Smruti Dash

Determination of standard molar Gibbs' energy of formation of Sr3MoO6(s)

An electromotive oxygen concentration cell using calcia-stabilized zirconia (CSZ) electrolyte has been constructed and used to determine ΔfGmo(SrMoO4, s, T). The e.m.f. of cell (II), Pt/SrMoO3(s) + SrMoO4(s)/CSZ/air/Pt, could be represented by E (V) ± 0.006 = 1.515-4.299 × 10−4T (K), while the e.m.f. of cell (III), Pt/SrMoO3(s) + SrMoO4(s)/ CSZ/FexO(s) + Fe(s)/Pt, could be represented by E (V) ± 0.002 = 0.1459 - 5.859 × 10−5T (K). The e.m.f. values were combined with ΔfGmo(T) of SrMoO3(s) and FexO(s) from our earlier studies to get ΔfGmo(SrMoO4, s, T) values as ΔfGmo(SrMoO4, s, T) (kJmol−1) ± 1.2 = −1582.6 + 0.3692T (K) (1046.0 ⩽ T ⩽ 1255.5 K) for cell (II) and ΔfGmo(SrMoO4, s, T) (kJmol−1)± 0.4 = −1583.0 + 0.3699T (K) (1037.5 ⩽ T ⩽ 1278.0 K) for cell (III). From the variation in ΔfGmo(SrMoO4, s, T) (kJmol−1) with temperature and the relevant heat capacity values from the literature, ΔfHmo(298.15 K) of SrMoO4(s) has been obtained as −(1582.9±0.5) and −(1583.3±0.1) kJmol−1 respectively and the average value is −(1583.1±0.6) kJmol−1.

Phase diagram and thermodynamic calculations of alkali and alkaline earth metal zirconates

Abstract The ternary phase diagrams and partial pressures of various gaseous species over the equilibrium phase fields have been calculated for the MZrO (M = Li, Na, K, Rb, Cs, Sr and Ba) systems by using the SOLGASMIX-PV program, which computes equilibrium composition by direct minimization of the Gibbs energy of a system. The available experimental Gibbs energy data reported in the literature for binary and ternary compounds were used for these calculations. Where no data exist, values were estimated. These ternary phase diagrams are being reported for the first time, except for the lithium system.

Calorimetric studies on uranium molybdate

Abstract Enthalpy increment measurements on UMoO6(s) have been carried out using a high-temperature Calvet micro-calorimeter in the temperature range 299 to 1000 K. The enthalpy increments were least squares analyzed using Shomate’s method. The complete thermodynamic information for UMoO6(s) has been computed. The enthalpy increment expression for UMoO6(s) as a function of temperature is given by H o (T)−H o (298.15 K )( J mol −1 )=−53928.8+158.65T( K )+21.443×10 −3 T 2 ( K )+14.077×10 5 /T( K ).

Thermodynamic studies on SrThO3(s)

Abstract The Gibbs energy of formation of SrThO3(s) has been determined using e.m.f. and manometric techniques. In the e.m.f. method, two fluoride cells have been constructed to determine ΔfG0m(SrThO3,s,T) using CaF2(s) as a solid electrolyte. The cells used are: (−) O 2 ( g ), Pt / SrO ( s )+ SrF 2 ( s )// CaF 2 //( SrThO 3 ( s )+ ThO 2 ( s )+ SrF 2 ( s )/ Pt , O 2 ( g )(+), ( I ) (−) O 2 ( g ), Pt / SrThO 3 ( s )+ SrF 2 ( s )+ ThO 2 ( s )// CaF 2 ( s )// CaO ( s )+ CaF 2 ( s )/ Pt , O 2 ( g )(+). ( II ) The observed e.m.f. values are represented by following respective expressions: E ( V )±0.0001=0.0998+3.254×10 −5 T ( K ), ( Cell I ) E ( V )±0.0001=0.0285−6.37×10 −5 T ( K ). ( Cell II ) From the measured e.m.f. values of the cells and the ΔfG0m(T) values from the literature, ΔfG0m(SrThO3,s,T) have been calculated and are respectively given as Δ f G 0 m ( SrThO 3 , s ,T)±10 kJ mol −1 =−1829.2+0.2735T ( K ) (978⩽T ( K )⩽1154), ( Cell I ) Δ f G 0 ( SrThO 3 , s ,T)±20 kJ mol −1 =−1853.5+0.2867T ( K ) (1008⩽T ( K )⩽1168). ( Cell II ) In the manometric technique, equilibrium CO2(g) pressures are measured over the three phase mixture: {SrThO3(s)+SrCO3(s)+ThO2(s)} using a mercury manometer from 1075 to 1197 K. The corresponding Gibbs energy as a function of temperature is given by Δ f G 0 m ( SrThO 3 , s ,T)( kJ mol −1 )±14=−1865.4+0.3086T ( K ).

Enthalpy increment measurements of SrMoO4(s) and BaMoO4(s)

Abstract Enthalpy increment measurements on SrMoO 4 (s) and BaMoO 4 (s) were carried out using a Calvet micro-calorimeter. The enthalpy increment values were least squares analyzed with the constraints that H o ( T )− H o (298.15 K) at 298.15 K is equal to zero and C p o (298.15 K) is equal to the known value. The dependence of enthalpy increments with temperature can be given as: H o (T)−H o (298.15 K )( J mol −1 )=126.810 T( K )+24.571×10 −3 T 2 ( K )+21.782×10 5 /T( K )−47299. ( SrMoO 4 ( s ), 299.0≤T( K )≤1020.3) H o (T)−H o (298.15 K )( J mol −1 )=136.280T( K )+15.122×10 −3 T 2 ( K )+20.502×10 5 /T( K )−48852. ( BaMoO 4 ( s ), 299.0≤T( K )≤1020.3) Thermodynamic functions of SrMoO 4 (s) and BaMoO 4 (s) have been generated using the experimental Δ f H m o (298.15 K), Δ f G m o ( T ) and estimated S m o (298.15 K) values for SrMoO 4 (s) and BaMoO 4 (s) from the literature.

Calorimetric studies on Tl2Th(MoO4)3(s) and Tl2UO2(MoO4)2(s)

Abstract Enthalpy increment measurements on Tl2Th(MoO4)3(s) and Tl2UO2(MoO4)2(s) have been carried out using a high temperature Calvet microcalorimeter. Both compounds have shown transition in the experimental temperature range. The enthalpy increment data were least squares analyzed using Shomate’s method (Ttra>T) and origin programme version 6 (T>Ttra). In the Shomate method, the constraints used are Hom(T)−Hom (298.15 K) is zero at 298.15 K and Cop,m (298.15 K) equal to 364.6 and 269.44 J K−1 mol−1 for Tl2Th(MoO4)3(s) and Tl2UO2(MoO4)2(s), respectively. The resulting expressions are given below: H o m (T)−H o m (298.15 K) (J mol −1 )=−163 137+ 447.26T+19.286 ·10 −3 T 2 +83.669·10 5 /T (Tl 2 Th(MoO 4 ) 3 (s) , 299≤T≤500.6) H o m (T)−H o m (298.15 K) (J mol −1 ) =−86 844+237.13T+96.656·10 −3 T 2 +22.516 ·10 5 /T (Tl 2 UO 2 (MoO 4 ) 2 (s) , 299≤T≤496.5) Hom(T)−Hom (298.15 K) data, above the transition temperatures, were least squares analyzed using the origin program version 6 in which only the experimental Hom(T)−Hom (298.15 K) data points are taken. The corresponding expressions for Tl2Th(MoO4)3(s) and Tl2UO2(MoO4)2(s), respectively, are H o m (T)−H o m (298.15 K) (J mol −1 ) =−189 283+468.232T+19.350·10 −3 T 2 +18.878 ·10 6 /T (Tl 2 Th(MoO 4 ) 3 (s) , 775.5≤T≤828.0) H o m (T)−H o m (298.15 K) (J mol −1 ) =−78 811+235.391T+96.420·10 −3 T 2 +2.742·10 4 /T (Tl 2 UO 2 (MoO 4 ) 2 (s) , 626≤T≤835.4) The first differential of above equations with respect to temperature gives the molar heat capacities, Cop,m(T). The complete thermodynamic functions for the Tl2Th(MoO4)3(s) and Tl2UO2(MoO4)2(s) compounds have been computed for the first time.

Enthalpy increments of ThMo2O8(s) and Tl2MoO4(s)

Abstract Enthalpy increment measurements on Th(MoO 4 ) 2 (s) and Tl 2 MoO 4 (s) have been carried out using a high temperature Calvet micro-calorimeter. The enthalpy increments for Th(MoO 4 ) 2 (s) were least-squares analyzed using Shomate’s method. The constraints used are H o ( T )− H o (298.15 K) is zero at 298.15 K and C o p,m (298.15 K) equal 211.1 J K −1 mol −1 . The resulting expressions is given below: H o (T)−H o (298.15 K ) ( J mol −1 )=−66782+193.73 T ( K )+53.245×10 −3 T 2 ( K )+12.785×10 5 /T ( K ) ( Th(MoO 4 ) 2 (s) , 299≤T ( K )≤1000) Thallium molybdate shows two phase changes in the experimental temperature range 299–800.1 K. Thus, H o ( T )− H o (298.15 K) data for Tl 2 MoO 4 (s) were analyzed in the temperature range 400.3–669.9 (single phase) using origin program version 5 and the constraint used is H o ( T )− H o (298.15 K)=0. The corresponding expression is given as follows: H o (T)−H o (298.15 K ) ( J mol −1 )=−30169+95.99 T ( K )+54.060×10 −3 T 2 ( K ) (Tl 2 MoO 4 (s) , 400.3≤T ( K )≤699.9) The complete thermodynamic functions for Th(MoO 4 ) 2 (s) and Tl 2 MoO 4 (s) have been computed for the first time.

Thermodynamic properties of ZrMo2O8(s) and HfMo2O8(s)

Abstract The standard molar Gibbs energies of formation of AMo 2 O 8 (s) (where A  Zr, Hf) have been determined using a solid oxide galvanic cell. The dependence of e.m.f. on temperature of the cell Pt/ZrMo 2 O 8 (s)+MoO 2 (s)+ZrO 2 (s)/CSZ/air/Pt could be represented by E (V) ± 0.001 ± 0.7276 − 2.749×10 1 T(K) (870.5≤T(K)≤1159). Similarly the e.m.f. of the cell Pt/HfMo 2 O 8 (s) + MoO 2 (s) + HfO 2 (s)/CSZ/air/Pt could be represented by E (V)±0.001 0.7375−2.801×10 4 T(K) (906≤T(K)≤1148). In these cells CSZ represents 15 mol% calcia stabilized zirconia solid solution. The calculated Δ f G m 0 ( T ) values for ZrMo 2 O 8 (s) and HfMo 2 O 8 (s) can be given by Δ f G m 0 (ZrMo 2 O 8 s T )(kJ mol 1 ) ± 4.8 = −2535.4 + 0.6247T(K) Δ f G m 0 (HfMo 2 O 8 s. T )(kJ mol 1 ) ± 5.2 = −2551.4+0.6128T(K). Δ f H m 0 (298.15 K) of HfMo 2 O 8 (s) and ZrMo 2 O 8 (s) are calculated by the second-law method using the heat capacity values and can be given respectively as −(2568.3±2.0) and −(2588.6±2.2) kJ mol −1 . The standard molar Gibbs energy of formation of MoO 2 (s) required for the calculations has also been determined using the e.m.f. technique.

The standard molar Gibbs free energy of formation of SrMoO3(s)

Abstract ΔfGm°(SrMoO3,s, T) was obtained by measuring Sr(g) pressures over the mixture (SrMoO3(s) + Mo(s) + Cr(s) + Cr2O3(s)) in the temperature range 1185.5–1370.0 K and is given by the equation ΔfGm°(SrMoO3,s,T)(kJ mol−1) ± 0.6 = − 1290.3 + 0.2927T (K). Using the relevant heat capacity and entropy data from the literature and the variation of ΔfGm°(T) with temperature and Cp of SrMoO3(s) from the present work, the values of ΔfHm°(298.15 K) and Sm°(298.15 K) were calculated to be −(1294.6 + 3.6) kJ mol−1 and (84.2 ± 2.8) J K−1 mol−1 respectively.

The standard molar Gibbs free energies of formation of BaZrO3(s) and Ba(g)

Abstract The standard molar Gibbs free energy of formation ΔfGmo(BaZrO3, s) has been obtained by measuring Ba(g) pressures over the ternary phase field (BaZrO2(s) + Zr(s) + ZrO2(s)) in the temperature range 1203 K to 1347 K using the Knudsen-effusion mass-loss technique. Also ΔfGmo(Ba, g) has been obtained by measuring Ba(g) pressures over Ba(l) using transpiration and boiling-temperature techniques. The standard molar Gibbs free energies of formation of BaZrO3(s) and Ba(g) can be represented by Δ f G∘ m (BaZrO 3 , S, T)/(kJ·mol −1 ) = −1782.9+0.3236(T/K)±0.71, (1203 K to 1347 K) ; Δ f G∘ m (Ba, g, T)/(kJ·mol −1 ) = 178.7−0.0885(T/K)±0.15, (1177 K to 1435 K) By using the molar heat capacities of BaZrO3(s), Ba(g), Zr(s), and ZrO2(s) from the literature, ΔfHmo(BaZrO3, s, 298.15 K) has been calculated by second- and third-law methods and the corresponding values are −1776.1 kJ·mol−1 and −1721.7 kJ·mol−1. Smo(298.15 K) for BaZrO3 has been calculated to be 89.8 J·K−1·mol−1.