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The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ~33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations. [ABSTRACT FROM AUTHOR]

The dominant intensity of parity-forbidden intra- 4f transitions of europium(III) over O → Eu charge-transfer band (CTB) intensity is against common perceptions, yet this trend is observed in many germanate hosts and has not been rationalized so far. In search of a plausible explanation for this unusual trend, present work reports an experimental and theoretical investigations in conjunction on two sibling germanate host, namely, Y 2 GeO 5 and Y 2 Ge 2 O 7 having dopant Eu 3+ in their respective YO 7 polyhedra. Whereas for Y 2 GeO 5 :Eu 3+ , the CTB is more intense than the intra- 4f transitions in the excitation spectrum, in the case of Y 2 Ge 2 O 7 :Eu 3+ , the relative intensities of CTB and intra- 4f transitions are reversed. Comparative structural analysis reveals that Eu 3+ present in YO 7 of Y 2 GeO 5 has a greater number of tetra-coordinated oxygen (O tetra ) and yttrium atom as first and second neighbors, respectively (Eu 3+ -O tetra -Y 3+ linkages). Conversely, in Y 2 Ge 2 O 7 host, the Eu 3+ ion mostly has tricoordinated oxygen (O tri ) as its nearest neighbor and germanium ions next to O tri (Eu 3+ -O tri -Ge 4+ linkage). Theoretical calculations reveal that while Y 2 GeO 5 :Eu has O tetra (4Y) dominating at the Fermi level and the 4f state of Eu 3+ remains inert toward mixing, in Y 2 Ge 2 O 7 :Eu, the Fermi level has major contribution from O tri (2Y + 1Ge) with significant mixing with 4f states of Eu. The dominant control of Eu 3+ -O tri -Ge 4+ linkages in geometrical and electronic structure of Y 2 Ge 2 O 7 :Eu owing to the GeO 4 surrounding has been attributed to relative poor intensity of O → Eu CTB. Siege of Eu 3+ by GeO 4 and subsequent occurrence of Eu 3+ -O tri -Ge 4+ linkages play a dual role: First, it induces electronic rigidity to hinder excitation of electron at bridging (O tri ) oxygen by highly charged small Ge 4+ cation; second, the covalent character in Eu-O bond is achieved by intermixing of Eu's 4f and O tri 2p orbital which facilitates relaxing of the parity-selection rule thus enhancing the probability of intra-4f transitions. The inferences drawn remain valid when extrapolated to other inorganic oxides having EuO x polyhedra surrounded by covalent units like PO 4 , SiO 4 , etc. and have a prevailing number of low-coordinated oxygen atoms and highly charged small cation in the first and second coordination shells, respectively. The optical basicity concept is also found to endorse our explanation. These remarkable generic inferences will pave the rational way for designing efficient phosphors for solid-state lighting.

The present work involves the synthesis of a series of Sm(2-x)Dy(x)Zr(2)O(7) compounds (0.0 ≤ x ≤ 2.0) by a controlled gel combustion process. The powders were thoroughly analyzed by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, and diffuse-reflectance UV-visible spectroscopy. The powder XRD studies revealed the system to be single-phasic throughout with retention of pyrochlore-type ordering until 40 mol % of Dy(3+), beyond which the pyrochlore lattice gives way to the defect fluorite structure. Interestingly, Raman spectroscopic studies (as against XRD studies) showed retention of pyrochlore-type ordering throughout the homogeneity range of the compositions studied. This is perhaps the first study that reports retention of a weak pyrochlore-type superstructure in the Dy(2)Zr(2)O(7) system, which was otherwise known to crystallize in the defect fluorite system. The ionic conductivity measurements showed an increase in the activation energy (E(a)) with an increase in the mole percent of Dy(3+) owing to the decreased mobility with an increase in the degree of disorder. The system possesses a tunable band gap with varying amounts of Dy(3+). First-principles calculations were performed to support a decrease in the band gap of the doped system with an increase in the Dy(3+) content. The potential as photocatalysts of some of these compositions was explored, and they exhibited high photocatalytic activity for degradation of xylenol orange, with t(1/2) increasing from pure Sm(2)Zr(2)O(7) to pure Dy(2)Zr(2)O(7).