5 research outputs found
Structural Disorder in (Bi, M)<sub>2</sub>(Fe, Mn, Bi)<sub>2</sub>O<sub>6+<i>x</i></sub> (M = Na or K) Pyrochlores Seen from Reverse Monte Carlo Analysis of Neutron Total Scattering
The
average structures of the polycrystalline pyrochlores (Na<sub>0.60</sub>Bi<sub>1.40</sub>)Â(Fe<sub>1.06</sub>Mn<sub>0.17</sub>Bi<sub>0.77</sub>)ÂO<sub>6.87</sub> and (K<sub>0.24</sub>Bi<sub>1.51</sub>)Â(Fe<sub>1.07</sub>Mn<sub>0.15</sub>Bi<sub>0.78</sub>)ÂO<sub>6.86</sub> can
be refined through Rietveld refinement against Bragg scattering
data using cubic space group <i>Fd</i>3Ì
<i>m</i>, with off-centered 96<i>h</i> and 32<i>e</i> positions describing the A<sub>2</sub>OâČ network. Investigation
of their local structures through neutron total scattering confirms
the extent of disorder within these materials, and furthermore shows
significant deviation from the average structure, which is not accounted
for through analysis of Bragg data alone. Reverse Monte Carlo (RMC)
analysis with a 6 Ă 6 Ă 6 supercell was used to model accurately
this local disorder, revealing ellipsoidal distributions for A-site
potassium, distinctly different from the hollow torus-shaped distributions
for the sodium and bismuth cations. It is shown through bond valence
sum analysis that while these atomic displacements allow for the steric
preferences of Bi<sup>3+</sup>, they are also necessary to satisfy
the valence of both the bismuth and the alkali metals on the A sites.
Analysis of the final RMC configuration showed the BO<sub>6</sub> octahedra
for the separate B site metals to be more regular (OâBâO
â 90°) than those in the Rietveld model (OâBâO
â 85/95°) which describes an average of the three different
environments
Metastable (Bi, M)<sub>2</sub>(Fe, Mn, Bi)<sub>2</sub>O<sub>6+<i>x</i></sub> (M = Na or K) Pyrochlores from Hydrothermal Synthesis
The hydrothermal
syntheses, structures, and magnetism of two new pyrochlore oxides
of compositions (Na<sub>0.60</sub>Bi<sub>1.40</sub>)Â(Fe<sub>1.06</sub>Mn<sub>0.17</sub>Bi<sub>0.77</sub>)ÂO<sub>6.87</sub> and
(K<sub>0.24</sub>Bi<sub>1.51</sub>)Â(Fe<sub>1.07</sub>Mn<sub>0.15</sub>Bi<sub>0.78</sub>)ÂO<sub>6.86</sub> are described. With
preparation at 200 °C for 6 h in solutions of sodium or potassium
hydroxide, the alkali metals introduced from these mineralizers are
essential to the synthesis of the phases. The average long-range order
of the pyrochlore structure, with space group <i>Fd</i>3Ì
<i>m</i>, was investigated and refined against X-ray and neutron
diffraction data, and it was shown that disorder is present in both
the metal and coordinating oxygen positions, along with metal-mixing
across both the A and B sites of the structure. XANES analysis confirms
the presence of Mn<sup>4+</sup>, mixed valence Bi<sup>3+</sup> and
Bi<sup>5+</sup>, and Fe<sup>3+</sup>, the last also verified by <sup>57</sup>Fe MoÌssbauer spectroscopy. Magnetic measurements show
a lack of long-range magnetic ordering that is typical of geometrically
frustrated pyrochlores. The observed glasslike interactions occur
at low temperatures, with the onset temperature depending upon the
magnitude of the applied external field. Variable temperature X-ray
diffraction shows that these pyrochlores are metastable and collapse
on heating at ca. 395 °C, which suggests that their formation
by conventional solid-state synthesis would be impossible
Structures and Magnetism of the Rare-Earth Orthochromite Perovskite Solid Solution La<sub><i>x</i></sub>Sm<sub>1â<i>x</i></sub>CrO<sub>3</sub>
A new mixed rare-earth orthochromite
series, La<sub><i>x</i></sub>Sm<sub>1â<i>x</i></sub>CrO<sub>3</sub>, prepared through single-step hydrothermal
synthesis is reported. Solid solutions (<i>x</i> = 0, 0.25,
0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by the hydrothermal
treatment of amorphous mixed-metal hydroxides at 370 °C for 48
h. Transmission electron microscopy (TEM) reveals the formation of
highly crystalline particles with dendritic-like morphologies. Rietveld
refinements against high-resolution powder X-ray diffraction (PXRD)
data show that the distorted perovskite structures are described by
the orthorhombic space group <i>Pnma</i> over the full composition
range. Unit cell volumes and CrâOâCr bond angles decrease
monotonically with increasing samarium content, consistent with the
presence of the smaller lanthanide in the structure. Raman spectroscopy
confirms the formation of solid solutions, the degree of their structural
distortion. With the aid of shell-model calculations the complex mixing
of Raman modes below 250 cm<sup>â1</sup> is clarified. Magnetometry
as a function of temperature reveals the onset of low-temperature
antiferromagnetic ordering of Cr<sup>3+</sup> spins with weak ferromagnetic
component at NeÌel temperatures (<i>T</i><sub>N</sub>) that scale linearly with unit cell volume and structural distortion.
Coupling effects between Cr<sup>3+</sup> and Sm<sup>3+</sup> ions
are examined with enhanced susceptibilities below <i>T</i><sub>N</sub> due to polarization of Sm<sup>3+</sup> moments. At low
temperatures the Cr<sup>3+</sup> sublattice is shown to undergo a
second-order spin reorientation observed as a rapid decrease of susceptibility
Optimising motor learning in infants at high risk of cerebral palsy: a pilot study
Background: The average age for the diagnosis of cerebral palsy (CP) is 19 months. Recent neuroplasticity literature suggests that intensive, task-specific intervention ought to commence as early as possible and in an enriched environment, during the critical period of neural development. Active motor interventions are effective in some populations, however the effects of active motor interventions on the motor outcomes of infants with CP have not been researched thoroughly, but pilot work is promising. The aim of this study was to determine the short- term effects of âGAMEâ; a new and novel goal-oriented activity-based, environmental enrichment therapy programme on the motor development of infants at high risk of CP and test study procedures for a randomized controlled trial (RCT).
Methods: Pragmatic 2-group pilot RCT to assess motor outcomes, goal attainment, parent well-being and home environment quality, after 12-weeks of GAME intervention versus standard care. GAME included: creation of movement environments to elicit motor behaviours; parent training in motor learning and task analysis; frequent practice of motor tasks using a programme that was individualised to the child, was varied and focused on self-initiated movement. Data were analyzed using multiple regression.
Results: Thirteen infants were consented, randomised, treated and completed the study. At study conclusion, the GAME group (n = 6) demonstrated an advantage in Total Motor Quotient of 8.05 points on the Peabody Developmental Motor Scale-2 (PDMS-2) compared to the standard care group (n = 7) (p \u3c .001). No significant differences existed between groups on any other measure.
Conclusions: GAME appears to offer a promising and feasible new motor intervention for CP, with favourable short-term motor outcomes. A pressing need exists for an adequately powered RCT with long-term end points, to determine if GAME may advance these childrenâs motor trajectory
Control of Ionic Conductivity by Lithium Distribution in Cubic Oxide Argyrodites Li<sub>6+<i>x</i></sub>P<sub>1â<i>x</i></sub>Si<sub><i>x</i></sub>O<sub>5</sub>Cl
Argyrodite is a key structure type for ion-transporting
materials.
Oxide argyrodites are largely unexplored despite sulfide argyrodites
being a leading family of solid-state lithium-ion conductors, in which
the control of lithium distribution over a wide range of available
sites strongly influences the conductivity. We present a new cubic
Li-rich (>6 Li+ per formula unit) oxide argyrodite Li7SiO5Cl that crystallizes with an ordered cubic
(P213) structure at room temperature,
undergoing a transition at 473 K to a Li+ site disordered F4Ì
3m structure, consistent with
the symmetry adopted by superionic sulfide argyrodites. Four different
Li+ sites are occupied in Li7SiO5Cl (T5, T5a, T3, and T4), the combination of which is previously
unreported for Li-containing argyrodites. The disordered F4Ì
3m structure is stabilized to room temperature
via substitution of Si4+ with P5+ in Li6+xP1âxSixO5Cl (0.3 x < 0.85) solid solution. The resulting delocalization
of Li+ sites leads to a maximum ionic conductivity of 1.82(1)
Ă 10â6 S cmâ1 at x = 0.75, which is 3 orders of magnitude higher than the
conductivities reported previously for oxide argyrodites. The variation
of ionic conductivity with composition in Li6+xP1âxSixO5Cl is directly connected to structural changes
occurring within the Li+ sublattice. These materials present
superior atmospheric stability over analogous sulfide argyrodites
and are stable against Li metal. The ability to control the ionic
conductivity through structure and composition emphasizes the advances
that can be made with further research in the open field of oxide
argyrodites