powder neutron diffraction studies of oxide and...
TRANSCRIPT
Powder Neutron Diffraction studies of
Oxide and Oxynitride Materials
J. Paul Attfield
Department of Chemistry and
Centre for Science at Extreme Conditions (CSEC),
University of Edinburgh
X-rays Neutrons
Photons, constant velocity c
E = hn = hc/l
Particles, variable velocity v
l = h/mv; E = mv2/2
l = 1 Å; E ≈ 10 keV
core electron ionisation
l = 1 Å; E ≈ 100 meV
rotational, vibrational excitations
Charge>>Magnetic>Nuclear scattering
measure electron density
Nuclear ≈ Magnetic >> Charge
measure nuclear and magnetic densities
All elements accessible
but heavy >> light
Most elements scatter equally
a few incoherent (H) or absorbing (B, Gd)
sinl
Norm
ali
sed
Scat
teri
ng F
acto
r
b
fx
fm
Powder Neutron Diffraction Reactor Source
e.g. Institut Laue Langevin (ILL),
Grenoble, France
Pulsed Source
e.g. ISIS spallation facility, Rutherford
Appleton Laboratory, UK
Good for angle-dispersive diffraction, cf x-
ray diffraction
Good for wavelength-dispersive Time-of-
Flight neutron diffraction ;
l = h/mv and l = 2dsin ht/mL = 2dsin
Detector
Sample
Monochromator
Monochromatic X-rays/Neutrons
'White ' X-rays/Neutrons
Fixed detector
2White neutron pulse
Proton Pulse
Target
Moderator
L1
L2
L = L1 + L2
1 d(Å ) 3
2.5 d(Å ) 4.5
High 2
detector
Low 2
detector
t1 t2 (same time window)
Fixed detector
2White neutron pulse
Proton Pulse
Target
Moderator
L1
L2
L = L1 + L2
Fixed detector
2White neutron pulse
Proton Pulse
Target
Moderator
L1
L2
L = L1 + L2
1 d(Å ) 31 d(Å ) 3
2.5 d(Å ) 4.5
High 2
detector
Low 2
detector
t1 t2 (same time window)
Outline – examples of
energy/electronic materials
Q-range
Neutron flux/
Sample size
2. MnTiO3
and MnVO3
Contrast
4. Oxynitrides
Analysis
software
Competitive/complementary
methods; neutron + X-rays
1. Fe3O4
Sample
environments
3. BiNiO3
Powder
Neutron
Diffraction
1. The Verwey structure of magnetite
(Fe3O4) – a brief history
• 1939 - Fe3+[Fe2+Fe3+]O4 charge ordering at 125 K
transition proposed by Verwey.
• 1975 - monoclinic 2a x 2a x 2a Cc supercell
with 16 independent B-sites (Iizumi & Shirane).
• 1982 - Pmca subcell neutron refinement from
partially detwinned single crystal (Iizumi et al)
CO not seen.
• 2001 - Pmca subcell refinement used high
resolution powder neutron (HRPD@ISIS) and X-
ray (BM16@ESRF) data (Wright et al, 2001)
CO over 4 averaged sites.
• 2012 - full Cc structure from synchrotron
microcrystal (40 μm grain, 2 twin domains)
CO, OO, trimeron order
Mark Senn, Jon Wright & JPA, Nature (2012)
10
102
103
104
105
2.0 2.5 3.0 3.5 4.0
90
K I
nte
nsi
ty (
Co
un
ts)
d-spacing (?
*
**
-20
-10
0
10
20
Dif
f/e
sd
10
102
103
104
105
1.0 1.2 1.4 1.6 1.8 2.0
90
K I
nte
nsi
ty (
Co
un
ts)
d-spacing (?
***
*
-20
-10
0
10
20
Dif
f/e
sd
10
102
103
104
105
0.5 0.6 0.7 0.8 0.9 1.0
90
K I
nte
nsi
ty (
Co
un
ts)
d-spacing (?
-20
-10
0
10
20
Dif
f/e
sd
10-2
10-1
100
101
102
2.0 2.5 3.0 3.5 4.0
Inte
nsi
ty (
arb
.un
its)
d-spacing (?
*
*
-40
-20
0
20
Dif
f/e
sd
10-2
10-1
100
101
102
1.0 1.2 1.4 1.6 1.8 2.0
Inte
nsi
ty (
arb
.un
its)
d-spacing (?
***
-20
-10
0
10
20
Dif
f/e
sd
10-2
10-1
100
101
102
0.5 0.6 0.7 0.8 0.9 1
Inte
nsi
ty (
arb
.un
its)
d-spacing (?
-20
-10
0
10
20
Dif
f/e
sd
AB2O4 spinel
structure
130 K
90 K
High P-T large volume synthesis
High Pressure Materials SrCrO3
Orbitally driven phase separation
Ortega San Martin et al, PRL 2007
PbRuO3 Symmetry-reversing orbital transition
Kimber et al, PRL 2009
RFeAs(O,F)
New arsenide superconductors
Bos et al, Chem. Comm. 2008
MnVO3 Helimagnetic A site spin order
Markkula et al, PRB 2011
Also UK-Japan (EPSRC/JST)
collaboration with Kyoto/TIT/ISSP
2. MnTiO3 High pressure (8 GPa, 1000 °C) LiNbO3-type R3c polymorph
Spin order at Tc = 28 K – is it consistent with multiferroicity?
Neutron study of 70 mg sample on D20@ILL:
Antiferromagnetic order in xy-plane observed
Weak ferromagnetic component is symmetry allowed
Perpendicular coupling and switching of M(b) with P(c)
predicted (Fennie et al, PRL 2008)
Angel Arévalo-López and JPA, in preparation
MnVO3 A2+VO3 perovskites of 3d1 V4+ :
• A = Ca and Sr, and Cd(HP) - good metals.
• A = Pb(HP) - antiferromagnetic, ferroelectric insulator - cf PbTiO3.
• A = Mn(HP) (Y. Syono et al, J. Phys. Chem. Solids (1971)). Mn2+ is smaller
and has S = 5/2 3d5 Mn2+ moments. Physical properties unclear.
• Synthesis: Mn2V2O7 precursor reduced to MnVO3
perovskite at 1100 ºC under 8 GPa pressure for 30 min.
• SXRD (ID31@ESRF): orthorhombic Pnma perovskite
tilted superstructure 4-300 K, no structural transitions.
• Susceptibility (cf Syono et al):TN = 50 K; Curie-Weiss
moment of 5.80 μB, ( cf 5.91 μB for S = 5/2 Mn2+) = -154
K; partly frustrated antiferromagnet (||/TN ≈ 3).
• HP conductivity: metallic up to 67 kbar, small (Lorentz
force) magnetoresistance
localized Mn2+ 3d5 and itinerant B-site V4+ 3d1 electrons
MnVO3 Neutron Studies:
• PND (k 0 0) spin order (cf TbMnO3, but rare in metallic perovskites), D3(mz) and
D4(my) vectors two solutions:
– D3(mz) ± iD4(my) helical (non-polar point group 222, so not ferroelectric) 3.5 μB
– D3(mz) ± D4(my) SDW 4.9 μB
• Either/both of two exchange mechanisms possible:
– Superexchange (inequivalent Mn-O-Mn)
– RKKY (Ruderman-Kittel-Kasuya-Yosida)
Mn – V3d - Mn
Unusual system with localised and itinerant 3d channels
- interesting ambiguities
D20@ILL 1.7K
LDA band structure
with (000) spin order.
(kx 0 0) Fermi vectors
have kx = 0.15(↑) and
0.45(↓) .
Clemens Ritter Hua Wu
3. BiNiO3
• Made at 60 kbar, 1000 °C by Azuma et al, Kyoto, 2001
• Unexpectedly distorted structure, not ferroelectric
• High pressure neutron diffraction study (Paris-Edinburgh
cell) at ISIS.
(M. Azuma, Y. Shimakawa, M. Takano;
Kyoto; EPSRC-JST project)
Bi3+0.5Bi5+
0.5Ni2+O3
P-1 Bi3+Ni3+O3
Pnma
JACS, 2007
BiNiO3 and CNTE Large DV ≈ -3% at intervalence/M-I transition:
• 5% La substitution stabilises transition to ambient P,T
Useful NTE (Negative Thermal Expansion) materials for zero or
controlled expansion composites have smooth contraction over a
wide temperature range. Two types:
• Framework types such as ZrW2O8 and Cd(CN)2 contract over
very wide ranges, typical magnitude:
DL/L ≈ -1% over DT ≈ 1000 K aL = DL/(L.DT) = -10 x 10-6 K-1
e.g. ZrW2O8 has aL = -7 x 10-6 K over 0.3-1050 K.
• Electronic/magnetic transitions strongly coupled to the lattice;
e.g. previous record dilatometric aL = -25 10-6 K-1 for
(Mn0.96Fe0.04)3(Zn0.5Ge0.5)N anti-perovskite at 316-386 K. (Miller
et al, APL, 2005).
Colossal NTE (CNTE) paradigm; broaden
DL/L ≈ -1% transition to DT ≈ 100 K aL = -100 x 10-6 K-1 .
Bi0.95La0.05NiO3 provides proof-of-concept;
Crystallography aL = -137 x 10-6 K-1 and DT =70 K
Dilatometry aL = -82 x 10-6 K-1 and DT =60 K
Azuma et al, Nature Comm, 2011.
Bi0.95La0.05NiO3
High pressure synthesis R2O3 + Zr2ON2 → 2RZrO2N
Inorg Chem 2009
Oxynitrides (M. Yang, with ICMAB, Barcelona and NTU, Taiwan)
Eu Perovskites CMR in EuNbO2N and EuWON2
JACS 2008,2010; APL 2009
S = ½ Pyrochlores
R2Mo2O7 (Mo4+) R2Mo2O5N2 (Mo5+)
Chem Mater 2010
White-LED phosphors Cation tuning of (M,Eu)Si5-xAlxN8-xOx
JACS 2012
-8 -6 -4 -2 0 2 4 6 8
0
20
40
60
80
100
15K
10K
5K
D
R/R
H=
0(%
)
H (T)
2K
-0.2 -0.1 0.0 0.1 0.2
-900
-600
-300
0
300
600
900
x = 0.25
x = 0.50
x = 0.75
x = 1.00
Δr (Å )
1/λ
x-
1/λ 0
(cm
-1) Ca
Sr
Ba
Perovskite Oxynitrides
• AMO2N or AMON2 mostly of high valent d0 transition metals (A = Ca-Ba2+, R3+; M = Ti4+, Zr4+, Hf4+, Nb5+, Ta5+, Mo6+, W6+)
• Usually prepared by ammonolysis, e.g.
NH3, 950 oC
EuNbO4 EuNbO2N
• Useful properties mainly from large bandgaps (e.g. BaTaO2N photocatalysis, dielectrics; CaTaO2N-LaTaON2 red-yellow
pigments)
• O/N order in ABX3 perovskite lattice unclear ‘…The origin of the different ordering degrees therefore remains a
puzzling question.’ Ebbinghaus et al, Prog. Solid State Chem. (2009).
Powder neutron diffraction study of SrMO2N (M = Nb,Ta)
i. ‘Cubic’ high temperature structure
Neutron diffraction (D2B@ILL, l = 1.5943 Å ) exploits
high scattering contrast; bO= 0.581, bN= 0.936 fm.
Appears cubic but SrMO(O0.5N0.5)2 anion
distribution has tetragonal P4/mmm symmetry
with very small distortion (c/a = 0.9993).
Robust anion order up to >1100 C (~synthesis
temperatures).
Atom x y z Uiso (Å2) O/N
occupancy
Sr 0.5 0.5 0.5 0.0170(5)
0.0130(5)
Nb
Ta
0 0 0 0.0082(4)
0.0052(4)
X1 0 0 0.5 0.0225(4)
0.0187(3)
0.99(4)/0.01
0.96(4)/0.04
X2 (x2) 0.5 0 0 0.0225
0.0187
0.54(3)/0.46
0.51(3)/0.49
Nb a = 4.0541(2) Å c = 4.0511(4) Å
Ta 4.0442(3) Å 4.0421(5) Å
SrNbO2N
500 600 700 800 900 1000 1100
0.96
0.98
1.00
1.02
1.04
1.06
1.08
1.10
1.12
O1
Site
Occup
an
cy
Temperature (C)
• Apparent I4/mcm (ordered octahedral rotations) superstructure at 300 K.
Powder neutron diffraction study of SrMO2N (M = Nb,Ta)
ii. ‘Tetragonal’ room temperature structure
ii. ‘I4/mcm’ rotational superstructure at RT √2 x √2 x 2 superstructure from frozen rotations
Inequivalent anion sites Y1 and Y2 (x2).
Two possible orientations for anion and
rotational order.
Anion order directs rotational order
Lowers symmetry I4/mcm I112/m (≡ C2/m)
Predict loss of c-glide (observed by ED, Judith
Oro - ICMAB)
can
crot
can
Predicted
O/N
Y1 1/0 0.5/0.5
Y2 (x2) 0.5/0.5 0.75/0.25
Refined
(SrTaO2N)
Y1 0.48(4)/0.52
Y2 (x2) 0.76/0.24
Same distribution in refinements of SrTaO2N
(Clarke et al, Chem Mat, 2002) and Pnma
CaTaO2N (Gunther et al ZAAC, 2000).
[100]
b*
c*
[010]
c*
a*
SrNbO2N
What favours the SrMO(O0.5N0.5)2 anion distribution?
• difficult to rationalize from O2-/N3- electrostatic
repulsions.
• M-O/N covalency provides consistent model:
Structural Principles for Oxynitride Perovskites
Model for SrMO2N local structure
M(dp)-X(pp) covalency favours cis- (90º) configuration
in d0 complexes, e.g. L4MoX22+ (X2- = O, NR), MoF3O3
3-.
well-defined local structure of cis-MN2O4 octahedra
disordered -M-N- cis-chains (cf organic polymers)
chains segregate into layers (also CaTaO2N, EuTaO2N;
NdV4+O2N (3d1); not BaTaO2N)
X L
X
L L
L
F F
F
O O
O
M. Yang et al, Nature Chem, 2011
= M – N – M = M – O – M
• Increasing polymerisation of M-N-M units in MO3 matrix; for x = 0 1.5;
monomers oligomers chains crosslinked chains
• Symmetric polymerisation of M-O-M units in MN3 matrix for x = 3 1.5
evidence:
Structural Principles for AMO3-xNx Perovskites
O/N occs. SrTaO2N
LaNbON2
Pnma
Y1 0.48(4)/0.52
0.44(3)/0.56
Y2 (x2) 0.76/0.24 0.28/0.72
[010]
c*
a*
c*
b*
[100] EuWON2
All O/N local
coordinations
acentric
enhanced
dielectric
susceptibility and
optical transitions
Logvinovich et al, ZAAC 2010
SrMO2N – anion order or disorder?
= M – N – M = M – O – M
• Pauling (1935) estimates of configurational entropy:
S =R.lnZ; Z = w/22 w = number of local configurations
• Water (and spin) ices; n = 6 configurations per H2O molecule
Z = 1.5 S = 0.4R (zero-point entropy)
• SrMO2N (cis-square ice); n = 4 per MO2N2
Z = 1 S = 0 (ordered state??)
O
H
H O
H
H
O
H
H O
H
H
O H H O
H
H
‘Ice rules’ order in perovskite planes
(M-N x2, M-O x2 cf O-H x2, O…H x2)
Oxynitride Perovskite Entropies
Pauling ice rules configurational entropy for AMO3-zNz;
S ≈ NkBln[wffn/2(1-f)(1-f)n/2] ;
w = local configs, f = 2z/n where n = 4 (2D) or n = 6 (3D)
S = 0 for specific cases (e.g. SrTaO2N) despite structural
disorder - why?
Bonding rules- alternating M-N-M and M-O-M bridges along all rows.
only two possible sequences per row.
LLL lattice has configurations: W = 22L2
S = 2N2/3kBln2 S/N 0 as N ∞ (Pauling limit)
Entropy is ‘sub-extensive’, depends on particle size:
S≈ 10-7R as one mole single crystal
S≈ 0.1R per mole 40 nm nanoparticles (N≈ 106)
SrMO2N has ultra-highly correlated anion order
(cis-square ice, no spin analogues)
– crystallographically disordered on atomic scale
but with no macroscopic configurational entropy.
‘Open order’ (based on closure of correlation
vector sets) – first atomistic example.
Phil Camp, A Fuertes, JPA, JACS 2012.
↑
BaTaO2N
SrTaO2N LaNbON2
EuWO1.5N1.5
↓ ↓ ↓
Perovskite O/N Summary
• SrMO2N etc. structures have well-defined local anion
order - disordered cis –M-N- chains in xy planes – an
‘open order’ with sub-extensive entropy.
• Anion order controls the rotation/tilt order axis of
octahedra – new aspect of perovskite tilting.
• Anion order is robust but the resultant lattice distortions
are very small so that high resolution neutron diffraction
is needed to determine such structures.
• A wealth of similar local structures is expected across
the range of AMO3-xNx perovskites
• Challenge to control chains and tune physical properties,
including magnetism, orbital order in d1 V4+.
Local O/N tuning of WLED phosphors
Cation size tuning of (M,Eu)Si5-xAlxN8-xOx (M =
Ca,Sr,Ba) photoluminescence energy observed.
Structural mechanism;
1. Neutron diffraction (HRPD@ISIS) shows that O
substitute only for 2SiN (not 3SiN)
O’s coordinated to M/Eu
2. Size difference between Eu and host cation
controls local O-distribution
WT Chen, HS Sheu, RS Liu and JPA, JACS 2012
-0.2 -0.1 0.0 0.1 0.2
-900
-600
-300
0
300
600
900
x = 0.25
x = 0.50
x = 0.75
x = 1.00
Δr (Å )
1/λ
x-
1/λ 0
(cm
-1) Ca
Sr
Ba
400 450 500 550 600 650 700 750
Wavelength (nm)
x = 0.00
x = 0.25
x = 0.50
x = 0.75
x = 1.00
Rel
ati
ve
inte
nsi
ty (
a.u
.)
Ba
Sr
Ca
0.8 1.2 1.6 2.0 2.4
d
Observed
Calculated
Background
Inte
nsit
y (a
.u.)
Si4+/Al3+
O2-/N3-
N3-
M2+
(Å )
Ba Eu CaN O SiAlBaSi4AlN7O
Powder neutron diffraction
• Precise location of light atoms; H(D)/Li ion conductors, metal
carbides, nitrides, oxides and fluorides.
• Distinguishing neighbouring elements ordered over different
structural sites, e.g. O/N.
• Use combined X-ray+neutron data for precise refinements e.g Fe3O4
• Magnetic order e.g. MnVO3.
• Multiple powder patterns are easily recorded as a function of
temperature (decomposition, phase transitions, reactions), pressure
or time (solid state kinetics experiments) e.g. BiNiO3.
Thanks Wei-Tin Chen
Lucy Clark
Shigeto Hirai
Andrea Marcinkova
Mikael Markkula
George Penny
Marek Senn
Alex Sinclair
Congling Yin
Minghui Yang
Angel Arevalo-Lopez
Anna Kusmartseva
Jenny Rodgers
Amparo Fuertes, Judith Oro-Sole and
colleagues, ICMAB Barcelona
Jon Wright, ESRF
EPSRC
Leverhulme
STFC
Ministerio de Economía y Competitividad