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On the dimorphism of Pr6Mo10O39

Rudolph, Daniel, Laufer, Sonja, Hartenbach, Ingo
Zeitschrift für Naturforschung 2017 v.72 no.11 pp. 765-774
Raman spectroscopy, anions, carbonates, cations, crystal structure, crystals, dimorphism, geometry, thermal degradation
Attempts to synthesize Pr₄Mo₇O₂₇ using Pr, Pr₆O₁₁ and MoO₃ in a molar ratio of 8:6:77 led to a main product of scheelite-type Pr₀.₆₆₇[MoO₄] and few single crystals of the triclinic A-type Pr₆Mo₁₀O₃₉. The latter crystallizes in space group P1̅ (a=945.25(1), b=1058.49(2), c=1815.16(3) pm; α=104.149(1), β=95.220(1), γ=102.617(1)°, Z=2). Its crystal structure comprises six crystallographically independent Pr³⁺ cations, eight tetrahedral [MoO₄]²⁻ units, and one [Mo₂O₇]²⁻ entity. The cations display coordination numbers of seven (1×) and eight (5×), while the [MoO₄]²⁻ tetrahedra are surrounded by five Pr³⁺ cations each. The [Mo₂O₇]²⁻ anions exhibit a coordination environment of seven Pr³⁺ cations. The attempt to synthesize PrF[MoO₄] using PrOF (from in situ thermal decomposition of PrF[CO₃]) as reagent did not lead to the desired product but to monoclinic B-type Pr₆Mo₁₀O₃₉. This slightly less dense modification compared to its triclinic analogue crystallizes in space group C2/c (a=1247.93(3), b=1989.68(6), c=1392.52 (4) pm, β=100.505(2)°, Z=4) with three crystallographically independent Pr³⁺ cations, four [MoO₄]²⁻ tetrahedra, and again one [Mo₂O₇]²⁻ unit in the crystal structure. Thus, both Pr₆Mo₁₀O₃₉ modifications are better described with the structured formula Pr₆[MoO₄]₈[Mo₂O₇]. The coordination numbers around the Pr³⁺ cations are seven (1×) and eight (2×) while all four [MoO₄]²⁻ anions are again surrounded by five Pr³⁺ cations each. Six of the latter represent the coordination environment around the [Mo₂O₇]²⁻ entities. Besides the thorough comparison of the crystal structures single crystal Raman spectra were recorded for both Pr₆Mo₁₀O₃₉ phases.