Principles and Applications of Anion Order in Solid Oxynitrides

Review pubs.acs.org/crystal

Principles and Applications of Anion Order in Solid Oxynitrides Published as part of the Crystal Growth & Design virtual special issue on Anion-Controlled New Inorganic Materials J. Paul Attfield* CSEC and School of Chemistry, University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom ABSTRACT: Metal oxynitrides are emerging materials that may combine the advantages of oxides and nitrides. Anion order is important for controlling and tuning properties, and neutron diffraction provides good O/N contrast for experimental determinations of local or long-range O/N order in solids. Differences between oxide and nitride in charge, size, and covalent bonding are the important factors that drive anion order. An important example is the robust partial anion order in SrMO2N (M = Nb, Ta) and related oxynitride perovskites driven by covalency that results in disordered zigzag MN chains which segregate into planes within the perovskite lattice. This leads to unusual subextensive scaling of entropy, described as “open order”. Local anion order is important to optical materials. Size mismatch between host and dopant cations leads to local O/N clustering that tunes photoluminescence shifts systematically in M1.95Eu0.05Si5−xAlxN8−xOx phosphors, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu2+ dopant but a blue shift when the M = Ca host is smaller.

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INTRODUCTION Solid oxynitrides are emerging materials that can combine the advantages of transition metal oxides and nitrides. They generally have greater air and moisture stability than pure nitrides but with smaller bandgaps than comparable oxides leading to useful electronic or optical properties. Nitrogendoped TiO21 and (GaN)1−x(ZnO)x2 are good water-splitting catalysts and stoichiometric oxynitrides such as TaON3 and ATaO2N perovskites (A = Ca, Sr, Ba)4 also have photocatalytic applications. The perovskites SrTaO2N and BaTaO2N have high dielectric constants,5 and (Ba, Sr)TiO3−xNy thin films have been explored as possible high temperature dielectrics for telecommunications devices.6 CaTaO2N-LaTaON2 solid solutions are nontoxic red-yellow pigments,7 and many siliconbased oxynitrides such as MSi2O2N2 (M = Ca, Sr, Ba)8 are useful host materials for phosphors in white light light emitting diode (LED) devices. Oxynitrides with notable electronic transport properties include SrMoO2N with a high Seebeck coefficient9 and EuNbO2N10 and EuWON2,11,12 which show colossal magnetoresistances (CMR) at low temperatures. Oxynitrides are usually prepared by heating metal oxides under nitrogen or ammonia gases,13 but the spinel Ga3O3N14 and RZrO2N perovskites (R = Pr, Nd, and Sm)15 were synthesized by direct solid state reaction between oxides and nitrides or oxynitrides under high pressure. Full reviews of oxynitride perovskites,16 silicon oxynitrides,17 and general aspects of oxynitrides18,19 have recently been published. The possible order or disorder of anions is an important aspect of the engineering of oxynitride crystal structures and properties. The ordering principles will be © 2013 American Chemical Society

discussed in this review, and selected examples from recent studies of perovskites and silicon oxynitrides will be used to show how correlated local anion order influences structure and properties.

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ORDERING PRINCIPLES There are two ideal cases for anion ordering. Ideal order reflects crystallographically perfect long-range order of oxide and nitride over distinct structural sites. TaON and Si2ON2 are examples of ideally ordered ternary oxynitrides. Ideal disorder describes a solid solution in which anions are randomly distributed. These can have compositions of the type MO1−xNx if metal M has variable oxidation states for charge compensation, or MO1−3x/2NxVx/2 for a fixed valence where anion vacancies V are formed. Fully charge compensated solid solutions are rare, but the wurtzite-type (GaN)1−x(ZnO)x (x = 0.12) photocatalyst provides a quaternary example. ZrO2−3x/2NxVx/2 (0 < x < 0.29) is a solid solution for constant-valent Zr4+.20 Most solid solutions based on variable valence metals show partial charge compensation and so fall between the above limits. For example, nitrogen-doped ceria materials form a cubic fluorite-type solid solution Ce4+1−yCe3+yO2−(3x/2)−(y/2)NxV(x+y)/2 for 0 < x < 0.09 and 0 < y < 0.03,21 and the metastable anion-deficient pyrochlore Eu 2 Mo 2 O 5−x N 2+x−δ V δ obtained from ammonolysis of Eu2Mo2O7 has reported composition variables 0 < x < 1.74 and 0 < δ < 0.29.22 Received: July 24, 2013 Published: September 5, 2013 4623

dx.doi.org/10.1021/cg4011168 | Cryst. Growth Des. 2013, 13, 4623−4629

Crystal Growth & Design

Review

Attempts to rationalize the degree of O/N order in solid oxynitrides have used both ionic and covalent approaches. The electrostatic MAPLE (MAdelung Part of Lattice Energy) method32 has been applied to silicon oxynitrides,17 for example, to confirm the full anion order in CaSi2O2N2.33 A bond valence approach based on Pauling’s second rule demonstrated a general correlation between anion occupancies from diffraction studies of oxynitrides and oxyhalides, and the bond valence sums for each crystallographically distinct site.34 This approach has the advantage of simplicity as it requires knowledge of the network topology but not of the crystal structure details. The two cases shown in the following section are both recent studies that go beyond the above average structure methods and have evidenced locally correlated order of oxide and nitride within crystalline materials. They are rationalized using the above ionic-covalent principles, and the influence of the anion order upon materials properties is also discussed.

Few oxynitrides conform well to either the ideally ordered or disordered cases, and most have intermediate anion orders with local clustering or extended correlations that may give rise to nonrandom site occupancies in the averaged crystal structure. Neutron scattering is a powerful tool for determining such orders, as there is good contrast between the nuclear scattering lengths of oxygen (5.83 fm) and nitrogen (9.36 fm). The nature and degree of anion order in the ground state structure depends largely upon the lattice enthalpy and hence the bonding energies associated with the anion configuration. In the simplest hard sphere model of ionic bonding, differences in ion charge and size determine the degree of order. These parameters are shown for X = N, O, and F in Table 1. The Table 1. Comparison of Ionic and Covalent Sizes for X = N, O, and F Based upon Four-Coordinate Ionic Radii 4r and 1 X−2C Covalent Bond Distances X

N

O

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F

CORRELATED ORDERS IN OXYNITRIDES (i). Oxynitride Perovskites. A variety of AMO2N and AMON2 perovskites of alkaline earth or lanthanide metals A and high valent transition metals M are known. Significant ranges of nonstoichiometry are also found in some systems, such as EuWO1+xN2−x (−0.16 ≤ x ≤ 0.46).12 Notable photocatalytic, dielectric, or electronic transport properties have been reported in perovskite oxynitrides, as described in the Introduction. Oxide/nitride anion order is expected to be important, for example, in directing the M-cation displacements in dielectric materials, but initial neutron diffraction studies did not report consistent models, and a 2009 review of the field noted that “the origin of the different ordering degrees therefore remains a puzzling question.”16 Important insights were gained from a powder neutron diffraction study of SrMO2N (M = Nb, Ta) at high temperatures where these materials appear to be simple cubic perovskites.35 Structure refinement showed that these phases actually have tetragonal symmetry with inequivalent anion sites as shown in Figure 1a. The site on one edge of the perovskite cell (the unique tetragonal axis for anion order, can) is fully occupied by O, whereas sites in the two perpendicular directions have a statistical 50:50 O/N mixture. This average structure was interpreted as consisting of cis-MO4N2 octahedra

Ionic: Xn− charge n radius (4r)/Å Covalent: 1 X−2C bond distance/Å

3− 1.46

2− 1.38

1− 1.31

NCF 1.157

OCO 1.162

F−CN 1.264

differences in charge and radius are enough that N/O and O/F anions should usually be well ordered. However, there is substantial covalency in M−X bonding for high valent cations such as Si4+ or Ta5+ that are often found in functional oxynitrides. Bond lengths from the isoelectronic molecules CO223 and FCN24 are shown in Table 1 as an indicator of covalent effects upon X size. Sizes of ions in the solid state are based on comparison between different species in the same coordination environment, such as the standard four-coordinate anionic radii shown in Table 1.25 This implicitly leads to differences in bond order when the solid state approach is applied to covalently bonded structures. The distances shown in Table 1 are all for one-coordinated X connected to twocoordinated carbon and hence are comparable in a solid state analysis but impose a triple bond for N, a double bond for O, and a single bond for F. This reverses the size order found in the ionic limit. The above analysis shows that the effective size of anions in extended structures depends on the elements to which they are bonded. The 1X−2C bond distances also reveal a general difference between oxynitride and oxyfluoride structures. Covalent M−O bonds of high valent metals can be substantially shorter than M−F bonds, so oxide and fluoride anions tend to be long-range ordered in such crystals and give rise to useful or interesting functionalities, e.g., polar KNaNbOF5,26 the highcapacity battery material Ag4V2O6F2,27 antiferromagnetic chains in CsVOF3,28 and spin liquid behavior in the kagome layers of DQVOF (diammonium quinuclidinium vanadium(III,IV) oxyfluoride; [NH4]2[C7H14N][V7O6F18]).29,30 The difference between V4+−O bond distances of 1.58 Å and V4+−F distances of 1.96−2.15 Å in DQVOF illustrates the covalent shortening effect. In contrast, M−O and M−N bonds of high valent metals tend to be of similar length, for example, the ranges of Ta−O and Ta−N distances in TaON are respectively 1.99−2.15 and 2.07−2.15 Å.31 Although oxide and nitride are ordered in this material, crystallographic disorder between the anion sites is observed in many oxynitrides as described later.

Figure 1. (a) Unit cell of the pseudocubic phase of SrMO2N (M = Nb, Ta) perovskites. The true symmetry is tetragonal P4/mmm. Sr and M atoms are at the cell center and corners. The anion site in the vertical can direction is fully occupied by O, and the two sites in the perpendicular plane have a 50:50 O/N mixture. (b) Local cis-geometry of the coordination around M. (c) cis-MN chains arising from the local anion order in SrMO2N, with heavy/light lines representing M−N− M/M−O−M connections. 4624



Review

(Figure 1b) disordered four ways around the can axis. M(dπ)− X(pπ) covalency favors the cis-configuration of the two more strongly bonded ligands in such high valent d0 transition metal compounds; this geometry is well-known, for example, in MoO22+ complexes. The cis-coordination of each M cation by two nitrides and the linear coordination of each nitride by two M cations result in the formation of zigzag MN chains within two-dimensional layers of the SrMO2N perovskites. The chains are disordered within the layers as shown in Figure 1c, resulting in the average anion distribution of Figure 1a observed in the neutron experiment. Following the same covalent arguments, AMO1.5N1.5 perovskites are expected to have fac-MO3N3 octahedra in which all three N’s (and hence all three O’s) are mutually perpendicular, so the introduction of another N leads to cis-MO2N4 octahedra in AMON2 perovskites.35 Hence, a “chemical symmetry” between MN chain order in AMO2N and MO chain order in AMON2 perovskites is expected. An important aspect of the crystal structures of AMX3 perovskites is their tendency to form superstructures of longrange ordered tilts or rotations of the MX6 octahedra. A classification of the possible tilt systems was proposed by Glazer,36 and further descriptions of the structures and their phase transition relations have been reported.37,38 If the above anion order directs the octahedral tilting, then nonstatistical occupancies of the inequivalent sites created by the tilting are expected. This is illustrated for the room temperature structures of SrMO2N (M = Nb, Ta) which appear to have a common rotational perovskite superstructure with tetragonal I4/mcm space group symmetry. If the can unique axis for anion order in the high temperature P4/mmm structure is parallel to that for the octahedral roations, crot, then the same anion occupancies are expected for the rotationally inequivalent sites Y1 and Y2, as shown in Table 2 and Figure 2, and the I4/mcm symmetry is

Figure 2. Upper figure shows octahedral rotations around the unique crot axis in the pseudo-I4/mcm room temperature superstructure of the SrMO2N (M = Nb, Ta) perovskites, resulting in inequivalent anion sites Y1 and Y2. The two possible orientations of the anion order axis can within the simple perovskite cell with respect to crot are shown below.

anion order is expected to lower space group symmetry in these materials to monoclinic P21/m, and a partial refinement of NdVO2N in this space group gave a monoclinic angle of 90.07°. LaNbON2 demonstrates the chemical symmetry between the correlated orders in AMO2N and AMON2 materials  here the O/N ratio at the 2-fold anion site in the Pnma description is 25:75, and the local order is represented by Figure 1c with heavy lines representing cis-NbO chains within a perovskite nitride matrix. Electron diffraction has corroborated the above predictions of space group symmetry breaking due to anion order, as weak diffraction spots arising from loss of the glideplanes are consistently observed in the pseudo-I4/mcm and Pnma materials.35,43 Reported neutron and electron diffraction results show the above order of anions into planes of disordered cis-chains in many perovskite materials: AMO2N (A = Eu, Sr; M = Nb, Ta) and EuWON2;35 CaTaO2N,41 LaNbON2,42 and NdVO2N.43 Cubic BaTaO2N is perhaps the only well-characterized case in which two-dimensional chain order is not observed, although a neutron pair distribution function study concluded that ciscoordination of N atoms around Ta was present.44 This suggests that the cis-chains propagate in all three dimensions in BaTaO2N, so the average crystal symmetry remains cubic and all anion sites have statistical occupancies. The expected symmetry changes arising when anion cis-chain propagation changes from three to two dimensions is shown for several perovskite tilting superstructures in Table 3. For the pseudo-R3̅c, Imma, and Pnma superstructures of AMO2N, crystal order of distinct O and O0.5N0.5 sites arising from twodimensional chains breaks the equivalence between two

Table 2. Expected Anion Occupancies in SrMO2N (M = Nb, Ta) for the Two Orientational Relationships between the Anion Ordering and Rotational Ordering Axes Shown in Figure 2 and the Resulting Space Group Symmetries orientation

can∥crot

can⊥crot

Y1 O/N ratio Y2 O/N ratio space group

100:0 50:50 I4/mcm

50:50 75:25 Fmmm

preserved. In the alternative possibility with can perpendicular to crot, the occupancies take different values as Y2 averages over 100:0 and 50:50 O/N sites, and so takes an average 75:25 ratio. The different occupancies of these component anion sites also show that the 4-fold rotational symmetry is broken, and the space group symmetry is lowered with a splitting of the Y2 site, as discussed later. Refinements of the apparent I4/mcm room temperature superstructures of SrNbO2N and SrTaO2N against high resolution powder neutron diffraction data give the occupancies predicted for the can perpendicular to crot model in Table 2 for samples prepared under a variety of conditions.35,39,40 Occupancies close to the same characteristic 50:50 and 75:25 ratios at anion sites with respective 1- and 2-fold multiplicities have also been reported in CaTaO2N,41 LaNbON2,42 and NdVO 2 N,43 which have apparent orthorhombic Pnma symmetry. These observations show that the unique axis for two-dimensional anion order is again perpendicular to the unique (doubled) axis of the perovskite tilt superstructure. The 4625



Review

Table 3. Tilt Systems in Glazer Notation and Space Groups of Some Common Perovskite Superstructures, Showing How Confinement of cis-Anion Chains to Two-Dimensional Planes Lowers Symmetry for AMO2N or AMON2 Oxynitridesa three-dimensional cis-chains

two-dimensional cis-chains

tilt system

space group

tilt system

space group

0 0 0

Pm3̅m I4/mcm P4/mbm R3̅c Imma Pnma

aac a0b0c‑ a0b0c+ a−b−b− a0b−c− a+b−c−

P4/mmm Fmmm Cmmm C2/c C2/m P21/m

aaa a0a0c− a0a0c+ a−a−a− a0b−b− a+b−b−

0 0 0

a

Chains propagating in three dimensions give the same average symmetries as statistically disordered models and simple AMX3 perovskites. When chains are confined to two-dimensional layers, it is assumed that the unique axis of a tetragonal or orthorhombic tilt supercell is perpendicular to the unique axis for anion order, in keeping with known oxynitride structures. The three superstructures shown in bold type do not arise in the standard classification of AMX3 perovskites.

nonzero rotations, so the resulting structure has the same symmetry as a more highly tilted perovskite, e.g., anion order changes the GdFeO3-type Pnma superstructure, with a+b−b− tilts in Glazer notation, to P21/m symmetry where the out-ofphase tilts (denoted by the negative superscript) are inequivalent as a+b−c−. However, the superstructures resulting from two-dimensional chain confinement in some high symmetry perovskite tilt systems, including the two phases of SrMO2N (M = Nb, Ta), do not appear in the standard scheme.38 This is because the possibility of inequivalent rotations of zero magnitude does not arise for simple AMX3 perovskites but is needed to describe the distinct O and O0.5N0.5 sites in SrMO2N oxynitrides using Glazer notation. There are 15 standard tilt systems for simple AMX3 perovskites, of which three have degenerate zero rotations and so give rise to further superstructure symmetries when inequivalent zero rotations due to the above two-dimensional anion order are included. These are shown in bold type in Table 3. For example, the simplest two-dimensional anion ordered perovskite shown in Figure 1a is written a0a0c0 in Glazer notation to show that one zero-tilt anion site is inequivalent to the other two. Similarly, anion order lowers the symmetry of the SrMO2N (M = Nb, Ta) room temperature superstructure from apparent I4/mcm (a0a0c− tilts) to orthorhombic Fmmm with a0b0c− tilts in this extended notation, as shown in Figure 2. This superstructure was described in a lower symmetry monoclinic group I112/m in ref 35. Extending the general analysis of AMX3 perovskite tilting to include anion ordered AMX2X′ and AMXX′X″ materials gives a total of 19 distinct structures, as shown in Figure 3. In this description, the octahedra around M are centrosymmetric so the AMO2N materials with two-dimensional cis-chains fit the AMX2X′ tilting description with X = O0.5N0.5 and X′ = O (but not with X = O and X′ = N). The structures are classified by the number of equivalent trans-X−M−X units in each octahedron, Neq. Equivalence is lost through octahedral rotation or anion order. The aristotype structures for AMX3, AMX2X′, and AMXX′X″ perovskites are shown at the top of each Neq column in Figure 3. AMX3 perovskites adopt all of the superstructures except those shown with broken borders which

Figure 3. The 19 possible space groups for AMX3, AMX2X′, and AMXX′X″ perovskites that arise from combinations of anion order and octahedral rotations, adapted from ref 38. Structures are grouped by the number of equivalent X−M−X units in their octahedral, Neq. Tilts are described using the extended Glazer notation. Groupsubgroup relations are shown, with broken lines when the transition must be first order.

have inequivalent zero tilts, corresponding to the standard 15 tilt systems. AMX2X′ perovskites can have all Neq ≤ 2 tilts except for the a0b0c0 Pmmm structure with three inequivalent nonzero tilts and so also have 15 possible tilt systems. Only three of these (a0a0c0, a0b0c−, and a+b−c−) are verifed so far for oxynitrides. AMXX′X″ perovskites with three anion ordered sites are unknown at present. Their aristotype structure is orthorhombic a0b0c0 Pmmm, and a total of eight Neq = 1 structures is predicted. The supercells of the four additional structures, relative to the cubic Pm3̅m perovskite cell, are 1 × 1 × 1 for P4/mmm and Pmmm, 2 × 2 × 1 for Cmmm and 2 × 2 × 2 for Fmmm. The spontaneous segregation of the cis-chains into twodimensional planes in most oxynitride perovskites is surprising as the associated strains that result from differences between M−O and M−N bond lengths are very small due to covalency, as noted in the Ordering Principles section. For example, ideal Ta−O and Ta−N distances based on ionic radii25 are 2.04 and 2.12 Å, so the expected c/a lattice parameter ratio for the P4/ mmm structure of SrTaO2N shown in Figure 1a is 0.98, but the observed c/a = 0.9995 value is much closer to unity. Electronic factors are likely to be important, as band structure calculations showed that models of ATaO2N perovskites with planes of long-range ordered cis-chains (Figure 4a) minimize energy.45 The cis-TaN2 units have strong π-bonding, so their tendency to be coplanar may be rationalized as an effect of conjugation, like that of planar conjugated organic π-systems. The local anion order in AMO2N and AMON2 perovskites leads to loss of inversion symmetry at each M cation site although the average crystal structures are centrosymmetric. This may have a strong influence on the materials properties, 4626



Review

and previous studies showed that mixed A-cations have an ideal disorder where structure and properties correlate with the statistical properties of the cation size distribution.48 Simple long-range orders arise when cation sizes and charges are sufficiently different, for example, in double perovskites YBaMn2O6 with layered A-cations and the CMR material Sr2FeMoO6 with rocksalt-type B cation order. In contrast, oxide and nitride anions have a well-defined local order, although this does not develop into an ideal long-range order. (ii). Oxynitride Phosphors. Oxide/nitride order in several silicon crystalline silicon oxynitrides has been studied by neutron diffraction. Full anion order is observed in some materials such as the phosphor host material BaSi2O2N2,49 others such as β-sialons are fully anion disordered,50 and partial order is reported in a few cases such as lavenite type La4Si2O7N2.51 Optical properties such as photoluminescence are sensitive to the local environment around the emitting (activator) ion and hence to any local anion clustering in oxynitride phosphors. Direct structural evidence for local oxide/nitride clustering has not been reported, but a recent doping investigation of an important nitride phosphor reveals that such effects may be important.52 The M2Si5N8 nitrides (M = Ca, Sr, Ba) doped with Eu2+ are widely used as red phosphors to provide warm white-LED devices.17,53 Unusual variations in luminescent properties were discovered in a study of M1.95Eu0.05Si5−xAlxN8−xOx solid solutions, where x quantifies the charge-compensated substitution of AlO+ for SiN+, as summarized in Table 4. The

Figure 4. Models for the planes of cis-anion chains in AMO2N oxynitride perovskites where heavy/light lines correspond to M−N− M/M−O−M connections and the arrows show local electric dipoles. Panel (a) shows long-range ordered chains and panel (b) shows a disordered configuration. In both cases, the net dipole for a large region is zero.

for example, by increasing the intensities of optical transitions in pigments or luminescent oxynitrides.7 The anion order model also accounts for observations that ATaO2N (A = Ca, Sr, Ba) oxynitrides have high dielectric constants but do not show ferroelectric ordering transitions, unlike pure oxide analogues such as BaTiO3.5 Each cis-TaN2 unit has a local dipole associated with the difference between oxide and nitride charges, and the expected off-center shift of Ta. Linking of cisTaN2 units into chains results in overall cancellation of the local dipoles as shown in Figure 4. If the cis-chains are ordered as in Figure 4a, which has not yet been observed in any oxynitride perovskite, then all dipoles are parallel to one line, but if the chains are fully randomized within the plane then equal numbers of dipoles are parallel to two perpendicular lines, as in Figure 4b. The difference in population of dipoles parallel to the two directions is an order parameter for the possible order− disorder transition between the two structures. A fundamental property of interest in the correlated anion order or AMO2N and AMON2 perovskites is their configurational entropy. Disordered models like that of Figure 4b correspond to a cis variant of the square-ice lattice,46 where M− N/M−O bonds map on to short/long O−H bonds in ice, and only structures in which the two short O−H bonds are cis to one another are allowed. An exact calculation shows that a crystal containing N M cations has configurational entropy S = 2N2/3kB ln 2 where kB is Boltzmann’s constant.47 This unusual type is entropy is “subextensive” as the exponent of the number of atoms N is less than 1. Hence, entropy does not show the usual proportionality with the amount of sample. Rewriting the expression as Sm = (2R ln 2)/N1/3 shows that the molar configurational entropy Sm is strongly dependent on the particle size. For example, a one mole of single crystal of SrTaO2N has Sm ≈ 10−7R, which is effectively zero, but a powder of 40 nm nanoparticles which would each consist of N ≈ 106 formula units has a measurable entropy Sm ≈ 0.1R. Experimental demonstration of subextensive entropies in oxynitrides is a future challenge. The subextensive oxynitrides were also classified as showing an “open order”,47 related to the correlated order of displacements in ferroelectric perovskites such as BaTiO3. The above results show that anion order in oxynitrides provides new types of structural correlations that are distinct from the many other ordering phenomena in perovskites. Cation and anion orders appear quite different. Cations with similar sizes and charges tend to be disordered at A or B sites,

Table 4. Reported Changes in the Peak Emission Wavelength λ and the Activation Energy for Thermal Quenching of Photoluminescence Ea in M1.95Eu0.05Si5−xAlxN8−xOx Phosphors as x Increases from 0 to 1, from ref 52a M

Ca

Sr

Ba

Δr/Å Δλ (x = 0 → 1)/nm ΔEa (x = 0 → 1)/meV

−0.13 +440 −210

+0.01 −100 −30

+0.17 −780 +30

a

The cation size mismatch between the activator and host cations is calculated from standard ionic radii25 as Δr = r(8 M2+) − r(8Eu2+).

emission shifts to longer wavelengths (a red shift) with increasing AlO+ substitution, x, in the M = Ba and Sr series, but a pronounced blue shift to shorter wavelengths is observed in the Ca analogues. This indicates a significant cation sizemismatch effect, as the red shift is observed for M = Ba and Sr hosts larger than the emitting dopant Eu2+ ions, but for smaller Ca2+ cations this switches to a blue shift, and the wavelength shifts were found to have an approximately linear dependence on size mismatch. A strong size mismatch influence on thermal quenching of photoluminescence is also observed. The quenching activation energies decreases greatly from the ∼280 meV value at x = 0 in the M = Ca series, but changes little in the Sr and Ba materials. The correlation of photoluminescence changes with the size mismatch between host and activator cations suggested that some anion segregation also occurs, as the crystal field arising from the anion coordination around the Eu2+ activators is of prime importance. The host materials have framework structures, as shown in Figure 5 for Ba2Si5N8,54 with nitrides bonded to two or three silicon neighbors written as 2SiN or 3Si N. O/N order between these sites is important to optical 4627



Review

as many functional oxynitrides are based on high-valent cations such as Si4+ or Ta5+. Although neutron scattering provides good scattering contrast between oxygen and nitrogen, local and correlated orders are still difficult to establish experimentally, and combinations of diffraction, spectroscopy, and modeling methods will be needed to understand and design the anion orders within crystalline oxynitrides.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: j.p.attfi[email protected]. Notes

Figure 5. The framework structure of Ba2Si5N8 showing corner-linked SiN4 tetrahedra, with Ba2+ cations in channels.

The author declares no competing financial interest.

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ACKNOWLEDGMENTS I acknowledge collaborations with colleagues at the Universities of Edinburgh and St. Andrews, ICMAB Barcelona, NTU Taiwan, and elsewhere, who are shown as coauthors of the cited references. Support has been provided by EPSRC, STFC, and the Royal Society, UK.

properties as the M cation sites are coordinated mainly by the 2Si N anions. A powder neutron diffraction study of Ba1.95Eu0.05Si5−xAlxN8−xOx for x = 1 showed that oxide substitutes only at 2SiN sites, as expected from silicate chemistry where 3SiO coordination is not observed. This illustrates the greater covalency of N compared to O as in the “Pauling’s second rule” approach to anion distributions as lower valent oxide is less coordinated by high-valent silicon.34 A further local order of oxide and nitride across the 2SiN sites is evidenced by the size-mismatch variations of energy shift and thermal quenching. Ionic radii are an appropriate measure of anion size in considering their coordination to M2+ or Eu2+ cations here, and the 0.08 Å difference between anion radii shown in Table 1 is comparable to the cation size mismatches in Table 4. Hence local anion clustering reduces lattice strains that result from Eu-doping by equalizing the mean cation− anion distances. Eu2+ dopants are larger than the host cations in the Ca1.95Eu0.05Si5−xAlxN8−xOx series so clustering more small oxide anions around Eu2+ minimizes lattice strain, consistent with the blue shift and increased thermal quenching of photoluminescence as similar differences are observed when comparing nitride phosphors to oxide. Conversely, Eu2+ is smaller than the host cations in the Ba1.95Eu0.05Si5−xAlxN8−xOx series, so strain is minimized by coordinating nitride to Eu2+ while the oxide anions preferentially coordinate Ba2+. From changes in the photoluminescence wavelength, the local anion coordinations around Eu2+ in the x = 1 M1.95Eu0.05Si5−xAlxN8−xOx materials were estimated as O3.1N3.9 for M = Ca, although the average cation coordination is O1.25N5.75, and N9 (pure nitride coordination) for M = Ba, although the average is O1.5N7.5. The red shift observed in the Ba1.95Eu0.05Si5−xAlxN8−xOx series thus reflects only a lattice expanding effect from the AlO+ substitution, as Eu2+ dopants are not significantly coordinated by oxide anions. Overall, the study of M1.95Eu0.05Si5−xAlxN8−xOx phosphors indicates that substantial local anion clustering may occur in disordered oxynitride materials. Segregation resulting from both covalent bonding to silicon and interactions with nearby cations based on ionic size are evidenced, from the determination of the average crystal structure and optical probing of the local environment, respectively.

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REFERENCES

(1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269−271. (2) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286−8287. (3) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Commun. 2002, 1698−1699. (4) Highashi, M.; Abe, R.; Takata, T.; Domen, K. Chem. Mater. 2009, 21, 1543−1549. (5) Kim, Y.; Woodward, P. M.; Baba-Kishi, K. Z.; Tai, C. W. Chem. Mater. 2004, 16, 1267−1276. (6) Koutsaroff, I.; Higai, S.; Ando, A. Perovskite material with anioncontrolled dielectric properties, thin film capacitor device, and method for manufacturing the same. U.S. Patent Appl. US20130003254 A1, Jan 3, 2013. (7) Jansen, M.; Letschert, H. P. Nature 2000, 404, 980−982. (8) Li, Y. Q.; Delsing, A. C. A.; de With, G.; and Hintzen, H. T. Chem. Mater. 2005, 17, 3242−3248. (9) Logvinovich, D.; Aguiar, R.; Robert, R.; Trottmann, M.; Ebbinghaus, S. G.; Reller, A.; Weidenkaff, A. A. J. Solid. State Chem. 2007, 180, 2649−2654. (10) Jorge, A. B.; Oró-Solé, J.; Bea, A. M.; Mufti, N.; Palstra, T. T. M.; Rodgers, J. A.; Attfield, J. P.; Fuertes, A. J. Am. Chem. Soc. 2008, 130, 12572−12573. (11) Kusmartseva, A.; Yang, M.; Oró-Solé, J.; Bea, A. M.; Fuertes, A.; Attfield, J. P. Appl. Phys. Lett. 2009, 95, 022110. (12) Yang, M.; Oró-Solé, J.; Kusmartseva, A.; Fuertes, A.; Attfield, J. P. J. Am. Chem. Soc. 2010, 132, 4822−4829. (13) Elder, S. H.; Di Salvo, F. J.; Topor, L.; Navrotsky, A. Chem. Mater. 1993, 5, 1545−1553. (14) Soignard, E.; Machon, D.; McMillan, P. F.; Dong, J.; Xu, B.; Leinenweber, K. Chem. Mater. 2005, 17, 5465−5472. (15) Yang, M.; Rodgers, J. A.; Middler, L. C.; Oró-Solé, J.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Inorg. Chem. 2009, 48, 11498−11500. (16) Ebbinghaus, S. G.; Abicht, H. −P.; Dronskowski, R.; Müller, T.; Reller, A.; Weidenkaff, A. Prog. Solid State Chem. 2009, 37, 173−205. (17) Zeuner, M.; Pagano, S.; Schnick, W. Angew. Chem., Int. Ed. 2011, 50, 7754. (18) Fuertes, A. Dalton Trans. 2010, 39, 5942−5948. (19) Fuertes, A. J. Mat. Chem. 2012, 22, 3293−3299. (20) Collongues, R.; Gilles, J. C.; Lejus, A. M.; Perez, M.; Jorba, Y.; Michel, D. Mater. Res. Bull. 1967, 2, 837−848. (21) Jorge, A. B.; Fraxedas, J.; Cantarero, A.; Williams, A. J.; Rodgers, J.; Attfield, J. P.; Fuertes, A. Chem. Mater. 2008, 20, 1682−1684.

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CONCLUSIONS The above examples suggest that local segregation and correlated orders of oxide and nitride may be widespread, despite their apparent similarity when viewed as simple anions. Covalent bonding interactions play an important role, especially 4628



Review

(22) Yang, M.; Oró-Solé, J.; Fuertes, A.; Attfield, J. P. Chem. Mater. 2010, 22, 4132−4134. (23) Granar, G.; Rosseti, C.; Bailly, D. Mol. Phys. 1986, 58, 627−636. (24) Esposti, C. D.; Favero, P. G.; Serenellini, S.; Cazzoli, G. J. Mol. Struct. 1982, 82, 221−236. (25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751−767. (26) Marvel, M. R.; Lesage, J.; Baek, J.; Halasyamani, P. S.; Stern, C. L.; Poeppelmeier, K. R. J. Am. Chem. Soc. 2007, 129, 13963−13969. (27) Sorensen, E. M.; Izumi, H. K.; Vaughey, J. T.; Stern, C. L.; Poeppelmeier, K. R. J. Am. Chem. Soc. 2005, 127, 6347−6352. (28) Aldous, D. W.; Goff, R. J.; Attfield, J. P.; Lightfoot, P. Inorg. Chem. 2007, 46, 1277−1282. (29) Aidoudi, F. H.; Aldous, D. W.; Goff, R. J.; Slawin, A. M.; Attfield, J. P.; Morris, R. E.; Lightfoot, P. Nat. Chem. 2011, 3, 801− 806. (30) Clark, L.; Orain, J. C.; Bert, F.; De Vries, M. A.; Aidoudi, F. H.; Morris, R. E.; Lightfoot, P.; Lord, J. S.; Telling, M. T. F.; Bonville, P.; Attfield, J. P.; Mendels, P.; Harrison, A. Phys. Rev. Lett. 2013, 110, 207208. (31) Armytage, D.; Fender, B. E. F. Acta Crystallogr. B 1974, 30, 809−812. (32) Hübenthal, R. MAPLE, Programm zur Berechnung des Madelunganteils der Gitterenergie, Version 4; Universitat Giessen: Giessen, Hesse, Germany, 1993. (33) Henning, A. H.; Florian, S.; Oliver, O.; Wolfgang, S. Angew. Chem., Int. Ed. 2004, 43, 5540−5542. (34) Fuertes, A. Inorg. Chem. 2006, 45, 9640−9642. (35) Yang, M.; Oró-Solé, J.; Rodgers, J. A.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Nature Chem. 2011, 3, 47−52. (36) Glazer, A. M. Acta Crystallogr. 1972, B28, 3384−3392. (37) Woodward, P. M. Acta Crystallogr. 1997, B53, 32−43. (38) Howard, C. J.; Stokes, H. T. Acta Crystallogr. 1998, B54, 782− 789; 2002, B58, 565. (39) Clarke, S. J.; Guinot, B. P.; Michie, C. W.; Calmont, M. J. C.; Rosseinsky, M. J. Chem. Mater. 2002, 14, 288−294. (40) Zhang, Y. -R.; Motohashi, T.; Masubuchi, Y.; Kikkawa, S. J. Ceram. Soc. Japan 2011, 119, 581−586. (41) Gunther, E.; Hagenmayer, R.; Jansen, M. Z. Anorg. Allg. Chem. 2000, 626, 1519−1525. (42) Logvinovich, D.; Ebbinghaus, S. G.; Reller, A.; Marozau, I.; Ferri, D.; Weidenkaff, A. Z. Anorg. Allg. Chem. 2010, 636, 905−912. (43) Oro-Sole, J.; Clark, L.; Bonin, W.; Attfield, J. P.; Fuertes, A. Chem. Commun. 2013, 49, 2430−2432. (44) Page, K.; Stoltzfus, M. W.; Kim, Y. -I.; Proffen, Th.; Woodward, P. M.; Cheetham, A. K.; Seshadri, R. Chem. Mater. 2007, 19, 4037− 4042. (45) Wolff, H.; Dronskowski, R. J. Comput. Chem. 2008, 29, 2260− 2267. (46) Lieb, E. H. Phys. Rev. 1967, 162, 162−172. (47) Camp, P. J.; Fuertes, A.; Attfield, J. P. J. Am. Chem. Soc. 2012, 13, 6762−6766. (48) Attfield, J. P. Cryst. Eng. 2002, 5, 427−438. (49) Kechele, J. A.; Oeckler, O.; Stadler, F.; Schnick, W. Solid State Sci. 2009, 11, 537−543. (50) Gillott, L.; Cowlam, N.; Bacon, G. E. J. Mater. Sci. 1981, 16, 2263−2268. (51) Takahashi, J.; Yamane, H.; Hirosaki, N.; Yamamoto, Y.; Suehiro, T.; Kamiyama, T.; Shimada, M. Chem. Mater. 2003, 15, 1099−1104. (52) Chen, W. -T.; Sheu, H. S.; Liu, R. S.; Attfield, J. P. J. Am. Chem. Soc. 2012, 134, 8022−8025. (53) Höppe, H. A. Angew. Chem., Int. Ed. 2009, 48, 3572−3582. (54) Schlieper, T.; Milius, W.; Schnick, W. Z. Anorg. Allg. Chem. 1995, 621, 1380−1384.

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Principles and Applications of Anion Order in Solid Oxynitrides

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