Semiconducting properties and phase-matching nonlinear optical response of the one-dimensional selenophosphates ANb₂PSe₁₀ (A = K, Rb, and Cs)

The new compounds ANb₂PSe₁₀, where A = K, Rb and Cs, form from polyselenophosphate flux reactions and crystallize in the noncentrosymmetric space group Pc. They feature infinite one-dimensional ¹/_-[Nb₂PSe₁₀^-] chains separated by alkali cations. The chains consist of [Nb₂(Se₂)₂]⁴⁺ clusters bridged by a diselenide and a [PSe₄]^3- group. The chains pack differently depending on which alkali cation is present in the lattice. As a result, the analogs are not isostructural with respect to each other, and each has a different unit cell. The reaction chemistry involving a multitude of reaction conditions and their respective products is discussed. Other products from these reactions include NbSe₃ and Nb₂Se₉ in both crystalline and microfibrous morphologies. The ANb₂PSe₁₀ compounds are stable to oxidation in ambient air but decompose when heated above 350 °C. Their band gaps were measured to be 1.1-1.2 eV and the resistivity of the K analog at room temperature was measured at 4.5 ω·cm. Nonlinear optical second harmonic generation measurements were done on the Rb analog yielding a χ² of ∼7 pm/V and showing phase matching behavior.