Yoshio Kume

Low temperature heat capacities of ammonium hexachlorotellurate and its deuterated analogue

Ammonium hexachlorotellurate (NH4)2[TeCl6] and its deuterated analogue (ND4)2[TeCl6] were studied by calorimetry at temperatures between ca 10 and 300 K. The hydrogenous compound underwent a gradual phase transition of the displacive type at 88.0 K. Excess heat capacity was found in the critical region both for T ⩾ Tc and T < Tc. The deuterated compound behaved in a similar way around its transition temperature of 86.7 K. However, it underwent three more phase transitions at 31.9, 46.8 and 48.1 K, of which there were no counterparts in the hydrogenous compound. Order-disorder mechanisms are suggested from the magnitude of the entropy changes of these deuterium-induced phase transitions. A possible mechanism of this unusually strong isotope effect is proposed by taking the calorimetric and structural data into account. Standard thermodynamic functions were calculated from the heat capacities and tabulated in the Appendix.

A Strong Isotope Effect in the Low-Temperature Property of Ammonium Hexachlorotellurate

Heat capacity and neutron powder diffraction measurements have been performed on protonated and deuterated ammonium hexachlorotellurate. This powerful combination of techniques has provided a detailed picture of the phase transition behaviour in these materials. (NH4)2TeCl6 undergoes a single cubic-to-rhombohedral transition at 88 K. (ND4)2TeCl6 has a similar phase transition at 87 K but undergoes two further phase transitions at 32 K and 48 K. These deuteration-induced phase transitions involving tetragonal, monoclinic, rhombohedral and cubic symmetry are an unusually strong case of an isotope effect of which there have been no well-established cases among ammonium salts.

Heat capacities of copper(II) formate tetrahydrate and tetradeuterate

Abstract The heat capacities of copper(II) formate tetrahydrate and tetradeuterate have been measured from 12 to 300 K with an adiabatic calorimeter. They have sigmoidal temperature dependence except near the antiferroelectric-paraelectric transition temperatures, 235.78 ± 0.05 K and 245.64 ± 0.05 K, respectively. The corresponding enthalpy changes are 836.0 ± 1.0 J mol −1 and 936.9 ± 0.5 J mor −1 . The entropy changes are 3.546 ± 0.005 JK −1 mol −1 and 3.814 ± 0.002 JK −1 mol −1 . The heat capacities are larger in the high temperature phase than in the low temperature phase, the difference amounting to 5.74 JK −1 mol −1 and 7.15 JK −1 mol −1 for the hydrate and the deuterate, respectively. The heat capacity anomaly is compared with those in tin(II) chloride dihydrate and potassium hexacyanoferrate trihydrate and discussed in relation to the structure of the hydrogen bond networks in these substances. The discussion is extended to include possible properties of the hydrogen bond frameworks in ices I h and II.

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

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...

Deuteration effect in the phase transition of (NH4)2PbCl6

Abstract Heat capacities of ammonium hexachloroplumbate (ND 4 ) 2 PbCl 6 and its deuterated analogue (ND 4 ) 2 PbCl 6 were measured at temperatures between 13 and 300K. These compounds underwent lambda phase transitions at T c (H) = 78.1 K and T c (D) = 80.7 K respectively. The deuterated salt underwent another first order phase transition at 38.4K. The entropy change of this first order phase transition was 6.5 J K −1 mol −1 . The heat capacities at the lowest temperatures (13–22 22K) were larger for the hydrogenous crystal than for the deuterated analogue.

Neutron Diffraction Study on Orientational Disorder of ND+4 Ions in (ND4)2SeCl6 Crystal

A neutron diffraction experiment on the high-temperature phase of (ND 4 ) 2 SeCl 6 crystal has been performed with a time-of-flight powder diffractometer. The data were collected at 60 K in the d -spacing range 0.6–6 A. The Rietveld refinement revealed that the crystal has the antifluorite structure ( F m \bar3 m , a =9.8420(1) A) in which four D atoms of the ND + 4 ion at (1/4, 1/4, 1/4) are preferentially (but not exclusively) oriented toward the four unoccupied corners of the (1/2×1/2×1/2) cube surrounding the cation. The Fourier contour map ( F o b s ) of the (111) section suggested that the D atoms are disordered about the preferential orientations along the crystallographic 3-fold axes, with substantial density of the scattering centers distributed off the symmetry direction. The result is discussed in the light of previous thermodynamic data on the phase transition at 46 K.

Deuteration-induced phase transition in ammonium hexachloroplumbate

The heat capacities of (NH4)2PbCl6 and (ND4)2PbCl6 were measured at temperatures between 13 K and 300 K. A gradual phase transition was found at 77.8±0.3 K for (NH4)2PbCl6 with an associated transition entropy of 3.0 J K−1 mol−1. The transition temperature shifted to 81.0±0.3 K in (ND4)2PbCl6. A sharp first order transition occurred at 38.40±0.03 K for (ND4)2PbCl6 accompanied by a transition entropy of 7.5 J K−1 mol−1. There was no corresponding phase change in (NH4)2PbCl6. The mechanism of the deuteration-induced transitions in the present compound and similar transitions in (ND4)2TeCl6, (ND4)2PtCl6, (ND4)2PdCl6, and (ND4)2SeCl6 is discussed. Tunneling energies involved in the rotational motion of the ND4+ ion in the three-minimum potential were derived from the experimental entropy data. They correlate smoothly with the metal-chlorine bond distance for these five isomorphous compounds. Standard thermodynamic functions calculated from the heat capacity data are tabulated in the Appendix.

Structural phase transition in deuterated ammonium hexachloroplatinate (IV) studied by 35Cl NQR

Abstract The temperature dependences of the 35 Cl NQR frequencies and spin-lattice relaxation times in (ND 4 ) 2 PtCl 6 were measured from 4.2 to 400 K. The phase transition at 27.5 K induced by deuteration was confirmed to be a first-order one, although the transition was associated with rotary soft modes like other deuterated ammonium hexachlorometallates. It was suggested from the NQR spectra observed in the low temperature phase that the low temperature structure is orthorhombic (F mmm ).

A strong isotope effect in the phase transition behaviour of ammonium hexachlorotellurate

Abstract Heat capacity and neutron powder diffraction measurements have been performed on protonated and deuterated ammonium hexachlorotellurate. This powerful combination of techniques has provided a detailed picture of the novel phase transition behaviour in these materials.

Calorimetric, spectroscopic and dielectric studies of a phase transition in methylammonium hexachloroplatinate

Abstract The heat capacity of (CH 3 NH 3 ) 2 [PtCl 6 ] was measured from 15 to 300 K. A phase transition was found at T c = 123.82±0.05 K. An anomalous λ-type heat capacity was separated from the vibrational baseline using normal mode frequencies of the ions and lattice vibrations. The enthalpy and entropy of the transition are 1.49 kJ mol −1 and 14.5 J K −1 mol −1 respectively. The excess heat capacity Δ C diverged at T c following the equation Δ C = A ¦( T - T c / T c ¦ −α where A = 24.3±0.4 J K −1 mol −1 , T c = 123.87±0.01 K, α = 0.221 ±0.004 for T T c and A = 3.0±0.1 J K −1 mol −1 , T c = 123.77±0.02 K, α = 0.36±0.01 for T > T c . The methylammonium ion rotates rapidly about its molecular axis, as deduced from IR line broadening, the orientational correlation time being about 1 ps at 140 K. The activation energy of the reorientation was 3.5±0.1 kJ mol −1 as derived from three different modes of vibration of CH 3 NH 3 + and (CH 3 ND 3 + . The dielectric permittivity followed the excess entropy, but not the excess enthalpy, when plotted as a function of temperature in a normalized form. In contrast with Curie-Weiss behaviour, most of the change in permittivity associated with the transition occurred below T c . This may involve a hitherto unknown mechanism of dielectric response of a molecular ionic system.