RbEu[AsS4], Rb4Eu[AsS4]2 and CsEu[AsS4]: Three europium(II) thioarsenates(V) with the heavy alkali metals

Abstract

Three novel europium(II) thioarsenates(V)

(1),

(2), and

(3) were synthesized by using the molten polysulfide-flux method.

and

crystallize isotypically in the orthorhombic space group Pnma with $a=1729.53\left(9\right)\mathrm{pm}$, $b=679.18\left(4\right)\mathrm{pm}$, $c=670.46\left(4\right)\mathrm{pm}$ for (1) and $a=1796.48\left(9\right)\mathrm{pm}$, $b=686.54\left(4\right)\mathrm{pm}$, $c=663.35\left(4\right)\mathrm{pm}$ for (3) with $Z=4$. An attempt to obtain single-phase

by changing the molar ratio of the reactants and optimizing the synthesis temperature led to

as main product and

as a second phase, which crystallizes orthorhombically in the space group Ibam with $a=913.46\left(5\right)\mathrm{pm}$, $b=1873.08\left(11\right)\mathrm{pm}$, $c=1010.23\left(6\right)\mathrm{pm}$ for $Z=4$. All crystal structures contain isolated

anions with sulfur corners and edges building

-centered

polyhedra, whose condensation leads to $\frac{2}{\infty }$ $\left\{{\left[\mathrm{Eu}{\mathrm{(S1)}}_{1/1}^t{\mathrm{(S2)}}_{3/3}^e{\mathrm{(S3)}}_{4/2}^e\right]}^{6-}\right\}$ layers separated by

and

cations for (1) and (3) or

-linked $\frac{1}{\infty }$ $\left\{{\left[\mathrm{Eu}{\mathrm{(S2)}}_{4/2}^e{\mathrm{(S3)}}_{4/1}^t\right]}^{6-}\right\}$ chains in the case of (2). Single-crystal Raman spectroscopy was performed for all compounds showing the typical vibrational modes of discrete tetrahedral

groups.