Structural research on non transition metal double nitrites: Crystal structure, optical absorption and emission of TlBa2(NO2)5

The thallium-barium double nitrite, TlBa₂(NO₂)5, is pyroelectric in the 77-600 K range and crystallizes in thePca2 ₁ space group. The lattice constants at 293 K are: a = 17.868(12),b = 4.934(3),c = 13-426(11) A (MoKa, λ = 0.71069 A). There are four stoichiometric units in the unit cell of volume V= 1184(1) A³ (D _o = 3.98,D _x = 3.979 Mgm⁻³),F (000) = 1232, μ = 20.36mm⁻¹. The crystal structure was solved by Patterson and Fourier methods and refined by least-squares to a final conventional agreement indexR = 0.053 for 1371 independent reflections collected in a θ range of 3–30°, using MoKα radiation. There are two independent barium atoms surrounded by NO ₂ ⁻ groups, both with coordination number 10 and distances in the ranges Ba-O = 2.69(4)-3.18(4) A and Ba-N = 3.01(4)-3.18(4) A. The environment of thallium is clearly affected by the lone-pair stereoactivity and involves 12 Tl-O and Tl-N contacts less than 3.5A, but only four Tl-O distances are shorter than 3 A (min. 2.76(2), max. 2.85(3) A), with a pyramidal coordination and thallium at the apex of the pyramid. All these coordination polyhedra are joined in chains running along the shortest lattice vector [010]. The single crystal electronic spectra, studied in absorption with polarized light and in photostimulated emission, are interpreted as due to transitions involving NO ₂ ⁻ electronic levels perturbed by Tl⁺, whose spin-orbit interaction makes probable also the forbidden singlet-triplet transitions, in agreement with the interpretative picture given for post-transition metal nitrites.