skip to main content

Title: X-ray diffraction and density functional theory studies of R(Fe0.5Co0.5)O3 (R = Pr, Nd, Sm, Eu, Gd)
The structure of a series of lanthanide iron cobalt perovskite oxides, R (Fe 0.5 Co 0.5 )O 3 ( R = Pr, Nd, Sm, Eu, and Gd), have been investigated. The space group of these compounds was confirmed to be orthorhombic Pnma (No. 62), Z = 4. From Pr to Gd, the lattice parameter a varies from 5.466 35(13) Å to 5.507 10(13) Å, b from 7.7018(2) to 7.561 75(13) Å, c from 5.443 38(10) to 5.292 00(8) Å, and unit-cell volume V from 229.170(9) Å 3 to 220.376(9) Å 3 , respectively. While the trend of V follows the trend of the lanthanide contraction, the lattice parameter “ a ” increases as the ionic radius r ( R 3+ ) decreases. X-ray diffraction (XRD) and transmission electron microscopy confirm that Fe and Co are disordered over the octahedral sites. The structure distortion of these compounds is evidenced in the tilt angles θ, ϕ , and ω , which represent rotations of an octahedron about the pseudocubic perovskite [110] p , [001] p , and [111] p axes. All three tilt angles increase across the lanthanide series (for R = Pr to R = Gd: θ increases from 12.3° to 15.2°, ϕ from 7.5° to 15.8°, and ω from 14.4° to 21.7°), indicating a greater octahedral distortion as r ( R 3+ ) decreases. The bond valence sum for the sixfold (Fe/Co) site and the eightfold R site of R (Fe 0.5 Co 0.5 )O 3 reveal no significant bond strain. Density Functional Theory calculations for Pr(Fe 0.5 Co 0.5 )O 3 support the disorder of Fe and Co and suggest that this compound to be a narrow band gap semiconductor. XRD patterns of the R (Fe 0.5 Co 0.5 )O 3 samples were submitted to the Powder Diffraction File.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Powder Diffraction
Page Range / eLocation ID:
259 to 266
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract The post-stishovite transition is a classic pseudo-proper typed ferroelastic transition with a symmetry-breaking spontaneous strain. This transition has been studied using high-pressure spontaneous strains, optic modes, and elastic moduli (Cij) based on the Landau modeling, but its atomistic information and structural distortion remain poorly understood. Here we have conducted synchrotron single-crystal X-ray diffraction measurements on stishovite crystals up to 75.3 GPa in a diamond-anvil cell. Analysis of the data reveals atomic positions, bond lengths, bond angles, and variations of SiO6 octahedra across the transition at high pressure. Our results show that the O coordinates split at ~51.4 GPa, where the apical and equatorial Si-O bond lengths cross over, the SiO6 octahedral distortion vanishes, and the SiO6 octahedra start to rotate about the c axis. Moreover, distortion mode analysis shows that an in-plane stretching distortion (GM1+ mode) occurs in the stishovite structure at high pressure while a rotational distortion (GM2+ mode) becomes dominant in the post-stishovite structure. These results are used to correlate with elastic moduli and Landau parameters (symmetry-breaking strain e1–e2 and order parameter Q) to provide atomistic insight into the ferroelastic transition. When the bond lengths of two Si-O bonds are equal due to the contribution from the GM1+ stretching mode, C11 converges with C12, and the shear wave VS1[110] polarizing along [110] and propagating along [110] vanishes. Values of e1–e2 and Q are proportional to the SiO6 rotation angle from the occurrence of the GM1+ rotational mode in the post-stishovite structure. Our results on the pseudo-proper type transition are also compared with that for the proper type in albite and improper type in CaSiO3 perovskite. The symmetry-breaking strain, in all these types of transitions, arises as the primary effect from the structural angle (such as SiO6 rotation or lattice constant angle) and its relevant distortion mode in the low-symmetry ferroelastic phase. 
    more » « less
  2. While there has been rapid progress in the performance of perovskite solar cells, the details of film formation, effect of processing parameters and perovskite crystal structure are still under discussion. The details of the X-ray diffraction (XRD) pattern of the tetragonal phase of CH 3 NH 3 PbI 3 perovskite existing at room temperature are often overlooked, with unresolved (002) (at 2 θ = 13.99° for CuK α and q = 0.9927 Å −1 ) and (110) (at 2 θ = 14.14° and q = 1.003 Å −1 ) peaks considered to be one peak at 14°, leading to an inaccurate estimation of lattice parameters. In this study, we use an electrospray deposition technique to prepare perovskite films at room temperature, oriented in (002) and (110) directions, with (002) as the preferred orientation. The results of a detailed study on the emergence of the two orientations during perovskite formation are reported. The effect of process parameters, such as substrate temperature during deposition and annealing temperature, on the grain orientation was established using XRD and grazing incidence wide angle X-ray scattering (GIWAXS). The study suggests that an irreversible crystal reorientation from (002) to (110) occurs at high temperature during rapid annealing, whereas a reversible crystal thermal expansion is seen during slow annealing. Finally, the results of the grain reorientation are correlated with the film properties, and it is shown that the film with the dominant (110) orientation has improved morphology and optoelectronic properties. The detailed structural investigation and characterization presented in this study are important for the precise determination of crystal orientation and achievement of desirable photovoltaic properties of the absorber material by carefully observing the adjacent crystal plane peaks in the XRD pattern of the perovskite thin films. 
    more » « less
  3. Abstract

    As a new class of multi-principal component oxides with high chemical disorder, high-entropy oxides (HEOs) have attracted much attention. The stability and tunability of their structure and properties are of great interest and importance, but remain unclear. By using in situ synchrotron radiation X-ray diffraction, Raman spectroscopy, ultraviolet–visible absorption spectroscopy, and ex situ high-resolution transmission electron microscopy, here we show the existence of lattice distortion in the crystalline (Ce0.2La0.2Pr0.2Sm0.2Y0.2)O2−δHEO according to the deviation of bond angles from the ideal values, and discover a pressure-induced continuous tuning of lattice distortion (bond angles) and band gap. As continuous bending of bond angles, pressure eventually induces breakdown of the long-range connectivity of lattice and causes amorphization. The amorphous state can be partially recovered upon decompression, forming glass–nanoceramic composite HEO. These results reveal the unexpected flexibility of the structure and properties of HEOs, which could promote the fundamental understanding and applications of HEOs.

    more » « less
  4. Redox stabilities of the hydrogen electrode with in situ exsolved Fe–Ni nanoparticles from Sr 2 Fe 1.4 Ni 0.1 Mo 0.5 O 6−δ (SFMNi) perovskite are studied by analyzing the evolution of the phase composition and morphology during the redox cycles. It is found that certain amount of the exsolved nanoparticles have been oxidized to the transition metal oxide (Ni,Fe)O instead of reincorporating into the parent perovskite lattice upon re-oxidizing at 800 °C in air. However, the (Ni,Fe)O secondary phases show no adverse effect on the subsequent reduction treatment. The redox reversibility mechanism is explained by the regular-solution model. The electrodes are almost fully recovered in the reducing atmosphere, and the symmetrical cells measured under 9.7% H 2 –3% H 2 O–87.3% N 2 conditions show a stable specific area polarization resistance of around 1.93 Ω cm 2 at 800 °C during 13 redox cycles. Single cells using the Ni–Fe nanoparticles structured electrode exhibit a stable electrode polarization resistance of about 0.598 Ω cm 2 at 800 °C under open circuit voltage conditions and a steady electrolysis current density of about −653 mA cm −2 at 1.5 V during the steam electrolysis process over 5 redox cycles. These results indicate that the SFMNi material is a very promising electrode candidate for steam electrolysis application with robust redox reversibility. 
    more » « less
  5. Perovskite chalcogenides are gaining substantial interest as an emerging class of semiconductors for optoelectronic applications. High-quality samples are of vital importance to examine their inherent physical properties. We report the successful crystal growth of the model system, BaZrS 3 and its Ruddlesden–Popper phase Ba 3 Zr 2 S 7 by a flux method. X-ray diffraction analyses showed the space group of Pnma with lattice constants of a = 7.056(3) Å, b = 9.962(4) Å, and c = 6.996(3) Å for BaZrS 3 and P 4 2 / mnm with a = 7.071(2) Å, b = 7.071(2) Å, and c = 25.418(5) Å for Ba 3 Zr 2 S 7 . Rocking curves with full width at half maximum of 0.011° for BaZrS 3 and 0.027° for Ba 3 Zr 2 S 7 were observed. Pole figure analysis, scanning transmission electron microscopy images, and electron diffraction patterns also establish the high quality of the grown crystals. The octahedral tilting in the corner-sharing octahedral network is analyzed by extracting the torsion angles. 
    more » « less