Amazing Oxides - Ferromagnets, Superconductors, and...
Transcript of Amazing Oxides - Ferromagnets, Superconductors, and...
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