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At least 829 records · Page 46
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Transport properties of the SnBi{sub 2}Te{sub 4}–PbBi{sub 2}Te{sub 4} solid solution

We report on the electrical and thermal transport properties of the Sn{sub 1−x}Pb{sub x}Bi{sub 2}Te{sub 4} series and we discuss the potential of these materials for thermoelectric conversion applications. From the evolution of the XRD patterns, we can confidently conclude that a complete solid solution exists between SnBi{sub 2}Te{sub 4} and PbBi{sub 2}Te{sub 4}, with no miscibility gap. A crossover from p-type conduction in Sn-rich samples to n-type conduction in Pb-rich ones has been observed, with a transition between x=0.3 and 0.4. A concomitant increase of the electrical resistivity and of the Seebeck coefficient has been observed in the solid solution, which leads to almost constant values of the thermoelectric power factor. Moreover, the thermal conductivity is slightly reduced in the solid solution. The best figure of merit ZT values at room temperature have been observed for p-type Sn{sub 0.8}Pb{sub 0.2}Bi{sub 2}Te{sub 4} with ZT=0.25 and for n-type Sn{sub 0.3}Pb{sub 0.7}Bi{sub 2}Te{sub 4} with ZT=0.15. - Graphical abstract: Seebeck coefficient in (Pb/Sn)Bi{sub 2}Te{sub 4} solid solution. - Highlights: • A complete solid solution exists between PbBi{sub 2}Te{sub 4} and SnBi{sub 2}Te{sub 4.} • A crossover between p-type and n-type is observed for 0.3<Pb fraction <0.4. • The lattice thermal conductivity is hardly reduced in the solid solution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Effect of Na substitution for La on Li ion conductivity of Li5La3Nb2O12 garnets by sol–gel process

Highlights: • The effect of Na substitution for La on Li ion conductivity in the Li5La3Nb2O12 solid state electrolyte is reported. • The power XRD patterns reveal that the partial substitutions of Na for La in Li{sub 5+2x}La{sub 3-x}Na{sub x}Nb{sub 2}O{sub 12} electrolytes are isostructural. • The new solid state electrolyte is very stable in potential range of −0.8 to 5.0V vs. Li/Li+. - Abstract: In this paper, the effect of Na substitution for La on Li ion conductivity in the well-known Li{sub 5}La{sub 3}Nb{sub 2}O{sub 12} parents with the cubic garnet framework prepared by sol-gel process is reported. The optimum condition is at 750 °C for 6 h in air with 10% excess lithium salt. The power XRD patterns reveal that the partial substitutions of Na for La in Li5 + 2{sub x}La{sub 3-x}Na{sub x}Nb{sub 2}O{sub 12} electrolytes are isostructural. The cubic lattice parameters decrease as the molar fractions of Na increasing. Among the Li5 + 2{sub x}La{sub 3-x}Na{sub x}Nb{sub 2}O{sub 12} compounds investigated, the Na-substituted Li{sub 6.4}La{sub 2.3}Na{sub 0.7}Nb{sub 2}O{sub 12} exhibits the highest ionic conductivity of 1.6 × 10{sup −4} S cm{sup -1} at room temperature with an activation energy of 0.41 eV. The new solid state electrolyte is very stable in potential range of −0.8 to 5.0 V vs. Li/Li{sup +}, and is also chemically stable against LiMn{sub 2}O{sub 4} and LiCoO{sub 2} compounds up to 900 °C, indicating that Li{sub 6.4}La{sub 2.3}Na{sub 0.7}Nb{sub 2}O{sub 12} is a promising solid state electrolyte for all-solid-state battery application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Significant enhancement in structural, dielectric, piezoelectric and ferromagnetic properties of Ba0.9Sr0.1Zr0.1Ti0.9O3-CoFe2O4 multiferroic composites

Highlights: • (1-x)Ba{sub 0.9}Sr{sub 0.1}Zr{sub 0.1}Ti{sub 0.9}O{sub 3}-(x)CoFe{sub 2}O{sub 4} multiferroic composite has been synthesized using mechano-chemical activation technique. • An improvement in dielectric, ferroelectric and piezoelectric properties over the pristine BSZT sample has been observed. • The composite samples also show a substantial enhancement in ferromagnetic characteristics compare to bare BSZT sample. - Abstract: Multiferroic composites (1 − x)Ba{sub 0.9}Sr{sub 0.1}Zr{sub 0.1}Ti{sub 0.9}O{sub 3}-(x)CoFe{sub 2}O{sub 4} (x = 0.0, 0.05, 0.10, 0.20 & 0.30) were prepared using mechanochemical activation method. Rietveld refined XRD patterns confirm the diphasic nature of composite samples. All of the diffraction peaks of (1 − x)BSZT-(x)CFO can be identified for the separate perovskite (BSZT) and inverse spinal (CFO) phases and no other impurity phase were observed for the composites. (1 − x)BSZT-(x)CFO sample with x = 0.05 have shown significant improvement in microstructural, dielectric, ferroelectric and piezoelectric properties over the pristine BSZT sample. Such an improvement in electrical properties can be attributed to larger grain size, improved density and easier ferroelectric domain wall motion inside the grain. Dielectric breakdown strength of the composites also shows a similar trend and is observed to be maximum for the sample with x = 0.05 CFO concentration. The composite samples also show a substantial enhancement in ferromagnetic characteristics and increases progressively with increase in ferrite (CFO) content.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Distribution and mobility of lithium in NASICON-type Li1-xTi2-xNbx(PO4)3 (0 ≤ x ≤ 0.5) compounds

Highlights: • Li{sub 1-x}Ti{sub 2-x}Nb{sub x}(PO{sub 4}){sub 3} (0 ≤ x ≤ 0.5) were prepared via solid-state reaction. • The substitution of Ti{sup 4+} by Nb{sup 5+} in LiTi{sub 2}(PO{sub 4}){sub 3} increases unit cell dimensions. • The Rietveld analysis of XRD patterns was performed to deduce location of Li ions. • LiTi{sub 2}(PO{sub 4}){sub 3} have the highest total conductivity and low activation energy. - Abstract: The NASICON-type Li{sub 1-x}Ti{sub 2-x}Nb{sub x}(PO{sub 4}){sub 3} (0 ≤ x ≤ 0.5) has been prepared using a conventional solid-state reaction and characterized by powder X-ray (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and impedance spectroscopy (IS). The substitution of Ti{sup 4+} by Nb{sup 5+} in LiTi{sub 2}(PO{sub 4}){sub 3} increases unit cell parameters of NASICON phases (S.G. R 3-bar c). The parallel XRD and NMR analysis confirmed the formation of secondary phases besides NASICON compounds. The NMR components detected in {sup 93}Nb and {sup 31}P MAS-NMR spectra have been ascribed to formation of crystalline NbPO{sub 5} and NASICON phases. The substitution of Ti{sup 4+} by Nb{sup 5+} produces detection of three equally spaced {sup 31}P MAS-NMR P(OTi){sub 4-n}(ONb){sub n} components, whose intensity changes with the Nb content of the NASICON phase. On the other hand, the broad {sup 31}P NMR band detected at ∼−130 ppm has been ascribed to the formation of an amorphous Nb phosphate that strongly affects conductivity of samples. At 355 K, total conductivity decreased from 1.2 × 10{sup −7} S cm{sup −1} (Ea = 0.49 eV) to 6.0 × 10{sup −9} S cm{sup −1} (Ea = 0.56 eV) when going from LiTi{sub 2}(PO{sub 4}){sub 3} to Li{sub 0.5}Ti{sub 1.5}Nb{sub 0.5}(PO{sub 4}){sub 3}. This important decrease has been related to the increment of activation energy (stronger Li-network interaction); however, further studies are required to understand the deleterious effect of niobium in Li{sub 1-x}Ti{sub 2-x}Nb{sub x} (PO{sub 4}){sub 3} series.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Mechanism of zeolite X crystallization from diatomite

Highlights: • The intermediate species of zeolite X appears on the surface of solid particles. • The β cages build up the intermediate species, which was consist of four-membered rings (4R) and six-membered rings (6R). • The double six-membered ring (D6R) played a critical role for the crystallization process. • The β cages interconnect via double six-membered rings to form the framework of zeolite X. - Abstract: Here we systemically investigated the crystallization process of zeolite X using diatomite, a natural cost-effective silica precursor by the hydrothermal method. To decipher the mechanism, solids were separated from the mixtures at various stages of crystallization and were characterized by XRD, SEM, solid state NMR, FT-IR, UV-Raman and XRF. The XRD patterns revealed that the crystallization started between 3 and 3.5 h and ended by 5 h. The SEM images showed that the intermediate species of zeolite X appear on the surface of particles. The FT-IR and UV-Raman spectra suggested the presence of double six-membered rings (D6R) in the framework of zeolite X and they played a critical role in the crystallization process. Furthermore, the UV-Raman spectra indicated that the secondary building units of β cages formed the intermediate species and they were interconnected via double six-membered rings to form the framework of zeolite X.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Cu(II) Stability and UV-Induced Electron Transfer in a Metal–Organic Hybrid: An EPR, DFT, and Crystallographic Characterization of Copper-Doped Zinc Creatininium Sulfate

Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopic experiments, complemented by quantum chemical DFT calculations, were carried out on the copper-doped metal–organic hybrid and Tutton salt analogue zinc creatininium sulfate to determine its crystal structure, to characterize the electronic structure of the doped Cu(II) binding site, and to propose a pathway for an excited-state, proton-coupled electron transfer (PCET) process in UV-exposed crystals. The crystal structure is isomorphous to that of cadmium creatininium sulfate, which has the transition ion, not in direct coordination with the creatinine, but forming a hexahydrate complex, which is bridged to a creatininium through an intervening sulfate ion. The EPR g (2.446, 2.112, 2.082) and copper hyperfine (A Cu : -327, -59.6, 10.8 MHz) tensor parameters are consistent with doped copper replacing host zinc in the metal–hexahydrate complex. These parameters are similar to those observed for copper hexahydrate in doped Tutton salt systems at low temperature, where the unpaired electron occupies mainly the copper 3d x 2 –y 2 orbital. At room temperature in the Tutton systems, vibration couplings stemming from a dynamic Jahn–Teller effect cause tensor averaging which results in a reduction in their maximum g-tensor and hyperfine tensor values. However, like for the doped isomorphous Cd creatinine crystal, the Cu(II) EPR exhibits little, or no room temperature averaging compared to its low temperature pattern. Samples exposed to 254 nm UV light generate a carbon-centered free radical species, characterized by an isotropic g-tensor (g = 2.0029) and an alpha-proton hyperfine coupling (-24 -14 +4 G). These parameters identify it as a creatinine radical cation formed by the oxidative release of one of its C2 methylene hydrogens. DFT calculations confirm the unpaired electronic structures of both the Cu(II) site and free radical. The growth in radical concentration with an increase in the UV exposure time coincides with a decrease in the copper EPR signal, indicating a coupled light-induced oxidation reduction process. A comparison of the crystal structure with the EPR parameters and DFT results provides evidence for a UV-induced PCET.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Crystalline 1D Coordination Polymer Inhibitor Layer Leads to Vertical Sidewalls in Selectively Deposited ZnO on Nanoscale Patterns

Area-selective atomic layer deposition (AS-ALD) is a promising technique for the fabrication of next-generation nanoelectronics. There are two main challenges in AS-ALD: (1) achieving high selectivity of deposition on the growth regions, and (2) preventing mushrooming of the growth material onto the nongrowth regions and achieving well-defined interfaces. In this work, we use benzenethiol (BT) as an inhibitor in the selective deposition of ZnO on SiO 2 in the presence of copper with and without a native oxide (Cu/CuO x ). We observe that BT forms a monolayer on the Cu surface and a Cu-thiolate multilayer structure on CuO x . Using grazing incidence X-ray diffraction combined with simulations, we find that the multilayer structure is crystalline and composed of 1D coordination polymers of Cu-thiolate. Here, using ellipsometry and X-ray photoelectron spectroscopy, we show that the BT consumes the entirety of the CuO x during multilayer formation, allowing the multilayer thickness to be tuned by the thickness of the original oxide. Both the monolayer BT and the multilayer BT prove to be effective inhibitors of ZnO ALD, blocking nearly 500 ALD cycles, which is more than twice that achieved with other thiol inhibitors. Finally, we demonstrate that the multilayer structure can prevent mushrooming of the ALD material onto the nongrowth surface of nanoscale patterns, creating vertical sidewalls with well-defined material interfaces and providing excellent pattern transfer, even for a relatively thick deposited film. As such, these results demonstrate that BT is not only an effective inhibitor but also that its ability to form tunable multilayers makes it well-suited for highly precise nanopatterning applications.

Layers↗
DOE OSTI2019

A Natural 2D Heterostructure [Pb 3.1 Sb 0.9 S 4 ][Au x Te 2– x ] with Large Transverse Nonsaturating Negative Magnetoresistance and High Electron Mobility

We report the two-dimensional (2D) natural hetero-structure [Pb 3.1 Sb 0.9 S 4 ][Au$_x$Te$_{2-x}$] ($x$ = 0.52-0.36) which shows anomalous, transverse nonsaturating negative magnetoresistance (MR). For $x$ = 0.52, the material has a commensurately modulated structure with alternating [Pb3.1Sb0.9S4] rocksalt layers and atomically thin [Au$_x$Te$_{2-x}$] sheets, as determined by single-crystal X-ray diffraction using a (3 + 1)-dimensional space group; for other $x$ compositions, the modulated structure is absent and the Au and Te atoms are disordered. The transport properties in this system at low temperature (<100 K) are dominated by an unusual 2D hopping mechanism, while at room temperature a high carrier mobility of similar to ~1352 cm 2 V -1 s -1 obtained ($x$ = 0.36). The confined electrons within the [Au$_x$Te$_{2-x}$] layers are also exposed to interlayer coupling with the insulating [Pb 3.1 Sb 0.9 S 4 ] layers, and as a result, the properties of the heterostructures emerge not only from the constituent layers but also the interactions between them. Furthermore, the various Au and Te coordination patterns found in the [Au$_x$Te$_{2-x}$] sheets as a function of $x$ further contribute to a unique electronic structure that leads to the anomalous nonsaturating negative MR with different field dependent behaviors. First-principles calculations indicate that the [Au$_x$Te$_{2-x}$] sheets are responsible for the unusual electrical transport properties in this 2D system.

2D materials↗
DOE OSTI2019

Synthesis and Characterization of Single Phase Metal Dodecaboride Solid Solutions: Zr 1-x Y x B 12 and Zr 1-x U x B 12

Single-phase metal dodecaboride solid solutions, Zr 0.5 Y 0.5 B 12 and Zr 0.5 U 0.5 B 12 , were arranged by arc melting from pure elements. The phase purity and composition were established by powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and 10 B and 11 B solid-state nuclear magnetic resonance (NMR) spectroscopy. The effects of carbon addition to Zr 1-x Y x B 12 were studied and it was found that carbon causes fast cooling and as a result rapid nucleation of grains, as well as “templating” and patterning effects of the surface morphology. The hardness of the Zr 0.5 Y 0.5 B 12 phase is 47.6 ± 1.7 GPa at 0.49 N load, which is ~17% higher than that of its parent compounds, ZrB 12 and YB 12 , with hardness values of 41.6 ± 2.6 and 37.5 ± 4.3 GPa, respectively. The hardness of Zr 0.5 U 0.5 B 12 is ~54% higher than that of its UB 12 parent. The dodecaborides were confirmed to be metallic by band structure calculations, diffuse reflectance UV–vis, and solid-state NMR spectroscopies. The nature of the dodecaboride colors—violet for ZrB 12 and blue for YB 12 —can be applied to charge-transfer. XPS indicates that the metals are in the following oxidation states: Y 3+ , Zr 4+ , and U$^{{5+}/^{6+}}$. The superconducting transition temperatures (T c ) of the dodecaborides were determined to be 4.5 and 6.0 K for YB 12 and ZrB 12 , respectively, as shown by resistivity and superconducting quantum interference device (SQUID) measurements. The T c of the Zr 0.5 Y 0.5 B 12 solid solution was suppressed to 2.5 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Growth of GaP1 − x − yAsyNx on Si substrates by chemical beam epitaxy

Chemical beam epitaxy has been used to grow layers of GaP1 − xNx, GaP1 − yAsy, and nearly lattice-matched layers GaP1 − x − yAsyNx on Si substrates. To address the issue of antiphase domain generation associated with the growth of polar semiconductors on Si, misoriented Si(001) substrates have been used combined with a carefully designed GaP buffer layer growth. The reflection high-energy electron diffraction pattern exhibits a (2 × 4) surface reconstruction after GaP buffer layer and GaP(As,N) graded layer growth, indicating the good surface quality and planarity of the grown layers. Sample composition was obtained by simultaneous acquisition of Rutherford backscattering spectrometry and nuclear reaction analysis, indicating a linear dependence of N and As mole fractions on the flux of their respective precursor. GaP1 − x − yAsyNx layers grown on Si substrates have a lattice mismatch not larger than ±0.005 for N contents in the range 0.02 < x < 0.05. High-resolution X-ray diffraction reciprocal space maps demonstrate a good crystalline quality. Intense photoluminescence spectra have been measured in all GaP1 − xNx and GaP1 − x − yAsyNx layers, as it is expected for direct bandgap materials. Two wide overlapped emission peaks are observed in all the spectra, most likely related to near bandgap recombination. The position of the higher energy peak for GaP1 − xNx and GaP1 − x − yAsyNx layers has been compared to bandgap energy calculations using the band anticrossing model, showing good agreement.

Ben Saddik, K.↗
DOE OSTI2022

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction Al x Ga 1-x N on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of Al x Ga 1-x N superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the Al x Ga 1-x N superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa 1-x N/GaN quantum-well laser heterostructure designed for operation at ~370 nm.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Crystal structure and magnetic properties in semiconducting Eu 3-δ Zn x Sn y As 3 with Eu-Eu dimers

Magnetic structure and crystal symmetry, which primarily determine the time-reversal and inversion symmetry, may give rise to numerous exotic quantum phenomena in magnetic semiconductors and semimetals when arranged in different patterns. Here, a new layered magnetic semiconductor, Eu 3-δ Zn x Sn y As 3 , was discovered and high-quality single crystals were grown using the Sn flux. According to structural characterization by x-ray diffraction and atomic-resolution scanning transmission electron microscopy, Eu 3-δ Zn x Sn y As 3 is found to crystallize in a hexagonal symmetry with the space group P6 3 /mmc (No. 194). After examining different specimens, we conclude that their stoichiometry is fixed at ~Eu 2.6 Zn 0.65 Sn 0.85 As 3 , which meets the chemical charge balance. Eu 3-δ Zn x Sn y As 3 is composed of septuple (Eu 1-δ Sn y As 2 )-Eu-(Zn x As)-Eu sequences. The shortest Eu–Eu distance in the system is between two Eu layers separated by Zn x As along the c-axis. Magnetization measurement shows an antiferromagnetic ordering in Eu 3-δ Zn x Sn y As 3 at T N ~ 12 K, where the magnetic easy-axis is along the c-axis, and Mössbauer spectroscopy observes magnetic hyperfine splitting on Eu and Sn at 6 K. Magnetic anisotropy is significantly different from the ones along the ab-plane in other layered Eu-based magnetic semimetals. Heat capacity measurements confirm the magnetic transition around 12 K. Electrical resistivity measurement indicates semiconductor behavior with a band gap of ~0.86 eV. Finally, various Eu-based magnetic semiconductors could provide a tunable platform to study potential topological and magnetic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Robust superconductivity and the suppression of charge-density wave in the quasi-skutterudites Ca 3 ( Ir 1 – x Rh x ) 4 Sn 13 single crystals at ambient pressure

Single crystals of the quasi-skutterudite compounds Ca 3 (Ir 1-x Rh x ) 4 Sn 13 (3–4–13) were synthesized by flux growth and characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, magnetization, resistivity, and radio frequency magnetic susceptibility techniques. The coexistence and competition between the charge density wave (CDW) and superconductivity was studied by varying the Rh/Ir ratio. The superconducting transition temperature, T c , varies from 7 K in pure Ir (x = 0) to 8.3 K in pure Rh (x = 1). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, T CDW (x). The CDW starts in pure Ir, x = 0, at T CDW ≈ 40 K and extrapolates roughly linearly to zero at x c ≈ 0.53–0.58 under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above T c shows signs of non-Fermi liquid behavior in a cone-like composition pattern. We conclude that the Ca 3 (Ir 1-x Rh x ) 4 Sn 13 alloy is a good candidate for a composition-driven quantum critical point at ambient pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Doping effects of Cr on the physical properties of BaFe 1.9 - x Ni 0.1 Cr x As 2

We present a systematic study on the heavily Cr doped iron pnictides BaFe1.9_xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x = 0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Neel temperature T-n coincides with the tetragonal-to-orthorhombic structural transition temperature T-s, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x = 0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x = 0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.

Gong, Dongliang↗
DOE OSTI2024

Exploring the impact of Cr-doping on the crystallographic and magnetic structure of Mn 5 Si 3 antiferromagnetic alloy

Here, the role of Cr-doping on the structural and magnetic ground states of Mn 5 Si 3 alloy has been investigated through temperature-dependent neutron powder diffraction (NPD) and X-ray absorption fine structure (XAFS) techniques. All the Cr-doped alloys of nominal composition Mn 5-x Cr x Si 3 (for x = 0.05, 0.1, and 0.2) undergo two first-order magneto-structural phase transitions from hexagonal (space group: P6 3 /mcm) paramagnetic → orthorhombic (space group: Ccmm) collinear antiferromagnetic → orthorhombic (space group: Cc2m) non-collinear antiferromagnetic phase on cooling from room temperature. NPD studies at different constant temperatures indicate that both anti ferromagnetic phases are commensurate in nature and can be represented by q = (0,1,0) magnetic propagation vector for all the Cr-doped alloys. Such doping at the Mn-site results in a significant modification of the non-collinear antiferromagnetic structure (both moment size and orientation) and hence affects the unusual magnetic properties, like inverted hysteresis loop, thermomagnetic irreversibility, etc. The XAFS measurements were performed to interpret the local environment of doped Cr atoms in detail, which is critical for a microscopic understanding of the unusual properties of this class of Cr-doped Mn 5 Si 3 alloys. The analysis confirms the elemental state of Cr in the doped alloys and indicates a high degree of preservation of local crystallographic structure with varying Cr concentration and sample temperature. Doping induces intriguing changes in XAFS patterns, elucidated through different types of scattering mechanisms associated with the central absorbing Cr atom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1991

Organometallic chemical vapor deposition and characterization of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates

In this paper, we report the epitaxial growth of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates by open tube OMCVD. The chemical composition of the alloys characterized by energy dispersive X-ray spectroscopy shows that alloys with x up to 0.13 can be deposited on (001) GaP. Epitaxial growth with mirror smooth surface morphology has been achieved for x less than or equals to 0.05. Selected area electron diffraction pattern of the alloy shows that the epitaxial layer crystallizes in the chalcopyrite structure with relatively weak superlattice reflections indicating certain degree of randomness in the cation sublattice. Hall measurements show that the alloys are p-type, like the unalloyed films; the carrier concentration, however, dropped about 10 times from 2 x 10 exp 18 to 2 x 10 exp 17/cu cm. Absorption measurements indicate that the band tailing in the absorption spectra of the alloy has been shifted about 0.04 eV towards shorter wavelength as compared to the unalloyed material.

Xing, G. C.↗
NASA NTRS1993

Organometallic chemical vapor deposition and characterization of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates

The epitaxial growth of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates by open tube organometallic chemical vapor deposition (OMCVD) is reported. The chemical composition of the alloys characterized by energy dispersive X-ray spectroscopy shows that alloys with x up to 0.13 can be deposited on (001) GaP. Epitaxial growth with mirror smooth surface morphology was achieved for x less than or equal to 0.05. Transmission electron microscopy (TEM) micrographs of these alloys show specular epitaxy and the absence of microstructural defects indicating a defect density of less than 10(exp 7) cm(sup -2). Selected area electron diffraction pattern of the alloy shows that the epitaxial layer crystallizes in the chalcopyrite structure with relatively weak superlattice reflections indicating certain degree of randomness in the cation sublattice. Hall measurements show that the alloys are p-type, like the unalloyed films; the carrier concentration, however, dropped about 10 times from 2 x 10(exp 18) to 2 x 10(exp 17) cm(sup -3). Absorption measurements indicate that the band tailing in the absorption spectra of the alloy was shifted about 0.04 eV towards shorter wavelength as compared to the unalloyed material. Diodes fabricated from the n(+)-GaP/p-ZnSiP2-ZnGeP2-Ge heterostructure at x = 0.05 have a reverse break-down voltage of -10.8 V and a reverse saturation current density of approximately 6 x 10(exp -8) A/sq cm.

Xing, G. C.↗