Now here's an interesting molecule, CsZrF4(IO3)

[NNadir]

It's described here: CsZrF4(IO3): The First Polar Zirconium Iodate with cis-[ZrO2F6] Polyhedra Inducing Optimized Balance of Large Band Gap and Second Harmonic Generation (Lin Lin, Xingxing Jiang, Chao Wu, Zheshuai Lin, Zhipeng Huang, Mark G. Humphrey, and Chi Zhang Chemistry of Materials 2021 33 (14), 5555-5562)

From the introductory text:

Second-order nonlinear optical (NLO) materials are of great commercial and academic significance owing to their applications as optical frequency multipliers, optical parametric amplifiers, electro-optic rectifiers, etc. (1−3) For a practical NLO crystalline material, it is necessary to simultaneously satisfy several capability requirements, including strong second-harmonic generation (SHG; >3 × KH2PO4 (KDP)), large optical band gap (>3.0 eV), high laser damage threshold (LDT), wide transparency window for broad-spectrum adaptability, moderate birefringence for phase-matching (PM) behavior, and good chemical and physical stability. (4−6) However, there are intricate relations between these key functions: SHG performance and optical band gap are mutually conflicting (6,7) and a wide band gap usually corresponds to a high LDT but small birefringence. (8) As a result, it is highly challenging to optimize these diverse properties in one material...

...n contrast to other d0 TM cations, the Zr4+ cation has long been neglected as an NLO-functional constituent in the development of NLO materials because it displays the weakest SOJT distortion of the d0 TM family; zirconium-based NLO materials therefore remain underexploited. Nevertheless, the Zr4+ cation is a popular choice as a metallic constituent in the synthesis of novel MOF materials because the oxophilic cation Zr4+ forms strong coordination bonds with oxygen-based organic donors, resulting in a high chemical and thermal stability of the structural framework. (22,23) Although the SOJT distortions of Zr4+-centered polyhedral are weak, they do result in asymmetric coordination, and this may enhance the prospects of constructing the NCS structures needed for an SHG response, so the Zr4+ cation may be a promising NLO-active component in the design of NLO iodate materials with high structural stability. The Zr4+ cation also manifests diverse coordination modes, affording multifarious [ZrOxFy] (x + y = 6–8) polyhedra with ordered O/F atoms...

SOJT = Second Order Jahn Teller distortion. The Jahn-Teller effect refers to a quantum mechanical effect whereby "degenerate orbitals" reorganize in such a way as to remove symmetry.

This molecule is "noncentrosymmetric" - which improves its non-linear optical properties.

This is not my field of interest, and so I have no real comment, but I would like to remark on some of the molecules properties.

First its synthesis:

Hydrothermal Synthesis of CsZrF4(IO3)
A mixture of Zr(NO3)4·5H2O (0.429 g, 1.0 mmol), I2O5 (0.501 g, 1.5 mmol), CsF (0.334 g, 2.2 mmol), 0.15 mL of hydrofluoric acid, and 2.0 mL of deionized water were loaded into a 23 mL Teflon-liner autoclave, which was kept at 210 °C for 72 h and then cooled to 30 °C at a rate of 4 °C per hour. After washing with deionized water to remove the excess acid, colorless CsZrF4(IO3) crystals were obtained in a yield of 71% (based on Zr). The pH values were 2.0 for the initial mixture and 2.3 for the final product. The purity of the crystalline sample was confirmed by powder X-ray diffraction (PXRD; Figure S1).

The molecule crystalizes from water; it is thus insoluble in water, and in fact, nitric acid, since nitric acid is a product of the reaction.

Then its thermal properties:

Thermogravimetric analysis reveals that crystalline CsZrF4(IO3) is thermally stable up to 430 °C (Figure S3). The thermogravimetric curve then exhibits a dramatic weight-loss in the temperature range of 430 to 580 °C, accompanied by an endothermic peak at around 470 °C in the DSC curve, indicating decomposition of the title compound; the weight-loss of 49.8% corresponds to the loss of 0.5 I2, 2 F2, and 1 O2 per formula unit (49.4% for theoretical value). When the temperature increases beyond 650 °C, the weight decreases gradually, which may result from continuous release of O2.

I oppose the dumping of so called "nuclear waste," but what is interesting is that this molecule contains three elements each of which has a very long lived fission product among the isotopes formed for the element.

Significant amounts of nonradioactive zirconium isotopes Zr-90, Zr-91, Zr-92 are formed during fission, the first being the decay product of Sr-90, with a half-life of around 28 years. (Thus pure non-radioactive Zr-90 can be obtained from fission products. Zr-93 is also formed along with the stable Zr isotopes, and it is radioactive with a very long half-life, 1.53 million years.

Three cesium isotopes are formed in fission: Cs-133 (the natural non-radioactive isotope), Cs-135 and Cs-137. Cs-137 is highly radioactive (and therefore very useful) with a half-life of 33.08 years, a reasonable rate of decay, but Cs-135 is radioactive and long lived, with a half-life of around 3 million years. (It's formation is suppressed in fission because it's precursor isotope, Xe-135, has the highest known neutron capture section of any readily available nuclide.)

Finally, besides the stable iodine-127 most radioactive iodine isotopes decay rather rapidly, the most famous of these being I-131 with a half-life of 8 days; arguably it is the most problematic isotope in a nuclear accident; it is also used in certain medical treatments such as hyperthyroidism and thyroid cancers and a few other cancers. Less problematic but far longer lived is iodine-129, with a half life of around 15 million years. (Much of the Xenon in the Earth's atmosphere is thought to have originated from this isotope decaying in the primordial earth.

Because these three isotopes are long lived, they are not highly radioactive and thus have a fairly low associated heat load, in cesium's case when much of the Cs-137 has decayed to stable barium. Thus this molecule, with its high thermal stability and insolubility might represent a "waste form," if one was - I don't recommend this - to declare fission products "waste."

As it happens, I can think of excellent uses for zirconium - particularly the tetrafluoride which has some remarkable properties, and cesium isotope mixtures, and iodine can easily be transmuted into valuable xenon.

Nevertheless this molecule might prove a useful way to store these elements for the short term, until their recovery is required for use.

Esoteric, I know, but cool.