Thermal expansion of solid solutions Kr-CH4 at temperatures of liquid helium

A negative contribution of the CH4 impurity to the thermal expansion of the solution has been revealed in dilatometric studies of solid Kr+0.76% CH4, Kr+5.25% CH4 and Kr+10.5% CH4 solutions at 1-23 K. It is shown that the negative contribution results from changes in the occupancy of the ground state of the A-modifications of isolated CH4 molecules. Assuming that the CH4 impurity singles and clusters contribute to the thermal expansion independently, we can estimate their contributions. The contribution of the singles to the thermal expansion of the solid solution is negative. The energies of the first excitational rotational states were determined for singles and two-body and three-body clusters of CH4 molecules.Comment: 10 pages, 7 figure

TEMPERATURE VARIABLE LOW-FREQUENCY RAMAN SPECTRA AND ORIENTATION DISORDER IN NH4IO3NH_{4}IO_{3} CRYSTAL

1. D. F. Baisa, A. I. Barabash, E.A. Shadcin, Fisika Tverdogo Tela (in Russian), 27, 2357 (1985). 2. A.I. Barabash, J. Mol. Struct., 210, 517 (1992).Author Institution: National Academy of Sciences of Ukraine, Prosp. Nauki, 46, 252650 Kiev, Ukraine.The ammonium iodate $NH_{4}IO_{3}$ crystal is $known^{1}$ as ferroelectric with displacing type phase transition $(T_{c} = 367K)$, Polarized temperature variable lowfrequency Raman scattering $(W < 200 cm^{-1})$ studies in the phase transition region are presented. Measurements of peak positions of ``external'' rotational and translation optical modes and their half-widths allow to detect the orientation disorder and help to determine its character as well as a mechanism of the phase transition to the ordered phase. It appeared to be continuous and of the mixed displacing and order-disorder character. Quantitative analysis of the spectra according to lattice dynamics theories and using $^{127}I$ NQR $analysis^{2}$ have been performed. Previous literature data are reviewed and the apparent discrepancies concerning the transition processes are elucidated on the basis of the present results

SPECTROSCOPIC INVESTIGATIONS OF PECULARITIES OF STRUCTURE AND LATTICE DYNAMICS NH4IO32HIO3NH_{4}IO_{3}2HIO_{3}

Author Institution: Institute of Physics of National Academy of Sciences of UkraineThe $NH_{4}IO_{3}2HIO_{3}$ belongs to a hydrogen-bonded ion-covalent crystals and is characterized by a low symmetry. The space group of the crystal is $P1 (z = 2)$. At the room temperature the crystal unit cell contains four inversion centers which lays in geometrical centers of bifurcated hydrogen bonds. The investigations of temperature dependence of the dielectric properties shown that crystal may undergo a second order phase transition at $213 = T_{c}$ But the ferroelectrics properties of the title crystal were not founded at temperatures below $T_{c}$ where the crystal retained its symmetry $P1$ or $C^{i}_{1}$. The analysis of temperature dependencies of the$^{127}I$ NQR spectra has shown that at 77 K the befurcations of hydrogen bonds fails, and the observed number of NQR lines is twice more than that predicted in accordance with crystal symmetry at 300 K. The analysis of IR spectra of the title crystal in wide temperature range of 100-300 K shows that for all frequency intervals which include the regions of covalent stretching vibrations $I-O (500-900 cm^{-1})$ and N-H $(1000-1500 cm^{-1})$ and also including the region of covalent stretching vibrations O-H of hydroxyls of hydrogen bonds. These experimental facts lead us to conclusion that the phase transition has the attributes of antiferrelectric phase transition with doubling of unit cell volume at $T < T_{c}$. This phase transtion do not changes the crystal symmetry $(P1)$

SPECTROSCOPIC INVESTIGATION OF THE LATTICE DYNAMICS IN HYDROGEN-BONDED CRYSTAL NH4IO3NH_{4}IO_{3}

Author Institution: Institute of Physics, National Academy of Sciences of Ukraine; Institute of Low Temperature and Structure Research of the Polish Academy of Sciences, National Academy of Sciences of UkraineThe lattice dynamics of the ammonium iodate crystal $NH_{4}IO_{3}$ has been investigated by means of IR, Raman and NQR methods near the phase transition temperature 367 K $(T_{c})$. It is known, that $NH_{4}IO_{3}$ crystal grows from water solution and has orthorhombic syngony $(C^{0}_{2v} z = 4)$. In contrast with other hydrogen-bonded iodate crystals, which undergo the order-disorder phase transitions, the phase transition in $NH_{1}IO_{3}$ crystal has clear marks of the displacive type phase transition. First, $^{127}I$ NQR measurements allow to assign the low frequency bands observed in IR and Raman spectra of $NH_{4}IO_{3}$ crystal to the lattice vibrational associated with librational and translational vibrations of the $NH^{+}_{4}$ and $IO^{-}_{3}$ ions $(\nu < 120 cm^{-1})$ and with internal vibrations of N-H and I-O ions $(v < 850 cm^{-1})$, whereas high frequency bands were associated with $NH{\ldots}O$ hydrogen bond. Second, the potential barriers for such vibrations were estimated from the spectroscopic data. Further, it was shown, that the temperature dependence of the asymmetry parameter of the $^{127}I$ in the pretransition temperature range $((T_{c}-50) \div T_{c})$ can not be described by a simple Arrheniuse's law. In the present paper we attempted to explain this deviation by the decreasing of potential barriers for reorientational vibrations of $NH^{+}_{4}$ and $IO^{-}_{3}$ ions due to the temperature expansion of the crystal volume and the change in the hydrogen bonds network in the pretransition temperature range

FT-IR AND NQR STUDIES OF PHASE TRANSTION IN α−KIO3\alpha-KIO_{3} CRYSTAL

1. A. Petrosyan, A. Busch, V. Chechkin, V. Volkov, Yu. Venevtsev, Ferroelectrics, 21, 525 (1978) 2. A. Barabash, D. Baisa, E. Shadchin, Khimilcheskaya Fizika (in Russian), 5, 1506 (1986).Author Institution: National Academy of Sciences of Ukaraine, 46 Nauki Prosp., 252650 Kiev, Ukraine; Polish Academy of Sciences, 50-950 Wroclaw 2, P.O. Box 937, PolandFerroelectric $\alpha-kIO_{3} \cdot HIO_{3} (T_{c} = 223k)$ at T = 300k belongs to monoclinic $syngony^{1}$ (space group $p21/c, z = 4$) and consists of two sets of dimers $H(Io3)2^{-}$, connected by strong $(R_{O\ldots O} = 2, 5A)$ hydrogen bonds. In order to clarify the structure of low-temperature ferroelectric phase and the dynamics of phase transition the temperature variable FT - IR study $(4000-400 cm^{-1})$ of polycrystalline $\alpha-KIO_{3} \cdot HIO_{3}$ have been performed from T = 300 to 13k. Pressure dependable NQR measurements (0 - 5kbar) have been performed $earlier^{2}$ at T=300 and 77K. On cooling to low temperatures the ABC structure of OH stretching vibrations $(1600 - 2800cm^{-1})$ slightly varies: the middle frequency band (B) shows the asymmetric contour and moves slightly to lower frequencies; the intensity of it decreases, whereas those of bands A, and particularly C, increase. Another striking feature in these spectra is a very strong absoraption culminating between 600 and $1200cm^{-1}$ which shows a doublet structure on cooling. Such spectral distribution is typical for symmetrical strong H-bonds between two equivalent molecules related by a center of inversion or a two-fold axis. These facts correlate well with structural $data^{1}$ and confirm the NQR conclusions about distortion of anion sublattice along with the crystal space group lowering to $P2_{1}$ at the phase transition temperature

SPECTROSCOPIC EVALUATION OF THE STRUCTURE AND DYNAMICS OF THE HYDROGEN BONDS IN DIFFERENT CRYSTAL MODIFICATION OF POTASSIUM BIIODATE

$^{a}$A. I.Barabash, J. Mol. Struct., 270, 517 (1992)Author Institution: National Academy of Sciences of Ukraine, Institute of Physics; Polish Academy of Sciences, Institute of Low Temperature and Structure ResearchThe $KIO_{3}$.$HIO_{3}$ crystals belong to the family of acidic iodate compounds which is well-known for its ferroelectric phase transitions and has four crystal modifications. Here we report the results of IR and Raman study of vibrational spectra ($4000-60 cm^{-1}$) of $\alpha$ and $\sigma$-modifications in the temperature range from 300 to 13 K. The $\alpha-KIO_{3}$ $HIO_{3}$ crystal ($P2_{1}/b$ space group, z=4 at room temperature) is built of two nonequivalent sets of dimers $IO_{3}\cdots H \cdots O_{3}I$, joined by strong symmetric hydrogen bond (O...O distance is about 2.5 \AA). At 223 K ($T_{e}$) the ferroelectric phase transition occurs in this crystal. The stable $\sigma$-modification (space group $Fdd2, z=24$ at room temperature) is formed from structure units of three distinct types:$I_{2}O_{5}(OH)$, which are joined by the H-bound network. In order to understand the normal modes of vibrations in $\alpha-KIO_{3}-HIO_{3}$.and $\sigma-KIO_{3}\cdot HIO_{3}$, we have carried out lattice dynamics calculations using a generalized valence force field for the dimers $[H(IO_{3})_{2}]^{-}$ and othe structure units. The introduction of the translation and libration coordinates related to the normal coordinate analysis leads to a precise assignment of the frequencies and shows that both stretching and bending vibrations of $IO_{3}$ groups are strongly coupled with the translational vibrations of the hydrogen atom in case of $\sigma$-modification. In the case of $\sigma$-modification those vibrations are localized on separate I-O bonds. A comparative analysis of the vibrational spectra and structure is presented. On the base of analysis of vibrational spectra and according to $measurements^{a}$ of pressure dependencies of the $^{127}I$ NQR spectra at 77 K it was concluded that the mechanism of ferroelectric phase transition in $\alpha$-modification of potassium biiodate crystal involves the proton ordering in the double-well potential at $T_{c}$