Ca14AlSb11 Structure Research Articles

Tuning the Intermediate Valence Behavior in the Zintl Compound Yb14ZnSb11 by Incorporation of RE3+ [Yb14-xRExZnSb11 (0.2 ≤ x ≤ 0.7), RE = Sc, Y, La, Lu and Gd

The solid solutions of Yb14-xRExZnSb11 (RE = Sc, Y, La, Lu, and Gd; 0.2 ≤ x ≤ 0.7) were prepared to probe the intermediate valency of Yb in Yb14ZnSb11. The substitution of Yb with RE3+ elements should reduce or remove the intermediate valency of the remaining Yb ions. Large crystals are grown from Sn-flux, and the structure and magnetic susceptibility are presented. All compounds crystallize in the Ca14AlSb11 structure type and the RE3+ ions show Yb site substitution preferences that correlate with size. Two compositions of Yb14-xYxZnSb11 were investigated [x = 0.38(3), 0.45(3)] by temperature-dependent magnetic susceptibility and the broad feature in magnetic susceptibility measurements at 85 K in pristine Yb14ZnSb11 attributed to valence fluctuation decreases and is absent for x = 0.45(3). In compounds with nonmagnetic RE3+ substitutions (Sc, Y, La, and Lu), temperature-dependent magnetic susceptibility shows a transition from intermediate valency fluctuation toward temperature-independent (Y, La, and Lu) or Curie-Weiss behavior and possibly low temperature heavy Fermion behavior (Sc). In the example of the magnetic rare earth substitution, RE = Gd, the Curie-Weiss-dependent magnetic moment of Gd3+ is consistent with x. Hall resistivity of Yb14-xYxZnSb11 showed that the carrier concentration decreases with x and the signature of the low-T intermediate valence state seen for x = 0 is suppressed for x = 0.38 and gone for x = 0.45.

Ca14FeAs11 – A structure comprising structural motifs of iron‐based superconductors and Ca‐As Zintl phases

AbstractCa14FeAs11 is the first iron‐containing representative of the Ca14AlSb11 structure type and the first compound with isolated FeAs4 tetrahedra. It can be regarded as a salt‐like compound containing isolated [FeAs4]9− tetrahedra, linear [As3]7− units, isolated As3− anions and Ca2+ cations and thus is a borderline compound between binary Fe−As intermetallics and salt‐like calcium arsenides. Ca14FeAs11 is obtained by a two‐step high‐temperature reaction, in which preformed FeAs alloy is heated with Ca in the presence of additional As in a sealed silica tube. Structure determination from single crystals at 150 K reveal the Ca14AlSb11 structure type with space group I41/acd (Z=8, a=15.6268(2) Å, c=20.9781(5) Å, and V=5122.79(15) Å3). Band structure calculations predict metallic properties with strong contribution of Fe orbitals to the density of states at the Fermi level.

Synthesis and structure of Sr14Zn1+xAs11 and Eu14Zn1+xAs11 (x ≤ 0.5). New members of the family of pnictides isotypic with Ca14AlSb11, exhibiting a new type of structural disorder

Systematic investigation of the ternary systems A–Zn–As (A = Sr, Eu) led to the discovery of two new ternary phases, Sr14Zn1+xAs11 and Eu14Zn1+xAs11 (x ≤ 0.5). Both title compounds were structurally characterized by single-crystal X-ray diffraction. The basic structure of these new arsenides is akin to that of the numerous other pnictides adopting the tetragonal Ca14AlSb11 structure type (space group I41/acd (No. 142); Z = 8). The main difference is the presence of an interstitial Zn atom, located in a nearly trigonal-planar coordination environment of three As atoms. Four such fragments share common edges with the regular ZnAs4 tetrahedron, to form an unusual new structural unit with the formula [Zn1+xSb8]. The small uptake of additional zinc is needed to augment the overall valence electron count, thereby driving the system to an electronically charge-balanced state. Upon partial substitution of the nominally divalent Sr with a trivalent La, the equivalent of electron doping, the structure rids itself of the interstitial zinc, as demonstrated on the example of Sr13.30(2)La0.70ZnAs11, which was also structurally characterized by single-crystal X-ray diffraction as part of this study. Electronic structure calculations for the idealized composition Sr14ZnAs11 confirm the speculated electron-deficient assignment, and suggest intrinsic semiconducting behavior for the one-electron richer Sr14Zn1+xAs11 (x = 0.5) and Sr13LaZnAs11, respectively.

Synthesis, crystal structure and physical properties of the solid solutions Ca14–x RExCdSb11 (RE = La–Nd, Sm, Gd–Yb, x ≈ 0.85 ± 0.15)

This article deals with the new antimonides represented with the general formula Ca14–xRExCdSb11 (RE = La–Nd, Sm, Gd–Yb, x ≈ 0.85 ± 0.15). The 12 studied compounds constitute a nearly complete series of rare-earth metal substituted variants of the ternary Ca14CdSb11 phase. All have been synthesized from the respective elements, employing high-temperature reactions under molten flux conditions. The structures have been fully characterized by single-crystal X-ray diffraction methods. All materials crystallize in the tetragonal Ca14AlSb11 structure type (space group I41/acd, No. 142, Z = 8). Rare-earth element atoms randomly substitute Ca atoms on the four available crystallographic sites, with a noted preference for the Ca2 site in case of the light (La–Nd) rare-earth elements and the Ca1 site in case of the heavier (Sm, Gd–Yb) ones. The electronic structure calculations and resistivity measurements indicate title compounds as degenerated semiconductors. Magnetization measurements at varied temperature show Curie-Weiss paramagnetic behavior consistent with local-moment magnetism due to the 3+ ground state for the rare-earth metal ions. In the case of the Yb-containing sample, a mixed-valence Yb2+/3+ state is apparent. The measured charge transport properties suggest small bandgap degenerate semiconductor-like behavior and suitability for thermoelectrics.

From the Ternary Phase Ca14Zn1+δSb11 (δ ≈ 0.4) to the Quaternary Solid Solutions Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd, x ≈ 0.9). A Tale of Electron Doping via Rare-Earth Metal Substitutions and the Concomitant Structural Transformations.

The ternary compound Ca14Zn1.37(1)Sb11 and its six rare-earth metal substituted derivatives Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd; x ≈ 0.90 ± 0.06) have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. All compounds formally crystallize in the tetragonal Ca14AlSb11 structure type (space group I41/ acd, No. 142, Z = 8). The crystal structure of Ca14Zn1.37(1)Sb11 subtly differs from the structure of the remaining six, as well as from the structure of the archetype, due to the presence of a partially occupied interstitial Zn position. The extra zinc atom is needed in this structure to alleviate the unfavorable number of valence electrons in the imaginary Ca14ZnSb11. Electron doping, via substitution of RE3+ ions on Ca2+ sites, is shown as an alternative route to achieve electron balance in these Zn-based analogs of the Ca14AlSb11 structure, which does not require the incorporation of interstitial atoms. Electrical resistivity measurements done on single-crystalline samples are in agreement with the notion that Ca14- xRE xZnSb11 moieties behave as either bad metals or heavily doped semiconductors. Magnetization measurements show Curie-Weiss paramagnetic behavior related to the local-moment magnetism of the RE3+ ions.

Seebeck and Figure of Merit Enhancement by Rare Earth Doping in Yb14-xRExZnSb11 (x = 0.5).

Yb14ZnSb11 has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca14AlSb11 structure type that show metallic behavior. While the solid solution of Yb14Mn1-xZnxSb11 shows an improvement in the high temperature figure of merit of about 10% over Yb14MnSb11, there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb14-xRExZnSb11 (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb2+ was employed as a means of optimizing Yb14MnSb11 for use as a thermoelectric material. Yb14ZnSb11 is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb3+ or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb3+. The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb14MnSb11. These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb14ZnSb11 so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb14ZnSb11 and provides a zT ~0.7 at 1275 K.

The remarkable crystal chemistry of the Ca14AlSb11 structure type, magnetic and thermoelectric properties

Yb14MnSb11 is a member of a remarkable structural family of compounds that are classified according to the concept of Zintl. This structure type, of which the prototype is Ca14AlSb11, provides a flexible framework for tuning structure-property relationships and hence the physical and chemical properties of compounds. Compounds within this family show exceptional high temperature thermoelectric performance at temperatures above 300 K and unique magnetic and transport behavior at temperatures below 300 K. This review provides an overview of the structure variants, the magnetic properties, and the thermoelectric properties. Suggestions for directions of future research are provided.

Structure, Magnetism, and Thermoelectric Properties of Magnesium-Containing Antimonide Zintl Phases Sr14MgSb11 and Eu14MgSb11.

New Mg-containing antimonide Zintl phases, Sr14MgSb11 and Eu14MgSb11, were synthesized from high-temperature solid-state reactions in Ta tubes at 1323 K. Their structures can be viewed as derived from the Ca14AlSb11 structure type, which adopt the tetragonal space group I41/acd (No. 142, Z = 8) with the cell parameters of a = 17.5691(14)/17.3442(11) Å and c = 23.399(4)/22.981(3) Å for the Sr- and Eu-containing compounds, respectively. The corresponding thermoelectric properties were probed, which demonstrated high potential of these compounds as new thermoelectrics for their very low thermal conductivity and moderate Seebeck coefficient. Magnetism studies and theoretical calculations were conducted as well to better understand the structure-and-property correlation of these materials.

Synthesis and Structural Characterization of Ca<sub>14</sub>Nb<i><sub>x</sub></i>In<sub>1<i>-</i></sub><i><sub>x</sub></i>As<sub>11</sub> (<i>x</i> ≈ 0.85)

Single-crystals of the new compound Ca14NbxIn1–xAs11 have been obtained from a solid-state reaction in a sealed Nb ampoule. The initial experiment had been set up with the aim to investigate the effect of electron doping (via In) on the crystal structure and physical properties of Ca14MnAs11. Subsequent single-crystal X-ray diffraction and elemental analysis work suggested that instead of Ca14MnxIn1–xAs11, the major product of the reaction is the phase Ca14NbxIn1–xAs11. This supposition was corroborated when the title compound was synthesized from a reaction of Ca, In and As in a sealed Nb ampoule, proving that, 1) Mn metal is not included in the structure, and 2) that the inadvertent side reaction of As with the walls of the Nb container is the source of the niobium. The overall structure is isotypic with the tetragonal Ca14AlSb11 structure type (space group I41/acd), although some marked differences between the two must be noted. Current ongoing work is focused on the synthesis of phase pure polycrystalline samples and determination of the physical properties of this unusual transition metal Zintl phase.

On the Extended Series of Quaternary Zintl Phases Ca13REMnSb11 (RE = La–Nd, Sm, Gd–Dy)

Eight new compounds with a general formula Ca14–xRExMnSb11 (RE = La–Nd, Sm, Gd–Dy; x ≈ 1) were synthesized by the molten flux method and their structures were established via single‐crystal X‐ray diffraction. The overarching goal of this study was to investigate the effect of electron doping, via substitution of RE3+ at Ca2+ sites, on the crystal structure and physical properties of Ca14MnSb11, which is a candidate for thermoelectric applications. All studied phases are isostructural, and crystallize in the tetragonal body‐centered space group I41/acd (Ca14AlSb11 structure type, Pearson index tI208). The structure is made up of MnSb410– tetrahedra, randomly mixed Ca2+ and RE3+ cations, Sb3– anions, and linear Sb37– polyanions. Comprehensive structural work confirms high rare‐earth metal content with Ca2+/RE3+ randomly mixed on all four cation sites, with the caveat that the larger RE atoms (La–Nd) prefer to occupy the Ca2 site, while the smaller RE atoms (Sm, Gd, Tb, and Dy) preferably occupy the Ca1 site. Nearly phase‐pure polycrystalline samples have been synthesized using solid‐state reactions, and have been used for physical property measurements. The resistivities of the bulk Ca14–xRExMnSb11 (RE = La–Nd, Sm, Gd; x ≈ 1) samples reveal semiconducting behavior, consistent with the notion of electron doping in the parent p‐type Ca14MnSb11 material. Magnetic susceptibilities suggest complex magnetic ordering at temperatures below ca. 50 K, likely originating from coupling between two or more magnetic sub‐lattices.

Magnetic remanence in Yb14−xRExMnSb11 (RE=Tb, Dy, Ho) single crystals

Single crystals of Yb14−xRExMnSb11 (x~0.1, 0.4; RE = Tb, Dy, Ho) have been prepared as a solid solution by Sn flux reactions of the elements. They crystallize in the Ca14AlSb11 structure type in the I41/acd space group. The RE3+preferentially substitutes on the Yb(1) site which is the smallest volume Yb containing polyhedron. In the case of Ho3+, a small amount of Ho3+ also substitutes on the Yb(4) site. The ferromagnetic ordering temperature of Yb14MnSb11 is reduced from 53K to 41K as x increases and dependent on the identity of the RE. This is attributed to the reduction in carriers and reduced screening of the Mn2+ local moment. The effective moments, μeff, agree well with the calculated moments assuming the RE substitutes as a trivalent cation. The largest coercive field is observed for RE = Dy (1000Oe). For the maximum x of Yb14−xRExMnSb11 there are enough carriers for the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism of magnetic coupling via conduction electrons to still be valid in describing the ferromagnetic ordering.

Magnetic and structural effects of partial Ce substitution in Y b14MnSb11

Single crystals of Y b14−xCexMnSb11 were grown from tin metal as a flux solvent with a maximum Ce incorporation of 0.6. The phases with x ∼ 0.1–0.6 crystallize in the tetragonal Ca14AlSb11 structure type with I41/acd space group. In this structure type, there are 4 crystallographically unique Yb sites and the structure can be described according to the Zintl concept as containing 14Y b2+ + [MnSb4]9− + [Sb3]7− + 4Sb3−. For x > 0.3, Ce is incorporated on specific Yb sites in the structure as a function of x, initially at x = 0.3 on the Yb(2) site followed by Yb(4) at higher values of x. These sites have the largest volume as indicated by Hirshfeld surface analysis of chemical bonding. As Ce content is increased, the ferromagnetic ordering temperatures decrease and effective paramagnetic moments increase. The magnetic ordering temperatures decrease from the undoped TC of 50 K until x ∼ 0.4, where the lowest TC of 39 K is reached. As the additional electron introduced by Ce3+ fills the hole associated with [MnSb4]9−, the screening of the Mn moments is reduced. This leads to an increase in overall moment attributed to Mn in addition to the moment from the Ce3+ f electron. Increasing Ce content also leads to an increase in electrical resistivity, an expected effect from reducing the carrier concentration.

Yb14−xTmxMnSb11 (0

The compounds, Yb14−xTmxMnSb11 (0<x<0.5), were synthesized as single crystals via Sn flux. The maximum amount of Tm was determined from wavelength dispersive microprobe analysis to be x=0.45 (1). Single crystal data are consistent with the Ca14AlSb11 structure type and one formula unit can be described as consisting of 14 Yb2+ cations, MnSb4 tetrahedron, Sb3 linear anion and 4 Sb3- anions. The site occupancy of the Tm atom was not determined because of the small amount and small difference in scattering between Tm and Yb. The difference in bond lengths or angles between the solid solution and pristine Yb14MnSb11 is less than 1%. The magnetic susceptibility of samples prepared by Sn flux with the nominal composition of Yb14−xTmxMnSb11 (x=0.1, 0.2, 0.3, and 0.4), show that these phases are ferromagnetic at low temperature with the TC decreasing from 56 to 39K with increasing x. Magnetic saturation for the x=0.4 composition at 5K is consistent with Tm being present as Tm3+ and a negative magnetoresistance ratio (MR=[(ρ(H)−ρ(0))/ρ(H)]×100%) of 18% at 3T is observed.

Enhanced High-Temperature Thermoelectric Performance of Yb14–xCaxMnSb11

The high temperature p-type thermoelectric material Yb(14)MnSb(11) has been of increasing research interest since its high temperature thermoelectric properties were first measured in 2006. Subsequent substitutions of Zn, Al, and La into the structure have shown that this material can be further optimized by altering the carrier concentration or by reduction of spin-disorder scattering. Here the properties of the Yb(14-x)Ca(x)MnSb(11) solid solution series where isovalent Ca(2+) is substituted for Yb(2+) will be presented. Crystals of the Yb(14-x)Ca(x)MnSb(11) solid solution series were made by Sn-flux (x = 2, 4, 6, 8) with the following ratio of elements: (14-x)Yb: xCa: 6 Mn: 11Sb: 86Sn, and their structures determined by single crystal X-ray diffraction. The density of the material significantly decreases by over 2 g/cm(3) as more Ca is added (from x = 1 to 8), because of the lighter mass of Ca. The resulting lower density is beneficial from a device manufacturing perspective where there is often a trade-off with the specific power per kilogram. The compounds crystallize in the Ca(14)AlSb(11) structure type. The Ca substitution contributes to systematic lengthening the Mn-Sb bond while shortening the Sb-Sb bond in the 3 atom linear unit with increasing amounts of Ca. Temperature dependent thermoelectric properties, Seebeck, electrical resistivity, and thermal conductivity were measured from room temperature to 1273 K. Substitution of Yb with Ca improves the Seebeck coefficient while decreasing the thermal conductivity, along with decreasing the carrier concentration in this p-type material resulting in an enhanced thermoelectric figure of merit, zT, compared to Yb(14)MnSb(11).

Colossal Magnetoresistance in the Transition-Metal Zintl Compound Eu14MnSb11

Magnetoresistance measurements have been carried out on single-crystal sample of the transition-metal Zintl compound Eu14MnSb11. Eu14MnSb11 is the first compound of the Ca14AlSb11 structure type in which magnetoresistive effects have been measured. Colossal magnetoresistance (CMR) at 92 K and 5 T with Δρ/ρ(0) = −36% and at 5 K and 5 T with Δρ/ρ(0) = −42% were observed. CMR is observed at the ferromagnetic−paramagnetic phase transition (92 K). A large negative magnetoresistance is observed below the second magnetic transition (15 K) and is attributed to a field-induced antiferromagnetic−ferromagnetic ordering of the Eu2+ lattice. The MR is highly anisotropic with respect to the crystallographic axes. A sign change in the curvatures of ρ(H) curves below and above 92 K has been observed, indicating that two different mechanisms are responsible for the magnetic scattering of the carriers in two separate temperature regions.

Sr13□NbAs11 and Eu13□NbAs11 – Defect Variants of the Ca14AlSb11 Structure with asymmetric [As3]7− anions

AbstractThe novel compounds Sr13NbAs11 and Eu13NbAs11 have been synthesized from SrAs, Eu5As4, Sr, Nb and As in niobium ampoules at 1173–1273 K. The tetragonal tI 200 phases are defect variants of the Ca14AlSb11 structure (space group I41/acd (no. 142); Sr13□NbAs11: a = 1649.8(2) and c = 2214.1(3); Eu13□NbAs11: a = 1632.9(8) and c = 2197.3(8) pm; Z = 8). The structures are built from the cations Sr2+, and Eu2+, respectively, and from the anions [NbAs4]7−, As3−, and the linear polyanion [As3]7−. This polyanion (isosteric to I3−) is asymmetric with d(AsAs) = 273.0 and 346.0 pm (Sr) and 274.7 and 335.6 pm (Eu), respectively. The bond lengths in the tetrahedral anions are d(NbAs) = 250.8 and 251.1 pm. The complete structural arrangement is related to that of Cu2O by forming two interpenetrating networks. The oxygen atoms are substituted by niobium centered As4 tetrahedra, and the Cu atoms are substituted by As6 octahedra (centered by Sr, Eu). The central As atoms of the polyanions connect the nets. Both As networks are enveloped by the remaining cations forming cubes, tetragonal antiprisms and capped trigonal prisms.