Oxygen ordering and superconductivity in La(Ba2−xLax)Cu3O7+δ
The observation of superconductivity above 90 K in the Y–Ba–Cu–O system1,2 has prompted a search for other structures that display high-temperature superconductivity. The discovery of superconductivity above 70 K in La3−xBa3+xCu6Ol4+y3 raises some questions concerning its relationship to the YBa2Cu3O7−δ structure and the origins of the high-temperature superconductivity. The original X-ray diffraction study of La3Ba3Cu6O14+y, indicated a tetragonal structure4,5, on the basis of which it has been proposed that La3−xBa3+xCu6O14+y is a new class of high-temperature superconductor which, in contrast to YBa2Cu3O7−δ, contains no Cu–O chains3. More recently, a neutron diffraction study of non-superconducting La3Ba3Cu6O14+y6has indicated that the correct structure of this compound is simply the tetragonal YBa2Cu3O7−δ structure with partial substitution of lanthanum on the barium site7–12. In the present neutron powder diffraction study of a series of La(Ba2−xLax)Cu3O7+δ compounds, we show that the superconducting phase is a similarly disordered isomorph of the orthorhom-bic YBa2Cu3O7−δ structure13–19. The region of solid solubility for this compound extends from x = 0.5 (La:Ba:Cu ratios of 3:3:6) to x≈ 0.25 and does not include the stoichiometric compound x = 0 (LaBa2Cu3O7−δ). The presence of Cu–O chains, and not just a formal copper charge state of ≥2, is vital for the occurrence of superconductivity.
The La (Ba2-xLax)Cu3- yOz single crystals : structures and conductivity
The crystal structure of the orthorhombic and tetragonal phases of La (Ba 2-x La x ) Cu 3-y O 6+x/2-y+z are determined on twinned crystals. The orthorhombic structure, obtained for low x, is close to the regular Y-Ba-Cu-O type (twin a*b*c-b*a*c), but is highly copper deficient in the Cu(1) site (≃30%). The local correlations (ξ≃20A) between copper atoms and vacancies, as deduced from X-ray diffuse scattering, correspond to a short-range segregation of vacancies in chains. As a consequence of the large amount of defects, these crystals are non-typical semiconductors. The tetragonal structure, x≃0.50, leads to tri-twinned crystals with 90˚ faulting, a*a*3a-a*3a*a-3a*a*a (a, the perovskite lattice constant). In these materials the copper sites are found to be strongly anharmonic. This is due to the disorder int Les structures des phases orthorhombique et quadratique de La(Ba 2-x La x ) Cu 3-y O 6+x/2-y+z ont ete determinees sur des cristaux macles. La structure orthorhombique, observee pour x petit, est proche de la structure Y-Ba-Cu-O (macle a*b*c-b*a*c), mais presente un tres fort taux de defauts sur le site Cu(1) (≃30%). Les correlations locales (ξ=20A) entre atomes de cuivre et lacunes, deduites de la diffusion des rayons X, correspondent a une segregation a courte distance des lacunes en chaines. Comme consequence du taux de defauts eleve, ces cristaux sont des semi-conducteurs non typiques. La structure quadratique, x≃0.50, conduit a des cristaux trimacles avec des fautes a 90˚. a*a*3a-a*3a*a-3a*a*a (a, le parametre de la perovskite). Dans ces materiaux les potentiels des sites cuivre sont fortement anharmoniques. Ceci est du au desordre introduit par la substitution La-Ba. Ces cristaux sont egalement semi-conducteurs avec une loi d'activation en T -1/4 , ce qui indique que la conductivite est de type variable range hopping, consequence d'une localisation par le desordre
Natural Radioactivity of Intrusive-Metamorphic and Sedimentary Rocks of the Balkan Mountain Range (Serbia, Stara Planina)
Stara Planina (also known as the Balkan mountain range) is known for numerous occurrences and deposits of uranium and associated radionuclides. It is also famous for its geodiversity. The geologic framework is highly complex. The mountain is situated between the latitudes of 43° and 44° N and the longitudes from 22°16′ to 23°00′ E. Uranium exploration and radioactivity testing on Stara Planina began back in 1948. Uranium has also been mined in the zone of Kalna, within the Janja granite intrusive. The naturally radioactive geologic units of Stara Planina are presented in detail in this paper. The main sources of radioactivity on Stara Planina can be classified as: 1. Granitic endogenous—syngenetic–epigenetic deposits and occurrences; 2. Metamorphogenic—syngenetic; and 3. Sedimentary, including occurrences of uranium deposition and fluctuation caused by water in different types of sedimentary rocks formed in a continental setting, which could be classified under epigenetic types. The area of Stara Planina with increased radioactivity (higher than 200 cps), measured by airborne gamma spectrometry, is about 380 square kilometers. The highest values of measured radioactivity and uranium grade were obtained from a sample taken from the Mezdreja uranium mine tailing dump, where 226Ra measures 2600 ± 100 Bq/kg and the uranium grade is from 76.54 to 77.65 ppm U. The highest uranium (and lead) concentration, among all samples, is measured in graphitic schist with high concentrations of organic (graphitic) material from the Inovska Series—99.47 ppm U and 107.69 ppm Pb. Thorium related radioactivity is the highest in granite samples from the Janja granite in the vicinity of the Mezdreja granite mine and the Gabrovnica granite mine tailing dump, and it is the same—250 ± 10 Bq/kg for 232Th, while the thorium grade varies from 30.82 to 60.27 ppm Th. In gray siltstones with a small amount of organic material, the highest radioactivity is related to potassium—2080 ± 90 Bq/kg for 40K.