Adam Pikul

Order–Disorder Transition and Weak Ferromagnetism in the Perovskite Metal Formate Frameworks of [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3] (M = Ni, Mn)

We report the synthesis, crystal structure, thermal, dielectric, Raman, infrared, and magnetic properties of hydrogen and deuterated divalent metal formates, [(CH3)2NH2][M(HCOO)3] and [(CH3)2ND2][M(HCOO)3], where M = Ni, Mn. On the basis of Raman and IR data, assignment of the observed modes to respective vibrations of atoms is proposed. The thermal studies show that for the Ni compounds deuteration leads to a decrease of the phase transition temperature Tc by 5.6 K, whereas it has a negligible effect on Tc in the Mn analogues. This behavior excludes the possibility of proton (deuteron) movement along the N-H···O (N-D···O) bonds as the microscopic origin of the first-order phase transition observed in these crystals below 190 K. According to single-crystal X-ray diffraction, the dimethylammonium (DMA) cations are dynamically disordered at room temperature, because the hydrogen bonds between the NH2 (ND2) groups and the metal-formate framework are disordered. The highly dynamic nature of hydrogen bonds in the high-temperature phases manifests in the Raman and IR spectra through very large bandwidth of modes involving vibrations of the NH2 (ND2) groups. The abrupt decrease in the bandwidth and shifts of modes near Tc signifies the ordering of hydrogen bonds and DMA(+) cations as well as significant distortion of the metal-formate framework across the phase transition. However, some amount of motion is retained by the DMA(+) cation in the ferroelectric phase and a complete freezing-in of this motion occurs below 100 K. The dielectric studies reveal pronounced dielectric dispersion that can be attributed to slow dynamics of large DMA(+) cations. The low-temperature studies also show that magnetic properties of the studied compounds can be explained assuming that they are ordered ferrimagnetically with nearly compensated magnetic moments of Ni and Mn. IR data reveal weak anomalies below 40 K that arise due to spin-phonon coupling. Our results also show that due to structural phase transition more significant distortion of the metal-formate framework occurs for the deuterated samples.

Perovskite metal formate framework of [NH2-CH(+)-NH2]Mn(HCOO)3]: phase transition, magnetic, dielectric, and phonon properties.

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH(+)-NH2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3(-)] is counterbalanced by formamidinium (FMD(+)) cations located in the cavities of the framework. These cations form extensive N-H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD(+) cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD(+) cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD(+) cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K.

Structural, magnetic and dielectric properties of two novel mixed-valence iron(II)–iron(III) metal formate frameworks

Two novel mixed-valence iron(II)–iron(III) formate frameworks templated by ethylammonium and diethylammonium cations have been prepared and characterized by DSC, X-ray diffraction and spectroscopic methods. We also report dielectric and magnetic properties of the obtained samples. Both MOFs crystallize in the P1c structure and exhibit magnetic order at 39 K. The analogue with diethylammonium cations undergoes a structural phase transition near 240 K into a triclinic phase. This transition has an order–disorder character and it is associated with pronounced dielectric anomaly. This compound is therefore the second discovered mixed-valence metal formate exhibiting multiferroic properties.

Effect of solvent, temperature and pressure on the stability of chiral and perovskite metal formate frameworks of [NH2NH3][M(HCOO)3] (M = Mn, Fe, Zn)

We report the synthesis, crystal structure, and thermal, Raman, infrared and magnetic properties of [NH2NH3][M(HCOO)3] (HyM) compounds (M = Mn, Zn, Fe). Our results show that synthesis from methanol solution leads to perovskite polymorphs while that from 1-methyl-2-pyrrolidinone or its mixture with methanol allows obtaining chiral polymorphs. Perovskite HyFe, chiral HyFe and chiral HyMn undergo phase transitions at 347, 336 and 296 K, respectively, with symmetry changes from Pnma to Pna21, P63 to P212121 and P63 to P21. X-ray diffraction and Raman studies show that the phase transitions are governed by dynamics of the hydrazinium ions. Low-temperature magnetic studies show that these compounds exhibit magnetic ordering below 9-12.5 K. Since the low-temperature structures of chiral HyMn and perovskite HyFe are polar, these compounds are possible multiferroic materials. We also report high-pressure Raman scattering studies of chiral and perovskite HyZn, which show much larger stiffness of the latter phase. These studies also show that the ambient pressure polar phases are stable up to at least 1.4 and 4.1 GPa for the chiral and perovskite phase, respectively. Between 1.4 and 2.0 GPa (for chiral HyZn) and 4.1 and 5.2 GPa (for perovskite HyZn) pressure-induced transitions are observed associated with changes in the zinc-formate framework. Strong broadening of Raman bands and the decrease in their number for the high-pressure phase of chiral HyZn suggest that this phase is disordered and has higher symmetry than the ambient pressure one.

Temperature- and pressure-induced phase transitions in the niccolite-type formate framework of [H3N(CH3)4NH3][Mn2(HCOO)6]

We report the synthesis, crystal structure, thermal, pyroelectric, Raman, infrared and magnetic properties of [NH3(CH2)4NH3][Mn2(HCOO)6] niccolite. Our results show that this compound crystallizes in a trigonal structure (space group P1c) with dynamically disordered [NH3(CH2)4NH3]2+ cations. It undergoes a phase transition near Tc = 350 K. The low-temperature structure is polar (space group Cc) and pyroelectric measurements confirm that it exhibits ferroelectric properties. Detailed analysis of the structural changes shows that both the spatial arrangement of the [NH3(CH2)4NH3]2+ dipole moments and distortion of the manganese formate framework contribute to the spontaneous polarization within the (a, c) plane. Based on Raman and IR data, assignment of the observed modes to the respective vibrations of atoms is also proposed. Dynamic disorder of organic cations in the high-temperature phase manifests in the vibrational spectra through very large width of bands corresponding to vibrations of the NH3 groups. Ordering of these cations is clearly observed in the spectra through a pronounced decrease in their bandwidths below the phase transition temperatures. Low-temperature magnetic studies show that this compound is a weak ferromagnet below 9.0 K. We also report high-pressure Raman scattering studies of this compound, which reveal the presence of two pressure-induced phase transitions between 0.5 and 0.9 GPa and between 1.3 and 1.9 GPa.

Heavy-fermion superconductivity in Ce2PdIn8

Abstract The compound Ce2PdIn8 is a recently discovered novel member of the series CenTIn3n+2, where T=d-electron transition metal, and n = 1 or 2. So far, only the phases with T=Co, Rh and Ir have been intensively studied for their unconventional superconducting behavior at low temperatures. By means of magnetic susceptibility, electrical resistivity and heat capacity measurements we provide evidence that Ce2PdIn8 also has a superconducting ground state with a strong heavy-fermion character. The clean-limit superconductivity sets in at T c = 0.7 K at ambient pressure, likely at a verge of a quantum phase transition that manifests itself in a form of distinct non-Fermi liquid features in the bulk normal state characteristics.

Temperature-dependent Raman and IR studies of multiferroic MnWO4 doped with Ni2+ ions.

Temperature-dependent Raman and IR studies of MnWO(4) crystal doped with Ni(2+) ions were performed in the 4.2-300 K range. These studies were complemented by magnetization and specific heat measurements in the 2-100K range, which revealed that MnWO(4) crystal doped with Ni(2+) ions exhibits two phase transitions at 13.9 and 12.5K. Temperature evolution of Raman wavenumbers and linewidths revealed anomalous behaviour at low temperatures. These anomalies have been attributed to spin-phonon coupling, which appear due to onset of antiferromagnetic spin ordering. The observed anomalies extend above T(N)=13.9 K. This behaviour is consistent with the fact that MnWO(4) is a moderately magnetically frustrated material.

Crystal structure, magnetic and electrical properties of new ternary indides RPtIn (R=Pr, Sm)

Abstract Two new ternary compounds PrPtIn and SmPtIn were synthesized and characterized by means of X-ray diffraction, magnetic and electrical measurements. The crystal structure of SmPtIn was refined from single-crystal X-ray data, down to R1=0.0406 for 297 F2 values and 15 variables. The structure of PrPtIn was established from powder diffractometer data. Both compounds crystallize with a hexagonal structure of the ZrNiAl-type (space group P62m) with the lattice parameters: a=7.6522(3) A, c=4.0455(1) A for PrPtIn, and a=7.5895(6) A, c=3.9540(1) A for SmPtIn. The Pr-based compound is paramagnetic down to 1.7 K, whereas SmPtIn orders ferromagnetically below 25 K. Both phases exhibit metallic character of the electrical conductivity.

Kaczorowskiet al.Reply

Kaczorowski et al. Reply: In the preceding Comment [1], Uhlirova et al. state that the single crystals of Ce2PdIn8 investigated in Ref. [2] were contaminated by some amount of CeIn3, which resulted in misinterpretation of the observed antiferromagnetic (AF) ordering below 10 K. They demonstrate the result of their energy dispersive X-ray (EDX) analysis that clearly indicates a sandwich-like character of single crystals grown by technique similar to that applied in Ref. [2], with well defined regions of Ce2PdIn8 and CeIn3. Indeed, our recent metallurgical investigations of the system do corroborate the findings by Uhlirova et al. and point out that the presence of CeIn3 in such crystals is hardly avoidable. Because of overlapping of the Bragg peaks due to CeIn3 with those of Ce2PdIn8 and possible residues of indium flux, it is not possible to detect small amount (about 10%) of the binary phase in X-ray diffraction experiments. Moreover, EDX study on single-crystalline surface is not capable to reveal the presence of a layer of CeIn3 located beneath a relatively thick layer of Ce2PdIn8. These unfortunate shortcomings of the standard sample characterization methods, as well as quite good reproducibility of the results of electrical resistivity and heat capacity measurements performed on a few crystals taken from different batches have led us to incorrect conclusion on the intrinsic character of the antiferromagnetic order in the compound studied. In Fig. 1 we present the lowtemperature characteristics of one of those crystals, in which the CeIn3 layer has been removed by polishing the sample down to the thickness of about 100 μm. Clearly, no phase transition other than that due to superconductivity (SC) is observed, thus definitively ruling out the concept of SC emerging out of AF state. The authors of the Comment agree with our statement in Ref. [2] that heavy-fermion (HF) superconductivity is a bulk property of Ce2PdIn8, yet they do not present any physical data to illustrate the superconducting behavior in their samples. Furthermore, Uhlirova et al. pronounce a large spread in the values of the critical temperature measured for their crystals (Tc = 0.45-0.7 K), which is at odds with our own findings. Actually, all the single crystals we studied thus far have been found to superconduct below Tc = 0.70±0.02 K (as examples see the data in Fig. 1 and in Ref. [2]). Most recently, the very same Tc has been observed for high-quality polycrystalline samples of Ce2PdIn8 [3]. In all cases, the SC transition has been well-defined in both the electrical resistivity and the heat capacity data, while the parameters describing the superconducting state have had values very similar to those reported in Ref. [2], which undoubtedly manifest its HF character. Most importantly, in an extended range above Tc the behavior of Ce2PdIn8 distinctly differs from the predictions for a Fermi liquid, namely the resistivity is a linear function of the temperature and the specific heat over temperature ratio diverges with decreasing the temperature (see Fig. 1 and Ref. [3]). These features suggest the presence of antiferromagnetic spin fluctuations and hint at an unconventional character of the superconducting state that possibly emerges in the proximity of a quantum critical point instability, alike in the related HF superconductors CeT In5 and Ce2T In8 with T = Co, Rh and Ir [4]. The latter hypothesis seems supported by recently obtained muon spin rotation spectroscopy and inelastic neutron scattering data [5], which clearly evidence magnetically-driven superconducting behavior in Ce2PdIn8, even if long-range AF order is actually absent as an intrinsic property of this compound.

Search for unconventional superconductors among the YTE2Si2compounds (TE = Cr, Co, Ni, Rh, Pd, Pt)

Motivated by the recent discovery of exotic superconductivity in YFe2Ge2 we undertook reinvestigation of formation and physical properties of yttrium-based 1:2:2 silicides. Here we report on syntheses and crystal structures of the YTE 2Si2 compounds with TE  =  Cr, Co, Ni, Rh, Pd and Pt, and their low-temperature physical properties measurements, supplemented by results of fully relativistic full-potential local-orbital minimum basis band structure calculations. We confirm that most of the members of that family crystallize in a tetragonal ThCr2Si2-type structure (space group I4/mmm) and have three-dimensional Fermi surface, while only one of them (YPt2Si2) forms with a closely-related primitive CaBe2Ge2-type unit cell (space group P4/nmm) and possess quasi-two-dimensional Fermi surface sheets. Physical measurements indicated that BCS-like superconductivity is observed only in YPt2Si2 (T c  =  1.54 K) and YPd2Si2 (T c  =  0.43 K), while no superconducting phase transition was found in other systems at least down to 0.35 K. Thermal analysis showed no polymorphism in both superconducting phases. No clear relation between the superconductivity and the crystal structure (and dimensionality of the Fermi surface) was observed.