Effect of isovalent substitution on the crystal structure and properties of two-slab indates BaLa2−x Sm x In2O7

Abstract Exploring the effect of isomorphic substitution of atoms on crystal structure and features of oxide compounds is one of the main tasks of modern materials science. This paper deals with the La/Sm isovalent substitution in a two-slab perovskite-like BaLa2In2O7 structure and its effect on the structural features and magnetic properties of BaLa2−x Sm x In2O7 indates synthesized. A complete characterization including data of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), structural calculations (Rietveld method), second optical harmonic generation of the laser radiation, calculations of valence bond sums (VBS), and magnetic susceptibility data of phases obtained is presented. The existence region of BaLa2−x Sm x In2O7 solid solutions with a two-slab perovskite-like structure (0 ≤ x ≤ 1.8) was established, and their coordinate parameters were refined. The character of barium and RE atoms distribution in BaLa2−x Sm x In2O7 structure has determined, and the correlation between substitution degree of lanthanum atoms and the length of Ln–O2 interblock distance has revealed. The magnetic properties of BaLa2−x Sm x In2O7 were considered in terms of the crystal field effect.

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 Srcontaining A n+1 B n O 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+1 B n O 3n+1 phases with n = 2 was only defined for Ba/Sr substitution [27] in Ba 1−x Sr x La 2 In 2 O 7 phases. At the same time, there is no data on the effect of isovalent substitution of RE atoms in BaLn 2 In 2 O 7 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 BaLa 2 In 2 O 7 SPS and to determine its effect on the structural features and magnetic properties of isovalently substituted BaLa 2−x Sm x In 2 O 7 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−x Sm x In 2 O 7 samples. The study of the Ln/Ln I substitution is a venture into the theory of the formation of Bacontaining A n+1 B n O 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.

Experimental details 2.1 Reagents
Chemically pure Ba(CH 3 COO) 2 (≥99 wt%) was purchased from Ostchem (Ukraine), and chemically pure La Preparation of the initial charge of co-crystallized salts for the subsequent synthesis of BaLa 2−x Sm x In 2 O 7 polycrystalline indates was carried out by co-crystallization (evaporation at intensive mixing) of the mixture of barium acetate, lanthanum, samarium, and indium nitrates aqueous solution (the ratio is equal to Ba:La:Sm:In = 1:2x:x:2 (increment x = 0.1)) with further thermal treatment of the product obtained with a gas-jet to remove the main part of the nitrogen oxides. The powder obtained was ground and pressed (under pressure p = 3 × 10 8 Pa) as the tablets with further heat treatment up to 1,570 K. Maximum heating temperature was reached in 2 h (heating rate didn't exceed 15 K/min), the cooling proceeded spontaneously together with the oven for 10 h. A two-stage (2 + 2 h) mode of tablet roasting at 1,570 K with intermediate grinding and repressing after the first stage was used. This procedure ensured the completeness of components interaction (according to XRD and SEM data).

Characterization
Morphology of the mechanically ground BaLa 2−x Sm x In 2 O 7 ceramic tablets was studied by the scanning electron microscopy technique (SEM, JEOL JAMP-9500F, Japan). JEOL JAMP-9500F is a field emission auger microprobe operated at 10 kV, which offers the flexibility of optional analysis functions such as energy-dispersive X-ray spectroscopy (EDS). EDS mappings of O, In, Ba, Sm, and La elements were performed on 15.3 μm × 10.2 μm areas of as-prepared powder sample, which was placed on a carbon film and put into apparatus.
The X-ray powder diffraction data of the samples were collected with Shimadzu XRD-6000 diffractometer (CuKα radiation) in a discrete mode: the scanning interval 2θ was (20-75)°, the step scan of 0.02°, counting time per step was 5 s. The peak positions and integrated intensities of the reflections observed were determined by full profile analysis.
To fulfill the preliminary data processing, as well as to carry out analysis and interpretation of the XRD data obtained, an original software package was applied, which includes a complete set of standard Rietveld procedures, namely, phase analysis of diffraction patterns using set of working base data, refinement of unit cell parameters, testing of the structure models proposed and crystal structure parameters refinement (including coordinates of atoms, atomic position filling, texture, etc.) [28].
Tests for generation of the second I 2ω were performed on polycrystalline samples using the Nd:YAG laser (λ ω = 1.064 μm and λ 2ω = 0.532 μm). The powder of four-slab La 4 Ti 4 O 14 ferroelectric was used as a standard material for estimating the non-centrosymmetric structure.
Temperature dependences of the magnetic susceptibility χ(T) of the indates synthesized were measured by the Faraday technique (using ABГ-5 magnetometer) in a temperature range of 300-900 K in a purified argon atmosphere. To improve the quality of the experimental curve a set of magnetometric measurements was carried out for each sample, and the data obtained were averaged. An error of χ determination did not exceed 5%.
Ethical approval: The conducted research is not related to either human or animal use.

Results and discussion
SEM micrographs (Figure 1a) showed that mechanically dispersed BaLa 2−x Sm x In 2 O 7 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−x Sm x In 2 O 7 (0 ≤ x ≤ 1.8) particles ( Figure 1b).
X-ray diffraction patterns of the heat-treated samples of co-crystallized salts have revealed the that existence range of BaLa 2−x Sm x In 2 O 7 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 BaLa 2 In 2 O 7 compound [29] and are indexing well in the tetragonal system.
Destruction of two-slab BaLa 2−x Sm x In 2 O 7 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 SmInO 3 with a perovskite-type structure and BaSmInO 4 indate with CaFe 2 O 4 -type structure. It should be noted that B-positions of the BaSmInO 4 phase are occupied by Sm and In atoms simultaneously [30].
The lattice parameters (a and c values) of the tetragonal BaLa 2−x Sm x In 2 O 7 phases with SPS decrease monotonically within the existence range of the BaLa 2 In 2 O 7 -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+1 B n O 3n+1 compound with SPS [21][22][23][24][25][26][27].
Systematic extinction of the BaLa 2−x Sm x In 2 O 7 with SPS diffraction pattern indicates the following possible space groups: one centrosymmetric P4 2 /mnm and two noncentrosymmetric P4 2 nm or [31]. Results of the second optical harmonic generation of the laser radiation for   Table 1. Interatomic distances in each BaLa 2−x Sm x In 2 O 7 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 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−x Sm x In 2 O 7 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−x Sm x In 2 O 7 SPS structures was established by analyzing the valence bond sums (VBS) of these atoms in BaO 9 , BaO 12 , (La 2−x Sm x )O 9 , and (La 2−x Sm x )O 12 polyhedron (Table 3). Moreover, the Me-O bond valency (s) was calculated as s = exp((R 0 − R)/B) [34], where R 0 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 R 0 . 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 R 0 was refined by the equation: where R 0 (La) and R 0 (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 BaO 9 and LnO 12 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−x Sm x In 2 O 7 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 LnO 9 polyhedron. Previously, such an atom arrangement was observed for unsubstituted BaLa 2 In 2 O 7 compound [29]. The likely reason for such character of distribution of barium and RE atoms in the BaLa 2−x Sm x In 2 O 7 of two-slab SPS is the tendency of relatively smaller RE atoms to occupy small MeO 9 polyhedra.
It was shown that the main structural units of the synthesized BaLa 2−x Sm x In 2 O 7 indate with SPS are the directly unrelated two-dimensional (infinite in both X   (2) (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 MeO 9 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 BaO 12 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 LnO 9 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−x Sm x In 2 O 7 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−x Sm x In 2 O 7 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 BaSm 2 In 2 O 7 compound.
An increase in the number of small samarium atoms in the (La,Sm)O 9 interblock polyhedra affects the structure of  perovskite-like block with SPS also. In particular, it slightly increases the deformation of the BaO 12 intra-block polyhedra ( Table 2). It should be noted that the structure features of SPS revealed here for the BaLa 2−x Sm x In 2 O 7 phases are similar to those for the unsubstituted BaLn 2 In 2 O 7 (Ln = La, Pr, Nd). In particular, a decrease in the crystalline ionic radius of RE atoms in BaLn 2 In 2 O 7 indates results in a gradual decrease of the Ln-O(2) interblock bond length [29].
To study the effect of La 3+/ Sm 3+ substitution on magnetic properties of BaLa 2−x Sm x In 2 O 7 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 Sm 3+ oxides [35][36][37][38][39][40]. The electronic configuration of Sm 3+ (4f 5 ), 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].
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 Sm 3+ [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−x Sm x In 2 O 7 indates. It was shown that the temperature increase leads to a change in the effective magnetic moment of the Sm 3+ 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].

Conclusions
In this work, the effect of isovalent substitution of lanthanum by samarium atoms in BaLn 2 In 2 O 7 indate with the formation of BaLa 2−x Sm x In 2 O 7 solid solution (0 ≤ x ≤ 1.8) and features of this SPS are defined. The ordered distribution of barium and REM atoms in BaLa 2−x Sm x In 2 O 7 SPS has been revealed. It has been shown that the isovalent substitution degree increasing leads to the convergence of two perovskite-like slabs of InO 6 octahedra connected by vertices. This feature rearranges the two-slab perovskite-like structure of BaLa 2−x Sm x In 2 O 7 indates to the structure of threedimensional 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 Sm 3+ BaLa 2−x Sm x In 2 O 7 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−x Sm x In 2 O 7 phases. All this information creates a background for the control of BaLa 2−x Sm x In 2 O 7 structural characteristics by consequent isovalent substitution of atoms in A position of SPS.  Figure 6: Temperature dependences of the magnetic susceptibility of BaLa 2−x Sm x In 2 O 7 .