A “Chimie Douce” Sythesis of Perovskite-Type SrTa₂O₆ and SrTa_2-xNb_xO₆¹

Abstract

Chimie douce", or soft chemistry, represents a simple and useful route to metastable solid-state compounds. These terms have historically been used to describe several types of low-temperature solid-solid transformations such as intercalation/deintercalation, ion exchange, hydrolysis, and redox reactions. 2 The common feature of soft chemical reactions is that they produce metastable compounds that are structurally related to the parent solid, by preserving the atomic connectivity of small building blocks and/or extended structural elements. Some interesting examples of unusual phases made by soft chemistry include ReO 3like MoO 3 , 3 Ti 2 Nb 2 O 9 , 4 hexagonal WO 3 , 5 TiO 2 -B, 6 VS 2 , 7 and layered double hydroxides. 8 In general, it is not possible to prepare these compounds using high-temperature routes. Some of these low-temperature compounds have special properties, such as reversible intercalation, photoconductivity, or catalytic activity, which are not found in high-temperature, stable phases of the same composition.We report in this paper a chimie douce reaction that converts a layered perovskite phase into a metastable three-dimensional perovskite. The topochemical transformation is illustrated in Figure 1. In this series of reactions, the lamellar compound K 2 SrTa 2 O 7 and isostructural K 2 SrTa 2-x Nb x O 7 (x ) 0.2, 0.4) are first ionexchanged to the corresponding acid forms and then topochemically dehydrated to yield metastable perovskite phases SrTa 2 O 6 and SrTa 2-x Nb x O 6 . The process is similar to that previously reported for ion-exchange and condensation of K 2 Ln 2 Ti 3 O 10 . 10 Further heating causes the transformation to the structurally unrelated tetragonal tungsten bronze phases of the same composition. This transformation is interesting because K 2 -SrTa 2 O 7 is a member of a large class of structurally related compounds 9 (the Ruddlesden-Popper phases,

Synthesis, Proton Exchange, and Topochemical Dehydration of New Ruddlesden–Popper Tantalates and Titanotantalates

“…The cubic perovskite peaks are broad, indicating some disorder during the topochemical collapse. Peak broadening has been observed previously in similar systems (16) Fig. 5c, adopts tetragonal symmetry, which is lower symmetry than in the other topochemically synthesized perovskites that have larger A-site cations.…”

“…While phases such as K La Ti O are easily synthesized by reacting the constituent oxides and carbonates at 11003C for one day (7), synthesizing other phases requires some modi"ed approaches. For example, the n"2 phase K SrTa O has an ABO impurity phase (16), which can be minimized using KOH as the alkali source (20). The n"1 phase NaLaTiO is unstable relative to the n"3 phase Na La Ti O , so a short 30-min reaction time isolates the desired kinetic phase (22).…”

Observation of octahedral tilted in Sr0.5TaO3 prepared by nanosheet processing: An EXAFS study

“…The lattice parameter a was 0.393 nm for Sr 0.5 TaO 3 and 0.392 nm for RE-Sr 0.5 TaO 3 , as listed in Table 1. These values of the lattice parameter a agree with the values of 0.393 nm for Sr 0.5 Ta 0.8 Nb 0.2 O 3 studied by Ollivier and Mallouk [37] and 0.392 nm for Sr 0.5 TaO 3 studied by Bhuvanesh et al [38]. panel.…”

Novel protonated and hydrated Ruddlesden–Popper phases, HxNa1−xLaTiO4·yH2O, formed by ion-exchange/intercalation reaction

“…These families have attracted considerable attention because of their interesting properties such as superconductivity [6], colossal magnetoresistance [7], etc. In recent years, the intensive study on the modification of the layered perovskites to improve their properties has been performed by softchemical reactions such as ion-exchange [8][9][10], intercalation [11], exfoliation [11], topochemical dehydration [12,13], and reductive transformation to higher members [14,15].…”