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