Hochtemperatur-Festkörperchemie von Verbindungen der seltenen Erden mit Valenzinstabilität: Nichtstöchiometrisches TmSe und Mischkristalle / High Temperature Solid State Chemistry of Rare Earth Compounds with Valence Instability: Nonstoichiometric TmSe and Alloys

Abstract

Abstract The phase diagram of TmSe has been measured in the composition range 0.6 ≦ x ≦ 1.2 (x = mole Tm/mole Se) up to 2300 °C. Both phase diagram and lattice constants show the existence of a gross homogeneity range at 0.90 < x < 1.05. X-ray and electron diffraction at room temperature confirm the homogeneity of this phase and show no superstructures in this range but only a correlation of vacancies. A dramatic change of the lattice constant (1.7%) with nonstoichiometry shows that the latter controls the valence of Tm between 3 + and 2.71+. Density measurements indicate that both the solution of excess Tm or Se in the lattice leads to the formation of vacancies in the other sublattice. The experimental results can be explained by the existence of approx. 1% Schottky defects for all samples and nonstoichiometric vacancies increasing with deviations from stoichiometry up to 10% for the Se-rich phase boundary (each tenth lattice site vacant!). With increasing temperature the nonstoichiometric samples show two phase transitions: the first at approx. 1100°C for Tm-rich samples or approx. 1700°C for Se rich samples (the stoichiometric sample shows both transitions); the second for all samples at 2003°. At 2020 °C a eutectic appears at the composition of x = 1.00, its components melting at 2032 °C and 2028 °C. This shows that in the temperature range between the phase transitions and the eutectic temperature a decomposition of the homogeneity range must take place. This decomposition might be due to a valence segregation, the two components of the eutectic possibly being Tm2,71+Se and Tm3+Se. Various measurements indicate changes of the lattice stability in TmSe as a function of stoichiometry. Thus stoichiometric samples show a minimum melting point and solution-reaction calorimetry (with 4N HCl) indicates that the composition with the highest melting point (Tm3+Se) has the highest stability. As it is briefly discussed the unstable states may have important implications as active states for surface phenomena and catalysis. Further change of the Tm-valence in a TmSe-containing compound exceeding the range Tm3+↔Tm2,71+ may be achieved by mixed-crystal formation. The lattice pressure in TmSe is changed by chemical substitution leading among others to the interesting mixed crystal systems TmSe1-xTex and Tm1-xEuxSe. The first shows two pressure-induced phase transitions accompanied by a change of the Tm-valence from 2.25+ → 2.52+ and 2.52+ → 2.93+. Density measurements show that when the lattice collapses the number of vacancies nears 10%.