Hydrides with the perovskite structure: General bonding and stability considerations and the new representative CaNiH3

The stability and electronic structure of perovskite hydrides ABH₃ were investigated by means of first-principles density functional calculations. Two types of perovskite hydrides are distinguished: (1) When A and B are alkali and alkaline earth metals, the hydrides are ionic compounds with calculated band gaps of around 2 eV and higher. Their stability trend follows basically the concept of Goldschmidt's tolerance factor. (2) When A is one of the heavier alkaline earth metals (Ca, Sr, Ba) and B a transition metal, stable compounds ABH₃ result only when B is from the Fe, Co, or Ni groups. This stability trend is basically determined by effects associated with d band filling of both the transition metal and the hydride. In contrast to group (1) perovskites, the transition metal-containing compounds are metals. The synthesis of CaNiH₃ and its structure determination from CaNiD₃ is reported. This compound is a type (2) perovskite hydride with a fully occupied hydrogen position (CaNiD₃: a=3.551(4) Å, d_Ni-D=1.776(2) Å). Its stability is discussed with respect to transition metal hydrides with complex anions (e.g., Mg₂NiH₄, Na₂PdH₂, Sr₂PdH₄).     

Catalytic properties of manganites in the oxidation of CO and hydrocarbon

Manganites with a spinel structure MMn₂O₄ (M = Co, Cu, Zn, Mo) and M¹ _0.5M² _0.5 Mn₂O₄ (M = Co, Cu, Zn, Mg) have been synthesized and tested in the catalytic oxidation of CO, C₃H₆, and ethylbenzene. The dependence of the catalytic activity of the manganites on the nature of the cation has been established. The spinels containing transition metal ions (Cu, Co) are more active. A relation between catalytic and adsorption properties of manganites has been established. The participation of the lattice oxygen in the oxidation of CO to CO₂ has been found. The mechanism of the oxidation is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 11, pp, 2686–2669. November, 1996.     

Magnesium substitution in Nd0.7Sr0.3MnO3

Magnesium substitution in Nd_0.7Sr_0.3MnO₃ has been studied by neutron powder diffraction. Polycrystalline samples of nominal compositions Nd_0.7Sr_0.3Mn_1−yMg_yO₃ with y=0.0, 0.1, 0.2 and 0.3 were synthesized by the standard solid-state reaction method. Rietveld refinements of the neutron powder diffraction data showed that all samples had distorted perovskite structure of orthorhombic symmetry. Mg initially preferred to substitute for Nd and only at Mg concentration greater than 0.1, a substantial substitution for Mn occurred. Our study also showed that Mg-substitution did not change the crystal structure of Nd_0.7Sr_0.3MnO₃.     

Structure and properties of (1−x)(0.6Pb(Zn1/3Nb2/3)O3-0.4Pb (Mg1/3Nb2/3)O3)-xPbTiO3 thin films with perovskite phase promoted by polyethylene glycol

The preferential formation of a pyrochlore structure is a knotty problem in the preparation of Pb(Zn_1/3Nb_2/3)O₃ (PZN)-based thin film materials and its presence is significantly detrimental to the dielectric and piezoelectric properties. In this study, 40 mol% of PZN was replaced with Pb(Mg_1/3Nb_2/3)O₃ (PMN) for obtaining a perovskite composition around a morphotropic phase boundary (MPB), (1−x)(0.6PZN-0.4PMN)-xPT ((1−x)PZMN-xPT, PT: PbTiO₃) where x = 0.23. The thin films with this composition were prepared with a polyethylene glycol (PEG) modi-fied sol-gel method on LaAlO₃ substrates. The microstructural evolution of the films on heat treatment was examined with X-ray diffraction. With the aid of PEG, the formation of the pyrochlore phase was suppressed and the perovskite phase formed directly from the amorphous gel film. The multilayer films with a thickness around 0.25 μm showed a single perovskite phase without any detectable pyrochlore structure. Microscopic images showed uniform grain size of a few tens of nanometers. The role of the polymer dramatically promoting the perovskite phase was investigated with the aid of X-ray photoelectron spectroscopy and thermal analysis. The dielectric constant of the obtained film was 4160 at 1 kHz. The film demonstrated typical ferroelectric hysteresis loops and exhibited excellent piezoelectric performance.     

A structural study of ternary lanthanide orthoscandate perovskites

Ternary lanthanide scandates (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) have been synthesized at ambient pressure. Their structure has been investigated at room temperature by Rietveld analysis of powder X-ray diffraction data. The Ln-scandates are orthorhombic perovskites, adopting space group Pbnm (? 62), a≈b≈√2a_p, c≈2a_p, Z=4. Heavy lanthanides (Er-Lu), and Y do not form perovskites at ambient conditions. Compositionally driven phase transitions were not observed. The unit-cell parameters decrease with increasing ScO₆ octahedron rotation and atomic number of the Ln cation. In common with lanthanide orthoferrites, the uniform structural evolution is interrupted at the middle-heavy part of the lanthanide sequence. This is probably due to an interplay between: (i) enlargement of the ScO₆ octahedra relative to BO₆ in other perovskites (e.g., FeO₆ in GdFeO₃); (ii) reduction in size of the first coordination sphere of Ln³⁺ coincident with the lanthanide contraction; (iii) coincident expansion of the second coordination sphere due to screening effects of O^I1 on O^I2, and entry of Sc to the lanthanide coordination sphere; (iv) complex mixing between oxygen and lanthanide lanthanide f- and scandium d-orbitals. In the series studied, Ln³⁺ are in eight-fold coordination (tetragonal antiprism), and are considerably displaced from the center of the LnO₈ polyhedron along [001]. Evolution of the crystallochemical characteristics through the Ln orthoscandate series is complex due to both the antipathetic distortions of A- and B-site coordination polyhedra and interaction of the orbitals of oxygen, Ln and Sc. Empirically obtained limits of Goldschmidt and observed ^viiit_o tolerance factors for ternary LnBO₃ compounds adopting the Pbnm structure are 0.795 and 0.841, respectively.     

Structural characterization and physical properties of the system La1.33LixCrxTi2−xO6

New phases which arise from partial substitution of Ti⁴⁺ by Cr³⁺ and Li⁺ of the compound La_2/3TiO₃ have been obtained, giving rise to the series La_1.33Li_xCr_xTi_2−xO₆ (x=0.66, 0.55 and 0.44). These phases adopt a perovskite-type structure as deduced from their structural characterization. Rietveld's analyses of neutron diffraction data show that it is orthorhombic (S.G. Pbnm) with ordered domains. Conductivity has been examined by complex impedance spectroscopy and it increases with increasing lithium and chromium content. These materials behave as mixed conductors with low activation energies. Magnetic susceptibility variation with temperature shows antiferromagnetic interactions at the lowest temperatures.     

The phase diagram and tetragonal superstructures of the rare earth cobaltate phases Ln1−xSrxCoO3-δ (Ln=La, Pr, Nd, Sm, Gd, Y, Ho, Dy, Er, Tm and Yb)

Single phase perovskite-based rare earth cobaltates (Ln_1−xSr_xCoO_3−δ) (Ln=La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Dy³⁺, Y³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺; 0.67?x?0.9) have been synthesized at 1100°C under 1 atmosphere of oxygen. X-ray diffraction of phases containing the larger rare earth ions La³⁺, Pr³⁺ and Nd³⁺ reveals simple cubic structures; however electron diffraction shows orientational twinning of a local, tetragonal (I4/mmm; a_p×a_p×2a_p) superstructure phase. Orientational twinning is also present for Ln_1−xSr_xCoO_3−δ compounds containing rare earth ions smaller than Nd³⁺. These compounds show a modulated intermediate parent with a tetragonal superstructure (I4/mmm; 2a_p×2a_p×4a_p). Thermogravimetric measurements have determined the overall oxygen content, and these phases show mixed valence (3⁺/4⁺) cobalt oxidation states with up to 50% Co(IV). X-ray diffraction data and Rietveld techniques have been used to refine the structures of each of these tetragonal superstructure phases (Ln=Sm³⁺-Yb³⁺). Coupled Ln/Sr and oxygen/vacancy ordering and associated structural relaxation are shown to be responsible for the observed superstructure.     

Ln0.5A0.5MnO3 (Ln=Lanthanide,A= Ca,Sr) Perovskites Exhibiting Remarkable Performance in the Thermochemical Generation of CO and H2 from CO2 and H2O

Perovskite oxides of the Ln_0.5A_0.5MnO₃ (Ln=lanthanide, A=Sr, Ca) family have been investigated for the thermochemical splitting of H₂O and CO₂ to produce H₂ and CO respectively. The amounts of O₂ and CO produced strongly depend on the size of the rare earth ions and alkaline earth ions. The manganite with the smallest rare earth possessing the highest distortion and size disorder as well as the smallest tolerance factor, gives out the maximum amount of O₂, and, hence, the maximum amount of CO. Thus, the best results are found with Y_0.5Sr_0.5MnO₃, which possesses the highest distortion and size disorder. Y_0.5Sr_0.5MnO₃ shows remarkable fuel production activity even at the reduction and oxidation temperatures as low as 1200 °C and 900 °C, respectively.     

Approaching compositional limits of perovskite – type oxides and oxynitrides by synthesis of Mg0.25Ca0.65Y0.1Ti(O,N)3, Ca1–xYxZr(O,N)3 (0.1 ≤ x ≤ 0.4), and Sr1–xLaxZr(O,N)3 (0.1 ≤ x ≤ 0.4)

Partial substitution of cations and anions in perovskite-type materials is a powerful way to tune the desired properties. The systematic variation of the cations size, the partial exchange of O²⁻ for N³⁻ and their effect on the size of the optical band gap and the thermal stability was investigated here. The anionic substitution resulted in the formation of the orthorhombic perovskite-type oxynitrides Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃, Ca_1-xY_xZr(O,N)₃, and Sr_1–xLa_xZr(O,N)₃. A two-step synthesis protocol was applied: i) (nano-crystalline) oxide precursors were synthesized by a Pechini method followed by ii) ammonolysis in flowing NH₃ at T = 773 K (Ti) and T = 1273 K (Zr), respectively. High-temperature synthesis of such oxide precursors by solid–state reaction generally resulted in phase separation of the different A-site cations. Changes of the crystal structures were investigated by Rietveld refinements of the powder XRD data, thermal stability by DSC/TG measurements in oxygen atmosphere, oxygen and nitrogen contents by O/N analysis using hot gas extraction technique, and optical band gaps by photoluminescence spectroscopy. By moving from Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃ via Ca_1–xY_xZr(O,N)₃ to Sr_1–xLa_xZr(O,N)₃, the degree of tilting of the octahedral network is reduced, as observed by an increase in the B–X–B angles caused by the simultaneously increasing effective ionic radius of the A-site cation(s). In general, increasing substitution levels on the A-site (Y³⁺ and La³⁺) are accompanied by an enhanced replacement of O²⁻ by N³⁻. In all three systems, this anionic substitution resulted in a reduction of the optical band gap by approximately 1 eV (Ti) and up to 2.1 eV (Zr) compared to the respective oxides. For Mg_0.25Ca_0.65Y_0.1Ti(O,N)₃ an optical band gap of 2.2 eV was observed, appropriate for a solar water splitting photocatalyst. The Zr-based oxynitrides required a by a factor of 2 higher nitrogen contents to significantly reduce the optical band gap and the measured values of 2.9 eV–3.2 eV are larger compared to the Ti-based oxynitride. Bulk thermal stability was revealed up to T = 881 K. In general, the thermal stability decreased with increasing substitution levels due to an increasing deviation from the ideal anionic composition as demonstrated by O/N analysis.