Deep ultraviolet (DUV) laser with wavelength shorter than 200 nm has important applications in the fields of semiconductor lithography, laser micromachining, modern scientific instruments and other national defense science and technology and national economy. DUV second order nonlinear optical (NLO) crystal is the core element of all-solid-state laser, which can expand the laser wavelength to DUV region by doubling the frequency, so it has attracted the attention of scientists. At present, only "Chinese brand" frequency-doubling crystal KBe2BO3F2 (KBBF) has been commercially used in DUV region, and can realize the sixfold frequency of 1064 nm Nd:YAG (Nd:Y3Al5O12, neodymium-doped yttrium aluminum garnet) laser, that is, the direct output of DUV coherent light 177.3 nm. However, due to its serious layered growth habit and the highly toxic raw material beryllium oxide, the actual applications of KBBF are limited. Therefore, the design and synthesis of new DUV second order NLO crystals with excellent performance has become an urgent need.
In recent years, monofluorophosphate, has become a popular candidate material for the design and exploration of DUV NLO crystals. Due to the introduction of an electronegative element F, [PO3F]2- has a quite large energy gap (Eg), a relatively strong second harmonic generation (SHG) response and a larger birefringence (Δn) compared with those of phosphates, showing excellent NLO properties. For example, DUV NLO crystals NaNH4PO3F·H2O (Eg > 6.21 eV, 1.1 × KH2PO4 (KDP), Δnobv. = 0.053 at 589.3 nm), [C(NH2)3]2PO3F (Eg > 6.21 eV, 1.0 × KDP, Δncal. = 0.038 at 532 nm) and Kx(NH4)2-xPO3F (x = 0–0.3) (Eg > 6.21 eV, 1.0 × KDP, Δncal. = 0.030 at 532 nm). However, the traditional synthesis method has problems such as F element loss and corrosion of production equipment, which hinders the preparation of monofluorophosphates. Therefore, it is urgent to establish a green and effective synthesis method. Further more, through structural adjustment and performance optimization, more excellent DUV NLO monofluorophosphate crystals can be developed, thus realizing their practical applications in the field of NLO lasers.
Work 1: In order to optimize the preparation of monofluorophosphates, we establish a green, simple and effective solution synthesis method. (NH4)2PO3F, which has a simple, cheap and non-toxic preparation method, is selected as the raw material to undergo acid and single base neutralization reaction, acid and double bases neutralization reaction, and metathesis reaction with corresponding metal bases and salts, respectively. Three types of 12 cases of monofluorophosphate are successfully synthesized: alkali metals (Na+, K+, Rb+, Cs+; 1–4), mixed cations (Li+NH4+, Na+NH4+, Li+K+, Na+K+; 5–8), and alkaline earth metals monofluorophosphates (Mg2+, Ca2+, Sr2+, Ba2+; 9–12). This simple and effective solution synthesis method realizes the quantitative substitution of metal ions for NH4+, and overcomes the traditional methods of high energy consumption, easy loss of F element, highly toxic raw material HF or H2PO3F and corrosion of production equipment, etc. Such work builds a solid foundation for the effective synthesis of monofluorophosphates and their practical applications in the DUV NLO field. Simultaneously, we also systematically carry out differential thermal analysis and thermogravimetric analysis of these monofluorophosphates. Most of the detailed thermal stability data are reported for the first time, which will provide useful information for related research and help to explore more applications in other fields.
Work 2: There is no d–d and f–f electron transition that will make the band gap red shift in the outermost electrons of alkali metal cations and NH4+. Therefore, alkali metal monofluorophosphate is likely to be the best choice for DUV NLO materials. We use Na+ and Rb+ to quantitatively replace NH4+ in (NH4)2PO3F·H2O, in aqueous solution, thus synthesizing Na1.5Rb0.5PO3F·H2O (NRPF·H2O) crystallized in the noncentrosymmetric (NCS) orthorhombic space group of Pmn21. The absorption cutoff edge of NRPF·H2O is short than 200 nm, which can achieve the optical transparency down to the DUV region, and has a large birefringence Δnobv = 0.04 at 546 nm. Theoretical calculation shows that the shortest phase matching wavelength can reach 265 nm, which can realize the fourth harmonic generation of Nd:YAG laser (1064 nm). NRPF·H2O exhibits relatively strong SHG response at 1064 nm and 532 nm: 0.55 × KDP and 0.2 × β-BaB2O4 (BBO), respectively. More interestingly, the analysis of three closely related compounds (NH4)2PO3F·H2O, NaNH4PO3F·H2O and NRPF·H2O, shows that the NH4+ cations in (NH4)2PO3F·H2O are gradually quantitatively replaced by Na+, or Na+ and Rb+, so that the hydrogen bonds around NH4+ are gradually broken, resulting in the number of intra-layer hydrogen bonds of the three compounds changing from 8, 2 to 2, and the inter-layer hydrogen bonds changing from 2, 4 to 0. At the same time, the rearrangement of [PO3F]2- anions leads to the change of symmetric elements of the three compounds from P21/c, Pc to Pmn21, accompanied by changes in crystal structures and optical properties.
Work 3: Through the green solution method described above, we successfully synthesize a new Cd2+-containing monofluorophosphate, Cd2.5(NH4)2(PO3F)3Cl·2H2O (CNPFC), which is crystallized in the monoclinic P21/c centrosymmetric (CS) space group. It is a DUV birefringent crystal (Δncal. = 0.035). We further analyze the structure of three similar compounds, CdPO3F·2H2O, Cd5(PO4)3Cl and CNPFC, and verify the diversity of transition metal cationic coordination number (CN = 4–9) and the diversity of connection modes between these polyhedrons (dimer, trimer and hexamer, etc.). This makes the transition metal monofluorophosphates have rich structural adjustability, and can be easy to regulate and obtain the NCS structures, so as to obtain excellent performances of DUV NLO crystals.
Work 4: We continue to regulate the anion species of Cd2+ and the solution concentration, and observe a rare solution concentration driven isomerization crystallization phenomenon. Two cases of Cd(NH4)2(PO3F)2·2H2O (CdNPF) isomers transparent in the DUV region are obtained, which are the α phase crystallized in the monoclinic P21/n CS space group and the β phase crystallized in the orthomorphic Cmc21 NCS space group. We prepare 0.15 mol/L (M) and 0.20 M (based on CdI2) solutions of CdI2 and (NH4)2PO3F with 1:2 stoichiometric ratio, and diffuse them with the same amount of methanol. The UV-visible absorption spectra confirm that there are more [PO3F]2- anions around Cd2+ in the higher concentration CdI2-0.20 M solution, therefore, Cd2+ is precipitated from mother liquor as [Cd(H2O)2(PO3F)4] to obtain β-CdNPF. However, Cd2+ was precipitated as [Cd(H2O)3(PO3F)3] in the lower concentration solution to obtain α-CdNPF crystal. The dipole moment of the former octahedron is smaller than that of the latter (β: 1.06 D vs. α: 1.60 D), which eventually eliminates the unwanted symmetric center trap that is induced by the antiparallel dipole-dipole interactions as happening in α-CdNPF and giving rise to the formation of NCS β-CdNPF, and the NLO activity is successfully activated (0.78 × KDP). Interestingly, from α phase to β phase, it is the small difference in the number of [PO3F]2- around Cd2+ increasing from 3 to 4 that causes the fluorescence of 4d10 → 4d95s1 in Cd2+ to be slightly redshifted (β: 437 vs. α: 431 nm) and makes the energy gap smaller (cal.: β: 4.01 vs. α: 4.05 eV). β-CdNPF is the first NCS Cd2+-containing monofluorophosphate, which is expected to be a potential DUV NLO crystal material. These insights of regulating the coordination ligand number by controlling the concentration of the mother solution that alters the crystallization process offer a new and unconventional access to the design and searches for NLO materials.
谷歌
