ABSTRACT

Room and liquid nitrogen temperatures Mn K-edge extended X-ray fine structure (EXAFS) studies were carried out on powder samples of ALaMnRuO₆ (A = Ca, Sr, Ba) perovskites. The EXAFS analysis of Mn K-edge spectra showed MnO₆ octahedra are distorted. Among the three perovskites, the polyhedral distortion is highest for SrLaMnRuO₆ (D = 1.4 × 10^–3). For CaLaMnRuO₆ (D = 0.5 × 10^–3) and BaLaMnRuO₆, (D = 0.7 × 10^–3), the smaller values of distortion parameters indicate that the MnO₆ octahedra are relatively regular and undistorted in the latter two systems. In SrLaMnRuO₆, MnO₆ distortion appears to be of orthorhombic (Q₂) type whereas for CaLaMnRuO₆ and BaLaMnRuO₆, the distortion is of tetragonal or Q₃ type. The Mn-O distances as determined from the EXAFS measurements, increases with the increase in the size of the divalent cation. No structural changes occurred in the local environment around Mn atom when the temperature was lowered from room to liquid nitrogen temperature.

1. Introduction

The ideal perovskite structure with stoicheometry AMO₃, where A is typically a large, low oxidation state cation, M is a smaller transition metal or lanthanide cation, has cubic symmetry with space group [1]. The capacity of perovskite structure to incorporate almost all the elements in the periodic table except for noble gas elements, beryllium and phosphorus has resulted in materials exhibiting range of properties including dielectric, piezoelectric, ferroelectric, optical, superconducting and magnetoresistive properties [2]. The perovskite structure is viable to wide variations in compositions from the ideal formula AMO₃. These variations can be achieved by isomorphous substitutions at A or M sites, and cationic and anionic deficiencies. These can be represented as,. The physical properties of perovskites are shown to depend on the doping levels at the A or M sites [3-6]. For 50% substitution, such mixed system are classified as double perovskites with the formula AA′MM′O₆ or A₂MM′O₆ as reported by Anderson et al. (1993) in reference [7]. The lattice distortion and local structure plays an important role in the magnetoelectric transport properties of perovskite manganites, in particular its effect on the colossal magnetoresistive (CMR) properties [8-10].

Ruthenates perovskites has attracted  immense interest of the physicist and chemist over the past few years [11-15]. This surge of interest in ruthenates is because of their rich variety of magnetic states exhibited by these materials. For example, SrRuO₃ is a 4d ferromagnetic metal [14] while CaRuO₃ is metallic but not magnetic; the double perovskite Sr₂YRuO₆ is an antiferromagnetic insulator; and the layered perovskite Sr₂RuO₄ is superconducting [15]. The magnetic properties of double perovskite ruthenate oxides depends on the nature of MM′ distribution over the octahedral sites [16-18].

Though the crystal structures of ALaMnRuO₆ (A = Ca, Sr, Ba) are well established [19-24]. However, no information is available about the local structure around the manganese in these materials. In our earlier publications, we have reported crystal structure as well as the oxidation state of Mn in SrLaMnRuO₆, BaLaMnRuO₆ and CaLaMnRuO₆ [21-23]. Room temperature local strucutre around Mn in SrLaMnRuO₆ has also been reported [21]. In the present paper, we now report the local structure around Mn in CaLaMnRuO₆ (CLMRO), and BaLaMnRuO₆ (BLMRO) at room and liquid nitrogen temperatures. In addition we also report the loal structure in SrLaMnRuO₆ (SLMRO) at liquid nitrogen temperature.

2. Experimental

The single-phase powder samples of ALaMnRuO₆ (A= Ca, Sr, Ba) were synthesized by conventional solid-state reaction method as reported earlier in [21]. X-ray diffraction revealed that all the samples have orthorhombic symmetry (space group Pbnm), with the tilting scheme a^–a^–b⁺ which results in the unit cell dimension of ×× 2a (where a is the pseudocubic unit cell parameter).

Room and liquid nitrogen temperatur Mn K-edge Xray absorption spectroscopy experiments, which include both XANES and EXAFS techniques, were performed at the XAFS beam line at ELETTRA Synchrotron Radiation Source, Trieste, Italy. Well-dispersed powder samples in cyclohexane were deposited on porous membranes. The amount of the sample was pre-calculated to obtain optimum absorption jump (Dμ » 1). X-ray beam energies were tuned using a double crystal, focusing Si (111) monochromator crystals. The incident (I_o) and transmitted (I_t) beam intensities were recorded using ionization chambers filled with appropriate gases. Data were collected in three regions: 1) the pre-edge region with a step size of 5 eV to allow pre-edge background calculation and subtraction; 2) the XANES region from 30 eV below the edge to 50 eV above the absorption edge with a step size of 0.3 eV; and 3) the EXAFS region upto 7050 eV. All the edges were recorded at least twice. Raw data were corrected and converted to k-space, summed and background subtracted to yied the EXAFS function c(k) using the EXCALIB and EXBACK Programs [25]. The edge profiles were separated from the EXAFS data and, after subtraction of the linear pre-edge background, normalized to the edge step. The data were weighted by k³, where k is the photoelectron wavevector, to enlarge the oscillations at high k. The data were fitted using the non-linear square minimisation program EXCURV92 [26] which calculates the theoretical function using the fast curved wave theory [27]. Phase shifts corrections were calculated using Xalpha exchange and ground potentials.

3. Results and Discussion

Figure 1(a)-(b) represents the modulus of Fourier Transform (FT) extracted from the XAS data at 300 K and 80 K by measuring Mn K-edge EXAFS spectra for the three systems. The k³-weighted c(k) data were Fourier transformed (FT) in the k range of 2.5 - 12 Å^–1 using a Guassian window. Well defined peaks corresponding to the different coordination shells around the Mn absorbing atom are visible upto about 5 Å. The main intense peak at about 1.9 Å in the FT is associated with the MnO₆ octahedron. Further peaks around 3 - 4 Å in the FT are attributed to Mn-A/La and Mn-O-Mn interactions [8,28].

The first peak of FT, corresponding to the Mn-O, octahedra are identical for all the materials except for a

slight upward shift that indicates gradual increase of the Mn-O distances with increasing size of A atom. However, there are marked differences corresponding to Mn-A/La and Mn-O-Mn interactions as we move from CaLaMnRuO₆ to BaLaMnRuO₆. The shape of these peaks are found to be dependent either on the crystallographic symmetry or the different scattering power of the divalent cations. Since, laboratory X-ray diffraction studies have revealed that all of these materials have the same orthorhombic symmetry (space group Pbnm) [21-23], these differences can only be attributed to the different scattering powers of A cations. For all three samples, nearly constant amplitudes of Mn-O peaks at 300 K and 80 K indicates that no structural changes occur in the local environment around the Mn as the temperature is lowered.

Previous neutron diffraction [19,20] and synchrotron X-ray diffraction studies [21,23] have shown that the structure of ALaMnRuO₆ is orthorhombic with distorted Mn/RuO₆ octahedra (Figure 2). Owing to the Jahn Teller effects, caused by the 3d Jahn Teller ion Mn³⁺, distortion

of MnO₆ produces three Mn-O bonds with equal bond distances, long (l), medium (m) and short (s) bonds [29, 30]. In case of CLMRO and BLMRO, the MnO₆ octahedra are quite regular and the spread in the bond lengths is small [22,23] whereas in SLMRO the bondlengths in the basal plane are further split into two Mn-O bonds of equal bond lengths and the structure is considerably distorted [21].

Extended X-ray absorption fine structure (EXAFS) spectroscopy provides structural information about a sample by way of the analysis of its X-ray absorption spectrum. In order to extract structural information from experimental spectra, a simple analytical expression that relates the EXAFS signal to the structural parameters is required. An EXAFS analytical expression like the one suggested by Teo [31] is:

where k is the electron wave vector, N_j is the number of atoms in the j-th coordination shell, δ_j(k) is the phase shift, f_j(k) is the scattering amplitude, σ_j is the Debye Waller factor in terms of a Gaussian broadening resulting from thermal and static disorder and R_j is the M-O bond length in the Mn-O octahedral (Figure 2(b)). Various structural parameters can be extracted by an approximate Fourier analysis of the normalized EXAFS term c(k) [32].

In all of these perovskites oxides, the Mn ions are surrounded by six oxygen atoms forming an octahedron (Figure 2(b)). In cubic, undistorted perovskite structure, all the Mn-O bond distance are equal, i.e., R₁ = R₂ = R₃. As both La and A cations are distorted from the central position in the simple cubic unit cell, making their nearneighbor oxygen environment complicated. In addition to these distortions, presence of JT distortion is also present. The six otherwise equal, Mn-O bond lengths in the tilted octahedral consequently split into pairs of unequal bond length. Distortion caused by the Jahn-Teller effect in perovskites usually involves four of the octahedral bonds (bonds involving planar oxygen, R₂ and R₃) contracting and two of the octahedral bonds (apical oxygen) lengthening which gives an elongated octahedral shape.

Different models were employed for fitting the experimental EXAFS data. In the first model (2 + 2 + 2 model), we have used three subshells each with a coordination number N_Mn-O = 2 with unequal Mn-O bondlengths. Subsequently we employed models with two sub shells, one corresponding to a Mn-O coordination number N_Mn-O = 4 at shorter distance and the other corresponding to N_Mn-O = 2 at longer distance (4 + 2 model); a model with bimodal distribution of Mn-O bonds with three longer Mn-O bonds and three shorter Mn-O bonds (3 + 3 model) and a model with one Mn-O sub shell with N_Mn-O = 6. Owing to strong correlation between coordination number (N) and Debye-Waller factor (σ²), only the distance (R) and σ² were refined. Since electronic properties mainly depend on the nearest neighbours, we are focusing only on the first shell analysis.

For SLMRO, as reported previously [21] the best fit to EXAFS data were obtained for 2 + 2 + 2 model. In case of CLMRO and BLMRO, the best fits were obtained for 4 + 2 model, i.e. four short and two long bonds. The remanining models lead to relatively high values of Debye Waller Factors without any significant improvement in the fit. Typical fitted FT spectra at room temperature are shown in Figure 1(a). The summary of the fitted structural parameters are listed in Table 1. The calculated interatomic distances are in good agreement with values obtained from the crystallographic measurements [19-23]. This suggests that the local structure as determined from EXAFS mathches the average structure as determined from neutron or Synchrotron X-ray diffraction. The main result is that the Mn-O distances increases with the increasing size of the divalent cation A in agreement with the shifts observed in the first peak of the FT. The slightly higher values of Debye-Waller factors suggests a site to site variation of the bond lengths in the MnO₆ octahedra [28].

There are two types of distortion modes associated with the JT distortion: Q₂ and Q₃ which are defined in terms of long (l), medium (m) and short (s) Mn-O bonds as:

with l, s being the Mn-O distances in the ab plane and m is out of plane Mn-O bond length. Q₃ is a tetragonal whereas Q₂ is orthorhombic distortion [29]. In the tetragonal or Q₃ distortion the in-plane bonds extend and out of the plane bond shortens or vice versa implying that one bond length is larger than the other two. On the other hand, in the orthorhombic or Q₂ distortion, the two in plane bond lengths (Mn-O₂ ) split more symmetrically around the out of plane Mn-O₁ bond length. Examination of the bond lengths derived from the XAFS data reveals that in SLMRO, MnO₆ distortion appears to be of orthorhombic (Q₂) type whereas for CLMRO and BLMRO, the distortion is of tetragonal or Q₃ type.

The MnO₆ polyhedral distortion parameter D, frequently observed for e_g level degenerated systems such as Mn³⁺ () containing perovskites is defined as

where ádñ is average Mn – O bond distance. The distortion parameters of ALaMnRuO₆ are listed in Table 2. Among the three perovskites, the polyhedral distortion is the highest for SLMRO. For CLMRO and BLMRO, the smaller values of distortion parameters indicate that the MnO₆ octahedra are relatively regular and undistorted. This result is consistent with the neutron diffraction meas-

urements of Garanado et al. [19]. As the temperature is lowered, there is little change in the average bond distances and hence the distortion parameters.

4. Conclusion

We have carried out Mn-K-edge X-ray Absorption spectroscopy measurements to study valence state of Mn and local structure around it in single phase ALaMnRuO₆ perovskites prepared through solid state technique. The Mn-O distances as determined from the EXAFS measurements, increases with the increasing size of the divalent cation. No structural changes occur in the local envoirnment around Mn as the temperature is lowered from room to liquid nitrogen temperature.

5. Acknowledgements

Two of the authors (JB and RS) are grateful to the Abdus Salam International Center for Theoretical Physics (ASICTP) for the provision of financial support under the ICTP-ELETTRA user programme. The authors are also grateful to Dr. Luca Olivi for his skilful help in conducting the experiments at the XAFS beam line at the ELETTRA Synchrotron Radiation Source, Trieste, Italy.

REFERENCES

NOTES

^*Corresponding author.