Intermediate valency state of samarium chalcogenides under high pressure

Abstract

Samarium chalcogenides undergo isostructural phase transitions under pressure and samarium ions turn into an intermediate valency state (IVS).The thermo EMF S and electroresistance p of SmS, SmSe and SmTe under pressure to 12 GPa and in the temperature range 30-400 "C have been measured to investigate properties of the IVS and its stability limits. Hydrostatic pressure to 8 GPa and quasi-hydrostatic pressure to 12 GPa on monocrystals have been realised in 'toroid' cells. It is found that the IVS is stabilised after phase transitions at pressures of 0.65 GPa in SmS, 3.4 GPa in SmSe and 5.2 GPa in SmTe. Transition to IVS is accompanied by sharp anomalies in p and S. It is shown that IVS exists in a wide pressure range (at least to 12 GPa). The zone structure model for these compounds is suggested.

Phase transition properties of SmTe under pressure

“…B2 transition is accompanied by a valence change in the pressure range of 6-8 GPa. Sidorov et al [3] have reported similar valence transition at 5.2 GPa. To our knowledge, no theoretical studies have been performed on the structural properties at high pressures.…”

“…In the recent past, the rare-earth chalcogenides (RECs) have attracted experimental [1][2][3][4][5] and theoretical attention [6][7][8] due to their interesting structural, optical, mechanical and electronic properties. Out of these RECs, Samarium mono-chalcogenides SmX (X ¼ S, Se, Te) have been investigated in greater detail in view of their technological applications [1,2] in spintronic and spin filtering devices.…”

“…These studies show that these compounds undergo a pressure-induced transition from six-fold coordinated rock-salt (B1) to eight-fold coordinated CsCl (B2) structure. Besides, it has also been found [2][3][4] that some of these compounds show an isostructural valence transition from divalent (2 þ ) to trivalent (3 þ ) states due to promotion of highly correlated f electrons into the 5d conduction band states of the Sm ion. These compounds are semiconducting if Sm is divalent and they become metallic if the Sm ion is trivalent in nature.…”

Electronic structure of Sm and Eu chalcogenides

“…Photoemission experiments show distinctly different spectra for the two phases, which usually are interpreted on the basis of divalent f 6 ions in the ground state and mixed valent f 5 -f 6 ions in the high pressure metallic phase [17,18]. Similar valence instabilities are observed in SmSe and SmTe [13,17,19,2], which also crystallize in the NaCl structure. For these compounds the volume changes continuously, but anomalously, with pressure (at room temperature) [20,2].…”

Electronic structure of samarium monopnictides and monochalcogenides

“…7 The C 12 elastic constant decreases with pressure 17 and becomes negative in the high pressure phase, indicative of an intermediate valence system. 1 The pressure-induced semiconductor to metal transformation has also been traced by optical reflectivity studies, 18 Mössbauer measurements, 19 resistivity measurements, 20 and inelastic x-ray scattering. 8 Valence transitions can also be brought about by alloying with other trivalent ions, such as Y, La, Ce, or Gd, into the SmS lattice.…”