Zdeněk Jirák

Structure and magnetic properties of Pr1−xSrxMnO3 perovskites

Abstract A comprehensive account of structural features in mixed Mn3+, Mn4+ perovskites, Pr1−xAxMnO3 (A = Ca, Sr, Ba) is given. The newly investigated Sr system was studied at various temperatures by X-ray and neutron diffraction, magnetic, and conductivity measurements. A low-temperature transition from the activated type to the degenerate type of conductivity was observed in the Sr system for x = 0.3–0.4. It occurs 20–60 K below the ferromagnetic Curie point.

Neutron diffraction study of the modulated structure of Bi2(Sr, Ca)3Cu2O8+y

Abstract The modulated structure of the 80 K superconductor Bi 2 (Sr, Ca) 3 Cu 2 O 8+y is refined from the single-crystal neutron diffraction data on a monodomain block of dimensions 1.5 × 1.0 × 0.03 mm 3 . The modulation is recognized as commensurate with period 4.75 a , resulting in a supercell 19a × b × c (a = 5.397 (1), b = 5.401 (1), c = 30.716 (3) A ) and symmetry Pnaa. The essential feature of the structure is the periodic insertion of additional oxygen rows into BiO planes, which in turn causes large displacive modulation in all layers. The ideal oxygen stoichiometry is y = 4 19 ∽ 0.21 . Bonds in the BiO and SrO planes form characteristic chains running along the a -axis. The coordination of the Bi 3+ cations is uniform and consists of three mutually nearly perpendicular bonds to oxygens of 2.0–2.2 A lengths. The Sr 2+ cations are bonded only to eight oxygen atoms at the most. About 7% of the Sr sites, located in the vicinity of the inserted oxygen, are possibly vacant. The Ca site situated between the CuO 2 planes is substituted to 30% by strontium.

Electric transport and magnetic properties of perovskites LaMn1−xCoxO3 up to 900 K

The structural, magnetic and transport properties of the LaMn1−xCoxO3 system were investigated over a wide temperature range, 10–900 K. Two structural types, depending on x, were detected—orthorhombic Pbnm () and rhombohedral (x>0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn1−xCoxO3 system, the respective substituents are unambiguously characterized as Co2+ () and Mn4+ (). On the Mn-rich side, up to x<0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co2+ valence towards the prevailing Co3+, while for the Co-rich samples (), the Co3+ states tend to disproportionate at high temperatures to Co2++Co4+, which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below K and coagulate below K.

Character of the excited state of the Co3+ ion in LaCoO3

Generalized gradient approximation (GGA)+U calculations for LaCoO3 confirm the existence of a stationary state composed of a mixture of the low spin (LS) and high spin (HS) Co3+ ions in a 1:1 ratio, which is of insulating character. At low temperatures, this state is located about 25 meV above the homogeneous LS ground state if CoLS–O–CoHS bond length optimization is taken into account. The energy difference decreases with the lattice dilatation and the LS+HS(1:1) phase with bond length optimization becomes stable at some intermediate temperature. These results, together with previous electron spin resonance evidence for HS excitations in the LS ground state, lead to the conclusion that the diamagnetic–paramagnetic transition in LaCoO3 at 50–150 K is caused by a gradual population of HS Co3+ ionic states, provided that neighbouring sites are in the LS state. Consistency between experimental data for the paramagnetic susceptibility and anomalous thermal expansion is achieved for an HS–LS energy difference of 16 meV that corresponds to 180 K. With increasing HS population, the antiferromagnetic (AF) interactions HS–LS–HS become effective. The LS+HS(1:1) phase is saturated above 150 K and the susceptibility acquires a simple Curie–Weiss behaviour in which AF interactions are characterized by θ = −160 K.

Study of Pr1−xMn1+xO3 perovskites

Abstract The structural and magnetic properties of the Pr1−xMn1+xO3 perovskites were studied. The increase of x (i.e., Pr Mn ) leads to the decrease of the orthorhombic deformation and of the Neel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr3+1−xMn2+x)A(Mn3+1−xMn4+x)O2−3 by the double exchange of Mn3+Mn4+ pairs at the B—sublattice.

Crystal field and magnetism of Pr3+ and Nd3+ ions in orthorhombic perovskites

Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO3 perovskites (R=Pr, Nd, M=Ga, Co) are calculated using a first-principles electronic structure and the Wannier projection. The method contains a single adjustable parameter that characterizes the hybridization of R(4f) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of the trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the 4f electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within a few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO3 the theory is also compared with the magnetic measurements on a single crystal presented by Luis et al (1998 Phys. Rev. B 58 798). Good agreement between the experiment and theory is found.

Structural modulation, oxygen content and transport properties in Bi2.13Sr1.87CuO6+y and Bi2.05Sr1.54La0.41CuO6+y superconductors

Abstract A neutron diffraction study, electrical conductivity and thermopower measurements have been performed on single-crystal superconductors of the 2201-type - Bi 2.13 Sr 1.87 CuO 6+ y with T c =6 K ( γ =0.17) and Bi 2.05 Sr 1.54 La 0.41 CuO 6+ y with T c =4−11 K ( γ =0.28-0.33). The crystals exhibit an “oblique” modulation with wave vectors τ =[0.198, 0, 0.36] and τ =[0.221, 0, 0.75], respectively. The structure determination of the La-free compound located two extra oxygen sites in the BiO plane with periodicity ≈5 a . One is inserted into the Bi-O-Bi- ribbons and the second one between the ribbons, similarly as observed earlier in Bi 2 Sr 2.4 Y 0.6 Cu 2 O 8.5 . The modulation exhibits a phase shift between bismuth double layers, which is a consequence of interlayer Bi-O links and oxygen vacancies in the SrO planes. The measurements of transport properties relate the occurrence of the superconductivity to an interplay of two effects: (1) generation of charge carriers due to oxygen insertion and (2) low-temperature localization due to bismuth cations in the strontium sites.

Structural anomalies, spin transitions, and charge disproportionation in LnCoO3

The diamagnetic-paramagnetic and insulator-metal transitions in LnCoO3 perovskites (Ln=La,Y, rare earths) are reinterpreted and modeled as a two-level excitation process. In distinction to previous models, the present approach can be characterized as a low-high-intermediate spin LS-HS-IS scenario. The first level is the local excitation of HS Co3+ species in the LS ground state. The second excitation is based on the interatomic electron transfer between the LS/HS pairs, leading finally to a stabilization of the metallic phase based on IS Co3+. The model parameters have been quantified for Ln=La, Pr, and Nd samples using the powder neutron diffraction on the thermal expansion of Co–O bonds that is associated with the two successive spin transitions. The same model is applied to interpret the magnetic susceptibility of LaCoO3 and YCoO3.

Preparation and the crystal structure of a new manganate, Sr4Mn3O10

Abstract The growth of strontium manganates from melts based on SrCl2SrF2 as solvents yielded orthorhombic crystals of composition Sr4Mn2.964Pt0.036O10 (space group Cmca, a = 5.443(2), b = 12.427(4), c = 12.500(4) A, Z = 4, ϱcal = 5.34 g cm−3). The platinum incorporated in samples originates from the crucible and may be essential for the crystal stabilization. The structure can be derived from a close-packed arrangement of oxygen and strontium atoms, in which some positions are vacant. This results in a more open structure. Manganese and platinum cations occupy all octahedral voids coordinated by the oxygen atoms only. The characteristic feature of the structure is triple groups of face-shared octahedra which are interconnected by common corners into a two-dimensional framework perpendicular to the [010] direction. Stacking in the third dimension is established by SrO bonds.

Structural and transport properties of YBa2Cu3−yCoyO7−x solid solutions

Abstract The X-ray study of YBa 2 Cu 3- y Co y O 7 solid solutions showed that in addition to the orthorhombic-tetragonal transition existing in the compositional range of 0.0 y y >0.4. A model based on an arrangement of microwatwins, formed owing to a tendency of cobalt ions to be six-fold coordinated, was proposed to explain the observed structural changes. A comparison of the oxygen stoichiometry and Hall charge carriers density data showed that this tendency is decisive for the charge distribution in the (Me(1)O) sublattice. Besides the geometrical arrangement, the quality of the bonds and consequent character of the transport properties is influenced by a more ionic character of cobalt ions in comparison with copper ones.