k=0 magnetic structure and absence of ferroelectricity in SmFeO3.

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

Revision of the structure of Cs2CuSi5O12 leucite as orthorhombic Pbca.

The crystal structure of a hydrothermally synthesized leucite analogue Cs(2)CuSi(5)O(12) has been determined and refined using the Rietveld method from high-resolution synchrotron X-ray and neutron powder diffraction data. This structure is based on the topology and cation-ordering scheme of the Pbca leucite structure of Cs(2)CdSi(5)O(12), and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs(2)CuSi(5)O(12) is topologically identical to other known leucite structures and is different from that originally proposed by Heinrich & Baerlocher [(1991), Acta Cryst. C47, 237-241] in the tetragonal space group P4(1)2(1)2. The crystal structure of a dry-synthesized leucite analogue Cs(2)CuSi(5)O(12) has also been refined; this has the Ia3d cubic pollucite structure with disordered T sites.

Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucites.

The crystal structures of the leucite analogues Cs(2)MgSi(5)O(12), Cs(2)ZnSi(5)O(12) and Rb(2)MgSi(5)O(12) have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with (29)Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs(2)CdSi(5)O(12), thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. (29)Si MAS NMR shows five distinct Si T sites for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12), but six Si T sites for Cs(2)ZnSi(5)O(12). The refined structures for Cs(2)MgSi(5)O(12) and Rb(2)MgSi(5)O(12) were determined with complete T-site ordering, but the refined structure for Cs(2)ZnSi(5)O(12) was determined with partial disorder of Mg and Si over two of the T sites.

Remarkable impact of low BiYbO3 doping levels on the local structure and phase transitions of BaTiO3.

In situ Raman spectroscopy shows the simultaneous incorporation of small amounts of Bi3+ and Yb3+ into the lattice of BaTiO3 to break the average symmetry inferred from X-ray powder diffraction analysis and permittivity measurements. In particular, Bi3+ with a stereochemically active lone-pair of electrons induces severe lattice strain and the coexistence of different local crystal symmetries over a wide temperature range, effectively controlling the physical properties, such as the temperature dependence of the permittivity and the Curie temperature. These results show that compositional gradients based in small variations of these two dopants could successfully explain the enhanced thermal stability of the permittivity in core-shell type ceramics, whereas the lower capacitance of the shell can also cap the maximum permittivity at the Curie temperature.

Determination of Debye temperatures and Lamb-Mössbauer factors for LnFeO3 orthoferrite perovskites (Ln = La, Nd, Sm, Eu, Gd).

Lanthanide orthoferrites have wide-ranging industrial uses including solar, catalytic and electronic applications. Here a series of lanthanide orthoferrite perovskites, LnFeO3 (Ln = La; Nd; Sm; Eu; Gd), prepared through a standard stoichiometric wet ball milling route using oxide precursors, has been studied. Characterisation through x-ray diffraction and x-ray fluorescence confirmed the synthesis of phase-pure or near-pure LnFeO3 compounds. 57Fe Mössbauer spectroscopy was performed over a temperature range of 10 K-293 K to observe hyperfine structure and to enable calculation of the recoil-free fraction and Debye temperature (θ D) of each orthoferrite. Debye temperatures (Ln = La 474 K; Nd 459 K; Sm 457 K; Eu 452 K; Gd 473 K) and recoil-free fractions (Ln = La 0.827; Nd 0.817; Sm 0.816; Eu 0.812; Gd 0.826) were approximated through minimising the difference in the temperature dependent experimental centre shift and theoretical isomer shift, by allowing the Debye temperature and isomer shift values to vary. This method of minimising the difference between theoretical and actual values yields Debye temperatures consistent with results from other studies determined through thermal analysis methods. This displays the ability of variable-temperature Mössbauer spectroscopy to approximate Debye temperatures and recoil-free fractions, whilst observing temperature induced transitions over the temperature range observed. X-ray diffraction and Rietveld refinement show an inverse relationship between FeO6 octahedral volume and approximated Debye temperatures. Raman spectroscopy show an increase in the band positions attributed to soft modes of Ag symmetry, Ag(3) and Ag(5) from La to GdFeO3 corresponding to octahedral rotations and tilts in the [0 1 0] and [1 0 1] planes respectively.