Structural and magnetic study of order−disorder behavior in the double perovskites Ba2Nd1−xMnxMoO6

The synthesis and structural and magnetic characterization of the site-ordered double perovskites, Ba2Nd1−xMnxMoO6, 0 0.3, no deviation from the ideal cubic Fm3̅m symmetry is observed. Furthermore, dc-susceptibility measurements confirm that Mn2+ is being doped onto the Nd3+ site, and the associated oxidation of Mo5+ to Mo6+. For all compositions, the Curie−Weiss paramagnetic behavior above 150 K indicates negative Weiss constants that range from −24(2) and −85(2) K. This net antiferromagnetic interaction is weakest when x ≈ 0.5, where the disorder in cation site occupancy and competition with ferromagnetic interactions is the greatest. Despite these strong antiferromagnetic interactions, there is no evidence in the dc-susceptibility of a bulk cancellation of spins for x > 0.05. Low-temperature neutron diffraction measurements indicate that there is no long-range magnetic order for 0.1 ≤ x < 0.9. Ba2Nd0.10Mn0.90MoO6 exhibits additional Bragg scattering at 2 K, indicative of long-range antiferromagnetic ordering of the Mn2+ cations, with a propagation vector k = (1/2, 1/2, 1/2). The scattering intensities can be modeled using a noncollinear magnetic structure with the Mnthe Mn2+ moments orientated antiferromagnetically along the four different 111 directions

A neutron diffraction study of structural distortion and magnetic ordering in the cation-ordered perovskites Ba2Nd1−xYxMoO6

The cation ordered perovskites Ba2Nd1-xYxMoO6 (0.04 ≤ x ≤ 0.35) have been synthesised by solid-state techniques under reducing conditions at temperatures up to 1350°C. Rietveld analyses of X-ray and neutron powder diffraction data show that these compounds adopt a tetragonally-distorted perovskite structure. The tetragonal distortion is driven by the bonding requirements of the Ba2+ cation that occupies the central interstice of the perovskite; this cation would be underbonded if these compounds retained the cubic symmetry exhibited by the prototypical structure. The size and charge difference between the lanthanides and Mo5+ lead to complete ordering of the cations to give a rock-salt ordering of Nd3+/Y3+O6 and MoO6 octahedra. The I4/m space group symmetry is retained on cooling the x = 0.1, 0.2 and 0.35 samples to low temperature ca. 2 K. Ba2Nd0.90Y0.10MoO6 undergoes a gradual distortion of the MoO6 units on cooling from room temperature to give two long trans bonds (2.001(2) Å) along the z direction and four shorter apical bonds (1.9563(13) Å) in the xy plane. This extension is propagated through the structure and gives negative thermal expansion of -13×10-6 K-1 along c. With increasing Y3+ content this distortion is reduced in x=0.2 and eliminated in x=0.35 which contains largely regular MoO6 octahedra. The x=0.1 and x=0.2 show small peaks in the neutron diffraction profile due to long range antiferromagnetic order arising from ordered moments of ca. 2 μB

Structural and magnetic study of the cation-ordered perovskites Ba2−xSrxErMoO6

A series of perovskite phases have been prepared from the appropriate carbonates and oxides by heating under reducing conditions at temperatures up to 1300 °C. Complete ordering between ErO6 and MoO6 octahedra and a disordered distribution of Sr2+ and Ba2+ occur in all compounds. Neutron powder diffraction experiments show that the substitution of Sr2+ into Ba2ErMoO6 introduces a progressive reduction in symmetry from Fm3¯m (x=0) to I4/m (x=0.5, 0.8) to P21/n (x=1.25, 1.75, 2.0). Magnetic susceptibility measurements indicate that all of these compounds show Curie–Weiss paramagnetism and that for x<1.25 this behaviour persists down to 2 K. The monoclinically distorted compounds show magnetic transitions at low temperature and neutron diffraction has confirmed the presence of long-range antiferromagnetic order below 2.5 and 4 K in Ba0.25Sr1.75ErMoO6 and Sr2ErMoO6, respectively. Ba0.75Sr1.25ErMoO6, Ba0.25Sr1.75ErMoO6 and Sr2ErMoO6 do not undergo structural distortion on cooling from room temperature

Ion exchange and structural ageing in the layered perovskite phases H1-xLixLaTiO4

Grinding together the solid acid HLaTiO¬4 with stoichiometric quantities of lithium hydroxide monohydrate gives the solid solution H1-xLixLaTiO4. The structures of these crystalline phases have been refined against neutron powder diffraction data to show that all of these compounds crystallise in the centrosymmetric space group P4/nmm. The protons and lithium cations occupy sites between the perovskite layers; the former in hydroxide groups that hydrogen-bond to adjacent layers whilst Li+ is in four-coordinate sites that bridge the perovskite slabs with a geometry intermediate between square-planar and tetrahedral. The reaction proceeds rapidly but the unit cell size continues to evolve over the course of days with a gradual compression along the interlayer direction that can be modelled using a power law dependence reminiscent of an Ostwald ripening process. On heating, these materials undergo a mass loss due to dehydration but retain the layered Ruddlesden Popper structure up to 480°C before a substantial loss of crystallinity on further heating to 600°C. Impedance spectroscopy studies of the dehydrated materials shows that Li+ mobility in these materials is lower than the LiLaTiO4 end member, possibly due to microstructural effects causing large inter-grain resistance through the defective phases

Lithium dimer formation in the li-conducting garnets Li5+xBaxLa3-xTa2O12 (0&lt;x 1.6)

The garnet system Li5+xBaxLa32xTa2O12 shows an unprecedented Li+ content (x ¡ 1.6) and short Li–Li distances of ca2.44A° between majority occupied sites suggesting that the high Li+ mobility requires a complex cooperative mechanism

Magnetic dilution in magnetoresistive perovskites; cation doping in Ba2FeMoO6

Poster describing the magnetic dilution in magnetoresistive perovskites; cation doping in Ba2FeMoO6