EPR and vibrational studies of YVO4:Tm3+, Yb3+ single crystal

A well oriented YVO4 single crystal, with 5% Yb3+ and 2% Tm3+ nominal doping, was investigated using the Raman and EPR techniques.The EPR measurements suggest that Yb3+ ions occupy eight-coordinated Y3+ sites forming bisdisphenoids of the D2d symmetry. An inhomogeneous distribution of rare-earth ions leads to a significant distortion of the local point symmetry (C1). It seems that strong dipole–dipole interactions between Yb3+ ions are responsible for the distortion. As a result, two types of ytterbium magnetic centers appear. They correspond to paired magnetic centers and distorted isolated paramagnetic centers that are strongly sensitive to the magnetic field directions and some imperfections of the crystal. Pair centers can be recorded through the rotation around the c-crystal axis, whereas isolated centers can be measured when the crystal is rotated around the a-crystal axis. With the increasing temperature, the ytterbium signal disappeared at about 23 K and a group of narrow lines became visible. These lines, observed in the range of 240–550 mT, correspond to the Gd3+ (S = 7/2) ions, doped to the structure unintentionally from the basic materials.