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 in the crystal and emitted at half the wavelength and twice the frequency of incident light—for the sample with the largest cation, Cs⁺, had an intensity 15 times that produced by a typical commercial nonlinear optical material. Even the smallest cation, K⁺, mixed with Cs⁺, produced about double the SHG response of the commercial benchmark material. The new salts exhibit strong photoluminescence in solution; they are also optically transparent from the mid- to the near-IR region, which gives them potential for use in a range of applications, from broadband communication to medical devices. (S. Banerjee et al., J. Amer. Chem. Soc. 130 , 12270, 2008 http://dx.doi.org/10.1021/ja804166m .)