The influence of dopants (Co, Cu, Fe and Ni) on the optical, electronic and magnetic properties of multiferroic MnWO was studied using Raman spectroscopy, ultraviolet-visible spectroscopy (UV-Vis), magnetization measurements and density functional theory (DFT) calculations. The evolution of Raman spectra with different elemental substitutions at the Mn site was also studied, where the peak width increased with doping of higher mass elements (Co, Cu, Fe and Ni). UV-Vis diffuse reflectance spectroscopy on polycrystalline Mn A WO (A  =  Co, Cu, Fe and Ni) (0  ⩽  [Formula: see text]  ⩽  0. was performed. The evaluated electronic band gap decreasing with successive Co, Cu and Fe doping reflected the lower ionic radius of the substituted element, and for Ni-doped MnWO the band gap increased slightly compared to the parent MnWO. Bader charge transfer and a partial density of states (PDOS) analysis from DFT simulations predict the appearance of impurity states in the band gap region (of pure MnWO) from the d orbital of the dopant (Co, Cu and Fe) hybridized with the p orbital of the bonded O atoms due to charge transfer from O to the dopant, and reduced the band gap of Co, Cu and Fe-doped MnWO. On the other hand, for Ni-doped MnWO strong W-O hybridization occurring due to large charge transfer from oxygen to tungsten leads to an increase in the band gap. The band gap, computed using the GGA  +  U method, is close to the experimental value. The signature of the d-d transition observed in the UV spectra is explained in terms of the crystal field stabilization energy caused by the octahedral distortion present in the lattice. Three different antiferromagnetic phases (AF1, AF2 and AF3) are identified in MnWO and also for the Co (18.75%)-doped sample. For Cu-doped samples, suppression of the AF1 phase and stabilization of the AF2 phase is observed up to 2 K. Successive doping of Cu leads to the diminution of magnetic frustration. A new magnetic order is identified for Ni-doped MnWO in the temperature range 13.7-20 K.

Phase diagram of FeVO4-CrVO4 solid solutions pertinent with structural and magnetic phases is presented with unambiguous experimental evidences. Solid solutions Fe1–xCrxVO4 (0 ≤ x ≤ 1.0) were synthesized through the standard solid state route and studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectra of X-rays, Raman spectroscopy, d.c. magnetization, and 57Fe Mössbauer spectroscopic studies. FeVO4 and CrVO4 were found to be in triclinic (P-1 space group) and orthorhombic structures (Cmcm space group), respectively. Cr incorporation into the FeVO4 lattice leads to the emergence of a new monoclinic phase dissimilar to the both end members of the solid solutions. In Fe1–xCrxVO4 up to x = 0.10, no discernible changes in the triclinic structure were found. A new structural monoclinic phase (C2/m space group) emerges within the triclinic phase at x = 0.125, and with the increase in Cr content, it gets stabilized with clear single phase signatures in the range of x = 0.175–0.25 as evidenced by the Rietveld analysis of the structures. Beyond x = 0.33, orthorhombic phase similar to CrVO4 (Cmcm space group) emerges and coexists with a monoclinic structure up to x = 0.85, which finally tends to stabilize in the range of x = 0.90–1.00. The Raman spectroscopic studies also confirm the structural transition. FeVO4 Raman spectra show the modes related to three nonequivalent V ions in the triclinic structure, where up to 42 Raman modes are observed in the present study. With the stabilization of structures having higher symmetry, the number of Raman modes decreases and the modes related to symmetry inequivalent sites collate into singular modes from the doublet structure. A systematic crossover from two magnetic transitions in FeVO4, at 21.5 K and 15.4 K to single magnetic transition in CrVO4, at 71 K (antiferromagnetic transition), is observed in magnetization studies. The intermediate solid solution with x = 0.15 shows two magnetic transitions, whereas in the compound with x = 0.33 one of the magnetic transitions disappears. 57Fe Mössbauer spectroscopic studies show a finger print evidence for disappearance of non-equivalent sites of Fe as the structure changes from Triclinic–Monoclinic–Orthorhombic phases with the increasing Cr content in Fe1–xCrxVO4. Comprehensive studies related to the structural changes in Fe1–xCrxVO4 solid solutions lead us to detailed phase diagrams which shall be characteristic for room temperature structural and temperature dependent magnetic transitions in these solid solutions, respectively.

Insertion of lead and lead telluride in Bi2Te3 leads to a change in the thermal conductivity, frequency shift, and the broadening of phonon modes.

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