The effects of substituting nonmagnetic Mg 2+ and Zn 2+ ions for the Mn 2+ (S = 5/2) ions on the structural, magnetic and dielectric properties of the multiferroic frustrated antiferromagnet MnWO 4 were investigated. Polycrystalline samples of Mn 1-x Mg x WO 4 and Mn 1-x Zn x WO 4 (0 ≤ x ≤ 0.3) solid solutions were prepared by a solid-state route and characterized via X-ray and neutron diffraction, magnetization, and dielectric permittivity measurements. Mg and Zn substitutions give rise to very similar effects. The Néel temperature T N , the AF3-to-AF2 magnetic phase transition temperature T 2 , and the critical ferroelectric temperature T c = T 2 of MnWO 4 are reduced upon the nonmagnetic doping.At the lowest temperature (T = 1.5 K), the incommensurate magnetic structure for x(Mg) = 0.15 and x(Zn) = 0.15 corresponds to either a sinusoidal spin arrangement or an elliptical spin-spiral phase similar to the polar AF2 structure observed in MnWO 4 . These findings were discussed by considering the effects of the Mg and Zn substitutions on the crystal lattice and on the spin exchange network of MnWO 4 .

Ceramic samples of MnW 1−x Mo x O 4 (x ≤ 0.3) solid solution were prepared by a solid-state route with the goal of increasing the magnitude of the spin-exchange couplings among the Mn 2+ ions in the spin spiral multiferroic MnWO 4 . Samples were characterized by X-ray diffraction, optical spectroscopy, magnetization, and dielectric permittivity measurements. It was observed that the Neél temperature T N , the spin spiral ordering temperature T M2 , and the ferroelectric phase-transition temperature T FE2 of MnWO 4 increased upon the nonmagnetic substitution of Mo 6+ for W 6+ . Like pure MnWO 4 , the ferroelectric critical temperature T FE2 (x) coincides with the magnetic ordering temperature T M2 (x). A density functional analysis of the spin-exchange interactions for a hypothetical MnMoO 4 that is isostructural with MnWO 4 suggests that Mo substitution increases the strength of the spin-exchange couplings among Mn 2+ in the vicinity of a Mo 6+ ion. Our study shows that the Mo-doped MnW 1−x Mo x O 4 (x ≤ 0.3) compounds are spin-frustrated materials that have higher magnetic and ferroelectric phase-transition temperatures than does pure MnWO 4 .