Synthesis under high-oxygen pressure, magnetic and structuralcharacterization from neutron powder diffraction data ofYGa1�xMn1+xO5 (x = 0.23): A comparison with YMn2O5

C. de la Calle a,*, J.A. Alonso a, M.J. Martínez-Lope a,M. García-Hernández a, G. André b

a Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, E-28049 Madrid, Spainb Laboratoire Leon Brillouin, CEA Saclay, F-9119 Gif sur Ivette, France

Received 11 July 2007; received in revised form 26 September 2007; accepted 18 October 2007

Available online 1 November 2007

Abstract

A new material of nominal stoichiometry YGaMnO5 has been prepared in polycrystalline form from citrate precursors followedby thermal treatments under high-oxygen pressure. This compound has been characterized from neutron powder diffraction (NPD)data and magnetic measurements. For comparison, the parent compound YMn2O5 has also been synthesized and its crystal structurerefined by NPD data. The new oxide has an actual stoichiometry YGa1�xMn1+xO5 (x = 0.23), determined by NPD, showing animportant cationic disorder between both metal sites; it is orthorhombic, Pbam (SG), and its crystal structure contains chains ofMn4+O6 edge-sharing octahedra, linked together by Ga3+O5 pyramids and YO8 units. With respect to YMn2O5, containing axiallyelongated MnO5 pyramids due to the Jahn–Teller effect of Mn3+ cations, the GaO5 pyramidal units in YGa0.77Mn1.23O5 aresubstantially flattened. This compound has a paramagnetic behaviour with two weak anomalies at about 50 K and 350 K. Themagnetic structures, studied at 1.4 K and 100 K show a ferromagnetic coupling along the chains of MnO6 octahedra.# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Oxides; C. High pressure; C. Neutron scattering; D. Magnetic structure; D. Crystal structure

1. Introduction

The family of RMn2O5 (R = rare earths) oxides was first described in the 1960s by Bertaut et al. [1,2] that alreadyrecognized the extraordinary flexibility of the structure concerning the substitutions of both R and Mn atoms allowing,for instance, the preparation of RMnTiO5 phases, or RAlGeO5 oxides [3]. Some RMn2O5 materials have recently beenrevisited since they are among the few oxides that show a significant magneto-ferroelectric effect [4–8] implying acoupling between ferroelectricity and magnetic order in the system. The interplay between ferroelectricity andmagnetism in the RMn2O5 oxides suggests the possibility that the polarization can be controlled by the application ofan external magnetic field, what would allow the design of new devices for practical applications. A serious drawbackof these phases is the relatively low temperature at which this phenomenon occurs. Therefore, the search of newcompounds susceptible to show similar effects at higher temperatures is certainly appealing.

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Materials Research Bulletin 43 (2008) 197–206

* Corresponding author.E-mail address: c.delacalle@icmm.csic.es (C. de la Calle).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.materresbull.2007.10.029

All the RMn2O5 (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) oxides are isostructural (space group Pbam, Z = 4) andcontain infinite chains of Mn4+O6 octahedra sharing edges, linked together by Mn3+O5 and RO8 units [9]. Theorthorhombic crystal structure contains two crystallographic sites for Mn atoms, with different oxygen coordination.Mn4+ ions are located at the 4f sites, octahedrally coordinated to oxygens, whereas Mn3+ ions occupy the 4h sites andthey are bonded to five oxygen atoms, forming a distorted tetragonal pyramid. The structure of YMn2O5 was originallystudied by X-ray diffraction [10] but an accurate structural study by NPD is lacking. Its magnetic structure is a rathercomplex antiferromagnetic helicoidal structure with a long-range order of Mn3+ and Mn4+ below the transitiontemperature TN �45 K [11–13].

With the aim to induce new magnetic interactions in the members of the RMn2O5 family, we recently designed andprepared YFeMnO5 [14] and YCrMnO5 [15], which are obtained by replacing Mn3+ by Fe3+ and Cr3+, respectively.

In this paper we report on the preparation of a new material with nominal YGaMnO5 stoichiometry, which isisostructural with the RMn2O5 compounds and is obtained by substitution of Ga by Mn cations. YGaMnO5 is, thus, thefirst reported member of a novel RGaMnO5 (R = rare earths) series. In principle, the introduction of diamagnetic Ga3+

cations in the pyramidal units would simplify the magnetic interactions between Mn4+ at the chains of MnO6

octahedra, and would help to understand the magnetic coupling in this structural type. This new compound has beensynthesized from a citrate precursors procedure followed by annealing under high-O2 pressure and it has beencharacterized from the structural point of view from NPD. This study is completed with macroscopic magneticsusceptibility measurements. For the sake of comparison with the crystal structure of the parent YMn2O5 oxide, wehave also prepared and studied this phase by NPD.

2. Experimental

An oxide of nominal YGaMnO5 stoichiometry was prepared in powder form by a citrate technique. Stoichiometricamounts of analytical grade Y2O3, MnCO3 and Ga(NO3)3�xH2O were dissolved in a citric acid aqueous solution withsome droplets of HNO3; the solution was slowly evaporated, leading to an organic resin which was dried at 140 8C andslowly decomposed at 400 8C in air. All the organic materials and nitrates were eliminated in a subsequent treatment at800 8C in an O2 flow. Then, the precursor powders were slowly heated to 850 8C at a final pressure of 200 bar O2 and heldat this temperature for 12 h in a VAS furnace. The product was finally cooled, under pressure, at 300 8C h�1 down toroom temperature. Finally, the oxygen pressure was slowly released. A dark brown polycrystalline powder was obtained.

In the same way, YMn2O5 was obtained in powder form from citrate precursors; the precursor powders were heated inair up to1000 8C for12 h and the resulting materialwas annealed at 900 8C underhigh-oxygenpressure (200 bar) for12 h.

The reaction products were characterized by X-ray diffraction (XRD) for phase identification and to asses phasepurity. The characterization was performed using a Bruker-axs D8 diffractometer (40 kV, 30 mA) in Bragg–Brentanoreflection geometry with Cu Ka radiation (l = 1.5418 Å).

Neutron powder diffraction (NPD) diagrams were collected at the Institut Laue-Langevin, Grenoble (France) and atthe LLB, Saclay (France). The diffraction patterns were acquired at the high-resolution D2B (ILL) diffractometer withl = 1.594 Å, at room temperature (295 K) in the angular range 0.18 < 2u < 1568 with a 0.058 step; low-temperatureneutron diffraction (100 K, 1.4 K) patterns were collected at the G4.1 (LLB) diffractometer with l = 2.422 Å in theangular range 14 < 2u < 948 with a 0.058 step. The NPD patterns were analysed by the Rietveld method [16] by usingthe Fullprof program [17]. A pseudo-Voigt function was considered to generate the profile shape; the background wasfitted to a fifth-degree polynomial function. The coherent scattering lengths for Y, Ga, Mn and O were, 7.750 fm,7.288 fm, �3.730 fm and 5.803 fm, respectively.

The dc magnetic susceptibility was measured for YGaMnO5 with a SQUID magnetometer from Quantum Designequipped with a 70 kOe superconducting magnet, in the temperature interval 4 < T < 400 K under a 1 kOe magneticfield. An isothermal magnetization curve was obtained at 4 K for a magnetic field ranging from �50 kOe to 50 kOe.

3. Results

3.1. Synthesis

The preparation of YGaMnO5 has been favoured by the use of reactive precursors and by annealing in high-O2

pressure (850 8C under 200 bar of O2). This material cannot be obtained from ceramic mixtures or by thermal

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treatments in air at ambient pressure: mixtures of the competitive YMnO3 perovskite and Y3Ga5O12 garnet werealways identified by XRD after thermal treatments in air. This can be understood since high-oxygen pressure favoursthe full stabilization of the high-oxidation state of manganese, Mn4+. YGaMnO5 is not formed after treatment at800 8C in an O2 flow; high-oxygen pressure conditions are required to stabilize Mn4+ cations in this structure. Also, thepreparation of pure YMn2O5 was favoured by the high-O2 pressure conditions; otherwise mixtures with YMnO3

perovskite are obtained.

3.2. Room temperature X-ray and neutron powder diffraction

The XRD pattern of YGaMnO5 shows well-defined reflections corresponding to an orthorhombic unit cell, isotopicto YMn2O5, as displayed in Fig. 1. No impurity phases were detected from XRD data. The crystal structurerefinements were performed from NPD data collected at room temperature (RT).

The crystal structure of YMMnO5 (M = Ga, Mn) was defined in the orthorhombic space group Pbam (No. 55), withthe unit-cell parameters shown in Table 1. Yatoms were located at 4g positions, M atoms at 4h, Mn atoms at 4f and thefour crystallographically independent oxygen atoms at 4e, 4g, 4h and 8i positions. A good fit between the observed andthe calculated profiles was obtained after the respective Rietveld refinements for M = Ga, Mn, as shown in Fig. 2a

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Fig. 1. Observed (circles) and refined (full line) X-ray diffraction profiles of nominal YGaMnO5 and YMn2O5, refined in an orthorhombic unit cell,space group Pbam.

and b. For M = Ga the refinement was improved by introducing as secondary phases metal vanadium (coming from thesample holder) and b-Ga2O3, segregated from the main phase. From the scale factors, the amount of b-Ga2O3 wasestimated as 1.6(3)% in weight.

For M = Ga, the presence of a certain level of antisite disorder between Ga and Mn cations was checked: a 3% ofMn 4f positions are occupied by Ga cations (Ga3+) and a 26% of Ga 4h positions are occupied by Mn cations (probablyMn3+) as shown in Table 1. Additionally, the refinement of the oxygen occupancy factors led to identify a slightoxygen deficiency over the O1 sites. Table 1 also lists the structural and thermal parameters, and discrepancy factorsafter the final refinement from NPD at RT. Table 2 includes the mean interatomic distances and some selected bondangles. A view of the crystallographic structure along the c-axis is displayed in Fig. 3. There are two different oxygenenvironments for the atoms that occupy the 4f and 4h sites. At the 4f site, the Mn4+ cations are coordinated to sixoxygens in Mn4+O6 distorted octahedra, whereas at the 4h site the Ga3+/Mn3+ ions form Ga3+O5/Mn3+O5 distorted

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Table 1Structural parameters for YMMnO5 oxides after the Rietveld refinement in the orthorhombic Pbam space group from NPD data

Atom M = Ga M = Mn

RT (K) 100 (K) 1.4 (K) RT (K)

a (Å) 7.2832(7) 7.2808(15) 7.2823(15) 7.2639(2)b (Å) 8.4294(8) 8.4320(14) 8.4347(14) 8.4758(2)c (Å) 5.6571(5) 5.6574(10) 5.6568(10) 5.6673(1)V (Å3) 347.31(6) 347.31(11) 347.47(11) 348.91(8)Y (4g)

x 0.1378(9) 0.135(2) 0.128(2) 0.1370(4)y 0.1689(7) 0.1584(14) 0.164(2) 0.1712(3)B (Å2) 0.96(9) 0.9(1) 1.0(1) 0.64(3)

M (4h)x 0.383(1) 0.386(3) 0.377(3) 0.4122(6)y 0.354(1) 0.361(3) 0.359(3) 0.3485(5)focc 0.744(6) 0.710(8) 0.706(8) 1.00B (Å2) 0.3 0.9(1) 1.0(1) 0.33(7)

Mn (4f)z 0.246(7) 0.246(7) 0.250(7) 0.2561(9)focc 0.968(6) 0.94(2) 0.94(2) 1.00B (Å2) 0.3 0.9(1) 1.0(1) 0.34(6)mx (mB) – 1.4(1) 1.3(1) –

O1 (4e)z 0.259(3) 0.272(5) 0.276(4) 0.2701(6)focc 0.90(2) 0.98(2) 1.00(2)B (Å2) 0.94(10) 1.3(1) 1.4(1) 0.54(5)

O2 (4g)x 0.172(1) 0.155(3) 0.154(3) 0.1633(4)y 0.4396(9) 0.440(2) 0.441(2) 0.4443(4)B (Å2) 1.05(10) 1.3(1) 1.4(1) 0.43(5)

O3 (4h)x 0.144(1) 0.156(3) 0.155(3) 0.1499(5)y 0.430(1) 0.429(2) 0.427(2) 0.4292(4)B (Å2) 1.43(15) 1.3(1) 1.4(1) 0.51(5)

O4 (8i)x 0.3879(5) 0.389(2) 0.392(2) 0.3938(2)y 0.2013(6) 0.206(1) 0.205(1) 0.2064(5)z 0.2383(8) 0.246(2) 0.241(2) 0.2454(5)B (Å2) 0.35(8) 1.3(1) 1.4(1) 0.43(3)

x2 1.19 1.07 1.26 1.63Rp% 3.16 1.81 1.95 3.87Rwp% 3.94 2.32 2.47 4.88RBragg% 5.99 3.21 3.74 5.51

Y and O2 at 4g (x, y, 0), M and O3 at 4h (x, y,1/2), Mn at 4f (0,1/2, z), O1 at 4e (0, 0, z), and O4 at 8i (x, y, z) positions.

tetragonal pyramids. The Mn4+O6 octahedra share edges via O2 and O3 and form infinite chains. The pyramids shareedges to form dimer units Ga2O8 or Mn2O8 linked via O1 oxygens. The different chains of octahedra areinterconnected through the pyramidal dimer units via O3 and O4 oxygens. These features are depicted in Fig. 4a and b.

3.3. Magnetic measurements for nominal YGaMnO5

The dc susceptibility and reciprocal susceptibility vs. temperature data are shown in Fig. 5a. In the temperatureinterval 100 K < T < 300 K the reciprocal susceptibility increases linearly, revealing a paramagnetic behaviour. Twoweak anomalies are observed on the curve at about 50 K and 350 K. A linear Curie–Weiss fit in the range 350–400 Kgives a Curie constant of C = 2.02 emu mol�1 Oe�1 K, a Weiss constant of uweiss = 26 K and an effective momentmeff = 4.04mB/f.u., which is in reasonable agreement with the expected spin-only moment, 4.54mB/f.u., calculated bythe expression:

meff ¼ ½n4fðmMn4þÞ þ n4hðmMn3þÞ2�1=2

where n4f and n4h are the stoichiometric coefficients of Mn4+ and Mn3+ at the 4f and 4h sites (n4f = 0.968; n4h = 0.256).

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Fig. 2. Observed (circles), calculated (full line), difference (bottom line) and Bragg positions (vertical lines) NPD patterns at room temperature for(a) YGa0.77Mn1.23O5 (the second and third refined phases correspond to vanadium and b-Ga2O3) and (b) YMn2O5.

The magnetization isotherm at 4 K (Fig. 5b) shows a significant curvature, reaching a magnetization value of0.5mB/mol at the maximum field of 50000 Oe and a remnant moment at 0 Oe of 0.1mB/mol, suggesting the presence offerro- or ferrimagnetic interactions in the system.

3.4. Low-temperature neutron diffraction

Two NPD patterns were collected at 1.4 K and 100 K, below and above the 50 K anomaly observed in the magneticmeasurements, in order to identify the possible existence of long-range magnetic ordering. A ferromagnetic structurewas modelled with Mn moments at 4f positions along the x-direction. The structure converged with ordered magneticmoments slightly above the neutrons detection threshold of 1mB for ordered spins, as shown in Table 1. A similarordered moment was obtained for 1.4 K and 100 K, meaning that the ferromagnetic structure is established well above100 K. The simultaneous refinement of the crystal structures at both temperatures gave rise to the unit-cell andstructural parameters also included in Table 1. The standard deviations of the structural parameters are larger for the1.4 K and 100 K data since these patterns were collected with a longer wavelength, in a limited 2u region spanningfrom 148 to 948. For the same reason, overall B thermal factors were refined for metal and oxygen atoms. Fig. 6illustrates the goodness of the fit for the 1.4 K pattern.

4. Discussion

The oxide with nominal stoichiometry YGaMnO5 derives from the parent YMn2O5 oxide (containing one Mn3+ andone Mn4+ cation per formula) by replacement of Mn3+ by Ga3+ cations. The crystal structure of nominal YGaMnO5,refined by NPD data is tightly related to that of YMn2O5, also refined here by NPD, as expected from the similarity inionic radii between Ga3+ (0.62 Å) and Mn3+ (0.64 Å) in sixfold coordination and high-spin state for Mn3+ cation [18](no data are found for Mn3+ in fivefold coordination and high-spin state).

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Table 2Main bond distances (Å) and selected angles (8) for orthorhombic YMMnO5 oxides determined from NPD data

M = Ga M = Mn

RT (K) 100 (K) 1.4 (K) RT (K)

Distances (Å)Y–O1(�2) 2.279(11) 2.26(2) 2.28(2) 2.322(3)Y–O2 2.295(10) 2.37(2) 2.34(2) 2.323(4)Y–O2 2.376(10) 2.39(2) 2.46(2) 2.408(4)Y–O4(�2) 2.282(7) 2.35(2) 2.38(2) 2.345(3)Y–O4(�2) 2.515(7) 2.54(2) 2.46(2) 2.476(3)hY–Oi 2.353 2.38 2.38 2.377(4)M–O1(�2) 2.014(13) 1.93(2) 1.95(2) 1.937(4)M–O3 1.868(12) 1.77(3) 1.71(3) 2.024(5)M–O4(�2) 1.964(7) 1.95(2) 1.96(2) 1.912(2)hM–Oi 1.965 1.91 1.91 1.933(4)M–M 2.97(1) 2.86(3) 2.98(3) 2.867(6)Mn–O2(�2) 1.936(17) 1.87(3) 1.87(3) 1.933(4)Mn–O3(�2) 1.879(18) 1.93(3) 1.91(3) 1.858(4)Mn–O4(�2) 1.884(5) 1.915(12) 1.903(13) 1.912(2)hMn–Oi 1.899 1.91 1,89 1.901(3)Mn–Mn 2.77(3) 2.79(6) 2.82(5) 2.903(7)Mn–Mn 2.89(3) 2.87(6) 2.83(5) 2.763(7)

Angles (8)M–O1–M 95.0(7) 96(2) 99(2) 95.4(3)Mn–O2–Mn 91(1) 97(3) 98(3) 97.3(3)Mn–O3–Mn 100(2) 96(3) 95(3) 96.0(4)Mn–O3–M 130(1) 131(2) 132(2) 131.3(2)Mn–O4–M 125.5(7) 128(2) 127(2) 122.1(2)

As shown in Table 1, a 3% of the Mn positions at 4f sites are occupied by Ga cations, and a 26% of the Ga positionsat 4h are occupied by Mn cations. This feature could only be unravelled by a neutron diffraction study, thanks to thecontrasting neutron scattering lengths of Ga and Mn. The final crystallographic formula is Y[Mn0.968(6)Ga0.032(6)]oct[-Ga0.744(6)Mn0.256(6)]pyrO4.90(2). For the sake of simplicity, we will label this phase as YGa1�xMn1+xO5 (x = 0.23) orYGa0.77Mn1.23O5 hereafter. The trend of both cations to exhibit some degree of antisite disordering is also shown inother substituted structures like YFeMnO5 [14], ErFeMnO5 [19] and ErAlGeO5 [20] compounds. This antisitedisordering is specially remarkable in YCrMnO5 [15]; in this case the octahedral positions are occupied by roughly50% of Mn and Cr cations, and the pyramidal groups contain two thirds of Mn and one third of Cr cations. In the

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Fig. 4. (a) Detail of the YMMnO5 (M = Ga, Mn) structure, highlighting the interconnection of MnO6 chains via MO5 double pyramids. (b)Comparison of the dimer groups of MnO5 and GaO5 pyramids in YMn2O5 and YGaMnO5, respectively.

Fig. 3. A view of the crystallographic structure of YMMnO5 (M = Ga, Mn), approximately along the c-axis. Octahedra and tetragonal pyramidscorrespond to Mn4+O6 and M3+O5 polyhedra. Octahedra share edges, forming infinite chains along the c-axis. Pyramids form dimer units, linkingtogether the chains of octahedra. Spheres represent the Y atoms.

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Fig. 6. Observed (circles), refined (full line) and peak positions (vertical lines for three phases: nuclear, magnetic and b-Ga2O3) NPD profiles forYGa0.77Mn1.23O5 at 1.4 K.

Fig. 5. (a) Thermal evolution of the magnetic susceptibility measured under a 1 kOe magnetic field. The right axis corresponds to the reciprocalsusceptibility. (b) Isothermal magnetization curve at T = 4 K.

case of Fe and Cr, this antisite disordering was ascribed to the relative ease of Cr and Fe to adopt the tetravalentoxidation state for the incorporation into the octahedral positions; this is clearly not the case for Ga3+, which is thoughtto adopt the trivalent state in both pyramidal and octahedral positions. The departure of the nominal YGaMnO5

stoichiometry in a structure containing 26% less gallium at the pyramidal positions is probably driven by the higherstability of Jahn–Teller Mn3+ cations at these positions, and it is accompanied by the segregation of some b-Ga2O3,which could be partially detected in the neutron patterns. The slight oxygen deficiency of the final phase, alsodetermined from NPD data leads to a refined oxidation state for Mn at octahedral (4f) sites of 3.9(1)+, close to theexpected tetravalent state.

It is interesting to compare the crystal structure of YGa0.77Mn1.23O5 and YMn2O5 compounds. ForYGa0.77Mn1.23O5 the volume of the orthorhombic unit-cell is slightly smaller than for YMn2O5 (see Table 1),scaling with the Ga3+ vs. Mn3+ sizes. In YMn2O5 (and other RMn2O5 compounds) the Mn4+O6 octahedra are fairlyflattened, with two bonds significantly shorter than the remaining four bonds, e.g. Mn–O3 bond-length is 1.858(4) Å,and the average Mn–O distance is 1.901 Å (Table 2). This is also observed in YGa0.77Mn1.23O5, with Mn–O3 distancesof 1.88 Å and an average value of 1.90 Å. Regarding the tetragonal pyramids, Ga3+O5 units are flattened, as the axialGa–O3 bond length is the shortest one (1.87 Å), which is in contrast with that observed in YMn2O5, where the M–O3bond in the axial position is the longest one in the Mn3+O5 pyramids (2.024(5) Å), as illustrated in Fig. 4b.Additionally, the equatorial Ga–O1 (2.01 Å) and Ga–O4 (1.96 Å) bond distances (forming the square basis of thepyramid) are longer in Ga3+O5 than the corresponding Mn–O distances in the Mn3+O5 units of the YMn2O5

(1.937(4) Å and 1.912(2) Å, respectively). This is probably related to the Jahn–Teller character of Mn3+ cations,favouring an increase of the axial bond lengths in the Mn3+O5 pyramids, in contrast with the non Jahn–Teller characterof Ga3+ (3d10 configuration). It seems that the substitution of Ga3+ by Mn3+ induce the Ga3+ cations to shift towards thepyramid apex. Besides, this shift leads to a significant increase in the Ga–Ga distance within the dimer (2.97 Å),compared to the Mn–Mn distance in YMn2O5, of 2.867(6) Å. With respect to the oxygen coordination of Y3+ cations,in both cases it can be described as Y3+O8 bicapped prisms, with average hY–Oi distances of 2.353 Å, in goodagreement with the average hY–Oi bonds lengths of 2.377 Å observed in YMn2O5.

The magnetic susceptibility curve for YGa0.77Mn1.23O5 shows weak anomalies at 350 K and 50 K. In theparamagnetic region above 350 K a positive Weiss constant suggests the presence of ferromagnetic interactions, whichat low temperature cause a significant curvature in the magnetization isotherms at 4 K. It is tempting to propose, in afirst approach, the presence of ferromagnetic interactions along the chains of MnO6 octahedra sharing edges along thec axis, showing Mn–O–Mn angles very close to 908. For these Mn4+–O–Mn4+ paths, the Goodenough–Kanamori rules[21,22] indeed predict predominant ferromagnetic interactions. The refinement of the magnetic structures at 100 Kand 1.4 K confirm the presence of ordered magnetic moments at the Mn 4f positions, ferromagnetically aligned alongthe x-direction. We suggest that the anomaly observed at 350 K corresponds to the establishment of these interactions.The GaO5 pyramids, structurally linking the chains in the ab plane, contain 26% of Mn3+, which could partially couplethe adjacent chains accounting for a partial three-dimensional coherence: the refined Mn4+ moments at 4f sites aremuch lower than the expected 3mB, implying that the long-range ordering does not extend to the full structure. In anycase, this is in sharp contrast with that found for YMn2O5, where the primary trend of Mn4+ moments to adopt aferromagnetic coupling along the chains is overcome by the strong antiferromagnetic Mn4+–O–Mn3+ interactions(between the chains and pyramidal units), giving rise to an overall incommensurate magnetic structure along the c-axis. It seems that the presence of diamagnetic Ga3+ ions at the pyramidal positions dramatically simplify the magneticinteractions in this structural type. According to these ideas, YGa0.77Mn1.23O5 oxide could be considered as a partiallyordered one-dimensional ferromagnetic system.

5. Conclusions

A new oxide has been obtained by replacing Mn3+ by Ga3+ in the parent YMn2O5 compound. A NPD studyconfirms that YGa1�xMn1+xO5 (x = 0.23) is isotypic with the parent material (space group Pbam), and contains chainsof edge-linked Mn4+O6 octahedra connected via dimer groups of square pyramids Ga3+O5. High-oxygen pressureconditions are required to stabilize Mn4+ cations. In YGa0.77Mn1.23O5 the square pyramids are flattened, showingshorter axial distances with respect to the Mn3+O5 pyramids in YMn2O5. A considerable antisite disorder exists in thestructure, where a 3% of Mn at 4f positions are occupied by Ga cations (Ga3+) and a 26% of Ga at 4h positions areoccupied by Mn cations (probably Mn3+). The magnetic structure at 1.4 K and 100 K, studied by NPD, shows a

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ferromagnetic coupling of Mn4+ magnetic moments along the chains of MnO6 octahedra. The magnetic coherenceacross the three-dimensional structure is achieved via the Mn3+ cations occupying at random 26% of the Ga pyramidalpositions. The presence of diamagnetic Ga3+ ions at the pyramidal positions dramatically simplify the magneticinteractions in this structural type, allowing the manifestation of ferromagnetic interactions along the chains of MnO6

octahedra, which in the parent compound YMn2O5 are overcome by strong antiferromagnetic Mn4+–O–Mn3+

interactions, between the chains and pyramidal units, giving rise to an overall incommensurate magnetic structurealong the c-axis.

Acknowledgements

We thank the financial support of CICyT to the project MAT2004-0479. We are grateful to ILL for making allfacilities available.

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