New Alkaline-Earth Metal Fluoroiodates Exhibiting Large Birefringence and Short Ultraviolet Cutoff.

[NNadir]

The paper I'll discuss in this post is this one: New Alkaline-Earth Metal Fluoroiodates Exhibiting Large Birefringence and Short Ultraviolet Cutoff Edge with Highly Polarizable (IO3F)2– Units (Minqiang Gai,§ Tinghao Tong,§ Ying Wang,§ Zhihua Yang, and Shilie Pan, Chem. Mater. 2020, 32, 13, 5723–5728)

Organohalide contamination of the air, hydrosphere and land is a very serious environmental issue. From my perspective, the best and least demanding approach to ameliorating this long term problem is high energy radiation.

The highest energy radiation readily available on Earth is gamma radiation, however in most matrices this high energy radiation is down converted to lower forms of energy, x-rays and high energy UV. The ability to control and direct, that is to focus, this high energy radiation is more problematic than it is with visible light, and thus materials that have the ability to refract UV radiation is certainly of interest.

This is why this paper caught my eye.

I won't spend a lot of time discussing it, but just offer a brief excerpt and a few pictures to give a feel for the paper, which is, in any case, relatively short.

As a representative phenomenon in nature, the polarized light has led to plenty of discoveries and applications in which birefringent materials are one of core functional materials to regulate the polarization of light.(1,2) Currently, birefringent crystals, laser crystals, and nonlinear optical (NLO) crystals are the three fundamental crystal devices in all-solid-state lasers. Nevertheless, the difficulty in designing birefringent crystal materials is how to construct excellent fundamental building blocks (FBB) and realize the desirable arrangement of the anionic groups to increase the polarizability anisotropy of materials. The halogen atoms are introduced into metal borates, phosphates, carbonates, silicates, iodates, etc.,(3?15) leading to afford many opportunities for discovering new compounds with rich structural and compositional diversities.(16) Some commercial birefringent materials (YVO4,(17) TiO2,(18) LiNbO3,(19) CaCO3,(20) MgF2,(21) ?-BaB2O4 (?-BBO),(22) etc.) have been used widely in the near-infrared (NIR), visible, and ultraviolet (UV) regions. In view of structural chemistry, a main strategy for obtaining high optical anisotropy is to introduce the symmetric building units (SBU).(23?25) The triangular or planar structure group with p-conjugation is beneficial for large birefringence, including BO3, NO3, CO3, and C3N3O3.(26?32) Among these units, the coplanar BO3 groups are beneficial for having the large anisotropic polarizability...

...In this contribution, we reported one iodate Sr(IO3)2 (SIO) and two fluoroiodate compounds SrI2O5F2 (SIOF) and Ba(IO2F2)2 (BIOF). Among these compounds, SIOF and BIOF are the first reported cases in alkaline-earth metal fluoroiodates. It is noted that the (IO3F)2– units in fluoroiodates have not yet been reported. Interestingly, compared with the reported fluoroiodates, SIOF shows a large birefringence (cal. 0.203 at 532 nm) and its birefringence is about twice than that of SIO and BIOF. SIO, SIOF, and BIOF have short UV cutoff edges (255, 250, 230 nm) as well as good thermal stability. The mechanism of large birefringence source in SIOF and BIOF was discussed systematically...

Birefringent materials are materials that refract light - in this case UV light - differently - that is to a different degreen depending on orientation of the material with respect to beam path.

A few more comments from the paper about these properties:

In general, the birefringence is closely related to covalent behavior in the groups. Alkali and alkaline earth metal cations usually have weak covalency and make little direct contribution to the birefringence, such as Sr2+ and Ba2+. The total and partial density of states (TDOS and PDOS) of SIO, SIOF, and BIOF also reveal that the optical properties of above crystals are determined mainly by the anionic groups. The calculated frontier molecular orbital diagrams of (IO2F2)?, (IO3F)2–, (IOF4)?, (IO2F4)3–, (IO3)?, and (IO4)3– anionic groups from BIOF, SIOF, Cs(IOF4),(60) SbF5IF3O2,(62) SIO, and CsIO4(63) were investigated first by using the Gaussian 09 package at the LanL2DZ level without any structure optimization.(64) The presence of F atoms in the (IO3F)2– group, by the calculated frontier molecular orbital diagrams of the [IOF] and [IO] anionic groups, may cause the anisotropy of HOMO and LUMO, and this will affect the overall optical anisotropy of SIOF (Figure 2).

For most of my life, I've just thought of the fluoroiodates as potentially relevant materials to certain reprocessing schemes for used nuclear fuel and as, well, interesting curiosities. This paper points out I've been missing, overlooking, some things.

Some pictures from the text:



The caption:

Figure 1. Crystal structures of SIO, SIOF, and BIOF (a, b, c).

The caption:

Figure 2. Highest occupied molecular orbital (HOMO) and gaps from molecular orbitals of IO2F2, IO2F4, IO3, IO3F, IO4, and IOF4 units, respectively. Light blue and pink lines refer to the energy of the HOMO and lowest unoccupied molecular orbital (LUMO).

The caption:

Figure 3. (a) Calculated hyperpolarizabilities, HUMO–LUMO bandgaps, and static polarizability anisotropies of [IO] and [IOF] groups. (b, c) The reference axis of the minimum mean angle method in SIOF.

The conclusion:

In summary, we synthesized and characterized one iodate SIO and two alkaline-earth metal fluoroiodates SIOF and BIOF with large birefringences of 0.093, 0.18, and 0.092, respectively. These compounds exhibit short UV cutoff edges (255, 250, and 230 nm). The large birefringence in SIOF is mainly attributed to the highly polarizable (IO3F)2– anion groups through systematic analysis. Thus, we propose that the (IO3F)2– units may be a superior functional group to prompt us finding fabulous UV birefringent materials. These results indicate that the unexpectedly large birefringence of SIOF makes it to be a prospective UV birefringent material.

None of this means very much I suppose in the grand scheme of things, but I reflect on things of this nature thus: I often rail against what my generation has done to all future generations with terror and disgust. Irrespective of the fact that history will not forgive us, as some slight redeeming facet is that the world is left with all these tiny punctilios of knowledge that may be sign posts for the future to do what can be done to restore the world.

Being able to focus and move UV radiation certainly is something that may be of value.

I trust you are having a pleasant and safe summer evening in these very difficult times.

[eppur_se_muova]

1. Interesting ! nt