Martensitic phase transformation in TiNi

From temperature dependent perturbed angular correlation (PAC) measurements (77–873 K) in equiatomic TiNi intermetallic alloy, martensitic phase transformations have been observed. Three frequency components corresponding to three different phases of TiNi have been found in the temperature range 298–873 K. The results of quadrupole frequency and asymmetry parameters at room temperature are found to be: ωQ = 14(1) Mrad/s, η = 0 (33%), ωQ = 40.0(5) Mrad/s, η = 0.66(3) (52%) and ωQ = 56.7(3)Mrad/s, η = 0.39(2) (15%). The frequency component with η = 0 and which enhances to ~52% at 373 K can be attributed to the cubic austenite phase. The predominant component (~52%) found at room temperature has been attributed to monoclinic martensitic phase of TiNi and the third component with values of ωQ and η similar to those for the martensitic phase is attributed to the intermediate orthorhombic phase. At 77 K, no intermediate and austenite phases have been found but only the martensite phase is observed at this temperature. From XRD measurements at room temperature also, three phases of TiNi have been observed.

TOPOLOGICAL FEATURES OF THE ELECTRON DENSITY DISTRIBUTION IN COVELLITE: POSITION Cu1.

The synthesized sample of covellite was investigated by NQR 63,65Cu methods. The experimentally determined values of the quadrupole frequency νQ and the asymmetry parameter η have been used to study the distribution of electron density in the region of the quadrupole nuclei in position Cu1. It is found that Cu-S bond in position Cu1 is ionic, and the gap LUMO-HOMO decreases with increasing temperature.

Investigation of the Temperature Dependence of the NQR Parameters in Covellite

By a method NQR 63,65Cu the synthesized standard of covellite was studied. The experimental values of quadrupole frequency νQ and parameter of asymmetry η were used for research of distribution of electronic density and Laplacian of the electron density in area of quadrupole kernel in position of Cu1. It is set that connection of Cu - S in position of Cu1 is mainly ionic tipe, and size of gap of LUMO - HOMO diminishes with the increase of temperature.

Perturbed Angular Correlation Study of the Static and Dynamic Aspects of Cadmium and Mercury Atoms Inside and Attached to a C60 Fullerene Cage

Abstract30 keV 111mCd and 50 keV 199mHg beams from ISOLDE were used to implant on preformed targets of C60 with a thickness of 1 mg cm-2. Endofullerene compounds, viz. 111mCd@C60 and 199mHg@C60 formed during implantation were separated by filtration through micropore filter paper followed by solvent extraction. Dried samples of the endofullerene compounds were counted for the time differential perturbed angular correlation (TDPAC) measurement using the coincidence of the 151 - 245 keV cascade of 111mCd and the 374 - 158 keV cascade of 199mHg on a six LaBr3(Ce) detector system coupled with digital electronics. The results for 111mCd@C60 indicate a single static component (27%) and a fast relaxing component (73%), the latter implying that the cadmium atom moves rapidly inside the cage at room temperature. The quadrupole interaction frequency and asymmetry parameter of the cadmium atom occupying the static site in C60 are wQ=8.21(36) Mrad s-1 and η = 0.41(9), respectively. The fast relaxation constant is 0.0031(4) ns-1. Similarly, mercury atoms also exhibit a single static and a fast component. The static site has a quadrupole frequency wQ=283.0(12.4) Mrad s-1 and η =0 with a fraction of 30%. The fast relaxation constant is 0.045(8) ns-1 with a fraction of 70%, very similar to that of cadmium.

Interpretation of the Magnetic Order, Neutron Diffraction and 111Cd PAC Studies of NdScGe

NdScGe is a ferromagnet with TC = 194(2) K. The magnetic moments of the Nd substructure order along the tetragonal c-axis just below TC and undergo a gradual canting away from the c-axis upon cooling, commencing at around 165 K. We used neutron powder diffraction to show that the Nd magnetic moments lie 70(3)o off the c-axis at 4 K [1]. The apparent discrepancy between our neutron diffraction results and those obtained using PAC [2] (a canting angle of around 32o just below TC , increasing to 45o at 25 K) can be reconciled by placing the principal axis of the electric field gradient tensor in the tetragonal basal plane, rather than along the c-axis, as assumed in the PAC analysis. The unusual temperature dependence of the 111Cd PAC quadrupole frequency at the Sc site, which was interpreted in terms of a lattice softening occurring near the Curie temperature, is shown to be a consequence of the fact that one cannot determine the sign of the quadrupole frequency from a PAC experiment. Finally, we show that the Nd magnetic moments in the canted regime have a [110] basal component rather than [100].

Transport, AC-Susceptibility and PQR Measurements of Strongly Electron Correlated Compound YbInCu4 at High Pressures

We have investigated physical properties of the valence fluctuating compound YbInCu 4 at high pressures and low temperatures. The first-order valence transition temperature TV ~ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure-stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature TK ~ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. 63 Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T1 and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

139La NQR Study in La2CuO4 over Temperatures up to 800 K

We have measured 139La NQR spectra and the nuclear spin-lattice relaxation rate for the highest (±7/2↔±5/2) and the middle (±5/2↔±3/2) NQR lines in the parent compound of high-Tc superconductor, La2CuO4, in a wide temperature range up to 800 K. From the abrupt increase of the quadrupole frequency VQ just below the tetra-ortho structural transition temperature TOT which comes from the staggered tilt of the CuO6 octahedra, the critical exponent is evaluated to be 0.4 which is close to 0.5 expected β in the standard mean field approximation for second order phase transition. In higher temperature than TOT, the nuclear spin-lattice relaxation at La site is dominated by the quadrupole relaxation, not reflecting the critical spin dynamics in the CuO2 plane. We successfully discriminate the two types of quadrupole relaxation rate, WQ1 and WQ2. The relaxation rate WQ2 is one order of magnitude larger than 1 and increases with the critical exponent α ⁓1.19 toward TOT

Perturbed Angular Correlation Studies in SrTiO3 Single Crystals

The quadrupole hyperfine interactions of 111Cd/111 In probe nuclei in single-crystalline SrTiO3 perovskite samples were investigated using Perturbed Angular Correlation spectroscopy. Three electric field gradients were detected and their fractions and hyperfine parameters were measured in the temperature range from 26 to 700 K. The fraction f0 having a vanishing quadrupole frequency, as expected for 111Cd on substitutional sites in a cubic lattice, starts to develop around 300 K and reaches 100% at 700 K. Two well-defined EFG's having closely lying quadrupole frequencies and asymmetry parameters of ωQi = 49.1(3) Mrad/s, 77, η0= 0.10(2) and ωQ2 = 51.8(3) Mrad/s, η2 = 0.12(2), at room temperature, were identified in the temperature range from 250 to 530 K, and their (100) orientation in the lattice was determined. They are associated with electronic defects at the probe atoms.