Akiteru Watanabe

Is it possible to stabilize δ-Bi2O3 by an oxide additive ?

Abstract This paper discusses whether the high-temperature stable modification δ-Bi2O3 can really be stabilized with an oxide additive. As a result, there is little possibility of the stabilization. All the stabilized δ phases reported by many researchers were nothing but the quenched high-temperature stable phases which form a solid solution based on δ-Bi2O3. These quenched phases can easily be obtained in the bismuth-rich region of each system with Ln2O3 (Ln=lathanoid or Y), TeO2, Nb2O5 or Ta2O5. Since the se quenched phases are metastable at a lower temperature, than about 700°C of which a critical temperature varies with composition and an oxide additive, on annealing at that low temperature they transform gradually into the low-temperature stable modification in the systems with Ln2O3 or decompose into two other phases in the other systems.

Polymorphism in Bi2WO6

Abstract The polymorphism of Bi2WO6 has been studied using differential dilatometry and differential thermal analysis with polycrystalline specimens prepared by sintering the oxides Bi2O3 and WO3. Two reversible polymorphic transitions were observed, one at 662°C and one at 962°C. The former transition showed a very small change of enthalpy and very little dimensional change, while the latter showed a large thermal hysteresis, had a large change of enthalpy, and was accompanied by a sizable volume change. The high-temperature powder X-ray data indicated that the intermediate phase as well as the low-temperature form had orthorhombic symmetry and the high-temperature form had monoclinic symmetry. The 662°C transition is displacive and the 962°C transition reconstructive. A crystal structure of the high-temperature form is proposed and discussed in comparison with that of the low-temperature form.

Phase relations of hexagonal and cubic phases in holmia-doped bismuth sesquioxide, Bi2−2xHo2xO3 (x=0.205–0.245)

Abstract A new phase was found in the bismuth-rich portion of the system Bi 2 O 3 Ho 2 O 3 . This phase formed a solid-solution Bi 2−2 x Ho 2 x O 3 ( x =0.205–0.245 at 650°C) having a Bi 0.765 Sr 0.235 O 1.383 -type layered structure with hexagonal symmetry, and showed high oxide-ion conduction. Thermal analysis, isothermal heat treatments and electrical conductivity measurements revealed that the hexagonal phase transformed reversibly into a high-temperature face-centered cubic phase which is known as extrinsic δ-Bi 2 O 3 stabilized by Ho 2 O 3 . The hexagonal-cubic transformation temperature varied from 715° to 735°C depending on the composition x . Thus, the cubic phase existed stably only above these transition temperatures, so that the known stabilized δ phase with Ho 2 O 3 was metastable at lower temperatures. The oxide-ion conductivities of both hexagonal and cubic phases were measured and compared. Phase relations of the δ phase “stabilized” by other lanthanoid oxides are discussed.

Bi23M4O44.5 (M = P and V): new oxide-ion conductors with triclinic structure based on a pseudo-fcc subcell

Abstract New compounds, Bi23M4O44.5 (M = P and V), have been found in the systems Bi2O3-M2O5. They crystallize in the triclinic system with a = 11.366(1〈) A , b = 11.369(1) A , c = 20.453(3) A , α = 77.535(8) °, β = 86.214(9) °, and γ = 119.565(7) ° for M = P, and with a = 11.545(2) A , b = 11.547(1) A , c = 20.665(3) A , α = 76.27(1) °, β = 87.51 (1) °, and γ = 119.82(1) ° for M = V. Two formula weights are contained in the unit cell. The structure is based on a pseudo-fcc subcell with a′ ≈ 5.5 A . The axial relations between the triclinic cell and the subcell are a = b ≈ ( 3 2 2 )a′ and c ≈ ( 3 6 2 )a′ . The compounds show a good oxide-ion conduction. In particular, Bi23V4O44.5 is excellent: σ ≈ 10−2 mho cm−1, activation energy ≈ 0.78 eV, and O−2 transport number ≈ 0.9 at about 600 °C. Both compounds melt congruently at about 950 °C without polymorphic transformation.

New members of a family of layered bismuth compounds

Abstract Two new compounds, Bi 3 Ti 2 O 8 F and PbBi 3 Ti 3 O 11 F, were prepared and identified by X-ray diffraction analysis. These compounds are members of the family called layered bismuth compounds. Thermal properties of the new compounds were also studied. Besides the preparation and identification of these new compounds, a new method for preparing already known members, Bi 2 NbO 5 F and Bi 2 TiO 4 F 2 , was reported. Moreover, the possibility of the existence of other new members belonging to the family was discussed.

An outline of the structure of oxide-ion conductors Bi23V4-4xP4xO44.5 (0≤x≤1)

Abstract A complete solid-solution series, Bi 23 V 4−4 x P 4 x O 44.5 (0≤ x ≤1), has been formed between two isomorphous compounds, Bi 23 M 4 O 44.5 (M=P and V). The series crystallize in the triclinic symmetry and show a high oxide-ion conduction. The structure has been examined on single crystals of Bi 23 P 4 O 44.5 using crystal lattice images taken by high-resolution transmission electron microscope. As a result, Bi and P cations are orderly located at all pseudo-cube corners and face centers of pseudo-fcc subcells with a ′≈0.55 nm, and the ordering of these two kinds of cations leads to the triclinic superstructure. Using this structural model, positions of oxide ions are deduced from the fluorite-related type structure.

Polymorphic transformation of δ-Bi2O3 stabilized with Ln2O3 (Ln = Sm, Eu, Gd, Tb and Dy) into a new phase with a C-type rare-earth oxide-related structure

Abstract This paper describes the polymorphism of the novel phase Bi 1 − x Ln x O 1.5 ( Ln = Sm , Eu , Gd , Tb , and Dy with = 0.38, 0.375, 0.275-0.40, 0.275-0.35, and 0.30−0.35, respectively) which has a C-type rare-earth oxide-related structure. This C-type phase transformed without delay to the δ-Bi 2 O 3 type high-temperature phase around 900 °C upon heating. Upon cooling, however, the δ-phase never returned to the original C-type phase. The irreversibility is only apparent, because the δ-phase transformed gradually to the C-type phase by annealing around 800 °C; in other words, the rate of transition from the δ to the C-type phase is extremely sluggish. The electrical conductivity of the C-type phase was not good ( σ = 10 −3.9 S cm −1 at 700 °C), though the oxide-ion transport number was 70% over a temperature range 600 to 850 °C.

Phase stability of Bi0.765Sr0.235O1.383-type bismuth mixed oxides with hexagonal symmetry

Abstract This paper describes the hygroscopic properties and thermal stability of a Bi 0.765 Sr 0.235 O 1.383 -type hexagonal phase which exists with a layered structure in the bismuth-rich region of the binary system Bi 2 O 3 MO (M=Ca, Sr or Ba). The hexagonal phase absorbed moisture from the air at room temperature leading to gradual decomposition; as a result, αBi 2 O 3 and an unknown phase appeared for M=Sr or Ba, and only the unknown phase was recognized for M=Ca. Not only polycrystalline samples but single crystals were easily decomposed when they were soaked in hot water. Isothermal heat treatments at about 500°C revealed that the hexagonal phase for M=Ba underwent a sluggish decomposition to yield αBi 2 O 3 and another unknown phase, but the hexagonal phases of the other systems were stable.

Induced emission cross sections of near-stoichiometric LiNbO3: Mg, Nd

Optical absorption and emission spectra are measured on Nd3+ ion in near-stoichiometric LiNbO3 single crystals codoped with MgO. From the decomposition of the nonpolarized spectra, the Stark splitting scheme is established for 2wt% Nd3+ ion in the near-stoichiometric LiNbO3:Mg, Nd system. The induced emission cross sections are calculated to be 2.70×10−19 and 5.57×10−20cm2 for the π and σ polarizations, respectively, based on the decomposition of the polarized spectra. These values agree well with the reported values of 1.8×10−19 and 5.1×10−20cm2 for the π and σ polarizations of the conventional congruent-melt composition crystal through the branching ratio of emission spectra despite the different approach of calculations and difference in the near-stoichiometric and congruent-melt compositions. This fact shows that the slight difference in the host composition does not affect the cross section. The population inversion threshold is calculated to be 2.0×1016 for the Nd(2wt%) near-stoichiometric LiNbO3:Mg...

New monoclinic compounds, Bi3.24Ln2W0.76O10.14, having a pseudo-orthohexagonal cell based on a pseudo-fcc subcell in the systems Bi2O3–Ln2O3–WO3 (Ln=La, Pr, and Nd)

Abstract New compounds, Bi 3.24 Ln 2 W 0.76 O 10.14 , have been found in the systems Bi 2 O 3 – Ln 2 O 3 –WO 3 ( Ln =La, Pr, and Nd). They crystallize ostensibly in the orthorhombic (or orthohexagonal) symmetry, e.g. with a=6.9694(1) A , b=4.0242(1) A , c=9.3335(1) A , and Z =1 for Ln =La. At the same time, the lattice forms a superstructure based on a pseudo-fcc subcell with a′≈5.6 A , where the transformation matrix is (1/2,1/2,−1)/(−1/2,1/2,0)/(1,1,1). However, the cation configuration has proved that the true symmetry is monoclinic ( C 2/ m ) with β ≈90°. All new compounds undergo two reversible polymorphic transitions at about 980°C and at 1097–1210°C depending on Ln . Their electrical conductivity exhibited lower values (about 10 −4 S cm −1 at 500°C) despite the pseudo-fcc subcell in relation to the δ -Bi 2 O 3 type which is the well-known good oxide-ion conductor.