39K quadrupole coupling and spin-lattice relaxation in the high temperature phases in KLiSO4

³⁹K quadrupole perturbed nuclear magnetic resonance spectra show that in KLiSO₄ atT _c =743 K a phase transition from a room temperature hexagonal to a high temperature orthorhombic phase takes place. The high temperature phase is definitely not incommensurately modulated. The huge shortening of the³⁹K spin-lattice relaxation time on approachingT _c from below demonstrates that KLiSO₄ becomes a superionic conductor above 743 K. The self-diffusion coefficient of the Li-ions is estimated asD=10^–6 cm²/s at 780 K.     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

The ¹H NMR line-width and spin-lattice relaxation time T₁ of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T₁ near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T₁ around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH₃ groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the ¹H NMR relaxation behavior, and that ¹H nuclei play important roles in the phase transitions of the TSCC single crystal.     

Low temperature Raman spectra of KLiSO4

Raman experiments have been performed on KLiSO₄ below room temperature. In the intermediate temperature range (190 K < T < 265 K), the spectra are better understood on the basis of the C₃ rather than C₆ point group, but a lower symmetry cannot be rejected. In the low temperature region (T < 190 K) the occurrence of three ferroelastic domains should be taken into account to explain the corresponding Raman spectra.     

Multiple phase transitions in rare earth tetraborides at low temperature

We report the temperature dependence of the magnetic susceptibility of single crystals of PrB₄, GdB₄, TbB₄, HoB₄ and TmB₄, both parallel and perpendicular to the tetragonal c-axis. We also present low temperature resistance measurements on crytals of GdB₄ through TmB₄. Two magnetic phase transitions are found for TbB₄, DyB₄, HoB₄ and TmB₄. For the latter two compounds, the lower transitions appear to be first order. For HoB₄, we have measured the low temperature specific heat. The lower transition in TbB₄ and HoB₄ is rapidly depressed upon dilution with YB₄.     

Optical study of the phase transitions in LiKSO4

The birefringence of LiKSO₄ has been measured over the range 27–700°C. The change in birefringence with heating and cooling is seen to be very different. Observations have been made on domains in (001) and (100) plates near the phase transitions.     

Powder neutron diffraction study of low temperature phase transition in LiKSO4

The salient features of the neutron diffraction measurements on polycrystalline sample of LiKSO₄ at 170 K and 104 K are reported. The sample undergoes a phase transformation to a single phase as against “mixed phases” reported in single crystal studies. These differences are discussed on the basis of the kinetics of nucleation and growth processes.     

Exafs study of structural phase transitions in RbVF4

Successive structural transitions of RbVF₄ have been studied with extended X-ray absorption fine structure (EXAFS) of Rb ions. It is found that there are some modulations in the pre-edge and post-edge regions of the X-ray absorption spectrum in the vicinity of the structural phase transitions above room temperature. By analyzing the EXAFS, it is also found that local spatial distortions around the Rb ions are induced at the structural transitions.     

High temperature phase transitions in the crystal [4]of p-trimethylammoniumbenzenesulfonate zwitterion

Crystals of p-trimethylammoniumbenzenesulfonate (ZWT) have been obtained from the thermally induced soli d-state reaction of methyl p-dimethylaminobenzenesulfonate (MSE) or by recrystallization in aqueous solution. The characterization of four crystalline phases has been performed by calorimetric, birefringence, X-ray and Raman scattering studies. An unknown phase transition at room temperature yielding a new crystallographic phase has been found by calorimetric measurements. ZWT crystalline samples obtained from the solid state reaction are disordered as compared to ZWT single crystals obtained by growing from aqueous solution. This disorder is related to different arrangements of the phenyl ring orientations. Received 21 October 1998 and Received in final form 14 April 1999     

Piezoelectric resonance study of structural phase transitions in BaMnF4

Structural transitions of BaMnF₄ are studied by piezoelectric resonance. The results show that large structural modulations occur from 220 K to 250 K. It is found from the temperature-dependence of the resonance curves that successive phase transitions appear at about 226 K, 234 K and 244 K. The inconsistency of the transition temperature from a A2₁ am to an incommensurate phase is interpreted as intrinsic structural instability, but not by defects or impurities in the crystal of BaMnF₄ used.     

High pressure study of phase transitions inα-FePO4

High pressure behaviour of FePO₄ in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO₄ tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.     

A low temperature study of antiferromagnetic YbVO4 by NMR thermally detected by nuclear orientation

NMR-TDNO results using an external ⁶⁰CoCo (hcp) nuclear orientation thermometer for non-irradiated, single crystal, antiferromagnetic YbVO₄ are compared with those obtained earlier with neutron activated samples using both internal and external γ-ray thermometers. Detailed comparisons are made for the ¹⁷¹Yb (I=1/2, 14.31% abundant) stable nucleus. This strongly asymmetric, largely homogeneous, resonance lineshape was retained and is readily power broadened. Extremely broad, field-dependent homogeneous thermometric responses are observed in the expected frequency range for the quadrupolar stable nucleus ¹⁷³Yb (I=5/2, 16.13% abundant) for both irradiated and non-irradiated samples. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermodynamic studies of successive phase transitions in BaZnGeO4 and a new solid phase below 186.1 K

The successive phase transitions of BaZnGeO₄ have been studied on meltsolidified samples. A new solid phase (named phase VI) has been found below 186.1 K in samples of large particle size (diameter:D0.1 mm). The higher temperature crystalline phase V can be supercooled easily down to liquid helium temperature. On heating, however, it transforms into phase VI above 95 K in a slow exothermic process. Heat capacities have been measured by adiabatic calorimetry between 14 and 300 K. The enthalpy and entropy of the V–VI phase transition are 187.1 Jmol^–1 and 0.971 J K^–1 mol^–1, respectively. The corresponding data for the IV–V phase transition at 199.8 K are 229.3 J mol^–1 and 1.168 JK^–1 mol^–1. The phase VI does not appear in samples of smaller particle size (D0.1 mm).     

14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

The¹⁴N NMR spectra and spin-lattice relaxation timeT ₁ of [N(CH₃)₄]₂ZnI₄ have been studied between room temperature and 200 K. Two phase transitions atT _c 1=255 K and atT _c 1=217 K are observed. The¹⁴N NMR lineshape andT ₁ data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH₃)₄]₂ZnI₄.     

On the low temperature phase transition in (NH4)2SO4

An ultracryostat and multidecameter were used to determine the temperature dependence of the dielectric constant ?′ and dielectric loss ?″ over a wide range of frequencies of single crystals and polycrystalline samples of (NH₄)₂SO₄ in the region of the low temperature phase transition. A sharp increase was observed in the values of ?′ and ?″ at about ?50°C. In addition, a dielectric dispersion was detected and found to be more pronounced in the high temperature phase. This dispersion was attributed to piezoelectric resonance. The observed sudden increase in the values of the dielectric constant and dielectric loss below ? 50°C was attributed to the ferroelectric nature of the low temperature phase of (NH₄)₂SO₄.A DTA thermogram showed a sharp peak at ? 50°C which indicated that the phase transition is one of first order type. A TMA thermogram showed that this transformation was associated with a rapid increase in the expansion coefficient. Such an increase in the lattice parameter might be attributed to the enhanced rotation of electric dipoles associated with the distorted NH₄⁺ and SO₄^2? ions. The distortion of both the ammonium and sulfate ions in addition to their expected orientational motion are suggested to be responsible for the ferroelectric behaviour of ammonium sulfate below ?50°C.A transition to a metastable hexagonal state at about ?40°C is thought to occur, and this transformation is found to be irreversible.