A-site Size Disorder Research Articles

Investigations on structural disorder-induced modifications in the transport behaviour of rare-earth manganites

The results of the studies on structural disorder-induced modifications in the transport behaviour of La0.5Pr0.2Ca0.3−x Ba x MnO3 (LPCBMO) (0.05 ≤ x ≤ 0.30) manganites were reported. Structural studies using X-ray diffraction (XRD) measurements confirmed the single phasic nature of all the samples without any detectable impurities. The A-site size disorder ( ${ {\sigma }}_{\mathrm{A}}^{\mathrm{2}}$ ) increased from 3.81 × 10 −5 (x = 0.05) to 14.9 × 10−5 (x = 0.30). With the increase in structural disorder in LPCBMO system, the transport improved for the range: 0.15 ≤ x ≤ 0.30, which can be ascribed to the enhancement in one electron bandwidth which dominates over the structural disorder effect, while for lower values of x, strong competition existed between size disorder and one electron bandwidth. Below 50 K, all ρ–T plots showed resistivity minimum behaviour, which modified with disorder. This behaviour was discussed in detail on the basis of electron–electron interaction having the form: $ {\rho } {=} {[}\mathrm{1} {/} {(} {\sigma }_{\mathrm{0}} {+} {BT}^{\mathrm{1 {/}2}} {)} {]} {+} {\rho }_{ {n}} {T}^{ {n}}$ . Variation in temperature sensitivity with disorder was also discussed in context of granular morphology and phase segregation scenario.

Synthesis method dependence of the lattice effects in Ln0.5M0.5FeO3 perovskites (Ln = La and (Nd or Gd); M = Ba and (Ca or Sr))

A new series of cubic iron perovskites with the composition Ln0.5M0.5FeO3 (Ln = La and Nd (or Gd); M = Ba and Sr (or Ca)) with the same average A-site radius (〈rA〉 = 1.3 Å) but different A-site size disorder, σ2(rA), from 0.0114 to 0.0230 Å2, has been prepared by three different synthesis routes: (a) ceramic method, (b) combustion method and (c) the Pechini method. A strong correlation of the structural parameters with σ2(rA) and the synthesis method is observed. The unit cell volume increases with σ2(rA) in all the cases but the overall isotropic displacement factor of the atoms and the lattice microstrain do not show the same trends. Samples synthesised by the low temperature routes show a σ2(rA)-dependence of the microstrain whereas in the oxides prepared by the ceramic method microstrain seems to be σ2(rA)-independent.

Magnetic phase coexistence in Tb +3- and Sr +2-doped La 0.7Ca 0.3MnO 3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La 0.325Tb 0.125)(Ca 0.3Sr 0.25)MnO 3 manganite sample. The simultaneous doping of magnetic Tb 3+ and divalent Sr 2+ in the La 0.7Ca 0.3MnO 3 system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈 r A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator–metal transition ( T IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature ( T C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb) 0.45(CaSr) 0.55MnO 3 sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.

Effect of size disorder on the magnetic and transport properties of the (LaR) 0.55(CaSr) 0.45MnO 3 (R=Nd, Sm & Tb) compounds

Electrical resistivity, magnetization and magnetoresistance (MR) measurements have been performed on ABO 3-type (LaR) 0.55(CaSr) 0.45MnO 3 (R=Nd, Sm and Tb) compounds, in which A-site size disorder varies but tolerance factor remains constant. The insulator–metal (I–M) transition temperature ( T p) and the Curie temperature ( T C) of these compounds decrease from ∼170 K for R=Nd to ∼120 K for R= Tb, which is consistent with increasing size disorder. All the samples exhibit maximum in MR at T p and an MR of 70–80% at 5 K. The reduced T p and T C and enhanced MR in these compounds vis-`a-vis the T p and T C of ∼230 K of the standard La 0.55Ca 0.45MnO 3 compound, may be attributed to the local structural distortions induced by enhanced A-site size-disorder.

Effect of the A Cation Size on the Structural, Magnetic, and Electrical Properties of Perovskites (La1−xNdx)0.7Sr0.3r003nMnO3

An intense effort has recently been devoted to studying the interplay between structure, magnetism, and transport in manganese perovskite Ln1−xAxMnO3 (Ln=La, Pr, Nd, Sm; A=Ca, Ba, Sr). As a function of temperature, applied magnetic field, doping, A-site ionic radius 〈rA〉, and A-site size disorder, this system displays a rich phase diagram for both magnetotransport and structural properties. We have investigated the structural, magnetic, and transport properties of (La1−xNdx)0.7Sr0.3MnO3. The crystal structure was examined by X-ray powder diffraction which indicated that all the samples were single phase and revealed a transition from rhombohedral to orthorhombic structure with increasing x. The magnetization and resistivity investigation shows that for all values of x, (La1−xNdx)0.7Sr0.3MnO3 are ferromagnetic–metallic at low temperatures and paramagnetic–semiconductor above the Curie temperature Tc.