80 I. A. Zvereva, a* E. A. Tugovab, V. F. Popovac, O. I. Silyukova, I. A. Minicha a Saint Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034, Russian Federation b Saint Petersburg State Technological Institute, 26 Moskovsky pr., St. Petersburg, 190013, Russian Federation c Institute of Silicate Chemistry of RAS, 2 Makarova nab., St. Petersburg, 199034, Russian Federation * irinazvereva@spbu.ru The impact of Nd3+/La3+ substitution on the cation distribution and phase diagram in the La 2 SrAl 2 O 7 -Nd 2 SrAl 2 O 7 system The effect of isovalent cation substitution of lanthanum atoms in the structure of La 2 SrAl 2 O 7 oxide, and phase equilibria (solidus-liquidus curves) in the binary system La 2 SrAl 2 O 7 -Nd 2 SrAl 2 O 7 were studied. It was found that Nd3+ substitution for La3+ has effect on the structure of La 2 SrAl 2 O 7 in respect of the character of cation distribution in the solid solution La 2–x Nd x SrAl 2 O 7 from statistically disordered to the ordered one where strontium cations predominantly occupied the rock-salt layers, as reflected by the solidus-liquidus lines. Keywords: perovskites, transition metal oxides, solid solutions, layered compounds, phase diagram Received: 28.04.2018. Accepted: 09.05.2018. Published: 10.05.2018. © Zvereva I. A., Tugova E. A., Popova V. F., Silyukov O. I., Minich I. A., 2018 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 1. 05 Zvereva I. A., Tugova E. A., Popova V. F., Silyukov O. I., Minich I. A. Chimica Techno Acta. 2018. Vol. 5, No. 1. P. 80–85. ISSN 2409–5613 Introduction The elements that occupy A-sites in the perovskite-related oxides play a sig- nificant role in both crystallographic and physical properties. A well-known fact is related to the heterovalent substitutions of large Ap+ cations which can trigger mixed valence state of B transition elements and induced superconductivity in cuprates or giant magnetoresistance in manganites. It is also generally acknowledged that elec- trical properties of a perovskite-like solid solution depend significantly on the mean size of cations located in the A-site posi- tions, and even more – on the size mis- match between them. The isovalent substitution of Ca2+ for Sr2+ was studied intensively in the super- conducting cuprates La2–xSr(Ca)xCu2O6–δ [1] and in the magnetoresistant manga- nites Ln0.7A0.3MnO3 (A = Ca, Sr) [2]. The crystal chemistry of such cationic substi- tution in the perovskite-like aluminates is quite similar, as exemplified by the re- sults obtained for two structural types of layered aluminates – LaSr(Ca)AlO4 and La2Sr1–xCaxAl2O7 [3, 4]. The most striking result is the evidence for the high destabi- lizing role of Ca2+. It is confirmed by the fact that LaCaAlO4 is thermodynamically unstable, it has demixed to LaAlO3 and 81 CaO instead of complex crystallographic transformation, which is originated in local ordering of Ca2+ and La3+ ions [5, 6]. The substitution of Ca2+ for Sr2+ in La2SrAl2O7 is more crucial for the layered structure, and brings progressive instability when the amount of Ca becomes more than x = 0.5 because of the positional ordering phenomena [4]. To clarify the different rare-earth (RE) cations’ presence in layered perovskite type oxides, it is important to understand their crystal chemistry and stability of the in- tergrowth structure that influences their properties and potential applications. The size of RE cation, as well as partial RE and Sr cation ordering, play a prominent role in the actual magnetotransport properties. This is the reason why it is worthwile to investigate the cation ordering and the sta- bility of structure in the system where the redox processes for the transition metal are eliminated, as in the case of the isostruc- tural aluminates. We herein report on the structure analysis for the series of solid solutions La1–xNdx)2SrAl2O7 and the investigation of solidus-liquidus equilibria in system La2SrAl2O7-Nd2SrAl2O7. Both RE stron- tium aluminates Ln2SrAl2O7 (Ln = La, Nd) belong to the n = 2 member of so-called Ruddlesden-Popper phases (An+1BnO3n+1) [7], in which double perovskite slabs (P2) alternate with the rocksalt layers (RS). The stability of the structure represented as (P2/ RS) intergrowth for the studied aluminates and their solid solutions at high tempera- tuteres are considered in the view of the RE–Sr cation ordering. Experimental Solid solutions (La1–xNdx)2SrAl2O7 were prepared by a solid state reaction of the stoichiometric amounts of RE2O3 (RE  = La, Nd), Al2O3, and SrCO3 according to the following equation: (1 – x)La2O3 + xNd2O3 + SrCO3 + Al2O3   (La1–xNdx)2SrAl2O7 + CO2. The mixtures were preheated at 900 °C in air, then grinded, pelletized and an- nealed at 1450 °C for 100 hours (140 hours for RE = La), and cooled in air. The as-prepared compounds were cha- racterized by the X-ray powder diffraction (XRPD) using a Philips PW3020 diffrac- tometer, in Cu Kα radiation. The structural analysis of the aluminates was made using the results collected in the angular range 2θ = 5–120°, with the step of 0.04° in 2θ and exposure time of 12 s. The structural parameters were refined by the Rietveld method using the FullProf software. The melting points for the samples were determined by the differential ther- mal analysis (DTA) using a VTA-982 high- temperature thermal analyzer in helium atmosphere with heating rate of 10 K/min. Results and discussion The results of X-ray diffraction (XRD) analysis show that the continuous series of solid solutions (La1–xNdx)2SrAl2O7 (0 ≤ х ≤1), crystallizing in the Sr3Ti2O7 struc- tural type, were obtained as single phases. The indexing of the XRPD patterns were performed within the conventional for the P2/RS intergrowth structure tetragonal unit cell (space group (S.G.) I4/mmm). The va- lues of unit cell parameters (Fig. 1) decrease monotonically with the increase of Nd con- tent in accordance with the difference in their ionic radii r(RE3+). Observed devia- tion from the Vegard’s law might indicate 82 the nonrandom distribution of neodymi- um atoms in the Lа2SrAl2O7 structure. It is worth to note that, contrary to the Ca2+  Sr2+substitution, the substitution of Nd3+  La3+ does not limit the homogeneity range of the (La1–xNdx)2SrAl2O7 solid solutions. It was shown earlier [8] that the dis- tribution of Ln3+ and Sr2+atoms over two non-equivalent structural positions, name- ly 12-coordinated AO12 and 9-coordinated AO9, is different in Lа2SrAl2O7 and Nd2SrA- l2O7. The distribution of La 3+ and Sr2+ in Lа2SrAl2O7 appears to be generally random, however, with some preferable La occupa- tion of AO12 polyhedra in the perovskite layers. On the contrary, the visible devia- tion from the random distribution towards to the positional ordering of Nd3+ ions in rock-salt layers is observed in Nd2SrAl2O7. The situation is more complicated in (La1–xNdx)2SrAl2O7 solid solutions where AO12 and AO9 polyhedra are occupied by the three types of ions: La3+, Nd3+ and Sr2+. In the case of the Nd3+  La3+ substitution, the determination of the sites’ occupation by the Rietveld refinement is more difficult, as compared to the case of the Ca2+  Sr2+ substitution, because of the smaller diffe- rence in the number of electrons between La3+ and Nd3+ (three f-electrons). Therefore, the full-profile X-ray diffraction analysis of solid solutions (La1–xNdx)2SrAl2O7 provides only the information about the redistribu- tion of Sr atoms over two sites with the change in the neodymium content. Al- though La3+ and Nd3+ could not be distin- guished by the Rietveld refinement, one can take into account the general tendency obtained for the whole series of Ln2SrAl2O7 oxides (Ln  = La-Ho) [8], which showed the evidence of favorable occupancy of AO9 polyhedra by the atoms with smaller atomic radii. Nevertheless, the structure of (La1–xNdx)2SrAl2O7 solid solutions was refined by means of the Rietveld method. The occupancy values for the non- equivalent sites calculated for the random distribution of cations and obtained from the XRPD results for two solid solutions (La1–xNdx)2SrAl2O7 and parent oxides La2SrAl2O7 and Nd2SrAl2O7 are shown in Table 1. It should be emphasized that the Rietveld refinement gives only the oc- cupancy for Sr atoms. The distribution of La and Nd atoms is undefinable; it could be assumed under consideration that redis- tribution of strontium from the rock-salt layers (AO9 polyhedra) into the perovskite layers (AO12 polyhedra) is compensated specifically by the transfer of Nd to the rock-salt layers. It should be noted that deviation from the random distribution of cations is observed independently of the distribution of RE atoms when neodymium content is х ≥ 0.6. No deviations for the random cation distribution were observed in the concentration range x ≤ 0.5. To summarize the aforementioned re- sults one can conclude that the transition from random distribution of A-site cations (RE3+ and Sr2+) between non-equivalent positions to the ordered one with certain Fig. 1. The unit cell parameters for the (La1–xNdx)2SrAl2O7 solid solutions versus Nd content (x) 83 preferred occupancy of perovskite layers by Sr2+ ions with the Nd content increase takes place in vicinity of x = 0.6. Therefore, the distribution of La3+, Nd3+, Sr2+ cations between two non-equivalent sites (AO9 and AO12) could be presented by the structural diagram shown in Fig. 2. Bearing in mind that the positional ordering when lanthanide atoms prefe- rably located in the rock-salt layers sites stabilizes the layered P2/RS type struc- ture, one can conclude that solid solu- tion (La2–xNdx)2SrAl2O7 should be more stable than lanthanum strontium alumi- nate La2SrAl2O7, but less stable than neo- dymium-containing analog Nd2SrAl2O7. The influence of cation ordering in the continuous series of solid solutions on the phase diagrams, which, in particular, can be observed on a shape of liquidus and soli- dus lines, was the issue of our further ex- perimental study. We applied the ceramic technique followed by the XRPD analysis in order to examine the phase equilibria in the pseudo-binary system La2SrAl2O7– Nd2SrAl2O7 within the entire concentra- tion range. Melting points were detected by means of a differential thermal analysis. The phase diagram (Fig. 3) was con- structed taking into account the particulari- ties of the Ln2SrAl2O7 melting mechanism [9]. It was found that both La2SrAl2O7 and Nd2SrAl2O7 melt incongruently: Ln2SrAl2O7  Liquid + LnAlO3, (Ln = La, Nd). As a result, two types of double-phase fields where liquid phase coexists with the solid, namely, (La2–xNdx)2SrAl2O7) + liquid and (La1–xNdxAlO3) + liquid, appear on the phase diagram. The main feature of phase equilibria in the La2SrAl2O7–Nd2SrAl2O7 system is the existence of the minimum on the solidus line. Considering the phase equi- libria in the pseudo-binary La2SrAl2O7– Nd2SrAl2O7 system in combination with the structural transformations of cation coordination during the formation of the Fig. 2. The structural diagram for the pseudo- binary La2SrAl2O7–Nd2SrAl2O7 system L a+ 3 A O 12 Statistically- disordered distribution La+3, Nd+3, Sr+2 Ordering Sr+2 AO12 0.4 0.6 0.8 La2SrAl2O7 Nd2SrAl2O7 Table 1 The occupancy for A-site cations in the AO12 and AO9 polyhedra in La2SrAl2O7, Nd2SrAl2O7 and (La1–xNdx)2SrAl2O7 solid solutions (x = 0.6 and 0.8) P2/RS AO12 AO9 Lа+3 Nd+3 Sr+2 Lа+3 Nd+3 Sr+2 Ln2SrAl2O7 random 0.67 – 0.33 1.33 – 0.67 calculated 0.73 – 0.27 1.27 – 0.73 (La0.4Nd0.6)2SrAl2O7 random 0.26 0.41 0.33 0.52 0.80 0.67 calculated 0.26 0.36 0.38 0.52 0.86 0.62 (La0.2Nd0.8)2SrAl2O7 random 0.13 0.54 0.33 0.27 1.06 0.67 calculated 0.13 0.47 0.40 0.27 1.13 0.60 Nd2SrAl2O7 random – 0.67 0.33 – 1.33 0.67 calculated – 0.54 0.46 – 1.46 0.54 84 (La2–xNdx)2SrAl2O7 solid solution, it is no- ticeable that the minimum on the solidus line correlates with the disordering-or- dering transition domain for La3+, Nd3+, Sr2+ distribution. The existence of minimum on the melting point plot may be related to the decrease of enthalpy of the solid solutions formation, which actually reflects the de- crease in their stability. Certainly, the stu- died solid solutions cannot be described in terms of regular solution model, since the entropy contribution to the Gibbs energy of solid solutions formation, accompanied by the cation redistribution between the non-equivalent 12-coordinated (cuboc- tahedron AO12) and 9-coordinated (anti- prism AO9) sites, should not be neglected. Conclusions It was shown that Nd3+  La3+ cation substitution in the crystal structure of La2SrAl2O7 allows to obtain continuous series of solid solutions (La1–xNdx)2SrAl2O7 (0 ≤ х  ≤ 1). Introduction of Nd3+ instead of La3+ changes the character of cation dis- tribution in the La2–xNdxSrAl2O7 solid solu- tions from a statistically disordered to the ordered with respect to the strontium cati- ons in the rock-salt layers. 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