Thermally Robust Anion-Chain Order in Oxynitride Perovskites

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Thermally Robust Anion-Chain Order in Oxynitride Perovskites Lucy Clark,†,∥ Judith Oró-Solé,‡ Kevin S. Knight,§ Amparo Fuertes,‡,* and J. Paul Attfield†,* †

CSEC and School of Chemistry, University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh, EH9 3JZ, United Kingdom Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193 Bellaterra, Spain § ISIS Facility, Rutherford Appleton Laboratory, Didcot, OX11 0QX, United Kingdom ‡

S Supporting Information *

ABSTRACT: The presence and thermal stability of anion order in the oxynitride perovskites SrTaO2N and LaTaON2 have been determined using high resolution powder neutron and electron diffraction data. Partial order of oxide and nitride anions consistent with the formation of planes of disordered cis-anion chains is observed in both materials, with a chemical symmetry between distributions in SrTaO2N and LaTaON2. No loss of anion order is observed up to 1100 °C and extrapolations based on lattice strains show the order to be stable to remarkably high temperatures >2000 °C, demonstrating that anions are segregated when the materials are synthesized. SrTaO2N has an apparent tetragonal I4/mcm superstructure at room temperature due to ordered octahedral tilts, but anion order lowers symmetry to an orthorhombic Fmmm supercell (with lattice parameters a = 8.0657(8), b = 8.0614(7), and c = 8.0775(4) Å). Anion order also lowers the symmetry of LaTaON2 from apparent orthorhombic Imma to monoclinic I2/m (a = 5.7140(6), b = 8.0595(6), c = 5.7506(5) Å, and β = 90.239(4)° at 20 °C) and this superstructure persists up to 1100 °C with an extrapolated loss of tilting at 1540 °C. Anion order appears to direct octahedral tilting such that the more rigid Ta−N−Ta bridges retain bond angles closer to 180° than the Ta− O−Ta connections in these superstructures. KEYWORDS: perovskites, oxynitrides, anion order, powder neutron diffraction

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INTRODUCTION Transition metal oxynitrides are an important group of mixed anion materials and are of interest due to their electronic, optical, and magnetic properties.1 Many of these are perovskites with ideal compositions AMO2N or AMON2 based on highvalent d0 cations such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+.2,3 LaTaON2−CaTaO2N solid solutions are nontoxic alternatives to cadmium-based red-yellow pigments,4 BaTaO2N is photocatalytically active for water splitting5 and both SrTaO2N and BaTaO2N have high dielectric constants.6 Doped materials show notable electronic transport properties such as colossal magnetoresistances (CMR) at low temperatures in EuNbO2N7 and EuWON2 8,9 and a high thermoelectric (Seebeck) coefficient for SrMoO 2 N. 10 The latter material and NdVO2N11 are examples of d1−cation oxynitride perovskites. The issue of whether and how anions in AMO2N and AMON2 perovskites are ordered has been controversial.2,3,12,13 In contrast with layered K2NiF4-type perovskites,14 anion order in pseudocubic AMO2N or AMON2 perovskites cannot be rationalized according to Pauling’s second crystal rule because all the anion sites show the same bond strength sums.15 An experimental difficulty is that the similarity of M−O and M−N bond distances results in very small lattice distortions from any anion order so that highly resolved powder neutron data are required to extract reliable site occupancies, despite the high © 2013 American Chemical Society

contrast between the scattering lengths of oxygen (5.83 fm) and nitrogen (9.36 fm). Evidence for partial order of nitrides as layers of disordered cis-MN chains (Figure 1a) was recently reported from a study of SrMO2N samples (M = Nb, Ta).16 Above a phase transition at 200−300 °C, these materials appeared to adopt the ideal cubic AMX3 perovskite structure (space group Pm3̅m), implying that anions are statistically disordered. However, neutron refinements showed that a very small tetragonal distortion lowers symmetry to P4/mmm, which allows for anion order over two inequivalent sites. Refinement of the P4/mmm model for SrNbO2N and SrTaO2N revealed full oxygen occupancy at the X1 anion site on one axis and a 50:50 O:N distribution at the other two X2 sites, as shown in Figure 1b. This average anion distribution evidences the presence of cisMO4N2 octahedra at all M sites, driven by the greater covalency of M−N bonds in comparison to M−O. The combination of 90° coordination of each M by two nitride anions and the linear coordination of each nitride anion by two M cations leads to the formation of cis-MN chains and rings. These were found to Received: November 8, 2013 Revised: December 6, 2013 Published: December 9, 2013 5004

dx.doi.org/10.1021/cm4037132 | Chem. Mater. 2013, 25, 5004−5011

Chemistry of Materials

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giving detailed structural insights into the links between Ta− O/N covalency, anion order, and octahedral tilting.

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EXPERIMENTAL SECTION High quality, phase pure, polycrystalline samples of the oxynitride perovskites SrTaO2N and LaTaON2 were prepared at high temperatures using flowing ammonia gas. LaTaON2 was prepared by treating LaTaO4 at 950 °C and a flow rate of 300 cm3/min during two 40 h cycles with intermediate regrinding. The LaTaO4 precursor was prepared from a stoichiometric mixture of La2O3 and Ta2O5 that was ground, pelletized, and heated in air at 1000 °C during 12 h followed by two 15 h treatments at 1500 °C with intermediate regrinding. SrTaO2N was prepared by treating stoichiometric mixtures of SrCO3 and Ta2O5 at 1000 °C under NH3 flow of 300 cm3/min for three 40 h cycles, with intermediate regrinding and repelletizing. Sample purities were confirmed by powder X-ray diffraction and nitrogen contents determined from elemental analyses using a Thermo Fisher Scientific Instrument were 1.03(2) for SrTaO2N and 1.97(2) for LaTaON2. Electron diffraction patterns from individual microcrystallites of LaTaON2 were obtained at room temperature using a JEOL 1210 transmission electron microscope operating at 120 kV equipped with a side-entry 60/30° double tilt GATHAN 646 specimen holder. The sample was prepared by dispersing the powder in ethanol and depositing a droplet of this suspension on a carbon-coated holey film supported on a copper grid. Neutron powder diffraction data were collected on the High Resolution Powder Diffractometer (HRPD) at the ISIS spallation neutron source, Rutherford Appleton Laboratory, U.K. Approximately 1 g of each sample was vacuum sealed in a quartz tube that was covered with a thin layer of vanadium foil. A high temperature vanadium element furnace was used to access temperatures up to 1100 °C. Powder diffraction data were recorded for SrTaO2N at 20, 500, 700, 900, and 1100 °C, and for LaTaON2 at 20, 150, 350, 600, 850, and 1100 °C, with collection times of 3 h per temperature. The powder diffraction profiles were analyzed by Rietveld refinement using the GSAS program.24 Structural models were refined simultaneously against data collected from backscattering and 90° detector banks, which provide the d-spacing ranges 0.6−2.6 Å and 0.9− 3.8 Å, respectively.

Figure 1. (a) cis-MN chains, depicted by the heavy lines, that propagate in two-dimensional layers in AMO2N perovskites as a result of local anion ordering within cis-MO4N2 octahedra. The lighter lines show the perovskite cells, in the same orientation as in panel b. (b) The high temperature average tetragonal AMO2N unit cell with A and M cations at the cell center and corners, and O (X1) and average O0.5N0.5 (X2) anion sites respectively shown as full and half-shaded atoms. O and N positions are reversed in AMON2 perovskites.

spontaneously segregate into two-dimensional (2D) planes within the perovskite structure, as shown in Figure 1a. Although the P4/mmm SrMO2N structure appears substantially disordered on an average, crystallographic length scale, it is highly constrained and ordered at a local level. As a result, such oxynitride perovskites are predicted to show unusual entropic behavior, with subextensive entropies that tend toward zero in a macroscopic sample.17 A chemical symmetry between the anion orders of AMO2N and AMON2 perovskites was predicted,16 such that planes of cis-MO chains in a perovskite nitride matrix are expected for AMON2, reversing positions of O and N atoms in Figure 1. A small loss of anion order in SrTaO2N was observed at the highest studied temperature of 750 °C in the previous investigation.16 This was interpreted as evidence of a 2D−3D crossover as the cis-chains become deconfined from the planes and propagate throughout the structure at high temperatures. Recent first-principles calculations have also favored 3D chain motifs.18 The room temperature structures of SrNbO 2N and SrTaO2N, in common with many AMX3 perovskites, have superstructures arising from ordered tilts of the MX6 octahedra. These were first described by Glazer who introduced a simple notation for their classification,19 and the nature of and relationships between the tilt systems have been further analyzed by several authors.20−22 There are 15 unique tilt systems for simple AMX3 perovskites where all anions are chemically equivalent. Coupling of the anion chain order to the tilting of the MO4N2 octahedra in the room temperature structures of SrNbO 2 N and SrTaO 2 N was evidenced previously,16 and subsequent analyses have shown that the average anion order in Figure 1b can introduce three further tilt symmetries that are not found for simple AMX3 perovskites,12,13 as will be elaborated later. To test the prediction of chemical symmetry between AMO2N and AMON2 anion orders and to investigate 2D−3D crossover of anion chains, we report here the results of high resolution powder neutron diffraction studies of SrTaO2N and LaTaON213,23 at temperatures up to 1100 °C. These are supported by room temperature electron diffraction patterns for the latter material. The neutron data have enabled the first full refinements of coordinates in the tilted and anion-ordered superstructures of SrTaO2N and LaTaON2 to be carried out,

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RESULTS (a). SrTaO2N. (i). Room Temperature Structure. At room temperature SrTaO2N has a rotationally ordered perovskite superstructure that is common in AMX3 perovskites.22 This is described as a0a0c− in Glazer notation to show that the MX6 octahedra undergo equivalent rotations of zero magnitude around the cubic perovskite a and b axes and a finite out-ofphase rotation around the c-axis. This gives rise to a √2ap × √2ap × 2ap supercell, where ap ≈ 4 Å is the simple cubic perovskite lattice parameter, with tetragonal I4/mcm space group symmetry. Rotation of the octahedra around the unique c-axis creates two anion sites, one (Y1) on the c-axis and the other (Y2) in the ab-plane, in a respective 1:2 ratio. (‘Y’ labels are used throughout in this paper for anion sites that are inequivalent due to octahedral rotations) Rietveld refinement of an I4/mcm model against room temperature neutron data for SrMO2N (M = Nb, Ta) in our previous study16 showed that the unique axis for average anion order (the vertical axis in Figure 1b) is perpendicular to the 5005



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unique c-axis for rotational order, as the Y1 anion site on the rotational c-axis had near 50:50 O:N occupancies while the Y2 site in the ab-plane refined close to 0.75O:0.25N, consistent with averaging over sites with 50:50 and 100:0 O:N occupations. The two equatorial sites in the rotationally ordered room temperature structure are therefore inequivalent, which breaks the c-glide symmetry operation, as confirmed by electron diffraction. This was originally reported as lowering the symmetry of the I4/mcm supercell to I112/m;16 however, subsequent analyses have shown that the structure can be expressed in a larger 2ap × 2ap × 2ap supercell with orthorhombic Fmmm space group symmetry.12,13 The average anion order introduces a change from a0a0c− to a0b0c− tilting in Glazer notation. This allows for the chemical inequivalence between O and O0.5 N0.5 anion sites in the ab-plane. Inequivalent tilts of zero magnitude are not possible for AMX3 perovskites where all anions are chemically identical so the a0b0c− Fmmm superstructure does not appear in the list of 15 standard perovskite tilt systems.22 A fit of the tetragonal I4/mcm model to the 20 °C HRPD data for SrTaO2N gave O:N occupancies of 0.45(3):0.55 and 0.78:0.22 for the Y1 and Y2 sites respectively, in agreement with several previous neutron refinements.16,25,26 The reduced goodness of fit parameter was χ2 = 4.40 and the overall weighted-profile R-factor was Rwp = 2.83%, for refinement against backscattering and 90° bank spectra. Following the above symmetry analysis, the orthorhombic Fmmm model was also refined to allow for the average anion order. This lowers the residuals to χ2 = 4.25 and Rwp = 2.78% and the fitted profiles are shown in Figure 2. All atomic coordinates were refined as allowed by the space group symmetry. Isotropic thermal parameters were refined for each atom, but those derived from the ab-plane Y2 site in the tetragonal model (Y21 and Y22) were constrained to be equal. O:N distributions at the three anion sites were refined within the constraint of the ideal chemical composition. The results of the refinement are summarized in Table 1. One of the ab-plane anion sites (Y21) has O:N occupancies of 0.95:0.05 while the other (Y22) has 0.46:0.54, confirming the ab-plane anion order. The Fmmm rotationally and anion ordered superstructure of SrTaO2N is displayed in Figure 3, and bond distances and angles are shown in Table 2. (ii). High Temperature Study. SrTaO2N undergoes a structural phase transition to a pseudocubic structure between room temperature and 200 °C, so the high temperature HRPD powder neutron data sets collected at 500−1100 °C were all analyzed using the P4/mmm model discussed in the introduction and shown in Figure 1b. All atom coordinates are fixed by symmetry in this model. The wide d-range of data enabled isotropic thermal parameters for all atoms to be refined independently in addition to anion occupancies at the two sites, subject to the constraint of the ideal composition. Good fits were obtained at each temperatureFigure 4 shows the 1100 °C fit. Results at each temperature are tabulated in Supporting Information, and the thermal variations of isotropic temperature factors and occupancies are plotted in Figure 5. Correlation between the temperature factors and site occupancies in the refinement results in slightly unphysical values of the oxygen occupancy above 100%, however, these remain within 3σ of full occupancy at all temperatures, where σ is the estimated standard deviation (e.s.d.). Hence, there is no evidence for anion disorder consistent with 2D−3D crossover of chains in SrTaO2N up to 1100 °C, showing that the two-

Figure 2. Room temperature SrTaO2N neutron diffraction data collected on the (a) backscattering and (b) 90° detector banks of HRPD. Lower and upper reflection markers are respectively for orthorhombic Fmmm SrTaO2N and vanadium from the sample environment.

Table 1. Refined Parameters for the Orthorhombic Fmmm SrTaO2N Model at 20 °Ca atom

coordinates x, y, z

Sr Ta Y1 Y21 Y22

0, 0, 0.2480(33) 1/4, 1/4, 0 1/4, 1/4, 1/4 0.2750(13), 0, 0 0, 0.2387(17), 0

occupancy (O/N)

Uiso/Å2

0.59(8)/0.41 0.95(3)/0.05 0.46/0.54

0.0028(40) 0.0014(33) 0.0120(30) 0.0065(14) 0.0065

a

Lattice parameters are a = 8.0657(8), b = 8.0614(7) Å, c = 8.0775(4) Å. Estimated standard deviations for independent variables are shown in parentheses.

dimensional confinement of cis-chains shown in Figure 1a is robust at high temperatures. The unit cell volume of SrTaO2N increases linearly with temperature in the studied range and no anomaly is apparent, as shown in Figure 6. A 2D−3D crossover of the anion chains at higher temperatures would result in a transition from tetragonal P4/mmm to cubic Pm3̅m symmetry. The spontaneous strain eac = 2(a − c)/(a + c) thus falls to zero at the transition temperature Tc for the 2D−3D crossover. Assuming that eac follows a variation with (1 − T/Tc)1/2 for temperatures T/K, as described later for LaTaON2, gives a crude estimate of 2100 ± 600 °C for the transition temperature by fitting the eac points in Figure 6. (b). LaTaON2. (i). Room Temperature Structure. In agreement with a recent report,13 we found that the room temperature powder neutron diffraction profiles from La5006



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Figure 5. Thermal variations of fractional oxygen occupancies for the two anion sites (labeled X1 and X2) and isotropic thermal parameters for the P4/mmm structure of SrTaO2N. The dashed black lines show ideal oxygen occupancies for the cis-chains anion order model in Figure 1. Figure 3. Room temperature orthorhombic Fmmm crystal structure of SrTaO2N, with atoms shaded as in Figure 1b. The Fmmm axes and cell are shown as solid lines, and the tetragonal I4/mcm cell, which averages over the ab-plane anion order, is shown by the dashed lines.

Table 2. Selected Bond Lengths (Å) and Angles (deg) from the Room Temperature Fmmm SrTaO2N and I2/m LaTaON2 Structures Ta−Y1 × 2 Ta−Y21 × 2 Ta−Y22 × 2 ⟨Ta−Y⟩ Y1−Ta−Y21 Y1−Ta−Y22 Y21−Ta−Y22 Ta− Y1−Ta Ta−Y21−Ta Ta−Y22−Ta

SrTaO2N

LaTaON2

2.019(1) 2.025(1) 2.019(1) 2.022(1) 90 90 93.1(6) 180 168.6(6) 174.8(8)

2.058(1) 2.039(1) 2.051(1) 2.049(1) 92.2(2) 90.7(2) 91.4(2) 156.4(1) 165.5(3) 164.0(5)

Figure 6. Temperature variations of the spontaneous strain parameter eac derived from the refined tetragonal lattice parameters a and c, and the cell volume of the P4/mmm phase of SrTaO2N. The solid line shows a tricritical fit to eac for a transition at ∼2100 °C.

similar manner to the I4/mcm model described above, the Imma superstructure has two distinct anion sites, Y1 close to the b-axis and Y2 near the ac-plane, in a 1:2 ratio. Initial refinements showed that the La, Ta, and Y1 sites had small thermal parameters but Y2 had a large value indicating some local disorder, so isotropic thermal parameters of the La, Ta, and Y1 sites were subsequently constrained to be equal while that for Y2 was refined independently to reduce correlations with occupancy factors. The Imma fit is shown in Figure 7a, and refinement results are given in Supporting Information. The O:N occupancies from the Imma fit for LaTaON2 were 0.44:0.56 for Y1 and 0.27:0.73 for Y2 sites. These are analogous to the occupancies for the I4/mcm SrTaO2N model, and show that the anion order of Figure 1 is present but with O and N positions reversed through chemical symmetry between AMO2N and AMON2 structures. The unique tilt superstructure direction (the Imma b-axis) is again perpendicular to the anion order axis. Anion order breaks the equivalence of the two Y2 sites as they have N and O0.5N0.5 compositions. Hence, their octahedral rotations must be inequivalent so the Glazer tilt symmetry changes from a0b−b− to a0b−c− and the space group symmetry is lowered from orthorhombic Imma to monoclinic I2/m (a nonstandard setting of C2/m). This splits the Y2 site into inequivalent Y21 and Y22 positions. The I2/m super-

Figure 4. Rietveld plot of the refinement of the pseudocubic P4/mmm model to 1100 °C powder neutron diffraction data collected on the backscattering detector bank of HRPD for SrTaO2N. The ticks mark the reflection positions for the P4/mmm phase.

TaON2 are fitted well by an orthorhombic √2ap × 2ap × √2ap perovskite superstructure with space group Imma, which describes a0b−b− tilting of octahedra in Glazer notation. In a 5007



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Table 3. Results of the Refinement of the Monoclinic I2/m Model for LaTaON2 to Room Temperature HRPD Data with Refined Lattice Parameters a = 5.7140(6), b = 8.0595(6), c = 5.7506(5) Å, β = 90.239(4)°a atom

x

y

z

occupancy (O/N)

La Ta Y1 Y21 Y22

0.7469(20) 3/4 0.7394(13) 0.0 1/2

0.0 1/4 0.0 0.7818(7) 0.2148(12)

0.2525(8) 3/4 0.6775(4) 0.0 0.0

0.41(2)/0.59 0.0/1.0 0.59/0.41

Isotropic thermal parameters were fixed at zero for La, Ta, and Y1 sites and refined to 0.0140(4) Å2 for the Y21 and Y22 sites. a

comparison to 90.07° in NdVO2N for example,11 and visibly improves the fit to some diffraction peaks as shown in the insets to Figure 7. Figure 8 displays a view of the I2/m structure, and bond distances and angles are shown in Table 2.

Figure 7. Rietveld fits of (a) the orthorhombic Imma and (b) the monoclinic I2/m model to neutron diffraction data from the backscattering HRPD detector for LaTaON2 at room temperature. Lower and upper reflection markers are for LaTaON2 and vanadium from the sample environment. Insets show the intense peak at 2.02 Å where the improvement to the fit obtained with the monoclinic model is evident in panel b.

structure is one of the standard 15 tilt models for AMX3 perovskites as out-of-phase tilts of different magnitude can also arise in the absence of anion order. The starting model for I2/m refinement of LaTaON2 was obtained from a distortion mode analysis of the refined Imma structure using the ISODISTORT program.27 The atomic positions are equivalent to those shown in ref 21 via an origin shift. All variable coordinates were refined independently but isotropic thermal parameters for the La, Ta and Y1 sites were fixed at zero (as the combined variable otherwise refined to a slightly negative value), and values for Y21 and Y22 sites were constrained to be equal. Refinement of this model gave O/N occupancies of 0.42(2)/0.58, −0.06(3)/1.06, and 0.64(4)/0.36 at the Y1, Y21, and Y22 sites, respectively, in good agreement with the average ordering model of Figure 1b. However, correlation between occupancies and thermal parameters in the analysis of high temperature data described below led to unphysical occupancies in excess of 100% and so the Y21 occupancy was fixed at 100% N in all I2/m refinements. Parameters for the room temperature I2/m structure of LaTaON2 are shown in Table 3. The unit cell is equivalent to that used in a previous C2/m refinement but the atom positions are different.23 The I2/m model gave a significant improvement compared to the Imma fit, with residuals falling from χ2 = 3.99 and Rwp = 1.90% for Imma to χ2 = 3.56 and Rwp = 1.79% in I2/m. The monoclinic distortion due to anion order as represented by the β-angle of 90.24° is relatively large, in

Figure 8. Room temperature monoclinic I2/m structure of LaTaON2 with La in yellow, Ta in blue, N in gray, and O0.5N0.5 in red/gray.

The lowering of space group symmetry from Imma to I2/m as a result of anion order is also supported by electron diffraction patterns from individual crystallites for LaTaON2. The presence of very weak (hk0) spots for odd h and k values as shown in Figure 9 demonstrates that the a-glide symmetry is lost. (ii). High Temperature Study. No structural phase changes are apparent in the powder neutron diffraction patterns of LaTaON2 between 20 and 1100 °C, so all data were fitted using the above monoclinic I2/m model. Initial refinements in which the O/N occupancies at all three anion sites were varied gave N occupancies at the Y21 site increasing above 100% with temperature. This results from a slight correlation of anion occupancies with thermal parameters but also demonstrates that there is no 2D−3D crossover of the cis-anion chains between layers up to 1100 °C. Hence, the anion order is thermally robust to at least this temperature, as is that in SrTaO2N. The thermal evolutions of lattice parameters a, b/√2, c, and β, and the geometric average cell length 5008



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reflecting the small monoclinic distortion due to anion order is taken to quantify the 2D−3D cis-chains crossover, and pseudoImma strains29 e1 = (b/√2 − a0)/a0, e2 + e3 = (a + c − 2a0)/a0, and e4 = (a − c)/a0 describe the octahedral tilts. These strains are proportional to the square of the thermodynamic order parameters and so are expected to vary with (1 − T/Tc)n with n = 1 for a second order transition and n = 1/2 if the transition is tricritical.28 The curvature of the e1, e2 + e3, and e4 variations in Figure 10b is incompatible with second order behavior but good fits are obtained assuming a tricritical dependence with n = 1/2 as shown. The fits to e1, e2 + e3, and e4 approach zero at 1540 ± 70 °C, which is an estimate for the temperature at which all octahedral tilts become thermally randomized. eβ shows much less thermal variation, and the temperature dependence of this strain, and eac for SrTaO2N in Figure 6, is unclear. We have assumed tricritical variations in both casesif the transitions are second order then the projected 2D-3D transition temperatures are much higher. Even the tricritical fit for LaTaON2 gives the projected temperature at which the cisanion chains become fully randomized between layers as 3000 ± 200 °C, far above the synthesis temperature. This demonstrates that the segregation of chains into two-dimensional layers in LaTaON2 is extremely robust to high temperatures. AMX3 perovskites with the orthorhombic Imma tilt superstructure do not usually transform directly to the cubic Pm3̅m aristotype on heating, but instead undergo a first-order phase transition to an intermediate tetragonal I4/mcm (e.g., in SrSnO3)30 or rhombohedral R3̅c phase (e.g., in doped RMnO3).31 Hence, the I2/m (pseudo-Imma) structure of LaTaON2 observed up to 1100 °C may transform on heating to an intermediate Fmmm (pseudo-I4/mcm) or C2/c (pseudoR3c̅ ) phase that then changes to the basic P4/mmm (pseudoPm3̅m) anion ordered cell of Figure 1b at ∼1540 °C. Further high temperature experiments will be needed to provide experimental confirmation of any intermediate tilt phases.

Figure 9. [100] and [001] electron diffraction images from the same LaTaON2 crystallite with c* or a* vertical and b* horizontal. The observation of faint superstructure peaks (arrowed) shows that a-glide symmetry is broken, consistent with the change from Imma to I2/m due to anion order.

a0 = (V/√2)1/3, where V is cell volume, are plotted in Figure 10a. Further refinement results are shown in Supporting Information. Thermal changes in the octahedral tilts and in the 2D/3D order of the cis-anion chains may be examined by calculating spontaneous strains from the lattice parameters.28 It is reasonable to assume that the two orders are not coupled, as observed for SrTaO2N above, so the strain eβ = −cos β

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DISCUSSION SrTaO2N and LaTaON2 are representative stoichiometric perovskite oxynitrides that enable the effects of octahedral tilting and anion order to be investigated. They also provide a test of the proposed chemical symmetry between anion orders in AMO2N and AMON2 perovskites. The use of high resolution powder neutron diffraction has enabled lattice parameters, O/N site occupancies and all coordinates to be refined together as allowed by symmetry for both materials, unlike in our previous refinements where pseudosymmetry constraints were applied to atomic coordinates.11,16 Analysis of neutron diffraction patterns from room temperature up to 1100 °C allows the degree of anion ordering under synthesis temperatures to be determined. The refinements provide a strong confirmation of the local anion ordering model shown in Figure 1, with planes of disordered cis-chains, in both SrTaO2N and LaTaON2. The partial long-range order of oxide and nitride in LaTaON2 is equivalent to that in SrTaO2N, with exchange of O and N sites, confirming the chemical symmetry between these orders. This symmetry is driven by the stability of cis-TaN2 bonds arising from dπ−pπ covalency. This favors the formation of cis-TaO4N2 octahedra in SrTaO2N and also stabilizes the cis-configuration of TaO2N4 octahedra in LaTaON2, as the cis-configuration gives five cis-TaN2 pairs per octahedron whereas the transconfiguration has only four.

Figure 10. (a) Temperature variations of the monoclinic LaTaON2 lattice parameters. (b) The spontaneous strains showing tricritical fits as described in the text. The fits to pseudo-Imma octahedral tilting strains e1, e2 + e3, and e4 converge at a critical temperature of 1540 °C, whereas that for anion order strain eβ is ∼3000 °C. The e1 and e2 + e3 points and curves overlie each other although they are not identical. 5009



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However, oxide-nitride order lowers symmetry further as the unique axis for anion order is perpendicular to the unique (doubled) axis in both superstructures. The resulting a0b0c− tilting introduces a larger orthorhombic Fmmm superstructure for SrTaO2N and a monoclinic distortion to I2/m symmetry for a0b−c− LaTaON2. The same perpendicular relationship between anion-ordering and tilt-doubled axis was also observed in NdVO2N where anion order lowers symmetry from apparent orthorhombic a+b−b− Pnma to a+b−c− P21/m.11 Electron diffraction confirms the loss of glide-planes of symmetry due to anion order in all of these materials. The orientational relationships between O/N anion order and octahedral tilting in SrTaO2N, LaTaON2 and NdVO2N evidence a further covalent influence. Pi-bonding is stronger for Ta−N than Ta−O bonds so bending of Ta−N−Ta bridges is more destabilizing than for Ta−O−Ta. Hence anion sites Y with higher N populations are those that are least tilted so Ta− Y−Ta angles remain close to 180°. This is consistent with the measured angles for SrTaO2N and LaTaON2 in Table 2 and those from the refined NdVO2N model.11 SrTaO2N has Ta− O0.5N0.5-Ta angles of 180 (at the Y1 site) and 175° (Y22) while the Ta−O−Ta angle is 169° (Y21). One of the N-rich O0.5N0.5 sites occupies the Y1 position on the rotational axis of the I4/ mcm superstructure, so the Y2 sites are split into O0.5N0.5 (Y22) and O (Y21) sites. In LaTaON2 the Ta−O0.5N0.5−Ta angles are 156 (Y1) and 164° (Y22) while the Ta−N−Ta angle is 166° (Y21). Here, the Y1 anion site in the Imma superstructure has the greatest tilt, and so, it is occupied by O0.5N0.5 which is Npoor in this material, and the Y2 sites are split into O0.5N0.5 (Y22) and N (Y21) as symmetry is lowered to I2/m. The reported model for NdVO2N has a V−Y1−V angle of 159° while V−Y2−V is 156°, consistent with O0.5N0.5 at Y1 and O0.5N0.5 and O at Y2 sites (although it was not possible to refine independent coordinates for the Y21 and Y22 sites in the P21/m cell).11 The above analysis suggests that the partial long-range anion order in oxynitride perovskites directs the perovskite tilting such that the M−Y−M angles at the most N-rich sites remain closest to 180°. If this is correct, then the chemical symmetry between the anion distributions in the SrTaO2N and LaTaON2 results from Y1 being the least tilted anion site in the pseudoI4/mcm SrTaO2N structure, but the most tilted site in the pseudo-Imma arrangement of LaTaON2. Hence, different distributions could be observed in other tilted oxynitride superstructures, for example, an AMON2 material with I4/mcm tilts should have N at the Y1 site on the rotational axis and O0.5N0.5 at the two Y2 sites in the ab-plane so that no further lowering of symmetry (to Fmmm) is expected. Further neutron analyses of perovskite oxynitrides will be needed to test this hypothesis. Finally, we note that the relative magnitudes of the octahedral tilt and anion order distortions in perovskites and their effects on experimental diffraction data are not related to their temperature scales. The room temperature tilt superstructures of SrTaO2N and LaTaON2 are very evident in powder neutron diffraction patterns, but they are lost above estimated temperatures of 200 and 1540 °C, respectively, whereas the anion order distortions are far more subtle but have critical transitions near 2000 and 3000 °C.

It is notable that the magnitude of the lattice distortion due to anion order in LaTaON2, as represented by the monoclinic angle β = 90.24° at 20 °C, is the largest reported so far. For comparison, the Fmmm lattice parameters of SrTaO2N in Table 1 are equivalent to a √2ap × √2ap × 2ap cell with angle γ = 90.03° in the I112/m setting used previously,16 and NdVO2N has monoclinic angle 90.07°.11 Refinements of anion site occupancies show no detectable loss of 2D chain order up to 1100 °C for either SrTaO2N or LaTaON2. However, the associated lattice distortions are a more precise measure of the anion order, and tricritical extrapolations of spontaneous strains obtained from the lattice parameters give very high temperatures (>2000 °C) as estimates for the 2D-3D transition to an arrangement of fully randomized chains (or fully randomized anions). This is far above the ranges of their reported preparation temperatures, which are 900−1500 °C for SrTaO2N and 900−1000 °C for LaTaON2,2 showing that anions spontaneously segregate into 2D layers of cis-chains when the materials are synthesized. The robustness of the 2D anion chain order to temperature is surprising as an energetically favorable local cis-MX2 geometry is preserved whether the chains propagate in 2 or 3 dimensions within the perovskite lattice. Although the 2D layers of cis-chains are not long-range ordered they have only a small, subextensive, configurational entropy that tends toward zero in large particles, whereas 3D chain order has a calculated entropy of 4.8 J K−1 mol−1.17 The estimated 2000−3000 °C range for the 2D−3D crossover temperature thus corresponds to an enthalpy difference of 11−16 kJ mol−1 between the 2D and 3D chains structures for macroscopic samples. The stability of the 2D order may result from conjugation like that in planar organic π-systems, given the strong π-bonding within cis-TaN2 units. Covalent effects are also evident from comparison of the bond distances and angles for SrTaO2N and LaTaON2 as shown in Table 2. Ta−O/N bonds in LaTaON2 (with distances d = 2.04−2.06 Å) are longer than those in SrTaO2N (d = 2.02− 2.03 Å). This is consistent with greater covalency of La−O/N compared to Sr−O/N slightly weakening the Ta−O/N bonds in the former material but is inconsistent with a simple ‘chemical pressure’ argument where small La3+ should shorten Ta−O/N distances relative to large Sr2+ in the perovskite structure. Furthermore, covalency reverses the order of Ta−O/ N distances compared to predictions based on ionic radii where oxide is smaller than nitride,12 for example, their respective 4coordinate ionic radii are 1.38 and 1.46 Å.32,33 Table 2 shows that d(Ta−O) > d(Ta−O0.5N0.5) in SrTaO2N and d(Ta− O0.5N0.5) > d(Ta−N) in LaTaON2. The covalent shortening effect is seen clearly here as the anions are in otherwise identical coordination environments and is equivalent to comparing C O and CN bonds in organic molecules. Despite the slight differences between Ta−O and Ta−N distances, the Ta(O,N)6 octahedra in SrTaO2N and LaTaON2 are very regular, with cis-O/N−Ta−O/N angles deviating by no more than 3° from the average of 90°. Hence, the crystal structures approximate well to the tilted superstructures of many AMX3 perovskites. SrTaO2N has a perovskite tolerance factor t = 0.98 that is close to the ideal value of unity, and the room temperature structure has ordered octahedral rotations perpendicular to only one axis (a0a0c− in Glazer notation, giving rise to an apparent tetragonal I4/mcm supercell) whereas LaTaON2 with t = 0.95 is more distorted and has tilts around two axes (a0b−b− and apparent orthorhombic Imma symmetry).

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CONCLUSIONS Partial order of oxide and nitride anions consistent with the formation of planes of disordered cis-anion chains is observed in 5010



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(12) Attfield, J. P. Cryst. Growth. Des. 2013, 13, 4623−4629. (13) Porter, S.; Huang, Z.; Woodward, P. M. Cryst. Growth. Des. 2013, DOI: 10.1021/cg401230a. (14) Tobias, G.; Beltrán-Porter, D.; Lebedev, O. I.; Van Tendeloo, G.; Rodríguez-Carvajal, J.; Fuertes, A. Inorg. Chem. 2004, 43, 8010. (15) Fuertes, A. Inorg. Chem. 2006, 45, 9640−9642. (16) Yang, M.; Oró-Solé, J.; Rodgers, J. A.; Jorge, A. B.; Fuertes, A.; Attfield, J. P. Nature Chem. 2011, 3, 47. (17) Camp, P. J.; Fuertes, A.; Attfield, J. P. J. Am. Chem. Soc. 2012, 134, 6762. (18) Hinuma, Y.; Moriwake, H.; Zhang, Y.-R.; Motohashi, T.; Kikkawa, S.; Tanaka, I. Chem. Mater. 2012, 24, 4343. (19) Glazer, A. M. Acta. Cryst. B 1972, 28, 3384. (20) Aleksandrov, K. S. Ferroelectrics 1976, 14, 801. (21) Woodward, P. M. Acta Crystallogr. 1997, B53, 32−43. (22) Howard, C. J.; Stokes, H. T. Acta Crystallogr. 1998, B54, 782− 789; 2002, B58, 565. (23) Günther, E.; Hagenmayer, R.; Jansen, M. Z. Anorg. Allg. Chem. 2000, 626, 1519. (24) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos National Laboratory Report No. LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 1994. (25) Clarke, S. J.; Guinot, B. P.; Michie, C. W.; Calmont, M. J. C.; Rosseinsky, M. J. Chem. Mater. 2002, 14, 288−294. (26) Zhang, Y. -R.; Motohashi, T.; Masubuchi, Y.; Kikkawa, S. J. Ceram. Soc. Japan 2011, 119, 581−586. (27) Campbell, B. J.; Stokes, H. T.; Tanner, D. E.; Hatch, D. M. J. Appl. Crystallogr. 2006, 39, 607. (28) Carpenter, M. A.; Salje, E. K. H.; Graeme-Barber, A. Spontaneous Strain As a Determinant of Thermodynamics and Phase Transitions in Minerals; Verlag: Stuttgart, Germany, 1998. (29) McKnight, R. E. A.; Howard, C. J.; Carpenter, M. A. J. Phys.: Condens. Matter 2009, 21, 015901. (30) Mountstevens, E. H.; Redfern, S. A. T.; Attfield, J. P. Phys. Rev. B 2005, 71, 220102. (31) Attfield, J. P. Int. J. Inorg. Mater. 2001, 3, 1147. (32) Shannon, R. D. Acta. Cryst. A 1976, 32, 751. (33) Baur, W. H. Cryst. Rev. 1987, 1, 59.

the oxynitride perovskites SrTaO2N and LaTaON2 up to 1100 °C. The anion order is estimated to be stable up to very high temperatures (>2000 °C), far above-reported preparation conditions for SrTaO2N and LaTaON2, showing that anions spontaneously segregate into layers of cis-chains when the materials are synthesized. Perovskite superstructures due to ordered octahedral tilts are observed in both materials at room temperature, but partial anion order lowers symmetry further, from apparent tetragonal I4/mcm to a larger orthorhombic Fmmm superstructure for SrTaO2N, and from apparent orthorhombic Imma to monoclinic I2/m symmetry in LaTaON2. Covalent effects on Ta−O/N bonding are observed as bond lengths are in the opposite order to predictions based on ionic radii. Anion order appears to direct octahedral tilting such that the more rigid Ta−N−Ta bridges retain bond angles closer to 180° than the Ta−O−Ta connections.

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ASSOCIATED CONTENT

S Supporting Information *

Further Rietveld refinement results and plots for SrTaO2N and LaTaON2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: j.p.attfi[email protected]. Present Address ∥

Department of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4M1, Canada Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by EPSRC, STFC, and the Royal Society, U.K., and The Ministry of Economy and Competitiveness (MINECO), Spain (Projects MAT2011-24757 and SAB2011-0047).

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REFERENCES

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Thermally Robust Anion-Chain Order in Oxynitride Perovskites

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