We report on the magnetic properties of disordered Li2Co2 – xNix(MoO4)3 (0 ≤ x ≤ 2) system with a lyonsite structure, where Li+, Ni2+, and Co2+ ions are distributed in a disordered manner in the trigonal prism and octahedral sites. All compositions were found to exhibit paramagnetic behavior at high temperature; the magnetic susceptibility was described by the modified Curie-Weiss law. The experimental effective moments were explained by spin and orbital contributions of the Co2+ ions and only the spin contribution for Ni2+ ions. The negative Curie-Weiss temperature θ indicates the existence of antiferromagnetic coupling for all compositions. At low temperature, the compounds with Co2+ ions (x = 0, 1, 1.6, and 1.8) exhibit a weak ferromagnetic state below 4 K due to intrinsic canting of antiferromagnetically coupled lattices. The compound containing only Ni2+ ions, Li2Ni2(MoO4)3, exhibits a pure antiferromagnetic behavior.

A new rare earth ferroelectric tetragonal tungsten bronze compound with general formula Ba2.15−x Na0.7+x Nb5−x W x O15 (x = 0.25) was elaborated as ceramic and investigated using X-ray diffraction and dielectric measurements. The results show a tetragonal phase with the space group P4bm. Special emphasis was done to characterize diffuse phase transition (DPT) that occurs close to 472 °C. Using dielectric measurements in a frequency range of (10 Hz–1 MHz) and in a temperature range (25–550 °C), we have shown that the real part of the permittivity close to DPT is well described by Santos–Eiras phenomenological model. Space charge polarization, relaxation phenomena and free charges conductivity have been analyzed using dielectric, impedance spectroscopy and Nyquist plots showed non-Debye (polydispersive) type relaxation. In paraelectric phase, the Arrhenius activation energies were determined. Frequency dependence of ac conductivity at different temperatures follows the Jonscher’s universal law (Jonscher et al. in J Mater Sci 20:4431, 1985).

A solid solution was found to exist in the quaternary Li2O–CoO–NiO–MoO3 system between the two phases Li2Co2(MoO4)3 and Li2Ni2(MoO4)3. Both Li2Co2(MoO4)3 and Li2Ni2(MoO4)3 are isostructural with the mineral lyonsite, and substitution according to the formula Li2Co2−xNix(MoO4)3 (0⩽x⩽2) demonstrates that a complete solid solution exits. Rietveld analysis revealed that solid solution crystallizes at room temperature in orthorhombic space group Pnma (D2h). The structure shows that if the corners and edges octahedra share in the leaves, they share faces columns. Nickel and cobalt are shared statically in the trigonal prism and octahedral sites. The results of these compositions are consistent with studies where the Raman spectra. Therefore, all compositions are similar and show linear changes frequencies depending on the composition due to the substitution of Co2+ by Ni2+ with a larger radius.

Crystal structures of Ba2.15−xNa0.7+xNb5−xWxO15 (BaNaNbW) materials were investigated by X-ray diffraction and refined using the Rietveld method. It is shown that all the samples crystallize in the tungsten tetragonal bronze (TTB) like structure and form a solid solution in the range (0⩽x⩽1). To study the temperature induced phase transition in these compounds, Raman spectra were collected in situ at room pressure and elevated temperatures, up to 330°. The Raman results show strong evidence that the materials under study go through a phase transition around Te=270°C. The value of the transition temperature depends on the chemical composition and increases with increasing tungsten content in the materials at a linear rate. The observation of this phase transition by Raman technique is in agreement with dielectric spectroscopy anomaly.