Structures and crystal chemistry of the double perovskites Ba2LnB′O6 (Ln=lanthanide B′=Nb5+ and Ta5+): Part I. Investigation of Ba2LnTaO6 using synchrotron X-ray and neutron powder diffraction

Color Controllable Inorganic Pigments with Ce3+ as a Color Source

The Ba2RE1-xCexTaO6 (RE = La, Gd, Y; 0 ≤ x ≤ 1) pigments were synthesized by a conventional solid-state reaction method to develop environmentally friendly reddish inorganic pigments. The host was the double-perovskite-type Ba2RETaO6 (RE: rare-earth elements), and the color source was Ce3+. All Ba2RE1-xCexTaO6 samples were obtained in a single-phase form as solid solutions. Rietveld refinement analysis of the Ba2RETaO6 (RE = La, Ce, Gd, and Y) samples revealed that the average bond length between RE3+ and O depends on the ionic radius of RE3+, and the shorter the RE-O length, the stronger the crystal field surrounding Ce3+. A photon energy at the maximum 4f-5d absorption of Ce3+ depended on the weighted average ionic radius (rRE) at the RE3+ site (i.e., the crystal field energy around the Ce3+ ions). In response to this phenomenon, the sample color was changed in order to orange, red, pink, and violet with a decrease in the rRE value, and a hue angle (h°) was roughly linearly related to that. For validation of the tendency, we demonstrated the synthesis and characterization of Ba2La0.5-yYyCe0.5TaO6 to obtain a more reddish color. As we exactly expected, a more reddish color was obtained while maintaining a high C value. Furthermore, the h° values for y = 0.1 and 0.2 were in good agreement with our estimation. In light of the above results, by controlling the intensity of the crystal field surrounding Ce3+ and/or the concentration of Ce3+, the optical absorption wavelength and absorption intensity of the Ba2RE1-xCexTaO6 system can be changed to adjust the color.

Effects of Codoping on Site-Dependent Eu3+ Luminescence in Perovskite-Type Calcium Zirconate and Hafnate

The single doping of Eu³⁺ and codoping with Ga³⁺ or La³⁺ into perovskite (ABO₃)-type CaZrO₃ and CaHfO₃ were carried out to investigate the effect of codoping on the photoluminescence (PL) features of Eu³⁺, attempting the site-selective Eu³⁺ doping at either A or B sites. To comprehend the Eu³⁺ PL features, the accurate locations of Eu³⁺ were examined by X-ray absorption near edge structure (XANES) and the proportions of Eu³⁺ located at A or B sites (Eu³⁺(A) or Eu³⁺(B); Eu_Ca^• or Eu_Zr^'/Eu_Hf^') were analyzed. No considerable differences in the XANES spectra and in the proportions of Eu³⁺(A) and Eu³⁺(B) were observed in the single-doped samples. On the other hand, the shape of the XANES spectra and the Eu³⁺ proportion markedly differed in the codoped samples. Ga³⁺ codoping (Ga_Zr^'/Ga_Hf^') achieved Eu³⁺(A) proportions of more than 90%, and La³⁺ codoping (La_Ca^•) resulted in the largest Eu³⁺(B) proportions of approximately 40%. In both PL and PL excitation spectra, the site-dependent spectral features were changed depending on the proportion of Eu³⁺(A) and Eu³⁺(B), especially the PL peaks at 595 nm were intensified most in the La³⁺ codoped samples. Consequently, the site-dependent Eu³⁺ PL features in CZO and CHO were found to become more conspicuous by the effect of Ga³⁺ or La³⁺ codoping. The results also revealed that the codoped ions work not only to compensate for the charge of aliovalent Eu³⁺ but also to drive Eu³⁺ to the intentional doping sites.

Tunable Magnetic Properties in Sr2FeReO6 Double-Perovskite

Double-perovskite oxides have attracted recent attention due to their attractive functionalities and application potential. In this paper, we demonstrate the effect of dual controls, i.e., the deposition pressure of oxygen (P_O2) and lattice mismatch (ε), on tuning magnetic properties in epitaxial double-perovskite Sr₂FeReO₆ films. In a nearly lattice matched Sr₂FeReO₆/SrTiO₃ film, the ferrimagnetic-to-paramagnetic phase transition occurs when P_O2 is reduced to 30 mTorr, probably due to the formation of Re⁴⁺ ions that replace the stoichiometric Re⁵⁺ to cause disorders of B-site ions. On the other hand, a large compressive strain or tensile strain shifts this critical P_O2 to below 1 mTorr or above 40 mTorr, respectively. The observations can be attributed to the modulation of B-site ordering by epitaxial strain through affecting elemental valence. Our results provide a feasible way to expand the functional tunability of magnetic double-perovskite oxides that hold great promise for spintronic devices.

Observation of Anisotropic Thermal Expansion and the Jahn-Teller Effect in Double Perovskites Sr2-xLaxCoNbO6 Using Neutron Diffraction

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr_2-xLa_xCoNbO₆ (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P2₁/n) is observed between x = 0.4 and 0.6 samples. Interestingly, the temperature evolution of the unit cell parameters follows the Grüneisen approximation, and the analysis suggests an isotropic thermal expansion in the case of the x = 0.4 sample, whereas the x = 0.6 sample shows the anisotropy where the thermal expansion along the c-axis significantly deviates from the Grüneisen function. We observe the z-out Jahn-Teller distortion in the CoO₆ and consequently z-in distortion in the adjacent NbO₆ octahedra. With an increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter Δ around the B-site atoms.

Site-Selective Doping and Site-Sensitive Photoluminescence of Eu3+ and Tb3+ in Perovskite-Type LaLuO3

Eu³⁺ and Tb³⁺ ions were site-selectively doped into LaLuO₃ with the orthorhombic perovskite-type structure (ABO₃), and their luminescence properties were examined considering the doping sites (A or B sites). The X-ray diffraction analysis revealed the expansion or contraction of the unit cell volumes of the materials depending on the doping sites. The spectra of X-ray absorption near edge structure for the Eu and Tb L_III edge exhibited different shapes for the ions at A and B sites, confirming the site-selective doping of Eu³⁺ and Tb³⁺ in LaLuO₃. The photoluminescence (PL) and PL excitation (PLE) spectra of the materials also showed some differences caused by the doping sites. The intensities of the Eu³⁺ PL peaks derived from the ⁵D₀-⁷F₁ transitions and those from the ⁵D₀-⁷F₂ transitions were markedly different between Eu³⁺ at A sites and those at B sites because of the different site symmetries. The splitting of the intense Tb³⁺ PL peaks originating from the ⁵D₄-⁷F₅ transitions and the absence of PL peaks from ⁵D₃-⁷F_J transitions were found only for Tb³⁺ at B sites because of the strong crystal field at B sites. In addition to the PL spectra, the positions of PLE peaks originating from charge transfer transitions in Eu³⁺ and the 4f-5d transitions in Tb³⁺ depended on the doping sites. The successful site-selective doping enabled us to clarify the site-sensitive luminescence properties of Eu³⁺ and Tb³⁺ doped in the perovskite-type LaLuO₃.

Ultralow thermal conductivity in highly anion-defective aluminates

Ultralow thermal conductivity (1.1 W/m.K, 1000 degrees C) in anion-deficient Ba2RAlO5 (R=Dy, Er, Yb) compounds was reported. The low thermal conductivity was then analyzed by kinetic theory. The highly defective structure of Ba2RAlO5 results in weak atomic bond strength and low sound speeds, and phonon scattering by large concentration of oxygen vacancies reduces the phonon mean free path to the order of interatomic distance. Ba2DyAlO5 exhibits the shortest phonon mean free path and lowest thermal conductivity among the three compositions investigated, which can be attributed to additional phonon scattering by DyO6 octahedron tilting as a result of a low tolerance factor. The Ba2RAlO5 (R=Dy, Er, Yb) compounds have shown great potential in high-temperature thermal insulation applications, particularly as a thermal barrier coating material.