J O U R N A L O F C H E M I S T R Y Materials A variable temperature neutron diVraction study of the layered perovskite YBaMn2O5 Judith A. McAllister and J. Paul Attfield*† Department of Chemistry, University of Cambridge, L ensfield Road, Cambridge, UK, CB2 1EW, and the Interdisciplinary Research Centre in Superconductivity,Madingley Road, Cambridge, UK, CB3 0HE A variable temperature neutron diVraction study has been carried out on the layered perovskite YBaMn2O5 between 100 and 300 K.A broad peak indexed as (1/2, 1/2, 1) on the nuclear cell is consistent with short range MnII/MnIII valence ordering. The magnetic structure below the ferrimagnetic ordering temperature of 167 K has been determined and is in agreement with a previously proposed model. Changes in lattice parameters, bond lengths and angles show evidence of an exchange striction at TC.Introduction Perovskite type Ln1-xAxMnO3 phases (Ln=trivalent lanthanide, A=Ca, Sr, Ba) exhibit giant magnetoresistances (GMR).1 The features important to this phenomenon are the threedimensional MnMOMMn network and Mn of mixed valency with the mean Mn oxidation state between +3 and +4, i.e.hole-doped MnIII. This gives rise to ‘double-exchange’,2 a transfer of spin-polarised electrons from Mn3+ to Mn4+, below the Curie temperature. A new perovskite-related Mn oxide, YBaMn2O5, was recently reported.3 The structure has a layered arrangement with oxygen vacancies in the yttrium plane and the manganese –oxygen network consists of double layers of identical MnO5 square pyramids linked through their apical oxygens.This structure is similar to those of YBaCuFeO5+d4 and Fig. 1 Proposed3 valence and magnetic ordering model for YBaMn2O5 YBaCo2O5+d.5 Magnetic measurements on YBaMn2O5 showing alternating MnII with S=5/2 (up) spins and MnIII with S=2 showed a ferrimagnetic ordering temperature of 167 K with (down) spins the measured ferromagnetic component of about 0.5 mB, this is consistent with a magnetic and valence (charge) order of MnII and MnIII, as shown in Fig. 1. Each MnII is linked via were assigned to Ba2SiO4 resulting from reaction with the oxygen to five MnIII ions and vice versa which if extended over silica tube, due to a slight rupture in the Au foil. long distances would give a Ó2a×Ó2a×c superstructure. In Constant wavelength neutron powder diVraction data were this study we have investigated the magnetic and crystal collected at 100, 140, 190 and 300 K on instrument D2B at structures and the extent of superstructure formation in the Institut Laue-Langevin, Grenoble, France. The wavelength YBaMn2O5 using variable temperature powder neutron selected for this experiment was from the (335) Bragg reflection diVraction.of the Ge crystals, with l=1.594 A ° . Rietveld refinement6 was carried out using the General Structure Analysis System (GSAS).7 Experimental A stoichiometric mixture of YMnO3, BaO and MnO was used Results in the preparation of a 10 g sample of YBaMn2O5 for neutron diVraction. YMnO3 was prepared by heating a mixture of The starting model for YBaMn2O5 in this refinement was Y2O3 (Aldrich 99.99%) and MnO2 (Aldrich 99.99%) at 1350 °C taken from the previous X-ray study,3 the structural parameters for 8 h.BaO and MnO were prepared by heating BaO2 are given in Table 1. The impurity phases were fitted using (Aldrich 99.99%) and MnO2 under flowing H2–N2 at 900 °C models were taken from Yakel et al.,8 Grosse et al.,9 and Sasaki for 8 h. These precursors were then ground together, pressed et al.10 for YMnO3, MnO and Ba2SiO4 respectively.A good into 13 mm pellets, wrapped in gold foil and sealed in an fit to the data was obtained using a pseudo-Voigt peak shape evacuated silica tube which was heated at 1000 °C for 4 days. function and a cosine Fourier series background function, This procedure was repeated twice. After each heating the tube despite secondary phases being present. The refined phase was quenched into air to avoid possible decomposition of proportions by mass were YBaMn2O5 32.1%, YMnO3 36.3%, YBaMn2O5 on cooling. Despite many attempts with varying Ba2SiO4 24.9% and MnO 6.7%.The refined YBaMn2O5 experimental conditions it has not been possible to prepare a structural model is in good agreement with that of the previous phase pure sample of this phase.YMnO3 and MnO were X-ray diVraction study.3 Fig. 2 shows the calculated, observed observed in the X-ray powder diVraction pattern and and diVerence plots for the 300 K data. The oxygen content additional peaks subsequently observed in the neutron profile was confirmed as being stoichiometric. Results of the refinement are given in Table 1 and Fig. 4 and 5 ( later). Both the 300 and 190 K data showed a broad peak at 2h= †E-mail: jpa14@cam.ac.uk J. Mater. Chem., 8(5), 1291–1294 1291Table 1 Refinement results for YBaMn2O5 a with e.s.d.s in parentheses T/K 100 140 190 300 x2 3.2 3.9 2.9 2.5 RWP(%) 6.9 7.6 6.5 6.0 cell parameters a/A ° 3.9114(2) 3.9121(2) 3.9146(2) 3.9186(2) c/A ° 7.6241(1) 7.6287(6) 7.6351(6) 7.6540(5) atomic parameters Uiso/A° 2 0.0009(6) 0.0067(5) 0.0017(5) 0.0041(6) Mn z 0.234(1) 0.233(1) 0.236(1) 0.236(1) m/mB 1.78(2) 1.17(6) — — O(1) z 0.1868(5) 0.1870(5) 0.1858(4) 0.1870(4) distances BaMO(1)×8 3.087(3) 3.087(3) 3.096(3) 3.095(3) BaMO(2)×4 2.7658(1) 2.7662(1) 2.7681(1) 2.7709(1) YMO(1)×8 2.419(2) 2.421(2) 2.418(2) 2.426(2) MnMO(1)×4 1.989(7) 1.988(2) 1.995(2) 1.995(2) MnMO(2)×1 2.026(9) 2.034(10) 2.012(8) 2.020(8) Mn,Mn×1 3.57(2) 3.56(2) 3.61(2) 3.61(2) angles O(1)MMnMO(1)×4 88.10(9) 88.19(10) 87.85(9) 87.97(8) O(1)MMnMO(1)×2 159.0(5) 159.5(6) 157.6(5) 158.3(5) O(1)MMnMO(2)×4 100.5(3) 100.2(3) 101.2(2) 100.9(2) MnMO(1)MMn 159.0(2) 159.5(6) 157.6(5) 158.3(5) MnMO(2)MMn 180 180 180 180 aSpace group P4/mmm, Y 1(c) 1/2,1/2,0, Ba 1(d) 1/2,1/2,1/2, Mn 2(g) 0,0,z, O(1) 4(i) 1/2,0,z, O(2) 1(b) 0,0,1/2.Fig. 2 Calculated, observed and diVerence neutron diVraction plots for 300 K. The reflection markers from bottom to top represent YBaMn2O5, YMnO3, Ba2SiO4 and MnO respectively. 20.6°, which is not attributable to any impurity phase and can be indexed as (1/21/2 1) on the nuclear cell. The extent of the ordering was calculated using the Scherrer equation (1): t= 0.9l Ó(C2 pos-C2 std)cosh (1) where t is the domain size, l is the neutron wavelength, Cpos is the full width at half maximum (FWHM) of the peak, Cstd is the FWHM of a standard peak from the diVraction pattern (which is assumed to be instrumentally resolution-limited) and h is the peak position.This gave t#50 A ° . Fig. 3 DiVracted neutron intensity at the 1/21/2 1 position (2h=20.5°) Below 167 K a magnetic peak appears at the (1/21/21) for YBaMn2O5 as a function of temperature.The Rietveld fits to the position showing that the magnetic cell parameters are background and magnetic peaks are shown. The intensity scale for Ó2a×Ó2a×c, where a and c are of those of the nuclear model. the 140, 190 and 300 K data is twice that for the 100 K data.In the 140 and 100 K data the sharp peak due to long range magnetic order is superimposed on the broad valence order peak as shown in Fig. 3. The magnetic model in the refinement Discussion is as shown in Fig. 1, but with the Mn moments constrained to have equal magnitude, as attempts to refine the Mn2+ and The basic structure of YBaMn2O5 and the proposed valence and magnetic order models are confirmed by this variable Mn3+ moments independently were unsuccessful. 1292 J. Mater. Chem., 8(5), 1291–1294temperature neutron diVraction study. The oxygen content of YBaMn2O5 was confirmed by the refinement to be five oxygen atoms per unit cell. The presence of a broad peak above TC and a sharp peak below, indexing on a Ó2a×Ó2a×c supercell, give evidence for long range magnetic order below TC but a valence ordering that extends only over ca. 50 A ° domains. Fig. 4 shows the variation of YBaMn2O5 lattice parameters a and c with temperature. Both parameters show a decrease with decreasing temperature which is significantly steeper about TC. Fig. 5(a) shows the variation of MnMO(1) and MnMO(2) bond lengths with temperature.MnMO(1) decreases below TC whereas MnMO(2) increases. These changes in bond lengths arise due to a change in the MnMO(1)MMn bond angle which shows an anomalous Fig. 6 Schematic one-dimensional representation of the valence and magnetic ordering of Mn2+ and Mn3+ ions in YBaMn2O5 through oxygen atoms (not shown). The broken lines represent domain walls due to disruption of the valence ordering; (a) shows the antiferromagnetic ordering of adjacent Mn2+ (5 mB) and Mn3+ (4 mB) moments, (b) shows the same situation with the moments separated into the average 4.5 mB and diVerence (±0.5 mB) components.The former order antiferromagnetically and are unaVected by the domain walls, whereas the latter order ferromagnetically and the direction of the magnetisation is switched at the domain boundary. Fig. 4 Variation with temperature of a and c parameters of YBaMn2O5 increase as the temperature decreases below TC [Fig. 5(b)]. These changes evidence an exchange striction at TC. The changes in lattice parameters, bond lengths and angles promote superexchange via the MnMO(1)MMn bridges mainly by increasing the MnMO(1)MMn bond angle towards the most favourable value of 180 °.Fig. 6 shows how the valence ordering can be disrupted without aVecting the long range antiferromagnetic ordering. The ideal average Mn moment is 4.5 mB and for the Mn2+ moment there is an additional 0.5 mB in the same direction as the average, but for the Mn3+ ion the extra 0.5 mB opposes the average. Antiferromagnetic MnMOMMn interactions lead to ferromagnetic ordering of the 0.5 mB components with unit cell a×a×c, but antiferromagnetic ordering of the average 4.5 mB with unit cell Ó2a×Ó2a×c.If the valence ordering is disrupted, leading to two adjacent Mn3+ or Mn2+ ions, Fig. 6 shows that the direction of the ferromagnetic component is reversed but the antiferromagnetic order is ideally unaVected. However, the average antiferromagnetic moment at 100 K is only 1.8 mB, showing that valence disorder also causes some disruption of this order.The ferromagnetic component should give rise to weak broad magnetic reflections at the hkl positions of the structural cell, and so are not seen beneath the structural Bragg intensities. In conclusion, YBaMn2O5 shows long range structural order (periodicity a×a×c) and short range valence order with a Ó2a×Ó2a×c superstructure up to at least 300 K.Below the ferrimagnetic ordering temperature of 167 K, these structural features respectively give rise to a long range antiferromagnetically ordered component (periodicity Ó2a×Ó2a×c) and a short range ferromagnetic component that is commensurate with the basic cell. Fig. 5 Variation with temperature of (a) MnMO(1) and MnMO(2) bond lengths and (b) MnMO(1)MMn bond angle in YBaMn2O5 We thank EPSRC for financial support for J.A.M.and pro- J. Mater. Chem., 8(5), 1291–1294 12935 W. Zhou, Adv.Mater., 1993, 5, 735. vision of neutron facilities at the ILL, and Dr Paolo Radaelli 6 H. M. Rietveld, J. Appl. Crystallogr., 1969, 35, 1. for technical assistance with data collection. 7 A. C. Larson and R. B. Von Dreele, Los Alamos National Laboratory Rep. No. LAUR-86-748, 1987. 8 H. L. Yakel, W. C. Koehler, E. F. Bertaut and E. F. 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