Smaller Sm Research Articles

Oxygen nonstoichiometry in LnBaFe2O5+w across Ln = Nd, Sm, Gd

In double-cell perovskites LnBaFe2O5+w, only lanthanoids Ln ​= ​Pr to Gd support wide equilibrium nonstoichiometry w from 0 to >0.5. This study investigates point-defect equilibria behind the variation in w along Ln ​= ​Nd, Sm, Gd by quenching the oxides from flowing gas of controlled oxygen partial pressures and temperatures. The experimental data of w as a function of pO2 and T are least-squares fit via standard enthalpies and entropies of the defect-equilibrium reactions of formation for electronic-defect pair e′ and h• and for anion-Frenkel pair Oi′′ and vO••, and of the Fe2+ oxidation. NdBaFe2O5+w is closest to behaving as a pure undoped oxide at 1000 ​°C, where it has a single plateau in the w versus pO2 plot at the w ​= ​0.5 level. A second plateau, at w ​= ​0, emerges at lower temperatures. The smaller Sm and Gd increase the size difference versus Ba, and their defect equilibria show two plateaus even at 1000 ​°C. The temperature-dependent defect equilibria across this series are therefore approximated by a formal Ln3+ for Ba2+ donor-doping model, and the results are presented in Brouwer diagrams of defect concentrations. The decrease of the anion-Frenkel defect-equilibrium constant KaF due to decreasing ionic size from Nd3+ towards Gd3+ is magnified upon decrease in temperature that feeds the entropy of the anion-Frenkel disorder; at 400 ​°C, Ln ​= ​Nd has KaF ​= ​10−5.4, Sm 10−15.0 and Gd 10−24.3.

Lattice collapse in mixed-valence samarium fulleride Sm(2.75)C(60) at high pressure.

Sm(2.75)C(60) displays large negative thermal expansion behaviour in the temperature range 4.2-32 K at ambient pressure as a result of a quasi-continuous valence change from the larger Sm(2+) towards the smaller Sm(2.3+) ion. Here we use the powder synchrotron X-ray diffraction technique at ambient temperature and elevated pressures to study the compression behaviour in the pressure range 0-6 GPa. An abrupt hysteretic phase transition, accompanied by a dramatic volume decrease (approximately =6.0%) and a change in colour from black to golden was found at approximately =4 GPa induced by a sudden Sm valence transition from +2.3 towards +3. Such behaviour is typical of highly correlated Kondo insulators like SmS and makes Sm(2.75)C(60) the first known molecular-based member of this fascinating class of materials.

Temperature-induced valence transition and associated lattice collapse in samarium fulleride.

The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-T(c)) copper oxide superconductors. Metal fulleride salts-metal intercalation compounds of C60--and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system--they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + epsilon) + to nearly 2 +. The negative thermal expansion--heating from 4.2 to 32 K leads to contraction rather than expansion--occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm(2+) towards the smaller Sm((2+epsilon)+), analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).