Molecular Germanium selenophosphate salts: Phase-change properties and strong second harmonic generation

A new series of germanium chalcophosphates with the formula A ₄GeP₄Q₁₂ (A = K, Rb, Cs; Q = S, Se) have been synthesized. The selenium compounds are isostructural and crystallize in the polar orthorhombic space group Pca2₁. The sulfur analogues are isostructural to one another but crystallize in the centrosymmetric monoclinic space group C2/c. All structures contain the new molecular anion [GeP ₄Q₁₂]^4-; however, the difference between the sulfides and selenides arises from the change in crystal packing. Each discrete molecule is comprised of two ethane-like P₂Q₆ units that chelate to a central tetrahedral Ge⁴⁺ ion in a bidentate fashion. The selenides were synthesized pure by stoichiometric reaction of the starting materials, whereas the sulfides contained second phases. The band gaps of the molecular salts are independent of the alkali metal counterions and have a value of 2.0 eV for the selenides and 3.0-3.1 eV for the sulfides. All A ₄GeP₄Se₁₂ compounds melt congruently, and the potassium analogue can be quenched to give a glassy phase that retains its short-range order as shown by Raman spectroscopy and powder X-ray diffraction. Interestingly, K₄GeP₄Se₁₂ is a phase-change material that reversibly converts between glassy and crystalline states and passes through a metastable crystalline state upon heating just before crystallizing into its slow-cooled form. Initial second harmonic generation (SHG) experiments showed crystalline K₄GeP₄Se₁₂ outperforms the other alkali metal analogues and exhibits the strongest second harmonic generation response among reported quaternary chalcophosphates, ∼30 times that of AgGaSe₂ at 730 nm. A more thorough investigation of the nonlinear optical (NLO) properties was performed across a range of wavelengths that is almost triple that of previous reports (λ = 1200-2700 nm) and highlights the importance of broadband measurements. Glassy K ₄GeP₄Se₁₂ also exhibits a measurable SHG response with no poling.