Synthesis, structural, magnetic and electrical properties of nominal La0.67Ba0.33-xNaxMnO3 \(0 ≤ x ≤ 0.33\) manganites

Complete nominal solid solutions La0.67Ba0.33-xNaxMnO3 (0 ≤ x ≤ 0.33) have been elaborated by ceramic route at 1200°C. ICPAES chemical analysis and XRD Rietveld structure refinement revealed that the nominal Na content is not achieved by our ceramic method. The chemical formula should be rather written as LaαBaβNaλMnO3δ with a significantly lower Na content (λ), than the nominal one (x), and with a slight deficit (δ) in oxygen content, leading to the appearance of two minor secondary phases identified as Mn3O4 and Na0.55Mn2O4 .1.5H2O. Magnetization and electrical resistance vs. temperature show paramagnetic/semiconductor – ferromagnetic/metallic transitions with only a slight decrease in magnetic and electrical transition temperatures when Na content increases. Such amount of this decrease is not expected according to the nominal Na content giving a significant difference between nominal and experimental Mn/Mn ratios.


Introduction
The mixed-valence state perovskites, Ln 1-x A x MnO 3 (Ln = rare earth, A = Ca, Sr, Ba ...) have taken considerable interest and have been extensively studied over the past fifteen years, following the discovery of colossal magnetoresistance (CMR) in these compounds [1].Although the CMR was discovered for the first time in thin films of La 0.67 Ba 0.33 MnO 3 compound [1], the manganite system La 1-x Ba x MnO 3 remains less studied than La 1-x Ca x MnO 3 and La 1-x Sr x MnO 3 homologous.Furthermore La 1-x Ba x MnO 3 is one of the prototype materials with a large size difference ("mismatch" effect) between La 3+ (r La 3+ =1.36Å) and Ba 2+ (r Ba 2+ =1.61Å) [2].The composition La 0.67 Ba 0.33 MnO 3 (x= 0.33) shows the highest Curie temperature (T C = 350K [3]) in La 1-x Ba x MnO 3 system.Recently we have studied the structural, magnetic and electrical properties of some manganites doped at Mn site by Ti, La 0.67 Ba 0.33 Mn 1-x Ti x O 3 [4], or by Fe, La 0.67 Ba 0.33 Mn 1-x Fe x O 3 [5], and we have showed for these compounds that the amount x= 0.1 of Ti or Fe is a limiting value of doping level above that we can't observe ferromagnetic/metallic -paramagnetic/semiconductor transitions and particularly with a Curie temperature value nearing the room temperature for x= 0.05 concentration, which is required for potential applications based on CMR and magnetocaloric effects.Another way to control the a e-mail: hcini_sobhi@yahoo.frmagnetic and electrical transitions temperatures is the substitution of barium by a monovalent cation such as sodium, with smaller radius, which reduces in particular the "mismatch" effect between La and Ba and may lead to significant changes in physical properties of the substituted compounds.Recently Das and Dey [6] have studied the magnetocaloric properties of nominal La 0.7 Ba 0.3- z Na z MnO 3 (z= 0.05, 0.1 and 0.15) manganites and have concluded that their materials are single phase rhombohedral c 3 R , though the only X-ray diffraction pattern for z= 0.05, showed in their work, contains extra peaks of not identified secondary phases.Moreover, no quantitative chemical analysis of the prepared samples was presented and no special care was taken to reduce Na volatilisation.Besides there isn't a significant change of Curie temperature as a function of Na content in their work.These circumstances prompted us to examine the synthesis, quantitative chemical analysis, structural, magnetic and electrical properties of complete nominal solid solutions La 0.67 Ba 0.33-x Na x MnO 3 (0 ≤ x ≤ 0.33).

Experimental procedures
Samples were prepared by the traditional solid-state reaction method.Mixed powders of La 2 O 3 , BaCO 3 , Na 2 O 3 and MnO 2 precursors was heated at 800°C for 24h and then subsequently pulverized.The obtained powder was ground, pressed into pellets and sintered for 24h at 1000°C.This process was repeated at 1100°C and 1200°C, respectively, followed by furnace cooling in atmosphere.Powder X-ray diffraction (XRD) analyses were carried out with a "PANalytical X'Pert Pro" diffractometer with filtered (Ni filter) Cu radiation.Microstructure of specimens and their composition were studied by scanning electron microscopy (SEM) using a Philips XL30 microscope with an energy dispersive X-ray spectrometer, and with inductively coupled plasma atomic emission spectrometry (ICP-AES).Magnetizations (M) vs. temperature (T) and vs. magnetic field (H) were measured using extracting sample magnetometer.M(T) data were obtained at 0.05 Teslas, in 10 -500 K temperature range.M(H) data were obtained at T= 50K, up to 10 Teslas.The electrical resistance vs. temperature was measured in 80 -300K temperature range, using two electrical contacts done on samples having a disk shape with a diameter of 10mm and a thickness of about 2mm.

Results and discussion
Figure 1 exemplifies SEM micrographs of our samples.Below x= 0.1 Na amount, the micrographs show a unique chemical contrast corresponding to the manganite phase (see figure 1.a).For x= 0.15, 0.2 and 0.25 compositions SEM micrographs show a coexistence of two phases: a major primary phase with an homogeneous clear contrast corresponding to the manganite phase and a minor secondary phase with dark contrast identified as Hausmannite (Mn 3 O 4 ) phase.Besides the two previously mentioned phases, SEM micrographs show a third phase (see figure 1.b) identified as Nabirnessite Na 0.55 Mn 2 O 4 .1.5H 2 O [7,8] for x = 0.3 and 0.33.Results of ICPAES analysis indicate that the compositions of La, Ba and Mn are very close to the nominal ones, whereas Na content is significantly lower than the nominal one.Based on morphological characterization, EDX and XRD analyses we suggest that this loss of Na leads to lacunar manganite phase and to the formation of Mn 3 O 4 and Na 0.55 Mn 2 O 4 .1.5H 2 O secondary phases, especially for higher Na amount.To quantify the proportion of the major manganite phase in our specimens we have reported in table 1 the results of ICPAES analysis corrected considering the presence of the Mn 3 O 4 and Na 0.55 Mn 2 O 4 .1.5H 2 O minor secondary phases estimated by XRD quantitative analysis using the Rietveld [9] and EVA [10] methods.This comparison shows that our compound presents two extra peaks between 10° and 20°, which are not observed in the spectrum of Kalyana et al. [11], since such spectrum begins at 20°.However one can clearly see a third extra peak marked by asterisk (*) at 25° in the two diagrams.These suggestions give evidence that the nominal amount of Na in prepared manganites does not be achieved in [6,11] works even the synthesis methods used by these authors differ from the one used by us.Detailed results of Rietveld refinement for some of our samples are listed in Table 2.One can see in this table that cell volume, bond length d Mn-O and bond angle θ Mn-O-Mn exhibit only a slight decrease, with the increase of Na, due to the small size of Na substituting the bigger Ba ion and especially to the important loss of Na.This is consistent with recent works [12,13] showing that lattice parameters are mainly affected by the size effect of A perovskite site, inducing a significant change in the Mn-O-Mn bond angle.00022-p.5  EMM-FM2011 figure 5).The T C values obtained for all samples are listed in Table 3.It is observed that the compound λ= 0.028 (x= 0.1) has a Curie temperature T C = 334K, practically the same of that of the undoped compound (λ= 0, T C = 335K), then T C decreases to 326K for λ= 0.043 (x= 0.15) and remains almost constant around 331K for λ= 0.076 and 0.093 (x= 0.20 and 0.25).This weak decrease of T C with the increase of Na content is very consistent with the results of Das and Dey [6] work showing that T C diminishes only by 5K when Na nominal content reaches z= 0.15.This result is a clear evidence that the chemical formula shouldn't be the same of the nominal one also in ref. [6].The b -inset of figure 5 is an enlarged region of M(T) curves for temperatures below T C .We can clearly see in that region a slight decrease in the magnitude of M(T) curves for the undoped compound (λ= 0) and for the doped one (λ= 0.028, x= 0.1), which should be ascribed to the contribution of the orthorhombic antiferromagnetic phase [14,15].Contrarily, beyond λ= 0.043, the magnetization exhibits a slight increase for all rhombohedral samples.Such increase is suddenly reinforced at 47K and below (see the b-inset of figure 5).This reinforcement is due to the contribution of the ferrimagnetic Mn 3 O 4 secondary phase whose transition temperature is 46K as reported in ref. [16].Figure 6 shows the M(H) curves measured at 50K for samples with (0 ≤ λ≤ 0.093).This temperature is selected just above the transition temperature (46K) of the ferrimagnetic phase Mn 3 O 4 whose contribution to the magnetization is only relevant below this temperature.The M(H) curves exhibit a rapid increase with H at low magnetic fields, which suggests rearrangement of FM domains, whereas above 1 Tesla the magnetization is almost constant.This behaviour is consistent with the assumption of a FM state.
The observed and calculated magnetic saturation moments coincide reasonably (see table 3) when we consider the Mn 4+ /Mn 3+ ratio given by La α Ba β Na λ MnO 3-δ formula.But the difference between meas s µ and cal s µ becomes important if we use the Mn 4+ /Mn 3+ nominal ratio.This is a second indication that the nominal content of sodium is not achieved in our samples synthesized by ceramic method and also in some works through the literature.Hence, using usually the nominal chemical formula without doing quantitative chemical analyses leads to misinterpretation of properties of such manganites.
00022-p.7 The temperature dependence of resistance R(T) of the samples has been investigated over a temperature range 80-300 K and the behaviour is shown in figure 7. Using the sign of the temperature coefficient of resistance dR/dT as a criterion (dR/dT < 0) for a semiconductor behaviour and dR/dT > 0 for a metallic behaviour), all samples, except λ= 0.02 one, exhibit a metal-like behaviour at low-temperature (T<T P ) and a semiconductor-like one above T P , where T P is the temperature peak of resistance, as indicated by arrows in figure 7. The maximum of resistance, R TP , and the electrical transition temperature, T P , values obtained for all samples are listed in Table 3.In this table we note a slight increase in R TP for samples with 0 ≤ λ ≤ 0.093, while beyond λ= 0.093 (λ= 0.107 and 0.122), R TP undergoes a sharp increase probably due to the contribution of the secondary Na 0.55 Mn 2 O 4 .1.5H 2 O phase.On the other hand T P decreases from 265K for λ= 0 to 240K for λ= 00022-p.8 EMM-FM2011 0.043, then remains nearly constant (around 250K) for λ= 0.073 and 0.093, thereafter T P decreases to 238K for λ= 0.122.One can notice that our T P value for λ= 0.122 (x= 0.33) Na content is in good agreement with the reported one of Kalyana et al. [17] for La 0.67 Na 0.33 MnO 3 compound (T P = 247K).This is a third indication that Na is partially reacted in LNMO compound synthesized by Kalyana et al. [11,17].We noted also that the T P values of our specimens are lower than T C values (see table 3).Such difference is often attributed to the grain boundary effects [18,19].Other studies suggest that the doping level in the A-site [20] as well as oxygen deficiency [19,21] play an important role for the observed electrical behaviour.1.5H 2 O for higher Na doping rate.The interpretation of magnetic and electrical properties was based on the Mn 4+ /Mn 3+ ratio determined according to La α Ba β Na λ MnO 3-δ formula.Such ratio is significantly different from the nominal one.Rietveld structure refinement revealed that compounds with λ= 0 and 0.02 exhibit mixed rhombohedral and orthorhombic phases but for λ≥ 0.028 the manganite phase exhibits single rhombohedral ( c R3 ) symmetry.Magnetic and electrical measurements show that these lacunar manganites exhibit ferromagnetic/metallicparamagnetic/semiconductor transitions with a weak decrease in Curie temperature, T C , and electrical transitions temperature, T P , with increasing Na content.Our magnetic and electrical results are consistent with those of Das and Dey [6] and Kalyana et al. [17], which give a clear evidence that the nominal chemical formula of manganites does not be achieved in their work and secondary phases shouldn't be excluded.Hence, using usually the nominal chemical formula without doing quantitative chemical analyses leads to misinterpretation of properties of such manganites.

Figure 2 .
Figure 2.a presents the XRD patterns for solid solutions La α Ba β Na λ MnO 3-δ (0 ≤ λ ≤ 0.122) showing sharp and intense peaks corresponding to the major manganite phase.No impurity peaks were observed below λ= 0.028 (x=0.1)Na content, in agreement with SEM observations.However beyond that concentration the XRD patterns show at 2θ= 36° a small peak (indicated by an asterisk in red color) corresponding to Hausmannite Mn 3 O 4 phase.For higher Na content (λ= 0.107 and

Figure 3 .
a exemplifies the Rietveld refinement of XRD data for La 0.71 Ba 0.15 Na 0.076 MnO 2.97 compound showing good agreement between observed and calculated profiles, if we consider the secondary Mn 3 O 4 phase in the refinement.In figure 3.b we gave XRD profiles of La 0.70 Ba 0.25 Na 0.05 MnO 3 of Das and Dey [6] work in which we have added asterisk showing extra peaks matching well with those of Mn 3 O 4 phase (compare Fig. 3.a and 3.b).These extra peaks are not considered for Rietveld refinement by Das and Dey [6].On the other hand for x= 0.33 Na nominal content we compared in figure 4 our X-ray diagram (figure 4.a) with the one of La 0.67 Na 0.33 MnO 3 (LNMO) of Kalyana et al. [11] (figure 4.b).

Table 1 .Fig. 3 .
Fig. 3. (a)-Observed and calculated XRD profiles obtained by the Rietveld analysis.Their difference is represented at the bottom of the figure (blue solid line).The vertical ticks show the positions of the calculated Bragg reflections for La 0.71 Ba 0.15 Na 0.076 MnO 2.97 (rhombohedral c R 3 ) and the minor Mn 3 O 4 phase (tetragonal I4 1 /amd).(b)-XRD profiles of La 0.70 Ba 0.25 Na 0.05 MnO 3 of Das and Dey [6] work in which we have added asterisk showing extra peaks matching well with those of Mn 3 O 4 phase (compare Fig. 3.a and 3.b).

Figure 5
Figure 5 shows magnetization vs. temperature, M(T), at 0.05 Teslas applied magnetic field for all samples.A clear paramagnetic (PM) -ferromagnetic (FM) phase transition, corresponding to a sharp increase of M(T), is observed in this figure.An estimation of the Curie temperature, T C , for PM -FM phase transition was determined by the temperature where dM(T)/dT is minimum (see the a-inset of

Fig. 5 .
Fig. 5. Temperature dependence of the magnetization measured at magnetic field µ 0 H = 0.05Teslas for La α Ba β Na λ MnO 3-δ (0 ≤ λ ≤ 0.122) manganites including, when present, the Mn 3 O 4 secondary phase.The ainset represents the dM/dT vs. T curve.The b-inset is an enlargement of data in low temperature region (below 150K)