Sr₂TMO₃ (TM = Ni, Co) Compounds with 1D TM–O Chains

Abstract

Recently, much attention has been paid to 1D correlated electron systems because some of the compounds, such as Sr 2 CuO 3 and Ca 2 CuO 3 , exhibit large and ultrafast nonlinear optical responses. [1,2] These properties are indispensable for all-optical switching devices to be used in next-generation, high-speed fiber-optic networks. In these compounds, a new concept in physics, called spin-charge separation, has been revealed as an inherent property. [3] These compounds have the crystal structure shown in Figure 1a ( been reported to exist in a 213 structure, and they were only synthesized in polycrystalline form. Therefore, exploring new materials systems with a 213 structure in a single-crystalline form is of great importance.For creating naturally unstable phases, epitaxial stabilization using a suitable template substrate is effective. In fact, we made a single-crystalline thin film of Sr 2 CoO 4 that had never been synthesized even in polycrystalline form using epitaxial stabilization.[6] The Sr 2 CoO 4 has a K 2 NiF 4 structure (Fig. 1a, center), referred to as the 214 structure hereafter, where 2D octahedral TMO 6 sheets are stacked in the direction of the c-axis with a half-phase (1/2, 1/2, 0) displacement along both the a-and b-axes. In the case of a-axis-oriented thin films with a 214 structure grown on the bc surface of an isostructural substrate, the half-phase displacement locks the crystalline structure and the in-plane orientation of the films as b film ʈb substrate and c film ʈc substrate , which is important for growing high-quality single-crystalline films and is known as "atomic graphoepitaxy". [7] The 213 structure can be considered to be a derivative of the 214 structure; it can be constructed by removing alternate oxygen ions in the 2D planes along the a-axis of the 214 structure. Therefore, if the films of a 213 structure are to be grown on the bc surface of a 214 crystal, as shown in Figure 1a, TM-O chains must be aligned to compensate for the half-phase displacement of the substrate surface, resulting in a-axis-oriented films. We have already demonstrated that this technique can be applied to the epitaxial growth of existing 213 compounds such as Sr 2 CuO 3 and Ca 2 CuO 3 on a LaSrAlO 4 substrate with a 214 structure. [8] In order to avoid a multidomain structure consisting of a-and b-axis orientations, the selection of a proper buffer layer (Sr 2 TiO 4 , b-axis lattice constant b 213film ∼ b buffer ) was shown to be essential. Here we report on the synthesis and characterization of single-crystalline thin films of novel Sr 2 NiO 3 and Sr 2 CoO 3 phases with a 1D TM-O chain structure that has never been synthesized even in polycrystalline form. To optimize the film growth conditions, we employed a combinatorial approach. [9] A 1D electronic structure and a sizable nonlinear optical susceptibility were revealed for Sr 2 NiO 3 by polarized absorption spectroscopy and third-harmonic-generation spectroscopy, respectively.Thin films were fabricated on a LaSrAlO 4 (100) substrate by a pulsed-l...

High‐Temperature Structural and Electrical Characterization of Reduced Oxygen‐Deficient Ruddlesden–Popper Nickelates

“…Interestingly, if plotted together, the conductivity values for the entire range of studied (La 1– x Sr x ) 2 NiO 4– δ compositions, reduced and oxidized, seem to follow a common exponential dependence on electron‐hole concentration under isothermal conditions (Figure B); some scattering is probably due to microstructural effects [porosity and microcracking (Table and Figures S1 and S6)]. Although tetragonal‐to‐orthorhombic transition itself does not seem to affect the electrical transport properties to a noticeable extent, progressive ordering of oxygen vacancies in the orthorhombic lattice on further reduction should eventually lead to a transition to 1D electronic transport, as expected for Sr 2 MO 3 (M = Cu, Ni, Co) , …”

Spontaneous phase segregation of Sr 2 NiO 3 and SrNi 2 O 3 during SrNiO 3 heteroepitaxy

“…1B can be assigned to (00l) reflections with an OOP (c) lattice parameter that is a multiple of 6.3 Å, a value much larger than that of either bulk hexagonal SrNiO 3 (a = 5.355 Å and c = 4.86 Å) or the expected cubic perovskite counterpart (a = 3.81 Å) (15,21). This expanded lattice parameter matches that of Sr 2 NiO 3 (21), for which the Sr/Ni ratio is clearly not 1:1. Scanning transmission electron microscopy (STEM) images in Fig.…”

“…The CaCu 2 O 3 structure is similar to that of SrCu 2 O 3 , but the Cu-O sheets in CaCu 2 O 3 are strongly puckered. On the contrary, Sr 2 NiO 3 films have previously been grown on Sr 2 TiO 4 -buffered LaSrAlO 4 (100) by pulsed laser deposition (21), indicating that Sr 2 NiO 3 phase is more stable and easier to form than SrNi 2 O 3 phase. Therefore, when the SNO film thickness exceeds 1 u.c., the large ionic radius ratio R(Sr 2+ )/R(Ni 4+ ) and highly unstable Ni 4+ could promote The diffraction peak for the SrNi 2 O 3 film is also weak, but the peak position is the same as those of the Sr 2 NiO 3 film and the SrNiO 3− film, indicating that all three films have the same c-axis lattice parameter (~12.6 Å).…”