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Structural and Magnetic Properties of Topotactically Reduced YSr2Mn2O7-x (0 < x < 1.5) M. A. Hayward† Department of Chemistry, Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford, OX1 3QR, U.K. ReceiVed July 27, 2005. ReVised Manuscript ReceiVed October 24, 2005
Reaction of the n ) 2 Ruddlesden-Popper phase YSr2Mn2O7 with NaH at 225 °C yields the topotactically reduced phase YSr2Mn2O5.43(3) (a ) 3.62043(4) Å, c ) 22.3570(3) Å). Combination of this reduced phase with the stoichiometric starting material at 400 °C yields intermediate phases: YSr2Mn2O6.54(3) (a ) 3.80900(7) Å, c ) 20.2691(5) Å) and YSr2Mn2O5.96(3) (a ) 3.81071(7) Å, c ) 20.5238(4) Å). The variation in the anion vacancy distribution of these reduced phases as a function of stoichiometry is discussed in relation to the coordination polyhedra of the metal cations. Temperaturedependent magnetization data indicate strong antiferromagnetic coupling interactions in all samples. Longrange magnetic order is suppressed by structural or charge disorder in all samples except YSr2Mn2O5.5, which adopts a G-type antiferromagnetic ordering scheme with an ordered moment of 4.61(5)µB per manganese, consistent with S ) 5/2 Mn(II).
Introduction Mixed-valent manganese III/IV perovskite and Ruddlesden-Popper oxides have received considerable attention due to the observation of large magnetoresistive ratios in these phases.1-3 Interest has been sustained by the broader observation of strong coupling between spin, charge, and lattice degrees of freedom in these materials.4,5 The majority of studies have focused on the influence of cation substitution on the structures and physical properties of these compounds.6-8 However, there have been few studies focused on the effects of manipulating the anion lattices of these phases, particularly with regard to the introduction of large numbers of oxide ion vacancies via topotactic reduction. Of the few studies reported for materials based on layered Ruddlesden-Popper phases, only modest anion vacancy concentrations are achieved, preventing access to manganese oxidation states below Mn(III).9-13 To investigate the structures and physical properties of mixed-valent mangaTel.: +44 1865 272623.
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(1) von Helmolt, R.; Wecker, J.; Holzapfel, B.; Schultz, L.; Sanwer, K. Phys. ReV. Lett. 1993, 71, 2331. (2) Chahara, K.; Ohno, T.; Kasao, M.; Kozono, Y. Appl. Phys. Lett. 1993, 63, 1990. (3) Schiffer, P.; Ramirez, A. P.; Bao, W.; Cheong, S.-W. Phys. ReV. Lett. 1995, 75, 3336. (4) Radaelli, P. G.; Marezio, M.; Hwang, H. Y.; Cheong, S.-W.; Batlogg, B. Phys. ReV. B 1996, 54, 8992. (5) Ibarra, M. R.; Algarabel, P. A.; Biasco, J.; Garcia, J. Phys. ReV. Lett. 1995, 75, 3541. (6) Rao, C. N. R.; Cheetham, A. K.; Mahesh, R. Chem. Mater. 1996, 8, 2421. (7) Raveau, B.; Maignan, A.; Martin, C.; Hervieu, M. Chem. Mater. 1998, 10, 2641. (8) Battle, P. D.; Rosseinsky, M. J. Curr. Opin. Solid State Mater. Sci. 1999, 4, 163. (9) Gillie, L. J.; Wright, A. J.; Hadermann, J.; Van Tendeloo, G.; Greaves, C. J. Solid State Chem. 2002, 167, 145. (10) Gillie, L. J.; Wright, A. J.; Hadermann, J.; Van Tendeloo, G.; Greaves, C. J. Solid State Chem. 2003, 175, 188. (11) Millburn, J. E.; Mitchell, J. F. Chem. Mater. 2001, 13, 1957.
nese(II/III) oxide phases, much larger numbers of oxide ion vacancies are required. Recent work has shown that binary metal hydrides act as powerful reducing agents at low temperatures in the solid state.14 These reagents allow the topotactic reduction of complex manganese oxides to phases with average manganese oxidation states below +3.15 This study focuses on the reduction chemistry of the n ) 2 Ruddlesden-Popper phase YSr2Mn2O7 in order to asses any parallels between the physical properties of Mn(II/III) oxides and the more studied Mn(III/IV) phases. The specific choice of YSr2Mn2O7 was motivated by the large difference in ionic radii between Sr(II) and Y(III) which leads to complete cation ordering between the two available A-cation sites in the A3B2O7 structure. This avoids synthetic problems associated with partial A-cation ordering and phase separation associated with other LnSr2Mn2O7 phases.16-18 Experimental Section Five gram samples of YSr2Mn2O7 were prepared via a citrate precursor method. Suitable quantities of Y2O3 (99.999%, dried at 900 °C), SrCO3 (99.994%), and MnO2 (99.999%) were dissolved in 100 mL of 1:1 6 M nitric acid and distilled water. Three mole equivalents of citric acid and 5 mL of analar ethylene glycol were added and the solution was heated with constant stirring. The gel (12) Mitchell, J. F.; Millburn, J. E.; Medarde, M.; Argyriou, D. N.; Jorgensen, J. D. J. Appl. Phys. 1999, 85, 4352. (13) Ruck, K.; Sgraja, M.; Krabbes, G.; Door, K.; Muller, K.-H.; Khristov, M. J. Alloys Compd. 2000, 306, 151. (14) Hayward, M. A.; Green, M. A.; Rosseinsky, M. J.; Sloan, J. J. Am. Chem. Soc. 1999, 121, 8843. (15) Hayward, M. A. Chem. Commun. 2004, 170. (16) Battle, P. D.; Green, M. A.; Laskey, N. S.; Kasmir, N.; Millburn, J. E.; Spring, L. E.; Sullivan, S. P.; Rosseinsky, M. J.; Vente, J. F. J. Mater. Chem. 1997, 7, 977. (17) Battle, P. D.; Millburn, J. E.; Rosseinsky, M. J.; Spring, L. E.; Vente, J. F. Chem. Mater. 1997, 9, 3143. (18) Battle, P. D.; Green, M. A.; Laskey, N. S.; Millburn, J. E.; Murphy, L.; Rosseinsky, M. J.; Sullivan, S. P.; Vente, J. F. Chem. Mater. 1997, 9, 552.
10.1021/cm051659c CCC: $33.50 © 2006 American Chemical Society Published on Web 12/24/2005
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Figure 1. Relative mass gain as a function of temperature for samples heated under flowing oxygen. Table 1. Refined Lattice Parameters and Measured Stoichiometries of YSr2Mn2O7-x Phases YSr2Mn2O7-x 0.5 1 1.5
a (Å)
c (Å)
3.80900(7) 20.2691(5) 3.81071(7) 20.5238(4) 3.62043(4) 22.3569(3)
volume (Å3)
TGA composition
294.07(1) 298.03(1) 293.04(1)
YSr2Mn2O6.54(3) YSr2Mn2O5.96(3) YSr2Mn2O5.43(3)
thus formed was subsequently ground into a fine powder, placed in an alumina crucible, and heated at 1 °C min-1 to 1000 °C in air. The resulting black powder was then pressed into 13 mm pellets at 5 tonnes pressure and heated in air at 1350 °C for 2 × 2 days with regrinding between heating periods. X-ray powder diffraction data from the resulting material could be indexed on the basis of the reported tetragonal cell of YSr2Mn2O7 (space group P42/mnm) and gave lattice parameters (a ) 5.4034(3) Å, c ) 19.879(1) Å) in good agreement with previously reported values (a ) 5.40388(5) Å, c ) 19.9050(2) Å).17 Reduction of YSr2Mn2O7 was performed using NaH (>95%). A 2:1 stoichiometric excess of NaH was thoroughly ground with samples of YSr2Mn2O7 in an argon-filled glovebox (O2 and H2O < 1 ppm). The mixture was then sealed in an evacuated Pyrex tube and heated for 3 × 3 days at 225 °C with regrinding between heating periods. Samples were then washed with 4 × 100 mL of methanol under nitrogen to remove sodium-containing phases before being dried under vacuum. Samples of intermediate composition were prepared by combining 2:1 and 1:2 molar ratios of as-prepared YSr2Mn2O7 and hydride-reduced YSr2Mn2O7-x and annealing them in evacuated Pyrex ampules at 400 °C for 3 days to equilibrate their oxygen contents. Samples of intermediate oxygen composition were prepared from reduced and unreduced samples from the same “batch” of starting material. Thermogravimetric data were collected using a Rheometric Scientific STA 1500 Thermal Analyzer. Neutron powder diffraction data were collected at station D2B at the ILL neutron source, Grenoble (λ ) 1.59 Å), utilizing an ILL “orange” cryostat for lowtemperature measurements. Zero-field-cooled and field-cooled direct-current magnetization measurements were collected from powdered samples using a Quantum Design MPMS SQUID magnetometer between 5 and 300 K. In addition, field-cooled magnetization isotherms were collected from samples at 5 and 300 K.
Results Thermogravimetric reoxidation data collected while heating samples under flowing oxygen are shown in Figure 1,
Figure 2. X-ray powder diffraction data collected from YSr2Mn2O7-x samples. Diffraction indices are marked for x ) 0.5 with broken lines to indicate the analogous reflections in more reduced samples. Impurities are marked with an asterisk.
with the calculated stoichiometries shown in Table 1. Complete oxidation back to YSr2Mn2O7 was confirmed by powder X-ray diffraction. The thermogravimetric data confirm that the three phases are of approximate composition YSr2Mn2O7-x (x ) 0.5, 1, 1.5). Structural Characterization. Neutron powder diffraction data collected from all three YSr2Mn2O7-x samples can be readily indexed on the basis of the body-centered tetragonal unit cells detailed in Table 1. Figure 2 shows a comparison of the X-ray powder diffraction patterns of the three samples allowing the 2θ shifts of specific diffraction reflections to be tracked as a function of reduction level. Close inspection of the data sets from the x ) 0.5 and x ) 1 samples revealed a number of additional diffraction reflections which could not be modeled by simple tetragonal cells. These extra features could however be accounted for by the presence of small quantities of impurity phases, namely, SrMnO3 for the x ) 1 sample and SrMnO3, Y2O3, and MnO for the x ) 0.5 sample. In contrast to the YSr2Mn2O7 starting material,17 electron diffraction data provide no evidence for a larger tetragonal unit cell corresponding to a doubling of the area of the ab plane for any of the reduced samples. Figure 3 shows representative electron diffraction patterns for x ) 1 and 1.5 samples consistent with the simple body-centered unit cells described in Table 1. Structural Refinement of YSr2Mn2O6.5 and YSr2Mn2O6. Models based on a simple n ) 2 Ruddlesden-Popper structure were refined against the neutron powder diffraction data collected from the x ) 0.5 and x ) 1 samples. The atomic positions, displacement parameters, and oxygen site occupancies of the majority YSr2Mn2O7-x phases were allowed to refine independently. Additional phases, corresponding to cubic SrMnO3 for the x ) 1 data set and SrMnO3, Y2O3, and MnO for the x ) 0.5 data set, were added to account for weak reflections observed in the diffraction patterns as described above. The structural refinements readily converged with good agreement between observed and calculated diffraction patterns, as shown in Figure 4, and with overall stoichiometries in good agreement with those obtained from thermogravimetric analysis. Full details
Topotactically Reduced YSr2Mn2O7-x (0 < x < 1.5)
Chem. Mater., Vol. 18, No. 2, 2006 323
Figure 3. Electron diffraction data collected from (a,b) YSr2Mn2O6 and (c,d) YSr2Mn2O5.5 confirming simple body-centered tetragonal structures for both phases.
of the refined models are given in Tables 2 and 3, with local metal coordinations and bond lengths in Figure 5. Structural Refinement of YSr2Mn2O5.5. Refinement of a simple model based on an n ) 2 Ruddlesden-Popper structure, against neutron powder diffraction data collected from YSr2Mn2O5.5, did not result in a good fit between observed and calculated patterns (χ2 ) 7.2). Calculated Fourier difference maps revealed significant scattering density at (1/2, 1/2, ∼0.095), which was interpreted as being due to a defect oxide ion site lying in the MnO2 layer. An oxide ion was inserted into the model at this position (O(4)), and the position, occupation, and displacement parameters of this site were refined. This resulted in a significant improvement to the fit (χ2 ) 4.1). The thermal displacement parameters of the 2b Sr(1) site and the 4e Y/Sr(1) site were observed to be significantly extended parallel to the z-axis, and both metal sites were seen to have nonphysically short contacts to the new defect O(4) oxygen site in this direction. To resolve this situation, a 4e Sr(2) site was added to the model at (1/2, 1/2, z) with an occupancy constrained to be the same as O(4), and the occupancy of the original 2b Sr(1) site lowered and constrained to retain the overall cation stoichiometry. An additional 4e Y/Sr(2) site was added to the model displaced along the z-axis from the original; the positions of the two Y/Sr sites were refined freely. The occupancy of the new Y/Sr(2) site was constrained to be equal to that of O(4), with the occupancy of the original Y/Sr(1) site constrained such that the combined occupancy of both sites corresponded to one fully occupied 4e site. The Y:Sr ratio of both sites was fixed at 1:1. Examination of the thermal displacement parameters of the 8g oxygen site showed that they were significantly elongated perpendicular to the Mn-O-Mn bond vector. This elongation was too large to be due to thermal motion and was interpreted as being a result of static displacement due
Figure 4. Observed calculated and difference plots obtained from the structural refinement of YSr2Mn2O7-x phases. Table 2. Crystallographic Data Obtained by Refinement against Neutron Powder Diffraction Data Collected from YSr2Mn2O6.54 at 300 Ka atom site Sr Y/Sr Mn O(1) O(2) O(3)
2b 4e 4e 8g 2a 4e
x
y
z
fraction
U11
U22
U33
0.5 0.5 0 0.5 0 0
0.5 0.5 0 0 0 0
0 0.1844(1) 0.0982(2) 0.1002(1) 0 0.2007(1)
1 0.5/0.5 1 0.916(7) 0.93(1) 0.96(1)
1.3(1) 1.62(6) 0.46(7) 0.9(1) 3.3(2) 5.7(2)
1.3(1) 1.62(6) 0.46(7) 0.9(1) 3.3(2) 5.7(2)
1.7(1) 1.17(9) 0.5(1 3.3(1) 1.9(2) 3.7(2)
space group: I4/mmm, a ) 3.80900(7) Å, c ) 20.2691(5) Å composition ) YSr2Mn2O6.51(6) χ2 ) 6.8, wRp ) 4.64%, Rp ) 3.46% a Refined phase fractions: YSr Mn O 2 2 6.54, 73.3(2)%; SrMnO3, 13.5(2)%; Y2O3, 2.7(1)%; MnO, 10.5(1)%.
to a disordered twisting of the Mn-O polyhedra around the z-axis to alleviate bond strain. To better model this structural feature, the 8g oxygen site was split into a 16n site and the position and thermal parameters of this oxide site were allowed to vary. This resulted in a slight reduction in the thermal displacement parameter perpendicular to the Mn-
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Table 3. Crystallographic Data Obtained by Refinement against Neutron Powder Diffraction Data Collected from YSr2Mn2O5.96 at 300 Ka atom
site
x
y
z
fraction
U11
U22
U33
Sr Y/Sr Mn O(1) O(2) O(3)
2b 4e 4e 8g 2a 4e
0.5 0.5 0 0.5 0 0
0.5 0.5 0 0 0 0
0 0.18560(8) 0.0984(1) 0.1001(1) 0 0.2006(1)
1 0.5/0.5 1 0.800(2) 0.862(6) 0.963(7)
2.24(7) 2.67(5) 1.41(7) 2.1(1) 4.5(1) 10.0(2)
2.24(7) 2.67(5) 1.41(7) 11.7(1) 4.5(1) 10.0(2)
2.2(1) 2.69(9) 1.7(1) 6.1(1) 2.6(2) 3.5(2)
space group: I4/mmm a ) 3.81071(7) Å, c ) 20.5238(4) Å composition ) YSr2Mn2O5.98(5) χ2 ) 5.20, wRp ) 5.49%, Rp ) 4.31% a
Refined phase fractions: YSr2Mn2O5.96 96.1(3)%; SrMnO3 3.9(3)%.
Figure 5. (a) Coordination polyhedra of metal ions in YSr2Mn2O6.54 (top) and YSr2Mn2O5.96 (bottom). Weighted bond valence sums are shown under each polyhedron. (b) The distribution of anion vacancies in YSr2Mn2O6.54 (left) and YSr2Mn2O5.96 (right). Fractional occupancies are given for the three crystallographically distinct oxide ions with the number of vacancies per unit cell given in square brackets. Table 4. Crystallographic Data Obtained by Refinement against Neutron Powder Diffraction Data Collected from YSr2Mn2O5.43 at 300 K atom
site
x
y
z
fraction
Uequiv
Sr(1) Sr(2) Y/Sr(1) Y/Sr(2) Mn O(1) O(2) O(3) O(4)
2b 4e 4e 4e 4e 16n 2a 4e 4e
0.5 0.5 0.5 0.5 0 0.5 0 0 0.5
0.5 0.5 0.5 0.5 0 0.0940(6) 0 0 0.5
0 0.0204(1) 0.1848(1) 0.1960(1) 0.0953(1) 0.1044(1) 0 0.20143(9) 0.0942(2)
0.452(3) 0.274(3) 0.137(3)/0.137(3) 0.363(3)/0.363(3) 1 0.246(3) 1 1 0.274(3)
0.0159(6) 0.0159(6) 0.0321(6) 0.0321(6) 0.0429 0.0542 0.0203 0.0649 0.0388
Mn O(1) O(2) O(3) O(4)
U11
U22
U33
0.054 (1) 0.016(1) 0.0259(8) 0.077(1) 0.055(3)
0.054(1) 0.093(8) 0.0259(8) 0.077(1) 0.055(3)
0.020(1) 0.052(2) 0.008(1) 0.039(1) 0.004(3)
U23 -0.005(2)
space group: I4/mmm a ) 3.62050(4) Å, c ) 22.3573(3) Å composition ) YSr2Mn2O5.51(3) χ2 ) 3.096, wRp ) 3.54%, Rp ) 3.37%
O-Mn bond vector, but the site remained significantly elongated. The occupation factors of the two axial oxide ions (O(2) and O(3)) were observed to have refined to values slightly greater than unity (∼1.02) and so were fixed at complete occupancy. Observed, calculated, and difference plots for the final fit to the neutron powder diffraction data are shown in Figure 4, with full details of the refined models given in Table 4, with local metal coordinations and bond lengths in Figure 6. Magnetic Characterization. Zero-field-cooled and field-cooled magnetization measurements collected from YSr2Mn2O7-x (0 < x < 1.5) samples in a measuring field of 1000 Oe are shown in Figure 7. It can be seen that samples with 0 e x e 1 have the same general form: in the
temperature range 100 < T/K < 300 they appear to exhibit Curie-Weiss type behavior, (1/χ vs T is linear (Figure 7)) with negative Weiss constants consistent with antiferromagnetic interactions. It should be noted however that the large Weiss constants obtained (Table 5) show a strict CurieWeiss description is invalid. Below 100 K, samples exhibit a positive deviation before the zero-field-cooled and fieldcooled data diverge at T ∼ 15 K. The total magnetization of samples diminishes with increasing x contrary to the expected increase in spin-only moment on reduction from an average manganese oxidation state of +3.5 to +2. This trend is observed most strikingly in the YSr2Mn2O5.5 sample, which exhibits a very weak susceptibility with an almost temperature-independent form. Magnetization-field isotherms collected from all samples are linear at 300 K but similar field-cooled isotherms collected at 5 K (Figure 8) display magnetic hysteresis for 0 e x e 1 and are slightly offset with respect to the origin, suggesting the possibility of spin glass behavior. The 5 K isotherm collected from YSr2Mn2O5.5 passes through the origin without displaying hysteresis and is linear in the (1T region before showing a negative deviation from linearity at higher fields. Taking all four data sets as a whole, it can be seen that the size of the observed magnetization and the degree of hysteresis decreases with increasing x. To further characterize the magnetic behavior of samples, low-temperature neutron powder diffraction data were collected. Diffraction data sets collected from YSr2Mn2O6.5 at 5 K show no additional diffraction features which could be assigned to the Ruddlesden-Popper majority phase. Additional diffraction features due to the onset of magnetic order in the MnO impurity were observed and were readily accounted for with an antiferromagnetically ordered model.
Topotactically Reduced YSr2Mn2O7-x (0 < x < 1.5)
Chem. Mater., Vol. 18, No. 2, 2006 325
Figure 6. Coordination polyhedra of the metal ions in YSr2Mn2O5.43. A superposition of both Sr and Y/Sr sites is shown (top) with the multiplicity and fractional occupancy of each site indicated. The separated coordination polyhedra of the distinct A-cation sites is shown (bottom). Bond lengths and [fractional occupancies] are shown for each metal oxygen contact. Weighted bond valence sums are shown under each polyhedron.
Figure 7. Zero field-cooled and field-cooled magnetization data collected from YSr2Mn2Mn2O7-x (0 < x < 1.5). Inset shows a plot of the reciprocal magnetization versus temperature. Data extracted from fits to the CurieWeiss law are given in Table 5. Table 5. Results of Fits to Magnetic Data Collected from YSr2Mn2O7-x (0 < x < 1.5) Samples YSr2Mn2O7 YSr2Mn2O6.5 YSr2Mn2O6
C
µ (µB)
µ expected (µB)
θ (K)
5.90 3.87 4.64
6.87 5.56 6.09
3.87/4.89 4.89 4.89/5.91
-135 -139 -400
There is therefore no evidence for long-range magnetic order in YSr2Mn2O6.5. The low-angle regions of neutron diffraction data collected from YSr2Mn2O6 (Figure 9) show no additional Bragg peaks on cooling to 5 K. There is however a significant shift in the diffuse scattering on cooling, leading to a local maximum centered around 2θ ∼ 18°, reminiscent of the behavior observed for YSr2Mn2O7.17 This suggests the onset of magnetic order in the sample, but with an ordered length too short to yield sharp diffraction reflections. Figure 10 shows a comparison of neutron powder diffraction data collected at 300 and 5 K from YSr2Mn2O5.5. A number of additional Bragg reflections are observable in the 5 K data set which can be indexed on the basis of an amag ) x2 × a, cmag ) c magnetic cell. A simple G-type antiferromagnetically ordered model, as shown in Figure 11, with
Figure 8. Magnetization-field isotherms collected from YSr2Mn2O7-x (0 < x < 1.5) at 5 K. Inset shows expended region around the origin.
spins aligned parallel to the z-axis, was refined against these data. An ordered moment of 4.61(5)µB per manganese was obtained, consistent with a high-spin S ) 5/2 Mn(II). Close inspection of the 300 K data set reveals weak diffuse scattering centered around 2θ ) 20°, similar to that observed at low temperature for YSr2Mn2O6. In combination with the low observed magnetic susceptibility of this phase, this suggests strong antiferromagnetic coupling is present at this temperature, which then orders into a three-dimensional antiferromagnetic lattice as the temperature is lowered. The relatively poor agreement between the observed and calculated diffraction patterns at 2θ ∼ 20° in the 5 K data set suggests this process is not fully complete at this temperature. Discussion Reduction of YSr2Mn2O7 to YSr2Mn2O7-x 0 e x e 1.5 results in the topotactic deintercalation of oxide ions from the host Ruddlesden-Popper structure. At low levels of reduction oxide ions are removed from all three crystallographic sites (Figure 5); however, the majority are removed from the 8g sites in the formerly MnO2 sheets (∼2/3 of the total) rather than the 2a or 4e sites in the AO layers (∼1/6 each of the total). This preference for removing oxide ions
326 Chem. Mater., Vol. 18, No. 2, 2006
Figure 9. Observed, calculated, and difference plots from the low-angle region of the fit to neutron powder diffraction data collected from YSr2Mn2O5.96 at 300 K (top) and 5 K (bottom) using the structural model described in Table 3. The dark line is added as a guide to the eye.
from the 8g site increases with further reduction. In the structure of YSr2Mn2O6 79% of the oxide ion vacancies occur on the 8g site compared with 13.6% and 7.3% for the 2a and 4e sites, respectively. On reduction to YSr2Mn2O5.5 all oxygen vacancies occur in the MnO2 sheets, leaving the AO sites fully occupied (Table 4). This anion vacancy distribution is similar to that observed in other reduced layered manganese oxides. Sr3Mn2O6,10 Sr2MnO3.5,9 and Ca2Mn2O3.519 all have oxide ion vacancies located exclusively in the MnO2 sheets. However, there are also counter examples; for instance, 80% of the anion vacancies in the structure of Nd0.8Sr2.2Mn2O6.42 are located in AO layers,11 suggesting a more complicated picture. The oxygen vacancy distribution and the associated changes in the lattice constants on reduction of YSr2Mn2O7 can be rationalized by considering the coordination requirements of the different metal cations. Low-temperature reduction leads to the deintercalation of the more mobile oxide ions, leaving the metal cations in place, with insufficient free energy to reorganize to the most thermodynamically stable arrangement. As oxide ions are removed from the structure, the oxidation state of the manganese ions falls such that the ideal geometry around the manganese ions changes, fewer and/or longer Mn-O bonds are required. Conversely, the coordination requirements of the A cations (Sr2+, Y3+) do not change; so the addition of oxide ion (19) Leonowicz, M. E.; R., P. K.; Longo, J. M. J. Solid State Chem. 1985, 59, 71.
Hayward
Figure 10. (Top) Observed, calculated, and difference plots from the lowangle region of the fit to neutron powder diffraction data collected from YSr2Mn2O5.43 at 300 K using the structural model described in Table 4. The dark line is added as a guide to the eye. (Bottom) The observed, calculated, and difference plots from the low-angle region of the fit to neutron powder diffraction data collected from YSr2Mn2O5.43 at 5 K including the antiferromagnetically ordered model shown in Figure 11.
Figure 11. Antiferromagnetically ordered model refined from neutron powder diffraction data collected from YSr2Mn2O5.43 at 5 K. The refined ordered moment of 4.61(5)µB per manganese center is consistent with S ) 5/ Mn(II). 2
vacancies, which lowers their average coordination numbers, means the remaining A-O bond lengths need to contract to maintain sensible coordination polyhedra. These opposing forces expanding and contracting the lattice lead to significant strain in both sets of cation-oxygen bonds. Close inspection of the metal oxygen bond lengths in the x ) 0.5 and x ) 1 structures reveals the 2a O and 4e O sites form short bonds to the A cations (Figure 5). Thus, on reduction oxide ions can be readily removed from the 8g sites without removing short A-cation oxygen interactions
Topotactically Reduced YSr2Mn2O7-x (0 < x < 1.5)
which would have a large energetic penalty. To compensate for the lower coordination numbers, the 2a and 4e oxide ions are drawn toward the A-cation positions, contracting the average A-O bond length and leading to a reduction in the a lattice parameter on reduction to YSr2Mn2O6.5. This contraction of the ab plane is opposed by the expected elongation of the equatorial Mn-O bonds on reduction. This competition between the bond geometry requirements of the metal ions leads to the observed contraction of the ab plane on reduction to x ) 0.5 and then a slight expansion at x ) 1 (Table 1). Close inspection of both reduced structures reveals an elongation of the thermal displacement parameters of the 8g oxide ion site perpendicular to the Mn-O-Mn bond vector, consistent with static displacement. Such a displacement suggests a disordered twisting of the Mn-O polyhedra around a vector parallel to the z-axis, which would act to relieve the Mn-O bond strain. In addition to the contraction in the ab plane, there is considerable expansion of the c-lattice parameter on reduction. This lengthening is required to avoid short anion-anion contacts on contraction of the structure in the ab plane. The calculated bond valence sums20 for the metal cations in YSr2Mn2O6 (Figure 5) show the structure is under considerable strain: the Mn sites under compression (sum is too large) and the A-cation sites under extension (sum is too small). Therefore, it is not surprising that further reduction leads to significant changes in the structure to relieve this strain. In contrast to the structures of YSr2Mn2O6.5 and YSr2Mn2O6, reduction of YSr2Mn2O7 to YSr2Mn2O5.5 results in the removal of oxide ions exclusively from the MnO2 sheets, leaving the AO layers fully occupied. In addition, there is significant structural disorder in both the oxygen and metal positions. This disorder is accompanied by large changes in the lattice parameters on reduction: a contracts by 5.8% compared to 0.9% for x ) 1 and c expands by 12.4% compared with 3.2% for x ) 1 (Table 1). These changes in structure can be seen as continuing the trend observed in the x ) 0.5 and x ) 1 phases. With localization of the anion vacancies in the MnO2-x sheets, none of the short A-O bonds are lost on reduction and the observed contraction in the ab plane of the structure significantly shortens these bonds further. The disorder present in the MnO2-x layers reflects the need to relieve the bond strain in the contracted equatorial Mn-O bonds and can be thought of as an extension of the elongated oxygen thermal displacement parameters seen in the less reduced samples. It should be noted that this significant structural disorder leads to poor location of some atoms in the structure (the equatorial oxide ions for example) and an underestimation of some metal oxygen bond lengths. The presence of defect O(4) site oxide ions in this layer can be rationalized as a response to the coordination requirements of the A-cations as this site forms short contacts to these metal cations. The resulting structure contains average metal coordination numbers of 5.06, 8.48, and 7.25 for Mn(II), Sr(II), and Y(III), respectively. The average bond valence sums for these metal coordination (20) Brese, N. E.; O’Keeffe, M. Acta Crystallogr., Sect. B: Struct. Sci. 1991, B47, 192.
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polyhedra (Figure 6) show that the large bond strain present in YSr2Mn2O6 has been reduced in YSr2Mn2O5.5. Temperature-dependent magnetization data collected from YSr2Mn2O7-x samples (Figure 7) are consistent with strong antiferromagnetic coupling between spins in these phases over the whole temperature range studied. The lack of any evidence of magnetic order in the Mn(III) phase YSr2Mn2O6.5 is somewhat surprising as analogous layered Mn(III) oxides with anion vacancies exhibit long-range antiferromagnetically ordered structures (e.g., Sr3Mn2O6 10). One structural contrast between YSr2Mn2O6.5 and Sr3Mn2O6 is the crystallographic order in the anion vacancies in the later phase. This suggests the disorder in the anion lattice of YSr2Mn2O6.5 quenches any long-range magnetic order in this phase. The broad feature in the neutron powder diffraction data collected from YSr2Mn2O6 at 5 K (Figure 9) is strongly reminiscent of that observed in the parent phase YSr2Mn2O7.17 It suggests that short-range (antiferromagnetic) order exists in these phases, which appears to be frustrated by the disordered array of Mn(II/III) centers in YSr2Mn2O6, in direct analogy to the Mn(III/IV) array in YSr2Mn2O7, although it should be noted that the disorder in the anion lattice of YSr2Mn2O6 will also have some influence. The low value and weak temperature dependence of the magnetic susceptibility of YSr2Mn2O5.5 suggests strong antiferromagnetic interactions exist between the S ) 5/2 Mn(II) centers in this phase. Indeed, the broad feature in the 300 K neutron powder diffraction data collected from this phase (Figure 10) provides evidence for short-range (antiferromagnetic) order at this temperature. The lack of any evidence for a supercell, in electron diffraction data (Figure 3), make a structural origin for this diffuse scattering unlikely. On cooling to 5 K, long-range antiferromagnetic order develops (Figures 10 and 11) in contrast to the other phases in this series, indicating that the magnetic coupling between Mn(II) centers is sufficiently strong to overcome the effects of structural disorder in this phase. Conclusion Topotactic reduction of the n ) 2 Ruddlesden-Popper phase YSr2Mn2O7 with NaH yields a Mn(II) phase YSr2Mn2O5.5. With equilibration of the oxygen contents of this reduced phase with the fully stoichiometric starting material, YSr2Mn2O7, phases of intermediate stoichiometry can be prepared. The anion vacancy distributions in these reduced materials can be rationalized by considering the competing coordination requirements of all three metal cations present. Strong antiferromagnetic coupling is present in all members of the series; however, long-range magnetic order is suppressed by structural and charge disorder in all but the most reduced phase YSr2Mn2O5.5. Acknowledgment. I thank T. Hansen and E. Suard for assistance collecting the neutron powder diffraction data and the Royal Society for funding this work. CM051659C
