Sol-Gel-Prepared Nanoparticles of Mixed Praseodymium Cobaltites-Ferrites.

Two series of nanocrystalline powders of PrCo1 - x Fe x O3 (x = 0.1, 0.3, 0.5, 0.7 and 0.9) of high purity were obtained by sol-gel citrate method at 700 and 800 °C. The formation of continuous solid solution with an orthorhombic perovskite structure (sp. group Pbnm) was observed. A peculiarity of the PrCo1 - x Fe x O3 solid solution is the lattice parameter crossovers, which occurred at certain compositions and revealed in the pseudo-tetragonal or pseudo-cubic metric. An average crystallite size of the PrCo1 - x Fe x O3 samples estimated from the analysis of the angular dependence of the X-ray diffraction (XRD) line broadening varies between 30 and 155 nm, depending on the composition and synthesis temperature.

Concentration- and Temperature-Induced Phase Transitions in PrAlO3-SrTiO3 System.

Single-phase mixed aluminates-titanates Pr1-x Sr x Al1-x Ti x O3 (x = 0.1, 0.2, 0.3, 0.5, 0.7) with rhombohedral perovskite structure were prepared by solid-state reaction technique at 1823-1873 K. Morphotropic rhombohedral-to-cubic phase transition in Pr1-x Sr x Al1-x Ti x O3 series is predicted to occur at x = 0.88. The temperature-induced structural phase transition R [Formula: see text] с - Pm [Formula: see text] m in Pr0.5Sr0.5Al0.5Ti0.5O3, detected at 930 K by in situ high-temperature X-ray synchrotron powder diffraction, occurs at considerably lower temperature as the corresponding transformation in the parent compound PrAlO3 (1770 K). Such remarkable drop of the transition temperature is explained by gradual decrease of the perovskite structure deformation in the Pr1-x Sr x Al1-x Ti x O3 series with increasing Sr and Ti contents as a consequence of the increasing Goldschmidt tolerance factor. For Pr0.3Sr0.7Al0.3Ti0.7O3 phase, a sequence of the low-temperature phase transformation R [Formula: see text] с - Immb(C2/m) - I4/mcm was detected.

Spatial separation of covalent, ionic, and metallic interactions in Mg11Rh18B8 and Mg3Rh5B3.

The crystal structures of Mg11Rh18B8 and Mg3Rh5B3 have been investigated by using single-crystal X-ray diffraction. Mg11Rh18B8: space group P4/mbm; a=17.9949(7), c=2.9271(1) Å; Z=2. Mg3Rh5B3: space group Pmma; a=8.450(2), b=2.8644(6), c=11.602(2) Å; Z=2. Both crystal structures are characterized by trigonal prismatic coordination of the boron atoms by rhodium atoms. The [BRh6] trigonal prisms form arrangements with different connectivity patterns. Analysis of the chemical bonding by means of the electron-localizability/electron-density approach reveals covalent BRh interactions in these arrangements and the formation of B-Rh polyanions. The magnesium atoms that are located inside the polyanions interact ionically with their environment, whereas, in the structure parts, which are mainly formed by Mg and Rh atoms, multicenter (metallic) interactions are observed. Diamagnetic behavior and metallic electron transport of the Mg11Rh18B8 and Mg3Rh5B3 phases are in agreement with the bonding picture and the band structure.

Sn(20.5) square(3.5)As(22)I(8): a largely disordered cationic clathrate with a new type of superstructure and abnormally low thermal conductivity.

Sn(20.5)As(22)I(8), a new cationic clathrate, has been prepared by using an ampoule technique. According to the X-ray powder diffraction data, it crystallizes in the face-centered cubic space group F23 or Fm(-)3 with a unit-cell parameter of a=22.1837(4) A. Single-crystal X-ray data allowed solution of the crystal structure in the subcell with a unit-cell parameter of a(0)=11.092(1) A and the space group Pm(-)3n (R=5.7 %). Sn(20.5)As(22)I(8) (or Sn(20.5) square(3.5)As(22)I(8), accounting for the vacancies in the framework) possesses the clathrate-I type crystal structure, with iodine atoms occupying the cages of the cationic framework composed of tin and arsenic atoms. The crystal structure is strongly disordered. The main features are a random distribution of vacancies, and shifts of the tin and arsenic atoms away from their ideal positions. The coordination of the tin atoms has been confirmed by using (119)Sn Mössbauer spectroscopy. Electron diffraction and high-resolution electron microscopy (HREM) analyses have confirmed the presence of the superstructure ordering, which results in a doubling of the unit-cell parameter and a change of the space group from Pm(-)3n to either F23 or Fm(-)3. Analysis of the crystal structure has led to the construction of four ordering models for the superstructure, which have been corroborated by HREM, and has also led to the identification of disordered regions originating from overlap of the different types of ordered domains. Sn(20.5)As(22)I(8) is a diamagnetic semiconductor with an estimated band gap of 0.45 eV; it displays abnormally low thermal conductivity, with the room temperature value being just 0.5 W m(-1) K(-1).

Ce4(P1-xSi x)3-z: a first example for the stabilization of the anti-Th3P4 type structure by substitution in the non-metal substructure.

A first rare-earth phosphide silicide Ce4(P(1-x)Si(x))(3-z) and its analogues with La, Pr, and Nd were synthesized and characterized. The compounds crystallize in the anti-Th3P4 structure type. The cerium compound shows a mixed occupation of the 12a site with Si and P and possesses a wide homogeneity range with respect to x and z variation. The electronic configuration of Ce, deduced from magnetic susceptibility and X-ray absorption spectroscopy data, remains 4f(1) (Ce3+) independently from x and z. The cerium valence and the phase stability region are discussed employing electronic band-structure calculation and chemical bonding analysis with electron localization function. Atomic interactions are shown to remain nearly unchanged, while the change of the excess electron concentration with P/Si substitution is considered to play the main role for the stabilization of the structural motif.