Synthesis and characterisation of In-doped MnWO4-type solid-solutions: Mn1−3xIn2x□xWO4 (x=0–0.11)

• Investigation of Mn(1−3x)In2x□xWO4 solid solutions (x=0–0.11).
• An unexpected lattice expansion by In-doping.
• Strong change in local environment around bridging oxygens.
• Enhancement of atomic bonds in W2O4 and WO2 units with increasing In-doping.

Substitution of In3+ for Mn2+ in MnWO4, which is one of the most studied multiferroics, was conducted systematically by solid-state reactions at 1050 °C to obtain a series of Mn(1−3x)In2x□xWO4 solid solutions (x=0–0.11; the box indicates vacancy). The successful In-doping could be proven by electron microprobe, and statically disordered structures of this solid solution series were elucidated by Rietveld analyses with X-Ray powder diffraction (XPD) data. Its average structure maintains the space group P2/c at least up to the replacement of 33 at% Mn. The tendency of their unit cells proportionally enlarging with In-doping is an unexpected observation because In3+ is smaller than Mn2+. This could not be explained alone by average atomic bonding distances and angles evaluated from XPD data analyses. Complimentarily, micro-Raman spectra of Mn(1−3x)In2x□xWO4 materials feature a hardening of Raman modes due to stronger atomic bonds in W2O4 and WO2 units with increasing In-doping. This is considered as an inevitable consequence of broken bonding in directly neighbouring Mn–O bonds as the presence of In3+ in MnWO4 accompanies the creation of defects at Mn sites. The current study using XRD and micro-Raman suggests the enlarging local space around the bridging oxygen as the reason for the unexpected cell expansion when smaller In3+ along with defects replace Mn2+ in MnWO4.

Despite of substitution of smaller In3+ for larger Mn2+, the In-rich solid solution Mn0.71(2)In0.19(1)□0.10WO4 undergoes a volume expansion of 0.4% with respect to MnWO4. This could be explained by a local space expansion of 0.13% around bridging oxygen at O2 sites, as illustrated by dashed lines for clarity. Figure optionsDownload as PowerPoint slide