Helical polymer 1/∞[P₂Se₆^2-]: Strong second harmonic generation response and phase-change properties of its K and Rb salts

The selenophosphates A₂P₂Se₆ (A = K, Rb) crystallize in the chiral trigonal space group P3₁21, with a = 7.2728(9) Å, c = 18.872(4) Å, and Z = 3 at 298(2) K and a = 14.4916(7) Å, c = 18.7999(17) Å, and Z = 12 at 173(2) K for K ⁺ salt and a = 7.2982(5) Å, c = 19.0019(16) Å, and Z = 3 at 100(2) K for Rb⁺ salt. The A₂P₂Se ₆ feature parallel one-dimensional helical chains of 1/∞[P ₂Se₆^2-] which depict an oxidative polymerization of the ethane-like [P₂Se₆]^4- anion. On cooling well below room temperature K₂P₂Se ₆ exhibits a displacive phase transition to a crystallographic subgroup and forms a superstructure with a cell doubling along the a- and b-axes. The Rb analogue does not exhibit the phase transition. The compounds are air stable and show reversible glass-crystal phase-change behavior with a band gap red shift of 0.11 and 0.22 eV for K⁺ and Rb⁺ salts, respectively. Raman spectroscopy, ³¹P magic angle spinning solid-state NMR, and pair distribution function (PDF) analysis for crystalline and glassy K₂P₂Se₆ give further understanding of the phase transition and the local structure of the amorphous state. K ₂P₂Se₆ exhibits excellent mid-IR transparency and a strong second harmonic generation (SHG) response. The SHG response is type-I phase-matchable and in the wavelength range of 1000-2000 nm was measured to be 50 times larger than that of the commercially used material AgGaSe ₂. Glassy K₂P₂Se₆ also exhibits an SHG response without the application of electric field poling. In connection with the NLO properties the thermal expansion coefficients for K ₂P₂Se₆ are reported.