Temperature symmetry breaking and properties of lead-free organic-inorganic hybrids: bismuth(III) iodide and antimony(III) iodide: (S(CH3)3)3[Bi2I9] and (S(CH3)3)3[Sb2I9].

We have synthesized and characterized two novel lead-free organic-inorganic hybrid crystals: (S(CH3)3)3[Bi2I9] (TBI) and (S(CH3)3)3[Sb2I9] (TSI). Thermal DSC, TG, and DTA analyses indicate structural phase transitions (PTs) in both compounds; TBI undergoes two structural phase transitions at 314.2/314.8 K (cooling/heating) and at 181.5 K of first (I ↔ II) and second order (II ↔ III), respectively. The crystal structures of TBI are refined for phases I (325 K), II (200 K) and III (100 K). TBI exhibits ferroelastic properties since both PTs are accompanied by a change in the symmetry of crystals: P63/mmc → C2/c (I → II) and C2/c → P1̄ (II → III). The presence of a ferroelastic domain structure has been confirmed by optical observations. In turn, TSI also reveals two PTs: I ↔ II (at 303.9/304.1 K) and II ↔ III (212.9/221.4 K). To compare and obtain insight into the mechanism of the PTs of TBI, we have carried out temperature dependent single crystal X-ray diffraction studies. Additionally, to confirm the change in the dynamical states of molecules in PTs, dielectric measurements have been carried out between 100 K and 400 K in the frequency range of 200 Hz to 2 MHz. Moreover, the measurements of the 1H NMR spin-lattice relaxation time, T1, and a second moment, M2, of the 1H NMR line have been undertaken in the temperature range between 100 and 300 K.

Dielectric-Optical Switches: Photoluminescent, EPR, and Magnetic Studies on Organic-Inorganic Hybrid (azetidinium)2MnBr4.

A new organic-inorganic hybrid, AZEMnBr, has been synthesized and characterized. The thermal differential scanning calorimetry, differential thermal analysis, and thermogravimetric analyses indicate one structural phase transition (PT) at 346 and 349 K, on cooling and heating, respectively. AZEMnBr crystallizes at 365 K in the orthorhombic, Pnma, structure, which transforms to monoclinic P21/n at 200 K. Due to the X-ray diffraction studies, the anionic MnBr42- moiety is discrete. The azetidinium cations show dynamical disorder in the high-temperature phase. In the proposed structural PT, the mechanism is classified as an order-disorder type. The structural changes affect the dielectric response. In this paper, the multiple switches between low- and high- dielectric states are presented. In addition, it was also observed that the crystal possesses a mutation of fluorescent properties between phase ON and OFF in the PT's point vicinity. We also demonstrate that EPR spectroscopy effectively detects PTs in structurally diverse Mn(II) complexes. AZEMnBr compounds show DC magnetic data consistent with the S = 5/2 spin system with small zero-field splitting, which was confirmed by EPR measurements and slow magnetic relaxation under the moderate DC magnetic field typical for a single-ion magnet behavior. Given the above, this organic-inorganic hybrid can be considered a rare example of multifunctional materials that exhibit dielectric, optical, and magnetic activity.

(C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9: ferroelastic lead-free hybrid perovskite-like materials as potential semiconducting absorbers.

We have synthesised and characterised novel organic-inorganic hybrid crystals: (C3N2H5)3Sb2I9 and (C3N2H5)3Bi2I9 (PSI and PBI). The thermal DSC and TG analyses indicate four structural phase transitions (PTs) at 366.2/366.8, 274.6/275.4, 233.3/233.3 and 142.8/143.1 K (on cooling/heating) for PSI and two reversible PTs at 365.2/370.8 and 252.6/257.9 K for PBI. Both analogues crystallize at room temperature in the orthorhombic Cmcm structure, which transforms, in the case of PBI, to monoclinic P21/n at low temperature. According to the X-ray diffraction results, the anionic component is discrete and built of face-sharing bioctahedra, [M2I9]3-, in both compounds, whereas cations exhibit distinct dynamical disorder over high temperature phases. Dielectric spectroscopy and 1H NMR spectroscopy have been used to characterise the dynamical state of the C3N2H5+ cations. The ferroelastic domain structure has been characterised by observations under a polarized optical microscope. Both compounds are semiconductors with narrow bandgaps of 1.97 eV (PBI) and 2.10 eV (PSI).

Advances and Property Investigations of an Organic-Inorganic Ferroelectric: (diisopropylammonium)2[CdBr4].

The preparation of materials featuring more than one ferroelectric phase represents a promising strategy for controlling electrical properties arising from spontaneous polarization, since it offers an added advantage of temperature-dependent toggling between two different ferroelectric states. Here, we report on the discovery of a unique ferroelectric-ferroelectric transition in diisopropylammonium tetrabromocadmate (DPAC, (C6H16N)2[CdBr4]) with a Tc value of 244 K, which is continuous in nature. Both phases crystallize in the same polar orthorhombic space group, Iab2. The temperature-resolved second-harmonic-generation (SHG) measurements using 800 nm femtosecond laser pulses attest to the polar structure of DPAC on either side of the phase transition (PT). The dc conductivity parameters were estimated in both solid phases. The anionic substructure is in the form of [CdBr4]2- discrete complexes (0D), while in the voids of the structure, the diisopropylammonium cations are embedded. The ferroelectric properties of phases I and II have been confirmed by the reversible pyroelectric effect as well as by P-E loop investigations. On the basis of the dielectric responses, the molecular mechanism of the PT at 244 K has been postulated to be of mixed type with an indication of its displacive nature.

Phase transition tuning by Fe(iii)/Co(iii) substitution in switchable cyano-bridged perovskites: (C3H5N2)2[KFexCo1-x(CN)6].

The single-crystals of mixed (C3H5N2)2[KFexCo1-x(CN)6] crystals, with different ratios of x = 0, 0.29, 0.42, 0.51, 0.63, 0.70, 0.85, 1, have been grown from aqueous solutions. The thermal stability of the crystals has been determined using both DTA and TGA analysis. DSC has revealed sequences of structural phase transitions (PTs). We have found four solid phases for the crystals with concentrations x < 0.6 and three phases above these concentrations. The Fe(iii) concentration has been estimated using the SEM technique. We have found the linear relationship of Tcversus the molar concentration of Fe(iii) for the IV → III PT. Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PTs has been discussed based on the results of dielectric relaxation, NIR, IR and Raman spectroscopyand X-ray measurements. Dielectric switching has been observed for all analysed mixed-crystals. All crystals exhibit ferroelastic properties.

Structural phase transitions coupled with prominent dielectric anomalies and dielectric relaxation in [(CH3)3NH]2[KCo(CN)6] and mixed [(CH3)3NH]2[KFexCo1-x(CN)6] double perovskite hybrids.

The crystals of pure [(CH3)3NH]2[KFe(CN)6] (TrMAFe) and [(CH3)3NH]2[KCo(CN)6] (TrMACo) as well as their mixed crystals (TrMAFexCo1-x), with different ratios of x = 0, 0.12, 0.18, 0.49, 0.56, 0.73, 0.81, 1.0, have been grown from aqueous solutions. The structure of TrMACo has been determined at 360 K to be cubic (Fm3[combining macron]m). In phase II (100 K), the TrMACo crystal is monoclinic (C2/c). The thermal stability of the pure and mixed crystals has been determined by using both DTA and TGA. Based on the DSC results, we have found a single phase transition (PT) for both pure and mixed crystals. The Fe(iii) concentration was estimated by using the SEM technique. We have found a linear relationship between the PT temperature (Tc) and the molar concentration of Fe(iii). Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PT has been discussed based on the results of dielectric relaxation and 1H NMR and X-ray spectroscopy measurements. The results confirmed that the PT mechanism of both pure and mixed crystals is related to the change in the dynamic state of the TrMA+ cations. The dielectric activation energy changes linearly with the mole fraction ranking from 35 to 38 kJ mol-1, for the mixed crystals.

Investigations of organic-inorganic hybrids based on homopiperidinium cation with haloantimonates(iii) and halobismuthates(iii). Crystal structures, reversible phase transitions, semiconducting and molecular dynamic properties.

A description of the thermal, structural, 1H NMR, electric and optical properties of four organic-inorganic hybrids, haloantimonates(iii) and halobismuthates(iii), based on homopiperidinium cation: (C6H14N)2SbCl5 (abbrev. HSC), (C6H14N)2SbBr5 (HSB), (C6H14N)2BiCl6[H3O] (HBC), (C6H14N)2BiBr5 (HBB), is presented. The common feature of the crystal structures of the studied compounds is the 1D (one-dimensional) chain for the anionic network in HSC, HSB and HBB, 1D hydrogen bond chain between 0D (zero-dimensional, isolated) BiCl6 octahedrons and hydronium moieties in HBC as well as a rich polymorphism in the solid state for all title compounds. The structures of the Sb(iii) and Bi(iii) derivatives are not isomorphous and they crystallize in the following space groups: HSC in P212121 both at 280 and 150 K, HSB in Pmna and P212121 at 310 and 150 K, respectively, HBC in C2/m, C2/m and C2/c at 300, 260 and 200 K, respectively, and HBB in P21/n both at 280 and 200 K. The anionic networks are in the forms of either pseudo- and zig-zag chains or a chain of a hydrogen bond. The band gap values, using the Tauc plot as well as the ac and dc conductivity parameters, were estimated. On the basis of the 1H NMR spin-lattice relaxation times, T1, and second-moment, M2, measurements and the dielectric responses, the molecular mechanisms of the phase transitions (PTs) have been postulated. The structural PTs are discussed in terms of the changes in cationic dynamics and distortions of the anionic sublattice. The powder technique of SHG (Kurtz and Perry method) has been used to analyze the second-order nonlinear optical properties of HSC.

The structure, methyl rotation reflected in inelastic and quasielastic neutron scattering and vibrational spectra of 1,2,3,5-tetramethoxybenzene and its 2:1 complex with 1,2,4,5-tetracyanobenzene.

X-ray diffraction studies show that molecules of the 1,2,3,5-tetramethoxybenzene (TMOB)(2) x 1,2,4,5-tetracyanobenzene complex form ...CCDCCDCC... columns with the short distances between molecular planes of C and D molecules equal to 3.186 A. The vibrational spectra recorded by using the inelastic neutron scattering, Raman, IR, and quasielastic neutron scattering (QENS) techniques aided by density functional theory calculations for the isolated molecules and the crystalline state enabled all four inequivalent librational modes, ascribed to the methoxy groups, to be analyzed. A rather good consistency was found between the experimental frequencies and those calculated for the crystal. The consistency was also achieved between the experimental structure of molecules and the theoretically reproduced one. A close similarity of the structures of the TMOB molecule isolated and in the complex is taken as a sign of dominating intramolecular interaction. The QENS spectra contain three Lorentzians of relative intensities of 1:1:2. Thus the two most strongly hindered of the four inequivalent methoxy groups in the crystalline lattice are characterized by rather similar barrier heights in good agreement with the packing analysis.

Elastic, quasielastic, and inelastic neutron-scattering studies on the charge-transfer hexamethylbenzene-tetracyanoquinodimethane complex.

The 1:1 hexamethylbenzene (HMB)-tetracyanoquinodimethane (TCNQ) complex shows a first-order phase transition at 230/218 K (heating/cooling) with no change of the space group. The neutron-diffraction studies reveal that this transition is related to a freezing of the rotation of methyl groups. The results for 100 K enabled precise determination of configuration of HMB.TCNQ complexes. The planes of HMB and TCNQ molecules from small angle (6 degrees) so that the dicyanomethylene group approaches the HMB molecule to a distance of 3.34 angstroms. The conformation of methyl groups was exactly determined. The quasielastic neutron-scattering spectra can be interpreted in terms of 120 degrees jumps with different activation barrier in low- and high-temperature phases, equal to 3.7 and 1.8 kJ/mol, respectively. These values are lower than that for neat HMB (6 kJ/mol). The conclusion can be drawn that the methyl groups can reorient more freely in the complex. This conclusion is in agreement with the results of inelastic neutron-scattering studies of low-frequency modes assigned to torsional vibrations of methyl groups. These frequencies are lower than those for neat HMB. The analyzed increase of frequencies of these modes as compared with free molecules can be interpreted as due to formation of unconventional C-H...Y hydrogen bonds which are more pronounced in crystals of neat HMB than in those of HMB.TCNQ. The low-frequency librational modes can be treated as a sensitive measure of unconventional hydrogen bonds formed by the CH3 groups.