The First Examples of Lithium‐Containing Mixed‐Alkali Strontium Borates with Different Dimensional Anionic Architectures and Short Cutoff Edges

Abstract

Three new mixed-alkali strontium borates, Li KSr (BO ) , Li Rb Sr (BO ) , and Li Rb SrB O , have been synthesized by the high-temperature solution method by adjusting the molar ratios of the reactants. They represent the first examples of lithium-containing mixed-alkali strontium borates. The structures of the three compounds show different dimensional anionic architectures, with Li KSr (BO ) and Li Rb Sr (BO ) possessing isolated BO units, whereas Li Rb SrB O has three-dimensional (3D) open frameworks with three types of channels occupied by the strontium and rubidium atoms. Interestingly, Li KSr (BO ) is the first example of a chain composed of LiO linked through the sharing of vertexes and edges. However, in Li Rb Sr (BO ) , isolated LiO units can be observed, which is also rare in borates. Detailed analysis and comparisons of the structures of the anhydrous mixed-alkali- and alkaline-earth-metal borates and nine-polymeric fundamental building blocks have been made. Furthermore, Li Rb Sr (BO ) and Li Rb SrB O present short cutoff edges (<190 nm), which was confirmed by the UV/Vis/NIR diffuse reflectance spectra. These attributes make them attractive for deep-UV optical materials.

Li4Ca2B8O16: A Borate with a Unique Fundamental Building Block and a Short Cutoff Edge

“…Taking the Li 4 Ca 2 B 8 O 16 structure into consideration, the reasons for the wide transparency range are as follows. First, the alkali and alkaline-earth (Li + and Ca 2+ ) metal cations are generally conducive to the cutoff edge shifting to the shorter wavelength due to no d–d and f–f transitions . Additionally, the [B 8 O 16 ] ∞ chain eliminates most of the terminal oxygen atoms in B–O configurations, leading to a shorter cutoff edge.…”

Al8(BO3)4(B2O5)F8: A F-Containing Aluminum Borate Featuring Two Types of Isolated B–O Groups

“…27,29−33 Owing to the Al atom exhibiting relatively strong electronegativity, it usually bonds with isolated B−O groups sharing O atoms, eliminating partially the dangling bonds. 34 Therefore, introducing the Al atoms can effectively hold back the polymerization of the B−O groups, and is in favor of obtaining wide transmission range. As we known, there is only one compound Al 6 (BO 3 ) 5 F 3 reported in the Al−B−O− F system, and its crystal structure has been determined.…”

“…The introduction of F not only enriches borate structures, but also is conducive to generating a large band gap. − Compared with the replacement of the O atoms with F, the introduction of the Al atoms is a wise choice to design novel compounds as well, and widen the transmission windows. As we known, the cutoff edge of compounds moves to red side due to dangling bond of the B–O groups. ,− Owing to the Al atom exhibiting relatively strong electronegativity, it usually bonds with isolated B–O groups sharing O atoms, eliminating partially the dangling bonds . Therefore, introducing the Al atoms can effectively hold back the polymerization of the B–O groups, and is in favor of obtaining wide transmission range.…”

Na2Rb2SrB18O30 with [B9O19]11− fundamental building block consisting of three [B3O7]5− groups

“…(1) Type I: two [B 3 O 7 ] 5− groups connect with third [B 3 O 7 ] 5− via vertex‐sharing of oxygens from BO 3 and BO 4 units, respectively, such as KB 3 O 5 (or A x A’ y (B 3 O 5 ) 3 (A=Na, A’=K, x + y =3)) [38–39] . According to topological structure, (Na 0.8 K 0.2 )K 2 (B 3 O 5 ) 3 , NaK 2 (B 3 O 5 ) 3 , KB 3 O 5 , and Na(Na 0.17 K 0.83 ) 2 (B 3 O 5 ) 3 belong to Schläfli symbol of {4 2 ;6 4 }2{6 6 }; (2) Type II: two [B 3 O 7 ] 5− groups connect with third [B 3 O 7 ] 5− via vertex‐sharing of oxygens only from BO 4 units, such as AMB 9 O 15 (A=Li and Na, M=Sr and Ba) [40] and AA'M'B 18 O 30 (A=Li and Na, A’=Rb and Cs, M’=Sr, Ba and Pb) [41–43] . Although these two series feature different 4‐ c nodes, namely, one unique [B 3 O 7 ] 5− node for the former ones and three unique [B 3 O 7 ] 5− nodes for the latter ones, the Schläfli symbols of all these compounds belong to {6 4 ;8 2 } according to the topological structure.…”