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The chemistry of hypercoordination has been a subject of fundamental interest, especially for understanding structures that challenge conventional wisdom. The small ionic radii of Fe ions typically result in coordination numbers of 4 or 6 in stable Fe‐bearing ionic compounds. While 8‐coordinated Fe has been observed in highly compressed oxides, the pursuit of hypercoordinated Fe still faces significant challenges due to the complexity of synthesizing the anticipated compound with another suitable anion. Through first‐principles simulation and advanced crystal structure prediction methods, we predict that an orthorhombic phase of FeF3 with exclusively 8‐coordinated Fe is energetically stable above 18 GPa—a pressure more feasibly achieved compared to oxides. Inspired by this theoretical result, we conducted extensive experiments using a laser‐heated diamond anvil cell technique to investigate the crystal structures of FeF3 at high‐pressure conditions. We successfully synthesized the predicted orthorhombic phase of FeF3 at 46 GPa, as confirmed by in situ experimental X‐ray diffraction data. This work establishes a new ionic compound featuring rare 8‐coordinated Fe in a simple binary Fe‐bearing system and paves the way for discovering Fe hypercoordination in similar systems. [ABSTRACT FROM AUTHOR]

• The Pt-H compounds were synthesized with the diamond anvil cell technique. • The experimental compression behavior of hcp PtH is determined above 100 GPa. • The structural relationship between PtH and SiH 4 is expounded directly. To date, the exact structure of PtH, the key to resolve the controversial hexagonal close-packed (hcp) phase of SiH 4 , is still absent above 100 GPa. Therefore, PtH has been synthesized and its compression behavior is determined through the angle dispersive X-ray diffraction method. We find that hcp PtH keeps stable up to at least 128 GPa and its experimental lattice parameters are highly consistent with those of the hcp one in the SiH 4 experiment. The present results provide reliable experimental information of PtH under high pressure, and directly clarify the debate between hcp PtH and "SiH 4 ". [ABSTRACT FROM AUTHOR]

Abstract: We investigated the high pressure phases of CdF2 by a joint theoretical and experimental study. The structural and electronic properties of CdF2 were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type (Fm-3m, ) structure to the cotunnite-type (Pnma, ) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF2 under high pressure. [Copyright &y& Elsevier]

Recent room temperature experiments on TiH2 [Kalita et al., J. Appl. Phys. 108, 043511 (2010)], an important compound in hydrogen storage research, revealed a cubic (fcc, Fm-3m) to tetragonal (bct, I4/mmm) phase transition at around 0.6 GPa, which was suggested to remain stable up to at least 90 GPa. However, the simulated X-ray diffraction (XRD) pattern of the I4/mmm structure cannot explain all the diffraction peaks observed at 90 GPa. In this article, we apply the recently developed particle swarm optimization algorithm for crystal structure prediction to propose that at 63 GPa TiH2 presents a further phase transition from I4/mmm to P4/nmm, which we have also confirmed is dynamically and enthalpically stable up to 294 GPa. Moreover, the XRD patterns and calculated lattice parameters of the P4/nmm structure are in good agreement with the experimental available data. Above 294 GPa, we predict a monoclinic P21/m structure to be stable. [ABSTRACT FROM AUTHOR]

• A new defective I 4 /mmm phase appears to be the high-pressure structure of Cu 1.875 Te above 2.9 GPa. • The high-pressure phase of Cu 1.875 Te has potential to be stabilized to ambient conditions as a metastable material. • We proposed a possible way to synthesize stoichiometric Cu 2 Te without deficiency. Cu 2−x Te is mainly used as conductive back contacting material in CdTe solar cells. Unavoidable Cu deficiency has traditionally been considered as the primary cause of structural complexity, which limited its practical applicability. Here, combining X-ray diffraction (XRD) measurement with extensive swarm structure search, we identify the ambient structure of Cu 1.875 Te to be a P- 3 m 1 phase, which transforms into a new defective I 4 /mmm phase at 2.9 GPa. Enthalpy calculations show that stoichiometric Cu 2 Te is expected to be synthesized above 5.3 GPa. The analysis of mechanical and dynamical stability implies that the high-pressure I 4 /mmm Cu 2 Te can potentially be stabilized to ambient conditions as a metastable material. Our results represent a significant step in understanding the crystal information of Cu 2−x Te. [ABSTRACT FROM AUTHOR]

The stabilities and phase evolutions of the binary Bi–S compounds are investigated by using synchrotron radiation X-ray diffraction in a diamond anvil cell in combination with laser heating technique. When it is heated to 1300 K at 32.6 GPa, Bi 2 S 3 decomposes into element S and a new BiS phase, which can be quenched to ambient conditions. The present results verify that cubic BiS, with an unprecedented stoichiometry and novel bonding characteristics, is at least metastable. It may shed new light on the fundamental sciences and potential technologies of the group V-VI binary compounds. • The stabilities and phase evolutions of the binary Bi-S compounds under extreme conditions are investigated in detail. • Advanced synchrotron radiation X-ray diffraction in combination with laser heating technique is used. • Bi 2 S 3 is thermodynamically unstable and decompose into a novel phase with an unprecedented BiS stoichiometry. • The present results verify that cubic BiS, with an unprecedented stoichiometry, is at least metastable. [ABSTRACT FROM AUTHOR]