ABSTRACT
Innovative materials for phosphor-converted white light-emitting diodes (pc-LEDs) are much sought after due to the huge potential of the LED technology to reduce energy consumption worldwide. One of the main levers for further improvements are the conversion phosphors. The system Ca1-x Srx AlSiN3 :Eu2+ currently provides one of the most important red-emitting phosphors for pc-LEDs. We report the discovery of the new polymorph ß-Ca1-x Srx AlSiN3 :Eu2+ which allows for significant improvements to LED efficacies. It crystallizes in the orthorhombic space group Pbcn with lattice parameters a=982.43(10)â pm, b=575.2(1)â pm and c=516.12(5)â pm. Compared to α-Ca1-x Srx AlSiN3 :Eu2+ , its emission shows a significantly reduced spectral full-width at half maximum (FWHM). With that, we demonstrated 3 % efficacy increase for white light-emitting pc-LEDs. The new polymorph can easily be industrialised, because the synthesis works on the same equipment as α-Ca1-x Srx AlSiN3 :Eu2+ .
ABSTRACT
A new structure type of nitridosilicates with an interrupted framework has been identified for M(7)Si(6)N(15) with M = La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnace at temperatures between 1550-1625 degrees C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce(7)Si(6)N(15) has been determined by using single-crystal X-ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P1, no. 2; a = 13.009(3), b = 25.483(5), c = 25.508(10) A; alpha = 117.35(3), beta = 99.59(3), gamma = 99.63(3) degrees; V = 7114(2) A(3); Z = 18; R1 = 0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R3, no. 148) have also been observed (a = 43.420(6), c = 6.506(2) A; V = 10 623(3) A(3); Z = 27; R1 = 0.0309). Pr(7)Si(6)N(15) (3) and La(7)Si(6)N(15) (4) are isostructural with 1 as evidenced by twinned single-crystal data for 3 (P1, no. 2; a = 12.966(3), b = 25.449(10), c = 25.459(10) A; alpha = 117.28(3), beta = 99.70(4), gamma = 99.60(4) degrees; V = 7068(4) A(3); Z = 18; R1 = 0.0526) and powder diffraction data for 4 (P1, no. 2; a = 13.109(9), b = 25.606(18), c = 25.609(18) A; V = 7223(12) A(3); Z = 18; R(P) = 0.0194; R(F)(2) = 0.0936). The crystal structure of M(7)Si(6)N(15) (M = La, Ce, Pr) is built up exclusively of corner-sharing tetrahedrons that appear as Q(2)-, Q(3)-, and Q(4)-type tetrahedrons forming different ring sizes within a less condensed three-dimensional network. Among the characteristic structural motifs are saw-blade-shaped 12-rings and finite chains consisting of four corner-sharing SiN(4) tetrahedrons. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce(7)Si(6)N(15) show paramagnetic behavior with an experimental magnetic moment of 2.29 mu(B) per Ce, thereby corroborating the existence of Ce(3+).
ABSTRACT
The mixed valence europium nitridosilicate Eu(2)SiN(3) has been synthesized at 900 degrees 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 Mössbauer 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 mu(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 approximately 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+).
