213 research outputs found
High-Temperature Transport Properties of the Zintl Phases Yb_(11)GaSb_9 and Yb_(11)InSb_9
Two rare-earth Zintl phases, Yb_(11)GaSb_9 and Yb_(11)InSb_9, were synthesized in high-temperature self-fluxes of molten Ga and In, respectively. Structures were characterized by both single-crystal X-ray diffraction and powder X-ray diffraction and are consistent with the published orthorhombic structure, with the space group Iba2. High-temperature differential scanning calorimetry (DSC) and thermal gravimetry (TG) measurements reveal thermal stability to 1300 K. Seebeck coefficient and resistivity measurements to 1000 K are consistent with the hypothesis that Yb_(11)GaSb_9 and Yb_(11)InSb_9 are small band gap semiconductors or semimetals. Low doping levels lead to bipolar conduction at high temperature, preventing a detailed analysis of the transport properties. Thermal diffusivity measurements yield particularly low lattice thermal conductivity values, less than 0.6 W/m K for both compounds. The low lattice thermal conductivity suggests that Yb_(11)MSb_9 (M = Ga, In) has the potential for high thermoelectric efficiency at high temperature if charge-carrier doping can be controlled
Magneto-optical behaviour of EuIn_2P_2
We report results of a magneto-optical investigation of the Zintl-phase
compound EuInP. The compound orders magnetically at =24 K and
exhibits concomitant large magnetoresistance effects. For 50 K and
increasing magnetic fields we observe a transfer of spectral weight in
from energies above 1 eV into the low-energy metallic
component as well as into a mid-infrared signal centered at about 600
cm. This latter absorption is reminiscent to what has been seen in a
large variety of so-called Kondo materials and ascribed to excitations across
the hybridization gap. The observed gain of Drude weight upon increasing
magnetic field suggests an enhancement of the itinerant charge-carrier
concentration due to the increasing magnetization, a phenomenon that was
previously observed in other compounds which exhibit colossal magnetoresistive
effects.Comment: 13 pages, 4 figure
Investigations on Tetragonally Distorted Sodium Thallide NaTl‐tI8
In-depth investigations of the long-time known Zintl phase NaTl revealed a phase transition of tetragonal NaTl-tI8 [I4(1)/amd; a = 5.2268(9) angstrom, c = 7.539(1) angstrom, V = 205.97(9) angstrom(3)] to Zintl's cubic NaTl-cF16 [Fd3m; a = 7.4697(6) angstrom, V = 416.79(5) angstrom(3)] between 351 and 355 K. This phase transformation was observed for NaTl prepared by two different synthetic routes including Zintl's original procedure. An excess of sodium applied during the synthesis in liquid ammonia also resulted in the formation of NaTl-tI8. DSC measurements suggest a first order phase transition. In addition to in-situ temperature dependent powder X-ray diffraction experiments, DSC measurements and solid-state NMR investigations, we also performed theoretical DOS and band structure calculations for the cubic and tetragonal phase, respectively. The results suggest Na-Tl interactions in the second coordination sphere being responsible for the observed tetragonal distortion of Zintl's cubic NaTl
Synthesis, structure, magnetism, and high temperature thermoelectric properties of Ge doped Yb_(14)MnSb_(11)
The Zintl phase Yb_(14)MnSb_(11) was successfully doped with Ge utilizing a tin flux technique. The stoichiometry was determined by microprobe analysis to be Yb_(13.99(14))Mn_(1.05(5))Sb_(10.89(16))Ge_(0.06(3)). This was the maximum amount of Ge that could be incorporated into the structure via flux synthesis regardless of the amount included in the reaction. Single crystal X-ray diffraction could not unambiguously determine the site occupancy for Ge. Bond lengths varied by about 1% or less, compared with the undoped structure, suggesting that the small amount of Ge dopant does not significantly perturb the structure. Differential scanning calorimetry/thermogravimetry (DSC/TG) show that the doped compound's melting point is greater than 1200 K. The electrical resistivity and magnetism are virtually unchanged from the parent material, suggesting that Yb is present as Yb^(2+) and that the Ge dopant has little effect on the magnetic structure. At 900 K the resistivity and Seebeck coefficient decrease resulting in a zT of 0.45 at 1100 K, significantly lower than the undoped compound
High thermoelectric efficiency in lanthanum doped Yb14MnSb11
Lanthanum doping of the high-temperature p-type thermoelectric material Yb_(14)MnSb_(11) enhances
the figure of merit zT through carrier concentration tuning. This is achieved by substituting La^(3+)
on the Yb^(2+) site to reduce the free hole concentration as expected from the change in valence. The
high-temperature transport properties (Seebeck coefficient, electrical resistivity, Hall mobility, and
thermal conductivity) of Yb_(13.6)La_(0.4)MnSb_(11) are explained by the change in carrier concentration
using a simple rigid parabolic band model, similar to that found in Yb_(14)Mn_(1−x)A_(lx)Sb_(11). Together, use of these two dopant sites enables the partial decoupling of electronic and structural properties in
Yb_(14)MnSb_(11)-based materials
Anisotropic effect of Cd and Hg doping on Pauli limited superconductor CeCoIn
We investigated the effect of Cd and Hg doping on the first order
superconducting (SC) transition and the high field-low temperature SC state of
CeCoIn by measuring the specific heat of CeCo(InCd) with x=0.0011, 0.0022 and 0.0033 and CeCo(InHg) with x=0.00016, 0.00032, and 0.00048 at temperatures down to 0.1 K and
fields up to 14 T. Cd substitution rapidly suppresses the cross-over
temperature , where the superconducting transition changes from
second to first order, to =0 K with x=0.0022 for [100], while
it remains roughly constant up to x=0.0033 for [001]. The
associated anomaly of the proposed FFLO state in Hg-doped samples is washed out
by x=0.00048, while remaining at the same temperature, indicating high
sensitivity of that state to impurities. We interpret these results as
supporting the non-magnetic, possibly FFLO, origin of the high field - low
temperature state in CeCoIn
Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11
The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by
Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number
(104) of atoms in the primitive cell has precluded any previous full electronic
structure study. Using an efficient, local orbital based method within the
local spin density approximation to study the electronic structure, we find a
gap between a bonding valence band complex and an antibonding conduction band
continuum. The bonding bands lack one electron per formula unit of being
filled, making them low carrier density p-type metals. The hole resides in the
MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment,
leaving a net spin near 4 \mu_B that is consistent with experiment. These
manganites are composed of two disjoint but interpenetrating `jungle gym'
networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within
the same network, and weaker couplings between the networks whose sign and
magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be
ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic)
the ferro- and antiferromagnetic states are calculated to be essentially
degenerate. The band structure of the ferromagnetic states is very close to
half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two
additional figures (Fig.8 and 11 of the paper) are provided in JPG format in
separate files. Submitted to Phys. Rev. B on September 20th 200
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