Synthesis, structure and magnetic properties of La₃Co₂SbO₉: A double perovskite with competing antiferromagnetic and ferromagnetic interactions

Abstract

The synthesis, structural characterization, and magnetic properties of La₃Co₂SbO₉ double perovskite are reported. The crystal structure has been refined by X-ray and neutron powder diffraction data in the monoclinic space group P2₁/n. Co²⁺ and Sb⁵⁺ have the maximum order allowed for the La₃Co₂SbO₉ stoichiometry. Rietveld refinements of powder neutron diffraction data show that at room temperature the cell parameters are a=5.6274(2) Å, b=5.6842(2)  Å, c=7.9748(2)  Å and β=89.999(3)°. Magnetization measurements indicate the presence of ferromagnetic correlations with T_C=55 K attributed to the exchange interactions for non-linear Co²⁺–O–Sb⁵⁺–O–Co²⁺ paths. The effective magnetic moment obtained experimentally is μ_exp=4.38 μ_B (per mol Co²⁺), between the theoretical one for spin only (3.87 μ_B) and spin-orbit value (6.63 μ_B), indicating partially unquenched contribution. The low magnetization value at high magnetic field and low temperature (1 μ_B/f.u., 5 T and 5 K) and the difference between ZFC and FC magnetization curves (at 5 kOe) indicate that the ferromagnetism do not reach a long range order and that the material has an important magnetic frustration.

Introduction

Perovskites and double perovskites in particular are very attractive because of the interest in applications and fundamental areas of advanced materials, catalysis and the many interesting physical properties they have [1].

The general formula of a simple perovskite is ABO₃ and is a highly flexible structure. It can accommodate almost all elements of the periodic table on its cuboctahedral (A) and octahedral (B) sites. The structure of a cubic ideal perovskite can be seen as a three-dimensional array of BO₆ octahedra linked by the vertex, with the A ion in the cuboctahedral cavity surrounded by 8 BO₆ octahedra. Usually an alkali-earth ion occupies the A site [2], [3], but a smaller ion, like a lanthanide, can also be accommodated in this site. However, several distortions can arise if the A ion is too small [4]. There are three common distortions: cation displacement within the BO₆ octahedra and distortions and tilting of these octahedra.

When two ions, B and B′, are located on two different crystallographic sites, a double perovskite, A₂BB′O₆, is formed. B and B′ ions can be completely or partially ordered among the two different sites. The degree of order depends on the size and charge differences between the B ions. In general, a big difference in size and charge facilitates ordering at the B-site. Nevertheless, other factors, such as specific bonding interactions and synthesis condition may affect the degree of order [5], [6]. The magnetic properties are largely affected by the nature of the B cations, for example, B could be a transition metal with unpaired electrons, and if B′ ion has a close shell configuration, then the interactions between B ions and their neighbors will determine the magnetic behavior. Consequently the B order-disorder relationship strongly affects the magnetic properties of these materials [7], [8], [9].

In the last years, the synthesis of new double perovskite has resulted in a very attractive topic of research, after it has been informed that Sr₂FeMoO₆ has room temperature colossal magnetoresistance (CMR) [10]. Besides, its half-metallicity, high magnetic transition temperature along with its spin polarized charge transport has a very large importance in the field of spintronics [11] and in applications in read heads, magnetic memories, recording media or field sensors [12].

If the ratio B/B′ in double perovskites is one, the general formula will be A₂BB′O₆. For this stoichiometry, in the most ordered case, one of the sites will be fully occupied by B ions, while the other fully occupied by B′. Usually, the formula in this case is written as A₂(B)(B′)O₆. But if the stoichiometry is A₃B₂B′O₉, the maximum order that can be present within B and B′ cations can be written as: A₂(B)(B_1/3B′_2/3)O₆ [7], a situation which has an intrinsic disorder, i.e., one of the sites is fully occupied by B but the other one has a disordered situation, additional disorder could be obtained by moving some B′ to the site fully occupied by B, strongly affecting the magnetic properties [7]. For example, anti-site disorder has been shown to be responsible for spin-glass behavior for different double perovskites, i.e., Sr₂FeTeO₆ [13], Sr₂Mn_0.7Fe_0.3O₆ [14], Sr₂FeMoO₆ [15] among others. The double perovskites La₃Co₂NbO₉ and La₃Co₂TaO₉ were prepared by our group [16], [17] by sol–gel and solid state methods and the correlation between the B order-disorder and magnetic behavior was reported. Samples prepared by solid state showed spontaneous magnetization bellow 72 K for La₃Co₂TaO₉ and 62 K for La₃Co₂NbO₉ and positive θ values, showing prevalence of ferromagnetic interactions, while those prepared by sol–gel present low spontaneous magnetization (below 40 and 20 K, respectively) and negative θ values, showing prevalence of antiferromagnetic interactions. This behavior was explained by a simple model which shows that the most B-site ordered compounds favors the ferromagnetic Co²⁺–O–M⁵⁺–O–Co²⁺ interactions while the most disordered ones favor the antiferromagnetic Co²⁺–O–Co²⁺ interactions [16]. These Co²⁺–O–M⁵⁺–O–Co²⁺ ferromagnetic interactions, according to this model, are present in nanoclusters, whose amount and size are highly dependent on the degree of disorder on the B sites.

Here, we describe the synthesis, structure and magnetic characterization of the double perovskite La₃Co₂SbO₉, completing in this way the synthesis and characterization of the double perovskites family La₃B₂B′O₉ (with B′⁵⁺ ions) containing Co²⁺ as magnetic ion. La₃Co₂SbO₉ was actually informed by the first time by G. Blasse [18] as an orthorhombic perovskite; however no detailed structural or magnetic studies were informed.