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The optical conductivity of the Kondo insulator compound CeOs 4 Sb 12 reveals the formation of a 70 meV charge gap below 160 K. It also exhibits a weak shoulderlike structure at 30 meV, below 60 K. We have investigated CeOs 4 Sb 12 by inelastic neutron scattering techniques using incident neutron energies ͑E i ͒ of 23 and 200 meV, at 5 and 176 K. Our measurements with E i = 200 meV show magnetic scattering centered near 50-60 meV and low-Q ͑ϳ3.8 Å −1 ͒ at 5 K, in addition to the continuum of magnetic scattering observed at 176 K. This clearly indicates the opening of a spin gap ͑or pseudogap͒ of the order of 50 meV at 5 K in the strongly renormalized band near the Fermi energy. Furthermore, measurements with E i = 23 meV did not reveal any evidence of magnetic excitations below 20 meV, consistent with the scenario of a spin gap in CeOs 4 Sb 12. However, the estimated magnetic scattering deduced from the E i = 200 meV data reveals the presence of broad magnetic scattering between 25 and 80 meV centered near 27 meV, indicating a second energy scale with a gap energy of 27 meV. We interpret the 50 meV energy scale in terms of a direct gap, while the 27 meV energy scale corresponds to an indirect gap across the two hybridized bands. This gap energy of 27 meV is in good agreement with the so-called universal energy scale versus single ion Kondo temperature plot. We present the general features of spin gap systems such as CeOs 4 Sb 12 and CeRu 4 Sb 12 , and discuss the role of crystal field excitations regarding the spin gap formation based on a theoretical model.

The structure of Ca0.37Sr0.63TiO3 perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]*) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]*). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the Δ line of symmetry (k = [0,0,ξ]*). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous modulation wavevectors for a value of ξ different from 0 and 1/2.

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