title: Phase transitions and chromium(iii) luminescence in perovskite-type [C2H5NH3][Na0.5CrxAl0.5-x(HCOO)3] (x = 0, 0.025, 0.5), correlated with structural, dielectric and phonon properties.
abstract: We report the synthesis, crystal structure, dielectric, vibrational and emission spectra of heterometallic MOFs, [C2H5NH3][Na0.5Cr0.5(HCOO)3] (EtANaCr), [C2H5NH3][Na0.5Al0.5(HCOO)3] (EtANaAl) and [C2H5NH3][Na0.5Al0.475Cr0.025(HCOO)3] (EtANaAlCr). These compounds crystallize in non-centrosymmetric monoclinic polar structures (space group Pn) and undergo order-disorder phase transitions upon heating to the monoclinic centrosymmetric structure (space group P21/n) at 369 (EtANaAl) and 373 K (EtANaCr). In principle, they are ferroelectric below these temperatures. In the high-temperature phase, ethylammonium (EtA+) cations are dynamically disordered over two symmetrically independent positions while upon cooling they begin to order. The ordering is accompanied by distortion of the metal formate framework. The hydrogen bonds (HBs) between the NH3+group and NaO6octahedral units are more robust than between the NH3+group and CrO6(AlO6) octahedral units and this feature explains a much stronger distortion of the former units and a weak effect of a trivalent cation type on the phase transition temperature. The dielectric studies have confirmed the occurrence of phase transitions of dipolar character and dipole relaxation processes. The optical studies show that EtANaCr and EtANaAlCr exhibit efficient Cr(iii)-based emission characteristics for intermediate-ligand field strength.
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland. m.ptak@int.pan.wroc.pl.
authors.forename: Maciej
authors.initials: M
authors.lastname: Ptak
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland. m.ptak@int.pan.wroc.pl.
authors.forename: Mirosław
authors.initials: M
authors.lastname: Mączka
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland. m.ptak@int.pan.wroc.pl.
authors.forename: Anna
authors.initials: A
authors.lastname: Gągor
authors.affiliation: Faculty of Fundamental Problems of Technology, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370, Wrocław, Poland.
authors.forename: Adam
authors.initials: A
authors.lastname: Sieradzki
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland. m.ptak@int.pan.wroc.pl.
authors.forename: Bartosz
authors.initials: B
authors.lastname: Bondzior
authors.affiliation: Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Box 1410, 50-950 Wrocław 2, Poland. m.ptak@int.pan.wroc.pl.
authors.forename: Przemysław
authors.initials: P
authors.lastname: Dereń
authors.affiliation: Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland.
authors.forename: S
authors.initials: S
authors.lastname: Pawlus