Gašper Tavčar

Weak ferromagnetism and ferroelectricity in K3Fe5F15

Here, we report on the observation of a weak ferromagnetic transition at TN=122K in a K3Fe5F15 system which is ferroelectric and ferroelastic below Tc=490K. The magnetization and the susceptibility continuously increase with decreasing T down to 2K. The Mossbauer spectra show a spontaneous magnetic ordering and at least three sites corresponding to Fe2+ and Fe3+. The ratio between Fe2+ and Fe3+ is 60:40. At 6K, there are two magnetically ordered sextets with internal fields of 585 and 263kOe. The “slim” hysteresis loops observed are as well characteristic of weak ferromagnetism. At 122K and at low frequencies, the system shows dielectric anomaly characteristic of magnetoelectric behavior.

Coordination compounds with XeF2, AsF3 and HF as ligands to metal ions: a review of reaction systematics, Raman spectra and metal, fluoro-ligand polyhedra

Abstract The reactions between Ln(AsF 6 ) 3 (Ln: lanthanide) and excess of XeF 2 in anhydrous HF (aHF) as a solvent yield coordination compounds [Ln(XeF 2 ) 3 ](AsF 6 ) 3 or LnF 3 together with Xe 2 F 3 AsF 6 or mixtures of all mentioned products depending on the fluorobasicity of XeF 2 and LnF 3 along the series. XeF 2 in a basic aHF is able to oxidize Pr 3+ to Pr 4+ besides Ce 3+ to Ce 4+ and Tb 3+ to Tb 4+ . The tetrafluorides obtained are weaker fluorobases as XeF 2 and are immediately exchanged with XeF 2 yielding Xe 2 F 3 AsF 6 and LnF 4 . The analogous reaction between Ln(BiF 6 ) 3 and XeF 2 in aHF yields [Ln(XeF 2 ) 3 ](BiF 6 ) 3 , Ln: La, Nd. Raman spectra of the compounds [Ln(XeF 2 ) n ](AF 6 ) 3 (A: As, Bi) show that no XeF + salts are formed. The interaction of XeF 2 with metal ion is covalent over the fluorine bridge. Analogous reactions of Ln(AsF 6 ) 3 with AsF 3 in aHF yield [Ln(AsF 3 ) 3 ](AsF 6 ) 3 which are stable in a dynamic vacuum at temperatures lower than 233 K. In reactions between M(AF 6 ) 2 (M: alkaline earth metal and Pb, A: As, Sb) and XeF 2 in aHF as a solvent, compounds of the type [M(XeF 2 ) n ](AF 6 ) 2 were synthesized. Analogous reactions with AsF 3 yield coordination compounds of the type [M(AsF 3 ) n ](AsF 6 ) 2 . During the preparation of M x (AsF 6 ) x (M: metal in oxidation state x +) by the reaction between metal fluoride and excess of AsF 5 in aHF it was found that HF could also act as a ligand to the metal ions (e.g. Ca(HF)(AsF 6 ) 2 ).

Electron paramagnetic resonance and Mössbauer study of antiferromagnetic K3Cu3Fe2F15

The electron paramagnetic resonance (EPR) spectra of K3Cu3Fe2F15 consist above 85 K of a superposition of two Lorentzians with different intensities, T-dependences, linewidths, and g-factors. The broad line shows an antiferromagnetic (AFM) behavior and the narrow one shows a paramagneticlike behavior. The Mossbauer spectra consist at room temperature of two quadrupolar doublets and below 77 K of two magnetic sextets and one quadrupolar doublet. Below 85 K the broad line EPR disappears and the intensity of the narrow line increases nearly exponentially with decreasing temperature. The broad EPR line originates from regions with local AFM order, whereas the narrow line comes from regions without AFM order, e.g., grain boundaries.

Polarons in low temperature phase of (NH4)3FeF6

The frequency and temperature dependence of the dielectric constant and the ac electrical conductivity in (NH4)3FeF6 was investigated. The temperature dependence of the dielectric constant reveals the phase transition from tetragonal to cubic phase at 270 K. At temperatures below 225 K, the ac conductivity and dielectric constant follow the universal dielectric response (UDR), being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 225 K this mechanism governs the charge transport in this fluoride system.