Spin, Charge, and Orbital Ordering in the B-Site Diluted Manganates

Philip P. C. Frampton, Peter D. Battle, and Clemens Ritter. Inorganic ... P. D. Battle , S. J. Blundell , J. B. Claridge , A. I. Coldea , E. J. Cussen...
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Chem. Mater. 2002, 14, 425-434

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Spin, Charge, and Orbital Ordering in the B-Site Diluted Manganates La2-xSrxGaMnO6 P. D. Battle,*,† S. J. Blundell,§ J. B. Claridge,‡ A. I. Coldea,§ E. J. Cussen,‡ L. D. Noailles,‡ M. J. Rosseinsky,*,‡ J. Singleton,§ and J. Sloan† Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K., Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, U.K., and Clarendon Laboratory, Department of Physics, University of Oxford, OX1 3PU, U.K. Received August 9, 2001

Oxidation of the ferromagnetic B-site diluted manganese(III) perovskite La2GaMnO6 is investigated by synthesis of the La2-xSrxGaMnO6 series. At the x ) 0.3 composition, which corresponds to diamagnetic element substitution at the B-site of metallic La0.7Sr0.3MnO3, the MnIII and MnIV valences order in real space at low temperature with an unusual lamellar motif. Orbital ordering at the MnIII centers in this array maintains ferromagnetism and enhances the static coherent Jahn-Teller distortion over that found for the pure MnIII endmember, facilitating eg electron hopping in the insulating state. Further oxidation to x ) 0.5 completely suppresses the charge and orbital ordering, leading to glassy rather than long-range ordered magnetism.

Introduction The strong electron-phonon coupling at the MnIII sites in the manganate perovskites plays an important role in charge and orbital ordering in MnIII/IV mixed valence compositions,1 which have been linked to the much-studied colossal magnetoresistance2 (CMR) effects. The orbital ordering3,4 produced by static JahnTeller distortions at the MnIII centers has a direct influence on the magnetic structures, for example stabilizing the unusual A-type magnetic order of LaMnO3. We have recently demonstrated that the substitution of diamagnetic B-site cations can produce unexpected magnetic and structural consequences. The RhIII low-spin d6 cation induces ferromagnetic correlations in La0.8Sr1.2Rh0.4Mn0.6O4, the first time this has been observed in an oxide with the K2NiF4 structure,5 while the d10 GaIII ion suppresses the coherent JahnTeller distortion at the MnIII sites in La2GaMnO6,6 surprisingly inducing ferromagnetism7 in place of the antiferromagnetism of LaMnO3. The observation of realspace valence ordering across the La1-xCaxMnO38 and Ln1-xSrxMnO3 (Ln ) Nd or Pr)9 series prompted us to †

Inorganic Chemistry Laboratory, University of Oxford. Department of Chemistry, University of Liverpool. Clarendon Laboratory, University of Oxford. (1) Rao, C. N. R.; Arulraj, A.; Cheetham, A. K.; Raveau, B. J. Phys. Condens. Matter 2000, 12, 83-106. (2) Ramirez, A. P. J. Phys. Condens. Matter. 1997, 9, 8171. (3) Goodenough, J. B. Magnetism and the Chemical Bond; Interscience: New York, 1963. (4) Kanamori, J. J. Appl. Phys. 1960, 31, 14S. (5) Battle, P. D.; Burley, J. C.; Cussen, E. J.; Hardy, G. C.; Hayward, M. A.; Noailles, L. D.; Rosseinsky, M. J. Chem. Commun. 1999, 1977. (6) Cussen, E. J.; Rosseinsky, M. J.; Battle, P. D.; Burley, J. C.; Spring, L. E.; Blundell, S. J.; Coldea, A. I.; Singleton, J. J. Am. Chem. Soc. 2001, 123, 1111. (7) Goodenough, J. B.; Wold, A.; Arnott, R. J.; Menyuk, N. Phys. Rev. 1961, 124, 373. (8) Wollan, E. O.; Koehler, W. C. Phys. Rev. 1955, 100, 545. ‡ §

investigate the influence of oxidation of the 50% B-site diluted MnIII array in La2GaMnO6. The introduction of MnIV centers will alter the local influence of incoherent Jahn-Teller distortions in the parent material on magnetic order while introducing both charge carriers and possible sources of charge and associated orbital ordering. This is particularly interesting given the absence of coherent long-range Jahn-Teller distortion in the parent material. The introduction of dopants at the B-site has been shown to introduce spin or cluster glass behavior without suppressing CMR in perovskites10 and to suppress both charge and orbital ordering in valenceordered manganates.11 Ru doping, and the associated Ru-O-Ru interactions, can introduce ferromagnetism and metallic behavior in charge-ordered12 and pure MnIV antiferromagnetic manganates.13 Here we present the detailed characterization of the average structure and electronic properties in B-site doped La2-xSrxGaMnO6 manganates with systematically varied mean B-site charge. This reveals new aspects of the interplay between charge, spin and orbital ordering in perovskites. Although charge ordering is not observed in undiluted La0.7Sr0.3MnO3 due to the width of the eg band, B-site doping produces the coexistence of unusual lamellar charge ordering with ferromag(9) Jirak, Z.; Krupicka, S.; Sinsa, Z.; Dlouha, M.; Vratislav, S. J. Magn. Magn. Mater. 1985, 53, 153. (10) de Teresa, J. M.; Ibarra, M. R.; Garcia, J.; Blasco, J.; Ritter, C.; Algarabel, P. A.; Marquina, C.; Moral, A. d. Phys. Rev. Lett. 1996, 76, 3392-3395. (11) Mahendiran, R.; Maignan, A.; Hervieu, M.; Martin, C.; Raveau, B. Appl. Phys. Lett. 2000, 77, 1517-1519. (12) Vanitha, P. V.; Arulraj, A.; Raju, A. R.; Rao, C. N. R. C. R. Acad. Sci. 1999, 2, 595. (13) Maignan, A.; Michel, C.; Hervieu, M.; Raveau, B. Solid State Commun. 1997, 101, 227-281.

10.1021/cm0103848 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001

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Table 1. Time (hours) at Temperature between Grinding and Pelletizing in the Synthesis of La2-xSrxGaMnO6: Q Indicates the Sample Was Removed from the Furnace at the Stated Temperature into the Ambient Laboratory Temperature or into Liquid Nitrogen (as indicated by lN2) T/°C 800 1000

1200

0.1 0.3a

24 12

24 12

12:24Q 34

0.5b 0.7 1

24 24 96

24 24

24:12 24 × 3

1250

5Q lN2 24 144

1300

120

1400

1450

0.5 then cooled at 0.4 °C/min

12

18:24Q lN2 62Q lN2

Final stage at 1400 °C after 1450° firing. b The x ) 0.5 sample was prepared from a citrate gel decomposed at 700 °C then fired at the temperatures listed. a

netism, two ground states normally mutually exclusive in manganates. These structural observations are important in rationalizing the dramatic effect of small substitutions at the B-site in the manganates.14

Figure 1. X-ray powder diffraction data showing the evolution from the Pbnm (x ) 0.1, lower solid line) to the I2/c (x ) 0.3 upper line) structures in La2-xSrxGaMnO6. (i) the loss of the 111 reflection (indicated by the arrow in the upper data set) in I2/c due to the onset of body-centring (ii) the splitting of the 112 and (iii) the 202 reflections. All indexing is given in the Pbnm cell.

Experimental Section Samples were prepared by solid-state reaction of La2O3 (Aldrich 99.999%, dried at 1000 °C for at least 12 h before weighing), Ga2O3 (Aldrich 99.99+%), SrCO3 (Alfa 99.99%), and MnO2 (Alfa 99.999%). Details of the precise preparative conditions for each sample are given in Table 1. The manganese oxidation states were determined by iodometric titration against sodium thiosulfate in degassed dilute HCl solution under flowing nitrogen. X-ray powder diffraction data were recorded using Siemens D5000 and Stoe Stadi-P diffractometers in Bragg-Brentano geometry with Cu KR1 radiation. Neutron powder diffraction data were measured at 300 and 1.7 K on the HRPD and GEM time-of-flight and D2B (λ ) 1.5942 Å) diffractometers at the Rutherford-Appleton Laboratory and the Institut Laue-Langevin, respectively. Rietveld analysis was performed with the GSAS15 software. Electron diffraction data were recorded on samples prepared by suspending the finely ground powder in hexane and subsequent evaporation onto lacey carbon-coated copper grids. Selected area electron diffraction (SAED) patterns were obtained by use of a double-tilting goniometer stage ((30°) to tilt the specimen in a JEOL 2000 FX transmission electron microscope (TEM). d.c. magnetization data were recorded on a Quantum Design MPMS-5T SQUID magnetometer, with samples contained in gelatine capsules. Muon spin relaxation and magnetotransport data on these materials are reported separately: for the purposes of the present paper, it is important to note that none of the materials are metallic.16

Results X-ray powder diffraction revealed that phase pure La2-xSrxGaMnO6 could be formed over the range 0 e x < 1. Substitution of strontium beyond x ) 0.1 requires significantly higher temperatures than those needed for La2GaMnO6. Quenching into liquid nitrogen was employed to avoid the formation of an impurity (LaSrGa3O7). At the x ) 0.3 composition, a final anneal with a slower cooling rate was required to obtain similar crystallinity to the other samples in the series. This did not affect the magnetic behavior or result in changes in the (14) Raveau, B.; Hervieu, M.; Maignan, A.; Martin, C. J. Mater. Chem. 2001, 11, 29-36. (15) Larson, A. C.; von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratories: Los Alamos, 1990. (16) Coldea, A. I.; Noailles, L. D.; Blundell, S. J.; Marshall, I. M.; Steer, C.; Battle, P. D.; Rosseinsky, M. J. Phys. Rev. B. Submitted for publication.

Figure 2. Electron diffraction pattern from the [100] zone of La1.5Sr0.5GaMnO6, revealing the weak 011 reflection at 4.5 Å, which is absent in R3 h c.

electron diffraction pattern. Iodometric titrations showed all compounds to be stoichiometric within error, yielding compositions of La1.7Sr0.3GaMnO5.98(1), La1.7Sr0.3GaMnO5.99(1), and La1.3Sr0.7GaMnO6.02(1) and the manganese oxidation state is henceforth discussed solely in terms of the strontium concentration. The structural characterization of the materials required a combination of X-ray, neutron and electron diffraction. Inspection and indexing of the X-ray data indicated that the Pbnm GdFeO3-type structure of La2GaMnO6 was not maintained beyond x ) 0.1, due to changes in the absence conditions and reflection splittings. (Figure 1) Satisfactory Rietveld analyses of both X-ray and neutron data could be obtained in both R3 hc (corresponding to an a-a-a- octahedral tilt scheme17 with equal rotations about all three Mn-Mn directions) and I2/c (a-b-b- tilting), with slightly but significantly improved agreement indices in the monoclinic space group. The choice between the two space groups was made using electron diffraction evidence (Figure 2) which could only be indexed in I2/c due to the presence of weak reflections absent in R3 h c. The monoclinic I2/c cell was therefore used in all refinements of diffraction (17) Glazer, A. M. Acta Crystallogr. 1972, B28, 3384.

Diluted Manganates La2-xSrxGaMnO6

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Table 2. (a) Positional and Displacement Parameters in the La2-xSrxGaMnO6 Series (space group I2/c (number 15)) Derived from Rietveld Refinement of Neutron Powder Diffraction Data as a Function of Temperature and (b) Positional and Displacement Parameters of La1.7Sr0.3GaMnO6 at 1.7 K (Space Group P21/a (Number 14)) Derived from Rietveld Refinement of Neutron Powder Diffraction Data (D2b): The Magnetic Moment per Mn Cation at the B-Site Is Quoteda 0.3, RT a/Å b/Å c/Å β/° V/Å3 Mn/Ga Uiso/Å2 La/Sr y Uiso/Å2 O1 y Uiso (Å2) O2 x y z Uiso/Å2

0.5, 1.7 K

0.5, RT

0.7, RT

(a) La2-xSrxGaMnO6 Seriesb 5.4749(3) 5.4518(1) 5.46529(9) 5.5250(3) 5.4932(1) 5.50478(8) 7.7756(5) 7.7484(2) 7.74997(14) 90.673(3) 90.674(1) 90.528(1) 235.768(3) 232.031(4) 233.149(4)

5.46288(9) 5.49017(9) 7.73790(9) 90.404(1) 232.071(2)

0.0033(2)

0.0039(6)

0.0095(5)

0.0086(6)

0.5014(7) 0.0055(1)

0.5007(7) 0.0009(2)

0.5009(6) 0.0053(2)

0.5012(9) 0.0060(2)

0.9446(6) 0.0046(4)

0.9580(7) 0.0081(9)

0.0490(7) 0.0057(8)

0.958(1) 0.007(1)

0.2311(8) 0.7301(6) 0.0233(3) 0.0119(5)

0.2644(7) 0.766(1) -0.0218(3) 0.0144(7)

0.2692(5) 0.2778(6) 0.7689(7) 0.7766(5) -0.0298(3) -0.0222(3) 0.01286 0.0039(4)

(b) La1.7Sr0.3GaMnO6 at 1.7 Kc a/Å 5.47637(5) b/Å 5.52179(5) c/Å 7.75175(8) β/° 90.045(2) V/Å3 234.408(4) Mn/Ga Uiso (Å2) 0.0004(5) µB/ Mn(1) 2.206(53) La/Sr x -0.0138(3) y 0.4967(2) z 0.2509(5) Uiso /Å2 0.0015(2) O1 x 0.0049(4) y 0.9372(2) x 0.2459(7) Uiso /Å2 0.0042(4) O2 x 0.2669(9) y 0.762(1) z -0.0350(6) Uiso /Å2 0.008(1) O3 x 0.7338(8) y 0.231(1) z 0.4692(6) Uiso /Å2 0.004(1) a Agreement indices: x ) 0.3, combined refinement of neutron (GEM, four detector banks) and X-ray data Rwp ) 5.14%, Rp ) 7.47%, and χ2 ) 1.012 for 61 variables. x ) 0.5 room temperature, combined refinement of neutron (D2B) and X-ray data Rwp ) 5.89%, Rp ) 5.44%, and χ2 ) 1.735 for 42 variables. x ) 0.5 1.7 K, refinement of neutron data (D2B) Rwp ) 5.53%, Rp ) 4.16%, R(F2) ) 2.67%, and χ2 ) 2.094 for 29 variables. x ) 0.7, combined refinement of neutron (HRPD) and X-ray data Rwp ) 9.09%, Rp ) 7.83%, and χ2 ) 1.425 for 30 variables. b The atomic positions are Mn/Ga, 4a 1 h (0,0,0) La/Sr; O1, 4e 2 (0,y,1/4); and O2: 8f 1 (x,y,z). Mn and Ga occupy the 4a site in a 1:1 ratio. La and Sr occupy the 4e site in a ratio determined by the composition. c The atomic positions are Mn/Ga1, 2a 1h (0,0,0); Mn/Ga2, 2d 1h (1/2,1/2,1/2); La/ Sr, O1, O2, and O3, 4e 1 (x,y,z). Mn and Ga occupy the 2a and 2d sites in a 1:1 ratio. La and Sr occupy the 4e site in a ratio determined by the composition. The magnetic moments at both Ga/Mn sites are constrained and spatially oriented as described in the text. Rwp ) 5.54%, Rp ) 4.36%, R(F2) ) 3.44%, and χ2 ) 2.308 for 39 variables.

Table 3. (a) Mn-O Distances and O-Mn-O and Mn-O-Mn Angles in I2/c Symmetry La2-xSrxGaMnO6 Phases and (b) Mn-O Distances and O-Mn-O and Mn-O-Mn Angles in La1.7Sr0.3GaMnO6 at 1.7 K: The Space Group Is P21/a 0.3, RT

0.5, 1.7 K

0.5, RT

(a) I2/c Symmetry La2-xSrxGaMnO6 Phases Mn1-O(1) × 2 1.9678(5) 1.9508(5) 1.9562(5) Mn1-O(2) × 2 1.966(4) 1.959(3) 1.957(4) Mn1-O(2) × 2 1.962(4) 1.949(3) 1.950(4) Mn1-O(1)-Mn2 161.1(2) 166.4(2) 164.1(2) Mn1-O(2)-Mn1 163.8(1) 164.0(1) 166.1(1) O(1)-Mn1-O(2) 91.4(1) 91.3(1) 91.1(1) O(1)-Mn1-O(2) 90.5(1) 90.7(1) 90.5(1) O(2)-Mn1-O(2) 91.32(3) 90.0(2) 90.88(2) La-O(1) 3.076(5) 2.981(5) 2.488(5) La-O(1) 2.449(5) 2.512(5) 3.017(5) La-O(1) × 2 2.7536(6) 2.7353(5) 2.7465(5) La-O(2) × 2 3.028(3) 3.019(2) 2.513(3) La-O(2) × 2 2.697(3) 2.769(4) 2.766(4) La-O(2) × 2 2.496(3) 2.480(2) 2.988(3) La-O(2) × 2 2.807(3) 2.711(4) 2.718(4)

0.7, RT 1.9481(7) 1.940(5) 1.954(6) 166.4(4) 167.8(2) 90.8(2) 90.4(2) 90.69(3) 2.982(7) 2.508(7) 2.7406(7) 2.944(4) 2.707(5) 2.544(4) 2.769(5)

(b) La1.7Sr0.3GaMnO6 at 1.7 K. The space group is P21/a Mn1-O(1) × 2 1.938(6) Mn1-O(2) × 2 1.983(5) Mn1-O(2) × 2 1.950(6) Mn1-O(1)-Mn2 159.67(7) Mn1-O(2)-Mn1 162.77(27) O(1)-Mn1-O(2) 90.4(2) O(1)-Mn1-O(2) 90.7(2) O(2)-Mn1-O(2) 91.60(4) Mn2-O(1) × 2 2.000(6) Mn2-O(3) × 2 1.978(6) Mn2-O(3) × 2 1.950(5) Mn2-O(3)-Mn2 163.87(26) O(1)-Mn2-O(3) 90.2(2) O(1)-Mn2-O(3) 90.1(2) O(3)-Mn2-O(3) 91.26(4) La-O(1) 2.8647(24) La-O(1) 2.4355(15) La-O(1) 2.6606(25) La-O(2) 3.072(6) La-O(2) 2.601(6) La-O(2) 2.850(7) La-O(2) 2.511(6) La-O(2) 2.850(7) La-O(3) 2.505(5) La-O(3) 3.053(6) La-O(3) 2.798(7)

data presented here. Refined parameters for all phases for which neutron powder diffraction data were collected are presented in Table 2a with refined bond lengths and angles in Table 3a. As expected, the unit cell volume decreases smoothly across the series upon oxidation from MnIII to MnIV, while the b axis shows a small maximum at x ) 0.1 just before the onset of the monoclinic phase (Figure 3). Temperature- and field-dependent magnetization measurements (Figure 4) show that the x ) 0.1 and 0.3 compositions retain the ferromagnetic ordering of the x ) 0 parent material, with similar Curie temperatures and saturation moments. (Table 4) The temperature dependence of the magnetization of the x ) 0.1 sample resembles that of La2GaMnO6, with a sharp rise in the magnetization below 100 K and Tc estimated from the second derivative of the magnetization as 75 K. In La1.7Sr0.3GaMnO6, the extra MnIV centers exert a qualitatively different influence on the temperature dependence of the magnetism. The ferromagnetic ordering transition at 66 K is evident from the raw FC/ZFC magnetization in Figure 4iii but careful inspection of

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Figure 3. Unit cell parameters: (i) a (squares), b (crosses), and c/x2 (triangles) and (ii) β (triangles) and V (circles) in the La2-xSrxGaMnO6 series at 298 K.

Figure 4. Magnetization versus temperature in a 100 G measuring field of the purely ferromagnetic members of the La2-xSrxGaMnO6 series (i) x ) 0, (ii) x ) 0.1, and (iii) x ) 0.3. Field-cooled (FC) data and zero-field cooled (ZFC) data are shown as filled and empty symbols, respectively. Table 4. Magnetic Transition Temperatures and Parameters Derived from Curie-Weiss Fits to La2-xSrxGaMnO6 x) Ttransition/K θ/K effective moment (µB Mn1-) moment at 2 T, 5 K (µΒMn1-)

0.1

0.3

0.5

0.7

75 140(7) 5.6(1) 3.80

66 160(7) 5.4(1) 3.43

31 150(7) 5.2(1) 1.12

21 160(7) 4.5(1) 0.47

the second derivative reveals a second transition at 16 K: these two distinct transitions are confirmed in muon spin relaxation data16 (Figure 5i). The origin of the transition at 16 K and the detailed nature of the

Figure 5. (i) The second temperature derivative of the magnetization of La1.7Sr0.3GaMnO6. (ii) Magnetization versus field at 6 K for La1.7Sr0.3GaMnO6.

magnetic phases which lie between the paramagnetic regime (T > 66 K) and the ferromagnetic structure observed at 1.7 K are currently under investigation; the discussion below is concerned primarily with the lowtemperature ground state. The 6 K magnetization isotherm (Figure 5ii) is characteristic of a soft ferromagnet, and tends to 3.6 µB/Mn in fields above 2 T; complete saturation is not observed in fields of up to 5 T in any of these materials. This ferromagnetic longrange order is confirmed by the refinement of neutron powder diffraction data at 1.7 K. However, the detailed low-temperature structural analysis presented later shows that this ferromagnetic state is quite different from that at x ) 0 in La2GaMnO6.6 Qualitatively different behavior is apparent at x ) 0.5 (Figure 6i). The sharp FC magnetization rise, a clear signature of ferromagnetic ordering at T g 70 K in the less doped members of the series, is lost. It is replaced by a divergence of the FC and ZFC magnetization at 31 K in what appears to be a spin-glass-like manner. The ZFC data show a sharp cusp while the FC magnetization becomes almost temperature-independent below this point, its value of 0.018 µB/Mn at 6 K in 100 G being 25 times smaller than for x ) 0.3. The magnetization at 6 K (Figure 6ii) does not saturate in 5 T where it reaches a value of 1.54 µB/Mn. This value is too large to be consistent with the pure spin-glass interpretation, as is the 0.078 µB/Mn remanent magnetization at 6 K. However, the 1.7 K neutron powder diffraction data do not require a magnetic component for satisfactory refinement, indicating that there is no long-range ferromagnetic order in this sample (Figure 6iii). Details of the crystal structure are given below. The x ) 0.7 sample in 100 G displays a similar lowtemperature FC/ZFC divergence at the reduced temperature of 21 K and a higher temperature FC/ZFC separation characteristic of the blocking of clusters of spins below 100 K (Figure 7i). The 100 G FC magnetization is now 0.005 µB/Mn at 6 K. The 5 K magnetization isotherm (Figure 7ii) has considerably reduced hysteresis and remanence (0.01 µB/Mn), and the 4 T value of

Diluted Manganates La2-xSrxGaMnO6

Chem. Mater., Vol. 14, No. 1, 2002 429

Figure 7. (i) Magnetization versus temperature for La1.3Sr0.7GaMnO6 in a 100 G measuring field. (ii) Magnetization versus field for La1.3Sr0.7GaMnO6 at 5 K.

Figure 6. (i) Magnetization versus temperature for La1.5Sr0.5GaMnO6 in a 100 G measuring field. Field-cooled (FC) data and zero-field cooled (ZFC) data are shown as filled and empty symbols, respectively. (ii) Magnetization versus field for La1.5Sr0.5GaMnO6 at 6 K. (iii) Rietveld refinement of neutron powder diffraction data from La1.5Sr0.5GaMnO6 at 1.7 K in space group I2/c. The observed data are shown as points, the fit as a solid line and the difference curve is plotted below. Tick marks indicate the positions of Bragg reflections.

0.7 µB/Mn is far from saturation, suggesting that the pure spin glass description is now appropriate. None of the samples studied obey the Curie-Weiss law over an extended temperature range, and the derived parameters are thus unreliable. The ferromagnetic sign of the Weiss constants (Table 4) derived from all the fits is a robust feature and consistent with the ferromagnetic long-range order observed for x < 0.5. Neutron powder diffraction is required for a detailed discussion of oxide positions and magnetic order in metal oxides. (The x ) 0.1 phase refines in the Pbnm space group and is isostructural with La2GaMnO6). At room-temperature La1.7Sr0.3GaMnO6 adopts the different and rare a-b-b- tilting scheme13,18 in the I2/c space (18) Ritter, C.; Radaelli, P. G.; Lees, M. R.; Barratt, J.; Balakrishnan, G.; Paul, D. M. J. Solid State Chem. 1996, 127, 276-282.

group. There is only one crystallographically distinct B-site and no Ga/Mn order (which would have been unambiguously detected due to the different signs of the neutron scattering lengths), so the structural change is driven solely by octahedral tilting in response to the changed A cation size and Mn valence. Using the commonly adopted approach to tolerance factor calculation with the nine-coordinate ionic radii of Shannon,19 the tolerance factor t increases from 0.91 to 0.92 on oxidation from Pbnm symmetry (a+b-b- tilting) La2GaMnO6. This would place the phase almost exactly on the Pnma/R3 h c boundary of 30% MnIV A0.7A′0.3MnO3 materials,20 but the consequence of the increased tolerance factor in this B-site diluted case is not the rhombohedral a-a-a- tilting scheme but the unusual a-b-bI2/c tilting, seen at low temperatures for Pr0.6Sr0.4MnO3 (t ) 0.93).21 This I2/c structure is maintained across the rest of the series. The room-temperature structure refinements involved simultaneous analysis of X-ray and neutron histograms to enhance the accuracy of the oxide site occupancy (which refined to 1.00 within the error of 0.03 in all cases), as shown in Figure 8. The variation of the three inequivalent Mn-O distances within the MnO6 octahedron and the Mn-O-Mn angles with x is shown in Figure 9 and Table 3a. The Jahn-Teller distortion expected due to the presence of MnIII centers can be parametrized using the quantities σJT (1), Q1, and Q2.

σJT )

x

1

[(Mn-O)i - 〈Mn-O〉]2 ∑ 3 i

(1)

〈Mn-O〉 is the mean Mn-O bond distance. Q1 ) 2/x6 (2m - l - s), and Q2 ) x2 (l - s) where m, l, and s are chosen as the medium, long, and short Mn-O distances (19) Shannon, R. D. Acta Crystallographica 1976, A 32, 751. (20) Hwang, H. Y.; Cheong, S.-W.; Radaelli, P. G.; Marezio, M.; Batlogg, B. Phys. Rev. Lett. 1995, 75, 914-917. (21) Radaelli, P. G.; Marezio, M.; Hwang, H. Y.; Cheong, S.-W. J. Solid State Chem. 1996, 122, 444-447.

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Figure 10. Rietveld refinement of neutron powder diffraction data on La1.7Sr0.3GaMnO6 at 1.7 K in space group P21/a presented as in Figure 6iii. The lower set of tick marks correspond to magnetic reflections. The starred reflections in the inset [212, 113, 120, 003, and 021] confirm the lowering of symmetry from I2/c on cooling.

Figure 8. Rietveld refinement of powder neutron (i) and X-ray (ii) diffraction data from La1.7Sr0.3GaMnO6 at 298 K in space group I2/c, presented as in Figure 6iii.

Figure 11. Crystal structure of La1.7Sr0.3GaMnO6 at 1.7 K. Alternating layers of MnIII-rich and MnIV-rich octahedral sites are shown along the z-axis.

Figure 9. (i) Mn-O distances within the MnO6 octahedron and (ii) Mn-O-Mn angles in the La2-xSrxGaMnO6 series at 298 K.

(Figure S1). The extent of static coherent Jahn-Teller distortion does not change significantly across the series at room temperature. The oxide anion isotropic displacement parameters across the series are shown in Figure S2 of the Supporting Information (unlike the pure MnIII end-member, anisotropic displacement parameters did not produce a significant improvement to the fits).

The 1.7 K neutron powder diffraction pattern of La1.7Sr0.3GaMnO6 is considerably changed from room temperature, with the appearance of extra reflections consistent with the development of two distinct B cation sites on cooling. The space group at 1.7 K is P21/a (Figures 10 and 11). The refined parameters are given in Table 2b and the derived bond lengths and angles at the two manganese sites are given in Table 3b. The refined magnetic moments at the two sites did not differ by more than three standard deviations when refined independently and so were constrained to have the same value in the final model, which refined to 2.22(5) µB/ Mn in the xz-plane. The data allow the conclusion that the moment does not lie along y, but the exact direction within the xz-plane cannot be identified. No structural transition was observed on cooling the x ) 0.5 sample, which retains the valence-disordered I2/c structure at 1.7 K. Discussion The interplay between charge, orbital and spin ordering is a key aspect of the electronic behavior of perovskites where both MnIII and MnIV oxidation states occur

Diluted Manganates La2-xSrxGaMnO6

on the B-sites. This paper is the first detailed study of the influence of strong (50%) random dilution of the B-site by a diamagnetic p-block cation on the oxidation of MnIII perovskites. The choice of a p-block element removes the need to consider the role of vacant dorbitals on the diluent. Introduction of MnIV centers into the ferromagnet La2GaMnO6, where there is no coherent static Jahn-Teller distortion to be suppressed, has a strikingly different effect to that found in antiferromagnetic Jahn-Teller ordered LaMnO3. Charge ordering of the MnIII and MnIV valences occurs, with a decisive effect on the physical properties, despite the random and diluted occupancy of the octahedral site by electronically inactive cations. Oxidation of MnIII to MnIV upon substitution of Sr2+ for La3+ produces a smooth decrease in all the Mn-O distances at 298 K as expected. Although the tilting scheme changes beyond x ) 0.1 to lower the symmetry to monoclinic, the Mn-O-Mn angles bridging the octahedra become considerably closer to linear upon oxidation (Figure 9ii and Table 3a). The O equivalent isotropic displacement parameters (Figure S2) vary significantly across the series, with a pronounced increase in the O(2) root-mean-square displacement to 0.12 Å at x ) 0.7: the increase in this parameter is most marked between Pbnm La2GaMnO6 and I2/c La1.7Sr0.3GaMnO6. This may reflect the increase in the distribution of bond lengths at individual octahedral sites arising from the increasing valence disorder across the series at room temperature, or reflect proximity to local or long-range charge-ordering instabilities. The La2GaMnO6 parent compound undergoes ferromagnetic ordering due to local incoherent static JahnTeller displacements favoring orthogonal orbital order at neighboring Mn-occupied B-sites.6 Because this ferromagnetism arises from correlations of the local metal environment from site to site, rather than from delocalized eg electrons mediating double exchange interactions, it might be expected to be sensitive to oxidation, which decreases the number of Jahn-Teller active centers. The magnetization and variable temperature neutron diffraction data presented here show an unusual correlation between the mean Mn valence, the real-space charge ordering and the magnetism. The Curie temperature and saturation moment of La2GaMnO6 are almost insensitive to doping of up to 30% MnIV. This differs from undiluted La1-xSrxMnO3 and suggests that the limiting moment is defined by the extent of B-site dilution rather than the concentration of MnIV. Detailed examination of the magnetization data at x ) 0.3 indicate a qualitative change, with a second transition at 16 K. The low-temperature structure of this phase reveals an unusual lamellar valence ordering stabilized by the dilution on the manganese sublattice. Our ongoing studies of the magnetic phase diagram of the x ) 0.3 composition have shown that the onset of the transition to the valence-ordered state is between 200 and 273 K, well above the observed magnetic ordering transitions. Although re-entrant ferromagnetic transitions from charge-ordered states are known,22,23 (22) Arulraj, A.; Biswas, A.; Raychaudhuri, A. K.; Rao, C. N. R.; Woodward, P. M.; Vogt, T.; Cox, D. E.; Cheetham, A. K. Phys. Rev. B 1998, 57, R8115-R8118. (23) Moritomo, Y. Phys. Rev. B 1999, 60, 10374-10377.

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in the present case the onset of ferromagnetism does not suppress the valence order or correspond to the onset of metallic behavior as in all previous examples. In the valence and spin disordered x ) 0.3 phase at 300 K, the small change in the mean Mn-O distance from 1.9832(9) Å in La2GaMnO6 to 1.965(2) Å and the increase in mean A-site radius produces the a-b-btilting scheme (Figure S3). Oxidation of 30% of the Jahn-Teller active MnIII centers produces a small decrease in the σJT parameter ( eq 1) at 298 K from 0.0048(7) to 0.0020(9). In Jahn-Teller-ordered LaMnO3, σJT ) 0.1163(4). The decrease in the coherent distortion is accompanied by a significant increase in the O2 displacement parameter, so the local distortion at individual MnO6 sites may not change. This contribution of local incoherent distortions to the displacement parameters has been demonstrated in La2GaMnO6 itself.6 At 1.7 K in the ferromagnetic state of x ) 0.3, the extra Bragg reflections indicate that there are two distinct B-sites in space group P21/a. The distinction between the octahedral sites is not due to Ga/Mn order (which is absent at room temperature and cannot arise on cooling due to elementary diffusion considerations). It arises from the preferential ordering of the +III and +IV oxidation states at distinct octahedral sites by motion and subsequent trapping of the eg electrons on cooling. This is in contrast to the ferromagnetic metal La0.7Sr0.3MnO3 where the +III and +IV valences are dynamically disordered and the ferromagnetism arises from double exchange interactions mediated by itinerant eg electrons. A 50% B-site dilution induces charge ordering in this metallic system and localization of the distinct valence states over the disordered array of B-sites produces an unusual lamellar ordering motif (Figure 12i), previously observed only in La0.85Ca0.15MnO3.24 In the valence-disordered high temperature state in the present diluted case, the cubic B-site array is occupied randomly by 50% electronically inactive GaIII, 35% MnIII, and 15% MnIV centers, so only half of the sites can have their occupancy altered in any valence ordering process on cooling. This simple change makes a dramatic qualitative difference to the valence ordering pattern, which is layered: the two distinct B-sites alternate along the z-direction. The mean bond lengths in the two layers are 1.957(3) and 1.976(3) Å, compared with 1.965(2) Å in the valence-disordered state at 300 K, consistent with charge ordering on cooling producing a higher MnIV concentration at site 1. The maximum possible extent of charge ordering is 50% MnIII at Mn(2) and 30% MnIV/20% MnIII at Mn(1), with 50% of both sites occupied by GaIII (V(Mn1) ) 3.21; V(Mn2) ) 3.05). Bond valence sums (V(Mn1) ) 3.05; V(Mn2) ) 3.21) are consistent with complete ordering of MnIII in the Mn(2) layer. The valence ordering is coupled with a significantly enhanced static coherent Jahn-Teller distortion in both layers, consistent with enhanced charge localization and the presence of localized MnIII centers at both Mn(1) and Mn(2) sites. Although the experimental errors do not allow the extent of Jahn-Teller distortion at the two (24) Lobanov, M. V.; Balagurov, A. M.; Pomjakushin, V. J.; Fischer, P.; Gutmann, M.; Abakumov, A. M.; D’yachenko, O. G.; Antipov, E. V.; Lebedev, O. I.; van Tendeloo, G. Phys. Rev. B 2000, 61, 8941.

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Figure 12. (i) Mn-O bond length alternation within the charge ordered sheets of the P21/a 1.7 K structure of La1.7Sr0.3GaMnO6. The long bonds at each Mn-O site are indicated as the thickest solid lines and the short bonds as the thinnest solid lines. Note the ferrodistortive interlayer arrangement of the long Mn(2)-O(1) bonds and the antiferrodistortive intralayer arrangement of the Mn(1)-O(2) bonds. (ii) Long-range averaged orbital ordering within the Mn(1) and Mn(2) layers inferred from the bond length distribution in the P21/a 1.7 K structure of La1.7Sr0.3GaMnO6. (iii) Possible local orbital orderings to maximize eg delocalization in response to differing Ga3+ environments of an MnIII center (hatched). The Ga3+ ions are shown as open and MnIV ions as filled spheres, respectively.

distinct B-sites to be distinguished (σJT is 0.018(3) at site 1 and 0.020(3) at site 2), there is clearly an orderof-magnitude increase in the static, coherent JahnTeller distortion from 0.002(1) at 298 K on cooling into the ferromagnetic charge ordered state. These distortions are now larger than those in the MnIII end-member

Battle et al.

La2GaMnO6. The orientations of the Jahn-Teller distortions within the two layers are different however, reflecting orbital ordering (Figure 12) accompanying the charge order. The MnO6 octahedra in the MnIII-rich Mn(2) layer are axially elongated along the interlayer z-direction and the MnIV-rich Mn(1)O6 octahedra are axially compressed. The eg orbitals in both layers are at least partially occupied and ordered to produce ferromagnetism. Within the MnIV-rich Mn(1) layer, the axial compression of the octahedra along z and the alternation of the longest Mn(1)-O(2) distance along the [110] and [11 h 0] directions (Figure 12i) within the layer is consistent with the antiferrodistortive occupation of 3x2-r2 and 3y2-r2 orbitals at alternate MnIII sites within the layer (Figure 12ii), in a manner similar to LaMnO3. As there is no long-range charge ordering within the layer (all the B-sites within the layer are symmetry-equivalent), these eg electrons are able to hop from site to site to maximize their delocalization and produce ferromagnetic exchange. In La1.7Sr0.3GaMnO6, the Ga substitution prevents metallic behavior due to Anderson localization but the eg spins are still able to hop locally and generate ferromagnetism. The empty eg orbitals in the MnIV-rich Mn(1) layer thus appear to be predominantly directed along z. The ferrodistortive elongation of the interlayer Mn(2)-O(1) bond and associated shortening of the Mn(1)-O(1) distance is the most pronounced structural change on entering the charge-ordered state, and indicates preferential occupancy of the eg orbitals with pronounced z directional character at the MnIII sites in the Mn(2) layer. This orbital ordering with a z-component allows eg carriers to hop from MnIII to MnIV centers between layers, producing ferromagnetic exchange. Within the Mn(2) layer the alternation of short and intermediate Mn-O bonds along the [110] and [11h 0] directions indicates that x2-z2 and y2-z2 orbitals are occupied at neighboring MnIII sites (Figure 12ii), giving ferromagnetic superexchange. Exchange with MnIV neighbors in the layer will be ferromagnetic via hopping to the empty eg orbitals, as there is no long-range charge order within the layer. The above is a rationalization of orbital ordering based on the average structure. Locally, orbital order will be strongly correlated with the distribution of diamagnetic Ga3+ centers and MnIII/IV correlations around the Jahn-Teller active MnIII site. 15% of the B-sites (more in the Mn(1) layer, fewer in the Mn(2) layer) will be MnIV centers with both z2 and x2-y2 eg orbitals empty, and will thus interact ferromagnetically by σ superexchange with suitably orbitally ordered MnIII neighbors. MnIII sites will actually generate a higher probability of MnIV than MnIII neighbors due to electrostatic considerations, and the layered charge ordering in the present diluted case can be thought of as occurring to allow effective delocalization of the eg electrons between the layers. Considering an MnIII center with three GaIII, two MnIV, and one MnIII neighbor (5/16 of the possible cases), local orbital ordering can occur to generate ferromagnetic exchange and the associated long/short bond alternation whether the 3 Ga neighbors are arranged in a fac or mer manner (Figure 12iii) allowing enhanced delocalization of the

Diluted Manganates La2-xSrxGaMnO6

eg electrons. The B-site dilution and disorder prevents metallic behavior, but the orbital order permits threedimensional ferromagnetic exchange because it is locally interrupted by Ga3+ dopants which alter the preferred direction of orbital ordering to maintain eg delocalization between MnIII and MnIV centers. The small averaged Jahn-Teller distortions (Q1) -0.045(9), Q2 ) 0.04(1) site 1, Q1 ) 0.00(1), and Q2 ) 0.07(1) site 2) are due to the dilution at the B-site. The average structure retains signatures of the local orbital ordering which gives rise to the ferromagnetism. Because the orbital order requires specific spatial interactions between neighbouring Mn cations, the establishment of this long-range order despite the dilution must be considered as more demanding than simple percolation. It is an example of high-density site percolation25 where the criterion for connectivity is that each site has a sufficient number of occupied neighboring sites and the percolation threshold is generally higher than for simple percolation. For a simple cubic lattice, 50% dilution is above the threshold for percolation of sites with three neighbors but below the value for four, suggesting that a minimum of three manganese neighbors for a given manganese site are required to define the orbital ordering pattern. The orbital ordering pattern with long Mn-O bonds alternating within and between layers is a consequence of the unusual lamellar charge-ordering seen in this B-site diluted system. The valence ordering pattern allows the long-range ferromagnetism of the parent compound to survive to 30% doping: the small mean bond-length differences between the two layers are reflected in the magnetic structure, where both sites carry the same ordered moment. The shape of the magnetization isotherm and the 5T saturation moment are also consistent with long-range ferromagnetism. Charge and orbital order are commonly observed in antiferromagnetic manganates. By site diluting a mixedvalence manganate, we have induced orbital and charge ordering in a ferromagnetic system. Recent theory26 indicates that orbital polarization energies (the dependence of energy on orbital ordering at neighboring sites) are much reduced in ferromagnetic manganates because there is no spin-induced restriction on electron hopping between neighbors: in an antiferromagnet, the orbital ordering is critical to the delocalization of the eg electrons. B-site dilution of the ferromagnetic La0.7Sr0.3MnO3 localizes the eg electrons by the Anderson disorderinduced mechanism and therefore enhances the importance of orbital order, as local correlation of Jahn-Teller distortions now allows maximum eg electron delocalization. Orbital polarization therefore increases with disorder despite the ferromagnetism, because the electrons coupling the t2g spins are localized, not itinerant. The introduction of further MnIV centers destabilizes this charge-ordered pattern, which is unusual as the Coulombic interactions favoring checkerboard Wigner crystal charge ordering are usually strongest at x ) 0.5: in the La1-xCaxMnO3 series, further charge ordered states occur beyond x ) 0.15. This is consistent with the charge order apparent at x ) 0.3 being driven by (25) Kogut, P. M.; Leath, P. L. J. Phys. C 1982, 15, 4225-4233. (26) Maezono, R.; Ishihara, S.; Nagaosa, N. Phys. Rev. B 1998, 58, 11583.

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Figure 13. Evolution of ordering temperature and type with x in the La2-xSrxGaMnO6 series. CD and CO refer to the charge-disordered and charge-ordered ferromagnetic phases. CG and SG indicate cluster and spin-glass ground states. The boundaries between these ground states are represented with broad lines to indicate that there is uncertainty concerning the compositions at the boundaries.

the orbital ordering at the MnIII sites to maximize the delocalization of the eg electrons, and once the number of MnIII centers is reduced beyond this value orbital ordering is no longer favored. The x ) 0.5 phase does not undergo a structural transition upon cooling, with the mean Mn-O and Jahn-Teller parameters unchanged within error, although the increase in Uiso for O1 may indicate local static distortions set in on cooling. This composition-induced change in structural behavior dramatically affects the magnetism, as the spontaneous magnetization and ordering temperature are sharply reduced. There is no long-range ordered magnetization at x ) 0.5 as proven by the absence of magnetic Bragg peaks. Instead, the majority of the moments are frozen in a glasslike fashion as indicated by the sharp cusp in the ZFC curve. This indicates that the suppression of the valence order, and associated long-range orbital correlations, on further oxidation prevents the onset of ferromagnetic long-range order. This is in contrast with systems where the eg electrons become itinerant upon melting of the charge-ordered lattice, and reflects the importance of superexchange between localized electrons, as opposed to double exchange mediated by itinerant electrons, in the present case. The random potential produced at the Mn B-site localizes the eg electrons in both the charge-ordered and disordered states. B-site dilution is thus shown to induce unusual charge and orbital ordering patterns. Local probes are required to investigate the correlation between this long-range charge modulation and the immediate environment of each MnIII and MnIV site. Conclusion The introduction of MnIV into La2GaMnO6 suppresses ferromagnetism because it reduces the driving force for locally correlated orbital order, which produces the ferromagnetic superexchange pattern in the pure MnIII phase. The suppression of ferromagnetism is not monotonic, however: long-range order is maintained at 30% MnIV doping by an unusual lamellar charge-ordering pattern which allows ferromagnetic exchange between distinct MnIII and MnIV centers. The charge and orbital

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ordering occurs to maximize the local hopping of eg electrons in Anderson-localized states, as the B-site dilution increases the importance of local orbital ordering correlations over the usual situation in pure B-site MnIII/MnIV manganates with double-exchange ferromagnetism produced by itinerant eg electrons. The x ) 0.3 composition is thus a rare example of a nonmetallic orbital and charge-ordered manganate ferromagnet. Once this charge-ordered state is suppressed upon further oxidation, the loss of the associated local orbital correlations means the long-range ferromagnetic order can no longer be sustained (Figure 13). There is a crossover to spin glass-like magnetism via an inhomogeneous low temperature state at x ) 0.5 whose magnetic response is characteristic of a cluster glass containing small ferromagnetic regions. The absence of charge order at x ) 0.5 and the enhanced static coherent Jahn-Teller distortions over the pure Mn(III) endmember both suggest that the orbital ordering at the

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MnIII centers drives the formation of the lamellar charge-ordered array. Acknowledgment. We thank the UK EPSRC for supporting this work under GR/M83322, Dr. T. Hansen, Dr. P. G. Radaelli, and Dr R.M. Ibberson for assistance on the D2B, GEM and HRPD diffractometers and Professor A. Harrison (University of Edinburgh) for discussions concerning percolation. Supporting Information Available: Figure S1 showing (i) Q1, (ii) Q2, and (iii) σJT parameters in the La2-xSrxGaMnO6 series at 298 K. The oxide displacements involved in the Q1 and Q2 modes shown. Figure S2 showing isotropic displacement parameters of the oxide anions in La2-xSrxGaMnO6. Figure S3 showing octahedral tilting schemes in the Pnma and I2/c space groups (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM0103848