Effect of aliovalent doping on the properties of perovskite-like multiferroic formates

Abstract

We report synthesis, thermal, dielectric, Raman, IR and luminescence studies of chromiumdoped multiferroic MOF, [(CH 3 ) 2 NH 2 ][Mn(HCOO) 3 ] (DMMn). These studies reveal that doping with chromium(III) leads to lowering of the ferroelectric phase transition temperature T c . The doping also changes the character of the phase transition from strongly first-order for undoped sample to partially diffused one for 3.1% of chromium doping. This behavior resembles behavior of inorganic ABO 3 perovskite ferroelectrics where doping often leads to decrease of T c and diffuse character of a phase transition. We also show that the chromium-doped sample exhibits efficient luminescence. Additional studies demonstrated that [(CH 3 ) 2 NH 2 ][M II (HCOO) 3 ] formates (M II =Mg, Mn, Co) may also be doped with other trivalent cations such as Al 3+ , In 3+ , Eu 3+ or Er 3+ .Doping with these ions also leads to decrease of T c and diffuse character of the phase transition.Additional optical studies show that europium-doped DMMn sample also exhibit luminescent properties. Thus our discovery opens up a new and simple route for synthesis of various multifunctional amine-templated metal formate frameworks with tunable multiferroic and luminescent properties by doping these frameworks with wide range of trivalent cations. 160 K. Firstly, many bands exhibit significant narrowing. This behavior is especially pronounced for the ρ(NH 2 ) IR mode observed near 920 cm -1 (Figure 6(b)) and the lattice modes ( Figure S7).Secondly, new bands appear and some bands split. For instance, the ν 3 (HCOO -) IR mode near 795 cm -1 and the ν 5 (HCOO -) mode near 1370 cm -1 split into doublets (Figure 6a and 6c). Third, some bands exhibit significant changes in intensity. This behavior is very clearly observed, for instance, for the Raman bands in the range 2800-2900 cm -1 (Figure 5a). Figure 5. (a) Detail of the Raman spectra results corresponding to the spectral range 3100-2750 cm -1 for DMMn: 3.1% Cr 3+ . For the comparison sake, data for previously reported DMMn are presented in (b). 18 Arrows indicate the bands that exhibit pronounced increase in intensity below the phase transition temperature.data shows also some significant differences. Firstly, the observed changes in DMMn: 3.1% Cr 3+ are not so sharp as in DMMn but they are smeared over 100-160 K range. This behavior is consistent with shift of the phase transition temperature to lower value and its diffuse character.Secondly, the observed frequency shifts of the νas(CNC) and νs(CNC) modes when going from room-temperature to 5 K are smaller for DMMn: 3.1% Cr 3+ compared to DMMn. Furthermore, Figures 5 and 6 show that bands of DMMn: 3.1% Cr 3+ are broader and splitting smaller when compared to DMMn. This behavior points to weaker distortion of the framework and smaller changes in the C-N bond lengths at T c upon doping this MOF with chromium ions. Optical Studies of DMMn: 3.1 % Cr 3+Emission measured at 250 K is broad, centered at 12909 cm -1 (787.6 nm) and its FWHM is equal 2841 cm -1 (Figure 9)...

Efficient synthesis and tailoring of magnetic and dielectric properties of Pb‐free perovskite‐like ABX3 metal‐organic frameworks

“…It has been recently demonstrated that aliovalent doping of DMMnF with Cr(III) can be used to tune the magnetic and dielectric properties of these compounds. On the other hand, the present work shows that these properties can be tuned on the basis of particle size and morphology, as opposed to compositional tuning . Finally, we emphasize the increased importance of inter‐particle interactions at small sizes, which results in magnetic as well as dielectric relaxor‐like behavior below average particle size ∼1 μm, demonstrating the use of size‐confinement to obtain a functionally and fundamentally interesting property in these materials.…”

“…This behavior has been previously reported in ABX 3 MOFs. However, it was obtained either by introducing disorder in the form of mixed‐valence B ions, i.e., aliovalent doping of the divalent MOFs or by slowing down the molecular dynamics using deuterated A ions . Below, we show evidence of this behavior in micron‐sized particles of the MOF DMMnF.…”

Raman‐scattering studies of pressure‐induced phase transitions in perovskite‐like acetamidninium manganese formate

“…The known formates have been extensively studied in a broad temperature range, and these studies showed that majority of them undergo phase transitions associated with orientational ordering of organic cations located in the cavities of the framework . Pressure is another thermodynamic parameter that can help to better understand structure–property relationship in this class of compounds .…”

Raman spectroscopic studies across the ferroelectric transition in cobalt substituted dimethyl amine manganese formate

“…The nature of this ferroelectric transition has been probed using several techniques such as electrical polarization, crystallographic methods, neutron scattering, dielectric measurements, thermal, magnetic, and electron paramagnetic resonance (EPR) methods . Optical techniques sensitive to local structural changes such as infrared (IR) and Raman spectroscopy have been carried out on DMMn to investigate the actual microstructural mechanism involved . All these earlier studies had laid emphasis on the response of the high‐wavenumber internal modes across the transition temperatures.…”