investigating and understanding ionic ammine materials · 2019-12-11 · investigating and...
TRANSCRIPT
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Investigating and Understanding Ionic Ammine Materials
Tuesday, 23rd September 2014
Martin Owen JonesSTFC, ISIS Neutron Spallation facility
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A (very) brief history of ammine materials
Alfred Werner1866 – 1919
Nobel Prize in Chemistry 1913
Werner Complexes
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Coordination Compounds
CoCl3•6NH3
[Co(NH3)6]Cl3
Conductivity and Cl- analysis
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Most Metals in Solution
LFSE, Cation radius and charge, nature of ligand....
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E. A. Sullivan, S. Johnson, J. Phys. Chem. 1959, 63, 233 –238
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E. A. Sullivan, S. Johnson, J. Phys. Chem. 1959, 63, 233 –238
Li(NH3)nBH4
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E. A. Sullivan, S. Johnson, J. Phys. Chem. 1959, 63, 233 –238
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Phasen =
AmmoniaContent
wt%
VolumeÅ3
Volume of NH3
MoleculeÅ3
Ammonia Density
gL-1
Vapour Pressure at 20°Cmbar
1 41 382 43 296 6.7
2 61 588 47 385 24
3 70 652 37 526 96
4 76 840 39 539 110
Liquid NH3
(-33°C)100 - - 682 1 017
C. S. Cragoe, C. H. Meyers, C. S. Taylor, J. Am. Chem. Soc., 1920, 42 (2), 206–229E. A. Sullivan and S. Johnson, J. Phys. Chem., 1959, 63, 233
Li[NH3]nBH4
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Stable just below room temperature under vacuum
Y. Guo, G. Xia, Y. Zhu, L. Gao, X. Yu, Chem. Commun., 2010, DOI: 10.1039/b924057h
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Using Neutron to characterise ammines
Simultaneous thermogravimetric and diffraction studies
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GEM bank 4 (50-74)All Runs 42824-43283
(42909-42946 (inc), 42985-43099 (inc) and 43188-43271 (inc) cut)
LiBD4 + ND3 Desorption RT
steel
n=2 n=2
n=3 n=3
n=1 n=1
n=4 n=4
n=4 n=3 n=2 n=1
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Li[NH3]BH4 - Structure
S. R. Johnson, W. I. F. David, et al, Chem. Asian J. 2009, 4, 849 – 854
LiBH4
• Tetrahedral coordination about Li atoms from 3 (BH4)- and 1 NH3
Li[NH3]BH4
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Tetrahedral Li coordination - 3 (BH4)- and 1 NH3
Edge-sharing chains of tetrahedra run along the b-axis
Li[NH3]nBH4n = 2
2nd NH3 orientation s similar to solid NH3
Single (η1) hydrogen bridge bonds seen for BH4
n = 3
Layers of B and Li atoms
Li sits in a B6 octahedra
Octahedra form face sharing chains along the c-axis
n = 4
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Mg(NH3)6Cl2 ↔ Mg(NH3)2Cl2 ↔ Mg(NH3)Cl2 ↔ MgCl2
R3-MHChloromagnesiteFM-3M
-6NH3
4NH3 NH3 NH3
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Mg[NH3]6X2 (X = Cl, Br, I)Structure
2 6 3 6 2 3 6 2 3 6 22 6 3 6 2 3 6 2 3 6 2
All cubic K2PtCl6 structure as per Mg[NH3]6Cl2ESRF X-ray diffraction, ID31
Mg[NH3]6Cl2 Mg[NH3]6I2
Mg[NH3]6Br2
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Structure
Lattice parameter that increases with increasing anion size Slight increase in Mg-N bond length
ESRF X-ray diffraction, ID31
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NMR
For MgX2, Decreasing Mg chemical shift with increasing anion atomic number
Increase in diamagnetic shielding / electron density around the Mg nucleus.
For Mg[NH3]6X2 chemical shift at lower field hence less electron density at Mg wrt MgX2
Very small change in chemical shift, with the X-ray data suggests very little change in Mg-N bonding with changing anion
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TGA
IncreasingDesorption
Temperature
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Starting Compound
Desorption Step 1 Desorption Step 2 Desorption Step 3
T1O T1P wt% T2O T2P wt% T3O T3P wt%
Mg(NH3)6Cl2 98 117 32 187 204 10 250 274 9
Mg(NH3)6Br2 130 155 23 223 243 6 270 292 9
Mg(NH3)6I2 182 212 18 - 282 - - 349 -
To = Onset, Tp = Peak
Increasing with anion radius
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Confusing. If no change in Mg-N bonding why the difference in Tdec?
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Assumption
The cation is [Mg(NH3)6]2+ / [Mg(NH3)2]2+ / [Mg(NH3)]2+
rather than Mg2+
2+
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Mg(NH3)6X2 (s) Mg(NH3)2X2 (s) + 4NH3 (g) Mg(NH3)X2 (s) + 5NH3 (g) MgX2 (s) + 6NH3 (g)
Mg(NH3)62+
(g) + 2X-(g) Mg(NH3)2
2+(g) + 2X-
(g) + 4NH3 (g) Mg(NH3) 2+(g) + 2X-
(g) + 5NH3 (g) Mg2+(g) + 2X-
(g) + 6NH3 (g)
DHr1 DHr2 DHr3
DHd1’ DHd2’ DHd3’
DLattH(Mg(NH3)6X2) -DLattH(Mg(NH3)2X2) -DLattH(Mg(NH3) X2) -DLattH(MgX2)
Hess Cycle
Kapustinskii EquationDlattH ≈
A |z+| |z-|u
(r+ + r-)
Increasing anion size reduces DLattH
DLattH (Mg(NH3)6X2 - DLattH (Mg(NH3)2X2 is always negative (favourable)
This favourable contribution decreases with increasing r- (X- radius)
therefore, Tdec increases with increasing anion size
Thus the enthalpy of ammonia desorption is a more significant factor in the overall enthalpy of reaction DHr1
Tdec∝∆Hr∝1
𝑟𝐿+ + 𝑟−−
1
𝑟𝑆+ + 𝑟−+ ∆Hd
Simple Ionic Model
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Mg(NH3)6X2 (s) Mg(NH3)2X2 (s) + 4NH3 (g) Mg(NH3)X2 (s) + 5NH3 (g) MgX2 (s) + 6NH3 (g)
Mg(NH3)62+
(g) + 2X-(g) Mg(NH3)2
2+(g) + 2X-
(g) + 4NH3 (g) Mg(NH3) 2+(g) + 2X-
(g) + 5NH3 (g) Mg2+(g) + 2X-
(g) + 6NH3 (g)
DHr1 DHr2 DHr3
DHd1’ DHd2’ DHd3’
DLattH(Mg(NH3)6X2) -DLattH(Mg(NH3)2X2) -DLattH(Mg(NH3) X2) -DLattH(MgX2)
Hess Cycle
Thus the enthalpy of ammonia desorption is a more significant factor in the overall enthalpy of reaction DHr1
The enthalpy of ammonia desorption (DHd1), which is found from our X-ray diffraction and NMR studies to be essentially
independent of X
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𝑟 Mg(NH3)6 2+ − 𝑟 Mg(NH3)2
2+ > 𝑟 Mg(NH3)2 2+ − 𝑟 Mg(NH3)
2+ ≈. 𝑟 Mg(NH3)1 2+ − 𝑟 Mg 2+
For constant change in anion size, Tp increases for subsequent desorption steps since
For increasing anion size, Tp1 increases
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Why is this interesting?
Design solid-state storage materials
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Characterising Solid State Ammines
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Another way to determine ammonia content?
Dielectric Constant (εr)
NaCl 3Ammonia 17
MgCl2 Mg(NH3)2Cl2
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Microwave dielectric resonator
2.45 GHz, TM010 mode of a resonant cavity
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Sample
Microwave
cavity resonator
Quartz tube
Mass
spectrometer Data storage Vector network
analyser
Mass flow
controller Ammonia
Gas line
Three way
valve
Argon
Gas line
Experimental Set-Up
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Experimental Set-Up
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0 500 1000 1500 2000 2500 3000-300
-250
-200
-150
-100
-50
0
Time (s)
Fre
quency s
hif
t (K
Hz)
1.006
1.005
1.004
1.003
1.002
1.001
1.000
Perm
ittivity
(1 )
Empty Tube under Flow
Ar
NH3
2.42 2.43 2.44 2.45 2.46
-70
-65
-60
-55
-50
-45
-40
-35
-30
S21 M
agnit
ude
(dB
)
Frequency (GHz)
(a)
(b)
(c)
(d)
AmmoniatedSalt
Ar or NH3
Salt
Sample under Flow
Decrease in frequency due to increasing dielectric losses (DB) and polarizability (DF) of cavity
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-3.5 107
-3 107
-2.5 107
-2 107
-1.5 107
-1 107
-5 106
0
5 106
-2 x 106
0
2 x 106
4 x 106
6 x 106
8 x 106
1 x 107
0 2 4 6 8 10 12
D F
req
uen
cy (
MH
z) D
Ba
nd
wid
th (M
Hz)
Time (hrs)
Dielectric Data
Ca(NH3)nBr2
CaBr2
Increasing Bandwidth
due to increasing dielectric losses (DB) and polarizability (DF)
as concentration of ammonia increases
DecreasingFrequency
Order-Disorder effects?
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Structural Data
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Good Correlation
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Use a simple ionic model tounderstand and predict theproperties of ionic ammines
Ammines are gravimetrically andvolumetrically dense hydrogen and ammoniastores
We can use simultaneous techniquesto develop greater a understanding ofthese materials as well as developingnew characterisation tools
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Acknowledgements
David Royse Oxford / STFC
Jon Hartley Cardiff
Adrian Porch Cardiff
Bill David STFC / Oxford
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Mg(BH4)2Mg(NH3)6(BH4)2
shares the same
K2PtCl6 structure
Lower Tp1, than Mg(NH3)6Br2 (2.03 for BH4- cf. 1.96 for Br-)
dihydrogen bonding in the intermediate Mg(NH3)2(BH4)2, as proposed in Soloveichik Inorg. Chem., 2008, 47 (10)
Mg[NH3]6(BH4)2
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Simultaneous neutron diffraction
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CaBr2