Elucidating Structure–Spectral Property Relationships of Negative Thermal Expansion Zr₂(WO₄)(PO₄)₂: A First-Principles Study with Experimental Validation

The phonon, infrared, and Raman spectroscopic properties of zirconium tungsten phosphate, Zr₂(WO₄)(PO₄)₂ (space group Pbcn, IT No. 60; Z = 4), have been extensively investigated using density functional perturbation theory (DFPT) calculations with the Perdew, Burke, and Ernzerhof exchange–correlation functional revised for solids (PBEsol) and validated by experimental characterization of Zr₂(WO₄)(PO₄)₂ prepared by hydrothermal synthesis. Using DFPT-simulated infrared, Raman, and phonon density-of-state spectra combined with Fourier transform infrared and Raman measurements, new comprehensive and extensive assignments have been made for the spectra of Zr₂(WO₄)(PO₄)₂, resulting in the characterization of its 29 and 34 most intense IR- and Raman-active modes, respectively. DFPT results also reveal that ν₁(PO₄) symmetric stretching and ν₃(PO₄) antisymmetric stretching bands have been interchanged in previous Raman experimental assignments. Negative thermal expansion in Zr₂(WO₄)(PO₄)₂ appears to have very limited impact on the spectral properties of this compound. Furthermore, this work shows the high accuracy of the PBEsol exchange–correlation functional for studying the spectroscopic properties of crystalline materials using first-principles methods.