Crystal growth and optical characteristics of beryllium-free polyphosphate, KLa(PO3)4, a possible deep-ultraviolet nonlinear optical crystal.

Pai Shan,Yongfa Kong,Jingjun Xu,Xuanwen Liu,Hongde Liu,Tongqing Sun,Hong Chen,Shaolin Chen

doi:10.1038/srep25201

Abstract

Deep-ultraviolet nonlinear optical crystals are of great importance as key materials in generating coherent light with wavelength below 200 nm through cascaded frequency conversion of solid-state lasers. However, the solely usable crystal in practice, KBe2BO3F2 (KBBF), is still commercially unavailable because of the high toxicity of beryllium-containing and the extreme difficulty of crystal growth. Here, we report the crystal growth and characteristics of an beryllium-free polyphosphate, KLa(PO3)4. Centimeter-sized single crystals have been easily obtained by the flux method and slow-cooling technique. The second-harmonic generation efficiency of KLa(PO3)4 powder is 0.7 times that of KH2PO4; moreover, the KLa(PO3)4 crystal is phase-matchable. Remarkably, the KLa(PO3)4 crystal exhibits an absorption edge of 162 nm, which is the shortest among phase-matchable phosphates so far. These attributes make KLa(PO3)4 a possible deep-ultraviolet nonlinear optical crystal. An analysis of the dipole moments of the polyhedra and theoretical calculations by density functional theory were made to elucidate the structure-properties relationships of KLa(PO3)4.

Highlights

To the difficulty of obtaining crystals with a large size and high optical quality
We pay more interest in phosphate—a novel promising candidate. It is well-known that both rare-earth La3+ and alkali ions have deep-UV transparency because of their empty f and d orbits. In light of this strategy, we have studied a condensed metaphosphate of cesium and lanthanum crystal, CsLa(PO3)[4], which exhibits a very short absorption edge (167 nm) and moderate powder second-harmonic generation (SHG) efficiency[22]
The crystals were analyzed by powder X-ray diffraction (PXRD) and Raman spectroscopy for determining their phase compositions and structural features

Summary

Introduction

To the difficulty of obtaining crystals with a large size and high optical quality. there is a quite urgent demand to discover new and beryllium-free deep-UV NLO materials now. The optical properties of the KLa(PO3)[4] crystal is more likely to differ from CsLa(PO3)[4], and are worth being investigated. The strong IR vibration peak around 908 cm−1 can indicate that the phosphoric anions of the as-grown crystals have long chain geometry of [PO3]∞.

Results

Conclusion