A. A. Yaroslavtsev

A Transition from Localized to Strongly Correlated Electron Behavior and Mixed Valence Driven by Physical or Chemical Pressure in ACo2As2 (A = Eu and Ca)

We demonstrate that the action of physical pressure, chemical compression, or aliovalent substitution in ACo2As2 (A = Eu and Ca) has a general consequence of causing these antiferromagnetic materials to become ferromagnets. In all cases, the mixed valence triggered at the electropositive A site results in the increase of the Co 3d density of states at the Fermi level. Remarkably, the dramatic alteration of magnetic behavior results from the very minor (<0.15 electron) change in the population of the 3d orbitals. The mixed valence state of Eu observed in the high-pressure (HP) form of EuCo2As2 exhibits a remarkable stability, achieving the average oxidation state of +2.25 at 12.6 GPa. In the case of CaCo2As2, substituting even 10% of Eu or La into the Ca site causes ferromagnetic ordering of Co moments. Similar to HP-EuCo2As2, the itinerant 3d ferromagnetism emerges from electronic doping into the Co layer because of chemical compression of Eu sites in Ca0.9Eu0.1Co1.91As2 or direct electron doping in Ca0.85La0.15Co1.89As2. The results reported herein demonstrate the general possibility of amplifying minor localized electronic effects to achieve major changes in materials properties via involvement of strongly correlated electrons.

Formation of Nanocrystalline Structures in the Ln 2 O 3 -MO 2 Systems (Ln = Gd, Dy; M = Zr, Hf)

The formation of (Ln3+)2(M4+)2O7 (Ln = Gd, Dy; M = Zr, Hf) nanocrystallites obtained by annealing mixed hydroxides LnM(OH)7 · nH2O (precursors) synthesized by coprecipitation has been studied by synchronous thermal analysis, X-ray diffraction (normal and anomalous diffraction of synchrotron radiation), and EXAFS. In the systems under consideration, heat treatment of the X-ray amorphous precursors leads to their dehydration, and at 600–700°C, nanocrystallites with an fcc structure of disordered fluorite start forming. A further increase in temperature is accompanied by crystallite growth (CDD) and considerable change in the local structure of the heat-treated compounds. The crystallization enthalpies and activation energies have been determined.

A study of the formation of Ln2 + xMe2 − xO7 − x/2 (Ln = Gd, Dy; Me = Zr, Hf) nanocrystals

It has been established that the process of producing the Ln2 + xMe2 − xO7 − x/2 (Ln = Gd, Dy; Me = Zr, Hf) nanocrystals by calcination of hydroxides, which, in turn, have been produced by coprecipitation of metal salts, includes several stages. At the beginning, the X-ray amorphous structure of the precursors remains unchanged during dehydration; during subsequent heating to 600–700°C, nanocrystals with a disordered fluorite structure begin to be formed. An increase in the temperature above 700°C leads to an increase in the size of crystallites (coherent scattering regions). This process is accompanied by changes occurring in their local structure. In the nanocrystalline powders of Cd2Hf2O7 and Gd2Zr2O7 synthesized at 1200°C (6 h), the pyrochlore-type superstructure with the lattice parameters doubled relative to fluorite has been revealed. It has also been found that, possibly, the Dy2HfO5 sample at 1600°C (3 h) has a modulated structure.

Features of the local structure of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Yb, Lu)

The parameters of the local crystal structure of dodecaborides RB12 (R = Ho, Er, Tm, Yb, Lu) have been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the vibrations of the rare-earth ion with respect to the boron cage are well described in the harmonic approximation. At the same time, the displacement of rare-earth ions from equilibrium positions of the crystal structure should be taken into account to determine the length of the R-B bond. The analysis of EXAFS spectra has revealed the displacement of 1–6% of rare-earth ions by about 0.2–0.3 Å in all compounds under investigation; this displacement at low temperatures results in the formation of a cage glass phase.

Ce valence in intermetallic compounds by means of XANES spectroscopy

Abstract The cerium effective valence was investigated by means of CeL3-XANES spectroscopy in some intermediate valence Ce-based systems with different magnetic ordering mechanisms: CeNi doped with Nd, Pr and Gd in the temperature range 5–300 K, and Ce3Fe17–xMnx intermetallics. The dependences obtained are considered within the frames of generally accepted mechanisms of rare-earth ions intermediate valence states. Possible correlations between the cerium effective valence and the magnetic properties of compounds are also noted.

Correlation of magnetic properties with the local features of the electronic and crystal structure in the Ce2Fe17 − xMnx intermetallide: XAFS data analysis

The rearrangement of the local environment of cerium has been examined using extended X-ray absorption fine structure spectroscopy above the K-Ce absorption edge for various manganese contents and temperatures in Ce2Fe17 − xMnx intermetallics (x = 0, 1, 2). The valence state of cerium has been studied simultaneously by X-ray absorption near-edge structure spectroscopy above the L3-Ce absorption under the same conditions. A correlation has been revealed between the changes observed in the local electronic and crystal structure of Ce2Fe17 − xMnx compounds and the types of magnetic states in them.

Coexistence of Long Range Magnetic Order and Intervalent State of Eu in EuCu 2 (Si x Ge 1- x ) 2 : Evidence from Neutron Diffraction and Spectroscopic Studies ¶

Experimental results of the X-ray absorption spectroscopy, Mössbauer spectroscopy (isomer shift) and neutron diffraction are presented for the series of EuCu2(SixGe1 − x)2 polycrystalline samples (0 < x < 0.75). Homogeneous intermediate valence state is established for Eu ions as well as long range magnetically ordered state at the temperatures below 10–15 K. Observation of the ordered magnetic moments at Eu site gives rise to the experimental statement for the coexistence of valence fluctuations and long range magnetic order takes place in the wide range of Ge concentrations for this substance.

Resonant inelastic X-ray scattering (RIXS) on magnetic EuCo2P2-based systems

The features of near-edge structure around the L3-Eu absorption edge are investigated in EuCo2P2, Pr1 − xEuxCo2P2 (x = 0.2, 0.4), and Nd0.6Eu0.4Co2P2 by means of resonant inelastic X-ray scattering (RIXS). Europium is shown to be in the intermediate valence state in all compounds. A comparison is made of the values of europium intermediate valence obtained by different methods employing the synchrotron radiation. The correlation between europium valence and the observed changes of magnetic ordering in the systems under investigation is discussed.

The combined study of Dy2O3-HfO2 nanocrystallites formation by means of EXAFS, PDF, XRD and SAXS

The process of double oxide Dy2O3-HfO2 nanocrystallites formation upon annealing to 1600°C was investigated with the combination off few X-ray and synchrotron methods. XRD and PDF show the mean crystal structure to be fluorite typical for the compounds of Ln2O3-MeO2 type. The ordering of structure and the growth of nanocrystallites upon annealing was estimated independently from XRD patterns, PDF functions and SAXS scattering curves. Neither transition to pyrochlore phase, nor signs of recently shown in related Gd2O3-HfO2 compounds pyrochlore-type superstructure is observed. However, EXAFS and the short range order of PDF clearly indicate the discrepancy between Dy and Hf local environment. Such a disorder of local crystal structure cannot be described by the fluorite model.

Local electronic and crystal structure of rare-earth cobalt phosphides RCo2P2 (R = La, Ce, Pr, Nd, Eu) studied by XAFS and RIXS

A detailed combined study of local electronic and crystal structures was performed for a series of rare-earth cobalt phosphides RCo2P2 (R = La, Ce, Pr, Nd, Eu) with peculiar itinerant magnetic ordering schemes using different methods employing the synchrotron radiation: RIXS, XANES and EXAFS. Europium and cerium are shown to be in the intermediate valence state in all compounds. The correlation between rare-earth valence and the observed changes of magnetic ordering in the systems under investigation is discussed.