Synthesis and Characterization of New AA′BWO₆ Perovskites Exhibiting Simultaneous Ordering of A-Site and B-Site Cations

Abstract

Six new AA′BB′O6 perovskites KLaMnWO6, NaLaMnWO6, NaNdMnWO6, NaTbMnWO6, NaNdCoWO6, and NaNdMgWO6 have been prepared. Each possesses the unusual combination of layered ordering of the A-site cations and rock-salt ordering of the B-site cations. The structure and properties of these samples have been characterized using monochromatic X-ray and neutron powder diffraction, UV–vis diffuse reflectance spectroscopy, and SQUID magnetometry. NaLaMnWO6, NaNdMnWO6, and NaTbMnWO6 adopt a structure with monoclinic P21 symmetry arising from the combination of cation ordering and a–a–c+ octahedral tilting. The structures of the other three compounds are similar, but the presence of satellite reflections in the neutron diffraction data suggests a more complicated superstructure. Each of the four AA′MnWO6 samples shows a paramagnetic to antiferromagnetic transition with Néel temperatures ranging from 6 to 15 K. The NaTbMnWO6 compound shows a second magnetic transition at ∼9 K. The origin of two magnetic phase transitions appears to arise from coupling between the Mn2+ sublattice and the Tb3+ sublattice.

Polar octahedral rotations: A path to new multifunctional materials

“…This was anticipated from the earlier work of Woodward and co-workers [43][44][45]. In this section, we discuss the discovery of octahedral rotation-induced ferroelectricity in several different materials.…”

New compounds and structures in the solid state

“…(NH 4 ) 2 Te 2 WO 8 forms a 2-dimensional structure by linking WO 6 octahedra and TeO 4 tetrahedra into layers separated by NH 4 + cations. 24 The presence of d 0 and d 10 configurations of W 6+ and Te 4+ , respectively leads to the asymmetric distortion of the environments around these cations due to second-order Jahn-Teller (SOJT) effects.…”

“…31 As well as the relatively dense materials Fe 2 As 2 O 7 Á 2H 2 O and LiFeAsO 4 OH, the use of diamines led to the formation of structures in which the doubly protonated amines occupy non-framework volume. The frameworks are composed of FeO 6 octahedra and tetrahedra of partially protonated As(O,OH) 4 33 The as-synthesised material has the natrolite topology and retains structural integrity in the temperature range 50 r T/K r 300 with the orthorhombic space group I2 1 2 1 2 1 . Heating the material above this temperature leads to partial loss of water and associated structural transformation to tetragonal symmetry, I 42d.…”

“…4 Each of these compounds contain W 6+ and the divalent cation in a rock-salt arrangement over the octahedral sites in the structure in a manner commonly observed in perovskites. However, these compounds are highly unusual in also containing an ordered arrangement of K + or Na + and Ln 3+ over layers in the structure.…”

“…16 This compound is isostructural with SrCuO 2 and crystallises in the space group P4/mmm [a = 3.926, c = 3.432 Å ]. This strong deviation from the cubic symmetry of the parent structure arises from the ordering of the anion vacancies, disrupting the three-dimensional connectivity of the perovskite structure to give a layered phase consisting of corner sharing squareplanar FeO 4 . This material contains high spin Fe 2+ (S = 2) aligned antiferromagnetically below a Ne´el temperature of 473 K. Despite the largely 2-dimensional nature of the magnetic interactions, the magnetic ordering temperature in this compound is considerably higher than other Fe 2+ compounds such as FeO, T N E 200 K. It is also noteworthy that the doubly degenerate (d xz ,d yz ) 3 ground state does not lead to Jahn-Teller instability.…”