High-pressure investigations of yttrium(III) oxoarsenate(V): Crystal structure and luminescence properties of Eu³⁺-doped scheelite-type Y[AsO₄] from xenotime-type precursors

Colourless, water- and air-stable single crystals of yttrium(III) oxoarsenate(V) Y[AsO₄] in the xenotime-type crystal structure were prepared by the reaction of yttrium sesquioxide (Y₂O₃) dissolved in aqueous nitric acid (13%) with a solution of arsenic(V) oxide hydrate (As₂O₅·3H₂O) and subsequent neutralization with 1 M caustic soda. Y[AsO₄] crystallizes tetragonally in the space group I4₁/amd with the lattice parameters a = 704.63(6) and c = 628.94(5) pm for Z = 4 and is isotypic to the minerals xenotime RE[PO₄] (RE: mainly Y and Yb) and chernovite RE[AsO₄] (RE: mainly Y and Ce). This xenotime-type yttrium compound was used as precursor in a high-pressure experiment (20 kbar) at 700 °C to create a new tetragonal modification of Y[AsO₄]. It shows the scheelite-type structure (space group: I4₁/a) with the lattice parameters a = 498.23(4) and c = 1120.71(9) pm for Z = 4, named after the mineral scheelite (Ca[WO₄]). Both tetragonal structures are characterized by only one crystallographically unique position for each of the Y³⁺, As⁵⁺ and O^2- ions with distances of d(Y–O) = 232 and 241 pm (C.N. = 8) as well as d(As–O) = 169 pm (C.N. = 4) in the case of the scheelite-type structure. The xenotime-type compound shows an unexpected slight decrease in average bond lengths for the yttrium to oxygen (d(Y–O) = 230 and 241 pm, C.N. = 8) as well as for the arsenic to oxygen distances (d(As–O) = 168 pm, C.N. = 4), accompanied by a drastic density increase from D_x = 4.85 (xenotime type) to D_x = 5.44 g ∙ cm^-3 (scheelite type). Luminescence spectroscopic measurements of the Eu³⁺-doped Y[AsO₄] samples, obtained in experiments at similar conditions as for the pure compounds, show a bright, reddish lighting for the scheelite type, which does not occur for the xenotime type of yttrium(III) oxoarsenate(V).