CaMnO 3 is a parent compound for numerous multicomponent manganese perovskite oxides. Its crystallographic data are of primary importance in the science and technology of functional CaMnO 3 -based materials. In the present study, data were collected for a CaMnO 3 sample at 302 K. The crystal structure refinement yields accurate absolute values of lattice parameters, a = 5.281 59͑4͒ Å, b = 7.457 30͑4͒ Å, and c = 5.267 48͑4͒ Å, leading to orthorhombic distortion of ͑c / a , ͱ 2c / b͒ = ͑0.997 33, 0.998 95͒. The orthorhombic distortion of the CaMnO 3 structure is discussed on the basis of comparison of our unit-cell size with data already published. At a graphical representation of the distortion, it is observed that there is a considerable scatter of the distortion values among the literature data but, interestingly, a considerable fraction of experimental results ͑including the present one͒ for stoichiometric samples are grouped around the distortion ͑c / a , ͱ 2c / b͒ = ͑0.9973, 0.9990͒, which lies close to a maximum in the extent of orthorhombicity. The influence of off-stoichiometry on the orthorhombic distortion is discussed on the basis of available experimental data. Simulations, employing a mean-field approach for low temperatures, predict an increase in cell volume and structural distortions with the concentration of oxygen vacancies when the additional electrons are localized on the manganese. A simple model of delocalization produced the opposite effect, which is expected to combine with lattice vibrations to recover the cubic phase at high temperatures.

The pressure dependence, up to 11.3 kbar, of basic parameters of the superconducting state, such as the critical temperature (T(c)), the lower and the upper critical fields, the coherence length, the penetration depth, and their anisotropy, was determined from magnetic measurements performed for two single-crystalline samples of FeTe(0.5)Se(0.5). We have found pressure-induced enhancement of all of the superconducting state properties, which entails a growth of the density of superconducting carriers. However, we noticed a more pronounced increase in the superconducting carrier density under pressure than that in the critical temperature which may indicate an appearance of a mechanism limiting the increase of T(c) with pressure. We have observed that the critical current density increases under pressure by at least one order of magnitude.

The issue concerning the nature and the role of microstructural inhomogeneities in iron chalcogenide superconducting crystals of FeTe0.65Se0.35 and their correlation with transport properties of this system was addressed. The presented data demonstrate that chemical disorder originating from the kinetics of the crystal growth process significantly influences the superconducting properties of an Fe–Te–Se system. Transport measurements of the transition temperature and critical current density performed for microscopic bridges allow us to deduce the local properties of a superconductor with microstructural inhomogeneities, and significant differences were noted. The variances observed in the local properties were explained as a consequence of weak superconducting links existing in the studied crystals. The results confirm that the inhomogeneous spatial distribution of ions and small hexagonal symmetry nanoscale regions with nanoscale phase separation also seem to enhance the superconductivity in this system with respect to the values of the critical current density. Magnetic measurements performed in order to determine, in an alternative way, the values of the critical current density, as well as to find the relaxation rate and to check the scaling of the pinning force, confirm the conclusions drawn from the transport measurements.

Pressure-induced changes in ferromagnetic transition temperature ͑T C ͒, saturation magnetization ͑M S ͒, remanent magnetization ͑M R ͒, and coercive field ͑H C ͒ as well as the changes in the lattice parameters were investigated for both stoichiometric and oxygen-deficient La 1−x Sr x CoO 3−␦ compounds in the wide range of substitutions, 0.3Ͻ x Ͻ 1 and charge doping 3.18Ͻ v Co =3+x −2␦ Ͻ 3.76. Comparison of our results with previous data for lightly substituted and stoichiometric oxygen compounds shows that dT C / dP depends mostly on x, changing the sign from negative to positive in the range of 0.2Ͻ x Ͻ 0.3 and remaining near 1.5 K/kbar for 0.5Ͻ x Ͻ 1. Decrease in saturation magnetization with pressure was observed for compounds with full oxygen content. In contrast, M S increases linearly with increasing pressure for compounds with the same level of charge doping but with varying oxygen contents. The 59 Co nuclear-magnetic-resonance study has shown that the ferromagnetic state of oxygen-deficient SrCoO 2.88 is partly due to double-exchange interactions between cobalt ions: Co 4+ with intermediate-spin ͑IS͒ ͑S =3/ 2͒ state and Co 3+ with the spin state switching between low-spin ͑S =0͒ and IS ͑S =1͒. Presence of localized IS Co 4+ indicates that ferromagnetism of SrCoO 2.88 is due to rather complex exchange interactions that include superexchange. For La 0.33 Sr 0.67 CoO 2.85 , no localized state of Co has been observed and all Co ions are in the mixed-valence state in agreement with the double-exchangedriven ferromagnetic metallic state observed for this composition. The value of bulk modulus for La 0.33 Sr 0.67 CoO 2.85 ͓K 0 = 1540͑25͒ kbar͔ was found to be much higher than that for SrCoO 2.88 ͓790͑15͒ kbar͔.

Magnetic measurements have been performed under pressure of 0–9.7 kbar and in the temperature range of 5–300 K for the hole-doped Nd1-xCaxBaCo2O5.5 system over the composition range x = 0–0.06, where a strong competition between antiferromagnetic and ferrimagnetic phases is present. The dependence of TN, i.e., of the temperature of a first order phase transition from the antiferromagnetic to the ferrimagnetic state, and of TC, i.e., of the temperature of the second order phase transition from the ferrimagnetic to the paramagnetic state on hydrostatic pressure was found to be a function of hole doping. The large pressure-induced increase of TN becomes enhanced with increasing of the hole doping level, while a small pressure-induced increase of TC is suppressed by increasing the hole doping level. This finding shows that the hydrostatic pressure stabilizes the antiferromagnetic phase, whereas the hole doping alone suppresses the antiferromagnetic phase. All investigated compounds have revealed the existence of a certain fraction of ferrimagnetic phase at temperatures much below TN. An increase of a magnetization observed in high magnetic field of 50 kOe at temperatures much below TN was explained as the high-field induced polarization of Nd3+ magnetic moments, appearing in the magnetic field exceeding the strength of Nd-Co antiferromagnetic exchange interactions.