Experimental and Theoretical Investigations on the Structural, Electronic, and Vibrational Properties of β-Bi ₂ Mo ₂ O ₉ Dibismuth Dimolybdenum

This study presents a comprehensive structural, vibrational, and electronic investigation of the monoclinic β-Bi2Mo2O9 compound, employing experimental and first-principles approaches. X-ray diffraction combined with Rietveld refinement confirms the crystallization of Bi2Mo2O9 in the P21/n space group. Density functional theory calculations within the LDA-D framework reveal a slight underestimation of the lattice parameters and unit cell volume, while preserving local geometries. The vibrational properties were examined through Raman and infrared spectroscopy, supported by group-theory analysis, indicating a rich phonon activity consistent with the complex symmetry and multiatom basis of the monoclinic lattice. Electronic structure calculations identify BMO as an indirect band gap semiconductor with a computed band gap of 2.23 eV, closely matching experimental optical data. Bader charge and electron localization function (ELF) analyses highlight a mixed ionic–covalent bonding character, particularly pronounced in the Mo–O sublattice. Low-temperature Raman spectra (12–300 K) revealed systematic redshifts, peak broadenings, and intensity reductions across low- and high-wavenumber phonon modes, indicative of pronounced anharmonic effects and thermal expansion of the lattice. On the contrary, high-pressure Raman spectra collected up to 9.08 GPa showed a general phonon hardening trend with increasing pressure, attributable to lattice compression. Several Raman modes exhibited anomalous behavior, such as mode appearance, disappearance, and slope changes, pointing to pressure-induced phase transitions and potential symmetry changes.