The Crystal Structure of Al4SiC4 Revisited.

Al4SiC4 is a ternary wide-band-gap semiconductor with a high strength-to-weight ratio and excellent oxidation resistance. It consists of slabs of Al4C3 separated by SiC layers with the space group of P63mc. The space group allows Si to occupy two different 2a Wykoff sites, with previous studies reporting that Si occupies only one of the two sites, giving it an ordered structure. Another hitherto unexplored possibility is that Si can be randomly distributed on both 2a sites. In this work, we revisit the published ordered crystal structure using experimental methods and density functional theory (DFT). Al4SiC4 was synthesized by high-temperature sintering at 1800 °C from a powder mixture of Al4C3 and SiC. Neutron diffraction confirmed that Al4SiC4 crystallized with the space group of P63mc, with diffraction patterns that could be fitted to both the ordered and the disordered structures. Scanning transmission electron microscopy, however, provided clear evidence supporting the latter, with DFT calculations further confirming that it is 0.16 eV lower in energy per Al4SiC4 formula unit than the former. TEM analysis revealed Al vacancies in some of the atomic layers that can introduce p-type doping and direct band gaps of 0.7 and 1.2 eV, agreeing with our optical measurements. Finally, we propose that although the calculated formation energy of the Al vacancies is high, the vacancies are stabilized by entropy effects at the high synthesis temperature. This indicates that the cooling procedure after high-temperature synthesis can be important in determining the vacancy content and the electronic properties of Al4SiC4.

Thermal Stability of the HfNbTiVZr High-Entropy Alloy.

The multicomponent alloy HfNbTiVZr has been described as a single-phase high-entropy alloy (HEA) in the literature, although some authors have reported that additional phases can form during annealing. The thermal stability of this alloy has therefore been investigated with a combination of experimental annealing studies and thermodynamic calculations using the CALPHAD approach. The thermodynamic calculations show that a single-phase HEA is stable above about 830 °C. At lower temperatures, the most stable state is a phase mixture of bcc, hcp, and a cubic C15 Laves phase. Annealing experiments followed by quenching confirm the results from thermodynamic calculations with the exception of the Laves phase structure, which was identified as a hexagonal C14 type instead of the cubic C15 type. Limitations of the applied CALPHAD thermodynamic description of the system could be an explanation for this discrepancy. As-synthesized HfNbTiVZr alloys prepared by arc-melting form a single-phase bcc HEA at room temperature. In situ annealing studies of this alloy show that additional phases start to form above 600 °C. This indicates that the observed HEA is metastable at room temperature and stabilized by a slow kinetics during cooling. X-ray diffraction analyses using different cooling rates and annealing times show that the phase transformations in this HEA are slow and that completely different phase compositions can be obtained depending on the annealing procedure. In addition, it has been shown that the sample preparation method (mortar grinding, heat treatment, etc.) has a significant influence on the collected diffraction patterns and therefore on the phase identification and analysis.

A cubic calcium oxynitrido-silicate, Ca(2.89)Si(2)N(1.76)O(4.24).

The title compound, tricalcium oxynitride silicate, with composition Ca(3-x)Si(2)N(2-2x)O(4+2x) (x ≃ 0.12), is a perovskite-related calcium oxynitrido silicate containing isolated oxynitrido silicate 12-rings. The N atoms are statistically disordered with O atoms (occupancy ratio N:O = 0.88:0.12) and occupy the bridging positions in the 12 ring oxynitrido silicate anion, while the remaining O atoms are located at the terminal positions of the Si(O,N)(4) tetrahedra. The majority of the Ca(2+) cations fill the channels along [100] in the packing of the 12-ring anions. The rest of these cations are located at several positions, with partial occupancy, in channels along the body diagonals.

The Hume-Rothery Compound Mn8 Ga27.4 Zn13.6 : Separated Zn13 -Clusters Interspersed in a Primitive Cubic Host Lattice.

Separated Zn13 cluster entities unexpectedly occur in the solid-state structure of Mn8 Ga27.4 Zn13.6 (the central building block is shown). They correspond to centered cuboctahedra, that is, small volumes of face-centered cubic metal. The intriguing segregation of Ga and Zn atoms in Mn8 Ga27.4 Zn13.6 was verfied by Rietveld refinement of neutron powder diffraction data.

Adopciones especiales: ¿Niños especiales para familias especiales? / No disponible

Desde el año 2005 venimos asistiendo a un descenso importante en la cifra anual de adopciones en España, así como de adopciones internacionales en el mundo que viene acompañado de un aumento de las adopciones especiales (Selman, 2010). Se entiende por adopción especial la adopción de niños cuya raza, mayor edad, pertenencia a un grupo de hermanos o discapacidad de tipo emocional, físico o conductual puede dificultar su adopción. El objetivo central de este artículo es revisar los resultados de la investigación acerca de los retos y riesgos asociados a las adopciones especiales y la estabilidad de estas adopciones y, más concretamente, a la adopción de niños con discapacidades, para posteriormente debatir sobre la conveniencia de promover este tipo de adopciones entre las actuales bolsas de solicitantes de adopción en espera y en qué condiciones (AU)

Since 2005 we have been witnessing a significant decline in the annual number of adoptions in Spain as well as international adoptions in the world, which is accompanied by an increase in special adoption (Selman, 2010). Special adoption is defined as the adoption of children whose race, older age, sibling group or emotional, physical, intellectual disability may hinder its adoption. The aim of this paper is to review specific research on the challenges, risks and stability of special adoptions and, more specifically, the adoption of children with disabilities. Finally we will discuss the convenience to promote this kind of adoption among awaiting adoption applicants and under what conditions (AU)

Adopciones rotas: el peligro de un nuevo maltrato / Disrupted adoptions: the danger of new maltreatment

La adopción internacional es una medida de protección para los menores a los que no se les puede ofrecer una familia en sus países de origen. A pesar de que la mayoría de las adopciones son exitosas, en un pequeño porcentaje la inadaptación familiar conduce a la ruptura de la adopción que quedan reflejadas en los expedientes en una muestra de 23 adopciones rotas o en riesgo de romperse, en comparación con un grupo equiparado de 28 adopciones intactas. Las quejas que discriminaron las adopciones rotas o en riesgo estuvieron relacionadas con la ausencia de vínculo entre padres e hijos y los problemas para establecer y aceptar los límites y las normas, junto con la sensación de la familia de haber sido engañados en el proceso. Se discute la relevancia de estos datos para los servicios adoptivos (AU)

International adoption is a child protection measure for those children that can´t find a family in their countries for origin. Most for the adoptions are successful but, in a small percentage, the family inadaptation leads to the disruption adoption. The aim of this paper is to analyse the adoptive parents complaints reflected in the case records, in a sample of 23 at risk or disrupted adoptions in comparison with a matched group of 28 intact adoptions. The complaints that discriminated at risk or disrupted adoptions were related to the attachment difficulties between parents and children and the problems to settle down and to accept norms and limits together with the feeling of the family of having been deceived in the process. The relevance of these findings for the adoption services is discussed (AU)