Discrete, Dispersible MnAs Nanocrystals from Solution Methods: Phase Control on the Nanoscale and Magnetic Consequences

Abstract

Nanocrystals of thermodynamically stable alpha-MnAs (hexagonal NiAs-type) and metastable beta-MnAs (orthorhombic MnP-type) have been synthesized by the reaction of triphenylarsine oxide (Ph(3)AsO) and dimanganesedecacarbonyl (Mn(2)CO(10)) at temperatures ranging from 250 to 330 degrees C in the presence of the coordinating solvent trioctylphosphine oxide (TOPO). Morphologically, both alpha- and beta-MnAs nanoparticles adopt a core-shell type structure with a crystalline core and low-contrast noncrystalline shell. In contrast to prior studies on MnAs particles, disks, and films, the present bottom-up synthesis yields discrete, dispersible MnAs nanoparticles without a structural support. Even in the absence of epitaxial strain, the lattice parameters of the nanocrystals are decreased relative to bulk MnAs, resulting in a volume decrease of 0.35% in alpha-MnAs and 0.38% in beta-MnAs nanoparticles. In contrast to bulk MnAs, where the ferromagnetic phase transition upon warming through 313-317 K is concomitant with a structure change from ferromagnetic alpha- to paramagnetic beta-MnAs, powder X-ray diffraction studies suggest there is no conversion of alpha-MnAs to beta over the temperature range 298-343 K. Moreover, magnetic measurements suggest that both alpha- and beta-MnAs are ferromagnetic with T(C) approximately 315 K. Partial phase transformation of beta-MnAs nanoparticles into thermodynamically stable alpha-MnAs occurs slowly over time (i.e., months) at room temperature. However, there is no associated change in magnetization, suggesting the ferromagnetism observed in beta-MnAs is intrinsic and cannot be attributed to alpha-MnAs impurities.

Nanoporous Metal Foams

“…Synthetic strategies for preparing nanoporous foams of many nonmetallic substances exist, including foams (aerogels) of silica; [7][8][9] main group, transition, lanthanide, and actinide metal oxides; [10][11][12][13][14][15][16] metal chalcogenides; [17,18] metal phosphides; [19] carbon; [20,21] organic polymers; [20,22] and even carbon nanotubes. [23,24] Additionally, numerous approaches for preparing macrocellular foams of various metals exist, [25,26] such as foaming of metal alloys using bubbled gas or decomposition of metal hydrides, [27,28] investment casting, [25,29] chemical vapor deposition onto polymer foam templates, [30] and cooling of liquid metal/hydrogen solutions through their eutectic points (the GASAR process).…”

Nanoporöse Metallschäume

“…Es gibt zahlreiche Synthesestrategien für die Herstellung nanoporöser Schäume aus nichtmetallischen Stoffen wie Quarz, [7][8][9] Hauptgruppenmetall-, Übergangsmetall-, Lanthanoid-und Actinoidoxiden, [10][11][12][13][14][15][16] Metallchalkogeniden, [17,18] Metallphosphiden, [19] Kohlenstoff, [20,21] organischen Polymeren [20,22] und Kohlenstoff-Nanoröhren. [23,24] Auch zur Synthese makrozellulärer Schaumstoffe aus Metallen sind zahlreiche Methoden bekannt, [25,26] z.…”

Diverse Structural and Magnetic Properties of Differently Prepared MnAs Nanoparticles

“…8 However, these epitaxial structures are subject to external pressure due to lattice strain from the substrate, resulting in significant shifts in the magnetostructural transition temperatures. Recently, a method for preparing MnAs as discrete unsupported nanoparticles 10 enables the effect of size on the first-order transition to be probed in the absence of external stress. Particles of 9-23 nm were prepared in solution by reaction of dimanganesedecacarbonyl (Mn 2 (CO) 10 ) and triphenylarsine oxide ((C 6 H 5 ) 3 AsdO) in coordinating solvents at temperatures ranging from 523 to 603 K. Intriguingly, small changes in the synthetic method led to two distinctly different products, as detected by powder X-ray diffraction conducted at room temperature.…”

“…Recently, a method for preparing MnAs as discrete unsupported nanoparticles 10 enables the effect of size on the first-order transition to be probed in the absence of external stress. Particles of 9-23 nm were prepared in solution by reaction of dimanganesedecacarbonyl (Mn 2 (CO) 10 ) and triphenylarsine oxide ((C 6 H 5 ) 3 AsdO) in coordinating solvents at temperatures ranging from 523 to 603 K. Intriguingly, small changes in the synthetic method led to two distinctly different products, as detected by powder X-ray diffraction conducted at room temperature. Notably, in one case, the metastable β structure, rather than the thermodynamic R structure, was observed.…”