From Phosphate Fluoride to Fluorophosphate: Design of Novel Ultraviolet/Deep-Ultraviolet Nonlinear Optical Materials for BePO₃F with Optical Property Enhancement

Abstract

Fluorine-containing compounds have stimulated the exploration of ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials. Alkali/alkaline-earth metal phosphates are one of the important potential systems as NLO materials, while the common small birefringence limits the phase-matching (PM) ability in the ultraviolet/deep-ultraviolet region. Herein, by applying a “fluorination synergy-induced enhancement of optical property” strategy, novel structures of phosphate fluoride/fluorophosphate in BePO3F with good thermodynamic/dynamic stability and promising NLO-related properties are discovered via performing crystal structure prediction combined with first-principles calculations. BePO3F-I–VI exhibit relatively large birefringence of 0.025, 0.048, 0.049, 0.049, 0.059, and 0.063 at 1064 nm, respectively. Simultaneously, BePO3F-I (Pc) is a new thermodynamically stable phosphate fluoride which possesses a wide band gap (E g = 8.03 eV), large second-harmonic generation (SHG) coefficient (d 11 = 0.67 pm/V, 1.7 × KDP), and the shortest PM wavelength of 292 nm. Other five thermodynamically metastable noncentrosymmetric (NCS) BePO3F structures (II–VI) belong to fluorophosphates and exhibit deep-ultraviolet PM wavelengths of 187, 183, 186, 188, and 196 nm. It reveals that the aligned nonbonding O 2p orbitals of [BeO2F2] and [PO4] units lead to a large SHG coefficient in the phosphate fluoride BePO3F-I. For fluorophosphates (BePO3F-II–VI), the synergy of [BeO3] planar units and [PO3F] units induces relatively large birefringence. Our research results provide an idea for exploring novel high-performance NLO materials.

Rational Design of the First Ammonium Magnesium Borate with Deep-Ultraviolet Cutoff Edge and Moderate Birefringence and Further Investigation into the Nature of Ammonium in the Borate System

“…In order to fulfill the requirement of transmission in the UV or even deep-UV region, the designs of NLO and birefringent optical materials focus on the combination of preferred anion groups (B–O/F, P–O/F, S–O) and ions without the influence of d – d and f – f electronic transition (mainly focused on alkaline and alkaline-earth metal elements). − Except for cation elements, NH 4 + was also widely applied not only because of its similar radii to K + and Rb + , which benefit the exploration of new compounds by cation substitution, but also for its contribution of good transmission in the UV or even deep-UV region. − ,− Moreover, in Becker’s work, when comparing NH 4 [B 5 O 6 (OH) 4 ]·2H 2 O with Rb­[B 5 O 6 (OH) 4 ]·2H 2 O, the additional hydrogen bonds introduced by NH 4 + reinforced the structure . In the previous research, many ammonium-containing compounds with excellent optical properties have been exploited, and they have been listed in Table S4 in the ESI.…”

Synthesis and characterization of a new isoxazolone-based nonlinear optical crystal: MPMOI

“…The SHG properties of organic nonlinear optical crystals with similar ketone-methoxy group structures reported in recent years are listed in Table S2. † When having the same electron donor CH 3 O-group, the SHG intensities of simple ketone structures, such as DMPP, 51 BMP, 52 and 6MN4MPP, 15 are 1-2 times that of KDP (d 36 = 0.39 pm V −1 at 1064 nm) 53,54 /urea (d 36 = 1.2 ± 0.1 pm V −1 at 1064 nm), 55 which are smaller than that of MPMOI. Besides, among molecule-type ketone-based nonlinear optical crystals, as shown in Table S2, † MPMOI shows a moderate second harmonic generation intensity.…”

Target‐Driven Design of Deep‐UV Nonlinear Optical Materials via Interpretable Machine Learning

“…Therefore, the birefringence property directly describes the corresponding ability of PM in the NLO process, that is to say, the shortest PM wavelength is usually beneficial from a suitable birefringence with a relatively small dispersion of refractive index. Through the unremitting efforts of researchers, a series of potential NLO materials which can realize PM in the deep-UV region owing to their moderate birefringence have been obtained, including borates, [8] fluorooxoborates, [9] fluorophosphates, [10] carbonates, [11] and so on. However, it still remains serious challenges in the process of exploration.…”