The high temperature enthalpies of high purity ScF3, CeF3, SmF3, GdF3 (redetermined), TbF3, DyF3, ErF3, TmF3, and YbF3 were measured from 100 to 1600°C and EuF3 from 100 to 975°C. The thermal data for the other fluorides in the rare earth series were reported earlier from this laboratory. The heat capacity, enthalpies of transition and fusion, and related thermodynamic functions were calculated. The smoothed values of H°T−H°298.15, Cp, S°T−S°298.15, and −(F°T−H°298.15)/T are tabulated at 100° intervals. The melting points, transition temperatures, and lattice parameters of the high purity fluorides prepared in this study were determined.

The pressure-assisted reaction sintering (PARS) method was used to prepare thermoelectric elements of the γ-phase lanthanum sesquisulfide solid solution alloys which exist over a wide homogeneity range: LaSy (1.33<y<1.5). A mixture of powders of stoichiometric lanthanum sesquisulfide (La2S3) and lanthanum trihydride (LaH3) was reaction sintered in a hot-press to make the desired composition of γ-LaSy. This method produced black dense single-phase γ-LaSy pellets with a theoretical density of 97% or better. The thermoelectric properties (electrical resistivity and Seebeck coefficient) of the samples prepared by the PARS technique were measured. The room temperature electrical resistivity and Seebeck coefficient of γ-LaSy varied in a systematic manner with the electron concentration indicating that γ-LaSy alloys behave like a simple metal. Although the current carriers appear to be quite heavy (drift mobility ∼3 cm2/V s), the temperature dependence (up to 1000 °C) of these properties of LaS1.42 were found to be linear functions, which is in accord with the above observations. The thermoelectric figure of merit based on the measured resistivity and Seebeck coefficient and an estimated thermal conductivity for LaS1.42 is 0.75×10−3 °C−1 at 1000 °C, which is comparable to those of the state-of-the-art materials such as silicon–germanium alloys at 1000 °C.

The Seebeck coefficient and electrical resistivity have been measured for the La3−x Mx S4 alloys with the Th3 P4 -type structure for M=Sm, Eu, Yb, and x=0.1 to 0.9 in order to determine their potential as high-temperature thermoelectric energy conversion materials. An analysis of the lattice parameters of these alloys suggests that Sm has a valence of 2.2 over the entire composition range. Furthermore, the variation of the Seebeck coefficient and electrical resistivity as a function of electron concentration is different for the Sm-doped alloys than for the Eu- and Yb-doped alloys, which are all different from that of the pure binary LaSy alloys. A maximum in the electrical power factor at 1000 °C is found to occur at x ≂ 0.3, but when a reasonable estimate is made of the thermal conductivity the maximum in the figure-of-merit at 1000 °C shifts to larger x values (∼0.7) in all alloy systems. The long term stability of the Sm and Eu ternaries is also reported.