Neodymium europium oxynitridosilicates of β-K2SO4 type: Structural, magnetic and red luminescence properties

Rare earth activated nitridosilicates have important applications as phosphor components in white light emitting diodes. (Oxy)nitridosilicates with β-K2SO4 structure doped with Eu2+ are luminescent materials with emission wavelengths ranging from green to red under excitation with blue-UV light. Here we report the synthesis of isostructural NdEuSiO3N and the new oxynitridosilicate NdSrSiO3N by solid state reaction between Nd2O3, NdN, Eu2O3 or SrO and Si3N4 under N2/H2 gas at temperatures of 1300 ​°C and 1500 ​°C respectively. These compounds are the limiting compositions of the solid solution NdSr1-xEuxO3N crystallizing in the space group Pmnb with cell parameters a ​= ​5.62293(1), b ​= ​7.02285(1), c ​= ​9.64784(2) Å for NdEuSiO3N, and a ​= ​5.63074(9), b ​= ​7.01658(10), c ​= ​9.65765(15) Å for NdSrSiO3N. The Nd3+and Eu2+ cations in NdEuSiO3N or Nd3+ and Sr2+ in NdSrSiO3N show partial order in the two available crystallographic sites of coordination numbers 10 (M1) and 9 (M2), with strongly preferred occupancy of the larger divalent cations Eu2+ (77%) and Sr2+ (78%) for the M1 site, whereas the M2 site is preferred by Nd3+. Magnetic susceptibility measurements down to 2 ​K show that NdEuSiO3N is ferromagnetic with Tc ​= ​3 ​K, in contrast to NdSrSiO3N that shows antiferromagnetic interactions at low temperatures. The band gaps of NdEuSiO3N and NdSrSiO3N determined from diffuse reflection spectroscopy are 2.37 and 3.72 ​eV respectively. NdEuSiO3N and the members of the solid solution NdSr0.98Eu0.02SiO3N and NdSr0.5Eu0.5SiO3N are orange-red light emitting luminescent materials, showing a broad emission band centered between 605 and 639 ​nm under excitation at 405 ​nm. These compounds are new phosphor materials with emission wavelengths shifted to the red with respect to the isostructural oxysilicates, which is induced by the introduction of nitride.