氧化鉍之δ相為目前導電率最高的氧離子導體，其導電率高於商用氧離子導體釔安定氧化鋯(YSZ)數十倍，純氧化鉍的δ相卻只穩定於730°C到其熔點825°C的溫度範圍，穩定δ相於室溫可藉由添加稀土元素例如氧化鉺，或者過度金屬元素例如氧化鈮等幾種元素，所以安定化氧化鉍可用於電化學元件上的固態電解質，例如氧氣感測器、氧氣產生器、固態燃料電池等，然而摻雜其他元素會使導電率降低。氧化鉍系電解質的導電率，會依據摻雜的元素種類、組成、結構而產生不同的變化，也有一些安定化δ相是由高溫淬火得來，藉由長時間退火熱處理會轉變成低導電率低溫穩定相。這些特徵對氧化鉍系電解質在電化學元件上的操作性能造成很大的影響。
摻雜鹼土元素Ca、Sr、Ba於Bi2O3中，由缺陷化學可預期氧空缺濃度的增加，但也增加了高溫氟化鈣結構的不穩定性，因而獲得菱方結構，其空間群為 。在本研究中，利用減少氧空缺的原理，探討Nb2O5摻雜對(Bi2O3)0.73(MO)0.27, (M=Ca, Sr, Ba)菱方結構相變抑制之影響。摻雜5mol%Nb2O5於(Bi2O3)0.73(CaO)0.27菱方結構，可獲得對稱性較高，空間群為 的菱方結構，藉由TEM的分析，此結構為δ-Bi2O3的 超結構(superstructure)，將此試片於900?C以液態氮淬火，其超結構會轉變成δ相，添加10及15mol%Nb2O5之組成，則析出第二相(β-Bi2O3)。添加5mol%Nb2O5於(Bi2O3)0.73(SrO)0.27菱方結構，部分Nb2O5可固溶於此菱方結構，而剩餘的Nb2O5與Bi-Sr-O反應形成第二相(Bi1.5Sr1.5Nb2O8.75)；摻雜10mol%Nb2O5於(Bi2O3)0.73(SrO)0.27菱方結構，主相由空間群為R-3m 的菱方結構，變成空間群為 P-3的菱方結構；摻雜15mol%Nb2O5於(Bi2O3)0.73(SrO)0.27菱方結構中，主相變成δ相。藉由SEM/EDS的分析，可區分兩相而獲得單一δ組成-(Bi0.887Sr0.06Nb0.053)2O3.046。藉由摻雜Nb2O5於(Bi2O3)0.73(MO)0.27，(M=Ca, Sr)，其相變過程為：菱方結構(R-3m )→菱方結構(P-3 )→δ相立方結構。添加5mol%Nb2O5於(Bi2O3)0.73(BaO)0.27菱方結構中，部分Nb2O5可固溶於此菱方結構，且析出第二相(Ba5Nb4O15.48)；摻雜10mol%Nb2O5於(Bi2O3)0.73(BaO)0.27菱方結構中，主相變成γ相；摻雜15mol%Nb2O5於(Bi2O3)0.73(BaO)0.27菱方結構中，主相變成δ相。由於BaO與Nb2O5容易反應形成第二相(Ba5Nb4O15.48)，因此藉由SEM/EDS的分析，此δ相不包含Ba元素。藉由摻雜Nb2O5於(Bi2O3)0.73(BaO)0.27，其相變過程為：菱方結構(R-3m )→γ相→δ相，其中間相非δ-Bi2O3的超結構(P-3 )，因為Ba2+陽離子半徑較大，陰陽離子吸引力較小，所以中間相以Bi2O3之低溫介穩相(γ相)存在。
添加Nb2O5於(Bi2O3)0.73(MO)0.27, (M=Ca, Sr, Ba)中，雖然會得到高導電率的晶體結構，但是會產生第二相使導電率下降。在Bi-Sr-Nb-O的系統中，配製出單一δ組成-(Bi0.887Sr0.06Nb0.053)2O3.046，其有良好的導電率，在450°C時，其導電率之數值為2.42*10-2S/cm。

It is well-known that the high-temperature modification of bismuth sesquioxide (δ-Bi2O3) is the best solid-state oxide ion conductor, the conductivity being about 2 orders of magnitude higher than that of conventional oxide conductors such as stabilized zirconia. The cubic δ-phase is stable between 730°C and the melting point at 825°C, although stabilization at room temperature of that phase can be attained by addition of several oxides such as Er2O3 or Nb2O5. However, the ionic conductivity of Bi2O3-based materials is strongly dependent on the dopant cation added as well as structural characteristics. It has also been shown that many of the stabilized δ-phases are metastable high-temperature phases, and by annealing they transformed into less conductive low temperature stable phases. All these aspects have a great influence on the performance of Bi2O3-based materials as solid electrolytes in electrochemical devices.
By doping alkaline-earth element Ca, Sr, Ba, the introduction of more oxygen vacancies may result in the formation of the rhombohedral structure (R-3m ) instead of δ phase. In this study, the effect of Nb2O5 on crystal structure of rhombohedral-type (Bi2O3)0.73(MO)0.27 (M=Ca, Sr, Ba) was discussed. In Bi-Ca-Nb-O system, new solid solution [(Bi2O3)0.73(MO)0.27]0.95(Nb2O5)0.05 with rhombohedral structure (P-3 ) was obtained. By TEM analysis, the structure can be described as superstructure of fluorite cell. As the amount of Nb2O5 increase, the rhombohedral structure (P-3 ) become less and less anisotropic, and another tetragonal phase was observed. By quenching in liquid N2, the fluorite structure was stabilized. In Bi-Sr-Nb-O system, by doping 5mol% Nb2O5 into (Bi2O3)0.73(SrO)0.27, some Nb2O5 dissolved in the rhombohedral structure and the excess Nb2O5 reacted with Bi-Sr-O to form Bi1.5Sr1.5Nb2O8.75. When the doped amount was 10mol% and 15mol%, the Bi-rich phase was rhombohedral (P-3 ) and δ-Bi2O3, respectively. In Bi-M-Nb-O system (M=Ca, Sr), the stabilization of phase transformation was rhombohedral (R-3m )→ rhombohedral (P-3 )→fluorite structure. In Bi-Ba-Nb-O system, by doping 5mol% Nb2O5 into (Bi2O3)0.73(BaO)0.27 rhombohedral structure, some Nb2O5 dissolved in the rhombohedral structure and the excess Nb2O5 reacted with BaO to form Ba5Nb4O15.48. When the doped amount was 10mol% and 15mol%, the Bi-rich phase observed was γ and δ, respectively. The distribution Bi/Ba/Nb element analysis of δ phase was 85/0/15(at %) by SEM/EDS analysis. It is difficult to form δ-Bi2O3 using Ba2+, because BaO reacted with Nb2O5 to form Ba5Nb4O15.48 easily. The corresponding mechanism of phase transformation from rhombohedral to cubic was rhombohedral (R-3m )→γ→δ.
By doping Nb2O5 into (Bi2O3)0.73(MO)0.27 rhombohedral structures, (M=Ca, Sr, Ba), the rhombohedral structures were able to stabilize to cubic structures which have high conductive property, but the second phases would lower their conductivity. In the Bi-Sr-Nb-O system, the composition of the δ phase was analyzed by SEM/EDS analysis, and found to be (Bi0.887Sr0.06Nb0.053)2O3.046 . The single δ phase (Bi0.887Sr0.06Nb0.053)2O3.046 shows higher conductivity of 2.42*10-2S/cm at 450°C.

1.M. J. Verkerk, K. Keizer and A. J. Burggraaf, “High oxygen ion conduction in sintered oxides of the bismuth oxide-erbium oxide system,” Journal of Applied Electrochemistry, 10 81 (1980)
2.A.S. Webster, D. Ling〝The structure and conductivity of new fluorite-type Bi2O3-Er2O3-PbO materials〞Solid State Ionics, 178, 1451-1457 (2007)
3.J. C. Boivin and D. J. Thomas, “Structural investigations on bismuth-based mixed oxides,” Solid State Ionics, 3/4 457 (1981)
4.W. C. Maskell, 〝Inorganic solid state chemically sensitive devices: electrochemical oxygen gas sensors〞Journal of Physics E: Scientific Instruments, 20,1156, (1987)
5.A. M. AZAD, S. LAROSE, S. A. AKBAR,〝Review Bismuth oxide-based solid electrolytes for fuel cells,〞Journal of materials science 29 4135-4151 (1994)
6.G. Mairesse, 〝Advances in oxygen pumping concept with BIMEVOX〞C.R.Acad. Sci. Paris , Serie Ⅱ c, t.2 , 651-660 , (1999)
7.S. A. Ivanov,〝Structural studies of alpha-Bi2O3 by neutron powder diffraction〞Powder Diffraction, 16, 227 (2001)
8.G. Gattow, H.Z. Schroder〝Die Kristallstructure der Hochtemperaturmodifikation von Wismuth-oxid〞Zeitschrift fur Anorganische und Allgemeine Chemie, 318, 176 (1962)
9.M. Yashima 〝Crystal structure and disorder fast oxide ion conductor cubic Bi2O3〞Chemical Physics Letters., 378, 395 (2003)
10. E.M. Levin, R. S. Roth, 〝Polymorphism of bismuth sesquioxide, Ι pure Bi2O3〞J. Res. Natl. Bur. Stand.,68A, 197 (1964)
11.H.A. Harwig, A.G. Gerards,〝Polymorphism of bismuth sesquioxide〞 Thermochimica Acta, 28, 121 (1979)
12.S.K. Blower, C. Greaves 〝The structure of beta-Bi2O3 from powder neutron-diffraction data〞 Acta Crystallographica section C-crystal structure communications, 44, 587 (1988)
13.H.A. Harwig 〝Structure of bismuthsesquioxide-alpha,beta,gamma and delta-phase〞Zeitschrift fur Anorganische und Allgemeine Chemie, 444, 151 (1978)
14.P. Shuk, H.-D. Wiemhofer, U. Guth, W. Gopel, and M. Greenblatt, “Oxide ion conducting electrolytes based on Bi2O3,” Solid State Ionics, 89 179. (1996)
15.C. N. R. Rao, G. V. Subba Rao and S. Ramdas, “Phase transformations and electrical properties of bismuth sesquioxide,” Journal of Physical Chemistry, 73 672 (1969)
16.T. Takahashi and H. Iwahara, “Oxide ion conductors based on bismuth sesquioxide,” Materials Research Bulletin, 13 1447 (1978)
17.T. Takahashi, T Esaka and H. Iwahara, “Conduction in bismuth (III) oxide-based oxide ion conductors under low oxygen pressure. I. Current blackening of the bismuth (III) oxide-yttrium oxide electrolyte,” Journal of Applied Electrochemistry, 7 299. (1977)
18.A. Laarif and F. Theobald, “The lone pair concept and the conductivity of bismuth oxides Bi2O3,” Solid State Ionics, 21 183 (1986)
19.G. Mairesse, “In Fast Ion Transport in Solids,” ed. B. Scrosati, Kluver, Amsterdam, , p. 271, (1993)
20.T. Takahashi, “H. Iwahara, High oxide ion conduction in sintered oxides of the system Bi2O3-Y2O3” Journal of Applied Electrochemistry, 5 187 (1975)
21.H. Iwahara, “ Formation of high oxide ion conductive phases in the sintered oxides of the system Bi2O3-Ln2O3 (Ln= La-Yd), ” Journal of solid state chemistry,39, 173-180.(1981)
22.T. Takahashi, H. Iwahara and T. Esaka, “High oxide ion conduction in sintered oxide of the system Bi2O3-M2O5” Journal of the Electrochemistry Society, 124 1563.(1977)
23.V. J. Powers, Ph D. thesis, Ohio State University, (1989).
24.A. Watanabe, “Is it possible to stabilize -Bi2O3 by an oxide additive?,” Solid State Ionics, 40/41 89. (1990)
25.A. Watanabe and T. Kikuchi, “cubic-hexagonal transformation of yttia-stabilized -bismuth sesquioxide, Bi2-2xY2xO3 (x=0.215-0.235),” Solid State Ionics, 21 287. (1986)
26A. Watanabe, “The dependence of polymorphism on the ionic radius of Ln3+ (Ln=La---Er, Y) in the oxide ion conductors Bi0.775Ln0.225O1.5” Solid State Ionics, 67 ,25-28 (1993)
27.M. Drache, P. Conflant, “Structural and conductivity properties of Bi0.775Ln0.225O1.5 oxide conductors (Ln=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy) with rhombohedral Bi-Sr-O type” Journal of Solid State Chemistry, 142, 349-359 (1999)
28.P. Conflant, J. C. Boivin, " Etude structure par diffractometrie X A haute temperature du conducteur anionique Bi0.844Ba0.156O1.422, " Solid State Ionics, 9&10, 925, (1983)
29.P. Conflant, J. C. Boivin, " Thermal Evolution of the crystal structure of the rhombohedral Bi0.75Sr0.25O1.375 phase: A single crystal neutron diffraction study" Journal of Solid State Chemistry, 112, 1-8, 1994
30.J. C. Boivin, D. J. Thomas, "Crystal chemistry and electrical properties of bismuth-based mixed oxides, " Solid State Ionics, 5, 523, (1981)
31.G. Meng, "Conductivity of Bi2O3-based oxide ion conductors with double stabilizers" Solid State Ionics, 28-30, 533, 1988
32.A.S. Webster, D. Ling “The structure and conductivity evolution of fluorite-type Bi2O3-Er2O3-PbO solid electrolytes during long-term annealing”Solid State Ionics, 179, 697 (2008)
33.K. Z. Fung and A. V. Vikar, “Phase stability, phase transformation kinetics, and conductivity of Y2O3-Bi2O3 solid electrolytes containing aliovalent dopants” Journal of the American Ceramic Society , 74, 1970. (1991)
34.K. Z. Fung and A. V. Virkar, “Effect of Aliovalent Dopants on the Kinetics of Phase Transformation and Ordering in RE2O3-Bi2O3(RE=Tb, Er, Y, or Dy) Solid Solution”, Journal of the American Ceramic Society, 76, 2403, (1993)
35.E. D. Waschsman, G. R. Ball, N. Jiang and D. A. Stevenson, “Structure and Defect studies in Solid Oxide Electrolytes”, Solid State Ionics, 52, 213-218, (1992)
36.J.M. Amarilla, R. M. Rojas, “Structural study of trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data”, Journal of Chemistry Society, Dalton Trans, 1137, (1999)
37.H. Zhao, S. Feng, “A rapid chemical route to niobates: hydrothermal synthesis and transport properties of ultrafine Ba5Nb4O15” Journal of materials chemistry, 10, 965, (2000)
38.B. Hallstedt, “Thermodynamic Assessment of the Bismuth-Strontium-Oxygen Oxide System” Journal of the American Ceramic Society, 80, 1085, (1997)
39.R.D. Shannon, “Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides” Acta Crystal, A32, 751, (1976)
40.D. Wolf, P. Keblinski, S. R. Phillpot, and J. Eggebrecht, “Exact method for simulation of Coulombic systems by spherically truncated, pairwise r-1 summation” Journal of Chemical Physics, 110, 8254-8282, (1999)
41.H.A. Harwig 〝Phase Relations in Bismuthsesquioxide〞Zeitschrift fur Anorganische und Allgemeine Chemie, 444, 167, (1978)
42.P. Conflant, “Bi-La-Based Oxide Conductors with Rhombohedral Bi-Sr-O Type: Structural and Conductivity Properties Optimization by Polycationic Substitutions for La” Journal of Solid State Chemistry, 149, 341, (2000)
43.D.Dhak, “Influence of substitution on dielectric and impedance spectroscopy of Sr1-xBi2+yNb2O9 ferroelectric ceramics synthesized by chemical route,” Applied Surface Science, 254, 3078, (2008)
44.M. J. Verkerk, “Structure and Ionic Conductivity of Bi2O3 substituted with lanthanide oxides” Journal of Physics and Chemistry of Solids, 43, 1129, (1982)