Atomic clusters and phase transitions in the metastable beta-Ta phase between 4.2 and 293 K

Atomic clusters were identified in the ground state of the non-equilibrium Ta phase (Frank-Kasper sigma-structure type) at 15, 120 and 293K. The evolution of the clusters with temperature leads to two phase transformations at 65 and 150K which are related to the electrical and magnetic properties. The magnetic phase transition at 65 K is associated with the magnetic symmetry group transformation P4'(2) /mn'm ( 65 K). It is shown that beta-U is also a two-component composite containing similar clusters. The nature of the stabilisation of beta-Ta at the cathode is discussed

A reinterpretation of the phase transitions in Na2CO3

Based on the structural data of phases alpha (hexagonal; 756-972 K), beta (monoclinic; 605-751 K), gamma (incommensurate, monoclinic; 295 K) and delta (lock-in, monoclinic; 110 K) of sodium carbonate, Na2CO3, we could draw a parallel between the phase transitions and the evolution of the second coordination sphere of the C atoms. The temperature-dependent structures observed in the beta phase are reproduced in the incommensurate gamma phase as a modulation wave, which relates to the content of the symmetrically equivalent {110} lattice planes in the alpha phase. By decreasing the temperature, the phase transitions are associated with a stepwise increase in the number of Na ions participating in the second coordination sphere of the C atoms. Over the full temperature range, this number increases from 3 to 7. The C-O distances and the mobility of the O atoms depends on the number of Na ions in the vicinity of the C atoms

The study of incommensurate structures as a probe to reveal atomic interactions in crystals

The structure of aperiodic crystals which includes incommensurate, quasi- and composite crystals is usually described in spaces of higher dimension, the so called superspace. The main advantage of the superspace formalism is that an aperiodic structure in three dimensions recovers its full periodicity in higher dimensions. The symmetry properties of aperiodic crystals are obviously more convenient to describe in superspace too. The origin of the incommensurate nature of structures can often be found in competing interatomic interactions. From molecular dynamics simulation of a simple three dimensional model with close-packed layers and a single degree of freedom for each particle, it is possible to find the existence conditions of commensurate and incommensurate phases. Incommensurate phases can already be predicted on the basis of nearest and next nearest neighbour particle interactions only. We illustrate this principle of interactions with two examples of structures, Na2CO3 and K3In(PO4)(2). These examples shows clearly the importance of non-oxygen interactions i.e. next nearest interactions for the formation of incommensurate structures

Cimetidine, C10H16N6S, formC: crystal structure and modelling of polytypes using suoperspace approach

An efficient method for modelling a polytypic family is presented with the example of cimetidine in the form C polymorph. The method exploits the (3 + 1)-dimensional superspace model, which is a powerful tool for the description, prediction and understanding of polytype modifications in small-molecule crystallography, as illustrated with this pharmaceutical example

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 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

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