Nonlinear optical (NLO) crystals play an important role in all-solid-state laser technology for their unique function of laser frequency conversion. Great efforts have been made to develop new NLO crystals from various types of traditional compounds such as borates, carbonates and iodates. To rationally design and synthesize such kind of crystals is still a challenge, and creating a new type of polar or non-centrosymmetric compounds will be of great significance.

Mixing two types of NLO-active anionic groups in the same crystal has been proved as a convenient strategy for designing new NLO materials. However, borate-iodate mixed NLO materials have never been reported yet.

In a study published in Angew. Chem. Int. Ed., the research group led by Prof. YE Ning at Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences reported the first metal borate-iodate NLO crystal, Be₂(BO₃)(IO₃).

The researchers designed Be₂(BO₃)(IO₃) based on a strategy of multicomponent modification of KBe₂BO₃F₂ (KBBF), which can be easily synthesized by subcritical hydrothermal technique.

They found that Be₂(BO₃)(IO₃) featured a KBBF-similar structure with unique [Be₂(BO₃)(IO₃)] composite-layers which were constructed by [Be₂BO₅] and [IO₃] layers.

The honeycomb [Be₂BO₅] layer maintained the structural features and NLO capability of [Be₂BO₃F₂] layer in KBBF. The (IO₃)^- groups were connected to [Be₂BO₅] layers, which were induced to organize in a polarized arrangement by this polar borate layer.

Benefiting from the introduction of (IO₃)^- groups, Be₂(BO₃)(IO₃) exhibited a very strong second harmonic generation (SHG) response of ~7.2 × KH₂PO₄ (KDP) and a large birefringence of 0.172 at 1064 nm. Besides, it unexpectedly showed the potential ability to optimize growth habits via van der Waals force.

As the first KBBF family member with mixed anionic groups, Be₂(BO₃)(IO₃) demonstrated the fact that heterogeneous anionic groups can be connected to the [Be₂BO₅] layers, which is of great significance to KBBF structural engineering.

This study not only provides a novel KBBF-similar NLO crystal, but also highlights borate-iodate as a new type of NLO material.