Graeme R. Blake

Spin structure and magnetic frustration in multiferroic RMn2O5 (R=Tb,Ho,Dy)

We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R=Tb,Ho,Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R=Tb,Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.

Ultrahigh sensitivity of methylammonium lead tribromide perovskite single crystals to environmental gases

Extremely low surface trap densities and ultrahigh sensitivity to oxygen and water molecules are demonstrated in perovskite single crystals. One of the limiting factors to high device performance in photovoltaics is the presence of surface traps. Hence, the understanding and control of carrier recombination at the surface of organic-inorganic hybrid perovskite is critical for the design and optimization of devices with this material as the active layer. We demonstrate that the surface recombination rate (or surface trap state density) in methylammonium lead tribromide (MAPbBr3) single crystals can be fully and reversibly controlled by the physisorption of oxygen and water molecules, leading to a modulation of the photoluminescence intensity by over two orders of magnitude. We report an unusually low surface recombination velocity of 4 cm/s (corresponding to a surface trap state density of 108 cm−2) in this material, which is the lowest value ever reported for hybrid perovskites. In addition, a consistent modulation of the transport properties in single crystal devices is evidenced. Our findings highlight the importance of environmental conditions on the investigation and fabrication of high-quality, perovskite-based devices and offer a new potential application of these materials to detect oxygen and water vapor.

Ferroelectricity Induced by Acentric Spin-Density Waves in YMn2O5

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.

Transition between Orbital Orderings in YVO3

Evidence has been found for a change in the ordered occupation of the vanadium d orbitals at the 77 K phase transition in YVO3, manifested by a change in the type of Jahn-Teller distortion. The orbital ordering above 77 K is not destroyed at the magnetic ordering temperature of 116 K, but is present as far as a second structural phase transition at 200 K. The transition between orbital orderings is caused by an increase in octahedral tilting with decreasing temperature.

Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO3 by high-temperature synchrotron x-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100 K and 1350 K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high-temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO3.

Modulated structure ofBa6ZnIr4O15; a comparison withBa6CuIr4O15 andSrMn1-xCoxO3-y

The new phase Ba 6 CuIr 4 O 15 has been shown by powder X-ray diffraction to be a member of the perovskite-related structural family A 3n+3 A′ n B n+3 O 6n+9 withn=1; space group R32, a=10.1196(3), c=13.4097(4) A. The trigonal-prismatic A′ sites and the octahedral B sites are both occupied by a disordered distribution of Ir and Cu. A combination of electron microscopy and X-ray diffraction has shown that Ba 6 ZnIr 4 O 15 has a related incommensurate structure with a≈10.1, c≈4.4 A and a modulation vector q = a *+0.35 c *. Alternatively, Ba 6 ZnIr 4 O 15 can be considered as a composite of trigonal (cell 1) and rhombohedral (cell 2) substructures with a 1 =a 2 =10.1228(9) c 1 =4.4099(6), c 2 =2.6982(2) A. It is shown that the same structural formalism can be used to describe the incommensurate hexagonal phase of SrMn 1-x Co x O 3-y .

Band gap narrowing of SnS2 superstructures with improved hydrogen production

Transition metal sulfides exhibit chemical and physical properties that are of much scientific and technological interest and can largely be attributed to their covalent bonding of 3d electrons. Hierarchical structures of these materials are suited for a broad range of applications in energy storage, as biological scaffold, and as sensors. In this work, hierarchical SnS2 structures have been synthesized and show excellent photocatalytic performance for the production of H2 under blue light (450 nm) irradiation. A combination of high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy indicates the formation of layered SnS2/SnS superstructures with a lattice mismatch between the two alternating layers. This indicates the presence of S vacancies and results in a drastic decrease of the band gap by 0.3 eV compared to bulk SnS2. This strategy of self-narrowing of the band-gap demonstrates its great potential for the design of new materials with visible light reactivity. Finally, we have extended this strategy to the synthesis of other transition metal sulfides (Ni3S4, CuS, CuS@C, and FeS2) with similar hierarchical structures, which have potential applications such as supercapacitors and electrode materials for sodium/lithium ion batteries.

Evidence for electronic phase separation between orbital orderings in SmVO3

We report evidence for phase coexistence of orbital orderings of different symmetry in SmVO3 by high resolution x-ray powder diffraction. The phase coexistence is triggered by an antiferromagnetic ordering of the vanadium spins near 130 K, below an initial orbital ordering near 200 K. The phase coexistence is the result of the intermediate ionic size of samarium coupled to exchange striction at the vanadium spin ordering.

Light‐Induced Bistability in Iron(III) Spin‐Transition Compounds of 5 X‐Salicylaldehyde Thiosemicarbazone (X=H, Cl, Br)

The iron(III) spin-crossover compounds [Fe(Hthsa)(thsa)]⋅H2 O (1), [Fe(Hth5Clsa)(th5Clsa)2 ]⋅H2 O (2), and [Fe(Hth5Brsa)(th5Brsa)2 ]⋅H2 O (3) (H2 thsa=salicylaldehyde thiosemicarbazone, H2 th5Clsa=5-chlorosalicylaldehyde thiosemicarbazone, and H2 th5Brsa=5-bromosalicylaldehyde thiosemicarbazone) have been synthesized and their spin-transition properties investigated by magnetic susceptibility, Mössbauer spectroscopy, and differential scanning calorimetry measurements. The three compounds exhibit an abrupt spin transition with a thermal hysteresis effect. The more polarizable the substituent on the salicylaldehyde moiety, the more complete is the transition at room temperature with an increased degree of cooperativity. The molecular structures of 1 and 2 in the high-spin state are revealed. The occurrence of the light-induced excited-spin-state trapping phenomenon appears to be dependent on the substituent incorporated into the 5-position of the salicylaldehyde subunit. Whereas the compounds with an electron-withdrawing group (-Br or -Cl) exhibit light-induced trapped excited high-spin states with great longevity of metastability, the halogen-free compound does not, even though strong intermolecular interactions (such as hydrogen-bonding networks and π stacking) operate in the system. For compound 2, the surface level of photoconversion is less than 35 %. In contrast, compound 3 displays full photoexcitation.

Surface-enhanced charge-density-wave instability in underdoped Bi2Sr2-xLaxCuO6+delta

Neutron and X-ray scattering experiments have provided mounting evidence for spin and charge ordering phenomena in underdoped cuprates. These range from early work on stripe correlations in Nd-LSCO to the latest discovery of charge-density-waves in YBa2Cu3O(6+x). Both phenomena are characterized by a pronounced dependence on doping, temperature and an externally applied magnetic field. Here, we show that these electron-lattice instabilities exhibit also a previously unrecognized bulk-surface dichotomy. Surface-sensitive electronic and structural probes uncover a temperature-dependent evolution of the CuO2 plane band dispersion and apparent Fermi pockets in underdoped Bi2 Sr(2-x) La(x) CuO(6+δ) (Bi2201), which is directly associated with an hitherto-undetected strong temperature dependence of the incommensurate superstructure periodicity below 130 K. In stark contrast, the structural modulation revealed by bulk-sensitive probes is temperature-independent. These findings point to a surface-enhanced incipient charge-density-wave instability, driven by Fermi surface nesting. This discovery is of critical importance in the interpretation of single-particle spectroscopy data, and establishes the surface of cuprates and other complex oxides as a rich playground for the study of electronically soft phases.