Triple A-Site Cation Ordering in the Ferrimagnetic Y₂CuGaMn₄O₁₂ Perovskite

Abstract

A new member of A-site columnar-ordered A 2 A′A″B 4 O 12 quadruple perovskites with the composition of Y 2 CuGaMn 4 O 12 was prepared by a high-pressure, high-temperature method at 6 GPa and about 1500 K. Its crystal structure and cation distributions were studied by powder synchrotron X-ray and neutron diffraction. There is a triple A-site cation ordering with some degrees of anti-site disorder among sites occupied by 3d transition metals: [Y 2 ] A [Cu 0.8 Mn 0.2 ] A′ [Ga 0.8 Mn 0.2 ] A″ [Mn 3.6 Cu 0.2 Ga 0.2 ] B O 12 . It has the space group P 4 2 / nmc (no. 137) between 1.5 and 873 K with a = 7.33884 Å and c = 7.66251 Å at 297 K. Despite anti-site disorder, it exhibits a long-range ferrimagnetic order at T C = 115 K with the ordered moment of 2.19 μ B at each B site and 0.89 μ B at the A′ or A″ site. Magnetic moments are aligned along the c axis; all moments are ordered ferromagnetically at the B sites, and the moments at the A′ or A″ site are ordered in the opposite direction. Cu 2+ doping drastically changes magnetic properties as “parent” Y 2 MnGaMn 4 O 12 just shows spin-glass magnetic properties without long-range ordering. Anisotropic thermal expansion was observed in Y 2 CuGaMn 4 O 12 : the lattice parameter a almost linearly decreases from 1.5 K to T C and then monotonically increases up to 873 K (almost linearly from 300 K); the parameter c monotonically increases from 1.5 to 300 K and then decreases up to 600 K.

A-Site Columnar-Ordered Perovskite CaZnV2O6 as a Pauli-Paramagnetic Metal

“…The VO 6 octahedron is compressed in the uniaxial direction. Notably, this distortion is just a feature of A-site columnar-ordered perovskites and does not significantly affect the electronic state of V, as described in a previous symmetry analysis . The small V–O–V angles (145–149°) of CaZnV 2 O 6 are specific to A-site columnar-ordered perovskites and are much smaller than those of CaVO 3 (∼160°) and SrVO 3 (180°). , …”

“…Notably, this distortion is just a feature of A-site columnarordered perovskites and does not significantly affect the electronic state of V, as described in a previous symmetry analysis. 60 The small V−O−V angles (145−149°) of CaZnV 2 O 6 are specific to A-site columnar-ordered perovskites and are much smaller than those of CaVO 3 (∼160°) and SrVO 3 (180°). 4,11 We also examined the temperature dependence of the lattice constants using low-temperature SXRD data, expecting a centrosymmetric−noncentrosymmetric transition at low temperatures (Figure S2).…”

Anisotropic Thermal Expansion and a Second-Order Charge Order Transition in the Ferrimagnetic Dy2CuZnMn4O12 Perovskite with Triple A-Site Cation Ordering

“…Small antisite disorders are often realized in A 2 A′A″B 4 O 12 perovskites with A = rare earths, A′ = Mn, and A″ = Mn. Such antisite disorders could be observed with synchrotron XRPD data as small (but detectable) deviations of occupation factors from unity (for ideal cation distribution models). − , However, during the structural analysis of Dy 2 CuZnMn 4 O 12 , all cation occupation factors converged to values close to unity. We report here the occupation factors at 600 K as the symmetry at 600 K is higher (see below) and the number of refined structural parameters is smaller.…”

“…The orthorhombic splitting of reflections also disappeared ( Figure 4 ). Therefore, at 450 K and above, we used the P 4 2 / nmc model 3 to obtain temperature dependence of the lattice parameters. The refined structural parameters and bond lengths at 600 K are summarized in Tables 3 and 4 , and the fitting patterns are shown in the inset of Figure 1 .…”

“…In comparison with classical ABO 3 perovskites with one type of A sites and A-site-ordered quadruple perovskites, AA′ 3 B 4 O 12 , with intrinsically two types of A sites (A with a 12-fold coordination and A′ with a square-planar coordination), A 2 A′A″B 4 O 12 perovskites have an additional degree of freedom in the presence of the A″ site with a tetrahedral coordination (as the first coordination sphere). Therefore, a principally different, new set of cations, which are not usually located at A sites of perovskites, can be used for the A′′ site, such as Ga 3+ and Zn 2+ . − …”