Investigation of the Evaporation Thermodynamics of Erbium Trifluoride and Thulium Trifluoride by Knudsen Effusion Mass Spectrometry and Matrix Isolation Infrared Spectroscopy

Abstract

The vaporization of ErF3 and TmF3 was investigated in the temperature range between 1266 and 1635 K by Knudsen effusion mass spectrometry and matrix isolation infrared spectroscopy. From the ionization efficiency curves the appearance energies of all ions formed from the equilibrium vapour were determined using Vogt's deconvolution method. For the determination of vapour pressure two methods, viz. mass-loss Knudsen effusion and Knudsen effusion mass spectrometry were applied. Comparing our investigation to former ones, the phase transitions on the Clausius–Clapeyron plots of ErF3 and TmF3 could be identified for the first time using Knudsen effusion mass spectrometry since this latter method is usually not sensitive for small heat effects. The mean natural logarithm of equilibrium vapour pressure as a function of temperature for all condensed phase–vapour regions can be expressed as follows: ln (p/Pa)=−(56600±2100)/T+(38.31±1.57) between 1266 and 1363 K ln (p/Pa)=−(51700±1900)/T+(34.61±1.38) between 1323 and 1423 K ln (p/Pa)=−(43320±340)/T+(28.74±0.23) between 1408 and 1624 K for TmF3, and ln (p/Pa)=−(52000±2400)/T+(34.60±1.69) between 1288 and 1383 K ln (p/Pa)=−(46300±1000)/T+(30.61±0.73) between 1378 and 1443 K ln (p/Pa)=−(43500±1200)/T+(28.61±0.75) between 1421 and 1635 K for ErF3. The vibration frequencies of the symmetric and asymmetric stretching modes of DyF3, ErF3 and TmF3 were measured for the first time, using matrix isolaton infrared spectroscopy. The thermochemical quantities such as enthalpy and entropy changes of sublimation and evaporation were calculated using both second and third laws of thermodynamics and the deviation was discussed in detail.