BaCa2MgSi2O8:Eu2+ phosphors have been prepared by a conventional high temperature solid state reaction technique. The emission and excitation spectra as well as the luminescence decays were investigated, showing that Eu2+ ions enter both Ba2+ and Ca2+ sites in the host. The structural refinement reveals that about 70% Eu2+ ions occupy Ba2+ sites in samples Ba1−xEuxCa2MgSi2O8 and BaCa2(1−x)Eu2xMgSi2O8. By investigation of the thermal-quenching, the schematic energy levels for Eu2+ in BaCa2MgSi2O8 were proposed. Intense blue emission was observed under 147, 172 and 254 nm excitation in comparison with the commercial blue phosphor BaMgAl10O17:Eu2+ (BAM), demonstrating the potential application of the phosphors in plasma display panels (PDPs) and tri-color fluorescent tubes.

The 12-layer hexagonal perovskite Ba4YMn3O11.5 (Rm, referred to as 12R) was transformed to a 6-layer mixed valent Mn3+/4+ hexagonal perovskite Ba4YMn3O10.7 (P63/mmc, referred to as 6H) via a partial-reduction in a N2 flow. This phase transformation between the 12R and 6H phases is redox-reversible. In contrast with the 12R-type Ba4YMn3O11.5 structure containing c-BaO2.75 and h-BaO3 layers with a (cchh)3 stacking, the 6H-type Ba4YMn3O10.7 structure consists of the cubic (c) BaO2.87 and hexagonal (h) BaO2.33 layers with a (cch)2 stacking, showing a preference of oxygen vacancy distribution in the hexagonal layers over the cubic layers. The h-BaO2.33 layer in the 6H-type Ba4YMn3O10.7 transforms two-thirds of face-sharing octahedral Mn2O9 dimers into edge-sharing pyramidal Mn2O8 units, sharing corners with Y octahedra/pyramids. Impedance measurements suggested that the 6H-type material is insulating with a bulk electrical resistivity of ∼107 Ω cm at 303 K, significantly higher than that for the 12R-type Ba4YMn3O11.5 by ∼4 orders of magnitude.

Phase relationships in the BaO–Ga2O3–Ta2O5 ternary system at 1200 °C were determined. The A6B10O30 tetragonal tungsten bronze (TTB) related solution in the BaO–Ta2O5 subsystem dissolved up to ∼11 mol % Ga2O3, forming a ternary trapezoid-shaped TTB-related solid solution region defined by the BaTa2O6, Ba1.1Ta5O13.6, Ba1.58Ga0.92Ta4.08O13.16, and Ba6GaTa9O30 compositions in the BaO–Ga2O3–Ta2O5 system. Two ternary phases Ba6Ga21TaO40 and eight-layer twinned hexagonal perovskite solid solution Ba8Ga4–xTa4+0.6xO24 were confirmed in the BaO–Ga2O3–Ta2O5 system. Ba6Ga21TaO40 crystallized in a monoclinic cell of a = 15.9130(2) Å, b = 11.7309(1) Å, c = 5.13593(6) Å, β = 107.7893(9)°, and Z = 1 in space group C2/m. The structure of Ba6Ga21TaO40 was solved by the charge flipping method, and it represents a three-dimensional (3D) mixed GaO4 tetrahedral and GaO6/TaO6 octahedral framework, forming mixed 1D 5/6-fold tunnels that accommodate the Ba cations along the c axis. The electrical property of Ba6Ga21TaO40 was characterized by using ac impedance spectroscopy.

An 8-layer B-site deficient twinned hexagonal perovskite Ba8Ga4–xTa4+0.6xO24 has been synthesized and its structure and microwave dielectric properties characterized. This hexagonal perovskite consists of eight close-packed BaO3 layers stacked by a sequence of (ccch)2, where c and h refer to cubic and hexagonal BaO3 layers, respectively. The Ba8Ga4–xTa4+0.6xO24 ceramic materials exhibit composition-independent dielectric permittivity εr ≈ 29, improved Qf value with the B-site vacancy content increase, and tunable temperature coefficient of resonant frequency τf from negative to positive. An optimum microwave dielectric performance was achieved for Ba8Ga0.8Ta5.92O24: Qf ≈ 29 000 GHz and τf ≈ 11 ppm/°C. The factors controlling the microwave dielectric properties are discussed in comparison with 8-layer twinned analogues and related 10-layer twinned hexagonal perovskites based on their structural and property data.Keywords: B-site deficiency; hexagonal perovskite; microwave dielectric ceramic; site ordering;

BaCa2MgSi2O8:Eu2+ phosphors have been prepared by a conventional high temperature solid state reaction technique. The emission and excitation spectra as well as the luminescence decays were investigated, showing that Eu2+ ions enter both Ba2+ and Ca2+ sites in the host. The structural refinement reveals that about 70% Eu2+ ions occupy Ba2+ sites in samples Ba1−xEuxCa2MgSi2O8 and BaCa2(1−x)Eu2xMgSi2O8. By investigation of the thermal-quenching, the schematic energy levels for Eu2+ in BaCa2MgSi2O8 were proposed. Intense blue emission was observed under 147, 172 and 254 nm excitation in comparison with the commercial blue phosphor BaMgAl10O17:Eu2+ (BAM), demonstrating the potential application of the phosphors in plasma display panels (PDPs) and tri-color fluorescent tubes.