Thermal
behavior of the orthorhombic (α) and triclinic (β)
polymorphs of BiNbO4 was studied by the methods of high-temperature
powder X-ray diffraction (HTPXRD) and differential scanning calorimetry
(DCS) in the temperature range 25–1200 °C. The study revealed
the sequence of thermal phase transformations and the new high-temperature
modification, γ-BiNbO4, which was formed above 1001
°C and existed up to the melting temperature of BiNbO4. The incongruent melting of BiNbO4 was characterized
by the formation of the cubic phase with the approximate composition
Bi3NbO7. The HTPXRD method was used in this
study to evaluate thermal deformations and to calculate thermal-expansion
coefficients (TEC) of the three modifications of BiNbO4 (α, β, and γ). The average volumetric TECs of
these three modifications were in the range 19–36 × 10–6 °C–1. The triclinic phase
β-BiNbO4 demonstrated the highest anisotropy of thermal
expansion. α-BiNbO4 was characterized by the minimal
TEC and anisotropy, which indicated its greatest stability. The crystal
structure of γ-BiNbO4 was determined at 1100 °C
using powder data and was refined using the Rietveld method (the α-LaTaO4 structural type, the space group Cmc21, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) Å3, R
wp = 4.82, R
Bragg = 3.61%). The
methods of thermal analysis and high-temperature powder X-ray diffraction
revealed that, during the heating, bismuth orthoniobate underwent
the following sequence of phase transitions: α-BiNbO4 → γ-BiNbO4 → β-BiNbO4 and β-BiNbO4 → γ-BiNbO4 → β-BiNbO4 or, at slow heating, β-BiNbO4 → (α-BiNbO4) → γ-BiNbO4 → β-BiNbO4, where γ-BiNbO4 is the high-temperature phase of bismuth orthoniobate.
The samples of Ni-doped
bismuth magnesium tantalate pyrochlores
with the general formula Bi
1.4
(Mg
1–
x
Ni
x
)
0.7
Ta
1.4
O
6.3
(
x
= 0.3, 0.5, 0.7) were
obtained by solid-phase synthesis. The crystal structure of the pyrochlore
type (sp. gr.
Fd
3̅
m:2
) was
clarified by the Rietveld method on the basis of X-ray powder diffraction
data. The unit cell parameters increase with the decreasing nickel
content in the range from 10.5319(1) to 10.5391(1) Å. The electronic
state of atoms is established by the XPS method. According to XPS
analysis, bismuth atoms have an effective charge of +3, nickel atoms
+(2 + δ), and tantalum ions +(5 – δ). The coefficient
of thermal expansion of the lattice of the samples was calculated
from high-temperature X-ray structural measurements in the range of
−180 to 1050 °C. The average values of linear TECs α
in the temperature ranges of 30–570 and 600–1050 °C
are 5.1 × 10
–6
and 8.1 × 10
–6
°C
–1
, respectively. The monotonicity of the
change in the thermal expansion coefficient in the temperature range
from −100 to 1050 °C indicates the absence of phase transformations.
All samples are dielectric and exhibit high activation energies ∼2.0
eV, moderately high dielectric constants ∼24–28, and
tangent dielectric losses ∼0.002 at 1 MHz and 21 °C. The
electrical properties of the samples are described by a simple parallel
equivalent scheme. The chemical composition of the materials has little
effect on the polarizability of the medium or on the value of the
activation energy of the conductivity. Ionic processes in investigated
materials at frequencies 200–10
6
Hz and at temperatures
100–450 °C were not detected.
A phase-pure nickel bismuth tantalate with pyrochlore structure was synthesized by a solid-phase synthesis method for the first time. The crystal structure of pyrochlore (refined formula Bi 1.58 Ni 0.60 Ta 1.40 O 7 , sp. gr. Fd-3m, a = 10.5343 Å, Z = 8) was clarified by the Rietveld method on the basis of X-ray powder diffraction (XRD) data. The crystallite size determined by the Scherrer method is ∼46 nm. The sample has an atypical pink-purple color. The electronic state of the atoms was investigated by XPS. According to XPS analysis, bismuth atoms have an effective charge of +3; nickel atoms, +(2 + δ); tantalum ions, +(5 − δ). The thermal expansion coefficient of the cell is calculated from high-temperature X-ray measurements in the range of 30−1200 °C. The thermal expansion coefficient (TEC) increases monotonically from 3.79 × 10 −6 °C−1 (30 °C) to 8.32 × 10 −6 °C−1 (990 °C). Above 1080 °C, the TEC decreases due to the thermal dissociation of pyrochlore with new NiTa 2 O 6 phase formation. Ni-doped bismuth tantalate refers to dielectrics and exhibits a moderately high dielectric constant, ∼32, and low dielectric losses, ∼2 × 10 −3 at 1 MHz and ∼30 °C. Above 300 °C, the dielectric losses and dielectric permittivity increase in the low-frequency region due to the activation of oxygen anions. It is found that the electrical characteristics of the sample are significantly affected by the ambient air humidity. An equivalent scheme which satisfactorily describes the electrical properties of the sample has been proposed.
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