Structural Evolution of CoMoO4 under Pressure: Multiphase Transformations and Reconstructive Behaviors

Binary transition metal molybdates are of significant research interest due to their distinctive properties, structural diversity, and promising applications. However, comprehensive investigations into the high-pressure structural behaviors and phase transition resistance of their multiphase forms are limited. This study examines the structural characteristics of the representative isostructural molybdate CoMoO4 under high pressure, using a diamond anvil cell combined with in situ laboratory Raman spectroscopy, X-ray diffraction, and synchrotron radiation X-ray diffraction techniques. β-CoMoO4 was found to undergo two gradual phase transitions up to 28 GPa, with the strip-shaped, thin samples exhibiting enhanced structural resistance compared to previous compression studies. The initial transition to the α-phase initiates at a relatively low pressure of approximately 1 GPa. A subsequent phase transition is observed, likely marking the emergence of a high-pressure phase (HP-phase) around 12 GPa, which then as the main phase at approximately 17–18 GPa with pronounced hysteresis. Upon pressure release from above 20 GPa, CoMoO4 predominantly reverts to the β-phase, with only a trace amount of the HP-phase retained. This study elucidates the pressure-induced structural reconstruction of thin CoMoO4 across its two-stage phase transitions, thereby enriching the fundamental understanding of binary transition metal molybdates under extreme conditions.Diamond anvil cells (DAC) were applied to compress samples to high pressures. T301 steel gaskets were used for pressure sealing. The 4:1 methanol-ethanol mixture was used as the pressure-transmitting medium (PTM) in laboratory Raman and X-ray diffraction experiments to achieve quasi-hydrostatic conditions up to 10 GPa. For synchrotron radiation X-ray diffraction experiments, silicon oil was used as the PTM for the lower-pressure range experiment, and 4:1 methanol-ethanol mixture was used for the higher-pressure range, respectively. Silicone oil deviates from hydrostatic state above 4 GPa. It is noteworthy that beyond the hydrostatic limit of the PTM, nonhydrostatic stresses inevitably develop. Pressure inside the DAC was determined by monitoring the fluorescence shift of Ruby balls (BETSA). In situ pressures were calibrated based on the IPPS-Ruby formula.In-situ powder Raman spectra data were obtained using a Horiba LabRAM HR Evolution Raman system, equipped with an SLWD 50× Olympus objective, a 532 nm laser excitation at 25 mW, and a 1800 grooves/mm grating. The Raman shift calibration error was within ±0.5 cm-1. To ensure accurate in-situ analysis, test zones were selected using an X, Y, Z-axis digital controller to avoid optical deviation caused by the diamond anvil.Powder X-ray diffraction patterns at ambient pressure were acquired using a PANalytical Empyrean powder diffractometer equipped with a Cu anode. Measurements were performed at room temperature in the 2θ range of 10° ‒ 50° and a collection time of 30 minutes. High-pressure Laboratory X-ray diffraction pattern were collected using a Bruker D8 Venture diffractometer with an Ag anode. Before data collection, alignment corrections were performed on both sides of the DAC to ensure the sample was positioned as close to the beam center, zero point, as possible, and aligned with the X-ray beam and detector. Debye-Scherrer ring images were collected with an exposure time of 60 s per image and then converted to 2D intensity data using DIFFRAC.EVA with 2θ range 3.8° ‒ 14.5° to avoid the diffraction from the gasket. Synchrotron radiation X-ray diffraction data were collected at the Beamline BL17UM station at the Shanghai Synchrotron radiation Radiation Facility (SSRF). The storage ring was operated at 3.5 GeV with a beam current of 220 mA in top-up mode. Crystallographic diffraction was performed at an energy of 13.422 keV in DMM mode, with a beam size of 75 µm and a photon flux of 3.23 × 1012 phs/s. The wavelength was calibrated to 0.4834Å using CeO2 as a standard. Diffraction patterns were collected using a DectrisEiger2 X 16M detector with an exposure time of 60 s per image. Debye-Scherrer ring images were processed into 2D intensity data by DIOPTAS. Rietveld refinements of lab and synchrotron radiation X-ray diffraction patterns were performed using GSAS II as well to determine unit cell parameters under pressure.