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Oxynitride Perovskites: Synthesis and Structures of LaZrO2N, NdTiO2N, and LaTiO2N and Comparison with Oxide Perovskites Simon J. Clarke,*,† Benjamin P. Guinot,§ Charles W. Michie,‡ Mathieu J. C. Calmont,§ and Matthew J. Rosseinsky*,‡ Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdon, and Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom, and School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom Received June 25, 2001. Revised Manuscript Received October 2, 2001
A new oxynitride, LaZrO2N, has been synthesized by the reaction of a highly reactive X-ray amorphous La2Zr2O7 precursor with flowing ammonia at between 900 and 1000 °C. The white insulating material crystallizes in the GdFeO3 distorted perovskite structure type with orthorhombic space group Pnma and with a ) 5.875 25(5) Å, b ) 8.250 31(7) Å, c ) 5.810 08(5) Å, and Z ) 4, as determined by neutron powder diffraction. A detailed structural analysis of the isostructural NdTiO2N is also reported: Pnma, a ) 5.5492(1) Å, b ) 7.8017(1) Å, c ) 5.529 01(9) Å, and Z ) 4. LaTiO2N crystallizes in a different distorted perovskite structure, which is triclinic, and consequently has two distinct Ti(O4N2) octahedra (LaTiO2N: I1 h , a ) 5.6097(1) Å, b ) 7.8719(2) Å, c ) 5.5752(1) Å, R ) 90.199(2)°, β ) 90.154(3)°, γ ) 89.988(8)°, Z ) 4). Oxide and nitride ions are completely disordered in all three phases. The M(O,N)6 octahedra (M ) Ti, Zr) in these phases and a number of perovskite titanates and zirconates become more regular as the tilting distortion of the octahedral framework becomes more pronounced, as a result of quenching of the second-order JahnTeller distortion of the octahedra.
Introduction A common synthetic route to nitrides is the reaction of a metal salt (oxide,1,2 sulfide,3 or halide4) with flowing ammonia at elevated temperatures. The nature of the product is determined by the electropositivity of the metals involved, and DiSalvo and co-workers2 have developed a rule-of-thumb for estimating the equilibrium constants for reactions between oxides and ammonia and thus whether full nitridation is feasible in the laboratory. The binary oxides of very electropositive metals such as the lanthanides do not react with ammonia, while the binary oxides of the early transition metals may often be fully nitrided with little or no reduction of the metal. Later transition and main group oxides are often reduced to the metal, perhaps via the formation of a thermally unstable nitride. In ternary systems, partial nitridation to form a ternary oxynitride may be possible if the elements are not too electropositive. The equilibrium favors the nitrided product at high temperatures and when the ammonia flow is fast, which lowers the partial pressure of H2O above the * To whom correspondence should be addressed. † University of Oxford. ‡ University of Liverpool. § University of Exeter. (1) Marchand, R.; Laurent, Y.; Guyader, J.; L’Haridon, P.; Verdier, P. J. Eur. Ceram. Soc. 1991, 8, 197. (2) Elder, S. H.; DiSalvo, F. J.; Topor, L.; Navrotsky, A. Chem. Mater. 1993, 5, 1545. (3) Marchand, R.; Tessier, F.; DiSalvo, F. J. J. Mater. Chem. 1999, 9, 297. Tessier, F.; Marchand, R. J. Alloys Compd. 1997, 410, 262. (4) Lerch, M.; Fu¨glein, E.; Wrba, J. Z. Anorg. Allg. Chem. 1996, 622, 367.
solid. LaTiO2N may be obtained by the ammonolysis of La2Ti2O7 at 900 °C,5 but further replacement of oxide by nitride is not possible even at higher temperatures. Zr is substantially more electropositive than Ti, so while TiN (Ti3+) may be obtained by reacting TiO2 with ammonia at 800 °C, the synthesis of Zr2ON2 (Zr4+) from ZrO2 requires temperatures of 800-1000 °C,6 and the thermodynamics of forming ZrN from ZrO2 are very unfavorable even at higher temperatures.2 Consequently, in the synthesis of LaZrO2N by the ammonolysis of La2Zr2O7, we demonstrate the need to use an X-ray amorphous precursor and a faster flow of ammonia than is required for the analogous synthesis of LaTiO2N. For comparison, we present structural analyses of LaTiO2N and NdTiO2N and show that in none of these oxynitrides is there significant ordering between the oxide and nitride anions. Other perovskite oxynitrides, BaTaON2,7 LaWO0.6N2.4,8 and SrWO1.7N1.3,9 also exhibit disordered anions. Complete or partial O/N ordering has been reported in the oxynitride perovskites SrTaO2N and LaTaON2 when synthesized with the aid of a halide mineralizer.10 This may indicate that O/N ordering is thermodynamically favored but that attainment of the (5) Marchand, R.; Pors, F.; Laurent, Y. Ann. Chim. Fr. 1991, 16, 553. (6) Clarke, S. J.; Michie, C. W.; Rosseinsky, M. J. J. Solid State Chem. 1999, 146, 399. (7) Pors, F.; Marchand, R.; Laurent, Y.; Bacher, P.; Roult, G. Mater. Res. Bull. 1988, 23, 1447. (8) Bacher, P.; Antoine, P.; Marchand, R.; L’Haridon, P.; Laurent, Y.; Roult, G. J. Solid State Chem. 1988, 77, 67. (9) Weller, M. T.; Skinner, S. J. Int. J. Inorg. Mater. 2000, 2, 463. (10) Gu¨nther, E.; Hagenmayer, R.; Jansen, M. Z. Anorg. Allg. Chem. 2000, 626, 1519.
10.1021/cm010577v CCC: $22.00 © 2002 American Chemical Society Published on Web 11/30/2001
Oxynitride Perovskites: Synthesis and Structures
Figure 1. The X-ray powder diffraction pattern of the “amorphous” La2Zr2O7 precursor used in the synthesis of LaZrO2N. The inset shows the pattern for crystalline La2Zr2O7 obtained by firing the amorphous precursor at 1300 °C.
thermodynamic limit is hampered by slow diffusion in the solid state. In less isotropic structures such as the K2NiF4-type phases Nd2AlO3N11 and Sr2TaO3N,12 O/N ordering is the norm. One motivation for the investigation of these d0 transition metal perovskites is to compare their structures and dielectric properties13 with the analogous technologically important oxides, notably the ferroelectrics, BaTiO3 and PbTiO3. Experimental Section Synthesis of LaZrO2N. A total of 5 g of amorphous La2Zr2O7 were prepared in the following way: appropriate quantities of dried (900 °C in air, overnight) La2O3 (Aldrich Chemical Co., 99.99%) and ZrO(NO3)2‚6H2O (Aldrich Chemical Co., 99.9%) were dissolved separately in dilute nitric acid. These solutions were mixed, and 7.4 g of saturated citric acid solution, containing as many citric acid molecules as the total number of metal ions, was added. A total of 20 cm3 of ethylene glycol was added, and the solution was heated with constant stirring until it formed a moist brown paste. This citrate gel was dried at 100 °C overnight, crushed, and then fired overnight at 600 °C in air to form a fine white powder containing no residual carbon, as determined by combustion analysis. The X-ray powder diffraction pattern (Figure 1) shows only extremely broad features coincident with the main diffraction peaks of La2Zr2O7 indicating that the oxide contains ordered regions of approximately 2 nm in size. Between 0.3 and 1 g of the amorphous oxide was placed in a sintered alumina boat, which was placed in a 22 mm internal diameter silica tube, which was in turn placed inside a split tube furnace. The tube was capped with a stopcock on each end. Ammonia gas (British Oxygen Company, 99.98%, and used as delivered) was flowed through the tube at a rate of approximately 40 dm3 h-1, and the sample was heated to 950 °C. After 24 h, the tube was isolated from the ammonia flow and removed from the furnace so that the sample cooled to room temperature in a few minutes under an ammonia atmosphere. The sample was found not to be air sensitive and was ground in air, and then the reaction was repeated until the transformation to the white oxynitride phase was complete. This typically required up to 20 regrinding and 24 h firing (11) Marchand, R.; Pastusyak, R.; Laurent, Y.; Roult, G. Rev. Chim. Miner. 1982, 19, 684. (12) Diot, N.; Marchand, R.; Haines, J.; Leger, J. M.; Macaudiere, P.; Hull, S. J. Solid State Chem. 1999, 146, 390. (13) Gouin, X.; Marchand, R.; Laurent, Y.; Gervais, F. Solid State Commun. 1995, 93, 857.
Chem. Mater., Vol. 14, No. 1, 2002 289 cycles, and while it was possible to obtain 0.3 g of material in a phase-pure form by this method, larger samples, such as those of approximately 1 g required for neutron diffraction, always contained a few percent of La2Zr2O7 pyrochlore impurity. Reaction of 0.5 g samples of crystalline La2Zr2O7 (obtained by firing the amorphous precursor at 1300 °C for 24 h; see inset to Figure 1) with ammonia at 950 °C under the conditions described above resulted in only about 10-20% conversion to the oxynitride after 10 grinding and heating cycles of 24 h. Because our amorphous La2Zr2O7 precursor crystallizes at the reaction temperature, competition between crystallization of the oxide pyrochlore and formation of the oxynitride perovskite means that it is difficult to effect full conversion to the oxynitride in a 1 g sample. Combustion analysis for nitrogen of phase-pure material revealed a stoichiometry of LaZrO2.09(5)N0.91(5); this material was used for X-ray powder diffraction investigations. Synthesis of LnTiO2N (Ln ) La, Nd). The analogous lanthanide titanium oxynitrides were prepared as described previously.5 Pale blue Nd2Ti2O7 and white La2Ti2O7 were prepared on a 5 g scale by the solid-state reaction between the predried (900 °C overnight in air) lanthanide oxides (La2O3, Aldrich, 99.999%; Nd2O3, ALFA AESAR, 99.99%) and anatase TiO2 (Aldrich, 99.9+%) at 1050 °C for 24 h and then, after regrinding and pelletizing, at 1310 °C for a further 24 h. The brown oxynitrides were prepared by reacting these crystalline precursors with ammonia at 950 °C for periods of 20 h, 6 h, and 6 h, with intermediate regrinding, in an ammonia flow of 20 dm3 h-1 using the apparatus described above. The phase purity of the products was confirmed by laboratory powder X-ray diffraction. The mass change during synthesis indicated that the ideal LnTiO2N composition was reached in each case, and combustion analysis for nitrogen confirmed this, producing compositions of LaTiO2.02(2)N0.98(2) and NdTiO2.03(3)N0.97(3). Powder Diffraction. Routine powder X-ray diffraction (PXRD) data to assess phase purity were collected using a Philips PW 1050 diffractometer operating in Bragg-Brentano geometry with Cu KR radiation. PXRD data for Rietveld refinement were collected for LaZrO2N and LaTiO2N on station 2.3 of the Synchrotron Radiation Source at the Daresbury Laboratory, U.K. The instrument was used in flat plate parallel beam Hart-Parrish geometry14 with the powder sprinkled on a Si(520) wafer and with an X-ray wavelength of 1.399 70(1) Å (LaZrO2N) and 1.399 61(1) Å (LaTiO2N) achieved using a water-cooled Si(111) monochromator and calibrated using a silicon standard. Data on LaZrO2N were collected in the 2θ range of 5°-107° with a step size of 0.01° and for 2 s per point. Data on LaTiO2N were collected in the 2θ range of 5°-115° with a constant step size of 0.01°, with the count time per point increasing with 2θ: 2 s per point in the range of 5°-15°, 3 s in the range of 15°-60°, 6 s in the range of 60°85°, and 9 s in the range of 85°-115°. Counts were measured using a scintillation detector. Neutron powder diffraction data were collected on all three compounds using the diffractometer POLARIS at the ISIS spallation source, Rutherford Appleton Laboratory, U.K. Samples were contained in 6 mm diameter cylindrical vanadium cans, and data were collected using detector banks located at angles 2θ of 35° (3He tube detector), 90° (ZnS scintillator), and 145° (3He tube detector, highest resolution bank: ∆d/d ) 5 × 10-3) in the d-spacing range of 0.5-8 Å for a total integrated proton current at the production target of 1008, 280, and 650 µAh for 1 g of LaZrO2N, 2 g of LaTiO2N, and 2 g of NdTiO2N, respectively. X-ray and neutron powder diffraction data were analyzed using the Rietveld profile refinement suite GSAS.15 Refinement against neutron data was carried out using all three detector banks simultaneously. (14) Parrish, W.; Hart, M.; Erickson, C. G.; Masciocchi, N.; Huang, T. C. Adv. X-ray Anal. 1986, 29, 243. Hart, M.; Parrish, W. Mater. Sci. Forum 1986, 9, 39. (15) Larson, A.; von Dreele, R. B. The General Structure Analysis System; Los Alamos National Laboratory: Los Alamos, NM, 1985.
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Table 1. Refinement Results for LaTiO2N, NdTiO2N, and LaZrO2Na formula radiation instrument T, K space group fw a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Fcalcd, kg m-3 no. of variables χ2 wRpb Rpb
LaZrO2N neutron POLARIS 298 Pnma 276.1 5.875 25(5) 8.250 31(7) 5.810 08(5) 90 90 90 281.630(3) 4 6.513(1) 72 1.316 0.0112 0.0210
LaZrO2N X-ray (1.4 Å) 2.3 298 Pnma 276.1 5.877 25(5) 8.252 89(7) 5.811 43(5) 90 90 90 281.879(3) 4 6.507(1) 22 1.274 0.0933 0.0745
NdTiO2N neutron POLARIS 298 Pnma 238.12 5.5492(1) 7.8017(1) 5.529 01(9) 90 90 90 239.370(3) 4 6.608(1) 70 2.497 0.0122 0.0236
LaTiO2N neutron POLARIS 298 I1 h 232.8 5.6097(1) 7.8719(2) 5.5752(1) 90.199(2) 90.154(3) 89.988(8) 246.190(5) 4 6.284(2) 74 2.944 0.0175 0.0369
a All neutron refinements against data from three detector banks at 145°, 90°, and 35° 2θ. b R factors are given for the highest resolution neutron diffraction data (145° detector bank).
Figure 2. The results of the refinement of the structure of NdTiO2N against powder neutron diffraction data (145° detector bank). The measured (points), calculated (line), and difference (lower line) profiles are shown. Tick marks indicate allowed reflections for NdTiO2N (lower set) and the TiN impurity (upper set). The inset shows a magnification of the low d-spacing region. Table 2. Atomic Parameters for NdTiO2N from Refinement in Pnma against Neutron Diffraction Dataa atom site Nd Ti O/N(1) O/N(2)
4c 4b 4c 8d
x
y
z
0.03175(9) 0.5 0.4853(2) 0.2850(1)
0.25 0 0.25 0.03934(7)
0.9955(3) 0 0.0755(2) 0.7161(1)
oxide 100 × Uiso,eq, Å2 fraction 0.64(2) 0.79(1) 1.04(4) 0.99(2)
0.77(2) 0.70(2)
a Equivalent isotropic displacement parameters are given; anisotropic displacement parameters are shown as ellipsoids in Figure 3.
Results Structure of NdTiO2N. NdTiO2N has previously been reported by Marchand and co-workers5 to crystallize in the GdFeO3-type distorted perovskite structure on the basis of laboratory PXRD. Our neutron data were clearly consistent with that model, and refinement, using ref 5 as a starting point, was carried out in the space group Pnma. A small amount of TiN impurity (1.06(2)% by mass) was present in this sample according to the refinement. The refinement results are presented in Table 1 and Figure 2, the structural parameters are presented in Table 2, and selected bond lengths and angles are presented in Table 3. The displacement ellipsoids were refined anisotropically for all atoms
except Ti (which is in an almost regular octahedral environment) and are shown in the structure diagram in Figure 3. Oxygen and nitrogen can readily be distinguished using neutron diffraction on account of their very different scattering lengths (N, 9.6 fm; O, 5.8 fm). The occupancy of the two anion sites by oxide and nitride was refined with each site constrained to be fully occupied but with no constraint on the overall composition. Our results suggest that the sample has a composition of NdTiO2.17(6)N0.83(6) and is thus marginally oxiderich compared with the stoichiometry obtained from chemical analysis. The results also show that the anions are distributed statistically over the available sites and there is negligible O/N order. This accords with the results of Raman spectroscopy16 carried out by Marchand and co-workers in which the lattice modes are broad due to the O/N disorder. Structure of LaZrO2N. The sample synthesized on a 0.3 g scale and used in the synchrotron X-ray powder diffraction study was pure white in color, and there were no impurities visible in the X-ray powder diffraction pattern. The larger sample used for neutron powder diffraction was pale blue-grey in color and contained a small amount of La2Zr2O7 pyrochlore by laboratory PXRD; the neutron diffraction measurement further revealed small amounts of Zr2ON2 and ZrNx impurities due to a Zr/La ratio slightly exceeding 1 in the batch of starting oxide used in this synthesis. It was apparent from laboratory PXRD that LaZrO2N adopts a structure similar to that of NdTiO2N. Refinement of such a model in the space group Pnma using starting lattice parameters obtained from laboratory PXRD was successful, and the results of the refinements against the synchrotron X-ray and neutron data are presented in Table 1. A joint refinement was not attempted in this case because two different samples were used. The refinement against the neutron data incorporated the three cubic impurity phases, which were accounted for with a total of nine parameters: a mass fraction of the sample (Zr2ON2, 2.5(1)%; ZrN, 3.2(1)%; La2Zr2O7, 3.4(1)%), a lattice parameter, and a peak-width parameter for each phase. As well as refining the cell constants and atomic positions for the main LaZrO2N phase (90.8(2)% by mass), the atomic displacement parameters, for all atoms except Zr, were refined anisotropically, and the O/N ratio on the two anion sites was refined with no constraint on the overall stoichiometry but assuming full occupancy of each anion site. The structural models obtained for LaZrO2N using both powder neutron and X-ray diffraction agree very closely as shown in Table 4. Plots of the refinements against X-ray and neutron data are shown in Figures 4 and 5, respectively. Selected bond lengths and angles are given in Table 5. The structure of LaZrO2N, showing the anisotropic displacement ellipsoids, is depicted in Figure 6. Our neutron results suggest a stoichiometry of LaZrO1.94(2)N1.06(2), consistent with the results of chemical analysis, and show negligible O/N order between the two available anion sites. We attempted to synthesize other lanthanide-zirconium oxynitrides. With the smaller Nd3+ replacing La3+, we obtained a gray product with broad diffraction peaks, (16) LeGendre, L.; Marchand, R.; Piriou, B. Eur. J. Solid State Inorg. Chem. 1997, 34, 973.
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Table 3. Selected Bond Distances (Å) and Angles (deg) for NdTiO2Na
a
Ti-O/N(1) Ti-O/N(2) Ti-O/N(2)
1.9963(2) [2] 1.9951(7) [2] 2.0060(7) [2]
O/N(1)-Ti-O/N(2) O/N(1)-Ti-O/N(2) O/N(1)-Ti-O/N(2)
89.42(3) [2] 90.58(3) [2] 89.58(4) [2]
Nd-O/N(1) Nd-O/N(1) Nd-O/N(2) Nd-O/N(2)
2.386(2) 2.556(1) 2.438(1) [2] 2.658(1) [2]
Nd-O/N(2) Nd-O/N(1) Nd-O/N(1) Nd-O/N(2)
2.760(1) [2] 3.064(1) 3.168(1) 3.276(1) [2]
90.42(4) [2] 88.813(7) [2] 91.187(7) [2]
The numbers in square brackets indicate the number of bonds or angles of a particular type.
Figure 3. The structure of NdTiO2N depicting the TiO6 octahedra and Nd3+ ions (open circles). Thermal ellipsoids are shown at the 99.9% confidence level. Table 4. Atomic Parameters for LaZrO2N from Refinement in Pnma against Neutron Diffraction Dataa atom site La
O/N(1)-Ti-O/N(2) O/N(2)-Ti-O/N(2) O/N(2)-Ti-O/N(2)
4c
x
0.03523(7) [0.0342(1)] Zr 4b 0.5 [0.5] O/N(1) 4c 0.4712(1) [0.468(2)] O/N(2) 8d 0.291 67(8) [0.291(1)]
y
z
0.25 [0.25] 0 [0] 0.25 [0.25] 0.049 19(6) [0.053(1)]
0.9922(1) [0.9914(2)] 0 [0] 0.0926(1) [0.091(2)] 0.708 30(8) [0.710(1)]
100 × oxide Uiso,eq, Å2 fraction 0.84(2) [0.16(3)] 0.59(1) [0.0]b 1.11(3) [0.0]b 1.17(3) [0.0]b
0.692(6) [0.667]b 0.625(4) [0.667]b
a Values in square brackets are for refinement of the structure of a different sample, containing no impurity phases, against synchrotron X-ray diffraction data. Equivalent isotropic displacement parameters are given; anisotropic displacement parameters obtained from the neutron refinement are shown as ellipsoids in Figure 6. b Not refined.
Figure 4. The results of the refinement of the structure of LaZrO2N against synchrotron X-ray powder diffraction data. The measured (points), calculated (line), and difference (lower line) profiles are shown. Tick marks indicate allowed reflections. The inset shows a magnification of the high-angle region.
which we were unable to index and which was probably multiphase. With the even smaller Y3+ ion, direct substitution of Y3+ for Zr4+ was observed, but an amorphous “YZrO3.5” precursor did not yield a single
Figure 5. The results of the refinement of the structure of LaZrO2N against powder neutron diffraction data (145° detector bank). The measured (points), calculated (line), and difference (lower line) profiles are shown. The sets of tick marks indicate allowed reflections for LaZrO2N (lower set), La2Zr2O7, TiN, and Zr2ON2 (upper set). A weak reflection at d ) 2.14 Å, due to the vanadium sample holder, was excluded from the refinement. The inset shows a magnification of the low d-spacing region.
phase oxynitride but only a mixture of fluorite-related phases. Structure of LaTiO2N. Inspection of our neutron diffraction data indicated that this material is not isostructural with NdTiO2N. On the basis of the unit cell of NdTiO2N (a x2a × x2a × 2a expansion of the cubic perovskite cell with cell parameter a), systematic absences corresponding to body centering were apparent. Possible body centered tetragonal (I4/mcm) or orthorhombic (Imma and Immm) space groups were rejected on the basis of poor LeBail-type modelindependent fits to the diffraction profiles, which indicated that the cell was distorted at least to monoclinic symmetry. Model-independent fits indicated that the I2/m setting of C2/m and the I2/a setting of C2/c were possible space groups. Our data did not allow us to firmly identify the systematic absences resulting from the glide plane in I2/a. However, Rietveld refinements in I2/m, including the C2/m model found by Jansen and co-workers10 for anion-ordered LaTaON2, resulted in unstable atomic positions, and the only stable monoclinic refinement was in I2/a (LeBail, χ2 ) 2.55, wRp ) 0.0164; Rietveld, χ2 ) 2.87, wRp ) 0.0173). Close investigation of the profile and comparison of the goodness of fit parameters, χ2, and the weighted profile R factors, wRp, for LeBail and Rietveld fits in I2/a and the triclinic space group, I1 h , against the highest resolution data (145° detector bank) suggested that the true symmetry is triclinic I1 h (LeBail, χ2 ) 2.04, wRp ) 2 0.0147; Rietveld, χ ) 2.45, wRp ) 0.0159). The synchrotron data supported this assignment producing superior Lebail and Rietveld fits in I1 h (LeBail, χ2 ) 1.68, 2 wRp ) 0.0419; Rietveld, χ ) 1.94, wRp ) 0.045) than in I2/a (LeBail, χ2 ) 1.78, wRp ) 0.0432; Rietveld, χ2 ) 2.11, wRp ) 0.047). Strain broadening of the diffraction
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Table 5. Selected Bond Distances (Å) and Angles (deg) for LaZrO2N Derived from the Refinement against Neutron Diffraction Dataa
a
Zr-O/N(1) Zr-O/N(2) Zr-O/N(2)
2.1383(2) [2] 2.1296(4) [2] 2.1368(4) [2]
O/N(1)-Zr-O/N(2) O/N(1)-Zr-O/N(2) O/N(1)-Zr-O/N(2)
88.33(3) [2] 91.67(3) [2] 88.70(3) [2]
La-O/N(1) La-O/N(1) La-O/N(2) La-O/N(2)
2.442(1) 2.6269(8) 2.4798(7) [2] 2.7812(7) [2]
La-O/N(2) La-O/N(1) La-O/N(1) La-O/N(2)
2.9502(7) [2] 3.3648(8) 3.418(1) 3.5791(6) [2]
O/N(1)-Zr-O/N(2) O/N(2)-Zr-O/N(2) O/N(2)-Zr-O/N(2)
91.30(3) [2] 88.51(1) [2] 91.49(1) [2]
The numbers in square brackets indicate the number of bonds or angles of a particular type.
Figure 6. The structure of LaZrO2N depicting the ZrO6 octahedra and La3+ ions (open circles). Thermal ellipsoids are shown at the 99.9% confidence level.
peaks and the insensitivity of X-ray diffraction to the anion positions, which are the major source of distortion, meant that the neutron data were the better arbiter of the correct model. Our final refinement in I1 h against data from all three detector banks had χ2 ) 2.94 (cf. χ2 ) 3.19 in I2/a). Our data allowed us to obtain precision in the anion positions factors of 2-5 larger than in previously reported isostructural triclinic perovskites,17 and anisotropic refinement of the displacement parameters for all atoms except Ti was possible and resulted in chemically sensible values for these parameters. A small amount of TiN impurity (0.57(2)% by mass) was included in the refinement. The O/N ratios on each of the three anion sites were refined independently producing the stoichiometry LaTiO1.99(6)N1.01(6), consistent with chemical analysis; no strong site preferences were indicated. The final refinement data are presented in Table 1 and Figure 7, the refined atomic positions are in Table 6, and selected bond distances and angles are in Table 7. The structure, including depiction of the anisotropic displacement ellipsoids, is shown in Figure 8. Discussion NdTiO2N and LaZrO2N both have the common GdFeO3-type distortion of the cubic perovskite structure. Glazer18 has classified the relative magnitudes and phases of the tilts about the three orthogonal axes of an undistorted cubic perovskite AMX3 (A ) 12-coordinate cation; M ) 6-coordinate cation; X ) anion), which can give rise to the families of distorted perovskites. The GdFeO3-type distortion is classified as a+b-b- (tilt system number 10) with two angles of tilt identical and adjacent layers of octahedra tilted in phase about one (17) Battle, P. D.; Gibb, T. C.; Jones, C. W.; Studer, F. J. Solid State Chem. 1989, 78, 281. (18) Glazer, A. M. Acta Crystallogr. 1972, B28, 3384.
Figure 7. The results of the refinement of the structure of LaTiO2N against powder neutron diffraction data (145° detector bank). The measured (points), calculated (line), and difference (lower line) profiles are shown. Tick marks indicate allowed reflections for LaTiO2N (lower set) and the TiN impurity (upper set). A weak reflection at d ) 2.14 Å, due to the vanadium sample holder, was excluded from the refinement. The inset shows a magnification of the low d-spacing region. Table 6. Atomic Parameters for LaTiO2N from Refinement in I1 h against Neutron Diffraction Dataa atom
site
x
y
La Ti(1) Ti(2) O/N(1) O/N(2) O/N(3)
4i 2a 2c 4i 4i 4i
0.4989(3) 0 0 -0.0620(2) 0.260(1) 0.255(1)
z
0.2516(7) 0.0010(8) 0 0 0.5 0 0.2495(8) -0.0006(9) 0.0358(4) 0.243(1) 0.4730(4) 0.757(1)
100 × oxide Uiso,eq, Å2 fraction 0.72(2) 0.77(1) 0.80(1) 1.15(5) 1.40(9) 1.59(9)
0.72(1) 0.67(3) 0.60(2)
a Equivalent isotropic displacement parameters are given; anisotropic displacement parameters are shown as ellipsoids in Figure 8.
cubic axis and out of phase about the other two axes. In NdTiO2N and LaZrO2N, the tilts around the three orthogonal directions are very similar in magnitude. The distortion in LaTiO2N is classified as a-b-c- (tilt system number 12), and this scheme requires the symmetry to be triclinic, with the consequence that there are two distinct octahedral sites for Ti: each octahedral vertex is shared between one octahedron of each type. In the case of LaTiO2N, two of the tilt angles are very similar in magnitude and larger than the one about the b axis. Figure 9 compares the two tilting schemes. The distortion from the ideal cubic perovskite structure in NdTiO2N and LaZrO2N is relatively large compared with that in LaTiO2N. This can be expressed by comparing the different cation-anion bond lengths using the “observed tolerance factor”, tobs, defined by Sasaki et al.19 as
tobs )
〈A-(O,N)〉
x2〈M-(O,N)〉
(19) Sasaki, S.; Prewitt, C. T.; Liebermann, R. C. Am. Miner. 1983, 68, 1189.
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Table 7. Selected Bond Distances (Å) and Angles (deg) for LaTiO2Na
a
Ti(1)-O/N(1) Ti(1)-O/N(2) Ti(1)-O/N(3) Ti(2)-O/N(1) Ti(2)-O/N(2) Ti(2)-O/N(3)
1.992(6) [2] 2.014(6) [2] 2.000(7) [2] 2.004(6) [2] 1.974(6) [2] 1.987(7) [2]
O/N(1)-Ti(1)-O/N(2) O/N(1)-Ti(1)-O/N(2) O/N(1)-Ti(1)-O/N(3) O/N(1)-Ti(2)-O/N(2) O/N(1)-Ti(2)-O/N(2) O/N(1)-Ti(2)-O/N(3)
89.6 (2) [2] 90.4(2) [2] 89.7 (2) [2] 88.7 (2) [2] 91.3(2) [2] 89.1(1) [2]
La-O/N(1) La-O/N(2) La-O/N(3) La-O/N(2) La-O/N(3) La-O/N(1)
2.465(2) 2.561(6) 2.578(6) 2.639(6) 2.689(7) 2.795(7)
La-O/N(1) La-O/N(3) La-O/N(2) La-O/N(3) La-O/N(2) La-O/N(1)
2.823(7) 2.895(6) 2.957(6) 3.008(6) 3.035(6) 3.144(2)
O/N(1)-Ti(1)-O/N(3) O/N(2)-Ti(1)-O/N(3) O/N(2)-Ti(1)-O/N(3) O/N(1)-Ti(2)-O/N(3) O/N(2)-Ti(2)-O/N(3) O/N(2)-Ti(2)-O/N(3)
90.3(2) [2] 88.8(3) [2] 91.2(3) [2] 90.9(1) [2] 88.8(3) [2] 91.2(3) [2]
The numbers in square brackets indicate the number of bonds or angles of a particular type. Table 8. Tolerance Factors and Octahedral Distortions for Precisely Determined Titanate and Zirconate Perovskites compound NdTiO2N LaTiO2N site Ti(1) LaTiO2N site Ti(2) CaTiO324 CaTiO325 CdTiO325 BaTiO323 Sr0.25La0.5TiO326 LaZrO2N SrZrO327 CaZrO324
Figure 8. The structure of LaTiO2N depicting the TiO6 octahedra and La3+ ions (open circles). Thermal ellipsoids are shown at the 99.9% confidence level. Ellipsoids are shown for the Ti(2) octahedron, which shares corners with six Ti(1) octahedra; both Ti sites are coordinated by all three anions.
Figure 9. Comparison of the octahedral tilting schemes in NdTiO2N/LaZrO2N and LaTiO2N viewed down the directions corresponding to the axes of a cubic perovskite.
where the mean A-(O,N) bond length refers to the twelve nearest neighbors and therefore tobs ) 1 for an undistorted cubic perovskite. The values of tobs are 0.986 for NdTiO2N, 0.978 for LaZrO2N, and 0.996 for LaTiO2N. This may partially account for the different tilting schemes adopted, although the a-b-c- tilt system is rare17,20 and there are likely electronic factors in operation as discussed below, which account for the adoption of a structure with two inequivalent sites for the M cation in LaTiO2N. Both tilting schemes have a single
space method group neutron neutron neutron neutron X-ray X-ray neutron X-ray neutron neutron neutron
mean M-O, Å
rms dev M-O, Å
Pnma 1.9991(3) 0.0049(4) I1 h 1.985(3) 0.009(3) 1.988(6) 0.012(3) Pnma 1.956(2) 0.005(2) Pnma 1.9547(7) 0.0040(7) Pnma 1.9658(9) 0.0027(9) Amm2 2.000(5) 0.098(4) Pban 1.943(2) 0.018(4) Pnma 2.1349(2) 0.0038(2) Pnma 2.091(1) 0.005(1) Pnma 2.096(1) 0.004(1)
tIR 0.884 0.898 0.898 0.886 0.886 0.879 0.992 0.912 0.851 0.887 0.840
A cation site in accord with the observation of Woodward21 that all AMO3 perovskites with one chemical type of A cation belong to tilting schemes with only one crystallographic A cation site. In a detailed analysis of the structure-directing factors in a wide range of perovskites,21,22 Woodward shows that neither of these tilting schemes requires the octahedra themselves to distort, although this is permitted. Indeed, in all three compounds, as well as the distortion of the MO2N framework, the MO4N2 octahedra are themselves distorted. The distortions may be quantified according to the root-mean-square (rms) deviation of the M-O bond lengths from their mean value (i.e., their standard deviation), and these values are tabulated (Table 8) for the oxynitrides described here and for comparable oxides.23-27 These distortions are often ignored because they are more than 1 order of magnitude smaller than the static distortions in the low-temperature polymorphs of BaTiO3,23 which lead to the ferroelectric nature of that compound below 393 K. The d0 cations such as Ti4+ in octahedral coordination can undergo large ferroelectric distortions as a result of HOMO (O2pπ)-LUMO (Ti3dπ) mixing, which precipitates a second-order Jahn-Teller distortion (SOJT).28 Goodenough and Longo29 point out (20) Woodward, P. M.; Sleight, A. W.; Vogt, T. J. Phys. Chem. Solids 1995, 56, 1305. (21) Woodward, P. M. Acta Crystallogr. 1997, B53, 44. (22) Woodward, P. M. Acta Crystallogr. 1997, B53, 32. (23) Kwei, G. H.; Lawson, A. C.; Billinge, S. J. L.; Cheong, S.-W. J. Phys. Chem. 1993, 97, 2368. (24) Koopmans, H. J. A.; van de Velde, G. M. H.; Gellings, P. J. Acta Crystallogr. 1983, C39, 1322. (25) Sasaki, S.; Prewitt, C. T.; Bass, J. D.; Schulze, W. A. Acta Crystallogr. 1987, C43, 1668. (26) Battle, P. D.; Bennett, J. E.; Sloan, J.; Tilley, R. J. D.; Vente, J. F. J. Solid State Chem. 2000, 149, 360. (27) Ahtee, A.; Ahtee, M.; Glazer, A. M.; Hewat, A. W. Acta Crystallogr. 1976, B32, 3243. (28) Cohen, R. E. Nature 1992, 358, 136. (29) Goodenough, J. B.; Longo, J. M. In Landolt-Bo¨ rnstein, Neue Serie III/4a; Hellwege, K. H., Ed.; Springer-Verlag: Berlin, 1970.
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Figure 10. The correlation of the root-mean-square deviation from the mean of the M-O lengths with the tolerance factor, tIR, based on ionic radii, for all precisely determined titanate and zirconate oxide and oxynitride perovskites.
that when the A cation decreases in size, leading to cooperative tilting of the octahedra with respect to one another, the shortening of the A-O distances enhances the σ-bonding between the A cations and those O2p orbitals, which interact in a π-sense with the M d orbitals. This has the effect of stabilizing the O2p orbitals, reducing the HOMO-LUMO mixing in the octahedra and thus quenching the ferroelectric distortion of the octahedra. Careful analysis of the electron density in CaTiO3 by Buttner and Maslen30 indicates that a SOJT is responsible for the distortion of the octahedra in that compound, although there is never any ferroelectric (i.e., static) displacement of Ti from the center of its octahedron. The degree of distortion of the octahedra in M4+ d0 perovskites should therefore decrease as the tilting distortion increases or the polarizing power of the A cation increases or both. There are sufficient precise determinations (esd in the anion positions in the 4th decimal place or better) of the atomic positions of distorted titanate and zirconate perovskites, tabulated in Table 8, for us to attempt such a correlation. For noncubic perovskites with either tilting distortions or static displacements of M from the centers of the octahedra, Figure 10 plots the distortion of the octahedra, MO6 or MO4N2 (M ) Ti, Zr), as measured by the standard deviation of the six M-(O,N) bond lengths, against the tolerance factor, tIR, defined using standard ionic radii31 as
tIR )
rA + rO,N
x2(rM + rO,N)
where we use for rA the cationic radius for 8 coordination, because this value is usually better defined than that for 12 coordination19 and use a weighted mean value of 1.43 Å for rO,N for the oxynitrides based on radii of 1.40 and 1.50 Å, respectively, for six-coordinate oxide31 and nitride32 ions. Figure 10 shows that there is a strong correlation between distortion of octahedra (30) Buttner, R. H.; Maslen, E. N. Acta Crystallogr. 1992, B48, 644. (31) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000. Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
Clarke et al.
and the tolerance factor, tIR, for all noncubic perovskitetype Ti4+ oxides and oxynitrides with precisely determined crystal structures. Fewer data exist for zirconate perovskites than for titanates, but Figure 10 shows no strong dependence of the rms distortion in Zr-(O,N) bond lengths on tolerance factor. This is a consequence of the Zr-(O,N) covalency being smaller than the Ti(O,N) covalency and the SOJT effect consequently being smaller for zirconates than for titanates. Comparing the oxynitrides, LaTiO2N has the largest rms distortion of M-(O,N) lengths. This is because La3+ is less electronegative than Nd3+ and Ti4+ is more electronegative than Zr4+, making the Ti-(O,N) covalency more marked than that in NdTiO2N or than the Zr-(O,N) covalency in LaZrO2N and making the SOJT effect largest in LaTiO2N. The triclinic tilting scheme of LaTiO2N, which is rare in perovskites17,20 and results in the only example of an oxynitride perovskite with two distinct M(O,N)6 octahedra, may be a consequence of the partial relaxation of the larger SOJT distortion, which should be afforded by the creation of two distinct sites. Conclusion In conclusion, we have shown that the synthesis of an oxynitride, LaZrO2N, by ammonolysis of an oxide, La2Zr2O7, containing elements as electropositive as La and Zr is possible, provided that an X-ray amorphous oxide is used, together with a rapid ammonia flow to reduce the partial pressure of the water produced in the reaction above the solid and thus drive the reaction in favor of the oxynitride product. From our structural studies, we conclude that perovskite oxynitrides synthesized as described often show oxide/nitride disorder, although ordering of the anions is not unknown in oxynitride perovskites10 or in compounds with related structures.11,12 We are able to correlate a decrease in the octahedral irregularity in Ti4+ d0 perovskite oxides and oxynitrides with a quenching of the second-order Jahn-Teller distortion as the octahedral framework becomes more distorted by tilting. In particular, LaTiO2N adopts a triclinic structure in which the two distinct octahedral sites are considerably more distorted than those in NdTiO2N and LaZrO2N. This can be directly correlated with the relative sizes and competing electronegativities of the A and M site cations. Acknowledgment. S.J.C. and M.J.R. thank the EPSRC for financial support including the award of a studentship for C.W.M. and for access to ISIS and the SRS. S.J.C. thanks the Universities of Exeter and Oxford, The Royal Society, and the Nuffield Foundation for funding. B.P.G. acknowledges support from the ERASMUS exchange scheme, and M.J.C.C. is grateful to the Universite´ de Rennes I for facilitating an exchange with the University of Exeter. We are grateful for valuable discussions with Professor R. G. Denning, University of Oxford. We thank Dr. Ron Smith of ISIS and Drs. Chiu Tang and Mark Roberts of the SRS for assistance with the diffraction investigations. CM010577V (32) Baur, W. H. Crystallogr. Rev. 1987, 1, 59.
