In the present work protonic forms of layered
Layered perovskite-like oxides are compounds which consist of intergrowths of perovskite and other structures, and they are formed by two-dimensional nanosized perovskite slabs interleaved with cations or cationic structural units. Perovskite-like compounds can exhibit a wide range of important physical and chemical properties, such as superconductivity [
In recent years, there has been a growing interest in using the soft chemistry methods for development of new layered perovskite-like compounds with specified physicochemical properties, as well as design of these materials based on the perovskite structure [
Layered compounds HLnTiO4 and H2Ln2Ti3O10 (Ln = La, Nd) are representatives of the Ruddlesden-Popper layered perovskite-type phases. They are formed by polyanion sheets of transition metal oxides (in case of HLnTiO4 polyanion sheets consist of a single layer of oxygen octahedral [
Protonic forms of transition metal-layered perovskite-like oxides attract great attention due to their interesting and variable properties, in particular the proton conductivity [
Alkaline forms NaLnTiO4 and K2Ln2Ti3O10 (Ln = La, Nd) of a powder were produced by ceramic technique as precursors for further synthesis of protonic forms. Stoichiometric mixtures of Ln2O3 (Ln = La, Nd), TiO2, and sodium or potassium carbonate (40% excess, since it is volatilized at high-temperature synthesis) were taken as initial compounds; all substances were calcined to remove traces of moisture.
In case of NaLnTiO4, following temperature mode has been used: fast heating to 780°C, calcinating at 780°C for 2 hours, fast heating to 900°C, and calcinating at 900°C for 3 hours [
Protonic forms HLnTiO4 (Ln = La, Nd) were obtained by substitution of Na+ ions in NaLnTiO4 (Ln = La, Nd) to H+, according to the following reaction:
Initial alkali metal-containing layered compounds were treated in 0.1 H HCl acid medium, as described in [
XRD analysis (powder X-ray diffraction, CuK
X-ray powder patterns: (a) precursor NaLaTiO4; (b) HLaTiO4; (c) precursor NaNdTiO4; (d) HNdTiO4.
Calculated unit cell parameters of obtained samples (a) NaLaTiO4, tetragonal, P4/nmm:
The degree of substitution of Na+ by H+ is estimated by means of TGA analysis using TG 209 F3 Iris, Netzsch [
The weight loss of the sample results from the sorbed water release (intercalated/adsorbed) and from water formed by decomposition of protonic form HNdTiO4. The content of sodium ions remaining after release of all water substance depends on the degree of substitution of the sample. Dependence between the degree of substitution of
From (
Calculated degrees of substitutes were found to be nearly 100% (±2%) for both HLaTiO4 and HNdTiO4.
SEM images (Zeiss Supra 40VP) show that the protonated samples keep the morphology of their layered Na precursors, where the particles are composed of layered plates that are typical for all layer compounds. The average thickness of the particles is about 300 nm. According to the X-ray microanalysis data, Na reflexes were not observed for all protonated substances.
Cation-deficient perovskites Ln2/3TiO3 (Ln = La, Nd) were obtained for the first time by leaching of Ln3+ ions from HLnTiO4 in acid solution (10x excess of HCl solution (0.1 H) for 7 days):
During the leaching in HCl solution of varying concentrations, it was found that the right product can be obtained only under conditions. Ln2/3TiO3 dissolved at higher concentration and reaction does not have time to pass through at lower concentration. In case of Nd2/3TiO3 pure phase of the product was obtained, while for La2/3TiO3 the impurity of stable HLnTiO4*
X-ray powder patterns: (a) HNdTiO4; (b) HNdTiO4 (4 days, excess of 0.1 H HCl); (c) Nd2/3TiO3 (7 days, excess of 0.1 H HCl); (d) HLaTiO4,; (e) HLaTiO4*
The SEM shows that the initial morphology is not preserved for defect perovskites Ln2/3TiO3 obtained by acid leaching. The resulting phase is characterized by irregularly shaped particles with dimensions less than 100 nm (Figure
SEM images: (a) NaNdTiO4; (b) HNdTiO4; (c) Nd2/3TiO3.
The process of acid leaching of Ln3+ cations was also observed in reaction of a perovskite-oxide K2Ln2Ti3O10 (Ln = La, Nd) with a solution of hydrochloric acid of different concentrations. Initial compound K2Ln2Ti3O10 was treated by 5.5x excess of 2 M hydrochloric acid for 5 days. In the process, K+ ions are replaced by H+, according to the following reaction:
XRD analysis data showed that in case of the initial K2La2Ti3O10 the resulting compound contains a large number of impurity cation-deficient perovskite La2/3TiO3 (Figure
X-ray powder patterns: (a) K2La2Ti3O10 + 10x excess of HCl (0.1 M), 24 h; (b) K2La2Ti3O10 + 5.5x excess of HCl (2 M), 5d (H2La2Ti3O10 (rhombus), La2/3TiO3 (star)); (c) K2La2Ti3O10 + 10x excess of HCl (0.1 M), 5d; (d) K2Nd2Ti3O10 + 10x excess of HCl (0.1 M), 24 h; (i) K2Nd2Ti3O10 + 5.5x excess of HCl (2 M), 5d, (H2Nd2Ti3O10 (rhombus), Nd2/3TiO3 (star)); (f) K2Nd2Ti3O10 + 10x excess of HCl (0.1 M), 5d.
Further, it was proposed to reduce the concentration to 0.1 M solution of 10x excess HCl. This concentration was chosen by analogy with the reaction of a single-layer perovskite NaLnTiO4 with hydrochloric acid solution, where the result is a pure phase HLnTiO4. But in the case of three-layer perovskite oxides the desired product was also obtained with impurities of Ln2/3TiO3 (Figure
The SEM images of samples show that the leaching results in the formation of square holes on the surface of the particles; the initial morphology of the layered oxide particles is preserved.
Treatment of NaLnTiO4 (Ln = La, Nd) by aqueous solution of VOSO4 leads to partial substitution of Na+ by (VO)2+ as described in [
SEM images: (a) NaNdTiO4 + VOSO4; (b) (VO)
Exfoliated HLnTiO4 (Ln = La, Nd) were prepared by treatment of initial HLnTiO4 in a double excess of VOSO4 aqueous solution at 80°C for 3 days. As a result a dark green powder was obtained. The particles of the obtained samples consist of a number of interconnected flat crystallites less than 10 nm width (Figure
The X-ray powder patterns (b) of obtained substances are close to the initial HLnTiO4 (Figure
X-ray diffraction patterns: (a) HNdTiO4; (b) HNdTiO4 exfoliated; (c) HNdTiO4 exfoliated, calcinations at 300°C, 30 min; (d) Nd2□Ti2O7 by pyrolysis at 680°C.
TGA shows that samples lose water during heating from 30 to 700°C, and in both La and Nd cases the mass loss is nearly the same, 12-13% of the total mass of the substance. The porous nature of the particles explains the continuous nature of mass loss on the TGA curves. This is in contrast with HNdTiO4 and HLaTiO4 dehydration from 250°C and 350°C, respectively (Figure
TGA analysis: (a) HNdTiO4; (b) HLaTiO4; (c) (VO)
XRD patterns of dehydrated (VO)
In case of protonic form of three-layer perovskite-like oxide H2Nd2Ti3O10*
XRD analysis showed that formation of new phases was not observed during the exfoliation synthesis; diffraction patterns of the initial hydrated and protonated H2Nd2Ti3O10*
X-ray diffraction patterns: (a) H2Nd2Ti3O10*
TGA curve of H2Nd2Ti3O10 exfoliated by VOSO4 ((VO)H2−2
TGA analysis: (a) H2Nd2Ti3O10*
Surface analysis by SEM demonstrated that the compound in general keeps the particle size of the layered precursor, but there are significant amounts of particles with exfoliated surface layers (Figure
SEM images: (a) H2Nd2Ti3O10; (b) (VO)
Cation-deficient perovskites Ln2/3TiO3 (Ln = La, Nd) were obtained for the first time by leaching of Ln3+ ions from HLnTiO4 in acid solution (10x excess of HCl solution (0.1 H) for 7 days) (Scheme
Structural transformation of HLnTiO4 (Ln = La, Nd):
Exfoliated (VO)
The process of acid leaching of Ln3+ cations was also observed in reaction of a perovskite-type layered oxide K2Ln2Ti3O10 (Ln = La, Nd) with a solution of hydrochloric acid of different concentrations. Resulting compounds contain a large number of impurity cation-deficient perovskites Ln2/3TiO3.
In case of three-layer perovskite-like oxide H2Nd2Ti3O10, the treatment with VOSO4 aqueous solution at 80∘C for 3 days leads to the compound with significant amount of exfoliated surface layers. But in general, it keeps the particle morphology of the precursor.
This work has been supported by the RFBR (Grant 12-03-00761). Powder X-ray study is carried out in the X-ray Diffraction Centre of Saint Petersburg State University. SEM study is carried out in the Interdisciplinary Resource Center for Nanotechnology of Saint Petersburg State University.