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Yb 14 MnSb 11 and Yb 14 MgSb 11 are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca 14 AlSb 11 structure type have not matched their stability and efficiency. First-principles computations show that the features in the electronic structures that have been identified to lead to high thermoelectric performances are present in Yb 14 ZnSb 11 , which has been presumed to be a poor thermoelectric material. We show that the previously reported low power factor of Yb 14 ZnSb 11 is not intrinsic and is due to the presence of a Yb 9 Zn 4+ x Sb 9 impurity uniquely present in the Zn system. Phase-pure Yb 14 ZnSb 11 synthesized through a route avoiding the impurity formation reveals its exceptional high-temperature thermoelectric properties, reaching a peak zT of 1.2 at 1175 K. Beyond Yb 14 ZnSb 11 , the favorable band structure features for thermoelectric performance are universal among the Ca 14 AlSb 11 structure type, opening the possibility for high-performance thermoelectric materials.

A rare-earth-containing compound, ytterbium aluminium antimonide, Yb3AlSb3(Ca3AlAs3-type structure), has been successfully synthesized within the Yb–Al–Sb system through flux methods. According to the Zintl formalism, this structure is nominally made up of (Yb2+)3[(Al1−)(1b– Sb2−)2(2b– Sb1−)], where1band2bindicate 1-bonded and 2-bonded, respectively, and Al is treated as part of the covalent anionic network. The crystal structure features infinite corner-sharing AlSb4tetrahedra, [AlSb2Sb2/2]6−, with Yb2+cations residing between the tetrahedra to provide charge balance. Herein, the synthetic conditions, the crystal structure determined from single-crystal X-ray diffraction data, and electronic structure calculations are reported.

With the emergence of graphene, the first two-dimensional (2D) material, many other 2D materials have been discovered and examined for novel applications. Various synthesis approaches have been emp...

Thermoelectric materials can convert heat into electricity. They are used to generate electricity when other power sources are not available or to increase energy efficiency by recycling waste heat. The Yb21Mn4Sb18 phase was previously shown to have good thermoelectric performance due to its large Seebeck coefficient (∼290 μV K−1) and low thermal conductivity (0.4 W m−1 K−1). These characteristics stem respectively from the unique [Mn4Sb10]22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb21Mn4−xCdxSb18 (x = 0, 0.5, 1.0, 1.5) and Yb21−yCayMn4Sb18 (y = 3, 6, 9, 10.5) have been prepared, their structures characterized and thermoelectric properties from room temperature to 800 K measured. A detailed look into the structural disorder for the Cd and Ca solid solutions was performed using synchrotron powder X-ray diffraction and pair distribution function methods and shows that these are highly disordered structures. The substitution of Cd gives rise to more metallic behavior whereas Ca substitution results in high resistivity. As both Cd and Ca are isoelectronic substitutions, the changes in properties are attributed to changes in the electronic structure. Both solid solutions show that the thermal conductivities remain extremely low (∼0.4 W m−1 K−1) and that the Seebeck coefficients remain high (>200 μV K−1). The temperature dependence of the carrier mobility with increased Ca substitution, changing from approximately T−1 to T−0.5, suggests that another scattering mechanism is being introduced. As the bonding changes from polar covalent with Yb to ionic for Ca, polar optical phonon scattering becomes the dominant mechanism. Experimental studies of the Cd solid solutions result in a max zT of ∼1 at 800 K and, more importantly for application purposes, a ZTavg ∼ 0.6 from 300 K to 800 K.

The mineral Zlatogorite, CuNiSb2, was synthesized in the laboratory for the first time by annealing elements at ambient pressure (CuNiSb2-AP). Rietveld refinement of synchrotron powder X-ray diffraction data indicates that CuNiSb2-AP crystallizes in the NiAs-derived structure (P3m1, #164) with Cu and Ni ordering. The structure consists of alternate NiSb6 and CuSb6 octahedral layers via face-sharing. The formation of such structure instead of metal disordered NiAs-type structure (P63/mmc, #194) is validated by the lower energy of the ordered phase by first-principle calculations. Interatomic crystal orbital Hamilton population, electron localization function, and charge density analysis reveal strong Ni-Sb, Cu-Sb, and Cu-Ni bonding and long weak Sb-Sb interactions in CuNiSb2-AP. The magnetic measurement indicates that CuNiSb2-AP is Pauli paramagnetic. First-principle calculations and experimental electrical resistivity measurements reveal that CuNiSb2-AP is a metal. The low Seebeck coefficient and large thermal conductivity suggest that CuNiSb2 is not a potential thermoelectric material. Single crystals were grown by chemical vapor transport. The high pressure sample (CuNiSb2-8 GPa) was prepared by pressing CuNiSb2-AP at 700 °C and 8 GPa. However, the structures of single crystal and CuNiSb2-8 GPa are best fit with a disordered metal structure in the P3m1 space group, corroborated by transmission electron microscopy.

We report on a solution-growth based method to synthesise single crystals of Y$_2$Co$_3$ and on its structural and magnetic properties. We find that Y$_2$Co$_3$ crystallizes in the La2Ni3-type orthorhombic structure with space group Cmce (No. 64), with Co forming distorted kagome lattices. Y$_2$Co$_3$ orders antiferromagnetically below $T_N$ = 252 K. Magnetization measurements reveal that the moments are primarily aligned along the b axis with evidence for some canting. Band-structure calculations indicate that ferromagnetic and antiferromagnetic orders are nearly degenerate, at odds with experimental results. Magnetization measurements under pressure up to 1 GPa reveal that the N/'eel temperature decreases with the slope of -1.69 K/GPa. We observe a field-induced spin-flop transition in the magnetization measurements at 1.5 K and 21 T with magnetic field along the b direction. The magnetization is not saturated up to 35 T, indicating that the antiferromagnetic ordering in Y$_2$Co$_3$ is quite robust, which is surprising for such a Co-rich intermetallic.

FeAs2−xSex (x = 0.30–1.0) samples were synthesized as phase pure powders by conventional solid-state techniques and as single crystals (x = 0.50) from chemical vapor transport. The composition of the crystals was determined to be Fe1.025(3)As1.55(3)Se0.42(3), crystallizing in the marcasite structure type, Pnnm space group. FeAs2−xSex (0 < x < 1) was found to undergo a marcasite-to-arsenopyrite (P21/c space group) structural phase transition at x ∼ 0.65. The structures are similar, with the marcasite structure best described as a solid solution of As/Se, whereas the arsenopyrite has ordered anion sites. Magnetic susceptibility and thermoelectric property measurements from 300–2 K were performed on single crystals, FeAs1.50Se0.50. Paramagnetic behavior is observed from 300 to 17 K and a Seebeck coefficient of −33 μV K−1, an electrical resistivity of 4.07 mΩ cm, and a very low κl of 0.22 W m−1 K−1 at 300 K are observed. In order to determine the impact of the structural transition on the high-temperature thermoelectric properties, polycrystalline FeAs2−xSex (x = 0.30, 0.75, 0.85, 1.0) samples were consolidated into dense pellets for measurements of thermoelectric properties. The x = 0.85 sample shows the best thermoelectric performance. The electronic structure of FeAsSe was calculated with DFT and transport properties were approximately modeled above 500 K.

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.

The Zintl phases, Yb14 MSb11 (M = Mn, Mg, Al, Zn), are now some of the highest thermoelectric efficiency p-type materials with stability above 873 K. Yb14MnSb11 gained prominence as the first p-type thermoelectric material to double the efficiency of SiGe alloy, the heritage material in radioisotope thermoelectric generators used to power NASA's deep space exploration. This study investigates the solid solution of Yb14Mg1-x Al x Sb11 (0 ≤ x ≤ 1), which enables a full mapping of the metal-to-semiconductor transition. Using a combined theoretical and experimental approach, we show that a second, high valley degeneracy (N v = 8) band is responsible for the groundbreaking performance of Yb14 MSb11 This multiband understanding of the properties provides insight into other thermoelectric systems (La3-x Te4, SnTe, Ag9AlSe6, and Eu9CdSb9), and the model predicts that an increase in carrier concentration can lead to zT > 1.5 in Yb14 MSb11 systems.

Correction for ‘Measured and simulated thermoelectric properties of FeAs2−xSex (x = 0.30–1.0): from marcasite to arsenopyrite structure’ by Christopher J. Perez et al., Mater. Adv., 2020, 1, 1390–1398, DOI: 10.1039/D0MA00371A.

${\mathrm{Yb}}_{14}\mathrm{Mn}{\mathrm{Sb}}_{11}$ is a magnetic Zintl compound as well as being one of the best high temperature $p$-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{MnSb}}_{4}^{9\ensuremath{-}}$, 1 ${\mathrm{Sb}}_{3}^{7\ensuremath{-}}$, and 4 ${\mathrm{Sb}}^{3\ensuremath{-}}$ atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{Yb}}^{3+}$, 1 (Mg, Zn)${\mathrm{Sb}}_{4}^{10\ensuremath{-}}$, 1 ${\mathrm{Sb}}_{3}^{7\ensuremath{-}}$, and 4 ${\mathrm{Sb}}^{3\ensuremath{-}}$. The predicted existence of ${\mathrm{Yb}}^{3+}$ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of ${\mathrm{Yb}}_{14}{\mathrm{MgSb}}_{11}$. In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate ($M=\mathrm{Zn}$), mostly 2+ ($M=\mathrm{Mn}$), to 3+ ($M=\mathrm{Mg}$). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for $M=\mathrm{Mn}$, Mg and temperature dependent for $M=\mathrm{Zn}$. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

BaFe2As2 superconductors doped with Co or Ni were synthesized by heat treatment of the reactive BaH2 precursor and elemental Fe, Co, Ni, and As in a sealed niobium container. The hydride route prov...

Since the report of high zT in Si95Ge5 there has been significant interest in low Ge alloy compositions for thermoelectric applications. The application of YbP was explored as a means to lower thermal conductivity. A series of 3% phosphorus (P) doped n-type Si95Ge5 (SiGe) alloy was reacted with YbH2 (0, 1, 2%). YbP was formed in the SiGe alloy matrix from the reaction between YbH2 and P during the Spark Plasma Sintering (SPS) process. Thermoelectric property measurements were performed on sintered pellets from room temperature to 1273 K. X-ray diffraction patterns were collected from the ground powder samples and confirmed the main phase possessed diamond structured Si95Ge5 (space group: Fd 3 ¯ m) as well as the presence of YbP (space group: Fm 3 ¯ m). The carrier concentration of the sample was controlled by the amount of YbH2 added, removing some of the phosphorus to form YbP. n-type Si95Ge5 alloy samples with higher YbP amounts showed higher electrical resistivity and lower thermal conductivity attributed to loss of the P dopant. Another composite series of n-type Si95Ge5 with YbP were synthesized with additional P compositions (1, 2%). The thermoelectric properties were characterized from room temperature to 1273 K, and the samples possess electrical resistivity, carrier concentrations, and thermal conductivity as expected from the additional P dopant. The presence of YbP lowered lattice thermal conductivity when the sample was appropriately doped. The Seebeck coefficients were measured with both off-axis and uniaxial axis experimental configurations. These results show that the off-axis measurements overestimate the Seebeck coefficients of the Si95Ge5 alloy samples. This is attributed to a cold finger effect and therefore only the uniaxial data are combined for zT calculations. The n-type Si95Ge5 samples with YbP and less than 3% P dopant show similar zT compared with the Si95Ge5 sample with no YbP inclusions and 3% P dopant with a peak zT of 0.6 at 1200 K.

Large crystals of Yb14MgSb11 were grown through a Sn flux method. Magnetic susceptibility measurements yield an effective magnetic moment of 3.4(1) μB, revealing the presence of both divalent and trivalent Yb in Yb14MgSb11. Previously assumed to only contain Yb2+ as in Yb14MnSb11, the mixed valency demonstrates that Yb14MgSb11 is a Zintl phase.

Zintl phases are promising thermoelectric materials because they are composed of both ionic and covalent bonding, which can be independently tuned. An efficient thermoelectric material would have regions of the structure composed of a high-mobility compound semiconductor that provides the “electron–crystal” electronic structure, interwoven (on the atomic scale) with a phonon transport inhibiting structure to act as the “phonon–glass”. The phonon–glass region would benefit from disorder and therefore would be ideal to house dopants without disrupting the electron–crystal region. The solid solution of the Zintl phase, Yb2–xEuxCdSb2, presents such an optimal structure, and here we characterize its thermoelectric properties above room temperature. Thermoelectric property measurements from 348 to 523 K show high Seebeck values (maximum of ∼269 μV/K at 523 K) with exceptionally low thermal conductivity (minimum ∼0.26 W/m K at 473 K) measured via laser flash analysis. Speed of sound data provide additional suppo...

Nanogermanium is a material that has great potential for technological applications, and doped and alloyed Ge nanocrystals (NCs) are actively being considered. New alloys and compositions are possible in colloidal synthesis because the reactions are kinetically rather than thermodynamically controlled. Most of the Group V elements have been shown to be n-type dopants in Ge to increase carrier concentration; however, thermodynamically, Bi shows no solubility in crystalline Ge. Bi-doped Ge NCs were synthesized for the first time in a microwave-assisted solution route. The oleylamine capping ligand can be replaced by dodecanethiol without loss of Bi. A positive correlation between the lattice parameter and the concentration of Bi content (0.5–2.0 mol %) is shown via powder X-ray diffraction and selected area electron diffraction. X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), scanning TEM, and inductively coupled plasma–mass spectroscopy are consistent with the Bi solubility up to ...

As surface ligands play a critical role in the colloidal stability and optoelectronic properties of semiconductor nanocrystals, we used solution NMR experiments to investigate the surface coordination chemistry of Ge nanocrystals synthesized by a microwave-assisted reduction of GeI(2) in oleylamine. The as-synthesized Ge nanocrystals are coordinated to a fraction of strongly bound oleylamide ligands (with covalent X-type Ge–NHR bonds) and a fraction of more weakly bound (or physisorbed) oleylamine, which readily exchanges with free oleylamine in solution. The fraction of strongly bound oleylamide ligands increases with increasing synthesis temperature, which also correlates with better colloidal stability. Thiol and carboxylic acid ligands bind to the Ge nanocrystal surface only upon heating, suggesting a high kinetic barrier to surface binding. These incoming ligands do not displace native oleylamide ligands but instead appear to coordinate to open surface sites, confirming that the as-prepared nanocrystals are not fully passivated. These findings will allow for a better understanding of the surface chemistry of main group nanocrystals and the conditions necessary for ligand exchange to ultimately maximize their functionality.

Recent experimental and theoretical work has demonstrated significant potential to tune the properties of silicon and germanium by adjusting the mesostructure, nanostructure, and/or crystalline structure of these group 14 elements. Despite the promise to achieve enhanced functionality with these already technologically important elements, a significant challenge lies in the identification of effective synthetic approaches that can access metastable silicon and germanium-based extended solids with a particular crystal structure or specific nano/meso-structured features. In this context, the class of intermetallic compounds known as Zintl phases has provided a platform for discovery of novel silicon and germanium-based materials. This review highlights some of the ways in which silicon and germanium-based Zintl phases have been utilized as precursors in innovative approaches to synthesize new crystalline modifications, nanoparticles, nanosheets, and mesostructured and nanoporous extended solids with properties that can be very different from the ground states of the elements.

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.

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.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM-1 s-1 and r2 relaxivity of 32.7 ± 4.7 mM-1 s-1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM-1 s-1 and r2 relaxivity of 1078.5 ± 1.9 mM-1 s-1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP's. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP's.

Within the field of thermoelectric materials for energy conversion magnesium silicide, Mg2Si, is an outstanding candidate due to its low density, abundant constituents and low toxicity. However electronic and thermal tuning of the material is a required necessity to improve its Figure of Merit, zT. Doping of Yb via reactive YbH2 into the structure is performed with the goal of reducing the thermal conductivity. Hydrogen is released as a by-product at high temperatures allowing for facile incorporation of Yb into the structure. We report on the properties of Yb- and Bi-doped Mg2Si prepared with MgH2 and YbH2 with the focus on the synthetic conditions, and samples’ microstructure, investigated by various electron microscopy techniques. Yb is found in the form of both Yb3Si5 inclusions and Yb dopant segregated at the grain boundary substituting for Mg. The addition of 1 at% Yb concentration reduced the thermal conductivity, providing a value of 30 mW/cm K at 800 K. In order to adjust carrier concentration, the sample is additionally doped with Bi. The impact of the microstructure on the transport properties of the obtained material is studied. Idealy, the reduction of the thermal conductivity is achieved by doping with Yb and the electronic transport is adjusted by doping with Bi. Large grain microstructure facilitates the electronic transport. However, the synthetic conditions that provide the optimized microstructure for electrical transport do not facilitate the additional Yb dopant incorporation. Therefore, the Yb and Bi containing sample with the optimized microstructure provides a zT=0.46 at 800 K.

Yb14MnSb11 is the most efficient bulk p-type thermoelectric material for high temperature applications. Materials with Y and Sc substitutions in Yb14MnSb11 were made both in Sn-flux and by ball milling. These small 3+ rare earth (RE) cations were introduced with the goal of providing chemical pressure on the structure. The RE3+ cation is smaller than Yb2+ and also donates one additional electron to this p-type semiconductor. In Yb14−xRExMnSb11 (RE = Sc, Y) the maximum x was about 0.5. X-ray diffraction experiments on the single crystals obtained from Sn-flux showed that Sc preferentially substitutes for Yb(1) and Yb(3), and decreases the size of the unit cell by about 0.3%. Y substitutes on all Yb sites and increases the size of the unit cell by about 0.2%. Samples with Yb14−xRExMnSb11 (x~0.3) were prepared via powder metallurgy and spark plasma sintering for transport and thermal conductivity measurements. Electron microprobe of the Sc-substituted sample showed small regions (≤1 μ m) containing greater amounts of Sc, and X-ray powder diffraction of the ball milled Sc sample could be fitted as phase pure Yb14−xScxMnSb11. Y-substituted samples showed larger regions of excess Y in electron microprobe, and small amounts of Yb4Sb3 in X-ray powder diffraction. The Sc sample has slightly reduced carrier concentration over optimized Yb14MnSb11, while the Y samples have even lower carrier concentrations. These carrier concentrations lead to comparable resistivity to Yb14MnSb11 in the Sc-substituted material, and higher resistivities in the Y-substituted material. All materials had similar Seebeck coefficients that slightly exceed Yb14MnSb11 at high temperatures, with the Sc-substituted sample having the highest despite having a higher carrier concentration. Sc-substituted samples also had a slightly higher thermal conductivity over the Y-substituted samples, which had comparable thermal conductivity to Yb14MnSb11. The zT values of the Sc and Y substituted samples are similar (zT1000 K~0.8), however below that of Yb14MnSb11 due to the compensation of Seebeck and resistivity.

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 53 K to 41 K 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 (1000 Oe). 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.

The thermal stability and thermoelectric properties of type I clathrate K8Al8Si38 up to 873 K are reported. K8Al8Si38 possesses a high absolute Seebeck coefficient value and high electrical resistivity in the temperature range of 323 to 873 K, which is consistent with previously reported low temperature thermoelectric properties. Samples with Ba partial substitution at the K guest atom sites were synthesized from metal hydride precursors. The samples with the nominal chemical formula of K8–xBaxAl8+xSi38–x (x = 1, 1.5, 2) possess type I clathrate structure (cubic, Pm3n), confirmed by X-ray diffraction. The guest atom site occupancies and thermal motions were investigated with Rietveld refinement of synchrotron powder X-ray diffraction. Transport properties of Ba-containing samples were characterized from 2 to 300 K. The K–Ba alloy phases showed low thermal conductivity and improved electrical conductivity compared to K8Al8Si38. Electrical resistivity and Seebeck coefficients were measured over the tempera...

EuGa4 hosts a magnetic Eu2+ sublattice surrounded by a network of covalently-bound Ga atoms with the BaAl4 structure type (space group I4/ m m m ). In this study, we present the synthesis and characterization of three new single crystal substitutional series Eu A x 1 − x Ga4 with A ​= ​Ca, Sr, or La. X-ray diffraction and resistivity measurements show that Ca substitution induced a structural phase transition from the tetragonal crystal structure at high temperatures to the monoclinic crystal structure (CaGa4 type, space group C2/m) at low temperatures and suppressed the antiferromagnetic ordering temperature to 8.8 ​K for x ​= ​0.45. Comparatively, La or Sr substitution maintained the tetragonal crystal structure and suppressed the antiferromagnetic ordering temperatures to 6.7 ​K and 1.6 ​K for (A, x) = (La, 0.37) and (Sr, 0.91), respectively. In addition to suppressing the magnetic order, magnetization and specific heat measurements indicate the onset of anisotropic metamagnetic transitions in (La, 0.18), (La, 0.37), and (Sr, 0.63), along with an incommensurate-to-commensurate magnetic transition in (Sr, 0.38). By comparing these effects of doping EuGa4, we show how size, disorder, and charge determine the structure-physical property relations in EuGa4.

The solid solution $\mathrm{Eu}{({\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x})}_{4}$ was grown in single crystal form to reveal a rich variety of crystallographic, magnetic, and electronic properties that differ from the isostructural end compounds ${\mathrm{EuGa}}_{4}$ and ${\mathrm{EuAl}}_{4}$, despite the similar covalent radii and electronic configurations of Ga and Al. Here we report the onset of magnetic spin reorientation and metamagnetic transitions for $x=0--1$ evidenced by magnetization and temperature-dependent specific heat measurements. ${T}_{\mathrm{N}}$ changes nonmonotonously with $x$, and it reaches a maximum around 20 K for $x=0.50$, where the $a$ lattice parameter also shows an extreme (minimum) value. Anomalies in the temperature-dependent resistivity consistent with charge density wave behavior exist only for $x=0.50$ and 1. Density functional theory calculations show increased polarization between the Ga-Al covalent bonds in the $x=0.50$ structure compared to the end compounds, such that crystallographic order and chemical pressure are proposed as the causes of the charge density wave behavior.

Two series of crystals were prepared via Sn flux synthesis with the compositional fluxes of Yb14−xLnxMnSb11, where x = 0.1–0.9 and x = 0.4. By X-ray structural measurements and microprobe analysis, the maximum amount of Ln incorporated in the unit cell matrix were determined to be 0.37 ± 0.04 for the La–Nd and 0.45 ± 0.04 for Sm–Tm elements with solid substitution solution formation. The Ln incorporation did not change the unit cell significantly but the cell volume decreased going from the largest La–Nd to the smallest Tm–Lu cations. The Ln0.30–0.40 samples demonstrated congruent melting and their melting points increased by ∼30–50 °C compared to the pristine matrix. The temperatures were attributed to the ordered structural state due to the Ln distribution in the unit cell only through the one regular system site. Based on geometrical fitting between crystal radii of Yb2+ and Ln3+ in six-coordination, the Yb(2) sites were found to be more preferable for substitution by La–Nd, Yb(1) by Sm–Ho and Yb(3) by Tm and Lu atoms. Thermal losses as a temperature function of the alloyed by La and Lu samples were determined by a step-by-step heating procedure with analysis of the vapor condensate deposited on the viewing window of the chamber. This experiment demonstrated a high mobility of the tetrahedral Mn and Sb along with Yb ions in the Yb14MnSb11 matrix with incongruent sublimation beyond 1300 °C and a decrease of the thermal weight losses by half if the matrix was alloyed by La. Occupation of the Yb sites by Ln atoms varied the geometry of the MnSb4 tetrahedron as well as electron properties and bonding in this structural fragment, and these changes are considered in the context of the coupling between chemical structure and thermal stability of the compounds. The improved thermal stability due to increasing the total ionic state of the alloyed samples was found to possibly be a useful factor for the long-time use of these materials for space applications.

The syntheses of YbTe1−x Sb x (x = 0, 0.05, 0.2, 0.5, 0.8, 1) were investigated by solid state reactions and formed into dense pellets by spark plasma sintering. X-ray powder diffraction and microprobe analysis indicated no solubility of Sb in YbTe, and these phases are better described as composite phases (YbTe)1−x (YbSb) x (x = 0, 0.05, 0.2, 0.5, 0.8, 1). Thermal conductivity, electrical resistivity, and Seebeck coefficients were acquired for the larger values of x (x = 0.2, 0.5, 0.8, 1) from room temperature to 773 K, and the figure of merit was calculated. Thermal conductivities for x = 0, 0.05 are also reported; however, measurements of Seebeck coefficients and electrical resistivity were not possible due to large resistivity. The figure of merit for all samples was low, and the maximum zT measured was zT 791K = 0.018 for YbSb. Low figures of merit were primarily the result of very high resistivity in YbTe rich samples, and high thermal conductivity, and a small Seebeck coefficient in all samples.

Solid-solution Zintl compounds with the formula Eu11Cd6–xZnxSb12 have been synthesized from the elements as single crystals using a tin flux according to the stoichiometry Eu:Cd:Zn:Sb:Sn of 11:6–xp:xp:12:30 with xp = 0, 1, 2, 3, 4, 5, and 6, where xp is the preparative amount of Zn employed in the reaction. The crystal structures and the compositions were established by single-crystal as well as powder X-ray diffraction and wavelength-dispersive X-ray analysis measurements. The title solid-solution Zintl compounds crystallize isostructurally in the centrosymmetric monoclinic space group C 2/m (No. 12, Z = 2) as the Sr11Cd6Sb12 structure type (Pearson symbol mC58). There is a miscibility gap at 3 ≤ xp ≤ 4 where the major product crystallizes in a disordered structure related to the Ca9Mn4Bi9 structure type; otherwise, for all other compositions, the Sr11Cd6Sb12 structure is the majority phase. Eu11Cd6Sb12 shows lower lattice thermal conductivity relative to Eu11Zn6Sb12 consistent with its higher mean atomi...

A series of alkali metal containing compounds with type I clathrate structure, A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38, were synthesized and characterized. Room temperature lattice parameters of A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38 were determined to be 10.424916(10), 10.470174(13), 10.535069(15), and 10.48071(2) A, respectively. The type I clathrate structure (cubic, Pm3n) was confirmed for all phases, and in the case of K8Al8Si38 and K8Ga8Si38, the structures were also refined using synchrotron powder diffraction data. The samples were consolidated by Spark Plasma Sintering (SPS) for thermoelectric property characterization. Electrical resistivity was measured by four probe AC transport method in the temperature range of 30 to 300 K. Seebeck measurements from 2 to 300 K were consistent with K8Al8Si38 and K8Ga8Si38 being n-type semiconductors, while Rb8Ga8Si38 and Cs8Ga8Si38 were p-type semiconductors. K8Al8Si38 shows the lowest electrical resistivity and the highest Seebeck coefficient. This phase a...

Bulk nanocrystalline Mg2Si thermoelectric materials were synthesized and consolidated in a one-step process through a solid-state reaction between magnesium hydride and silicon, using the spark plasma sintering (SPS) method. The hydrogen produced in the process alleviates the problem of the oxidation of Mg. The samples were reactively sintered at temperatures in the range 723–823 K and under a uniaxial pressure in the range of 71–164 MPa in 5 min. Powder X-ray diffraction (XRD) analysis showed the products to be pure Mg2Si. The grain size of the consolidated samples was less than 500 nm, as determined by transmission electron spectroscopy (TEM). Residual nano-pores were observed by scanning electron microscopy at grain boundaries; their presence is believed to be the consequence of hydrogen evolution during the reactive sintering. The effect of synthesis temperature and pressure on crystallite size, density, and transport properties was determined. The results showed that use of MgH2 instead of Mg in the onestep method prevents the formation of MgO. The addition of 1 at.% Bi as a dopant improved the power factor significantly. Samples with 1 at.% Bi had a ZT of 0.6 at 775 K.

Na + extraction from Na 2 Ti 2 Sb 2 O has been attempted using polytetrafluoroethylene (PTFE; empirical formula CF 2 ) as the extraction reagent. Systematic increase in both lattice parameters a along the intralayer direction and c along the interlayer direction was observed with respect to the reacted PTFE amounts, suggesting the successful deintercalation of the interlayer Na + . In addition, ferromagnet-like behavior, which is rare for systems consisting of ions with low magnetic moments, such as Ti 3+ (d 1 ), was observed in the Na + -deintercalated Na 2 Ti 2 Sb 2 O. This result suggests that the method of alkali–metal ion extraction using polytetrafluoroethylene as an extraction reagent (AEP) can be utilized in order to induce interesting and important properties in compounds including highly air- and solvent-sensitive mixed-anion compounds such as Na 2 Ti 2 Sb 2 O.

We have developed a fast, easy, and scalable synthesis method for Ba1xKxFe2As2 (0 ≤ x ≤ 1) superconductors using hydrides BaH2 and KH as a source of barium and potassium metals. Synthesis from hydrides provides better mixing and easier handling of the starting materials, consequently leading to faster reactions and/or lower synthesis temperatures. The reducing atmosphere provided by the evolved hydrogen facilitates preparation of oxygen-free powders. By a combination of methods we have shown that Ba1xKxFe2As2 obtained via hydride route has the same characteristics as when it is prepared by traditional solid-state synthesis. Refinement from synchrotron powder X-ray diffraction data confirms a linear dependence of unit cell parameters upon K content as well as the tetragonal to orthorhombic transition at low temperatures for compositions with x < 0.2. Magnetic measurements revealed dome-like dependence of superconducting transition temperature Tc upon K content with a maximum of 38 K for x close to 0.4. Electron diffraction and high-resolution high-angle annular dark-field scanning transmission electron microscopy indicates an absence of Ba/K ordering, while local inhomogeneity in the Ba/K distribution takes place at a scale of several angstroms along [110] crystallographic direction.

A thermochemical study of three germanium allotropes by differential scanning calorimetry (DSC) and oxidative high-temperature drop solution calorimetry with sodium molybdate as the solvent is described. Two allotropes, microcrystalline allo-Ge (m-allo-Ge) and 4H-Ge, have been prepared by topotactic deintercalation of Li_7Ge_(12) with methanol (m-allo-Ge) and subsequent annealing at 250 °C (4H-Ge). Transition enthalpies determined by differential scanning calorimetry amount to 4.96(5) ± 0.59 kJ/mol (m-allo-Ge) and 1.46 ± 0.55 kJ/mol (4H-Ge). From high-temperature drop solution calorimetry, they are energetically less stable by 2.71 ± 2.79 kJ/mol (m-allo-Ge) and 5.76 ± 5.12 kJ/mol (4H-Ge) than α-Ge, which is the stable form of germanium under ambient conditions. These data are in agreement with DSC, as well as with the previous quantum chemical calculations. The morphology of the m-allo-Ge and 4H-Ge crystallites was investigated by a combination of scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. Even though the crystal structures of m-allo-Ge and 4H-Ge cannot be considered as truly layered, these phases retain the crystalline morphology of the layered precursor Li_7Ge_(12). Investigation by diffuse reflectance infrared Fourier transform spectroscopy and UV–vis diffuse reflectance measurements reveal band gaps in agreement with quantum chemical calculations.

Applications of Ge nanocrystals (NCs) are limited by the stability and air reactivity of the Ge surface. In order to promote stability and increase the diversity of ligand functionalization of Ge NCs, the preparation of thiol-passivated Ge NCs via a ligand exchange process was investigated. Herein a successful replacement of oleylamine ligands on the surface of Ge NCs with dodecanethiol is reported. The successful ligand exchange was monitored by FTIR and NMR spectroscopy and it was found that dodecanethiol provided a better surface coverage, leading to stable Ge NC dispersions. Dodecanethiol capping also enabled band gap determination of the NCs by surface photovoltage (SPV) spectroscopy. The SPV measurements indicated an efficient charge separation in the ligand-exchanged Ge NCs. On the other hand, oleylamine-terminated Ge NCs of similar sizes exhibited a very small photovoltage, indicating a poorly passivated surface.

The compounds, Yb14−xTmxMnSb11 (0

Zintl phases have garnered a great deal of attention for many applications. The term “Zintl phase” recognizes the contributions of the German chemist Eduard Zintl to the field of solid-state chemistry. While Zintl phases were initially defined as a subgroup of intermetallic phases where cations and anions or polyanions in complex intermetallic structures are valence satisfied, the foundational idea of electron counting to understand complex solid-state structures has provided insight into bonding and a bridge between solid-state and molecular chemists. This Special Issue, “Advances in Zintl Phases”, provides a collage of research in the area, from solution to solid-state chemistry.

Single crystals of Ce3Ni25.75Ru3.16Al4.1B10 were obtained from a process in which a polycrystalline sample of CeRu2Al2B was annealed in an excess of a Ni–In flux. The initial phase, CeRu2Al2B, does not recrystallize, instead, crystals of a new phase, Ce3Ni25.75Ru3.16Al4.1B10, could be isolated once the flux was removed. The title compound crystallizes in the tetragonal space group P4/nmm (No. 129) with a=1139.02(8), c=801.68(6) pm (c/a=0.70) in the Nd3Ni29Si4B10 structure type. Electrical resistivity measurements reveal metallic behavior with a minimum of 700 µΩ cm and a small residual resistivity ratio of RRR=1.4 indicating a large amount of disorder scattering. The cerium atoms are either in the 4+ or an intermediate valence state with a valence fluctuation temperature far above room temperature.

Water-soluble poly(allylamine) Mn2+-doped Si (SiMn) nanoparticles (NPs) were prepared and show promise for biologically related applications. The nanoparticles show both strong photoluminescence and good magnetic resonance contrast imaging. The morphology and average diameter were obtained through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM); spherical crystalline Si NPs with an average diameter of 4.2 ± 0.7 nm were observed. The doping maximum obtained through this process was an average concentration of 0.4 ± 0.3% Mn per mole of Si. The water-soluble SiMn NPs showed a strong photoluminescence with a quantum yield up to 13%. The SiMn NPs had significant T1 contrast with an r1 relaxivity of 11.1 ± 1.5 mM–1 s–1 and r2 relaxivity of 32.7 ± 4.7 mM–1 s–1 where the concentration is in mM of Mn2+. Dextran-coated poly(allylamine) SiMn NPs produced NPs with T1 and T2 contrast with a r1 relaxivity of 27.1 ± 2.8 mM–1 s–1 and r2 relaxivity of 1078.5 ± 1.9 mM–1 s–1. X-band electron paramagnetic resonance spectra are fit with a two-site model demonstrating that there are two types of Mn2+ in these NP’s. The fits yield hyperfine splittings (A) of 265 and 238 MHz with significant zero field splitting (D and E terms). This is consistent with Mn in sites of symmetry lower than tetrahedral due to the small size of the NP’s.

Earth abundant element clathrate phases are of interest for a number of applications ranging from photovoltaics to thermoelectrics. Silicon-containing type I clathrate is a framework structure with the stoichiometry A8-xSi46 (A = guest atom such as alkali metal) that can be tuned by alloying and doping with other elements. The type I clathrate framework can be described as being composed of two types of polyhedral cages made up of tetrahedrally coordinated Si: pentagonal dodecahedra with 20 atoms and tetrakaidecahedra with 24 atoms in the ratio of 2:6. The cation sites, A, are found in the center of each polyhedral cage. This review focuses on the newest discoveries in the group 13-silicon type I clathrate family: A8E8Si38 (A = alkali metal; E = Al, Ga) and their properties. Possible approaches to new phases based on earth abundant elements and their potential applications will be discussed.