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At least 1,225 records · Page 68
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DOE OSTI2023

Structural evolution of iodine on approach to the monatomic state

Here, we applied single-crystal x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to 36 GPa and first-principles theoretical calculations to study the molecular dissociation of solid iodine at high pressure. Unlike previously reported, we find that the familiar Cmce molecular phase transforms to a Cmc2 1 molecular structure at 16 GPa, and then to an incommensurate polymeric Fmmm⁡(00⁢$\gamma$)⁢s⁢00 structure, which can be viewed as a mixture of molecular I 2 and zigzag chains of three iodine atoms, at 20 GPa. The available data are consistent with metallization of iodine at the boundary between the commensurate Cmc⁢2 1 structure and the incommensurate Fmmm⁡(00⁢$\gamma$)s⁢00 structure at 20 GPa.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

K 2 Co 2 TeO 6 : A layered magnet with a S = 1 2 Co 2 + honeycomb lattice

Recent observations of Kitaev interactions in d 7 electron configurations have generated research interest in Co 2+ based honeycomb magnets. Here, we report the synthesis of a K 2 Co 2 TeO 6 single crystal. X-ray diffraction and electron microscopy experiments show that K 2 Co 2 TeO 6 crystallizes in a $P$6 3 /$mcm$ space group with undistorted honeycomb layers of Co 2+ ions. Anisotropic magnetic and thermodynamic characterizations demonstrate an effective S = $\frac{1}{2}$ state of Co 2+ at low temperature, competing magnetic phases, and weak magnetic transitions that can be suppressed by applying fields. Moreover, the interlayer K disorder level is tunable by thermal treatments, and it affects the magnetic features obviously. In conclusion, our findings provide a new and promising platform for studying Kitaev physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Hidden anomalies in topological t – PtBi 2 – x probed by second harmonic generation

The noncentrosymmetric Weyl semimetal PtBi 2–x (t-PtBi 2–x ) exhibits various interesting technologically important physical properties. We report the experimental investigation of PtBi 1.6 via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at T* ~ 60 K and another at T x ~ 200 K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the 3m symmetry and flat Bi2 surface termination with the m symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) T* marks the onset of surface states which is also manifested in the c-axis resistivity drop and (2) T x corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. Furthermore, this study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Charge density waves and pinning by lattice anisotropy in 214 cuprates

The detection of static charge density waves (CDWs) in La 2-x ⁢Sr x ⁢CuO 4 (LSCO) with x ~ 0.12 at relatively high temperatures has raised the question of what lattice feature pins the CDWs. Some recent structural studies have concluded that some form of monoclinic distortion, indicated by the appearance of certain weak Bragg peaks (type M peaks) at otherwise forbidden positions, are responsible for CDW pinning. Here, as a test of this idea, we present neutron diffraction results for a single crystal of La 2-x ⁢Ba x CuO 4 (LBCO) with x = 1/8, which is known to undergo two structural transitions on cooling, from high-temperature tetragonal (HTT) to low-temperature orthorhombic (LTO) near 240 K, involving a collective tilt pattern of the corner-sharing CuO 6 octahedra, and from LTO to low-temperature tetragonal (LTT) near 56 K, involving a new tilt pattern and the appearance of intensity at peaks of type T. We observe both type M and type T peaks in the LTT phase, while the type M peaks (but not type T) are still present in the LTO phase. Given that CDW order is observed only in the LTT phase of LBCO, it is apparent that the in-plane Cu-O bond anisotropy associated with the octahedral tilt pattern is responsible for charge pinning. We point out that evidence for a similar, but weaker, bond anisotropy has been observed previously in LSCO and should be responsible for CDW pinning there. In the case of LBCO, the monoclinic distortion may help to explain previously reported magneto-optical evidence for gyrotropic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Anharmonicity and atomic distribution of SnTe and PbTe thermoelectrics

The structure and lattice dynamics of rock-salt thermoelectric materials SnTe and PbTe are investigated with single crystal and powder neutron diffraction, inelastic neutron scattering (INS), and first-principles simulations. Our first-principles calculations of the radial distribution function (RDF) in both SnTe and PbTe show a clear asymmetry in the first nearest-neighbor (1NN) peak, which increases with temperature, in agreement with experimental reports (Ref. 1,2). We show that this peak asymmetry for the 1NN Sn–Te or Pb–Te bond results from large-amplitude anharmonic vibrations (phonons). No atomic off-centering is found in our simulations. In addition, the atomic mean square displacements derived from our diffraction data reveal stiffer bonding at the anion site, in good agreement with the partial phonon densities of states from INS, and first-principles calculations. In conclusion, these results provide clear evidence for large-amplitude anharmonic phonons associated with the resonant bonding leading to the ferroelectric instability.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Physical properties of the Ce 2 M Al 7 Ge 4 heavy-fermion compounds ( M = Co , Ir , Ni , Pd )

Here, we report the synthesis, crystal structure, and characterization by means of single-crystal x-ray diffraction, neutron powder diffraction, and magnetic, thermal, and transport measurements of the new heavy-fermion compounds Ce 2 MAl 7 Ge 4 (M=Co,Ir,Ni,Pd). These compounds crystallize in a noncentrosymmetric tetragonal space group P$\bar{4}$2 1 m, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. Ce 2 CoAl 7 Ge 4 ,Ce 2 IrAl 7 Ge 4 , and Ce 2 NiAl 7 Ge 4 order magnetically below TM=1.8, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in Ce 2 PdAl 7 Ge 4 down to 0.4 K. Furthermore, the small amount of entropy released in the magnetic state of Ce 2 MAl 7 Ge 4 (M = Co, Ir, Ni) and the reduced specific heat jump at T M suggest a strong Kondo interaction in these materials. Ce 2 PdAl 7 Ge 4 shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Magnetic structures and excitations in a multiferroic Y-type hexaferrite BaSrCo 2 Fe 11 AlO 22

Here, we have investigated magnetic orders and excitations in a Y-type hexaferrite BaSrCo 2 Fe 11 AlO 22 (BSCoFAO), which was reported to exhibit spin-driven ferroelectricity at room temperature. By means of magnetization, electric polarization, and neutron-diffraction measurements using single-crystal samples, we establish a H-T magnetic phase diagram for magnetic field perpendicular to the c axis (H ⟂c ). This system exhibits an alternating longitudinal conical (ALC) magnetic structure in the ground state, and it turns into a non-co-planar commensurate magnetic order with spin-driven ferroelectricity under H ⟂c . The field-induced ferroelectric phase remains as a metastable state after removing magnetic field below 250 K. This metastability is the key to understanding of magnetic field reversal of the spin-driven electric polarization in this system. Inelastic polarized neutron-scattering measurements in the ALC phase reveal a magnetic excitation at around 7.5 meV, which is attributed to spin components oscillating in a plane perpendicular to the cone axis. This phasonlike excitation is expected to be an electric-field active magnon, i.e., electromagnon excitation, in terms of the magnetostriction mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of nickel substitution on magnetism in the layered van der Waals ferromagnet Fe 3 GeTe 2

We have grown a series of nickel-substituted single crystals of the layered ferromagnet (FM) Fe 3 GeTe 2 . The large single-crystalline samples of (Fe 1–x Ni x ) 3 GeTe 2 with x = 0–0.84 were characterized with single-crystal x-ray diffraction, magnetic susceptibility, electrical resistance, and muon spin spectroscopy. We find Fe can be continuously substituted with Ni with only a minor structural variation. In addition, FM order is suppressed from T C = 212K for x = 0 down to T C = 50K for x=0.3, which is accompanied by a strong suppression of saturated and effective moments, and Curie-Weiss temperature. Beyond x=0.3, the FM order is continuously smeared into a FM cluster-glass phase, with a nearly full magnetic volume fraction. Here, we attribute the observed change in the nature of magnetic order to the intrinsically disordered structure of Fe 3 GeTe 2 and subsequent dilution effects from the Ni substitution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Single-crystal investigation of the proposed type-II Weyl semimetal CeAlGe

We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in a noncentrosymmetric I4 1 md space group, in line with predictions of nontrivial topology. Magnetization, specific heat, and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of Δ = 9.11 K from heat capacity and hole-like carrier density of 1.4 × 10 20 cm –3 from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal ab plane below 7 K. A spin-flop transition to less than 1 μ B /Ce is observed to occur below 30 kOe at 1.8 K in the M(H) (H ∥ a) data. Furthermore, small magnetic fields of 3 and 30 kOe are sufficient to suppress magnetic order when applied along the a and c axes, respectively, resulting in a complex T–H phase diagram for H ∥ a and a simpler one for H ∥ c.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Order-Disorder Transitions in ( Ca x Sr 1 - x ) 3 Rh 4 Sn 13

The classification of structural phase transitions as displacive or order-disorder in character is usually based on spectroscopic data above the transition. Here, we use single crystal x-ray diffraction to investigate structural correlations in the quasiskutterudites, (Ca x Sr 1-x ) 3 Rh 4 Sn 13 , which have a quantum phase transition at x ~ 0.9. Three-dimensional pair distribution functions show that the amplitudes of local atomic displacements are temperature independent below the transition and persist to well above the transition, a signature of order-disorder behavior. The implications for the associated electronic transitions are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Filling the holes in the CaFe 4 As 3 structure: Synthesis and magnetism of CaCo 5 As 3

Here, we investigate single crystals of CaCo 5 As 3 by means of single-crystal x-ray diffraction, microprobe, magnetic susceptibility, heat capacity, and pressure-dependent transport measurements. CaCo 5 As 3 shares the same structure of CaFe 4 As 3 with an additional Co atom filling a lattice vacancy and undergoes a magnetic transition at T M = 16 K associated with a frustrated magnetic order. CaCo 5 As 3 displays metallic behavior and its Sommerfeld coefficient (γ = 70 mJ/mol K 2 ) indicates a moderate enhancement of electron-electron correlations. Transport data under pressures to 2.5 GPa reveal a suppression of T M at a rate of - 0.008 K/GPa. First-principles electronic structure calculations show a complex three-dimensional band structure and magnetic moments that depend on the local environment at each Co site. Our results are compared with previous data on CaFe 4 As 3 and provide a scenario for a magnetically frustrated ground state in this family of compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin-glass behavior and vacancy order in van der Waals layered β - MoCl 4

Two-dimensional ${\beta}\text{-}{\mathrm{MoCl}}_{4}$ is an attractive material from the perspectives of magnetism in $4d$ transition metal compounds, geometrically frustrated lattices, and magnetic van der Waals layered materials, but the magnetism in this compound has not been particularly well studied to date. In this paper the magnetic properties and crystal structure of ${\mathrm{MoCl}}_{4}$ are revisited, and results of ac and dc magnetic measurements and single crystal x-ray diffraction are reported. Crystals grow as well-formed and easily cleaved hexagonal plates that are unstable in air. The revised structural model comprises ${\mathrm{CdCl}}_{2}$-type layers with 50% Mo vacancies distributed over the sites of the triangular cation net. Interestingly, a structural ambiguity regarding the vacancy distribution is identified in the analysis of the diffraction data. The orbital moment is not expected to be quenched in this $4{d}^{2}$ compound. Accordingly, magnetization measurements indicate an effective moment that is about 20% lower than the spin-only value. The magnetic data reveal an anomaly near 5 K, below which a divergence of field-cooled and zero-field-cooled dc magnetization, a slow relaxation of thermoremanent magnetization, and enhanced frequency dependence of ac magnetization are observed. Finally and thus, ${\beta}\text{-}{\mathrm{MoCl}}_{4}$ represents an uncommon example of a cleavable spin-glass system.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Crystal structure and magnetic properties of the layered van der Waals compound VB r 3

Single crystals of the layered, van der Waals compound VBr 3 were synthesized by a vapor transport technique, and characterized by single-crystal x-ray diffraction, temperature- and field-dependent magnetization, and temperature-dependent specific-heat measurements. VBr 3 has a honeycomb-based BiI 3 -type structure with space group $R$-3 (no. 148) at 100 K, analogous to the structure of other honeycomb trihalides. Structural stacking faults were observed as evidenced by the streaking of reflections along $c*$ in the diffraction data. VBr 3 goes through a structural phase transition at 90.4 K and a subsequent antiferromagnetic phase transition at 26.5 K, below which the magnetic moments are primarily aligned along the $c$-axis.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Complex transport and magnetism in inhomogeneous mixed valence Ce 3 Ir 4 Ge 13

We report the discovery of Ce 3 Ir 4 Ge 13 , a Remeika phase with a complex array of structural, electronic, and magnetic properties. In this work, our single crystal x-ray diffraction measurements show that Ce 3 Ir 4 Ge 13 forms in the tetragonally distorted $I4_1/amd$ space group, which has three distinct rare earth sites. The electrical resistivity is almost temperature independent over three decades in temperature, from 0.4 K to 400 K, while the Hall coefficient measurements are consistent with a low-carrier semimetal. Magnetic susceptibility measurements reveal an effective moment, $μ^\text{exp}_\text{eff} = 1.87μ_B/$Ce, that falls significantly short of the expected value for Ce 3+ , suggesting that two of the rare earth sites are occupied by magnetic Ce 3+ while the third is occupied by nonmagnetic Ce 4+ . Upon cooling, Ce 3 Ir 4 Ge 13 first enters a short range magnetically ordered state below $T_\text{SRO}$ = 10 K, marked by a deviation from Curie-Weiss behavior in susceptibility and a broad field-independent heat capacity anomaly. At lower temperatures, we observe a second, sharper peak in the heat capacity at $T^\ast$ = 1.7 K, concurrent with a splitting of the field-cooled and zero-field-cooled susceptibilities. A small resistivity drop at $T^\ast$ suggests a loss of spin disorder scattering consistent with a magnetic ordering or spin freezing transition. Ce 3 Ir 4 Ge 13 is therefore a rare example of an inhomogeneous mixed valence compound with a complex array of thermodynamic and transport properties.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Observation of a C -type short-range antiferromagnetic order in layer spacing expanded FeS

Here, we report neutron diffraction studies of FeS single crystals obtained from Rb x Fe 2-y S 2 single crystals via a hydrothermal method. While no $\sqrt{5}$×$\sqrt{5}$ iron vacancy order or block antiferromagnetic order typical of Rb x Fe 2-y S 2 is found in our samples, we observe C-type short-range antiferromagnetic order with moments pointed along the c axis hosted by a different phase of FeS with an expanded interlayer spacing. The Néel temperature for this magnetic order is determined to be 170 ± 4 K. Our finding of a variant FeS structure hosting this C-type antiferromagnetic order demonstrates that the known FeS phase synthesized in this method is in the vicinity of a magnetically ordered ground state, providing insights into understanding a variety of phenomena observed in FeS and the related FeSe 1-x S x iron chalcogenide system.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tuning magnetic order in the van der Waals metal $Fe_5GeTe_2$ by cobalt substitution

Fe 5 - x GeTe 2 is a van der Waals material with one of the highest reported bulk Curie temperatures, T C ≈ 310 K . Here, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to ≈ 30 % enhances T C and changes the magnetic anisotropy, while ≈ 50 % cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for ≈ 46 % cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in Fe 5 - x GeTe 2 to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.

36 MATERIALS SCIENCE↗