31 research outputs found
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Thermochemistry study and improved thermal stability of Yb14MnSb11 alloyed by Ln3+ (La-Lu)
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
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The remarkable crystal chemistry of the Ca 14 AlSb 11 structure type, magnetic and thermoelectric properties
Yb MnSb 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 Ca AlSb , 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. 14 11 14 1
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Synthesis, structure, thermoelectric properties, and band gaps of alkali metal containing type I clathrates: A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38
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) Å, respectively. The type I clathrate structure (cubic, Pm3Ì...n) 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 also showed the largest thermal conductivity at room temperature of ∼1.77 W/Km. K8Ga8Si38 provides the lowest thermal conductivity, below 0.5 W/Km, comparable to the type I clathrate with heavy elements such as Ba8Ga16Ge30. Surface photovoltage spectroscopy on films shows that these compounds are semiconductors with band gaps in the range 1.14 to 1.40 eV
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Intermediate Yb valence in the Zintl phases Yb14MSb11(M=Zn,Mn,Mg): XANES, magnetism, and heat capacity
Yb14MnSb11 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 Yb2+, 1 MnSb49-, 1 Sb37-, and 4 Sb3- atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb2+, 1 Yb3+, 1 (Mg, Zn)Sb410-, 1 Sb37-, and 4 Sb3-. The predicted existence of Yb3+ 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 Yb14MgSb11. In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate (M=Zn), mostly 2+ (M=Mn), to 3+ (M=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=Mn, Mg and temperature dependent for M=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
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Charge density wave behavior and order-disorder in the antiferromagnetic metallic series Eu(Ga1-x Alx)4
The solid solution Eu(Ga1-xAlx)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 EuGa4 and EuAl4, 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. TN 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
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Charge density wave behavior and order-disorder in the antiferromagnetic metallic series Eu(Ga1-x Alx)4
The solid solution Eu(Ga1-xAlx)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 EuGa4 and EuAl4, 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. TN 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
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Magnetism and negative magnetoresistance of two magnetically ordering, rare-earth-containing zintl phases with a new structure type: EuGa 2Pn2 (Pn=P, As)
Single crystals of EuGa2Pn2 (Pn=P, As) were grown from a molten Ga flux and characterized by single-crystal X-ray diffraction at 100(1) K. They are isostructural and crystallize in a new structure type (monoclinic, P2/m, a=9.2822(9) A, =3.8967(4) A, c=12.0777(11) A, Î’=95.5220(10), R1= 0.0148, wR2=0.0325 (EuGa2P2) and a=9.4953(7) A, b=4.0294(3) A, c=12.4237(9) A, Î’ =95.3040(10), R1=0.0155, wR2=0.0315 (EuGa2As2)). The structures consist of alternating layers of two-dimensional Ga2Pn2 anions and Eu cations. The anion layers are composed of Ga2Pn6 staggered, ethane-like moieties having a rare Ga-Ga bonding motif; these moieties are connected in a complex fashion bymeans of shared Pn atoms. Both structures showsmall residual electron densities that can be modeled by adding a Eu atom and removing two bonded Ga atoms, resulting in structures (< 2%) wheremost of the atoms are the same, but there is a difference in bonding that leads to one-dimensional ribbons of parallel Ga2Pn6 staggered, ethane-like moieties. The compounds can be understood within the Zintl formalism, but show metallic resistivity. Magnetization measurements performed on single crystals show low-temperature magnetic anisotropy as well as multiple magnetic ordering events that occur at and below 24 and 20 K for the phosphorus and arsenic analogs, respectively. The magnetic coupling between Eu ions is attributed to indirect exchange via an RKKY interaction, which is consistent with the metallic behavior. The compounds display large negative magnetoresistance of up to-80 and-30%(MR=[(F(H)-F(0))/F(H)] 100%) for Pn=P,As, respectively,which is maximal at the magnetic ordering temperatures in the highest measured field (5T)
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Magnetism and negative magnetoresistance of two magnetically ordering, rare-earth-containing zintl phases with a new structure type: EuGa 2Pn2 (Pn=P, As)
Single crystals of EuGa2Pn2 (Pn=P, As) were grown from a molten Ga flux and characterized by single-crystal X-ray diffraction at 100(1) K. They are isostructural and crystallize in a new structure type (monoclinic, P2/m, a=9.2822(9) A, =3.8967(4) A, c=12.0777(11) A, Î’=95.5220(10), R1= 0.0148, wR2=0.0325 (EuGa2P2) and a=9.4953(7) A, b=4.0294(3) A, c=12.4237(9) A, Î’ =95.3040(10), R1=0.0155, wR2=0.0315 (EuGa2As2)). The structures consist of alternating layers of two-dimensional Ga2Pn2 anions and Eu cations. The anion layers are composed of Ga2Pn6 staggered, ethane-like moieties having a rare Ga-Ga bonding motif; these moieties are connected in a complex fashion bymeans of shared Pn atoms. Both structures showsmall residual electron densities that can be modeled by adding a Eu atom and removing two bonded Ga atoms, resulting in structures (< 2%) wheremost of the atoms are the same, but there is a difference in bonding that leads to one-dimensional ribbons of parallel Ga2Pn6 staggered, ethane-like moieties. The compounds can be understood within the Zintl formalism, but show metallic resistivity. Magnetization measurements performed on single crystals show low-temperature magnetic anisotropy as well as multiple magnetic ordering events that occur at and below 24 and 20 K for the phosphorus and arsenic analogs, respectively. The magnetic coupling between Eu ions is attributed to indirect exchange via an RKKY interaction, which is consistent with the metallic behavior. The compounds display large negative magnetoresistance of up to-80 and-30%(MR=[(F(H)-F(0))/F(H)] 100%) for Pn=P,As, respectively,which is maximal at the magnetic ordering temperatures in the highest measured field (5T)
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Localized states within the gap of Ce3 Au3 Sb4
The temperature dependence of the specific heat and of the resistivity under pressure has been measured for single crystals of the semiconductor Ce3 Au3 Sb4. The transport data follow an exponential activation and variable range hopping at low T, consistent with weak disorder and localization, while C / T has a - ln T dependence with large entropy. Thus the properties of Ce3 Au3 Sb4 are very different from those of ordinary Kondo insulators. © 2007