L. Szczepańska

Electronic structure, Jahn-Teller dynamics and electron spin relaxation of two types of octahedral Cu(II) complexes in cadmium formate dihydrate Cd(HCOO)2.2H2O. EPR and ESE studies

Cd(HCOO)2·2H2O single crystals weakly doped with Cu(II) ions have been studied by cw-EPR (4.2–300 K) and by electron spin echo – ESE (4.2–60 K). Copper(II) ions substitute Cd(II) in two different sites forming Cu(HCOO)6–Cuf and Cu(HCOO)2(H2O)4–Cuw octahedral complexes with strong preference to Cuf as shown by the intensity ratio of the EPR spectra (up to 20∶1). Despite different molecular structures both complexes have nearly identical EPR parameters at rigid lattice limit with gz=2.429, gy=2.092, gx=2.064, Az=120, Ay=32 and Ax=12×10−4 cm−1 for Cuf. This fact as well as strong axial deformation of the crystal field at Cu(II) sites indicate that the strong Jahn–Teller effect operates producing three wells in the potential surface with one having much lower energy than the others. In the Cuf complex the dynamic J–T-effect has been observed as a vibronic averaging of the two g and A parameters (along z and y axes). It indicates that only one of the higher energy wells is thermally accessible and the Silver–Getz model leads to the average energy difference between the two lowest energy wells δ12=500(60) cm−1. The δ12 is temperature dependent. For Cuw complex no vibronic effects were observed in EPR spectra indicating that higher energy wells are not populated up to 300 K. The spin–lattice relaxation time T1 and phase memory time TM were measured up to 60 K only, because for higher temperatures the ESE decay was too fast. Spin–lattice relaxation is governed by two-phonon Raman processes which allow one to determine the Debye temperature of the crystal as ΘD=193 K. The ESE decay was described as V(2τ)=V0exp(−τ/b−mτ2) indicating the contribution of the spectral diffusion (quadratic term). The ESE dephasing rate 1/TM is governed by spectral diffusion below 15 K. For higher temperatures the T1-processes and excitations to the higher vibronic levels of energy Δ=166 cm−1 give comparable contributions.

A reorientation of the PO3-groups and radiation damage effect in ferroelectric phase transition studied by CW-EPR of PO3 2− radical in glycine·H3PO3 single crystal

Abstract EPR of γ-ray generated PO3 2- radicals indicates that the four radical centers are equivalent in the paraelectric phase and become inequivalent in the ferroelectric phase due to a ±3° rotation. The phase transition temperature is shifted by irradiation from Tc = 224 K to Tc = 180 K. The molecular mechanism of the order-disorder phase transition is discussed

Raman scattering study of the ferroelectric phase transition in GPI and DGPI single crystals

Abstract The Raman spectra of GPI and DGPI crystals were investigated in the 400–3200 cm−1 internal vibrations range and in the temperature range 175–250 K and 295–355 K, respectively. Para-ferroelectric phase transition at 224 K for GPI and 322 K for DGPI was examined. The results obtained from Raman investigations confirm the order-disorder nature of a ferroelectric phase transition of second order type. One type of glycinium cations is observed in the paraelectric phase but two types in ferroelectric phase. The dynamical disorder of protons is coupled to the motions of glycinium cations.

Temperature study of the elastic properties of glycinium phosphite and deuterated glycinium phosphite crystals by brillouin scattering

Abstract Brillouin spectroscopy was used to study the elastic properties of glycinium phosphite (GPI) and deuterated glycinium phosphite (DGPI) crystals. The temperature measurements of the hypersonic velocity for acoustic modes propagating in various directions have been performed in the temperature range 170 K-300 K and 295 K-355 K for GPI and DGPI, respectively. Brillouin study has confirmed the existence of the order-disorder type ferroelectric phase transition (P21/a - P21) at 224 K for GPI and 322 K for DGPI. The obtained results are also discussed in terms of the reordering of the hydrogen bonds in the ferroelectric phase.

Ferroelectric domain structure of [N(CH3)4]2 ZnC14 single crystals

Abstract Ferroelectric domain structure of tetramethylammonium zinc chloride was revealed in the temperature range 275. 8R-279. OK by means of scanning electron microscopy. The domain patterns of lamellar type were observed. The mean width of the domains at 276.1 K amounted to 10 μm.

Nir-raman studies of poly(ethylene oxide) + (NH4)4H2(SeO4)3 polymer electrolyte

Abstract Infrared Raman scattering studies were made on a composite of poly(ethylene oxide) (PEO) polymer+(NH4)4H2(SeO4)3 crystal. Comparison of the spectra obtained for the composite of PEO+(NH4)4H2(SeO4)3 and the mixture of dry powders of the same contents suggests the existence of a complex in the composite. The degree of crystallinity of the polymer increases with growing content of (NH4)4H2(SeO4)3 in the composite.

Study of the elastic properties of glycine phosphite crystals (Gly·h3PO3) by Brillouin scattering

Abstract The results concerning the elastic properties of glycinium phosphite (GPI) crystals obtained by Brillouin scattering method are reported. The temperature measurements of the relative change AVN of the phase velocity for acoustic modes propagating in q = [100], q = [010], q = [110] and q = [011] directions have been performed in the temperature range 300 K - 170 K. Obtained results confirm the existence of the ferroelectric phase transition (P21/a - P21) at 224 K.

Thermal properties and radiation damage in NaNH4XO4·2H2O single crystals (XO4=SO4,SeO4)

Differential thermal analysis (DTA) studies of NaNH4SO4·2H2O, NaND4SO4·2H2O, and NaNH4SeO4·2H2O single crystals were performed in the temperature range of the ferroelectric-paraelectric phase transition and of the dehydration process. The first-order phase transition at the Curie point was confirmed for NaNH4SO4·2H2O, whereas the transition in the isomorphous NaNH4SeO4·2H2O at the Curie temperature was found to be similar to a contineous (second order) with considerable higher transition enthalpy. Dehydration process of the crystals studied was found to proceed in two stages, with maximum dehydration rates at ∼355 and ∼400K. A linear decrease in thermal energy related to the long-range ordering in NaNH4SO4·2H2O crystals was observed with the dose of γ-cobalt-60 irradiation and discussed with respect to the radiation induced decrease in spontaneous polarization.

Effect of domain wall clamping on the pyroelectric response of NaNH4SO4.2H2O

Abstract Temperature variation of the pyroelectric response Up was studied for NaNH4SO4.2H2O single crystals. In spite of the bulk pyroelectric response veryfied by flux modulation frequency dependence of Up in the ferroelectric phase, a small, frequency independent pyroelectric response was observed in the paraelectric phase up to ∼ 200 K. Moreover, the pyroelectric response is accompanied by Barkhausen pulses. The high temperature tail of Up(T) is attributed by us to the water defects in the crystals.

Dta and X-RAY studies of α-LiNH4SeO4 single crystals

Abstract The aim of the paper was a search for polymorphism in LiNH4SeO4 (LASe) single crystals and the study of the structure of the only known α-modification of LASe. We found the α-LASe to be isomorphous with α-LiNH4SO4 (the space group Pca21). Two kinds of polytypes, the A with cA = c and the B with cB = 2c, were observed. The two polytypes were found to exist simultaneously in each single crystalline sample with the B-polytype prevailing. This superstructure is found to affect the α rightarrow β polymorphous transformation.