Yu. N. Ivanov

Growth and characterization of p-type Cd1 − xZnxTe (x = 0.2, 0,3, 0.4)

Abstract Single crystals of p-type semi-insulating Cd1 − xZnxTe (CZT) with x = 0.2, 0.3 and 0.4 were grown using the high-pressure vertical Bridgman method. The crystals showed a high degree of axial homogeneity over most of the length of the ingot. The crystals were characterized by measuring the specific resistivity, X-ray spectral analysis, triaxial rocking curves, photoluminescence, etch-pit density and photocurrent kinetics. Despite the high homogeneity the photocurrent kinetics are worse than n-type CZT (x = 0.1 and 0.2) also grown by high-pressure vertical Bridgman.

Phase transitions of Cs3Sb2I9, Cs3Bi2I9, and Cs3Bi2Br9 crystals

The phase transitions of Cs3Sb2I9 , Cs3Bi2I9 , and Cs3Bi2Br9crystals were investigated by 133Cs nuclear magnetic resonance. Cs3Bi2Br9was also studied by 87Br nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs3Sb2I9 , the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs3Sb2I9has a trigonal structure. The observed temperature variation in the shape of the 133Cs NMR and 127I NQR signals suggests that Cs3Bi2I9may undergo a transition to an incommensurate phase.

Mapping high-pressure Bridgman Cd/sub 0.8/Zn/sub 0.2/Te

Single crystals of Cd/sub 0.8/Zn/sub 0.2/Te grown at the Institute of Solid State Physics, Chernogolovka, Russia, by the high-pressure vertical Bridgman method (HPVB) were mapped using X-ray fluorescence (XRF), X-ray diffraction (XRD), photoluminescence (PL), and leakage current measurements, most of the Russian samples which we refer to as p-type CZT were more uniform in Zn composition than U.S. commercially produced material. The Russian material had a poorer crystallinity and, in the best case, could only count nuclear radiation. Differences in the material properties between Russian (p-type) and U.S. (n-type) material will be described.

Homogeneity of CdZnTe detectors

Abstract We describe the current state of nuclear radiation detectors produced from single crystals of Cd 1− x Zn x Te(CZT), with 0.04 x

Evaluation of Russian-grown Cd0.8Zn0.2Te

More recent Russian grown single crystals of Cd0.8Zn0.2Te (CZT) were evaluated using proton induced x ray emission (PIXE), x ray diffraction (XRD), photoluminescence (PL), infra red (IR) transmission microscopy, leakage current measurements and response to nuclear radiation. Whereas in the past the Russian grown samples were not acceptable for gamma ray detectors application, the present samples had a somewhat better crystallinity and a higher resistivity, and did even show distinct photopeaks for an 241Am spectrum. Differences in the material properties between various Russian (n- and p-type) and U.S. (n-type) CZT are described.

Local and Average Structure of Relaxor Na1/2Bi1/2TiO3 from the Point of View of NMR

23 Na NMR spectra have been measured in a very low magnetic field at temperatures between 203 K and 780 K. The orientation dependences of the second-order quadrupole shifts of the central component were studied. The achieved spectral resolution is sufficient to test various models of short –range order in the Na 1/2 Bi 1/2 TiO 3 (NBT) structure using computer simulations of the spectral line shape. The polar monoclinic areas were found to coexist with the tetragonal phase insertions till the lowest investigated temperature 203 K. In the temperature range 203 K – 640 K the NMR spectra also display the presence of areas with very small deviation from the cubic structure (matrix).

Orientational dependence of the tails of dipole-broadened NMR spectra in crystals

This paper describes experimental and theoretical studies of the tails of the dipole-broadened nuclear magnetic resonance (NMR) absorption spectra of 19F in isomorphic single crystals of BaF2 and CaF2 with the magnetic field directed along three crystallographic axes. The results obtained by directly measuring the derivative of the tail of the NMR absorption spectrum and the falloffs of the Engelsberg-Lowe free precession after Fourier transformation qualitatively agree. It is shown that the shape of the tail is well described by an exponential function in which the orientational dependence of the exponent does not reduce to variation of the second moment. The observed shape of the tail and the orientational dependence of its parameters are explained on the basis of a self-consistent fluctuating-local-field theory. Nonlinear integral equations are derived for the correlation functions, taking into account the changes of the actual number of nearest neighbors caused by the anisotropy of the dipole-dipole interaction and the contribution of lattice sums with loops. The equations are solved numerically. Good agreement is obtained for the computed dropoffs of the free precession, the NMR spectra, and the cross-polarization rates with the experimental results.

Two-dimensional2H NMR exchange spectroscopy on conducting ionic crystals

The two-dimensional (2-D) deuterium nuclear magnetic resonance (NMR) exchange spectroscopy is applied to two types of conducting ionic crystals for the study of hydrogen mobility and conductivity, viz. partially deuterated ammonium hydrogen selenate, NH4HSO4 (AHSe), and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI), DBP1−xDBPIx. In both crystals chemical exchange processes of deuterons between different hydrogen bridges occur which are studied by the 2-D-2H-NMR technique over a wide temperature range in the slow-exchange regime. For AHSe exchange only occurs between two sites which are involved in hydrogen bonds. Two Arrhenius-like exchange processes were found the activation energies of which could be determined. For the case of DBP1−xDBPIx, with several deuteron sites taking part in the exchange, the analysis of the quantitative exchange behavior required a combination of time-domain analysis of our 2-D NMR data with mixing-time- and temperature-dependent measurements. Different exchange rates for each two-site exchange, all showing Arrhenius behavior, were obtained for DBP0.3, DBPI0.7. For crystals with different phosphite concentrationx the differences towards DBP0.3, DBPI0.3 were established. With the help of conductivity data from dielectric measurements quantitative relations between the exchange and conductivity processes are obtained for AHSe as well as for DBP1−xDBPIx. Finally, an estimation of effective charge carrier densities is discussed in view of possible conductivity models for both crystals.

23Na NMR study of the local order in the Na1/2Bi1/2TiO3 structure in a weak magnetic field

The orientation dependences of the second-order quadrupole shifts of the central component in the 23Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na1/2Bi1/2TiO3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111]p direction and a small displacement statistically or dynamically disordered over six equally probable [100]p-type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111]p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K.

Local structure of disordered PbSc1/2Nb1/2O3 in the region of the diffuse tetragonal phase-rhombohedral phase transition

The local structure of the ferroelectric-relaxor PbSc1/2Nb1/2O3 in the temperature range from 550 to 220 K has been investigated using 45Sc nuclear magnetic resonance. It has been found that, in the paraelectric phase at temperatures below 550 K, the crystal consists of regions of an ordered elpasolite structure and inclusions of the disordered tetragonal perovskite phase with displacements along directions of the [001] type. The relative weight of the tetragonal structure in the region of the paraelectric phase is approximately equal to 0.28. Below the temperature of the phase transition from the disordered modification to the polar phase, the relative weight of the tetragonal phase decreases with decreasing temperature. The tetragonal structure is replaced by the trigonal polar structure. In a wide temperature range (∼50 K), there exists a heterophase structure that is characteristic of relaxors. Note that the correlation length of displacements in the tetragonal phase should be very small to explain the absence of indications of the existence of this phase in the diffraction data.