Effect of isovalent substitution on the crystal structure and properties of two-slab indates BaLa_2−xSm_xIn₂O₇

1 Introduction

Compounds of A_n+1B_nO_3n+1 family (A = Sr, Ba, Ca, Ln, Na, K; B = Al, Ga, Fe, Ni, Cr, Sc, In, Ti, Sn, Zr, Hf, Pb, Mn; n is the amount of BO₆ octahedra slabs in a block, n = 1–3) with a slab perovskite-like structure (SPS) find a number of practically important electrophysical, optical, and catalytic properties [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. In particular, representatives of this family of compounds may find an application as humidity sensor materials with high sensitivities and fast response time [3], as oxygen sensors and solid electrolytes in solid-state fuel cells, showing a high degree of flexibility in controlling electrical and transport properties [4,5]. Thus, Ba₈In₆O₁₇ (with A_n+1B_nO_3n+1-type structure) exhibits high oxide ion conductivity comparable to stabilized zirconia, which is commercially used as a solid electrolyte [6]. Compounds of A_n+1B_nO_3n+1 exhibit photocatalytic activity of water decomposition into H₂ and O₂ under UV light [7,8] and luminescent parameters of the doped A_n+1B_nO_3n+1 compounds indicate the prospect of their use as matrices for luminophore and laser materials [9,10,11,12,13,14]. However, it should be noted that, unlike the Mn-, Co- or Fe-containing A_n+1B_nO_3n+1 compounds [15,16,17,18], the magnetic properties of Sm-containing A_n+1B_nO_3n+1 compounds with SPS have not been studied yet.

It is known that isomorphic substitution A/B of atoms in the crystallographic positions is one of the common ways of controlling the characteristics of oxides and oxide-based materials [5,7,16,19,20]. Note that the effect of isomorphic substitution is well studied for Sr-containing A_n+1B_nO_3n+1 phases with n = 1 and n = 2 [21,22,23,24,25,26]. However, the composition/structure relationship in Ba-containing A_n+1B_nO_3n+1 phases with n = 2 was only defined for Ba/Sr substitution [27] in Ba_1−xSr_xLa₂In₂O₇ phases. At the same time, there is no data on the effect of isovalent substitution of RE atoms in BaLn₂In₂O₇ indates (Ln = La, Pr, Nd) on their structure and properties.

The aim of present research was to study the conditions of La/Sm isovalent substitution in a two-slab BaLa₂In₂O₇ SPS and to determine its effect on the structural features and magnetic properties of isovalently substituted BaLa_2−xSm_xIn₂O₇ indates synthesized. To completely characterize this system, the present paper includes a systematic study of conditions of polymorphous substitution, crystal structure, and magnetic properties of BaLa_2−xSm_xIn₂O₇ samples. The study of the Ln/Ln^I substitution is a venture into the theory of the formation of Ba-containing A_n+1B_nO_3n+1 compounds with SPS and is of apparent interest for understanding the composition/structure/features correlation for other representatives of oxide compounds with perovskite structure.

2 Experimental details

3 Results and discussion

SEM micrographs (Figure 1a) showed that mechanically dispersed BaLa_2−xSm_xIn₂O₇ ceramic consists of irregularly shaped particles up to 10 μm in diameter with a smooth surface. Corresponding EDS maps showing the element’s distribution have revealed the homogeneous composition of BaLa_2−xSm_xIn₂O₇ (0 ≤ x ≤ 1.8) particles (Figure 1b).

Figure 1

SEM micrographs of BaLa_0,5Sm_1,5In₂O₇ powder ceramic: ×1000 (a); corresponding EDS maps showing the element’s distribution (b).

X-ray diffraction patterns of the heat-treated samples of co-crystallized salts have revealed the that existence range of BaLa_2−xSm_xIn₂O₇ phases with SPS is 0 ≤ x ≤ 1.8 also. Powder diffraction data and the results of Rietveld refinement are shown in Figure 2. Their diffraction patterns are similar to those for the unsubstituted BaLa₂In₂O₇ compound [29] and are indexing well in the tetragonal system.

Figure 2

Diffraction pattern (points), Rietveld refinement results (solid line) for BaLa_2−xSm_xIn₂O₇ with x = 0, 0.5, 1.0, 1.8 (P4₂/mnm) and diffraction pattern of “BaSm₂In₂O₇” (CuK_α radiation).

Destruction of two-slab BaLa_2−xSm_xIn₂O₇ indates with SPS takes place if the substitution degree x is higher than 1.8. Besides, if x = 2, this failure is accompanied by a formation of the two-phase system consisting of SmInO₃ with a perovskite-type structure and BaSmInO₄ indate with CaFe₂O₄-type structure. It should be noted that B-positions of the BaSmInO₄ phase are occupied by Sm and In atoms simultaneously [30].

The lattice parameters (a and c values) of the tetragonal BaLa_2−xSm_xIn₂O₇ phases with SPS decrease monotonically within the existence range of the BaLa₂In₂O₇-based solid solutions (Figure 3). Such dependence of the unit cell parameters is caused by the smaller size of the samarium atom and is typical for other solid solutions of the A_n+1B_nO_3n+1 compound with SPS [21,22,23,24,25,26,27].

Figure 3

Dependences of lattice parameters for BaLa_2−xSm_xIn₂O₇ phases on the degree of lanthanum atoms substitution (x).

Systematic extinction of the BaLa_2−xSm_xIn₂O₇ with SPS diffraction pattern indicates the following possible space groups: one centrosymmetric P4₂/mnm and two noncentrosymmetric P4₂nm or [31]. Results of the second optical harmonic generation of the laser radiation for BaLa_2−xSm_xIn₂O₇ (І_2ω) were compared P4̄n2d with those for the noncentrosymmetric La₄Ti₄O₁₄ phase with SPS I2ω′ and showed that the relative intensity of signal does not exceed 0.01I2ω′. According to ref. [32], insignificant І_2ω value unambiguously indicates that BaLa_2−xSm_xIn₂O₇ phases (as well as the initial BaLa₂In₂O₇ phase) belong to the centrosymmetric space group P4₂/mnm.

Taking into account the centrosymmetry of both BaLa_2−xSm_xIn₂O₇ indates (0 < x ≤ 1.8) synthesized here and their prototype BaLa₂In₂O₇ indate [29], the atomic parameters of this indate-prototype were chosen as starting values for BaLa_2−xSm_xIn₂O₇ structures. Crystallographic data for BaLa₂In₂O₇ indate according to ref. [29] are as follows: P4₂/mnm space group, a = 0.5915(2) nm, c = 2.086(1) nm, 4Ba atoms are placed in 4f x x 0 with x = 0.249; 8La atoms are placed in 8j x x z with x = 0.269, z = 0.185; 8 In atoms are placed in 8j x x z with x = 0.260, z = 0.400; 4O(1) atoms are placed in 4g xx¯ 0 with x = 0.806; 8O(2) atoms are placed in 8j x x z with x = 0.816, z = 0.290; 8O(3) atoms are placed in 8h 0 0.5 z with z = 0.096; 4O(4) atoms are placed in 4e 0 0 z with z = 0.126; 4O(5) atoms are placed in 4e 0 0 z with z = 0.383. Relevant structural calculations carried out using BaLa₂In₂O₇ as initial trial model led to a satisfactory agreement between the experimental and calculated intensities of reflections for each of the BaLa_2−xSm_xIn₂O₇ phases studied (the reliability factors R_B are about 0.04). Crystallographic data for the structures of several BaLa_2−xSm_xIn₂O₇ phases (with x = 0.5, 1.0, 1.6, and 1.8) obtained as a result of these calculations are listed in Table 1. Interatomic distances in each BaLa_2−xSm_xIn₂O₇ phase have been calculated based on the refined coordinate parameters of the atom. Moreover, a distortion degree (Δ) of the MeO_n polyhedra was calculated as Δ=1/nΣ[(Ri–R¯)/R¯]2 (where R_i is the Me–O distance, R¯ is the Me–O average distance, n is the coordination number for Me atom) [33] (Table 2).

Unfortunately, it was impossible to determine correctly the localization of Ba, La, and Sm atoms in BaLa_2−xSm_xIn₂O₇ phases from XRD data due to the proximity of their atomic scattering factors. Therefore, the distribution of barium, lanthanum, and samarium atoms within the 4f and 8j crystallographic positions of the BaLa_2−xSm_xIn₂O₇ SPS structures was established by analyzing the valence bond sums (VBS) of these atoms in BaO₉, BaO₁₂, (La_2−xSm_x)O₉, and (La_2−xSm_x)O₁₂ polyhedron (Table 3). Moreover, the Me–O bond valency (s) was calculated as s = exp((R₀ − R)/B) [34], where R₀ is the tabulated length of the monovalent bond, R is the experimental value of the bond length, and B is the tabulated value of the dispersion of the bond length, which is equal to 0.037 nm and was applied to find R₀. VBS was found from the valency of all bonds: VBS = Σs·n, where n is the number of bonds of this type. In the case of simultaneous filling of the position with lanthanum and samarium atoms, the above formula was applied to calculate VBS, while the standard value of R₀ was refined by the equation:

where R₀(La) and R₀(Sm) are the tabulated lengths of the monovalent bond, K(La) and K(Sm) are the occupancy of this position. As can be seen from the data listed in Table 3, the VBS values of barium and RE atoms in the hypothetical BaO₉ and LnO₁₂ polyhedron are significantly different from their chemical valences, while VBS values of these elements are much closer to their valences for the location of barium atoms in position 4f and RE atoms in position 8j.

Thus, the results of our calculations unambiguously indicate an ordered distribution of barium and RE atoms in BaLa_2−xSm_xIn₂O₇ phase with localization of barium atoms in the intrablock of cubo-octahedral voids of the perovskite-like block only and with localization of the RE atoms just in the LnO₉ polyhedron. Previously, such an atom arrangement was observed for unsubstituted BaLa₂In₂O₇ compound [29]. The likely reason for such character of distribution of barium and RE atoms in the BaLa_2−xSm_xIn₂O₇ of two-slab SPS is the tendency of relatively smaller RE atoms to occupy small MeO₉ polyhedra.

It was shown that the main structural units of the synthesized BaLa_2−xSm_xIn₂O₇ indate with SPS are the directly unrelated two-dimensional (infinite in both X and Y axes) perovskite-like blocks, each of which consists of two slabs connected by vertices of the deformed InO₆ octahedra (Figure 4a). Adjacent blocks are shifted by a half of the perovskite cube edge and alternate with each other in a direction of the XY plane diagonal. Neighboring perovskite-like blocks are separated by a slab of LnO₉ polyhedra and are held together by means of interblock bonds –O–Ln–O.

Figure 4

Crystal structure of BaLaSmIn₂O₇ as an arrangement of InO₆ octahedra and Ba (black circles) and La, Sm atoms (gray circles) (a); interblock boundary pattern for BaLa_0,2Sm_1,8In₂O₇ (La,Sm) atoms – gray circles) (b).

Eight oxygen atoms (four O(2), two O(3), one O(4), and one O(5)) of the LnO₉ polyhedron belong to the same block as Ln atoms, while the ninth oxygen atom (O(2)) refers to the neighboring block (Figure 4b). This Ln–O(2) interblock bond is the shortest (0.230(2)–0.214(1) nm) among all the bonds of LnO₉ polyhedron (Table 2). Moreover, its length approaches the minimum known for the (La,Sm)–O distances. This confirms a conclusion made on the base of VBS data on the impossibility of arrangement barium atoms, which are much larger than lanthanum and samarium atoms, in the 8j position in the MeO₉ polyhedron between perovskite-like blocks. It also indicates Ba atom localization in 4f position in intrablock voids of the perovskite-like block only, where its coordination polyhedron is a deformed BaO₁₂ cuboctahedron. It should be noted that the two O(2) atoms are located at significantly higher distances (0.352(2)–0.348(1) nm) from the RE atom among nine oxygen atoms of the LnO₉ polyhedron (Table 2); therefore, the coordination number of RE atoms could be considered as 7 + 2.

Analysis of the (La,Sm)–O(2) interblock bond length in BaLa_2−xSm_xIn₂O₇ SPS indicates that the replacement of lanthanum atoms by smaller samarium atoms leads to a significant decrease in the length of the (La,Sm)–O(2) interblock bond (from 0.230(2) nm to 0.214(1) nm) (Figure 5 and Table 2). This approaches the structure of BaLa_2−xSm_xIn₂O₇ indate to the structure of three-dimensional perovskite, and it is one of the main restrictive factors for these phases’ existence region and the absence of BaSm₂In₂O₇ compound.

Figure 5

Dependence of Ln–O interblock distance (d) in LnO₉ external block polyhedra on the of substitution of La atoms in BaLa_2−xSm_xIn₂O₇ phases with SPS.

An increase in the number of small samarium atoms in the (La,Sm)O₉ interblock polyhedra affects the structure of perovskite-like block with SPS also. In particular, it slightly increases the deformation of the BaO₁₂ intra-block polyhedra (Table 2).

It should be noted that the structure features of SPS revealed here for the BaLa_2−xSm_xIn₂O₇ phases are similar to those for the unsubstituted BaLn₂In₂O₇ (Ln = La, Pr, Nd). In particular, a decrease in the crystalline ionic radius of RE atoms in BaLn₂In₂O₇ indates results in a gradual decrease of the Ln–O(2) interblock bond length [29].

To study the effect of La^3+/Sm³⁺ substitution on magnetic properties of BaLa_2−xSm_xIn₂O₇ with SPS, we analyzed the temperature dependences of magnetic susceptibility χ(T) for samples with x = 1 and x = 1.8 (Figure 6). The χ(T) curves exhibited a moderately weak temperature dependence in temperature range (300–600) K, suggesting the presence of Van Vleck paramagnetism as usually observed for Sm³⁺ oxides [35,36,37,38,39,40]. The electronic configuration of Sm³⁺ (4f⁵), which has the ground J-multiplet (J = 5/2), on the site symmetry of C_2v indicates the ground state is a Kramers doublet [35]. For fitting the χ vs T curve at higher temperatures (up to 600 K), we thus used the Curie–Weiss law accompanied by a temperature-independent term χ_const is given as follows:

where C is the Curie constant and θ is the Curie temperature [35].

Figure 6

Temperature dependences of the magnetic susceptibility of BaLa_2−xSm_xIn₂O₇.

Besides, the χ(T) curves presented in Figure 6 clearly demonstrate a change in magnetic characteristics of the phases studied. In particular, one can see a change of χ(T) slope at 630, 770, and 810 K (Figure 6a). Processing the obtained χ(T) curves in (χ·T; T) coordinates (Figure 6b) and (χ⁻¹; T) (Figure 6c) reveals linear dependences of (χ·T)(T) curve in the temperature range 300–630 K, while χ⁻¹(T) curve demonstrates linear sections at higher temperatures.

The effective magnetic moments per samarium atom μ_eff were calculated taking into account the experimentally obtained C values, which are unchanged within a certain temperature range. The values calculated are listed in Table 4 and are much smaller (0.35–0.56) μ_B than the free-ion value of 0.83 µ_B, indicating that the Sm magnetic moment is under the influence of crystal electric field (CEF) of the perovskite structure. Note that a small μ_eff was often observed for Sm³⁺ oxides such as 0.44 µ_B for KBaSm(BO₃)₂ [39], 0.53 µ_B for Sm₃Sb₃Mg₂O₁₄ [38], 0.53 µ_B for Sm₂Zr₂O₇ [40], and 0.15 µ_B for Sm₂Ti₂O₇ [40]. The negative θ (Table 4) may indicate that the dominant magnetic interaction is antiferromagnetic even though it is very small and has to be interpreted carefully with the presence of CEF effects.

The data obtained allow us to speculate about the crystal filed effects in the BaLa_2−xSm_xIn₂O₇ indates. It was shown that the temperature increase leads to a change in the effective magnetic moment of the Sm³⁺ ion, as it is listed in Table 4. In our opinion, such alternation of the μ_eff magnetic moment may indicate not only the rearrangement of magnetic sublattices of the solid solution studied but also the prerequisite to the existence of polymorphic structural transformation of the phases close by structure [35,37].

4 Conclusions

In this work, the effect of isovalent substitution of lanthanum by samarium atoms in BaLn₂In₂O₇ indate with the formation of BaLa_2−xSm_xIn₂O₇ solid solution (0 ≤ x ≤ 1.8) and features of this SPS are defined. The ordered distribution of barium and REM atoms in BaLa_2−xSm_xIn₂O₇ SPS has been revealed. It has been shown that the isovalent substitution degree increasing leads to the convergence of two perovskite-like slabs of InO₆ octahedra connected by vertices. This feature rearranges the two-slab perovskite-like structure of BaLa_2−xSm_xIn₂O₇ indates to the structure of three-dimensional perovskite and is one of the main factors limiting their existence field. The magnetism of these compounds is understood by the localized 4f electrons of Sm³⁺ BaLa_2−xSm_xIn₂O₇ showed the reduced magnetic moment because of the crystal field effect and strong Van Vleck paramagnetism.

Analysis of the data obtained made it possible to establish the relationship between the composition, structure, and magnetic characteristics of the BaLa_2−xSm_xIn₂O₇ phases. All this information creates a background for the control of BaLa_2−xSm_xIn₂O₇ structural characteristics by consequent isovalent substitution of atoms in A position of SPS.