The phase transitions of the strongly correlated tetrahedral V 4 -cluster compound GeV 4 S 8 have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature ͑space group F43m͒ and transforms to orthorhombic ͑space group Imm2͒ at T S = 30 K. A Jahn-Teller distortion reduces the symmetry of the V 4 -cluster from 43m to mm2. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at T N . Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

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Abstract:The mixed valence europium nitridosilicate Eu 2 SiN 3 has been synthesized at 900°C in welded tantalum ampules starting from europium and silicon diimide Si(NH) 2 in a lithium flux. The structure of the black material has been determined by single-crystal X-ray diffraction analysis (Cmca (no. 64), a ) 542.3(11) pm, b ) 1061.0(2) pm, c ) 1162.9(2) pm, Z ) 8, 767 independent reflections, 37 parameters, R1 ) 0.017, wR2 ) 0.032). Eu 2 SiN 3 is a chain-type silicate comprising one-dimensional infinite nonbranched zweier chains of corner-sharing SiN 4 tetrahedra running parallel [100] with a maximum stretching factor f s ) 1.0. The compound is isostructural with Ca 2 PN 3 and Rb 2 TiO 3 , and it represents the first example of a nonbranched chain silicate in the class of nitridosilicates. There are two crystallographically distinct europium sites (at two different Wyckoff positions 8f) being occupied with Eu 2+ and Eu 3+ , respectively. 151 Eu Mö ssbauer spectroscopy of Eu 2 SiN 3 differentiates unequivocally these two europium atoms and confirms their equiatomic multiplicity, showing static mixed valence with a constant ratio of the Eu 2+ and Eu 3+ signals over the whole temperature range. The Eu 2+ site shows magnetic hyperfine field splitting at 4.2 K. Magnetic susceptibility measurements exhibit Curie-Weiss behavior above 24 K with an effective magnetic moment of 7.5 µ B /f.u. and a small contribution of Eu 3+ , in accordance with Eu 2+ and Eu 3+ in equiatomic ratio. Ferromagnetic ordering at unusually high temperature is detected at T C ) 24 K. DFT calculations of Eu 2 SiN 3 reveal a band gap of ∼0.2 eV, which is in agreement with the black color of the compound. Both DFT calculations and lattice energetic calculations (MAPLE) corroborate the assignment of two crystallographically independent Eu sites to Eu 2+ and Eu 3+ .

850 1090A uniform array of gallium nitride core-shell microrod (MR) lightemitting diode (LED) structures was grown by metalorganic vapor phase epitaxy. Defects and the quantum well (QW) luminescence in an individual rod were investigated by scanning tunneling electron microscopy (STEM) and STEM cathodoluminescence. Luminescence with different wavelength was detected from the quantum wells on the semipolar tip facets and the nonpolar sidewalls of the MRs. Furthermore, the MR array is processed into LED chips. The electro-optical characteristics of the devices are analyzed. Two separate emission bands are distinguished, which are attributed to the QWs on the semipolar tip facets and the nonpolar sidewalls, respectively. To obtain white LEDs, micrograin phosphors were developed which fit in between individual MRs. By using electrophoretic particle deposition, these phosphors are deposited onto the MR LED chips. Color coordinates, color temperature, and device efficiency are evaluated.Blue (top) and phosphor-converted white (bottom) microrod LEDs on 4 00 wafer.

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