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.
Thermal analysis and X-ray phase
analysis under high temperature
have defined the order of the BiTaO4 phase transformations
in the range of temperature from 25 to 1200 °C: α-BiTaO4 → γ-BiTaO4 → β-BiTaO4 and β-BiTaO4→ γ-BiTaO4 → β-BiTaO4; the new high-temperature phase
of bismuth orthotantalate is γ-BiTaO4. It was shown
that the transition α-BiTaO4 → β-BiTaO4 is irreversible, in contrast to BiNbO4. The crystal
structure of the new bismuth orthotantalate high-temperature modification
(γ-BiTaO4) was studied by X-ray powder diffraction.
The possible space group of the high-temperature orthorhombic phase
was determined as Cmcm, and the parameters of the
unit cell were calculated at 900 °C: a = 3.92455(4), b = 15.06243(16), c = 5.63483(6) Å,
and V = 333.093(6) Å3. As an alternative,
the space group of γ-BiTaO4 was determined as P21/c, and the following unit
cell parameters were calculated: a = 7.78151(10), b = 5.63466(6), c = 7.84843(7) Å, β = 104.5858(17)°, and V = 333.033(7)
Å3. The method of HTPXRD was used to obtain thermal
expansion coefficients (TECs) and to study the thermal deformations
of the α, β, and γ modifications of BiTaO4. The average volumetric TECs of all modifications studied had the
range of 18.4–33.6 × 10–6 °C–1. The highest anisotropy of thermal expansion was
the characteristic of the triclinic phase, β-BiTaO4, whereas α-BiTaO4 had minimal anisotropy and TEC
indicating the highest stability of this modification.
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