Search NASA⌕ Search

Engineering topics

Douglas, Tyra C.

Publications and source records attributed to Douglas, Tyra C..

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Rare Earth Nitrate Hybrid Double Perovskites [Me 4 N] 2 [MLn(NO 3 ) 6 ] (M = Na–Cs; Ln = La–Gd, ex. Pm)

An exploration of the synthetic and structural phase space of rare earth hybrid double perovskites A 2 B'BX 6 (A = organocation, B' = M + , B = M 3+ , X = molecular bridging anion) that include X = NO 3 – and B' = alkali metal is reported, complementing earlier studies of the [Me 4 N] 2 [KB(NO 3 ) 6 ] (B = Am, Cm, La–Nd, Sm–Lu, Y) (Me 4 N = (CH 3 ) 4 N + ) compounds. In the present efforts, the synthetic phase space of these systems is explored by varying the identity of the alkali metal ion at the B'-site. Herein, we report three new series of the form [Me 4 N] 2 [B'B(NO 3 ) 6 ] (B = La–Nd, Sm–Gd; B' = Na, Rb, Cs). The early members of the Na-series crystallize in the trigonal space group R3̅ from La to Nd where a phase transition occurs in the phase between 273 and 300 K, going from R3̅ to the high-symmetry, cubic space group Fm3̅m. The preceding trigonal members of the Na-series also undergo phase transitions to cubic symmetry at temperatures above 300 K, establishing a decreasing trend in the phase-transition temperature. The remainder of the Na-series, as well as the Rb- and Cs-series, all crystallize in Fm3̅m at 300 K. The temperature-dependent phase behavior of the synthesized phases is studied via variable-temperature spectroscopic methods and high-resolution powder X-ray diffractometry. All phases were characterized via single-crystal and powder X-ray diffraction and Fourier transform infrared (FT-IR) and Raman spectroscopic methods. These results demonstrate the versatility of the perovskite structure type to include rare earth ions, nitrate ions, and a suite of alkali metal ions and serve as a foundation for the design of functional rare earth hybrid double perovskite materials such as those possessing useful multiferroic, optical, and magnetic properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Magnetic excitation linking quasi-one-dimensional Chevrel-type selenide and arsenide superconductors

The quasi-one-dimensional Chevrel phases, A 2 M o6 Se 6 (A=Tl, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogs undergo a superconducting transition whereas the alkali metal analogs show charge gapping of another, not well understood type. Here we report the results of inelastic neutron scattering on polycrystalline In 2 Mo 6 Se 6 (T c =2.85 K) and Rb 2 Mo 6 Se 6 (nonsuperconducting) samples, which reveal a column of intensity with linear dispersion from [0 0 1/2] to [0 0 1] in both compounds. The observed temperature and |Q| independence together suggest the presence of unconventional carriers with a spin contribution to the excitation. This is contrary to the prevailing model for these materials, which is that they are nonmagnetic. The excitation has similar dispersion and S(Q,E,T) behavior as one observed in the structurally related superconducting compounds A 2 Cr 3 As 3 and A 2 Mo 3 As 3 (A=K, Rb, Cs), which has been interpreted as magnetic in origin and related to Fermi surface nesting. The connection is unexpected because the calculated Fermi surface of the arsenides differs substantially from the A 2 Mo 6 Se 6 compounds, and many consider them distinct classes of materials. This observation suggests a hidden link in the physics between both classes of superconductors, perhaps originating from their quasi-low-dimensional character.

1-dimensional systems↗
DOE OSTI2022

The crystal chemistry of ZrSi

Reported here is a revised crystal structure of β–ZrSi (TlI/CrB structure type), correcting the atomic position and bond distances. The Si–Si bond length has been modified substantially from 2.723(6) Å to 2.4411(8) Å. The β-ZrSi single crystals were grown from an arc-melted button, and were characterized using single crystal X-ray diffraction. A survey of the TlI/CrB structure type shows that changes to the nomenclature would be useful, separating it into four chemically distinct subtypes: TlI, CrB, CaSi, and ThCo. β–ZrSi is an example of the CaSi subtype. α-ZrSi crystallizes in the related FeB structure type, which is also divided here into the subtypes FeB, CeSi, and YNi. The effect of electron count on the relative phase stabilities of the CaSi and CeSi subtypes is rationalized with the aid of the Zintl concept and electronic structure calculations in the Linear Muffin-Tin Orbital (LMTO) basis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Fragile 3D Order in V 1 - x Mo x O 2

The metal-to-insulator transition in rutile VO2 has proven uniquely difficult to characterize because of the complex interplay between electron correlations and atomic structure. Here, we report the discovery of the sudden collapse of three-dimensional order in the low-temperature phase of V1-xMoxO2 at x = 0.17 and the emergence of a novel frustrated two-dimensional order at x = 0.19, with only a slight change in electronic properties. Single crystal diffuse x-ray scattering reveals that this transition from the 3D M1 phase to a 2D variant of the M2 phase results in long-range structural correlations along symmetry-equivalent (11L) planes of the tetragonal rutile structure, yet extremely short-range correlations transverse to these planes. These findings suggest that this two dimensionality results from a novel form of geometric frustration that is essentially structural in origin.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Large Single Crystals of V 1– x Mo x O 2 from a Two-Step Chemical Vapor Transport Synthesis

This article details the synthesis of single crystals with the chemical formulas V 1-x Mo x O 2 ( 0 ≤ x ≤ 0.6) using the chemical vapor transport method at high temperatures (850 to 950 °C). These compounds are of interest due to the special relationship between their structure and electronic properties. Understanding this relationship will help guide future efforts in designing energy-saving devices. While some of these compounds have been synthesized before, this is the first time their synthesis has been reported for such a wide range of compounds in the form of bulk single crystals. The single crystal form is required for detailed characterization of the compounds. The materials were characterized chemically and structurally in order to validate the precise chemical compositions that were reported. Characterization methods included x-ray diffraction, inductively-coupled plasma atomic emission spectroscopy, and electron dispersive spectroscopy. The synthesis method is tunable, and the introduction of multiple steps both overcome the previous chemical challenges precluding synthesis of high molybdenum content crystals and allow for the creation of larger single crystals that were otherwise possible. The crystals synthesized this way are suitable for specialized measurements for determining the precise physical properties and atomic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗