Extreme pressure-induced shifts of emission energies in M[Au(CN)₂] and M₂[Pt(CN)₄]•nH₂O - Compounds with low-dimensional metal-metal interactions

Emission properties of dicyanoaurates(I) and tetracyanoplatinates(II) are investigated under high hydrostatic pressure up to ca. 20 kbar and ca. 40 kbar, respectively, at T = 295 K. One observes pressure-induced red shifts of the emission energies, which are the largest values known for solid state compounds: Δν/Δp (cm⁻¹/kbar) = -120 (Cs₂Na[Au(CN)₂]₃); -150 (Cs[Au(CN)₂]); -200 (K[Au(CN)₂]); -300 (E parallel c and -250 (E perpendicular c (Li₂[Pt-(CN)₄]•4H₂0). In the investigated pressure range the energy shifts are linear and no phase transition is observed. Interestingly, Li₂[Pt-(CN)₄]•4H₂0 is a very particular compound of the series of tetracyanoplatinates(II). Using this material it is possible to tune the emission in the visible energy range continuously over more than 6000 cm⁻¹ by high pressure application up to 20 kbar. For both types of compounds, the reported large values of pressure induced shifts are related to low-dimensional metal-metal interactions. These interactions lead to the formation of electronic energy bands, delocalized free exciton states, and self-trapped states. The importance of such states for the emission properties of these compounds is also discussed