Tunable magnetic and electrical behaviors in perovskite oxides by oxygen octahedral tilting

REVIEWS SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 20 April 2015 | doi: 10.1007/s40843-015-0047-0 Sci China Mater 2015, 58: 302–312 Tunable magnetic and electrical behaviors in perovskite oxides by oxygen octahedral tilting Ya Gao1, Jianjun Wang1,2, Liang Wu1, Shanyong Bao1, Yang Shen1, Yuanhua Lin1* and Cewen Nan1* The oxides with perovskite structure possess abundant physical properties, such as magnetism, dielectricity, photoelectricity, ferroelectricity, etc. The oxygen ions in the perovskite unit cell constitute an octahedral distribution. The deformation or tilting of the special oxygen octahedra structure leads to new performances or properties change. Here, we give a review of the relationship between magnetic and electrical behaviors and oxygen octahedral tilting in several typical perovskite oxides. An understanding of how to tune these properties by controlling the tilting during the sample growth can more effectively guide the design of new structures for high performance and inspiring their potential applications. INTRODUCTION The ABO3 type oxides have the atoms arranged into the so-called perovskite structure. One perovskite unit cell involves six oxygen atoms, which occupy the face-centered sites of the face-centered cubic (FCC) structure, forming an oxygen octahedron, and the B-site cations infill the central vacancy, as shown in Fig. 1a. In a centrosymmetrical perovskite such as SrTiO3 [1–3] with space group of Pm3m and lattice parameter of 3.905 Å, the strontium ions are located at the corners of the cubic unit cell, and the titanium ion is in the center, surrounding by oxygen ions. These a b TiO6 octahedra are perfectly piled up in three dimensions, with 90° angles and six equal Ti−O bonds at a length of 1.952 Å, as shown in Fig. 1b. For the non-ideal cases, the atom arrangement has diverse possible distortions, including: 1) deformation of the oxygen octahedra arising from Jahn-Teller effect, which changes the cubic symmetry and results in metal-insulator transitions in some oxides, or the arrangement of the spin in B-site atoms and then the change of magnetic properties [4]; 2) displacement of the center cation inside the oxygen octahedra giving rise to ferroelectricity, which is the situation of BaTiO3 [5,6] and Pb(Zr1−xTix)O3 [7]; 3) in a wide range, oxygen octahedral tilting or rotation which can define the symmetry of the material [8]. One of the structural distortions or the combination among them (Fig. 1c) can form a unique perovskite structure [9]. The structural distortions are related to diverse functionalities such as ferroelectricity [10–12], dielectricity [13,14], electroconductivity [15], thermal conductivity [16], superconductivity [17], catalysis oxidation [18], photoelectricity [19], magnetism [20,21], and multiferroicity [22,23]. As early as 1972, Glazer [8,24] defined the classification of the oxygen octahedral tilting in perovskites, which was called the Glazer notation: albmcn, where a, b and c reprec A B O Figure 1 (a) Schematic of an ABO3 perovskite unit cell and its oxygen octahedron (orange). (b) Ideally arranged unit cell and octahedral configuration. (c) Possible distortions for non-ideal structures: Jahn-Teller elongation, center cation displacement, and oxygen octahedral tilting. Reproduced with permission from Ref. [9] (Copyright 2010, American Physical Society). 1 School of Materials Science and Engineering, and State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China 2 Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA * Corresponding authors (emails: (Lin Y); (Nan C)) 302 April 2015 | Vol.58 No.4 © Science China Press and Springer-Verlag Berlin Heidelberg 2015 REVIEWS SCIENCE CHINA Materials sent the tilting magnitudes around the [100], [010] and [001] axes of the cubic or pseudo-cubic unit cell, while the superscripts l, m, and n are +, − or 0, expressing the tilting angle of the neighbor octahedron tilts in the same (+,“inphase” type) or opposite (−, “anti-phase type”) sense, or no tilts (0). The origin of the oxygen octahedron tilting is reflected by the deviation of the B−O−B bond which can be described by two quantitative definitions. The first is Goldschmidt Tolerance Factor [25]: rA  rO t 2(rB  rO ) , (1) where rA, rB and rO are radii of A, B cations and O anions, respectively. The second is the Intersection Angle of the two bonds: θ =  (2) For a centrosymmetrical perovskite wherein the A cation matches in size with the O anion to form cubic closepacked layers and the B cation matches the size of the interstitial sites of the BO6 octahedron, the tolerance factor (t) is 1. However, once a distortion happens, the ideal packing will be broken, and t will deviates from 1.0 with the deviation reflecting how far the ionic sizes can move and still be “tolerated” by the perovskite structure. When the deviation of t from 1.0 is small, e.g., −0.05 ≤ t−1.0 ≤ 0.04, the crystal structures were found to preserve cubic symmetry [26,27]. Whereas, if the deviation is positive and relatively large, for example in Ba5Ta4O5 [28], the B−O bonds are tensed while A−O bonds are compressed, and the B−O−B bond angle θ remains 180°, leading to hexagonal atomic stacking accompanied by ferroic properties. Likewise, if the deviation is negative and relatively large, the A−O bonds are tensed while the B−O bonds are compressed, resulting in the tilting of the oxygen octahedra and the bent of the B− O−B bonds. Such distortion gives rise to tetragonal stacking (rotate around [001] axis, I4/mcm or P4/mbm systems), rhombohedral stacking (rotate around [111] axis, R3–c or Im3– systems), or orthorhombic stacking (rotate around [110] or [001] axis, Pbnm or Pnma) [29], and the average bond angle θ continues to decrease as the symmetry change from tetragonal to rhombohedral to orthorhombic. Therefore the tolerance factor and the oxygen octahedral tilting are criteria for the structure symmetry. The tilting of oxygen octahedra is highly related to the tunable properties of the perovskite oxides. In this review, we use four prototypes as examples to present the relationship between the oxygen octahedral tilting and the magnetic and electric properties of the perovskite ABO3 oxides. MAJOR TECHNIQUES TO OBSERVE OXYGEN OCTAHEDRAL TILTING The magnitude of the oxygen octahedron tilting can be evaluated from both theoretical calculations and experiments. Theoretically, calculation methods [30–32] can work from atom arrangements and predict the connection between the oxygen octahedral tilting and the properties. Fig. 2a gives an example of a complete new family of stable phases in multiferroic BiFeO3 (BFO) and related compounds [33]. Besides the a0a0c+ (c axis in-phase type, namely pp or mm) and a0a0c− (c axis anti-phase type, namely pm) tilting patterns, it is also possible (...truncated)