Experimental facts about appearance of new crystalline phases at low temperature related to deuterium substitution are collected and mechanisms of the isotope effect discussed. Compounds considered are (i) tri-alkali hydrogen selenates and sulphates, (ii) ammonium hexachlorometallates, (iii) bromo- and iodo-hydroxyphenalenone and (iv) chromium hydrogen dioxide. In (i), (iii) and (iv), linear O-H-O hydrogen bonds are involved. Proton tunnelling is the likely mechanism of the peculiar phase behaviour of (i). Definitely the tunnelling plays an essential role in (iii) but not in the extreme situation in (iv). In (ii), rotational tunnelling of an ammonium ion is the likely ingredient of the deuteration-induced phase transitions. The term proton cloud is introduced to describe the quantum mechanical distribution of the nucleus in the hydrogen bond and in the multi-valley rotational potential for an ammonium ion.

Proton tunnelling and deuteration-induced phase transitions in hydrogen-bonded crystals

The synthesis of a new ionic plastic crystal, tetraethylammonium-d20 d-10-camphorsulfonate, is reported. The crystal has three solid phases, the structures of which were determined by single-crystal X-ray diffraction (XRD). XRD analysis revealed a phase transition from nonpolar space group P21212 to polar space group P21 with increasing temperature. The dynamics of the d-10-camphorsulfonate anion and the tetraethylammonium-d20 cation was investigated by variable-temperature 1H and 2H solid-state NMR spectroscopy. The anion showed swing motion in high-temperature phases, which is in good agreement with the disorder observed in XRD analysis. The cation showed uniaxial rotation even in the low-temperature phase. The rotation axis of the tetrahedral structure showed a transition from the two-fold axis to the three-fold axis with the phase transition.

Nonpolar-to-Polar Phase Transition of a Chiral Ionic Plastic Crystal and Switch of the Rotation Symmetry

Deuteration-induced structural phase transitions in some hydrogen-bonded crystals

Neutron powder diffraction experiments of (ND4)2SeCl6, (ND4)2PtCl6, and (ND4)2PtBr6 were performed to investigate the mechanism of the strong deuterium substitution effect on the phase transitions of the (NH4)2MX6 family. The isotope effect is strong in the first and second compounds and weak in the third. The data were collected in the d-spacing range 0.5–4.3 A by using a time-of-flight powder diffractometer VEGA installed at the pulsed cold neutron source in KEK. The intensity data of high-temperature phases (HTP) were measured at the temperatures corresponding to 1.3Tc (Tc: transition temperature), and the low-temperature phases (LTP) at 4 K. The HTPs of the three compounds have an antifluorite type cubic structure (a≈9.8 A and Z=4) as previously reported while the LTPs of (ND4)2PtCl6 and (ND4)2PtBr6 are tetragonal with a tentative space group P42/n and unit cell dimensions similar to those of the HTPs. The LTP of (ND4)2SeCl6 may have an orthorhombic structure with a larger unit cell. The Rietveld refi...

Neutron diffraction study of (ND4)2SeCl6, (ND4)2PtCl6, and (ND4)2PtBr6 crystals: The origin of the strong deuterium substitution effect on the phase transitions

Rotational barriers in ammonium hexachlorometallates as studied by NMR, tunnelling spectroscopy and ab initio calculations

Recent and novel data obtained from chlorine NQR measurements on natural and deuterated (NH 4 ) 2 MCl 6 compounds are discussed with special regard to the influence of the ammonium-ion dynamics on the structural stability of these crystals. The temperature dependence (4.2 K to 350 K) of the chlorine NQR frequency ν Q and relaxation rates T 1 −1 , T 2 −1 obtained from the natural ammonium salts of Sn, Pd, Os, Pb, Te, Se and from the deuterated salts of Sn, Te and Se are analysed. Slight deviations from the normal temperature behaviour of ν Q and T 1 −1 are found in Sn, Pd and Os compounds which stay cubic in the whole temperature range. The ammonium compounds of Pb and Te undergo a structural transformation between 80 K and 90 K from the cubic to a trigonal phase which is distinguished by the preservation of the single line spectrum of the chlorine NQR below T c1

C. Dimitropoulos

Papers

Effect of Deuteration on the Phase Transitions and on the Critical Dynamics in Ammonium Hexachlorometallates