The self-hosting structure of beta-Ta

Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) Angstrom, space group P (4) over bar2(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor similar to 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb. [References: 24] 2

Optically switched magnetism in photovoltaic perovskite CH3_3NH3_3(Mn:Pb)I3_3

The demand for ever-increasing density of information storage and speed of manipulation boosts an intense search for new magnetic materials and novel ways of controlling the magnetic bit. Here, we report the synthesis of a ferromagnetic photovoltaic CH$_3$NH$_3$(Mn:Pb)I$_3$ material in which the photo-excited electrons rapidly melt the local magnetic order through the Ruderman-Kittel-Kasuya-Yosida interactions without heating up the spin system. Our finding offers an alternative, very simple and efficient way of optical spin control, and opens an avenue for applications in low power, light controlling magnetic devices

The commensurate composite sigma-structure of beta-tantalum

The single-crystal investigation of the self-hosting sigma-structure of beta-tantalum (beta-Ta) at 120 K (low-temperature, LT, structure) and at 293 K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same ( or an integer multiple) lattice constants but different space groups. The space groups of both host ( H) and guest ( G) components cause systematic absences, which result from their intersection. The highest symmetry of a sigma-structure can be described as [H: P4(2)/mnm; G: P4/mbm (c(G) = 0.5c(H)); composite: P4(2)/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In beta-Ta, two components modify their symmetry during the thermal process 293 K (RT- I)) double right arrow 120 K (LT)) double right arrow 293 K (RT-II): [H: P (4) over bar2(1)m; G: P (4) over bar2(1)m; composite: P (4) over bar2(1)m]) double right arrow [H: P (4) over bar, G: P4/mbm (c(G) = 0.5c(H)), composite: P (4) over bar]) [H: P (4) over bar2(1)m, G: P4/mbm (c(G) = 0.5c(H)), composite: P (4) over bar2(1)m]. Thus, the phase transition is reversible with respect to H and irreversible with respect to G

Validating the model of a (3+1)-dimensional incommensurately modulated structure as generator of a family of compounds for the Eu2(MoO4)3 scheelite structure

The previously proposed model postulating that the incommensurately modulated KNd(MoO$_4$)$_2$ structure can act as a generator of the scheelite family members is validated here by refining the crystal structure of europium molybdate, Eu$_2$(MoO$_4$)$_3$. The initial structural model was derived from the superspace characteristics of KNd(MoO$_4$)$_2$ and the predicted parameters used in the simulation of Eu$_2$(WO$_4$)$_3$. The refinement was performed using both superspace and traditional supercell approaches in superspace group $I2/b(\alpha\beta 0)00 and space group A2/a, respectively. The results obtained by both approaches are in excellent agreement and coincide with those reported in the literature

LiZnNb4O11.5: A novel oxygen deficient compound in the Nb-rich part of the Li2O–ZnO–Nb2O5 system

A novel lithium zinc niobium oxide LiZnNb4O11.5 (LZNO) has been found in the Nb-rich part of Li2O–ZnO–Nb2O5 system. LZNO, with an original a-PbO2 related structure, has been synthesized by the routine ceramic technique and characterized by X-ray diffraction and transmission electron microscopy (TEM). Reflections belonging to the LZNO phase, observed in X-ray powder diffraction (XRPD) and electron diffraction, have been indexed as monoclinic with unit cell parameters a=17.8358(9) A˚ , b=15.2924(7) A˚ , c=5.0363(3)A˚ and g=96.607(5)1 or as a-PbO2-like with lattice constants a=4.72420(3) A˚ , b=5.72780(3) A˚ , c=5.03320(3) A˚ , g=90.048(16)1 and modulation vector q=0.3a*+1.1b* indicating a commensurately modulated a-PbO2 related structure. The monoclinic cell is a supercell related to the latter. Using synchrotron powder diffraction data, the structure has been solved and refined as a commensurate modulation (superspace group P1121/n(ab0)00) as well as a supercell (space group P21/b). The superspace description allows us to consider the LZNO structure as a member of the proposed a-PbO2-Z (3+1)D structure type, which unifies both incommensurately and commensurately modulated structures. HRTEM reveals several types of defects in LZNO and structural models for these defects are proposed. Two new phases in Li2O–ZnO–Nb2O5 system are predicted on the basis of this detailed HRTEM analysis

The modulated structure of Ba0.39Sr0.61Nb2O6. I. Harmonic solution

The structure of a crystal of Sr0.61Ba0.39Nb2O6 has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P4bm(pp1/2; p-p1/2), unit-cell parameters a = 12.4566 (9), c = 7.8698 (6) Angstrom, modulation vectors q(1) = 0.3075 (6) (a* +b*), q(2) = 0.3075 (6) (a* - b*). The data collection was performed on a KUMA-CCD diffractometer and allowed the integration of weak first-order satellite reflections. The structure was refined from 2569 reflections to a final value of R = 0.0479. The modulation affects mainly the positions of the O atoms, which are displaced by as much as 0.5 Angstrom, and the site 4c that is occupied by Sr and Ba atoms. Only a simplified model, in which this atomic position is occupied by an effective atom Sr/Ba, could be refined from the data set. The modulation of displacement parameters has been used to account for the modulated distribution of Sr and Ba. The whole refinement uses only first-order modulation waves, but there are strong indications that for a complete solution the use of higher-order satellites and a more complicated model is necessary