Structure of strongly nonlinear crystal unraveled

The recently elucidated crystal structure of a promising class of inorganic polymer salts reveals why these materials generate strong second-harmonic generation (SHG) responses to optical stimulation. In general, asymmetric inorganic polymer thin films with highly polarizable bonds exhibit strong nonlinear optical behavior, and are used in some tunable, coherent IR lasers to probe the electronic or structural properties of molecules or surfaces. A team from Northwestern University and Argonne National Laboratory used Argonne’s Advanced Photon Source to study the quaternary salts formed from the zirconium selenophosphate (ZrPSe₆ ⁻) polyanion and its complementary metal cation (K⁺, Rb⁺, or Cs⁺)—this class of salt tends to crystallize as microneedles (see figure 1). The crystal structure (see schematic in figure 2) revealed a distortion in the molecular backbone from its ideal geometry, which contributes to the salt’s high polarity. The second harmonic—a beam generated...