Charge and Zener polaron order in Bi0.75Sr0.25MnO3

We have characterized the RT structure of Bi 0.75Sr0.25MnO3 by means of electron diffraction, synchrotron x-ray, and neutron powder diffraction. These data evidence that a;b;A2ap and c;2ap average structure presents a modulation that doubles the a andc lattice parameters. The temperature evolution of this compound above RT has been investigated using magnetization, calorimetry measurements, and neutron powder diffraction. These data reveal that the superstructure disappears above ;575 K where a structural transition takes place. Susceptibility data show that, concomitant to the formation of the superstructure ~on cooling!, there is an enhancement of the effective paramagnetic moment as has also been found in other Bi-(Sr,Ca)MnO 3 bismuth manganites. The observed features and cell evolution signals that a new type of charge and polaron ordering occurs at this composition.


I. INTRODUCTION
The phenomena of charge ordering ͑CO͒ and the formation of modulated structures is one of the most studied phenomena in mixed-valence manganites (R 1Ϫx A x MnO 3 ,R ϵ rare earth AϵCa,Sr).From the early study of Goodenough, in half doped manganites, CO has been understood as the real-space ordering of Mn 3ϩ and Mn 4ϩ ions forming a Wigner crystal, 1 in a way similar to the Verwey transition in magnetite. 2It has been argued that there are mainly two energy terms contributing to the stabilization of CO: the minimization of the Coulomb repulsion, and the reduction of the elastic energy coming from the accommodation of the Jahn-Teller distortions of Mn 3ϩ ions due to orbital order ͑OO, an ordered occupancy of d z 2 or d x 2 Ϫy 2 e g orbitals͒ concomitant with CO.Generally, CO/OO states compete with ferromagnetic metallic ͑FMM͒ states.The latter are favored by the reduction of the kinetic energy of e g electrons when their mean free path enlarges.As far as the bandwidth of the e g electrons is governed by steric effects, the Mn-O-Mn bond angle governs the transfer integral between e g (Mn)-2p (O)-e g (Mn) orbitals. 3,4The mean size of R and A cations (͗R A ͘) is thus governing the competition between CO/OO and FMM states.The ionic picture of CO proposed by Goodenough was initially corroborated by means of synchrotron x-ray powder diffraction, 5,6 and later on by means of neutron powder diffraction, 7,8 but, in both cases, data were refined imposing hard constraints.Usually, two features are taken as unquestionable signs of the presence of CO.The first one is the presence of superlattice spots or peaks in electron-diffraction and powder diffraction patterns.These spots or peaks signal the existence of a subtle supercell modulation superimposed to the average cell of shorter period.The second one is the stabilization, at low temperature, of a CE-type antiferromagnetic structure.When the doping level x is not the ideal value 1/2, a great asymmetry between underdoped ͑excess of Mn 3ϩ ) and overdoped ͑excess of Mn 4ϩ ) cases has been extensively reported in the literature.In the former case it has been interpreted that the excess of e g electrons are randomly located in the Mn places occupying d z 2 orbitals perpendicular to the planes.This implies that the superstructure formed does not change with respect to the xϭ1/2 case.These d z 2 orbitals introduce a ferromagnetic coupling between the planes that stabilizes the pseudo-CEtype magnetic structure. 9In contrast, in the overdoped region, the excess of holes changes continuously the propagation vector describing the superstructure. 10This drives to the characteristic charge and magnetic order for the rational value xϭ2/3. 11,12lthough the ionic picture of CO (Mn 3ϩ /Mn 4ϩ ) proposed by Goodenough ͑Ref.1͒ is, at present, widely accepted, a series of papers, both theoretical and experimental, evidencing the deficiencies of this picture have appeared.In fact a first indication that this ionic picture must not be completely accurate can be found in Radaelli's work where the reported Mn-O bond distances for Mn 3ϩ and Mn 4ϩ are more similar than expected. 57][8] In some of these works the situation is even worse and the same size for hypothetical Mn 3ϩ and Mn 4ϩ is obtained. 7Other evidences against the CO picture are the x-ray absorption near-edge structure where a unique edge is found ͑instead of two͒ thus indicating a unique valence of Mn. 13 Very recently, with the aid of single-crystal neutrondiffraction data, a new picture has been depicted. 14 -16The CO model proposed by Radaelli et al. in Ref. 5 fails in the refinement of these data.Interestingly, the successful refinement indicates that e g electrons are not confined in a single Mn ion ͑ruling out the Mn 3ϩ configuration͒ but they are located in a Mn-O-Mn triad with a quite open Mn-O-Mn bond angle.These two neighboring Mn ions thus present a mixed valence state 3.5ϩ.The shared electron couples ferromagnetically both Mn ions through the double exchange Zener mechanism forming a Zener polaron ͑ZP͒.These ZP's are spatially ordered, thus forming a ZP ordered ͑ZPO͒ phase.This order naturally induces the characteristic structural modulation of the so called CO phase.The formation of the CE-type magnetic structure is also explained by this model, but it implies a high amount of frustrated bonds.For the underdoped region the scheme proposed by the same group is quite amazing.For that region, it is proposed that two ZP's placed in consecutive stacking planes ͑one above the other͒ decompose forming two Mn 3ϩ ions and one ZP pointing in between the planes. 17Each ZP formed by this mechanism ͑called ZP c , because it points to c axis in the P bnm setting͒ introduces a FM coupling between four consecutive planes, and the formation of Mn 3ϩ reduces considerably the magnetic frustration.The consequence is the stabilization of the pseudo-CE structure found in the underdoped region. 9,18More recently, by means of resonant x-ray-diffraction measurements, an incomplete charge disproportionation Mn 3.5Ϫ␦ -Mn 3.5ϩ␦ with ␦Ӷ0.5 has been proposed but, in fact, authors were not able to set a lower limit of ␦. 19 Besides this general new ZPO picture, other recent unexpected findings related to the CO, and to the effect of 6s 2 lone pair of Bi 3ϩ ions, were found in Bi-Sr manganites.Although Bi 3ϩ presents an ionic radius similar to that of La 3ϩ , the ground state of Bi 0.5 Sr 0.5 MnO 3 is still controversial.Its transition temperature (T CO or T ZPO Ӎ525 K, the greatest ever reported in half doped manganites͒ and structural characterization suggest that the 6s 2 lone pair is only weakly screened in this compound; 20,21 a situation different, for instance, to Bi 0.5 Ca 0.5 MnO 3 (T CO Ӎ325 K). 20,22 Theoretical studies indicate that when the lone pair is weakly screened 6s-Bi orbitals can hybridize with 2p-O orbitals thus reduc-ing the mobility of Mn e g electron through Mn-O-Mn bridges. 23ith the objective of exploring the unknown phase diagram of Bi 1Ϫx Sr x MnO 3 family, we present here an investigation of the temperature-dependent electronic behavior of Bi 3/4 Sr 1/4 MnO 3 .Based on electron diffraction and neutron and synchrotron powder-diffraction measurements up to 750 K, our data confirm a singular behavior of Bi 3/4 Sr 1/4 MnO 3 compared to any other R 3/4 A 1/4 MnO 3 .The results obtained are discused in the framework of CO and ZPO models.

II. EXPERIMENTAL DETAILS
A polycrystalline sample of Bi 0.75 Sr 0.25 MnO 3 has been synthesized by sol-gel method.Stoichiometric quantities of Bi 2 O 3 ͑99.9%,Quality Chemicals͒, MnO 2 ͑99.99%,Quality Chemicals͒, and SrCO 3 ͑99.99%,DIOPMA͒ were dissolved in concentrated nitric acid.The solution was then gellified according to the acrylamide polymerization method, 24 and subjected to self-ignition.The quality of the sample was initially checked by laboratory x-ray powder diffraction.The sample is very well crystallized and only very small traces ͑less than 2%͒ of Bi 2 O 3 and Mn 3 O 4 ͑hausmannite͒ have been detected. 25hin specimens for transmission electron microscopy were prepared simply by crushing the sample with an agate mortar and pestle.Crushed fragments were mixed with alcohol, and the small flakes deposited on a holey carbon film, supported on a copper grid.The reconstruction of the reciprocal space was carried out at room temperature with a JEOL 200CX electron microscope, working at 200 kV and equipped with an energy dispersive spectroscopy ͑EDS͒ analyzer.
Neutron powder diffraction ͑NPD͒ measurements were carried out on the D2B high-resolution diffractometer ͑ILL, Grenoble͒ in the 10Ј collimation mode using ϭ1.594Å.Data were collected at several temperatures on heating: 300,500,525,575,600, and 750 K. Synchrotron x-ray powder diffraction ͑SXRPD͒ measurements were done on the BM16 ultra-high-resolution diffractometer ͑ESRF, Grenoble͒.A short wavelength ͓ϭ0.540092(1)Å͔ was selected with the double-crystal Si͑1 1 1͒ monochromator and calibrated with Si NIST.In order to minimize the x-ray absorption due to the high absorption coefficient of the compound, the sample was mixed with diamond powder and placed in a thin borosilicate glass capillary (ϭ0.3 mm).For both diffractometers ͑D2B and BM16͒, the instrumental resolution function was obtained from a pattern of Na 2 Ca 3 Al 2 F 14 .
Magnetic measurements have been done in a Quantum Design superconducting quantum interference device magnetometer under an applied field of 1 T in the range 300 K рTр660 K. Differential scanning calorimetry ͑DSC͒ measurements have been performed using a TA-Instruments commercial calorimeter ͑model 2920͒ using a temperature ramp of Ϯ5 K/min.

Figure 1͑a͒
shows the temperature dependence of the DC magnetic susceptibility and its inverse.A clear anomaly can be appreciated to start ͑on cooling͒ at 600 K.This anomaly is accompanied by a change in the slope of Ϫ1 curves.By fitting Curie laws above and below the transition we have found a change in the effective paramagnetic moment and Curie temperatures from eff ϭ4.68(4) B /Mn and C ϭϩ217(5) K (TϾ610 K) to eff ϭ5.26(2) B /Mn and C ϭϩ104(1) K (TϽ575 K). 25 An increment of the effective moment in the low-temperature phase is a characteristic feature of half doped manganites exhibiting CO, and a proper interpretation was given in the framework of the ZPO model. 14Qualitatively similar changes in the effective magnetic moment have also been reported in Bi 0.5 (Ca 1Ϫy Sr y ) 0.5 MnO 3 for yϭ0,0.33,0.67 and 1. 20 The calorimetry results are displayed in Fig. 1͑b͒, which shows the heat flow obtained when cooling and heating the sample through the transition found by susceptibility data.A moderate hysteresis of about 10 K is visible between the heating and cooling processes.These curves can be adequately integrated thus revealing an entropy change ⌬sϭ0.25(1)R.

A. Electron diffraction
The composition determination was performed by quantitative EDS analysis using a variable spot size and averaging over a large number of crystallites.The cationic analysis evidences a very slight loss of bismuth during the synthesis, but the ratio ͓(BiϩSr)/Mn͔ remains very close to the nominal value.The mean value of the actual cationic composition is Bi 0.71Ϯ0.04Sr 0.29Ϯ0.04, calculated for one manganese per unit cell ͑0.04 refers to the maximum deviation with regard to the average composition͒.The reciprocal space was reconstructed at RT by tilting around the different equivalent ͓100͔ P * and ͓110͔ P * directions of the perovskite sub-cell.
This study evidences complex ED patterns, observed for all the crystallites.The system of intense reflections is characteristic of an orthorhombic distorted perovskite sub-cell with aϷbϷͱ2a p and cϷ2a p (a p is the parameter of the ideal perovskite cell͒; the conditions of reflection found are hkl:hϩkϩlϭ2n; 0kl: k,lϭ2n; h0l: hϩlϭ2n; h00: h ϭ2n; 0k0: kϭ2n; 00l: lϭ2n.These are consistent with I bmm ͑n.74͒ and I bm2 ͑n.46͒ space groups ͑SG͒.Lattice parameters are similar to those reported for La 0.5 Ca 0.5 MnO 3 , 5 and also to those previously reported for the subcell of Bi 0.5 Sr 0.5 MnO 3 , which exhibits at room temperature a supercell related to CO/ZPO. 20,21Besides the main spots, attributable to the I-centered orthorhombic structure, a second set of weaker extra reflections is observed in the ED patterns collected at RT.In order to index them, a doubling of both the a and c parameters is necessary, i.e., the parameters of the supercell are aϷ2ͱ2a p , bϷͱ2a p , and c Ϸ4a p .The observed reflections also imply that there is no condition limiting the reflections.The occurrence of this supercell is illustrated in Fig. 2͑a͒, showing the ͓110͔ pattern, and in Fig. 3͑a͒ the reflections in the supercell are indicated in italics.This supercell is similar to that reported for Bi 2/3 Sr 1/3 MnO 3 . 26esides the superstructure ͑SS͒, the complexity of the ED patterns is reinforced by the existence of multiple small twinning domains, generated by the orthorhombic distortion of the subcell.This is illustrated in Fig. 3͑a͒ where two very weak extra reflections ͑shown by white lines͒ are generated by the presence of a tiny ͓110͔ oriented area in the ͓001͔ matrix.

B. Synchrotron x-ray and neutron powder diffraction
The results shown above are confirmed by the joint Rietveld analysis of NPD and SXRPD data collected at RT.These data can be jointly refined using I bmm SG (R B ϭ4.9%, 2 ϭ2.0 for SXRPD data and R B ϭ5.1%, 2 ϭ1.7 for NPD data͒, but not using I bm2 SG.The refined patterns are shown in Fig. 4 ͑the regions of the SXRPD pattern where the diamond peaks are localized have been removed͒.Together with the ͑sub͒cell reflections a large number of SS reflections can be observed.These have been successfully indexed as satellites of the mean I bmm structure using propagation vectors k 1 ϭ( 12 00) and k 2 ϭ( 1 2 0 1 2 ) ͑second line of reflections in Fig. 4͒.An enlarged view of the SXRPD pattern showing two SS peaks, and their indexes, is plotted in Fig. 5͑a͒.In addition to these peaks, those corresponding to a small impurity of Mn 3 O 4 ͑hausmannite͒ are also marked in Fig. 4 ͓the quantitative analysis renders that its fraction is properly with the propagation vector (0 0 1 2 ).We have not detected in the powder-diffraction patterns superstructure peaks with this propagation vector.This is consistent with the weak intensity of SS peaks in Fig. 2 when compared with those in Fig. 3.
Above the transition evidenced by magnetic and calorimetry measurements, the situation is quite different.The NPD diffraction pattern at 750 K can be satisfactorily indexed with a tetragonal ͓aϭ5.53631(5)Å and cϭ7.8118( 1) Å] primitive lattice, and no superlattice reflections are found.These reflections disappear ͑on heating͒ at about 575 K as is shown in Fig. 5͑b͒.However, the refinement using tetragonal SG's were not satisfactory.The metric of the sample is pseudotetragonal but the symmetry is clearly orthorhombic.The data at Tϭ750 K can be very well refined using orthorhombic P bnm SG ͓with aϭ5.5386(2)Å, b ϭ5.5310(2) Å, and cϭ7.8183(3)Å, which renders R B ϭ6.4% and 2 ϭ1.8], as shown in Fig. 6.It is worth mentioning that although at low temperature the ͑sub͒cell is I centered, several peaks violating the hϩkϩlϭ2n reflection condition are found at high temperature.These peaks disappear below 575 K as shown for (2 1 2)-(1 2 2) doublet in Fig. 5͑b͒.The structural details obtained by the Rietveld refinement of NPD data at different temperatures are reported in Tables I and II. Figure 7͑a͒ shows the temperature evolution of the lattice parameters.When entering the lowtemperature phase, a increases considerably while b and c slightly decrease.The anisotropic evolution displayed is very different from that found in half doped manganites ͑typically that of La 0.5 Ca 0.5 MnO 3 ) ͑Ref.5͒ where c strongly decreases and a and b slightly increase ͑in the P bnm setting͒; and also different from that reported in underdoped manganites ͑for instance Pr 0.7 Ca 0.3 MnO 3 ). 9In contrast, the evolution is very similar to that reported for LaMnO 3 when going through the OO transition. 27rom Table II no significant differences are found between d Mn-O1 and ͗d Mn-O2 ͘ in the studied temperature range.
This contrasts with the highly anisotropic evolution ͑strong apical compression͒ of Mn-O bond distances in half doped manganites. 5In the present case, such an anisotropic evolution could be hindered in the average structure, and would only become visible by solving the superstructure.The thermal evolution of ͗d Mn-O ͘ bond length, Mn-O1-Mn, and Mn- O2-Mn bond angles are shown in Fig. 7͑b͒.It evidences that when entering the low-temperature phase, the MnO 6 octahedron shrinks and that the Mn-O1-Mn bond angle becomes more bent while Mn-O2-Mn is straightened.In contrast with the usual situation in rare-earth manganites, the mobility of e g electrons is not directly governed by the Mn-O-Mn angle ͑although ͗ Mn-O-Mn ͘ is near 167°in Bi 0.5 Sr 0.5 MnO 3 at RT, this compound is insulating͒. 20

IV. DISCUSSION
The whole set of results presented indicates that there is a localization of e g electrons below about 600 K to give an FIG. 6. Rietveld refinement of NPD data at Tϭ750 K.The inset shows the high angle region in detail.Data have been refined using P bnm SG. ordered pattern.The first main observation from this study relies on susceptibility data.We have measured an enhancement of eff at the transition as predicted by the formation of localized ZP in the low temperature phase. 14Qualitatively the value of eff found above the transition is consistent with a mixture of Mn 3ϩ and Mn 4ϩ ͓in a ratio 0.71͑4͒:0.29͑4͔͒eff theor ϭ4.62(4) B /Mn ͓ eff exp ϭ4.68(4) B /Mn͔; and the value found below the transition matches the value that one can expect for the coexistence of ZP and Mn 3ϩ ions proposed in Ref. 17.To assess this, let us assume that a fraction ␣ of the manganese ions are in the 3ϩ valence state ͑with Sϭ2) and the rest ͓a fraction (1Ϫ␣)] form ZP ͑with S ϭ7/2).So, for each ␣ Mn 3ϩ there will be (1Ϫ␣)/2 ZPs.To find ␣ it is enough to consider the mean valence of Mn ions, which will be 3ϫ␣ϩ3.5ϫ(1Ϫ␣).So, in the present case ͓average valence 3.29͑4͔͒ it must be ␣ϭ0.42(8) and then eff ϭ ͱ ␣( eff Mn 3ϩ ) 2 ϩ͓1Ϫ␣/2͔( eff ZP ) 2 ϭ5.32(6)B /Mn which matches very well with the value found: eff exp ϭ5.26(2) B /Mn.From an CO picture one would expect eff theor ϭ4.62(4) B /Mn.These data indicate that below 600 K many paramagnetic units are not single Mn ions.The fact that there are ͑almost͒ no deviations of Ϫ1 curve from the Curie law evidences that these units do not change significantly between 300 and 600 K.The CO/OO picture predicts strong FM coupling inside the FM zig-zag chains of the CE magnetic structure above T N .This could drive to an apparent eff larger than the mentioned value 4.62 B /Mn, but the value would display a clear temperature dependence.It must be mentioned that a ''double-rows-type'' contrast was reported in Ref. 26 ͑for Bi 2/3 Sr 1/3 MnO 3 ) and interpreted as an alternating arrangement of Mn 3ϩ and polarons in the a-b plane.
From high-resolution NPD and ultra-high-resolution SXRPD data, our Bi 3/4 Sr 1/4 MnO 3 sample corresponds to a single, well crystallized phase even below the ordering temperature, where we have observed no evidence of phase separation or segregation.To be emphasized is the fact that coexistence of two phases ͑at RT͒ has been always found when studying other compositions of the Bi 1Ϫx Sr x MnO 3 series such as xϭ1/2 or xϭ2/3. 20,28This is an indication that the present composition is particularly stable and far from the next most stable composition giving a homogeneous long-range-ordered phase.The well defined structural transition gives rise to a modulated structure ͑as observed by ED, SXRPD, and NPD at RT͒ that, in the light of the magnetization data, appears to be more consistent with an ordered pattern of Zener polarons and the remaining (Ϸ40%) manganese atoms in the Mn 3ϩ state than with a pure coexistence of Mn 3ϩ and Mn 4ϩ ions ͑CO model͒.The ordered structure doubles a and c lattice parameters of the ͑sub͒cell.Concomitant with the ordering there is a completely unexpected anisotropic evolution of cell parameters, characterized by a marked expansion ͑on cooling͒ of a, and a small compression of b and c.We recall that a similar evolution of the lattice parameters has been recently reported to occur in Bi 2/3 Sr 1/3 MnO 3 , which shows a doubling of b and c lattice parameters ͑in the I bmm setting͒. 26These similarities and the proximity of both concentrations ͑0.29 and 0.33͒ indicate that probably both compounds present the same type of ordering.This drives us to the question of which is the ideal doping for the present ground state.It must be mentioned  II. that the structural changes are more pronounced in the present case and that the structural modulation persists up to higher temperature ͑it disappears at 500 K for Bi 2/3 Sr 1/3 MnO 3 ). 26In addition to this, DSC measurements on Bi 2/3 Sr 1/3 MnO 3 render an entropy change ⌬sϭ0.20(1)Rsmaller than the value found in the present case ͓⌬s ϭ0.25(1) R͔.So, we are led to conclude that the degree of order is stronger in Bi 0.71(4) Sr 0.29(4) MnO 3 than in Bi 2/3 Sr 1/3 MnO 3 .Namely, the imperfections, attributable to the nonideal stoichiometry, are smaller in the former than in the latter.Consequently, the ideal composition for new order or electronic configuration is closer to xϭ0.29 than to xϭ0.33.Now we concentrate on key aspects of the structural changes.ED, SXRPD, and NPD data show that the SG describing the average structure changes from P bnm, characteristic of an a ϩ b Ϫ b Ϫ rotation of MnO 6 octahedra ͑in Glazer's notation͒, to I bmm resulting in a 0 b Ϫ b Ϫ rotation of octahedra, 29 on cooling.The whole set of structural modifications accompanying the transition to the new electronic phase is completely different to that reported for Bi 1/2 Sr 1/2 MnO 3 and to that usually found in manganites.First, there is a contraction of MnO 6 octahedra ͓Fig.7͑a͔͒ when entering the CO phase on cooling.This evolution contrasts with the characteristic expansion of Mn-O mean distance during electronic localization in manganites and other transition-metal oxides.However such a contraction has been found in other bismuth containing manganites. 30In contrast with the xϭ1/2 case, the electronic reorganization taking place when e g electrons localize in a single Mn ion or form polarons is not reflected in the Mn-O distances found by fitting the average structure.In the xϭ1/2 case the basal ͗d Mn-O2 ͘ bond length is significantly larger than the apical d Mn-O1 bond length. 5,31In the present case there are no differences between these lengths.This dissimilarity can be explained by either the formation of ZP c ͑along c axis͒ or, in the ionic picture, the stabilization of d z 2 orbitals ͑along c axis͒.In both cases one must expect an apical d Mn-O1 bond distance larger than in the xϭ1/2 case.It is of interest to underline that, in the present case, the structural transition does not consist in a slight modulation of the orthorhombic P bnm high-temperature phase (as commonly found in CO manganites) but in a more pronounced structural transformation that involves a marked symmetry change of the average cell.Corroborating this fact, the number of observed superstructure peaks is larger ͑and their intensities higher͒ than in most common CO Mn perovskites.Unfortunately the description available below the ordering temperature corre-sponds to the average structure but not to the true ͑super-͒structure, and a definitive picture of the electronic redistribution cannot be drawn precisely.
This new type of ordered structure is more stable ͑its onset temperature is 600 K͒ than that of Bi 0.5 Sr 0.5 MnO 3 ͑with onset temperature 525 K͒.It must be noted that this new type of ordered structure has not been reported in rare-earth based manganites, which, in general, are ferromagnetic and metallic or present phase separation, in this compositional range.Thus, it is the fact that bismuth strongly favors the localization of e g electrons that allows this structure to be formed.

V. CONCLUSIONS
Bi 0.71(4) Sr 0.29(4) MnO 3 displays an ordered localization of e g electrons at a very high temperature ͓the onset temperature, from Fig. 1͑a͒, is T ZPO Ӎ600 K, the highest ever re-ported͔.The ordering transition is characterized by a change in the SG describing the average structure and by a modulation of the low-temperature phase that doubles a and c lattice parameters of the orthorhombic ͑sub͒cell.This transition is also accompanied by a considerable enlargement of a lattice parameter ͑while b and c shrinks͒, by the contraction of the Mn-O bond distance, and by the bending ͑opening͒ of Mn-O1-Mn ͑Mn-O2-Mn͒ bond angles.It is likely that the same type of order is also present in Bi 1Ϫx Sr x MnO 3 with xϭ1/3, 26 but the present compound is closer to the ideal stoichiometry for this type of order.On the other hand, the structural evolution evidences important differences between the order found and that of half doped Bi 0.5 Sr 0.5 MnO 3 . 20,31The order found is stronger ͓T ZPO (Bi 0.71 Sr 0.29 MnO 3 ) ϾT ZPO (Bi 0.5 Sr 0.5 MnO 3 )͔ and, in contrast with the xϭ0.5 and 0.67 cases, the studied sample does not separate into two macroscopic phases at the ordering transition.In order to draw a more detailed picture of the order established at T ZPO , future research, involving the study of the magnetic order at low temperature, is needed.Such description must help us to understand how the polaron ordering takes place in this type of underdoped compounds.
FIG. 1. ͑a͒ Temperature dependence of the susceptibility () and its inverse.The straight lines are the best fit of the Curie law for the regions above and below the transition.͑b͒ DSC thermogram showing the exotermic and endothermic transitions on cooling and heating, respectively.A small hysteresis can be observed.

1. 5
FIG. 3. ͑a͒ ͓001͔ room temperature ED pattern showing the extra reflections characteristic of the superstructure along a*.The reflections are indexed in the I bmm cell.The two small lines point to weak reflections coming from a twinning ͓110͔ oriented domain.͑b͒ Indexation of these spots.Italics corresponds to indexation in the supercell.

FIG. 7 .
FIG. 7. ͑a͒ Anisotropic temperature dependence of the ͑sub͒cell lattice parameters ͑left axis͒ and evolution of the unit-cell volume ͑right axis͒ through the transition.On cooling, the great expansion of a contrasts with the small shrink of b and c. ͑b͒ Temperature evolution of Mn-O-Mn bond angle and Mn-O bond distances from TableII.

TABLE I .
Structural details obtained by Rietveld refinement of NPD and SXRPD data at RT and from NPD data at Tϭ500,525,575,600, and 750 K ͑average structure͒.For I bmm SG, Mn is on a 4b ( 1

TABLE II .
Interatomic bond distances and angles found by Rietveld refinement of diffraction data at different temperatures.