SLIDE 1 Introduction −What is cobalt acetylide? −
Easy to synthesize (ion-exchange reaction in Ar atmosphere) CaC2 + CoCl2 CoC2 + CaCl2 CoC2
acetonitrile 78 oC, 48 h
Small particle (∼ 50 nm) and quite small crystallite size (∼ 5 nm)
2nm 2nm
TEM images of CoC2 (hydrous phase): The crystal lattice is highly distorted 1 washing H2O
- cf. Eur. Phys. J. D24 (2003), 97-100 and Chem. Phys. Lett. 369 (2003), 198-203
SLIDE 2
2
Rod-shape large crystallite particles can be synthesized at 100 oC
The particle shows ferromagnetism even in room temperature!
Ferromagnetism of smaller particles
SLIDE 3 Question: What's the role of the absorbed water?
3
CoC2 ··· Highly absorbent of water (ca. 1-2 H2O / Co2+)
CoC2 is small, therefore such a large amount of the absorbed water must changes the crystal structure and the physical properties of CoC2.
Here, we reveal the effect of the absorbed water Sample preparation
Anhydrous CoC2: CaC2 (5 mmol) and CoCl2 (5.2mmol) in 300 ml of acetonitrile was heated at 78 oC, 130 h in grove-box. *Anhydrous CoC2 contains ca. 35 % of CaC2 Hydrous CoC2: Anhydrous CoC2 was exposed to air (25 oC, humidity 70 %)
- r washed with water (for XRD to remove CaC2 peaks).
SLIDE 4
4
Crystal structure of the anhydrous CoC2
CoC2 (calc) CaC2 (calc) exp. d-value / Intensity / 104 counts 1 1.5 2 2.5 3 1 2 3
Å
Synchrotron radiation, 0.9988 Å
Anhydrous CoC2: Cubic → Orientation disorder of C2
2−
Sharp peaks → Large domain Crystallite size > 45 nm (Single domain particle)
XRD result of anhydrous CoC2 3.41 Å 4.82 Å 3.41 Å
SLIDE 5
5
Crystal structure of the hydrous CoC2
EXAFS result
Structural model for calculated spectrum (MgC2-type structure)
·Tetragonal lattice ·C2
2− dianions are ordered
·Co2+−C2
2−−Co2+ chain || c-axis
SLIDE 6
6
XRD result of hydrous CoC2
3.78 Å 3.35 Å 3.78 Å EXAFS spectrum is consistent with XRD pattern Broad peaks → small domain Crystallite size: ∼ 10 nm (consistent with TEM images)
exp. calc. (based on EXAFS) d-value / Intensity / 102 counts 1 1.5 2 2.5 3 2 4 6 Å
Cu Kα, 1.5418 Å
SLIDE 7
anhydrous air 10 min air 30 min air 60 min FC, H = 10 Oe T / K χ / 10-3 emu g-1 10 20 30 40 5 10 15 anhydrous air 10 min air 30 min air 60 min ZFC, H = 10 Oe T / K χ / 10-3 emu g-1 10 20 30 40 1 2 3 4 5
7
Susceptibility of the anhydrous and hydrous CoC2
*FC: cooled with H = 10 Oe
Anhydrous CoC2: Paramagnetic
Air-exposure induce the ferromagnetism
SLIDE 8 8
Magnetization curves of the anhydrous and hydrous CoC2
anhydrous 10 min 30 min 60 min H / T M / emu g-1 1.8 K
0.1
2 4 6 anhydrous 10 min 20 min 30 min 1.8 K H / T M / emu g-1
0.5 1
10 20
Coercive force and remanent magnetization are raised by water absorption
SLIDE 9
9
Mechanism of the water-induced ferromagnetism
Possible Co2+−C2
2−−Co2+ configurations
Perpendicula Intermediate Parallel
Hydrous CoC2 (ferromagnetic) Anhydrous CoC2 (superparamagnetic)
Ferromagnetic interaction Weak interaction (FM or AFM)
SLIDE 10 10
C2
2−
Co2+ Ferromagnetic configuration (strong) Expanded by water Water absorption
Large ferromagnetic domain
Ferromagnetism!
Schematic model of the influence of water absorption
Small superparamagnetic clusters
SLIDE 11
Summary
Anhydrous CoC2 Hydrous CoC2 Lattice: Cubic Tetragonal Orientation of C2
2−: Disordered
Ordered Magnetism: Superparamagnet Ferromagnet Structural domain: Large (∼ 50 nm) Small (∼ 10 nm)
Water-absorption induce the orientation ordering of C2
2−
Water-induced ferromagnetism
11
SLIDE 12 12
The other feature of MC2 compound −easy synthesis of nanoalloy−
Heating the MC2 compounds gives "carbon-coated metal nanoparticle"
MC2 MC2 + C + metal Carbon-coated metal (5 - 20 nm) Δ ∼ 250 oC Δ ∼ 250 oC
20 nm
- cf. Co@C
- cf. Fe@C
- cf. Appl. Phys. Lett. 84 (2004), 1753-1755
SLIDE 13
13
If the mixture of MCl2 and M'Cl2 is used for synthesis, nanoalloy can be easily obtained only by heating!
MxM'1-xC2 Carbon-coated alloy
FeCo@C Fe:Co ∼ 1:1 NiPd@C Ni:Pd ∼ 1:1
