LiEuMo2O8 – crystal growth, structure, and optical properties

• Single crystalline LiEuMo2O8 with different color shades.
• Refined XRD data shows identical crystal structure for both types.
• Emission spectra of Eu3+ are the same for both crystals.
• ESR spectra and SQUID proofs the presence of oxygen defects due to Mo5+ formation.

Centimeter size LiEu(MoO4)2 bulk single crystals have been grown by top-seeded solution growth using Li2MoO4 as a flux. The resulting crystals exhibited different coloration shades from dark to bright violet. Small single crystal fragments were studied by X-ray diffraction: CaWO4 type, space group I41/a, a = 519.74(4), c = 1132.56(11) pm, wR2 = 0.0143, 270 F2 values at 300 K and a = 518.70(4), c = 1130.25(11) pm, wR2 = 0.0197, 268 F2 values at 90 K. The Li+ and Eu3+ cations statistically occupy the 4b site. Down to 90 K, the single crystal X-ray data give no hint for cation ordering. Some of the grown crystals show dark violet color. This is due to defects in the oxygen substructure accompanied by partial reduction of Mo6+ (d0) to Mo5+ (d1) which was detected by electron spin resonance. 151Eu Mössbauer spectra of the bright and also of the dark violet LiEuMo2O8 samples show the presence of trivalent europium solely. Temperature dependent magnetic susceptibility data exhibit weak van Vleck type paramagnetism as a consequence of trivalent europium ([Xe]4f6). The dark violet samples show slightly higher susceptibility due to the contribution of Mo5+. These data point to a formula LiEuMo2O8–x for the violet samples. 7Li solid state NMR spectra underline the single lithium sites in both samples. Both colored crystals show identical emission spectra, which indicate the same chemical surrounding of the trivalent europium ions. However, excitation spectra and thermal quenching and decay curves points to a higher defect density in the violet sample.