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Title: Structural phase transitions and crystal chemistry of the series Ba{sub 2} LnB'O{sub 6} (Ln=lanthanide and B'=Nb{sup 5+} or Sb{sup 5+})

Abstract

The structures of 28 compounds in the two series Ba{sub 2} LnSbO{sub 6} and Ba{sub 2} LnNbO{sub 6} have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba{sub 2} LnNbO{sub 6}, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by {pi}-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations. - Graphical abstract: Lattice parameters versus temperature for Ba{sub 2}NdNbO{sub 6}. The formation of the I4/m tetragonal phase contrasts with the antimonate series where a rhombohedral structure occurs instead. This difference is believed to be caused by the presence of {pi}-bonding present in the niobates but absent in the antimonates.

Authors:
 [1];  [2];  [3]
  1. School of Chemistry, University of Sydney, Sydney, NSW 2006 (Australia)
  2. School of Chemistry, University of Sydney, Sydney, NSW 2006 (Australia), E-mail: kennedyb@chem.usyd.edu.au
  3. Bragg Institute, ANSTO, Private Mail Bag 1, Menai, NSW 2234 (Australia)
Publication Date:
OSTI Identifier:
21015664
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Solid State Chemistry; Journal Volume: 180; Journal Issue: 2; Other Information: DOI: 10.1016/j.jssc.2006.10.017; PII: S0022-4596(06)00563-9; Copyright (c) 2006 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; AMBIENT TEMPERATURE; ANTIMONATES; ANTIMONY IONS; CUBIC LATTICES; LATTICE PARAMETERS; MONOCLINIC LATTICES; NEUTRON DIFFRACTION; NIOBATES; NIOBIUM IONS; OXIDES; PHASE TRANSFORMATIONS; RARE EARTH COMPOUNDS; SYNCHROTRON RADIATION; TETRAGONAL LATTICES

Citation Formats

Saines, Paul J., Kennedy, Brendan J., and Elcombe, Margaret M. Structural phase transitions and crystal chemistry of the series Ba{sub 2} LnB'O{sub 6} (Ln=lanthanide and B'=Nb{sup 5+} or Sb{sup 5+}). United States: N. p., 2007. Web. doi:10.1016/j.jssc.2006.10.017.
Saines, Paul J., Kennedy, Brendan J., & Elcombe, Margaret M. Structural phase transitions and crystal chemistry of the series Ba{sub 2} LnB'O{sub 6} (Ln=lanthanide and B'=Nb{sup 5+} or Sb{sup 5+}). United States. doi:10.1016/j.jssc.2006.10.017.
Saines, Paul J., Kennedy, Brendan J., and Elcombe, Margaret M. Thu . "Structural phase transitions and crystal chemistry of the series Ba{sub 2} LnB'O{sub 6} (Ln=lanthanide and B'=Nb{sup 5+} or Sb{sup 5+})". United States. doi:10.1016/j.jssc.2006.10.017.
@article{osti_21015664,
title = {Structural phase transitions and crystal chemistry of the series Ba{sub 2} LnB'O{sub 6} (Ln=lanthanide and B'=Nb{sup 5+} or Sb{sup 5+})},
author = {Saines, Paul J. and Kennedy, Brendan J. and Elcombe, Margaret M.},
abstractNote = {The structures of 28 compounds in the two series Ba{sub 2} LnSbO{sub 6} and Ba{sub 2} LnNbO{sub 6} have been examined using synchrotron X-ray and in selected cases neutron powder diffraction at, below and above ambient temperature. The antimonate series is found to undergo a sequence of phase transitions from monoclinic to rhombohedral to cubic symmetry with both decreasing ionic radii of the lanthanides and increasing temperature. Compounds in the series Ba{sub 2} LnNbO{sub 6}, on the other hand, feature an intermediate tetragonal structure instead of the rhombohedral structure exhibited by the antimonates. This difference in symmetry is thought to be caused by {pi}-bonding in the niobates that is absent in the antimonates. The bonding environments of the cations in these compounds have also been examined with overbonding of the lanthanide and niobium cations being caused by the unusually large B-site cations. - Graphical abstract: Lattice parameters versus temperature for Ba{sub 2}NdNbO{sub 6}. The formation of the I4/m tetragonal phase contrasts with the antimonate series where a rhombohedral structure occurs instead. This difference is believed to be caused by the presence of {pi}-bonding present in the niobates but absent in the antimonates.},
doi = {10.1016/j.jssc.2006.10.017},
journal = {Journal of Solid State Chemistry},
number = 2,
volume = 180,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • The structure of 14 compounds in the series Ba{sub 2}LnTaO{sub 6} have been examined using synchrotron X-ray diffraction and found to undergo a sequence of phase transitions from I2/m monoclinic to I4/m tetragonal to Fm3-bar m cubic symmetry with decreasing ionic radii of the lanthanides. Ba{sub 2}LaTaO{sub 6} is an exception to this with variable temperature neutron diffraction being used to establish that the full series of phases adopted over the range of 15-500 K is P2{sub 1}/n monoclinic to I2/m monoclinic to R3-bar rhombohedral. The chemical environments of these compounds have also been investigated and the overbonding to themore » lanthanide cations is due to the unusually large size for the B-site in these perovskites. - Graphical abstract: The evolution of the structure across the series of double perovskites Ba{sub 2}LnTaO{sub 6} is established using a combination of synchrotron X-ray and neutron diffraction. The symmetry increases from monoclinic to tetragonal and then cubic as the size of the lanthanide decreases.« less
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  • The stabilization of the quadruple perovskite structure Ln{sub 2}Ba{sub 2}Cu{sub 2}Ti{sub 2}O{sub 11} as a function of the constituent lanthanides has been investigated. Powder X-ray data show that the mixed A-site compositions Ln`Ln``Ba{sub 2}Cu{sub 2}Ti{sub 2}O{sub 11} (Ln`Ln`` = LaY, LaHo, LaEr, and NdDy) concertedly order to form distinct CuO{sub 2}{sup 2-} and TiO{sub 2} double layers. In addition the A cations (Ln` Ln``, and Ba) order on three distinct sites, with the smaller lanthanide (Ln``) occupying the 8-coordinate site between contracted copper-oxygen planes. 22 refs., 5 figs., 2 tabs.
  • Compounds in the double perovskites series Ba{sub 2}LnSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba{sub 2}PrSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr{sup 3+} to Pr{sup 4+} with increasing x and that this is likely to be the cause of themore » loss of B-site ordering. Similarly, both Ba{sub 2}TbSbO{sub 6} and Ba{sub 2}TbSnO{sub 6-{delta}} are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb{sup 3+} to Tb{sup 4+}. Multiple linear regression analysis of the Pr and Tb L{sub III}-edge XANES indicates that the rate of Ln{sup 3+} transforming to Ln{sup 4+} is such that there are no oxygen vacancies in Ba{sub 2}PrSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} but in Ba{sub 2}TbSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} there is a small amount of oxygen vacancies, with a maximum of {delta}{approx}0.05 present. - Graphical abstract: Powder diffraction and spectroscopic techniques have been used to investigate the series Ba{sub 2}LnSn{sub x}Sb{sub 1-x}O{sub 6-{delta}} (Ln=Pr or Tb). It was found that increased Sn{sup 4+} doping leads to oxidation of the Ln{sup 3+} cations to Ln{sup 4+}. The X-ray absorption near-edge structure of the Ln L{sub III}-edge indicates that this oxidation state change occurs gradually such that there are few oxygen vacancies present.« less
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