In this article we study the effects of a partial substitution of Ba with the smaller cation Ca in the layered cobaltites YBaCo2O 5+δ for δ ≈ 0.5. Neutron thermodiffractograms are reported for the compounds YBa0.95Ca0.05Co2O5.5 (xCa = 0.05) and YBa0.90Ca0.10Co2O5.5 (xCa = 0.10) in the temperature range 20 K ≤ T ≤ 300 K, as well as high resolution neutron diffraction experiments at selected temperatures for the samples xCa = 0.05, xCa = 0.10 and the parent compound xCa = 0. We have found the magnetic properties to be strongly affected by the cationic substitution. Although the "122" perovskite structure seems unaffected by Ca addition, the magnetic arrangements of Co ions are drastically modified: the antiferromagnetic (AFM) long-range order is destroyed, and a ferrimagnetic phase with spin state order is stabilized below T ∼ 290 K. For the sample with xCa = 0.05 a fraction of AFM phase coexists with the ferrimagnetic one below T ∼ 190 K, whereas for xCa = 0.10 the AFM order is completely lost. The systematic refinement of the whole series has allowed for a better understanding of the observed low-temperature diffraction patterns of the parent compound, YBaCo2O5.5, which had not yet been clarified. A two-phase scenario is proposed for the xCa = 0 compound which is compatible with the phase coexistence observed in the xCa = 0.05 sample.

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This article presents the results of a neutron diffraction study of a series of quenched Ti-V alloys and an assessment of the composition dependence of the structural properties in the Ti-V system. Upon quenching to room temperature and atmospheric pressure, three metastable phases occur, viz., the hcp (␣ Ј) phase formed by a martensitic transformation, the omega (⍀) phase formed by a displacive transformation involving the collapse of the (111) planes of the bcc structure, and the untransformed bcc (␤ ) phase. The lattice parameters (LPs) of the ␣ Ј, ␤, and ⍀ phases are determined as functions of the V content in the composition range 3 Յ at. pct V Յ 70. This information is combined with a detailed analysis of the available experimental data on the ␣ Ј, ␤, and ⍀ phases in pure Ti and Ti-V alloys and the ␤ phase of V. New estimates for the LPs of ␤ and ⍀ Ti and expressions describing the composition dependence of the LPs are presented. Using the assessed values, various open questions are discussed, i.e., the composition range where the hexagonal to trigonal symmetry change is observed in the ⍀ phase, the applicability of an approximation involved in the plane collapse model for the ␤ → ⍀ transformation, and the extent to which the so-called Jamieson correlation for interatomic distances in the ⍀ phase holds for Ti.

In this paper we present a neutron diffraction study of the effects of replacing Ba with smaller Sr cations in the layered cobaltite YBaCo(2)O(5+δ) for δ≈0.5. Neutron thermodiffractograms are reported in the temperature range 20 K≤T≤300 K, as well as high-resolution neutron diffraction experiments at selected temperatures. The systematic refinement of the series of samples with Sr substituting at the Ba site permits the understanding of the complex magnetic behaviour of this system, and reinforces a two-phase scenario for the parent compound. We have found that Sr addition destabilizes the ferrimagnetic spin state ordered phase, as well as the '122' order characteristic of cobaltites with δ = 0.5, favouring instead a tetragonal and less ordered structure.

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The so-called Arrott plot, which consists in plotting H/M against M 2 , with H the applied magnetic field and M the magnetization, is used to extract valuable information in second-order magnetic phase transitions. Besides, it is widely accepted that a negative slope in the Arrott plot is indicative of a first-order magnetic transition. This is known as the Banerjee criterion. In consequence, the zero-field transition temperature T * is reported as the characteristic first-order transition temperature. By carefully analyzing the mean-field Landau model used for studying first-order magnetic transitions, we show in this work that T * corresponds in fact to a triple point where three first-order lines meet. More importantly, this analysis reveals the existence of two symmetrical second-order critical points at finite magnetic field (T c ,±H c). We then show that a modified Arrott plot can be used to obtain information about these second-order critical points. To support this idea we analyze experimental data on La 2/3 Ca 1/3 MnO 3 and discuss an estimate for the location of the triple point and the second-order critical points.

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