For decades, borides have been primarily studied as crystallographic oddities. With such a wide variety of structures (a quick survey of the Inorganic Crystal Structure Database counts 1253 entries for binary boron compounds!), it is surprising that the applications of borides have been quite limited despite a great deal of fundamental research. If anything, the rich crystal chemistry found in borides could well provide the right tool for almost any application. The interplay between metals and the boron results in even more varied material's properties, many of which can be tuned via chemistry. Thus, the aim of this review is to reintroduce to the scientific community the developments in boride crystal chemistry over the past 60 years. We tie structures to material properties, and furthermore, elaborate on convenient synthetic routes toward preparing borides.

Rediscovering the Crystal Chemistry of Borides.

The structural high-pressure properties of $\mathrm{Th}{\mathrm{O}}_{2}$ and $\mathrm{U}{\mathrm{O}}_{2}$ have been studied in diamond-anvil cells up to maximum pressures of 80 and 69 GPa, respectively. These results show that both thorium and uranium dioxides exhibit an identical sequence of structural transitions under pressure; both transform rather sluggishly to the cotunnite-type (orthorhombic $Pnma$) high-pressure structure. This study which was performed under better hydrostatic conditions than previous experiments has enabled us to determine reliable compressibility parameters for the two dioxides: $\mathrm{Th}{\mathrm{O}}_{2}$: ${B}_{0}=198(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.6(3)$; $\mathrm{U}{\mathrm{O}}_{2}$: ${B}_{0}=207(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.5(4)$. In the case of $\mathrm{U}{\mathrm{O}}_{2}$ the ambient pressure cubic phase was still found to be present at $69\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which is in contradiction with earlier measurements. With regards to these results and re-calculations performed on other actinide dioxides $\mathrm{Pu}{\mathrm{O}}_{2}$ and $\mathrm{Am}{\mathrm{O}}_{2}$, we obtain a different evolution of the bulk moduli through the actinide dioxide series.

Behavior of actinide dioxides under pressure: U O 2 and Th O 2

We report ab initio calculations of the structural, electronic, optical, and thermodynamic properties of plutonium oxides (${\text{PuO}}_{2}$ and $\ensuremath{\beta}{\text{-Pu}}_{2}{\text{O}}_{3}$). In order to describe the basic features of the electronic structure, a method suited to take into account strong local correlations has to be used. We apply the local density approximation/generalized gradient approximation ($\text{LDA}/\text{GGA})+U$ approximations to these compounds and compare them with the calculations of Sun et al. [J. Chem. Phys. 128, 084705 (2008)]. Whereas a good agreement is obtained for ${\text{PuO}}_{2}$, our LDA and $\text{LDA}+U$ results differ strongly from this study in the case of ${\text{Pu}}_{2}{\text{O}}_{3}$. In particular, the effect of the Hubbard parameter $U$ on the volume is qualitatively and quantitatively different. Moreover, thermodynamic quantities differ. We thus focus our study on ${\text{Pu}}_{2}{\text{O}}_{3}$ and emphasize the importance of a careful and systematic search of the ground state in $\text{LDA}+U$: In particular, different hints for the occupation matrices corresponding to the electronic configurations allowed by symmetry have to be tried. This procedure is absolutely necessary to find the absolute minimum of the energy. Reliable and accurate quantitative results are given for ${\text{Pu}}_{2}{\text{O}}_{3}$. We thus recover a more physical behavior coherent with calculations on other systems, such as cerium oxides.

Structural, thermodynamic, and electronic properties of plutonium oxides from first principles

Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure induced structural transition of PuO$_{2}$. To properly describe the strong correlation in the Pu $5f$ electrons, the local density approximation$+U$ and the generalized gradient approximation$+U$ theoretical formalisms have been employed. We optimize the $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground state fluorite structure and high pressure cotunnite structure. The best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within the LDA$+U$ approach. After carefully testing the validity of the ground state, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite PuO$_{2}$. Some thermodynamic properties, e.g., the Gibbs free energy, volume thermal expansion, and specific heat, are also calculated. As for cotunnite phase, besides the elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for PuO$_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75-133 GPa are predicted.

Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

CeO2 transforms to an orthorhombic PbCl2-type structure at a pressure of about 31 GPa. The phase transition is accompanied by a 9.8% volume contraction. The bulk modulus of the low-pressure fluorite-type structure is 236(4) GPa. Comparisons are made with the high-pressure behaviour of UO2 and ThO2.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0885715600017966

Powder diffraction analysis of cerium dioxide at high pressure

Thorium and plutonium dioxides were studied under pressure by the energy dispersive X-ray diffraction method. A double conical slit assembly was used to collect simultaneously the diffracted radiation at five and seven degrees. ThO2 undergoes a phase transformation at 40 GPa. The high-pressure phase remains stable up to 55 GPa, the highest pressure reached in the experiment. For PuO2, a structural transformation occurs near 39 GPa. The observed high-pressure phases of ThO2 and PuO2 exhibit similar diffraction spectra. Like for some other fluorite type compounds, the ThO2 and PuO2 high-pressure phase has been indexed in the PbCl2-type structure. The bulk modulus has been calculated as B0= 262 GPa with a pressure derivative of B0' = 6.7 for ThO2 and as B0 = 379 GPa with B0' = 2.4 for PuO2. The volume decrease at the transition is 12% for PuO2 and 8% for ThO2.

Pressure-induced phase transition in ThO2 and PuO2

Pressure-induced phase transition in α-Pu

High-pressure X-ray diffraction studies were performed on ThOS up to 43.3 GPa and on UOSe up to 47.5 GPa, at room temperature, using a diamond anvil cell and synchrotron radiation. The tetragonal structure ( P 4/ nmm ) of these compounds was retained over the whole pressure range. The bulk modulus B 0 and its pressure derivative B 0 ′ were determined for each compound.

High-pressure X-ray diffraction study of ThOS and UOSe by synchrotron radiation

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP1-x -Sx (X=0.1; 0.25; 0.4) up to 53 GPa UP1-x Sx is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP1-x Sx with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP1-x Sx 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.

High-pressure phase transitions in UP1-xSx

Equipment for high pressure experiments at variable temperatures is described. Measurements of U6Fe at ambient temperature up to 50 GPa show no phase transition. The bulk modulus B0=135 GPa and its pressure derivative (B′0=4.4) were obtained for U6Fe. Measurements of Ce were performed both at ambient temperature up to 64 GPa, and at 93 K up to 43 GPa. The tetragonal bc phase of Ce that forms above 26 GPa is unchanged in the extended pressure range, but shows less compressibility at low temperature.

Developments and new possibilities in high pressure powder diffraction with synchrotron radiation. Results for cerium metal and U6Fe

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