Thermal expansion, crystal structure, XPS and NEXAFS spectra of Fe-doped bismuth tantalate pyrochlore

Effect of Fe-Doping on Thermal Expansion and Stability of Bismuth Magnesium Tantalate Pyrochlorere

“…The cell parameter of pyrochlore calculated by us does not contradict the values given in articles [ 14 , 15 , 16 , 17 ] for iron-containing pyrochlores based on tantalate (10.4979–10.5033 Å) and (Bi 1.721 Fe 0.190 (Fe 0.866 Nb 1.134 )O 7 ) a = 10.508 of bismuth niobate.…”

“…An analysis of reflection extinctions made it possible to confirm that the symmetry of the crystal structure is indeed cubic with the space group :2. The structure of pyrochlore based on bismuth tantalate was refined by us earlier [ 17 , 18 , 19 , 20 ] and is used in this work to analyze the effect of iron atoms on the thermal expansion of samples. As a result of the calculation of the unit cell parameter of the samples, it was found that with an increase in the concentration of iron and a decrease in the content of magnesium, the cell constant decreases uniformly from 10.5225 ± 0.0007 (x = 0.225) down to 10.5009 ± 0.0001 Å (x = 0.525), obeying the Vegard law.…”

“…In its turn, the unit cell parameter of the solid solutions under consideration decreases with a decrease in the magnesium content, which may mean that magnesium ions are replaced by iron(III) ions in the crystallographic positions of tantalum(V) rather than bismuth(III) (R(Fe(III))c.n-6 = 0.645 Å, R(Bi(III))c.n-6 = 1.17 Å). It was previously established that the unit cell parameter of iron-containing solid solutions slightly increases with increasing iron content; for example, for Bi 3.36 Fe 2.08+x Ta 2.56-x O 14.56-x (−0.32 ≤ x ≤ 0.48) from 10.4979(8) up to 10.5033(1) Å, for Bi 3.36 Fe 2.08+x Nb 2.56-x O 14.56-x (−0.24 ≤ x ≤ 0.48) from 10.5071(4) up to 10.5107(7) Å, for Bi 3.36 Fe 2.08+x Sb 2.56-x O 14.56-x (0 ≤ x ≤ 0.64) from 10.4284(8) up to 10.4513(8) Å, emphasizing the possibility of replacing Nb/Ta/Sb ions with iron(III) ions [ 14 , 15 , 16 , 17 , 28 ]. In this case, the increase in the cell parameter is explained by the slightly larger ionic radius of iron(III) compared to the radii of niobium(V), tantalum(V), and antimony(V) (R(Fe(III))c.n-6 = 0.645 Å, R(Sb(V))c.n-6 = 0.60 Å, R(Nb(V))c.n-6 = 0.64 Å, R(Ta(V))c.n-6 = 0.64 Å) [ 27 ].…”

“…It should be noted that the geometric parameters of the Bi 1.5 Mg 0.375 Fe 0.375 Ta 1.5 O 7−Δ pyrochlore structure are slightly larger than those for Bi 1.5 Fe 0.75 Ta 1.5 O 7+Δ [ 17 ], which is due to the difference between the radii of Mg(II) and Ta(V), Fe(III). For example, the average Ta-O bond length for Bi 1.5 Fe 0.75 Ta 1.5 O 7+Δ is ~1.987 Å, while the Bi-O distances in the polyhedron vary from 2.30 up to 2.99 Å…”

“…An example of this is lanthanum zirconate La 2 Zr 2 O 7 , which is featured with a low thermal conductivity, thermal and phase stability above 1400 °C [ 13 ]. Iron-containing pyrochlores based on bismuth niobate and tantalate are also characterized by thermal stability and the absence of phase transitions up to 1140 °C according to the thermal analysis of samples [ 14 , 15 , 16 , 17 ]. Meanwhile, the thermal behavior of iron- and magnesium-containing pyrochlores in the high-temperature range remains unstudied.…”