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15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Antiferromagnetism in EuCu 2 As 2 and EuCu 1.82 Sb 2 single crystals

Single crystals of EuCu 2 As 2 and EuCu 2 Sb 2 were grown from CuAs and CuSb self-flux, respectively. The crystallographic, magnetic, thermal, and electronic transport properties of the single crystals were investigated by room-temperature x-ray diffraction (XRD), magnetic susceptibility χ versus temperature T, isothermal magnetization M versus magnetic field H, specific heat C p (T), and electrical resistivity ρ(T) measurements. EuCu 2 As 2 crystallizes in the body-centered tetragonal ThCr 2 Si 2 -type structure (space group I4/mmm), whereas EuCu 2 Sb 2 crystallizes in the related primitive tetragonal CaBe 2 Ge 2 -type structure (space group P4/nmm). The energy-dispersive x-ray spectroscopy and XRD data for the EuCu 2 Sb 2 crystals showed the presence of vacancies on the Cu sites, yielding the actual composition EuCu 1.82 Sb 2 . The ρ(T) and C p (T) data reveal metallic character for both EuCu 2 As 2 and EuCu 1.82 Sb 2 . Antiferromagnetic (AFM) ordering is indicated from the χ(T),C p (T), and ρ(T) data for both EuCu 2 As 2 (T N = 17.5 K) and EuCu 1.82 Sb 2 (T N = 5.1 K). In EuCu 1.82 Sb 2 , the ordered-state χ(T) and M(H) data suggest either a collinear A-type AFM ordering of Eu +2 spins S = 7/2 or a planar noncollinear AFM structure, with the ordered moments oriented in the tetragonal ab plane in either case. This ordered-moment orientation for the A-type AFM is consistent with calculations with magnetic dipole interactions. As a result, the anisotropic χ(T) and isothermal M(H) data for EuCu 2 As 2 , also containing Eu +2 spins S = 7/2, strongly deviate from the predictions of molecular field theory for collinear AFM ordering and the AFM structure appears to be both noncollinear and noncoplanar.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Metal Flux Growth, Structural Relations, and Physical Properties of EuCu 2 Ge 2 and Eu 3 T 2 In 9 ( T = Cu and Ag)

Single crystals (SCs) of the compounds Eu 3 Ag 2 In 9 and EuCu 2 Ge 2 were synthesized through the reactions run in liquid indium. Eu 3 Ag 2 In 9 crystallizes in the La 3 Al 11 structure type [orthorhombic space group (SG) Immm] with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. EuCu 2 Ge 2 crystallizes in the tetragonal ThCr 2 Si 2 structure type (SG I4/mmm) with the lattice parameters: a = b = 4.2218(1) Å, and c = 10.3394(5) Å. The crystal structure of Eu 3 Ag 2 In 9 is comprised of edge-shared hexagonal rings consisting of indium. The one-dimensional chains of In6 rings are shared through the edges, which are further interconnected with other six-membered rings forming a three-dimensional (3D) stable crystal structure along the bc plane. The crystal structure of EuCu 2 Ge 2 can be explained as the complex [CuGe] (2+δ)– polyanionic network embedded with Eu ions. These polyanionic networks present in the crystal structure of EuCu 2 Ge 2 are shared through the edges of the 011 plane containing Cu and Ge atoms, resulting in a 3D network. The structural relationship between Eu 3 T 2 In 9 and EuCu 2 Ge 2 has been discussed in detail, and we conclude that Eu 3 T 2 In 9 is the metal deficient variant of EuCu 2 Ge 2 . The magnetic susceptibilities of Eu 3 T 2 In 9 (T = Cu and Ag) and EuCu 2 Ge 2 were measured between 2 and 300 K. In all cases, magnetic susceptibility data followed Curie–Weiss law above 150 K. Magnetic moment values obtained from the measurements indicate the probable mixed/intermediate valent behavior of the europium atoms, which was further confirmed by X-ray absorption studies and bond distances around the Eu atoms. As a result, electrical resistivity measurements suggest that Eu 3 T 2 In 9 and EuCu 2 Ge 2 are metallic in nature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on EuCu(WO4)2 by Materials Project

EuCu(WO4)2 crystallizes in the triclinic P-1 space group. The structure is three-dimensional. Eu3+ is bonded in a 8-coordinate geometry to eight O2- atoms. There are a spread of Eu–O bond distances ranging from 2.41–2.75 Å. There are two inequivalent W6+ sites. In the first W6+ site, W6+ is bonded in a distorted octahedral geometry to six O2- atoms. There are a spread of W–O bond distances ranging from 1.82–2.17 Å. In the second W6+ site, W6+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of W–O bond distances ranging from 1.78–2.36 Å. Cu1+ is bonded in a 3-coordinate geometry to three O2- atoms. There are a spread of Cu–O bond distances ranging from 1.88–1.99 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a 3-coordinate geometry to one Eu3+, one W6+, and one Cu1+ atom. In the second O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Eu3+ and one W6+ atom. In the third O2- site, O2- is bonded in a 3-coordinate geometry to one Eu3+ and two W6+ atoms. In the fourth O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Eu3+ and one W6+ atom. In the fifth O2- site, O2- is bonded in a 2-coordinate geometry to two equivalent Eu3+ and one W6+ atom. In the sixth O2- site, O2- is bonded in a 4-coordinate geometry to three W6+ and one Cu1+ atom. In the seventh O2- site, O2- is bonded in a 2-coordinate geometry to one Eu3+, one W6+, and one Cu1+ atom. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to one Eu3+ and two W6+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thermal neutron diffraction determination of the magnetic structure of EuCu{sub 2}Ge{sub 2}

The magnetic structure of EuCu{sub 2}Ge{sub 2} has been determined by flat-plate neutron powder diffraction. Two magnetic phases are present in the neutron diffraction pattern at 3.5 K. They have the same moment, within error, and a common transition temperature. Both {sup 151}Eu and {sup 153}Eu Mössbauer spectroscopy show that the two magnetic phases belong to the same crystallographic phase. Both phases can be modelled by planar helimagnetic structures: one with a propagation vector of [0.654(1), 0, 0], the other with a propagation vector of [0.410(1), 0.225(1), 0].

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

The magnetic structure of EuCu 2 Sb 2

Antiferromagnetic ordering of EuCu 2 Sb 2 which forms in the tetragonal CaBe 2 Ge 2 -type structure (space group P4/nmm #129) has been studied using neutron powder diffraction and 151 Eu Mössbauer spectroscopy. The room temperature 151 Eu isomer shift of –12.8(1) mm/s shows the Eu to be divalent, while the 151 Eu hyperfine magnetic field (B hf ) reaches 28.7(2) T at 2.1 K, indicating a full Eu 2+ magnetic moment. B hf (T) follows a smooth $S=\frac{7}{2}$ Brillouin function and yields an ordering temperature of 5.1(1) K. Refinement of the neutron diffraction data reveals a collinear A-type antiferromagnetic arrangement with the Eu moments perpendicular to the tetragonal c-axis. As a result, the refined Eu magnetic moment at 0.4 K is 7.08(15) μ B which is the full free-ion moment expected for the Eu 2+ ion with $S=\frac{7}{2}$ and a spectroscopic splitting factor of g = 2.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Intermediate-valence state of the Sm and Eu in SmB 6 and EuCu 2 Si 2 : neutron spectroscopy data and analysis

For this study, magnetic neutron scattering data for Sm (SmB 6 , Sm(Y)S) and Eu (EuCu 2 Si 2 - x Ge x ) intermediate-valence compounds have been analysed in terms of a generalized model of the intermediate-radius exciton. Special attention is paid to the correlation between the average ion's valence and parameters of the low-energy excitation in the neutron spectra, such as the resonance mode, including its magnetic form factor. Along with specific features of the formation of the intermediate-valence state for Sm and Eu ions, common physical mechanisms have been revealed for systems based on these elements from the middle of the rare-earth series. A consistent description of the existing experimental data has been obtained by using the concept of a loosely bound hole for the Eu f-electron shell in the intermediate-valence state, in analogy with the previously established loosely bound electron model for the Sm ion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu 2 ( Si x Ge 1 - x ) 2

In mixed-valence or heavy-fermion systems, the hybridization between local f orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin uctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic eld, chemical composition). Recently, similar effects (mixed-valence, Kondo uctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its 4f shell, and the magnetic Eu 2+ state (4f 7 ) has no orbital component in the usual LS coupling scheme, which can lead to a quite different and interesting physics. In the EuCu 2 (Si x Ge 1-x ) 2 series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence uctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, Mossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the change in the Eu valence state. In particular, the results support the persistence of valence uctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration x c ≈ 0:65. In conclusion, the sequence of magnetic ground states in the series is shown to re ect the evolution of the magnetic spectral response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Materials Data on EuCu by Materials Project

CuEu is Tetraauricupride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Eu is bonded in a body-centered cubic geometry to eight equivalent Cu atoms. All Eu–Cu bond lengths are 3.14 Å. Cu is bonded in a body-centered cubic geometry to eight equivalent Eu atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on EuCu by Materials Project

CuEu crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Eu is bonded in a 7-coordinate geometry to seven equivalent Cu atoms. There are a spread of Eu–Cu bond distances ranging from 3.04–3.34 Å. Cu is bonded in a 9-coordinate geometry to seven equivalent Eu and two equivalent Cu atoms. Both Cu–Cu bond lengths are 2.57 Å.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on EuCu(MoO4)2 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2014

Materials Data on EuCu(MoO4)2 (SG:61) by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2017

Magnetocaloric behavior in ternary europium indides EuT 5 In: Probing the design capability of first-principles-based methods on the multifaceted magnetic materials

The most favorable structures and the types of magnetic ordering predicted from first-principles-based methods in a family of closely related transition-metal-rich indides EuT 5 In (T = Cu, Ag, Au) are gauged against relevant experiments. The EuT5In compounds adopt a different structure for each different coinage metal—EuCu 5 In ( hR 42; R $\overline{3}$m, a = 5.0933(7), c = 30.557(6) Å), EuAg 5 In ( oP 28; Pnma , a = 9.121(2), b = 5.645(1), c = 11.437(3) Å), and EuAu 5 In ( tI 14; I 4/ mmm , a = 7.1740(3), c = 5.4425(3) Å)—and crystallize with the Sr 5 Al 9 , CeCu 6 , and YbMo 2 Al 4 structure types, respectively. EuCu 5 In and EuAg 5 In order antiferromagnetically at T N = 12 and 6 K, respectively, whereas EuAu 5 In is ferromagnetic below T C = 13 K. EuCu 5 In exhibits complex magnetism: after the initial drop at T N , the magnetization rises again below 8 K, and a weak metamagnetic-like transition occurs at 2 K in μ 0 H = 1.8 T. The electronic heat capacity of EuCu 5 In, γ = ~400 mJ/(mol K 2 ), points to strong electronic correlations. Spin-polarized densities of states suggest that the magnetic interactions in the three materials studied are supported via mixing 4 f and 5 d states of Eu. As a result, a chemical bonding analysis based on the Crystal Orbital Hamilton populations reveals the tendency to maximize overall bonding as a driving force to adopt a particular type of crystal structure.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dynamic fingerprint of fractionalized excitations in single-crystalline Cu3Zn(OH)6FBr

Abstract Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu 3 Zn(OH) 6 FBr, and another is an antiferromagnetically ordered compound EuCu 3 (OH) 6 Cl 3 . In Cu 3 Zn(OH) 6 FBr, we identify a unique one spinon-antispinon pair component in the E 2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the E g magnetic Raman response once EuCu 3 (OH) 6 Cl 3 undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

Chanakian, Sevan↗
DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

36 MATERIALS SCIENCE↗