The effects of octahedral tilting of RbANb 2 O 7 (A = Bi, Nd) compounds was studied using density-functional theory. In this compound, the structural phase transition was correlated with two octahedral tilting modes (a − a − c 0 tilting and a 0 a 0 c + tilting), and the magnitude of the octahedral tilting mode was analyzed in the optimized structure. The theoretical results correlated well with the recent experimental results on the ferroelectricity of RbBiNb 2 O 7 . The hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes and off-center displacement mode was analyzed by group theory and symmetry mode analysis. The detailed relationship of the tilting modes to the structural phase transition and the detailed physical properties of ferroelectricity are also presented.

The octahedral tilting and ferroelectric-like structural transition of LiOsO 3 metallic perovskite [Y. Shi et al., Nat. Mater. 12, 1024(2013] is studied using first-principles density-functional theory. In LiOsO 3 , the a − a − a − octahedral tilting mode is responsible for cubic (F m3m) to rhombohedral structure (R3c), which is stable phase at room temperature, and at low temperature, noncentrosymmetric transition to R3c rhombohedral transition is realized as a result of zone center phonon softening. By using density-functional calculation and phonon calculation, the phase transition behavior of LiOsO 3 can be fully understood. We also present electronic structure change and Fermi surface change due to electron lattice coupling effect. The change of carrier density of state across phase transition is associated with experimental temperature dependence of resistivity, heat capacity, and susceptibility.Recently, there has been considerable interest in octahedral tilting transition and noncentrosymmetric transition in perovskite oxides not only for their technological importance as new ferroelectric materials and multistage memory devices but also from a fundamental point of view [1][2][3][4]. In this pursuit, first-principles density-functional theory has played a key role in explaining new physical states as well as predicting new realizations of physical states of materials [5][6][7]. LiOsO 3 is one of the important and unique additions to a new oxide material with pervoskite structure. According to a recent report from Shi et al. [8], LiOsO 3 is a perovskite oxide material with metallic properties. Interestingly, around 140 K, a centrosymmetric (R3c) phase to noncentrosymmetric (R3c) phase transition is reported. They also reported an anomaly in temperature dependence of heat capacity, susceptibility, and resistivity and indicate that such a structural phase transition is equivalent to the ferroelectric transition of LiNbO 3 materials.In both centrosymmetric (R3c) and non-centrosymmetric (R3c) phases, the octahedral tilting is one of the important ingredients [9][10][11][12]. The three-dimensional network of OsO 6 octaheral tilting in LiOsO 3 can be easily characterized by Glazer notation [9]. Glazer notation describes the octahedral tilting using the symbol a # b # c # , in which the literals refer to tilt around the [100], [010], and [001] directions of the cubic perovskite, and the superscript # takes the value 0, +, or − to indicate no tilt or tilts of successive octahedra in the same or opposite sense. The rhombohedral (R3c and R3c) structure of LiOsO 3 can be classified by a − a − a − tilting [8,9]. This tilting of LiOsO 3 is basically the same as LiNbO 3 [13-20] and originates from the rigidity of the OsO 6 octahedron and the large ionic size difference of the Li and Os ions.In addition to octahedral tilting, the zone center soft mode is the main source of the centrosymmetric (R3c) and noncentrosymmetric (R3c) phases. In LiNbO 3 , the phase transition from a centrosymmetric (R3c) to a noncentrosymmetric...

The effect of octahedral tilting of ASnO 3 (A = Ca, Sr, Ba) parent compound and bi-color ASnO 3 /BSnO 3 superlattice (A, B = Ca, Sr, Ba) is predicted from density-functional theory. In ASnO 3 parent compound, the structural phase transition as a function of A-site cation size is correlated with magnitude of the two octahedral tilting modes (aac 0 tilting and a 0 a 0 c + tilting).The magnitude of octahedral tilting modes in the superlattices is analyzed quantitatively and is associated with that of constituent parent materials. ASnO 3 /BSnO 3 superlattices show the hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes (aac 0 tilting and a 0 a 0 c + tilting), which are also responsible for the structural phase transition from the tetragonal phase to the orthorhombic phase. Ferroelectricity due to A-site mirror symmetry breaking is secondary order parameter for the orthorhombic phase transition in the bi-color superlattice and is related with Γ 5-symmetry mode. The coupling between tilting modes and ferroelectric mode in the bi-color superlattice of ASnO 3 /BSnO 3 is analyzed by group theory and symmetry mode analysis. PACS numbers: 71.15.Mb, 77.55.Px, 61.50.Ah, 63.20.D-, 77.84.Bw *Electronic Address: boggikim@pusan.ac.krRecently, there has been considerable interest in transparent conducting oxide (TCO) which exhibit optical transparency with high electrical conductivity not only for technological importance as touch screen and dye-synthesized solar cell but also scientific importance [1][2][3][4]. ASnO 3 is one of potential candidates for next generation TCO materials with perovskite structure [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In this material, Sn ion forms an octahedron with 6 neighboring oxygen atoms and SnO 6 octahedral are linked in three dimensional networks with corner sharing oxygen. ASnO 3 shows various structural forms depending on the size of A site atoms also depending on the external parameters, such as temperature and pressure.The most common structure of ASnO 3 is orthorhombic phase, in which octahedral tilting is important ingredient [23][24][25][26]. Three dimensional network of octahedral tilting in ASnO 3 can be characterized by so called Glazer notation [23]. The Glazer notation is describes the octahedral tilting using symbol a # b # c # , in which the literals refer to tilt around the [100], [010], and [001] directions of the cubic perovskite, and the superscript # takes the value 0, +, or -to indicate no tilt or tilts of successive octahedral in the same or opposite sense. The orthorhombic structure of ASnO 3 can be classified by aac + tilting [6-8, 23]. This tilting is basically originated from the interatomic forces and the size of A site atoms. When A site atoms are larger for BaSnO 3 , the tilting does not occur and the BaSnO 3 belongs to cubic perovskite [5, 22]. Whereas smaller size of A site atoms for Sr, Ca, the tilting occurs to maintain the environment of the B cation unchanged. The physical properties are clo...

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