High temperature perturbed angular correlation studies of Ln2NiO4+δ Ln=La, Pr, Nd

To read the full-text of this research, you can request a copy directly from the authors.


High temperature measurements on Ln2NiO4+δ (Ln=La, Pr, Nd) have been performed indicating a complex behavior in PAC experiments which can be attributed to fluctuating electric field gradients caused by the high mobility of excess oxygen.KeywordsPerturbed angular correlation (PAC)La2NiO4 Pr2NiO4 Nd2NiO4 Hight temperature measurements

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

ResearchGate has not been able to resolve any citations for this publication.
Full-text available
The lattice sites of implanted In atoms in Cr2O3 were investigated by means of electron emission channeling (EC) measurements using 111In probe atoms. EC spectra were recorded for several axes and compared to simulations. Indium atoms are most likely located at near-Cr sites. Small differences in the EC patterns for prompt and delayed electrons may be an indication for displacements of 111Cd emitter atoms following the electron capture decay of In.
Phase transition in La2NiO4+δ near stoichiometric oxygen content has been studied between 10 and 800 K by powder X-ray diffraction. In addition to a second-order transition from a high-temperature tetragonal (HTT) phase into a low-temperature orthorhombic (LTO1) phase at 700 K on cooling, a transition into another orthorhombic (LTO2) phase was observed at 75 K. The LTO2 phase developed into domains large enough for X-ray powder diffraction. We present evidence that the LTO2 phase is apparently an orthorhombic one and the second transition is of first order. The nature of phase transition in La2MO4 (M = Ni, Cu) is also discussed in relation to the temperature dependence of the orthorhombicity.
Nearly all elements can be oxidized and develop oxides, often with different oxygen contents and in different crystalline phases. Applying the classical perturbed angular correlation (PAC)-probes 111In/111Cd or 181Hf/181Ta, the probes are usually found on unperturbed cation lattice sites surrounded by oxygen atoms. Using different oxides of the same structure or comparing different crystal classes the position of the oxygen neighbours near the probe can be varied in a wide range. This allows testing theoretical concepts of electric field gradient (EFG) calculation.In general, the melting point of an oxide is very high, and the PAC experiments span a huge temperature range from Tm=10 to 1700K. Two temperature regions are known, where 111In/111Cd probes show dynamic hyperfine-interactions, which occur when the EFG changes direction or strength during the lifetime of the probe. At low temperatures the electron capture “after-effect” is observed, caused by a low availability of charge carriers in semiconducting or isolating oxides. At very high temperatures intrinsic defects or mobile atoms in ternary oxides move so fast, that undamped perturbation functions arise.Realizing the big impact of STM and AFM to the surface science, a probing technique like PAC for the next neighbours inside a sample seems to be attractive. In the past, numerous discussions asked whether the inserted PAC-probes are really spies—only observers—or if they actually change that neighbourhood, that they are supposed to analyse. Distortions in oxides are discussed.
Following doping with 111In in O2 gas at 1273 K, samples of La2CuO4+y were subjected to vacuum anneals of varying lengths to adjust the oxygen content. Susceptibility studies show that vacuum anneals of ∼13 h, starting at 1083 K and terminating at 873 K, consistently produce magnetic ordering at Néel temperature (TN)=317(3) K, the highest value yet published. Perturbed-angular-correlation (PAC) studies of the same samples, using the 171–245-keV γγ cascade of 111Cd populated via the decay of 111In, exhibit a combined magnetic-dipole–electric-quadrupole interaction. Analysis of this yields a magnetic hyperfine field and an electric-field-gradient asymmetry that follow the expected temperature dependence of the local magnetization and orthorhombic distortion, respectively. The samples have uniquely static and homogeneous hyperfine interactions with undetectable PAC line broadening. We argue that this implies a ‘‘defect-free’’ probe environment which we identify as stoichiometric with y≃0. Assuming that any residual defects are randomly distributed on oxygen lattice sites, we find ‖y‖≤0.016.
The structural behaviour at high temperature of the oxidized nickelates R2NiO4+delta(R=La, Pr and Nd) has been studied by means of neutron diffraction measurements. In the case of La2NiO4+delta the structure has tetragonal symmetry for the full measured temperature range. The temperature dependence of a and b cell-parameters shows the existence of a first order structural transformation, from an orthorhombic symmetry at room temperature, to a tetragonal symmetry at higher temperatures for Pr and Nd compounds. The value of this transformation temperature is about 690 K in the case of Pr2NiO4+delta and 790 K for Nd2NiO4+delta. In the latter case, keeping the sample under vacuum (almost-equal-to 10(-6) Torr) at a temperature around 1100 K, the structure becomes again orthorhombic, but different from the previous one. This effect is due to the loss of interstitial oxygen from the system at constant high temperature with the time.
Perturbed-angular-correlation (PAC) measurements with 111In(EC)111Cd probes, implanted at 400 keV into poly- or mono-crystalline Cr2O3, have been carried out as a function of temperature (290-1370 K). Two different electric-field gradients (e.f.g.'s) with identical orientations in the lattice have been found. Their temperature-dependent variations can be well described by a dynamical model which we propose to be connected with a hole trapping at a next-neighbour Cr ion.
Using Rietveld refinement of neutron powder diffraction data, we show that the excess oxygen in ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4+\mathrm{$\delta${}}}$ (0.13<$\delta${}<0.18) is incorporated as an interstitial oxygen defect. The defect is located near the ((1/4, 1/B) / 4 ,(1/4) site in the orthorhombic Fmmm structure, which provides a favorable coordination to four La atoms but requires four nearby oxygen atoms to be displaced from their normal positions. The defect concentration determined from structural refinement agrees well with the overall oxygen stoichiometry determined by hydrogen reduction. For intermediate oxygen contents ($\delta${}$\approxeq${}0.07) the system separates into two phases with different defect concentrations. Structural data suggest that the nearly stoichiometric phase, 0<$\delta${}<0.02, incorporates excess oxygen by forming a different defect. Except for large differences in the solubility of the oxygen defect, ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4+\mathrm{$\delta${}}}$ exhibits behavior remarkably similar to superconducting ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{$\delta${}}}$, suggesting that the oxygen defect structures are the same in both systems.
Solid State Ion. 63-65
  • M T Fernández-Díaz
  • J L Martínez
  • J Rodríguez-Carvajal
  • A Hayashi
  • H Tamura
  • Y Veda