Inelastic neutron scattering experiments were performed to investigate the crystalline electric field (CEF) excitations of Nd3+ (J = 9/2) in the iron pnictide NdFeAsO. The crystal field level structures for both the high-temperature paramagnetic phas
e and the low-temperature antiferromagnetic phase of NdFeAsO are constructed. The variation of CEF excitations of Nd3+ reflects not only the change of local symmetry but also the change of magnetic ordered state of the Fe sublattice. By analyzing the crystal field interaction with a crystal field Hamiltonian, the crystal field parameters are obtained. It was found that the sign of the fourth and sixth-order crystal field parameters change upon the magnetic phase transition at 140 K, which may be due to the variation of exchange interactions between the 4f and conduction electrons.
Thermoelectric figures of merit, ZT > 0.5, have been obtained in arc-melted TiNiSn-based ingots. This promising conversion efficiency is due to a low lattice thermal conductivity, which is attributed to excess nickel in the half-Heusler structure.
The phase stability of the (Bi2)m(Bi2Te3)n natural superlattices has been investigated through the low temperature solid state synthesis of a number of new binary BixTe1-x compositions. Powder X-ray diffraction revealed that an infinitely adaptive se
ries forms for 0.44 < x < 0.70, while an unusual 2-phase region with continuously changing compositions is observed for 0.41 < x < 0.43. For x > 0.70, mixtures of elemental Bi and an almost constant composition (Bi2)m(Bi2Te3)n phase are observed. Rietveld analysis of synchrotron X-ray powder diffraction data collected on Bi2Te (m = 2, n = 1) revealed substantial interchange of Bi and Te between the Bi2 and Bi2Te3 blocks, demonstrating that the block compositions are variable. All investigated phase pure compositions are degenerate semiconductors with low residual resistivity ratios and moderate positive magnetoresistances (R/R0 = 1.05 in 9 T). The maximum Seebeck coefficient is +80 muV K-1 for x = 0.63, leading to an estimated thermoelectric figure of merit, zT = 0.2 at 250 K.
