Abstract
Heat capacity and enthalpy increments of ternary bismuth tantalum oxides Bi(4)Ta(2)O(11), Bi(7)Ta(3)O(18) and Bi(3)TaO(7) were measured by the relaxation time method (2-280 K), DSC (265-353 K) and drop calorimetry (622-1322 K). Temperature dependencies of the molar heat capacity in the form C(pm)=445.8 + 0.005451T-7.489 x 10(6)/T(2) J K(-1) mo1(-1), C(pm)= 699.0+0.05276T-9.956 x 10(6)/T(2) J K(-1) mol(-1) and C(pm) =251.6+0.06705T-3.237 x 10(6)/T(2) J K(-1) mol(-1) for Bi(3)TaO(7), Bi(4)Ta(2)O(11) and for Bi(7)Ta(3)O(18), respectively, were derived by the least-squares method from the experimental data.
The molar entropies at 298.15 K,S degrees(m)(298.15 K)= 449.6 +/- 2.3 J K(-1) mol(-1) for Bi(4)Ta(2)O(11), S degrees(m)(298.15 K)= 743.0 +/- 3.8 J K(-1) mol(-1) for Bi(7)Ta(3)O(18) and S degrees(m)(298.15K)= 304.3 +/- 1.6 J K(-1) mol(-1) for Bi(3)TaO(7), were evaluated from the low-temperature heat capacity measurements.