Centrosymmetric CaBaMF₈ and Noncentrosymmetric Li₂CaMF₈ (M = Zr, Hf): Dimension Variation and Nonlinear Optical Activity Resulting from an Isovalent Cation Substitution-Oriented Design

Abstract

Zirconium/hafnium fluorides have recently been recognized as potential nonlinear optical (NLO) materials with short ultraviolet (UV) cutoff edges, which is significant in laser science and industry. The synthesis of noncentrosymmetric (NCS) materials based on centrosymmetric (CS) compounds by an isovalent cation substitution-oriented design is an emerging strategy in the NLO territory. Here, two isostructural and novel fluorides, CaBaMF8 (M = Zr (1), Hf (2)), have been synthesized through the combination of alkaline earth metals, zirconium/hafnium, and fluorine elements. They feature zero-dimensional and CS structures composed by an isolated MF8 (M = Zr, Hf) dodecahedron and dissociative Ca2+ and Ba2+ cations, and they display short UV cutoff edges (<200 nm) as well. Two three-dimensional fluorides Li2CaMF8 (M = Zr (3), Hf (4)) are obtained by replacing Ba with alkali metal Li atom, which not only represent phase-matchable second-harmonic-generation activities (0.36, 0.30× KH2PO4 (KDP)) at 1064 nm but also maintain short UV cutoff edges with high reflectance. This work has largely enriched the family of NCS zirconium/hafnium fluorides reaching the short UV region.

KBa3M2F14Cl (M = Zr, Hf): novel short-wavelength mixed metal halides with the largest second-harmonic generation responses contributed by mixed functional moieties

“…The theoretical birefringences at 546 nm are 0.12 for KBZFC and 0.10 for KBHFC, which are basically in alignment with the calculated birefringence values and are greatly enhanced compared with the reported DUV transparent fluorides BaMgF 4 (0.0077), SrAlF 5 (0.0242), BaZnF 4 (0.0164), Li 2 CaMF 8 (M = Zr, Hf) (0.05, 0.03), and K 3 Ba 2 Zr 6 F 31 (0.08). 41–45 The calculated electronic density difference (EDD) maps further illustrate the apparent charge transfer from Zr/Hf to F atoms and ordered dense electron cloud in the ac plane (Fig. 4c and f), which favors the superposition of optical anisotropy and illustrates the birefringence source of KBZFC and KBHFC.…”

“…To date, few metal fluorides have been reported with DUV cutoff edges, namely, BaMgF 4 , SrAlF 5 , BaZnF 4 , Li 2 CaMF 8 (M = zirconium (Zr), hafnium (Hf)), and K 3 Ba 2 Zr 6 F 31 . 41–45 However, BaMgF 4 , SrAlF 5 , and BaZnF 4 show weak SHG intensities (0.085, 0.65, 0.16 × KDP) and poor birefringences (0.0077, 0.0242, 0.0164) that are insufficient for PM behavior. Li 2 CaMF 8 (M = Zr, Hf) and K 3 Ba 2 Zr 6 F 31 were newly reported by our group, and were synthesized by introducing transition metals (TMs) Zr and Hf into halides for remarkable benefits, as follows.…”

Hg16O12(NO3)6F2(H2O): A Mercury-Rich Nitrate Oxyfluoride with Diverse Triple-Coordinated Mercury-Based Units That Exhibits an Unparalleled [(Hg16O12F2(H2O))6+]∞ Cationic Framework and a Large Birefringence

“…This may result in a transition from centrosymmetric to noncentrosymmetric structure units. Similarly, this strategy is also present in other compounds such as Li 2 CaMF 8 (M = Zr, Hf). , …”

From CaBaM2F12 to K2BaM2F12 (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity

“…The success of short-wavelength NLO oxyfluorides like A 4 B 4 O 6 F (A = NH 4 , Na, Cs), − Ba 4 Ca­(B 2 O 5 ) 2 F 2 , NaNH 4 PO 3 F·H 2 O, (NH 4 ) 2 PO 3 F, and K 3 Sc 3 (PO 4 )­(PO 3 F) 2 F 5 has greatly simulated our interest in synthesizing NCS fluorides. So far, our group has put vast effort into this system and obtained a certain result, including KNa 2 ZrF 7 [0.3 × KH 2 PO 4 (KDP); wavelength = 245 nm], K 3 Ba 2 Zr 6 F 31 (0.5 × KDP; wavelength < 200 nm) and Li 2 CaMF 8 (M = Zr, Hf) (0.36 and 0.3 × KDP, wavelength < 200 nm), by means of incorporating Zr/Hf atoms featuring the lowest electronegativities (1.4 and 1.3) among the d 0 transition metals into fluorides. Besides, alkali (A) or alkaline-earth (AE) metals are adopted for the absence of d–d or f–f electron transitions, which make for short cutoff edges. , …”