Noriko Onoda-Yamamuro

Calorimetric and IR spectroscopic studies of phase transitions in methylammonium trihalogenoplumbates (II)

Abstract Heat capacities of CH 3 NH 3 PbX 3 (X = Cl, Br, I) were measured between 13 and 300 K (365 K for the iodide). Two anomalies were found in the chloride and the iodide, and three in the bromide. All the phase transitions were of the first order, although the highest temperature transitions in the bromide and the iodide were close to second order. Their temperatures and entropies are as follows: CH 3 NH 3 PbCl 3 : 171.5 K (14.6 J K −1 mol −1 ), 177.2 K (10.0 J K −1 mol −1 ); CH 3 NH 3 PbBr 3 : 148.8 K(11.2 JK −1 mol −1 ), 154.0K(4.1 J K −1 mol −1 ), 236.3 K (8.2 J K −1 mol −1 ); CH 3 NH 3 PbI 3 : 161.4 K (19.0 J K −1 mol −1 ), 330.4 K (9.7 J K −1 mol −1 ). The transition entropies indicated that the phase transitions are of the order-disorder type. They are interpreted with three possible models involving the methylammonium ions disordered with respect to the orientation of the C-N axis itself and around the C-N axis. The infrared line width of the v 12 vibration of the methylammonium ion depended markedly on the temperature and was interpreted as caused by the hindered rotational motion in the cubic and tetragonal phases.

Dielectric study of CH3NH3PbX3 (X=Cl, Br, I)

Abstract Complex dielectric permittivities of CH3NH3PbX3 (X = Cl, Br, I) were measured at frequencies between 20 Hz and 1 MHz and at temperatures between 20 and 300 K (15 and 350 K for the iodide). Discontinuities or a sharp bend of the real part of the dielectric permittivity occurred at the phase transitions, except at the tetragonal (I4mcm-cubic phase transition where the permittivity showed no apparent change. The dielectric behavior in the cubic and tetragonal (I4mcm phases are described well by a modified Kirkwood-Frohlich equation. Dielectric dispersions were found in the orthorhombic phase of CH3NH3PbBr3 and CH3NH3PbI3 at temperatures between 30 and 120 K.

p-T phase relations of CH3NH3PbX3 (X = Cl, Br, I) crystals

Abstract Pressure-temperature phase relations of CH 3 NH 3 PbX 3 (X = Cl, Br, I) crystals were studied by using a high pressure DTA apparatus in the range between 0.1 Pa and 200 MPa. A triple point was found in each compound below 100 MPa. By pressurization, the low pressure phases disappeared at the triple point in the chloride and bromide while a new high-pressure phase appeared in the iodide. The pressures and temperatures of the triple points are 75.1 MPa, 175.7 K for CH 3 NH 3 PbCl 3 , 43.2 MPa, 152.9 K for CH 3 NH 3 PbBr 3 , and 84.8 MPa, 176.2 K for CH 3 NH 3 PbI 3 . All the boundaries between the cubic and tetragonal phases are upward convex and that of the iodide has a maximum at about 120 MPa. Other phase boundaries are essentially straight lines in the measured pressure and temperature ranges. By the use of the Clausius-Clapeyron equation, the transition volumes were calculated from the slopes of the phase boundaries and the transition entropies obtained in a previously published calorimetric experiment ( J. Phys. Chem. Solids 51 , 1383 (1990)).

New rotator phase revealed in di-n-alkylammonium bromides studied by solid-state NMR, powder XRD, electrical conductivity and thermal measurements

1 H NMR linewidth and relaxation times, powder XRD, electrical conductivity and differential scanning calorimetry have been performed on (C n H 2n+1 ) 2 NH 2 Br (n=2, 3 and 4). A new ‘rotator’ phase was revealed at 342481, 293538 and 254563 K, respectively, where rod-like cations perform uniaxial reorientation about the molecular long axis and, at the same time, translational self-diffusion at elevated temperatures. The highly disordered structure was also revealed by the transition entropies, the sum of which, at temperature above 130 K, approached the melting entropy, being as small as ca. 20 J K -1 mol -1 . These properties of the rotator phase are quite analogous to those detected in monoalkylammonium chlorides whose rotator phase can be described as a ‘low-dimensional plastic crystal’. The tetragonal crystal structure in the present rotator phase has space group I4/mmm close to the P4/nmm found for that of the monoalkylammonium salts.

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

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.

Thermal, electric, and dielectric properties of CH3NH3SnBr3 at low temperatures

The heat capacity of CH 3 NH 3 SnBr 3 was measured at temperatures between 13 K and 300 K. Four anomalies were found. Their temperatures and entropy changes are: 46.0 K, 0.37 J·K −1 ·mol −1 , (H); 188.2K, 6.02 J·K −1 ·mol −1 , (F); 213.0 K, (H), and 229.4 K, 18.98 J·K −1 ·mol −1 , (F), where H and F mean higher-order and first-order transitions, respectively. The last entropy value is the sum of the entropies of the two highest-temperature transitions. The transition entropies indicate that the transition at 46.0 K is of the displacive type and those at 188.2 K and 229.4 K of the order-disorder type. The total of the transition entropies is consistent with the orientational disorder of the methylammonium ion in a cubic environment. The electric properties changed from insulating to semiconducting at the 229.4 K transition where the conductivity increased by a factor of 10 4 as the transition temperature was traversed from below. A thermal activation energy of 29.1 kJ·mol −1 was determined for the conductivity of the high-temperature phase. A dielectric dispersion was found below 150 K for frequencies between 20 Hz and 1 MHz. A Cole-Cole analysis of the dielectric permittivities showed that the relaxation is characterized by a strong deviation from a single Debye process.

Neutron diffraction study on hydrogen bond structure in K3H(SeO4)2 and K3D(SeO4)2 crystals

Neutron diffraction experiments on the room- and low-temperature phases of K3H(SeO4)2 (Ttrs = 21 K) and K3D(SeO4)2 (Ttrs = 105 K) have been performed using the time-of-flight high-resolution powder diffractometer HRPD at the ISIS pulsed neutron source. Data were collected at 2 and 30 K for K3H(SeO4)2 and 2 and 130 K for K3D(SeO4)2 over the d-spacing range 0.7-2.4 A. The room-temperature phases of the crystals have monoclinic structure belonging to the space group A2/a. Rietveld refinement combined with the published dielectric and calorimetric studies reveals that H/D atoms are disordered over two symmetry-equivalent positions. The hydrogen bonds are non-linear with an O-H/DO angle of 175(1)°. The hydrogen bond length is 2.495(1) A in K3H(SeO4)2 and 2.533(2) A in K3D(SeO4)2. The separation between the half occupancy hydrogen positions is 0.32(1) A for K3H(SeO4)2 and 0.550(4) A for K3D(SeO4)2. The relation between these structural parameters and the isotope effect in the transition temperature is discussed. Diffraction patterns of the low-temperature phases indicated that the structure change accompanying the transition is very small.

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...

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.

Neutron powder diffraction study of the low-temperature phases of

We made neutron powder diffraction measurements for phase III (at 4, 120, 180, 220, and 250 K) and phase II (at 280 K) of . The structure of phase III was determined by the Rietveld method using the data obtained at 4 K; the initial structure parameters were obtained by a systematic trial-and-error method. Phase III is monoclinic (space group: ) with a = 4.4010(2) A, b = 9.5967(3) A, c = 6.9817(2) A, , and Z = 4. Both and ions are located at general positions, and the orientations of the ions are completely ordered. The structures of phase III at the higher temperatures were also examined on the basis of the structure at 4 K. The atomic displacement parameters of the ions increased anomalously with increasing temperature, indicating gradual but remarkable development of the orientational disorder of ions. The structure of phase II was determined to be rhombohedral (space group: ) with a = 5.027 90(4) A, c = 10.055 87(8) A, and Z = 3. The orientations of the ions were highly disordered about the axis.