Josie E. Auckett

Efficient up-conversion by triplet-triplet annihilation

Following experimental determination of kinetic parameters governing the up-conversion of light by triplet-triplet annihilation (TTA-UC), we develop a kinetic model to determine the conditions required for efficient up-conversion. We discuss the assumptions underpinning statistical arguments for an upper limit to TTA-UC and argue that no such limit exists.

Pressure-Induced Intersite BiM (M=Ru, Ir) Valence Transitions in Hexagonal Perovskites†

Pressure-induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba(3+n)BiM(2+n)O(9+3n) (n=0,1; M=Ir,Ru). These compounds show first-order, circa 1% volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M-M bonds contribute to the finely balanced nature of their electronic states.

Neutron Laue diffraction study of the complex low‐temperature magnetic behaviour of brownmillerite‐type Ca2Fe2O5

The atomic and magnetic structure of brownmillerite Ca2Fe2O5 has been refined against single-crystal neutron Laue diffraction data collected at 300, 100 and 10 K under zero-field and low-magnetic field (35 Oe = 35 × 103/4π A m−1) conditions. Ca2Fe2O5 is a canted G-type antiferromagnet with Pcm′n′ symmetry, the magnetic moments on Fe being directed approximately along the crystallographic c axis at room temperature. The refinement results show clearly that this magnetic structure persists down to T = 10 K, despite a previous suggestion that an anomalous magnetic susceptibility enhancement observed in Ca2Fe2O5 single crystals between 40 and 140 K might signify a reorientation of the antiferromagnetic easy axis from c to a below 40 K. Alternative explanations for this susceptibility anomaly are considered in terms of the evidence for partial or short-range loss of order in the anomalous regime, possibly due to the presence of multiple competing sublattice interactions.

Real-time powder diffraction studies of energy materials under non-equilibrium conditions

This topical review presents developments in the analysis of functional energy materials using real-time operando X-ray and neutron powder diffraction of non-equilibrium systems. Examples in the areas of porous framework materials and rechargeable battery electrodes are used to highlight improvements in structural detail gained for guest–host systems through advances in instrumentation and experimental approach.

Coexistence of spin glass and antiferromagnetic orders in Ba3Fe2.15W0.85O8.72

Ba(3)Fe(2.15)W(0.85)O(8.72) has been grown as large single crystals using the floating-zone method, permitting very precise characterization of the nuclear and magnetic structures by neutron and synchrotron diffraction methods. The results of our structural investigation are combined with dc and ac magnetization and heat capacity measurements to give an unusually complete and detailed picture of a complex magnetic system. The compound crystallizes in the hexagonal perovskite structure (space group P6(3)/mmc) and reveals antiferromagnetic order below T(N) = 290 K. Frequency-dependent ac susceptibility and the presence of magnetic viscosity suggest the onset of a spin glass component in this material below T(f) = 60 K. These findings are discussed on the basis of detailed analysis of the crystalo-chemical properties, supported by ab initio (density functional theory) calculations.

Single-Crystal Neutron Diffraction Study of Superstructure Ordering and Domain Behaviour in Brownmillerite-Type Ca2Fe2O5

We show that large single crystals of brownmillerite-type Ca2Fe2O5 can be grown using the floating-zone method under ambient pressure conditions, provided that the feed rods are pre-annealed to a very high density. Neutron diffraction data collected from these crystals show the emergence of a long-range ordered incommensurate phase at high temperature. The observation of this phase for the first time using neutrons proves that the incommensurate ordering of tetrahedral chains upon heating Ca2Fe2O5 is a truly long-range and bulk phenomenon. The results are used to compare and contrast the structures of Ca2Fe2O5 and Sr2Fe2O5, and are consistent with experimental observations of significantly higher oxide ionic conduction in the latter material.

Structural Disorder and Classical Spin-Glass Behaviour in Ba3Fe2SbO9

A new 6H-type perovskite Ba3Fe2SbO9 has been synthesised for the first time. Synchrotron and neutron powder diffraction data reveal complete structural disorder between Sb and Fe in the octahedral perovskite B sites. This results in classical spin-glass behaviour, which we characterise using magnetic susceptibility, magnetisation, and heat capacity measurements, although some evidence is seen for a transition to a partially ordered spin-glass like state below 24 K. The behaviour of Ba3Fe2SbO9 is compared with that of the 6H-type perovskite Ba3Fe2WO9, which displays antiferromagnetic character below TN = 290 K before entering a glassy state below Tf = 60 K. Differences between the magnetism in these two phases are discussed in terms of the complete structural disorder between the Fe and Sb ions in the former case, versus partial disorder (limited to the distribution and local orientation of Fe–W and Fe–Fe dimer units) in the latter.