Mizuhiko Ichikawa

Correlation between two isotope effects in hydrogen-bonded crystals: transition temperature and separation of two equilibrium sites

Abstract Isotope effects in hydrogen-bonded crystals exhibiting structural phase transitions are studied systematically. A correlation exists between the relative isotopic shift of the transition temperature Δ T C / T C,H and the symmetric hydrogen-bond length O⋯O in the high-temperature phase Δ T C / T C,H is proportional to Δδ, the isotopic variation in the distance between two equilibrium positions in the symmetric bond

Structure and nature of the low-temperature phase transition in M3H(XO4)2-type crystals with a zero-dimensional H-bond network

Abstract A review is given of the studies of M 3 H(XO 4 ) 2 -type crystals (M = K, Rb, Cs; H = H, D; X = S, Se) with a zero-dimensional (0-D) H-bond network, focusing on the characteristic features of their structures and the nature of the low-temperature phase transition. It is shown that a linear relation between the transition temperature T c and H-bond distance R (O⋯O) is valid as in the 1-D to 3-D cases ( T c = 4154 ( R − R 0 ), where T c is in K, R in A, and R 0 = 2.495 A). The large value of R 0 is also discussed. The (possible) antiferroelectric, higher-order and order—disorder characteristics of the transition are described. The origin of the disappearance of the transition in three hydrogenated crystals is discussed in connection with the quantum effect of the H-bond. The need to obtain a unified picture of the isotope effect including the geometric and the quantum aspects is emphasized.

Temperature dependence of lattice constants of Rb3H(SeO4)2, a ‘zero-dimensional’ h-bonded crystal

Abstract In view of the interest in the ‘zero-dimensional’ H-bond network and low transition temperature in M3H(XO4)2-type crystals, lattice constants of Rb3H(SeO4)2 have been measured in a temperature range between room temperature and 20 K using an automatic X-ray 4-circle diffractometer with a closed-cycle refrigerator in order to confirm whether it does not exhibit any phase transitions. With lowering temperature, a-, b-, and c-axes in monoclinic cell showed a monotonous decrease, while β decreased very little, with no remarkable changes suggesting phase transitions. These results raise a fundamental problem, i.e. why Rb3H(SeO4)2 does not exhibit phase transition although the crystallographically symmetric H-bond of 2.514(7) A lies in a range where disordering of the protons will almost certainly occur.

DEUTERATION-INDUCED STRUCTURAL PHASE TRANSITIONS IN SOME HYDROGEN-BONDED CRYSTALS

Abstract A review is given, mainly based on heat capacity measurements, of the deuteration-induced structural phase transitions recently found in some rather diverse types of hydrogen-bonded crystals: i.e. M 3 H(XO 4 ) 2 -type crystals with strong hydrogen bonds, 9-hydroxyphenalenone derivatives with strong intramolecular hydrogen bonds, and (NH 4 ) 2 MX 6 -type crystals with weak asymmetric hydrogen bonds. For M 3 H(XO 4 ) 2 -type crystals and 9-hydroxyphenalenone derivatives, the hydrogen-bond distance seems to be one of the determinant parameters, while for (NH 4 ) 2 MX 6 -type crystals it is as yet unclear. However, some common characteristics of the heat capacity behavior are noted: (1) the magnitude of the transition entropy is compatible with order-disorder-type phase transitions, and (2) unusually the values of the heat capacity of the undeuterated compound are larger than those of the deuterated compound at the lowest temperature range.

A calorimetric study of the deuteration effect on the phase behavior in Rb3H(SeO4)2 and Rb3D(SeO4)2: II. Heat capacities at very low temperatures

Abstract The heat capacities of Rb 3 H(SeO 4 ) 2 and Rb 3 D(SeO 4 ) 2 crystals were measured from 2 to 20 K with an adiabatic calorimeter. The results agreed excellently well with the previous data for T > 12 K . The heat capacities of Rb 3 H(SeO 4 ) 2 were larger than those of Rb 3 D(SeO 4 ) 2 in the temperature range 2.9 and 38 K. The excess heat capacity of Rb 3 H(SeO 4 ) 2 reached a maximum of 1.7 J K −1 mol −1 at 20 K. Its temperature dependence was analyzed with the Schottky heat capacity function. The low temperature limiting behavior of the excess heat capacity was reproduced well by a two-level Schottky scheme whose excited level is distributed according to a Gauss function centered at (52.5 ± 0.3) R K with the full width at half maximum of (29.9 ± 0.8) R K. Proton tunneling motion is discussed as a probable origin of the energy level splitting.

Linear relation between transition temperature and H-bond distance in M3H(SeO4)2-type crystals (M = K, Rb, Cs) with 0-dimensional H-bond network

Abstract A linear relation holds between the low-temperature transition point T c and H-bond distance R for M 3 H(SeO 4 ) 2 -type crystals (M = K, Rb, Cs) with 0-dimensional H-bond netowrk: T c = 4154 ( R - 4.495), where T c in K and R = 0 ⋯ 0 in A. Such a linear relation appears to be the general rule for crystals with 0- to 3-D H-bond networks. The most striking feature is that it gives an R value at T c = 0 K of 2.495 A, which is longer than the values 2.40—2.45 A in 1-D to 3-D H-bond networks and is incompatible with the knowledge so far obtained. The puzzling nature of Rb 3 H(ScO 4 ) 2 with no phase transition, its possible interpretations and the relation to the phase-transition model are discussed.

The effect of hydrogen bonding on the bond lengths and angles in the carboxyl group

The effects of hydrogen bonding on the geometry of the carboxyl group have been studied systematically based on accurate X-ray crystallographic data. In general, the C-O(H) bond length increases with increasing O ⋯ O hydrogen-bond length, while the C=O bond length decreases. These variations become less pronounced for longer O ⋯ O distances. The O=C-O(H) and C-C-O(H) angles decrease with increasing O ⋯ O separation, while the C-C=O angle increases. The sum of the three angles remains close to 360 °, testifying to the planarity of the carboxyl group. These correlations are not observed to hold for cyclic hydrogen-bonded dimers, indicating that a continuous variation in the degree of disorder of the protons may be present, ranging from the ordered case with easily distinguishable C=O and C-O(H), to the 50%-50% disordered case, where the two C-O distances become equal.

Calorimetric study of the deuteration effect on the phase behaviour in Rb3H (SeO4)2 and Rb3D (SeO4)2

Abstract The heat capacities of Rb 3 H (SeO 4 ) 2 and Rb 3 D (SeO 4 ) 2 crystals were measured from 12 to 304 K with an adiabatic calorimeter. In Rb 3 H (SeO 4 ) 2 , no thermal anomalies were observed and the experimental heat capacities were reproduced well by a sum of harmonic oscillator heat capacity functions. A higher-order phase transition was found at 95.4 K in Rb 3 D (SeO 4 ) 2 . The transition enthalpy and entropy were 3.4 × 10 2 J mol −1 and 4.0 J K −1 mol −1 , respectively. The magnitude of the entropy change suggests an order-disorder mechanism for the transition.

Geometric and quantum aspects of phase transition and isotope effect in hydrogen-bonded ferroelectrics and related materials

Structural phase transitions in hydrogen-bonded (H-bonded) ferroelectrics and related materials are reviewed from the geometrical viewpoint of H-bond system. It is pointed out that the isotope effect in the transition temperature Tc has a correlation with the symmetry of H-bond and its geometric isotope effect. It is shown that among isomorphous members, a liner relation between Tc and O c O distance R is valid in three dimensionally (3-D) H-bonded tetragonal KH2PO4-, 2-D squaric acid (H2C4O4)-, 1-D CsH2PO4-, 1-D PbHPO4-, 0-D M3H(SeO4)2-type crystals. Implication of the linear relation as negative evidence for proton-tunnelling model is discussed. It is emphasised that the geometric characteristic is responsible even for the low-temperature phase transition in M3H(XO4)2-type crystals of which the three members have no phase transition and are quantum ferroelectrics. The disappearance of the transition is discussed as a manifestation at low temperature of isotopic downward shift of the transition temperatu...

X-ray study of deformation density and spontaneous polarization in ferroelectric NaNO2

The deformation electron density of ferroelectric sodium nitrite has been determined from X-ray diffraction data at 30 K, using Hirshfeld deformation functions. Owing to the strong correlation between odd terms of the deformation coefficients, constraints were imposed in the refinement. The net charges for Na, N and O atoms were estimated to be 0.27, 0.20 and -0.24 e, respectively. The calculated spontaneous polarization using these net charges and atomic dipole terms, 7.8 microC cm(-2), is much closer to the recently measured value, 12 microC cm-2, as compared with the value calculated from the formal point charges (74 microC cm(-2)).