Magnetic spectral response and lattice properties in mixed-valenceSm1−xYxSsolid solutions studied with x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scatte

Abstract

Mixed-valence phenomena occurring in the "black" ͑B͒ and "gold" ͑G͒ phases of Sm 1−x Y x S have been studied by x-ray diffraction, x-ray absorption spectroscopy, and inelastic neutron scattering. Lattice-constant and phonon-dispersion results confirm that the valence instability occurs already inside the B phase. On the other hand, pronounced temperature anomalies in the thermal expansion ␣͑T͒, as well as in the Sm meansquare displacements, denote the onset of the B-G transition for the compositions x = 0.33 and 0.45. It is argued that these anomalies primarily denote an effect of electron-phonon coupling. The magnetic spectral response, measured on both powder and single crystals, is dominated by the Sm 2+ spin-orbit component close to 36 meV. A strongly overdamped Sm 3+ contribution appears only for x Ն 0.33 near room temperature. The quasielastic signal is strongly suppressed below 70 K, reflecting the formation of the singlet mixed-valence ground state. Quite remarkably, the signal around 36 meV is found, from the single-crystal spectra, to arise from two distinct, dispersive, interacting branches. The lower peak, confirmed to exist from x = 0.17 to x = 0.33 at least, is tentatively ascribed to an excitation specific to the mixed-valence regime, reminiscent of the "exciton" peak reported previously for SmB 6 .

Magnetic excitations in EuCu2(SixGe1−x)2: from mixed valence towards magnetism

“…The most reasonable interpretation is therefore the opening of a spin gap, accompanying the formation of a singlet ground state, which would be is a natural consequence of the excitonic model of IV. A similar behaviour was reported previously for the Sm-based IV compounds (Sm, Y)S and SmB 6 [4,6]. It is interesting to note, in this connection, that the temperature of 100 K above which the QE signal appears on heating also corresponds to a change in the slope of the energy shift of Ex1 shown in figure 5(a).…”

“…The value of the cross-section associated with Ex2, as well as its Q dependence following the Eu 3+ magnetic dipole form factor, strongly suggest that it corresponds to the SO transition shifted to a lower energy. Such a shift was previously observed in EuPd 2 Si 2 [10], but not in the Sm-based IV systems SmB 6 and (Sm, Y)S (gold phase), whose corresponding intermultiplet transitions occur close to the nominal ionic value 10 of 36 meV [4,5]. The latter materials, however, are categorized as 'IV semiconductors' in reference to their lowtemperature properties, whereas EuM 2 X 2 compounds have metallic ground states.…”

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

“…Some NTE materials have been found due to this effect, such as YbCuAl, 267 264 The NTE mechanism of temperature-induced charge transfer can be traced back to the studies on rare earths in early years of 1980s, such as the charge change of rare earth Yb in YbCuAl 267 and YbInCu 4 , 268 and Sm in (Sm 1Àx Y x )S (x = 0.33, a l = À3.0 Â 10 À5 K À1 , 10-255 K) 263,269 and (Sm 1Àx Gd x )S. 270 The large NTE was observed in A 0 Cu 3 Fe 4 O 12 (A 0 = larger rare earth elements La to Tb, Sr and Bi) which has A-site ordered double perovskites (Im% 3) of A 0 A 3 B 4 O 12 (Fig. Some NTE materials have been found due to this effect, such as YbCuAl, 267 264 The NTE mechanism of temperature-induced charge transfer can be traced back to the studies on rare earths in early years of 1980s, such as the charge change of rare earth Yb in YbCuAl 267 and YbInCu 4 , 268 and Sm in (Sm 1Àx Y x )S (x = 0.33, a l = À3.0 Â 10 À5 K À1 , 10-255 K) 263,269 and (Sm 1Àx Gd x )S. 270 The large NTE was observed in A 0 Cu 3 Fe 4 O 12 (A 0 = larger rare earth elements La to Tb, Sr and Bi) which has A-site ordered double perovskites (Im% 3) of A 0 A 3 B 4 O 12 (Fig.…”

Progress of Research in Negative Thermal Expansion Materials: Paradigm Shift in the Control of Thermal Expansion

“…For example, Sm 1−x Y x S (Jayaraman et al, 1975; Alekseev et al, 2006), Sm 2.75 C 60 (Arvanitidis et al, 2003), and YbGaGe (Salvador et al, 2003; Sleight, 2003) are known for negative or low thermal expansion by such a mechanism. In the case of Sm, two electronic configurations of (4 f ) 5 (5 d ) 1 and (4 f ) 6 compete energetically, but the atomic radius is determined mainly by the 4 f electron number.…”