# Halogen-ion-driven polymorphs for high-performance nonlinear optical crystalline materials

## TL;DR

Scientists developed a new method to create high-performance nonlinear optical crystals by using halogen ions to control crystal structure.

## Contribution

A halogen-driven strategy is introduced to achieve noncentrosymmetric polymorphs with record SHG efficiency in Sn-based hybrid halides.

## Key findings

- Two new polymorphs of [N(C2H5)4]SnBr3 (Cc and Cmc21) were formed using different halide sources.
- The Cmc21 phase shows an SHG efficiency of 5.6 × KH2PO4, the highest reported for Sn-based hybrid NLO materials.
- Theoretical calculations confirm the [SnBr3]− unit is responsible for the strong SHG response.

## Abstract

Noncentrosymmetric (NCS) crystalline materials are indispensable for nonlinear optical (NLO) applications, yet their rational design remains challenging due to thermodynamic preferences for centrosymmetric configurations. Herein, we demonstrate a halogen-driven strategy for obtaining NCS structures by leveraging competitive coordination between halide anions (X−) and stereochemically active Sn2+ centers. Precise halogen substitution induced the formation of two new polymorphs of [N(C2H5)4]SnBr3 (Cc and Cmc21 phase) using different halide sources (i.e., [N(C2H5)4]Cl or [N(C2H5)4]Br). Compared to the Cc phase (2 × KH2PO4), the resulting Cmc21 phase exhibits exceptional second-harmonic generation (SHG) efficiency (5.6 × KH2PO4). This performance ranks among the highest for all reported Sn-based organic–inorganic hybrid NLO materials to date. Theoretical calculations indicate that the [SnBr3]− unit is the primary source of the strong SHG response. This work establishes halogen-driven symmetry control as a viable strategy for achieving a dramatically enhanced SHG response, thereby providing a valuable reference for the rational design of high-performance NLO materials.

Halogen-driven polymorph selection: achieving tunable NCS crystallization and a record SHG response (5.6 × KDP) in Sn-based hybrid halides.