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 Yb 21 Mn 4 Sb 18 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 [Mn 4 Sb 10 ] 22− subunit and the large unit cell/site disorder inherent in this phase. The solid solutions, Yb 21 Mn 4− x Cd x Sb 18 ( x = 0, 0.5, 1.0, 1.5) and Yb 21− y Ca y Mn 4 Sb 18 ( 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 ZT avg ∼ 0.6 from 300 K to 800 K.
more »
« less
Tuning the magnetic ground state of Ce1-xYbxRhIn5 by Yb valence fluctuations
We characterize the properties of Ce1−xYbxRhIn5 single crystals with 0 x 1 using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. The Yb valence vYb, calculated from the magnetic susceptibility and measured using XANES, decreases from 3+ at x = 0 to ∼2.1+ at xact = 0.2, where xact is the measured Yb concentration. A transition from incommensurate to commensurate
antiferromagnetism is observed in neutron diffraction measurements along Q = (0.5, 0.5, l) between 0.2 xact 0.27; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature TI (TI < TN) for the same concentration range. Magnetic susceptibility measurements also
reveal features which provide additional evidence of magnetic ordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of Ce1−xYbxRhIn5.
more »
« less
- Award ID(s):
- 1810310
- NSF-PAR ID:
- 10105604
- Date Published:
- Journal Name:
- Physical review. B, Condensed matter
- Volume:
- 98
- ISSN:
- 1095-3795
- Page Range / eLocation ID:
- 195118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The mixed‐valence states of dilute Fe (0.2, 0.5, and 1.2 mol%) in bulk polycrystalline (SrTiO3) (STO) were determined. Specimens were sintered in air or O2at 1500°C or 1300°C with the intent to quantify the relative valence states of Fe in the context of understanding microstructure development of Fe‐doped STO. Mössbauer spectroscopy, electron paramagnetic resonance spectroscopy, and magnetometry were combined to track changes in Fe valence, and Raman spectroscopy and X‐ray diffraction were used to describe the associated structural changes. Small concentrations of defect associates were detected, but the majority of Fe′Tiacceptors are isolated from oxygen vacancies. As doping increases, negative exchange interactions increase, consistent with an increase in the concentration of configurations. Additionally, the fraction of dopants existing as isovalent increased with of the sintering atmosphere and with total Fe content.
-
We have measured the superconducting penetration depth (T ) in the heavy-fermion/intermediate-valent superconducting alloy series Ce1−xYbxCoIn5 using a transverse-field muon spin relaxation to study the effect of intermediate-valent Yb doping on Fermi-liquid renormalization. From (T ) we determine the superfluid density ρs (T ) and find that it decreases continuously with increasing nominal Yb concentration x, i.e., with increasing intermediate valence. The temperature-dependent renormalization of the “normal” fluid density ρN (T ) = ρs (0) − ρs (T ) in both the heavy-fermion and intermediate valence limits is proportional to the temperature-dependent renormalization of the specific heat. This indicates that the temperature-dependent Fermiliquid Landau parameters of the superconducting quasiparticles entering the two different physical quantities are the same. These results represent an important advance in understanding of both intermediate valence and heavy-fermion phenomena in superconductors.more » « less
-
Radial X‐Ray Diffraction Study of Superhard Early Transition Metal Dodecaborides under High Pressuremore » « less
Abstract The deformation behavior of the three metal dodecaborides (YB12, ZrB12, and Zr0.5Y0.5B12) is investigated using radial X‐ray diffraction under nonhydrostatic compression up to ≈60 GPa with a goal of understanding how bonding and metal composition control hardness. Zr0.5Y0.5B12, which has the highest Vickers hardness (
Hv = 45.8 ± 1.3 GPa at 0.49 N load), also shows the highest bulk modulus (K 0= 320 ± 5 GPa). The 0.49 N hardness for ZrB12and YB12are both lower and very similar, and both show lower bulk moduli (K 0= 276 ± 7 GPa, andK 0= 238 ± 6 GPa, respectively). Differential stress is then measured to study the strength and strength anisotropy. Zr0.5Y0.5B12supports the highest differential stress, in agreement with its high hardness, a fact that likely arises from atomic size mismatch between Zr and Y combined with the rigid network of boron cages. The (200) plane for all samples supports the largest differential strain, while the (111) plane supports the smallest, consistent with the theoretically predicted slip system of {111} [ ]. Strain softening is also observed for ZrB12. Finally, the full elastic stiffness tensors for ZrB12and YB12are solved. ZrB12is the most isotropic, but the extent of elastic anisotropy for all dodecaborides studied is relatively low due to the highly symmetric boron cage network. -
more » « less
Abstract Haldane topological materials contain unique antiferromagnetic chains with symmetry-protected energy gaps. Such materials have potential applications in spintronics and future quantum computers. Haldane topological solids typically consist of spin-1 chains embedded in extended three-dimensional (3D) crystal structures. Here, we demonstrate that [Ni(μ−4,4′-bipyridine)(μ-oxalate)]n(NiBO) instead adopts a two-dimensional (2D) metal-organic framework (MOF) structure of Ni2+spin-1 chains weakly linked by 4,4′-bipyridine. NiBO exhibits Haldane topological properties with a gap between the singlet ground state and the triplet excited state. The latter is split by weak axial and rhombic anisotropies. Several experimental probes, including single-crystal X-ray diffraction, variable-temperature powder neutron diffraction (VT-PND), VT inelastic neutron scattering (VT-INS), DC susceptibility and specific heat measurements, high-field electron spin resonance, and unbiased quantum Monte Carlo simulations, provide a detailed, comprehensive characterization of NiBO. Vibrational (also known as phonon) properties of NiBO have been probed by INS and density-functional theory (DFT) calculations, indicating the absence of phonons near magnetic excitations in NiBO, suppressing spin-phonon coupling. The work here demonstrates that NiBO is indeed a rare 2D-MOF Haldane topological material.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/DOI:10.1103/PhysRevB.98.195118
