EPR Study of V⁴⁺ Ions in Tungsten Trioxide WO₃

Abstract

The EPR spectra from V⁴⁺ ions in WO₃ single crystal were studied in the temperature range from 300 K to 77 K by using a K-band spectrometer. At 77 K, the g values and hf parameters A of V⁴⁺ ions were strong c-axis anistropy, so that g_X=1.928, g_Y=1.945, g_Z=1.815, (Eulerian angles; θ_g=3.9°, φ_g=45.8°, ψ_g=−0.2°), and A_X=−60.5 G, A_Y=−55.0 G, A_Z=−190.4 G, (θ_A=4.4°, φ_A=43.6°, ψ_A=1.5°). The ground state of 3d¹ electron of V⁴⁺ ion was determined to be |xy⟩ state by hf structure analysis. WO₃ is determined to be a ferroelectric material with the spontaneous polarization along the c-axis connected to the condensed ferroelectric Γ₁₅^z phonon modes of W⁶⁺ ions in the monoclinic II phase at 77 K. At the relations of g_av vs A_iso for V⁴⁺ and VO²⁺ ions in some host crystals, WO₃ is shown to be a remarkable material.