Old and New Insights into Structure and Properties of Eu₂[SiO₄]

Abstract

Lemon-yellow single crystals of Eu2[SiO4] were obtained by reaction of elemental europium with Eu2O3 in the presence of SiO2 carried out in evacuated silica ampules at 1373 K for 48 h followed by constant cooling to room temperature with 120 K per hour. Eu2[SiO4] crystallizes at room temperature in the larnite-type structure of β-Ca2[SiO4] (monoclinic, P21/n) with parameters a = 565.02(5), b = 709.15(6), c = 975.84(8) pm, β = 92.614(3)° for Z = 4. At 452 K it undergoes a reversible phase transition to an incommensurate structure similar to the isotypic compounds Sr2[SiO4] and K2[SeO4]. The phase transition energy was determined to 0.54(2) J·g–1 by DSC measurements and temperature dependent birefringence measurements show that the phase transition exhibits a significant hysteresis. Refinements of the structure of Eu2[SiO4] based on single crystal X-ray diffraction data show that the high temperature modification crystallizes in the orthorhombic space group Pnma­(α00)­0ss with a = 710.16(6), b = 566.93(5), c = 977.13(8) pm, and an incommensurate modulation along [100] with a modulation vector t = [0.293(4), 0, 0]. Eu2[SiO4] exhibits a bright yellow photoluminescence and its permittivity increases substantially above 300 K. Magnetic susceptibility and EPR measurements show that Eu2[SiO4] becomes ferromagnetic below 7 K with a g factor of 1.993.

The crystal structure and luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2+

“…One possible position would be the Li3 site, which forms an octahedral coordination to two O 2− anions with Li–O distances of 205.0(2) pm and to four N 3− anions with Li–N distances between 218.9(2) and 248.94(5) pm resulting in a polyhedron size of 14.19 Å 3 . Although the distances are too short regarding typical Eu–O/N coordination (Eu–O distances range from 240 to 300 pm and Eu–N distances range from 260 to 325 pm), 49–51 local spatial distortion or enlargement of the octahedron could occur in contrast to the average structure. To maintain charge neutrality, the inclusion of a divalent Eu 2+ cation may be compensated by Li-vacancies in the surroundings or by local O/N substitution.…”

Li2Eu3Br2[BO3]2: A New Europium(II) Halide Oxoborate with Yellow Luminescence

“…[ 43 ] Additionally, the yellow body color and emission of Li 2 Eu 3 Br 2 [BO 3 ] 2 also reminds of the optically similar europium(II) ortho ‐oxosilicate Eu 2 [SiO 4 ]. [ 40 ] As the luminescent properties of the latter have been subject to careful investigations by members of our group, similar studies are planned in collaboration with experts in this field for a future publication, since properties like emission maxima of bulk luminescence and the necessary theoretical considerations are beyond the scope of this article.…”

Low-temperature phase transition and magnetic properties of K3YbSi2O7

“…Actually, most of these phase transitions are reconstructive and occur at high temperatures (Felsche, 1973). However, there is also an example of a displacive phase transition, which was reported for Eu 2 SiO 4 (Busch et al, 1970;Funk et al, 2018). Eu 2 SiO 4 adopts monoclinic symmetry at room temperature and transforms to an orthorhombic structure at 438 K. Ananias and co-authors (2015) reported a lowtemperature phase transition in NaGd 0.99 Eu 0.01 SiO 4 .…”