4Mg1.5Zn8.5P12, the Zn2P4 chains are missing. The crystal structure of La4Mg1.5Zn8.5P12 can be derived from the structure of La4Zn7P10 by replacing the Zn2P4 chains with Zn4P6 chains and additional Mg atoms, as shown in Figure 7. In the structure of La4Mg1.5Zn8.5P12, layers are formed from Zn4P6 chains that are connected by P−P bonds. The layers are connected to each other by metal atoms situated in the 2a (0,0,0) crystallographic position. This position is jointly occupied by Mg (75%) and Zn (25%) atoms. Unlike in the structure of La4Zn7P10, no displacement of the metal atom from the center of the octahedron was detected. The metal in the center of the octahedron forms two short (2.57 Å) and four long equatorial (2.97 Å) bonds to P atoms. The shorter bonding interaction between Mg/Zn and P, 2.571(3) Å, agrees well with distances found in many binary and ternary magnesium−phosphorus compounds such as Mg3P2 (2.55− 2.60 Å),50 MgP4 (2.59−2.61 Å),51 and MgSiP2 (2.60 Å).52 The long Mg−P interaction, 2.967(2) Å, is similar to distances found in Mg1.37Zn0.62P2 (2.996 Å)53 and Mg3Ni20P6 (2.936 Å).54 The crystal structure of La4Mg1.5Zn8.5P12 is a new structure type with the Pearson symbol mS26, space group C2/m, and idealized Wyckoff sequence i6a1. Compounds that are closest in composition to La4Mg1.5Zn8.5P12 that crystallize in the C2/m space group with i6a1 Wyckoff sequence are Ba6(Li8−xMgx)8E12, E = Si, Ge.55,56 In those structures, E atoms form bonds to either one or two other E atoms forming E6 cis−trans isolated chains, while P atoms in the structure of La4Mg1.5Zn8.5P12 only form P2 dumbbells surrounded by metal cations. Thermoelectric Properties of La4Zn7P10. La4Zn7P10 exhibits good thermal stability without any melting or decomposition peaks up to 1123 K (Figure S2). No substantial changes in the PXRD pattern were detected in the sample after the DSC measurement (Figure S3). The densification temperature of La4Zn7P10 was set at 873 K based on the results of the thermal stability test. The charge and heat transport properties of La4Zn7P10 were investigated (Figure 8). The resistivity of La4Zn7P10 decreases with increasing the temperature across the whole measured temperature range, 2−400 K, indicating thermally activated behavior that is typical for semiconductors (Figure 8, top). Fitting the resistivity data to the equation ln(1/ρ) = ln(1/ρ0) − Ea/2kT (inset in Figure 8, top) gives an activation energy of 0.04 eV, which is in good agreement with the black luster of the single crystals and the bandgap predicted by electronic structure calculations. Despite the small activation energy, the resistivity of La4Zn7P10 at 300 K is 379 mΩ cm, which is about 10 times larger than the resistivity of a single crystal of Eu2Zn2As3 (25 mΩ cm), which has a bandgap of 0.02 eV.49 The high resistivity of La4Zn7P10 may be due to the presence of an admixture of LaZn3P3 or due to disorder in the Zn−P sublattice, which may affect the electron mobility.57 The absolute values of the thermopower of La4Zn7P10 increase with temperature (Figure 8, middle). The thermopower is negative in the whole temperature range indicating that electrons are the main charge carriers. At 300 K the Seebeck thermopower of La4Zn7P10 reaches a value of −110 μV/K, which is reasonable for the small bandgap semiconductor. The thermal conductivity of La4Zn7P10 is shown in Figure 8, bottom. The thermal conductivity first increases from 2 to 75 K reaching a maximum of 1.5 Wm−1 K−1, which is typical for crystalline solids. The thermal conductivity decreases with
Figure 5. COHP plot for the interactions between Zn4 and the equatorial P atoms in both the C2 and C2/m models. −ICOHP values are given in the legend.
atoms from the origin of the unit cell has a significant impact on the electronic properties of La4Zn7P10. Octahedral coordination is not typical for Zn atoms in phosphides but is quite common for Mg atoms.1,2 To produce a compound with octahedral coordination of metal atoms in the origin of the unit cell we attempted to substitute a portion of Zn atoms with Mg. We were not able to synthesize a quaternary compound La4Mg1Zn6P10. Instead, we produced La4Mg1.5Zn8.5P12, which has a similar crystal structure to La4Zn7P10 (Figure 6). The Zn4P6 chains present in the three-dimensional framework of La4Mg1.5Zn8.5P12 (red circle in Figure 6) are almost identical to the Zn4P6 chains found in the structure of La4Zn7P10 (Figure
Figure 6. General view of the crystal structure of La4Mg1.5Zn8.5P12 viewed along [010] together with the octahedral chain around the Mg atoms. The occupancy of the position shown as Mg is Mg0.75Zn0.25. La: green; Mg: magenta; Zn: blue; P: orange. Unit cell is shown in red lines. E
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 7. Comparison of the two crystal structures viewed along the [010] direction. Replacement of the Zn2P4 unit with Zn4P6+Mg results in the transformation of La4Zn7P10 into a hypothetical compound with the composition, La4Mg2Zn8P12. La: green; Mg: magenta; Zn: blue; P: orange.
coefficient.58 Due to the high electrical resistivity, the electronic contribution to the total thermal conductivity is negligible, e.g., the estimated value of κelectronic = 0.002 Wm−1 K−1 at 400 K, which is only 0.15% of the total thermal conductivity. The low lattice thermal conductivity may be attributed to the large unit cell57 and disorder in the Zn−P sublattice.57,59 Limited by the high resistivity, the final figure of merit of La4Zn7P10 is only ZT = 0.002 at 400 K. Despite the reasonable thermopower and thermal conductivity, future tuning of the electrical conductivity of La4Zn7P10 is required to make it a possible candidate for thermoelectric applications.
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CONCLUSIONS Two new zinc phosphide compounds, La4 Zn 7 P 10 and La4Mg1.5Zn8.5P12, have been synthesized and their structures characterized. The effect of the displacement of the zinc atom in La4Zn7P10 is further investigated by theoretical calculations and by properties characterization. Our efforts to replace zinc with magnesium resulted in the formation of La4Mg1.5Zn8.5P12. These new compounds demonstrate the high flexibility of zinc−phosphorus frameworks. La4Zn7P10 is shown to be an ntype narrow bandgap semiconductor with low thermal conductivity and good thermal stability up to 1123 K as measured by DSC. High thermal stability, low thermal conductivity, reasonable thermopower, and variable crystal structures demonstrate the potential for this system to contain materials that are viable thermoelectrics with doping to tune the electrical conductivity.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02216. Calculated and experimental powder X-ray diffraction patterns, EDS results of La4Zn7P10 and La4Mg1.5Zn8.5P12, differential scanning calorimetry plot of La4Zn7P10, tables with refined atomic coordinates, displacement parameters, and selected interatomic distances for La4Zn7P10 and La4Mg1.5Zn8.5P12, figures for the coordination of the split phosphorus positions, and additional ELF and COHP figures. (PDF) Crystallographic information file for La4Zn7P10 (CIF) Crystallographic information file for La4Mg1.5Zn8.5P12 (CIF)
Figure 8. Thermoelectric properties of La4Zn7P10: (top) electrical resistivity; (middle) Seebeck thermopower; (bottom) thermal conductivity. Inset in resistivity panel shows the ln(1/ρ) vs. 1000/T dependence and linear fits of high-temperature data (yellow dashed line).
increasing temperature down to a value of 1.3 Wm−1 K−1 at 300 K. The thermal conductivity of solids can be expressed as κtotal = κelectronic + κlattice = LT/ρ + κlattice, where L is the Lorentz number, which is estimated from the experimental Seebeck F
DOI: 10.1021/acs.inorgchem.6b02216 Inorg. Chem. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kathleen Lee: 0000-0002-1476-5384 Kirill Kovnir: 0000-0003-1152-1912 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. S. M. Kauzlarich for access to the DSC and 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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