Vaporization of LaCrO₃: Partial and Integral Thermodynamic Properties

Abstract

The vaporization of LaCrO3(s) and samples of the composition LaCrO3+ La2O3 was investigated in the temperature range of 1887‐2333 K by Knudsen effusion mass spectrometry using Knudsen cells made of tungsten lined completely with iridium. The species Cr(g), CrO(g), CrO2(g), and LaO(g) were identified in the vapor. Their partial pressures were determined by calibration with pure platinum solid. The thermodynamic activity of Cr2O3, acr2o3 in LaCrO3 for the Cr203‐poor phase boundary of this phase was In aCr2o3= ‐(17953/T) ‐ 0.485 (temperature T given in K) for the temperature range of the measurements with a probable overall error of ± 13%. The following values and temperature dependence of ΔG°f,T resulted for the formation of LaCrO3(s) according to the reaction 0.5Cr2O3(s) + 0.5La2O3(s) → LaCrO3(s): ΔG°f,2100= ‐78.9 ± 1.1 kj/mol, ΔH°f,298= ‐76.8 ± 5.2 kj/mol, and ΔG°r(kJ/mol) = ‐74.7 ‐ 0.00202T. Computations for the vaporization of LaCrO3 were conducted to show the volatility of this material in different atmospheres at high temperatures.

Thermodynamic Assessment of the La-Cr-O System

“…Peck et al [53] derived the Gibbs energy of formation of LaCrO 3 from the determination of the thermodynamic activity of Cr 2 O 3 in LaCrO 3 for the Cr 2 O 3 -poor phase boundary of LaCrO 3 in the temperature range from 1887 to 2333 K using Knudsen effusion mass spectrometry.…”

“…Enthalpy increments of LaCrO 3 at 1090 and 1350 K were measured by Suponitskii [30] using a high-temperature heatconducting calorimeter. [46] Standard entropy S LaCrO3 298 K ¼ 109:2 J mol À1 K À1 this work, calculated Gibbs energy of formation by 3 4 [49] solid oxide electrolyte-emf T ¼ 1273 K D G ¼ À42:29 AE 0:38 kJ mol À1 [52] CaF 2 -based emf T ¼ 2100 K D G ¼ À79:52 kJ mol À1 this work, calculated T ¼ 2100 K D G ¼ À78:9 AE 1:1 kJ mol À1 [53] Knudsen mass spectrometry D G ¼ À72:403 À 0:0034T ðkJ mol À1 Þ; 1273 À 2673 K this work, calculated D G ¼ À44:45 þ 0:002115T AE 0:4 ðkJ mol À1 Þ; 855 À 1073 K [50] CaF 2 -based emf D G ¼ À94:758 þ 0:08530T ðkJ mol À1 Þ; 700 À 885 K [51] CaF 2 -based emf Enthalpy increments H À H 298 K ; kJ mol À1 T ¼ 1090 K 98.19, this work, calculated 94.4 [30] HT(high temperature)-calorimetry T ¼ 1350 K 133.05 this work, calculated 139.2 [30] HT-calorimetry Activity of Cr 2 O 3 in LaCrO 3 T ¼ 2100 K a Cr2O3 ¼ 1:11 Â 10 À4 this work, calculated T ¼ 2100 K Transition temperature, K 540, this work, calculated 503-583 [33] adiabatic calorimetry 544 ± 1 [34] (a) DTA, DSC, thermogravimetry, dilatometry 536 [35] (a) adiabatic shield calorimetry, HT-XRD (air and vacuum) 563 ± 5 [36] DTA, dilatometry, HT-XRD, HT-microscopy, HT-X-ray photography 550 [37] HT-XRD 528-533 [38] (a) HT-XRD 533 ± 3 [38] (a) DTA 543 [39] XRD 533 [20] HT-XRD 540 ± 2 [40] (a) HT-XRD, DSC 533 ± 5 [40] (a) HT-XRD, dilatometry 545 [41] heating, DSC 535 [41] cooling DSC 550 [41] HT-XRD 523 [42] (a) starting transition, simultaneous DSC-XRD 541 [42] (a) completed transition, simultaneous DSC-XRD 533 [43] estimated from neutron powder diffraction 509 [44] DSC, XRD Enthalpy change of transition, J mol -1 340, this work, calculated 502.08 ± 41.84 at 503-583 K [33] calculated from adiabatic calorimetry 277 at 544 ± 1 K [34] (a) DSC 403.25 at 536 …”

“…The parameters A, B, and C are optimized using the enthalpy of formation from Cheng and Navrotsky, [47] activity-data of Cr 2 O 3 in LaCrO 3 from Peck et al, [53] heat capacity-data obtained by adiabatic calorimetry from Sakai and Stølen, [35] and enthalpy increment-data measured at high temperatures. [30] A phase diagram with congruent melting of lanthanum chromite and two eutectics [18,19] cannot be reproduced by using the emf-experiments.…”

“…Existing experimental data of lanthanum chromite perovskite structure, [33][34][35][36][37][38][39][40][41][42][43][44][45] thermodynamics, [30,[33][34][35]43,[46][47][48][49][50][51][52][53] phase stability, [54] and nonstoichiometry [55,56] along with the investigation techniques used are listed in Table 1.…”

“…A small peak which was found around 855 K can be explained most likely by the decomposition of an undetected impurity phase. [35] The calculated Gibbs energies of the formation of LaCrO 3 from the oxides are listed as a function of temperature together with data from the literature [49][50][51][52][53] in Table 1. The resulting Gibbs energies of formation from emf-measurements are remarkably less negative than the Gibbs energies of formation derived from Knudsen mass spectrometry.…”