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Title: The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M {sup 2+}=Ni{sup 2+} and Mg{sup 2+}) systems

Abstract

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi{sub 0.825}Ni{sub 0.125}{open_square}{sub 0.05}){sub 2}(Ni{sub 0.25}Nb{sub 0.75}){sub 2}O{sub 7} and (Bi{sub 0.835}Mg{sub 0.085}{open_square}{sub 0.08}){sub 2}(Mg{sub 0.235}Nb{sub 0.765}){sub 2}O{sub 7} at room temperature are found to be 116 and 151, respectively, while the dielectric loss tangents are 0.00065 and 0.00042, respectively, at 100 kHz. A highly structured characteristic diffuse intensity distribution apparent in electron diffraction is reported in both cases and partially interpreted in terms of large amplitude, {beta}-cristobalite-type tetrahedral rotations of the O'A {sub 2} tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to investigate the local crystal chemistry responsible for this displacive disorder. - Graphical abstract: The characteristic {beta}-cristobalite-type displacive disorder of the O'A {sub 2} sub-structure (of the ideal pyrochlore structure type) drawn relative to the surrounding B {sub 2}O{sub 6} octahedral framework sub-structure and responsible for the observed structured diffuse intensity distribution in the Bi{sub 1.65}Ni{sub 0.75}Nb{sub 1.50}O{sub 7} and Bi{sub 1.67}Mg{sub 0.64}Nb{sub 1.53}O{sub 7} misplaced-displacive cubicmore » pyrochlores.« less

Authors:
 [1];  [1];  [2]
  1. Research School of Chemistry, Australian National University, Canberra, ACT 0200 (Australia)
  2. Research School of Chemistry, Australian National University, Canberra, ACT 0200 (Australia), E-mail: withers@rsc.anu.edu.au
Publication Date:
OSTI Identifier:
21015682
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.2006.10.039; PII: S0022-4596(06)00571-8; 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; BISMUTH OXIDES; CRISTOBALITE; CUBIC LATTICES; DIELECTRIC MATERIALS; DISTRIBUTION; ELECTRON DIFFRACTION; KHZ RANGE 100-1000; MAGNESIUM IONS; MHZ RANGE; NICKEL IONS; NIOBIUM OXIDES; PERMITTIVITY; PYROCHLORE; TEMPERATURE RANGE 0273-0400 K

Citation Formats

Nguyen, Binh, Liu Yun, and Withers, Ray L. The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M {sup 2+}=Ni{sup 2+} and Mg{sup 2+}) systems. United States: N. p., 2007. Web. doi:10.1016/j.jssc.2006.10.039.
Nguyen, Binh, Liu Yun, & Withers, Ray L. The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M {sup 2+}=Ni{sup 2+} and Mg{sup 2+}) systems. United States. doi:10.1016/j.jssc.2006.10.039.
Nguyen, Binh, Liu Yun, and Withers, Ray L. Thu . "The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M {sup 2+}=Ni{sup 2+} and Mg{sup 2+}) systems". United States. doi:10.1016/j.jssc.2006.10.039.
@article{osti_21015682,
title = {The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M {sup 2+}=Ni{sup 2+} and Mg{sup 2+}) systems},
author = {Nguyen, Binh and Liu Yun and Withers, Ray L.},
abstractNote = {The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi{sub 2}O{sub 3} - M {sup 2+}O-Nb{sub 2}O{sub 5} (M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi{sub 0.825}Ni{sub 0.125}{open_square}{sub 0.05}){sub 2}(Ni{sub 0.25}Nb{sub 0.75}){sub 2}O{sub 7} and (Bi{sub 0.835}Mg{sub 0.085}{open_square}{sub 0.08}){sub 2}(Mg{sub 0.235}Nb{sub 0.765}){sub 2}O{sub 7} at room temperature are found to be 116 and 151, respectively, while the dielectric loss tangents are 0.00065 and 0.00042, respectively, at 100 kHz. A highly structured characteristic diffuse intensity distribution apparent in electron diffraction is reported in both cases and partially interpreted in terms of large amplitude, {beta}-cristobalite-type tetrahedral rotations of the O'A {sub 2} tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to investigate the local crystal chemistry responsible for this displacive disorder. - Graphical abstract: The characteristic {beta}-cristobalite-type displacive disorder of the O'A {sub 2} sub-structure (of the ideal pyrochlore structure type) drawn relative to the surrounding B {sub 2}O{sub 6} octahedral framework sub-structure and responsible for the observed structured diffuse intensity distribution in the Bi{sub 1.65}Ni{sub 0.75}Nb{sub 1.50}O{sub 7} and Bi{sub 1.67}Mg{sub 0.64}Nb{sub 1.53}O{sub 7} misplaced-displacive cubic pyrochlores.},
doi = {10.1016/j.jssc.2006.10.039},
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 disordered structures and low temperature dielectric relaxation properties of Bi{sub 1.667}Mg{sub 0.70}Nb{sub 1.52}O{sub 7} (BMN) and Bi{sub 1.67}Ni{sub 0.75}Nb{sub 1.50}O{sub 7} (BNN) misplaced-displacive cubic pyrochlores found in the Bi{sub 2}O{sub 3}-M{sup II}O-Nb{sub 2}O{sub 5} (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B{sub 2}O{sub 6} octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O'A{sub 2} tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (1/2more » , 1/2 , 1/2 ) to 96 h (1/2 , 1/2 -{epsilon}{sub A}, 1/2 +{epsilon}{sub A}) while the O' oxygen was shifted from position 8b (3/8 , 3/8 , 3/8 ) to Wyckoff position 32e (3/8 +{epsilon}{sub o}{sup '}, 3/8 +{epsilon}{sub o}{sup '}, 3/8 +{epsilon}{sub o}{sup '}). The refined displacement magnitudes off the 16d and 8b sites for the A and O' sites were 0.408 A/0.423 A and 0.350 A/0.369 A for BMN/BNN, respectively. - Graphical abstract: The final displacively disordered average structure of Bi{sub 1.667}Mg{sub 0.70}Nb{sub 1.52}O{sub 7} (BMN) projected along a close to <110> direction.« less
  • Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi{sub 2}(M {sup III}Nb{sup V})O{sub 7} Bi-pyrochlores, Bi{sub 2}InNbO{sub 7} and Bi{sub 2}FeNbO{sub 7}, as well as of their A site substituted Bi{sub 1.5}Y{sub 0.5}InNbO{sub 7} and Bi{sub 1.5}Y{sub 0.5}FeNbO{sub 7} variants. Highly structured diffuse distributions in the form of {l_brace}110{r_brace}* sheets of diffuse intensity perpendicular to the six <110> directions of real space along with <111>* rods of diffuse intensity perpendicular to the four {l_brace}111{r_brace} real space planes are observed. The existence of this structured diffuse scattering is interpreted in terms of large amplitude, {beta}-cristobalite-typemore » tetrahedral rotations of the O'A {sub 2} tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to understand the local crystal chemistry responsible for such displacive disorder. The frequency-dependent dielectric properties of Bi{sub 2}InNbO{sub 7} and Bi{sub 2}FeNbO{sub 7} are also investigated along with the effect upon them of A site doping with Y. - Graphical abstract: The pattern of correlated O'Bi{sub 4} tetrahedral rotation around the <111> axis responsible for the observed <111>* rods of diffuse intensity in Bi-pyrochlores.« less
  • Subsolidus phase relations have been determined for the Bi{sub 2}O{sub 3}-Fe{sub 2}O{sub 3}-Nb{sub 2}O{sub 5} system in air (900-1075 deg. C). Three new ternary phases were observed-Bi{sub 3}Fe{sub 0.5}Nb{sub 1.5}O{sub 9} with an Aurivillius-type structure, and two phases with approximate stoichiometries Bi{sub 17}Fe{sub 2}Nb{sub 31}O{sub 106} and Bi{sub 17}Fe{sub 3}Nb{sub 30}O{sub 105} that appear to be structurally related to Bi{sub 8}Nb{sub 18}O{sub 57}. The fourth ternary phase found in this system is pyrochlore (A{sub 2}B{sub 2}O{sub 6}O'), which forms an extensive solid solution region at Bi-deficient stoichiometries (relative to Bi{sub 2}FeNbO{sub 7}) suggesting that {approx}4-15% of the A-sites are occupiedmore » by Fe{sup 3+}. X-ray powder diffraction data confirmed that all Bi-Fe-Nb-O pyrochlores form with positional displacements, as found for analogous pyrochlores with Zn, Mn, or Co iad of Fe. A structural refinement of the pyrochlore 0.4400:0.2700:0.2900 Bi{sub 2}O{sub 3}:Fe{sub 2}O{sub 3}:Nb{sub 2}O{sub 5} using neutron powder diffraction data is reported with the A cations displaced (0.43A) to 96g sites and O' displaced (0.29A) to 32e sites (Bi{sub 1.721}Fe{sub 0.190}(Fe{sub 0.866}Nb{sub 1.134})O{sub 7}, Fd3-bar m (no. 227), a=10.508(1)A). This displacive model is somewhat different from that reported for Bi{sub 1.5}Zn{sub 0.92}Nb{sub 1.5}O{sub 6.92}, which exhibits twice the concentration of small B-type cations on the A-sites as the Fe system. Bi-Fe-Nb-O pyrochlores exhibited overall paramagnetic behavior with large negative Curie-Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. The single-phase pyrochlore with composition Bi{sub 1.657}Fe{sub 1.092}Nb{sub 1.150}O{sub 7} exhibited low-temperature dielectric relaxation similar to that observed for Bi{sub 1.5}Zn{sub 0.92}Nb{sub 1.5}O{sub 6.92}; at 1MHz and 200K the relative permittivity was 125, and above 350K conductive effects were observed.« less
  • A crystal chemical study has been carried out for the ternary systems Li/sub 2/O-M/sub 2/O/sub 5/-(M'O/sub 2/)/sub 2/ (M = Nb, M' = Ti; M = Ta, M' = Sn), in order to characterize and delimit the extension of domains of solid solutions in the vicinity of LiNbO/sub 3/ and LiTaO/sub 3/. The nature of the nonstoichiometry within these regions of solid solution has been found to be of a cationic excess or deficit. No evidence for anionic deficit has been found in this study. The ferroelectric Curie temperature T/sub C/ decreases as the composition deviates from LiNbO/sub 3/ ormore » LiTaO/sub 3/. The decline in T/sub C/ has been interpreted on the basis of the electrostatic interactions resulting from either cationic excess or deficit.« less
  • The accelerated life performance of a partially A-site substituted relaxor dielectric ceramic, (Pb[sub 0.875]Ba[sub 0.125])[sub A]-[(Mg[sub 1/3]Nb[sub 2/3])[sub 0.5](Zn[sub 1/3]Nb[sub 2/3])[sub 0.3]Ti[sub 0.2]][sub B]O[sub 3], was investigated for disk specimens with silver electrodes. The specimens were subjected to a humidity load life condition (85C, 95% rh, 700V), and the lifer performance was measured as a function of the stoichiometry of the relaxor. Stoichiometry was represented by the mole ratio A/B, which is the mole ratio of elements at A-sites to those at B-sites in the perovskite structure. The life performance was found to depend greatly on A/B, which was variedmore » between 0.95 and 1.05. When A/B was equal to or less than 0.99, the failure rate ar 500 h was 25% or below, while all the specimens failed within 350 h when A/B was equal to or greater than 1.00. All failed specimens had evidence of short-circuit failures. The life performance had a strong correlation with the microstructure of the dielectrics. Transmission electron microscopy revealed a PbO-rich secondary phase both at triple points and at grain boundaries in specimens with A/B greater than 1.00. Although a secondary phase was found at triple points in specimens with A/B = 1.00, it consisted of unreacted elements such as Pb, Ba, Nb, Zn, Mg, and Ti which did not form the perovskite structure. The results suggest that the lead component analyzed at triple points and grain boundaries is PbO, which has slight solubility in water. Thus, it appears that triple points and grain boundaries dissolve during the life test, and then migration of Ag ions through the dissolved grain boundaries leads to degradation.« less