The behaviors of transition metals and their oxides under high-temperature and high-pressure conditions have been studied extensively over a few decades, and knowledge about such material has important applications in physics, materials science, and engineering (Austin and Mott, 1970; Errandonea, 2006). Copper and its oxides are among the most investigated transition-metal materials. Cuprous oxide is a high-temperature semiconductor and one promising candidate materials for photo-electrochemical applications (Maksimov, 2000; Laskowski et al., 2003; Khanna et al., 2007). However, variations in physical properties and structural change of cuprous oxide Cu₂O at extreme conditions have not been fully investigated.

Cu₂O crystalizes in a simple cubic Bravais lattice with space group Pn-3m under normal thermodynamic conditions, while it has numerous structure forms at extreme conditions (Machon et al., 2003; Cortona and Mebarki, 2011; Feng et al., 2017). Most of previous studies have focused on the structural variations under high P-T conditions based on first-principles calculations. Cortona and Mebarki (2011) described the transition from cubic Cu₂O to the CdI₂-type structure (hexagonal, R-3m) at 10 GPa, while Feng et al. (2017) suggested two phase transitions, one at 5 GPa (Pn-3m→R-3m) and the other at 12 GPa (R-3m→R-3m1). However, this is hard to reconcile with some experimental results on the pressure-induced structural transformations of Cu₂O. A tetragonal phase was demonstrated by Machon et al. (2003) at pressures between 0.7 and 2.2 GPa using angle-dispersive powder X-ray diffraction (XRD), and another pseudocubic phase was detected at ∼8.5 GPa. What is more, Sinitsyn et al. (2004) found a new hexagonal phase with lattice parameters of a = 5.86 Å and c = 18.78 Å at 21 GPa which was significantly different from those reported earlier by Werner and Hochheimer (1982), who studied a hexagonal phase with CdCl₂-type structure with a = 2.82 Å and c = 12.7 Å measured at 18 GPa. Therefore, phase relations of Cu₂O at higher pressures are more sparse and show less mutual agreement. Further investigation is needed to study the exact high temperature and high pressures phases of Cu₂O.

There are limited studies on the behavior of cuprous oxides and copper chalcogenides under high pressure and high temperature conditions. In this present work, we report the phase transformations and elastic properties of Cu₂O up to ∼30 GPa at room temperature, by using synchrotron-based single-crystal XRD with diamond anvil cell (DAC). We confirmed the tetragonal phase Cu₂O observed between 10.4 and 13.8 GPa, and a novel monoclinic phase at higher pressures is also reported. As is well-known, physical properties of materials can be modified by tailoring either chemical composition or microstructure. These results can improve our knowledge of how pressure affect their elastic and physical properties. In this report we also present the measured compressibilities and equations of states of these high pressure phases of Cu₂O.

The cuprite measured in this study was originated from the Tonglushan Copper Miners in Daye, Hubei province. Single-crystal samples of natural, brick-red cuprite were selected as the starting material of our study. We screened several chips and polished them to ∼10 μm in thickness. At ambient conditions, the Cu₂O was characterized in an empty diamond anvil cell (DAC) by single crystal X-ray diffraction, at the GSECARS beamline 13-BM-C of the Advanced Photon Source (APS), Argonne National Laboratory (Zhang et al., 2017). Diffraction collected at ambient conditions showed that the cuprite crystal had a Pn-3m space group with a = 4.2733(7) Å, and the Cu₂O samples with a purity of 99.99% were used for the current study.

High-pressure compression measurements were performed, using short symmetric DACs fitted with Boehler-Almax diamond anvils with 300 μm flat culets and mounted into seats with 60° opening. Rhenium gaskets were preindented to ∼40 μm thickness, and holes were drilled to ∼170 μm diameter for the samples. On compression, the gasket thickness and sample chamber diameter both decrease to ∼20 μm at the highest pressures reached. The polished Cu₂O sample was loaded together into the sample chamber along with Pt foil for pressure calibration (Fei et al., 2007). To achieve quasi-hydrostatic conditions and maintain similar pressure environments everywhere in the sample chamber, we loaded the cell with neon as the pressure-transmitting medium using the COMPRES/GSECARS gas-loading system (Rivers et al., 2008).

In situ high-pressure single-crystal X-ray diffraction experiments on Beamline 13-BM-C used a monochromatic X-ray beam with a wavelength of 0.4340 Å and focused to a 15 × 15 μm² spot. The experimental details were also described previously (Qin et al., 2017; Zhang et al., 2017). To obtain adequate number of diffraction peaks of samples and increase the coverage of the reciprocal space, we collected data at four different detector positions. The diffraction images were analyzed using the ATREX/RSV software package (Dera et al., 2013). Integrated diffraction data were analyzing using the DIOPTAS software (Prescher and Prakapenka, 2015), the high-pressure synchrotron XRD patterns were indexed by Dicvol06 (Louër and Boultif, 2007). Lattice parameters were calculated by the program UnitCell and the Le Bail refinement by GSAS (Holland and Redfern, 1997; Toby, 2001).

The high-pressure behavior of cuprous oxide Cu₂O has attracted broad interests due to their various structures at different P-T conditions and its potential applications. Several previous studies have been published which focus on the phase transitions and decompositions of Cu₂O and its thermodynamic properties as well (Machon et al., 2003; Sinitsyn et al., 2004; Cortona and Mebarki, 2011; Liu et al., 2014; Feng et al., 2017). However, these previous results were unclear about the high-pressure crystal structure of Cu₂O (tetragonal or hexagonal phase) and it is obvious that a large discrepancy of transformation pressure between the cubic and high-pressure phases existed between literatures. In this study, we re-confirmed the phase transition sequence from cubic-to-tetragonal phase occurred between 8.4 and 10.4 GPa using single-crystal XRD, which is significantly more higher than previously estimated (Machon et al., 2003). A new high-pressure phase of Cu₂O was also indexed using Dicvol06 software (Louër and Boultif, 2007). A volume collapse of ∼7.7% in the region of 13.8–16.1 GPa was observed during the structural transformation from cubic to monoclinic phase (Figure 3). There is no indication of structural decomposition in this case. In addition, it is obvious that a considerable anisotropy in axial compressibility with β _b > β _c > β _a and we found the ratio of zero-pressure axial compressibility is 1.00:1.64:1.45 according to our data. Thus, it can be concluded that the largest anisotropy in compressibility is along the b axis, which behaves about twice as compressible than the a-axis in the structure (Figure 4). Further investigation should be done to investigate the exact high temperature phases of Cu₂O.

In some recent studies, several members of copper chalcogenides, such as Cu₂S and Cu₂Se have been theoretically proposed and experimentally exhibited as the thermoelastic materials (Danilkin et al., 2011; Santamaria-Perez et al., 2014; Zhang et al., 2018; Zimmer et al., 2018; Xue et al., 2019). The high-pressure phase Cu₂O, Cu₂Se and Cu₂S may adopt the same monoclinic structure at different pressure conditions, which indicates that these copper compounds may have a similar crystal chemistry configuration (Santamaria-Perez et al., 2014; Zhang et al., 2018). The structural complexity of Cu₂S have been studied previously, in which two phase transitions occurred at 3.2 and 7.4 GPa from the P21/c phase to two different monoclinic structures (Santamaria-Perez et al., 2014). Pressure-induced structural transition sequence is also identified in Cu₂Se. The initial low-pressure phase (C2/c) transformed to phase II and semimetallic phase III at 3.2 GPa, and then followed by a reconstructive transformation to bulk metallic phase IV (Pca21) at 7.4 GPa. These mentioned phases could be confidently associated with the electronic state transitions (Zhang et al., 2018; Chuliá-Jordan et al., 2020). As for the copper sulfide, three phase-transitions occurred at 3.2, 7.4 and 26 GPa, respectively, and there is a significant difference of the reported values of bulk modulus, ranging from 72 to 113 GPa (Santamaria-Perez et al., 2014). The determination of the phase stability of such stoichiometric copper oxides under compression will give more insight into possible systematic trends in group copper chalcogenides, and provide a direct comparison with such thermoelastic materials at extreme conditions where their phase behaviors could converge.

The high-pressure behaviors of cuprous oxide Cu₂O have been studied by synchrotron-based single-crystal XRD at pressures up to ∼30 GPa at 300 K conditions. The initial low-pressure cubic phase transforms to distorted P4₂/nnm phase between 10.4 and 13.8 GPa, and the tetragonal structure persisted at pressure up to ∼16 GPa. A new high-pressure phase of Cu₂O (monoclinic phase, P1a or P12/a1) was obtained at 16.1 GPa at room temperature and the high-pressure elastic properties was firstly measured in this study. It is expected that the three times more compressible high-pressure phase Cu₂O may process some advantages properties than previously thought. The findings contribute to broadening our knowledge of the crystal chemistry of cuprite at high-pressure conditions, thus giving a better understand of thermoelastic materials in the copper chalcogenide system.