M. Wojtaś

Ferroelectric Polarization in Nanocrystalline Hydroxyapatite Thin Films on Silicon

Hydroxyapatite nanocrystals in natural form are a major component of bone- a known piezoelectric material. Synthetic hydroxyapatite is widely used in bone grafts and prosthetic pyroelectric coatings as it binds strongly with natural bone. Nanocrystalline synthetic hydroxyapatite films have recently been found to exhibit strong piezoelectricity and pyroelectricity. While a spontaneous polarization in hydroxyapatite has been predicted since 2005, the reversibility of this polarization (i.e. ferroelectricity) requires experimental evidence. Here we use piezoresponse force microscopy to demonstrate that nanocrystalline hydroxyapatite indeed exhibits ferroelectricity: a reversal of polarization under an electrical field. This finding will strengthen investigations on the role of electrical polarization in biomineralization and bone-density related diseases. As hydroxyapatite is one of the most common biocompatible materials, our findings will also stimulate systematic exploration of lead and rare-metal free ferroelectric devices for potential applications in areas as diverse as in vivo and ex vivo energy harvesting, biosensing and electronics.

Polar and antiferroelectric behaviour of a hybrid crystal – piperazinium perchlorate

Monoprotonated piperazinium perchlorate, [NH2(CH2)4NH][ClO4], appeared to be a novel room temperature polar material (P1). Its acentric symmetry was confirmed by single-crystal X-ray diffraction, second harmonic generation (SHG) and pyroelectric measurements. Differential scanning calorimetry (DSC) measurements revealed a complex sequence of phase transitions above room temperature: I ↔ II at 433/422 K (heating–cooling), II ↔ III at 417/411 K, III ↔ IV at 403/395 K and IV ↔ V at 397 K (the lowest temperature phase transition recorded only upon heating). The characteristic feature of the structure of [NH2(CH2)4NH][ClO4] is the presence of two parallel cationic chains which are connected with each other by strong N–H⋯N hydrogen bonds. In phase V, these strongly polar non-equivalent chains contribute to spontaneous polarization. 1H NMR measurements disclosed the reorientational motions of the piperazinium ([NH2(CH2)NH]+) cations as well as the proton motion in the N–H⋯N hydrogen bonds along the piperazine chain. Over phase I, the overall motions of the ClO4− anions and reorientational motion of cations are postulated. The dielectric response, e′(T), accompanying the PT I ↔ II indicates possible antiferroelectricity in phase II.

Ferroelectric-Paraelectric Phase Transition in Triglycine Sulphate via Piezoresponse Force Microscopy

Triglycine sulphate (TGS) is a widely studied ferroelectric material due to its high pyroelectric coefficients and pure 2nd order phase transition of the order-disorder type. In this work, we report local piezoelectric properties and domain evolution in TGS via piezoresponse force microscopy (PFM) where the ferroelectric-paraelectric phase transition and its domain structure are visualized in a single image while slowly heating the crystal during scanning. Thus, the y-coordinate of the scan represents the temperature and serves as a temperature “lens” to zoom in peculiarities of the domain evolution and paraelectric-ferroelectric phase boundary. Strong shift of the Curie point and broad peak of the second harmonic PFM signal are rationalized in terms of the stabilization of ferroelectric phase at the surface and large contribution of the dielectric constant to the local electromechanical measurements.

Strong piezoelectricity in [H-β-(2-pyridyl)-Ala-OH][BF4] and [H-β-(2-pyridyl)-Ala-OH][ClO4] – new amino acid based hybrid crystals

New amino acid based [H-β-(2-pyridyl)-Ala-OH][ClO4] and [H-β-(2-pyridyl)-Ala-OH][BF4] crystals were synthesized and their structure and functional piezoelectric properties were investigated in detail. The former crystallizes in the piezoelectric P212121 space group whereas the latter belongs to the polar P1 space group. Piezoelectric force microscopy (PFM) measurements revealed that the piezoelectric coefficient, d15eff, of the [H-β-(2-pyridyl)-Ala-OH][BF4] crystal is more than twice that in the widely used transducer material lithium niobate, LiNbO3. The crystal structures of both [H-β-(2-pyridyl)-Ala-OH] derivatives are characterized by inter- and intramolecular hydrogen bond networks that are responsible for a high piezoresponse. The existence of intramolecular hydrogen bonding was confirmed by means of IR measurements. The thermogravimetric (TGA) technique was applied to study the thermal behavior of the title crystals. The piezoelectric properties are discussed in the context of the crystallographic structure and the microstructure of these crystals.

Structure and properties of tris(tetramethylammonium) nonabromodiarsenate(III), [(CH3)4N]3[As2Br9]

The new tetramethylammonium bromoarsenate(III) crystal [(CH3)4N]3 [As2Br9] has been synthesized and its phase behaviour established by differential scanning calorimetry and dilatometry. Three phase transitions are found: at 346/346 K , at 165/171 K and at 157/165 K on cooling/heating. Single-crystal x-ray diffraction studies of phases I (at 370 K) and II (at 298 K) have shown that the structure comprises discrete [As2Br9]3− anions and disordered [(CH3)4N]+ cations. Phase II crystallizes in the polar space group P31c; its polar nature was confirmed by pyroelectric measurements. Phase I has higher symmetry, P63mc or . Dielectric dispersion measurements revealed a relaxation process in the crystal. The ferroic (ferroelastic) properties were found below 165 K (the transition). The NMR studies indicate the subtle change in the motional state of the tetramethylammonium cations through all observed phase transitions.

Dielectric Relaxation in Ferroelectric Mixed Crystals System (CH3NH3)5Bi2Cl11(1 − x)Br11x

The dielectric response of the mixed crystal system, (CH3NH3)5Bi2Cl7.26Br3.74 (MAPCBB34) and (CH3NH3)5Bi2Cl6.6Br4.4 (MAPCBB40), in the frequency range between 500 Hz and 1 GHz is investigated in the vicinity of para-ferroelectric phase transition. The ferroelectric phase transition temperatures (Tc) found for MAPCBB34 and MAPCBB40 are shifted towards lower temperature in comparison to those found for pure analogues. The Cl → Br substitution strongly affects the dynamic dielectric properties of the crystals. The parameters of the dielectric relaxation in MAPCBB34 and MAPCBB40 are compared to those for closely related ferroelectric compounds.

Domain Structure, Local Hysteresis and Ferroelectric Phase Transition in (CH3NH3)5Bi2Br11 (MAPBBB) Single Crystals

Local ferroelectric properties of single crystals (CH3NH3)5Bi2Br11 (MAPBB) are investigated by Scanning Probe Microscopy (SPM) in a piezoelectric contact mode. The SPM signal proportional to the effective piezoelectric coefficient, deff, is measured along with the standard topography. Both ferroelectric domain imaging and local hysteresis measurements are performed at room temperature in the close vicinity of the phase transition temperature (Tc ≈ 39°C). Lamellar domain structure with the phase contrast due to 180° domains is observed for the surfaces perpendicular to the polar direction. Local hysteresis loops acquired by SPM are well saturated with the average coercive voltage of ∼15 V. The shape of the loops is found to depend on the stiffness of the SPM cantilever. Local studies are complemented with the macroscopic measurements (polarization, piezoelectric coefficient, dielectric permittivity) performed in the temperature range including Tc (20–39°C). The mechanisms of the domain contrast and prospects of using MAPBB for high-density data storage are discussed.

Dielectric Dispersion in [(CH3)3NH]3[Sb2Cl9(1-x)Br9x ] Mixed Crystals

The dielectric response of [(CH3)3NH]3[Sb2Cl9(1-x)Br9x ] mixed crystals (TMACBA, x = 0.42, 0.85) and the pure [(CH3)3NH]3[Sb2Br9x ] (TMABA) crystal was investigated in the frequency range 20 kHz–30 MHz. The mixed TMACBA crystals with 0.55 < x ≤ 1 are non-polar as opposed from those with x ≤ 0.55. They crystallize in trigonal space group, R c. Their anionic sublattice is built of discrete [Sb2X9]3- units distinct from infinitive layers as in ferroelectric TMACBA crystals. The effect of hydrogen bonds formation on dynamical properties of trimethylammonium cations was analysed.