We report that the 122 type Sr1−xCaxCo2P2 solid solution undergoes an anomalous structural transition from the uncollapsed to the collapsed ThCr2Si2 structure at a distinct onset composition near x = 0.5. Correlated with the structural changes, the electronic system evolves from a nearly ferromagnetic Fermi liquid to an antiferromagnetic metal, through a complex crossover regime. The structural collapse, driven by P-P bonding across the (Sr,Ca) layers, is much more pronounced in this system than it is in the analogous Fe-based system, indicating a strong sensitivity of structure to total electron count in the transition metal pnictide 122 family.

The charge-ordered perovskite Pr0.65Ca0.28Sr0.07MnO3 was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping. PACS. 63.20.-e Phonons in crystal lattices -71.30.+h Metal-insulator transitions and other electronic transitions -72.60.+g Mixed conductivity and conductivity transitions -78.30.-j Infrared and Raman spectra

A series of ceramic R 0.67 Sr 0.33 CoO 3 samples with RϭLa, Pr, Pr 0.5 Nd 0.5 , Nd, Nd 0.5 Sm 0.5 , and Sm have been prepared. We report on detailed structural, electrical, magnetic, and thermodynamic studies. All samples reveal magnetic order, spanning the range from a ferromagnetic metal to a ferrimagnet close to a metal-to-insulator transition. We found a clear correlation between structural properties and resistivity as well as between structure and magnetic exchange interactions. In this context the importance of double exchange is discussed. Heatcapacity experiments show a significant anomaly at the magnetic transition temperature of the La compound only. The samples with magnetic rare-earth ions reveal significant Schottky anomalies at low temperatures and allow for a rough estimate of the crystal-field splittings.

The orthorhombic rare-earth cobaltate Nd 0.67 Sr 0.33 CoO 3 is studied by neutron powder diffraction. The Co sublattice orders ferromagnetically below T c Ϸ200 K with the magnetic moments aligned along the c axis. On cooling, Nd 0.67 Sr 0.33 CoO 3 revealed an induced ferromagnetic order of the Nd sublattice coupled antiparallel to the Co lattice. Within a simple spin-only picture, steric considerations give evidence for a mixed-valent mixed-spin configuration of the Co ions. This is compared to an intermediate valence-state scenario that shows the necessity of detailed electronic-structure calculations.

We report on structural, magnetic and Electron Spin Resonance (ESR) investigations in the manganite system LaMn1−xCrxO3 (x ≤ 0.5). Upon Cr-doping we observe a reduction of the Jahn-Teller distortion yielding less distorted orthorhombic structures. A transition from the Jahn-Teller distorted O ′ to the pseudocubic O phase occurs between 0.3 < x < 0.4. A clear connection between this transition and the doping dependence of the magnetic and ESR properties has been observed. The effective moments determined by ESR seem reduced with respect to the spin-only value of both Mn 3+ and Cr 3+ ions.