Molecular Germanium Selenophosphate Salts: Phase-Change Properties and Strong Second Harmonic Generation

A new series of germanium chalcophosphates with the formula
A4GeP4Q12 (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 [GeP4Q12]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 A4GeP4Se12 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 K4GeP4Se12 also exhibits a measurable SHG response with
no poling.