Magnetic and structural transitions inLa1−xAxCoO3(

Abstract

We report thermal-expansion, lattice-constant, and specific-heat data of the series La 1−x A x CoO 3 for 0 Յ x Յ 0.30 with A = Ca, Sr, and Ba. For the undoped compound LaCoO 3 , the thermal-expansion coefficient ␣͑T͒ exhibits a pronounced maximum around T = 50 K caused by a temperature-driven spin-state transition from a low-spin state of the Co 3+ ions at low temperatures toward a higher spin state at higher temperatures. The partial substitution of the La 3+ ions by divalent Ca 2+ , Sr 2+ , or Ba 2+ ions causes drastic changes in the macroscopic properties of LaCoO 3 . The large maximum in ␣͑T͒ is suppressed and completely vanishes for x տ 0.125. For A = Ca three different anomalies develop in ␣͑T͒ with further increasing x, which are visible in specific-heat data as well. Together with temperature-dependent x-ray data, we identify several phase transitions as a function of the doping concentration x and temperature. From these data we propose an extended phase diagram for La 1−x Ca x CoO 3 .

Anomalous magnetic and spin glass behavior in Nb-substituted LaCo1−xNbxO3

“…The rare-earth cobaltite LaCoO 3 exhibit unusual magnetic and electronic phase transitions, which can be tuned by changing the temperature, pressure [1][2][3][4][5][6] as well as with chemical substitutions at the La/Co site [7][8][9][10][11][12][13][14][15][16]. The 3d orbitals of the Co 3+ ion, surrounded with octahedrally coordinated O 2− ions, are split into the t 2g (triply degenerate) and the e g (doubly degenerate) orbitals in the lower and upper energy levels, respectively.…”

“…The feature near 500 K is ascribed to an insulator to metal transition [32], where the electrical conductivity increases by two orders of magnitude. Interestingly, these transitions are strongly affected by substituting alkali-earth elements at La site and transition metal elements at Co site [7,8,[14][15][16]. For example, a well established case is the observation of a spin-cluster-glass insulator to ferromagnetic metal transition at about 20% concentration of Sr in bulk La 1−x Sr x CoO 3 [33].…”

Phase diagram and isotope effect in(Pr1−yEuy)0.7Kalinov1, Gorbenko2, Талденков3 et al. 2010Phys. Rev. BSelf Cite154200View full textAdd to dashboardCite

“…The heterovalent doping (R 1−x A x CoO 3 ; A = Ba, Sr, and Ca) causes hole doping and chemical pressure (arising from the change of the ionic radii in the R 1−x A x complex) and stabilizes a magnetic (IS or HS) state of Co 3+ . 30,33 For the relatively large atomic species R = La and A = Ba or Sr, the nonmagnetic (LS) insulating ground state of RCoO 3 changes to a ferromagnetic metal at high enough hole concentration, which is due to the enhancement of the double exchange interaction caused by the decrease of ∆E and an increase in the number of electrons at the e g orbitals. The isovalent doping with a smaller rareearth, i.e., chemical pressure without changing the Co valence, is usually realized by introducing other trivalent rare-earth ions R 3+ .…”

Evolution of Magnetic Oxygen States in Sr-DopedLaCoO3

“…For x < 0.18 the crystal becomes ferromagnetic (FM) and metallic, [3][4][5][6][7] With increasing x, the rhombohedral structural distortion gradually decreases and the crystal approaches the cubic perovskite structure [8].…”