Phase transitions in rare earth tellurides under pressure

Abstract

Using first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R(3+)) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R(2+)) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R(2+) → R(3+) valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

Structure-Dependent Phase Diagram of the Occupation Number of 4f Electrons in Cerium Monoarsenide

“…This occupation number also agrees well with the XPS spectroscopy data, a photoelectron spectroscopy measurement, a DMFT calculation ( n f ∼ 1) with non-crossing approximation (NCA) impurity solver, a linear muffin-tin orbit (LMTO) + DMFT (LMTO + DMFT) calculation at T = 300 K ( n f = 1.008), and a self-interaction-corrected local spin density approximation (SIC-LSDA) calculation, suggesting the localized (atomic-like) 4f 1 configuration for Ce ions at ambient pressure and transforming into a delocalized phase under high pressure . In fact, previous experimental observation and first principles calculations both suggest that the valence (or the electron occupation number) change might not be directly from an integer to another integer, but rather to an intermediate valence upon the applied pressure. , In contrast to the valence fluctuation of CeN, the localized 4f state in CeAs (B1-type structure) demonstrates that except for the applied pressure, the p–f mixing behavior, the nearly energetically degenerate f 0 and f 1 configurations, and the electron transition f 0 d 1 ↔ f 1 d 0 have a marginal effect on the localization and occupation number of Ce 4f electrons, namely, the latter electronic mechanisms do not induce Ce 4f electrons to turn into an intermediate configuration or a mixed-valence state. Experimentally, the intensity ratio of Ce 4+ (4f 0 )/Ce 3+ (4f 1 ) + Ce 2+ (4f 2 ) XPS peaks is proportional to the amplitude of Ce 4+ and Ce 3+ states, and the localized regime in CeAs (B1-type structure) could be attributable to the screening of core holes by Ce 4f electrons.…”

“…76 In fact, previous experimental observation and first principles calculations both suggest that the valence (or the electron occupation number) change might not be directly from an integer to another integer, but rather to an intermediate valence upon the applied pressure. 79,80 In contrast to the valence fluctuation of CeN, 73 the localized 4f state in CeAs (B1-type structure) demonstrates that except for the applied pressure, the p−f mixing behavior, the nearly energetically degenerate f 0 and f 1 configurations, and the electron transition f 0 d 1 ↔ f 1 d 0 have a marginal effect on the localization and occupation number of Ce 4f electrons, namely, the latter electronic mechanisms do not induce Ce 4f electrons to turn into an intermediate configuration or a mixed-valence state. Experimentally, the intensity ratio of Ce 4+ (4f 0 )/Ce 3+ (4f 1 ) + Ce 2+ (4f 2 ) XPS peaks is proportional to the amplitude of Ce 4+ and Ce 3+ states, 64 and the localized regime in CeAs (B1-type structure) could be attributable to the screening of core holes by Ce 4f electrons.…”

Temperature‐dependent electronic properties for 4f states in cerium mononitride

“…Additionally, temperature-enhanced entropy would increase Ce 4f electron localization, expand the lattice, increase the occupancy number of Ce 4f electrons, as well as decrease the valence state of Ce ions [51]. A GS model fitting of UPS spectra yields a localized 4f occupancy of 0.82 [40], and a metallic band manifesting itself in a narrow peak at the Fermi level in CeN [11], consistent with another similar GS fitting results based on UPS and BIS measurements as well as the effective electron occupancy number deduced from the lattice constants, namely n f $ 0.85 [105], while the latter scenario is consistent with the Kondo model picture, in which the trivalent state and Ce 4f localization at around 1000 K are due to the breakdown of the screening mechanism and the onset of fluctuating local 4f moments with increasing of temperature [37,51], however, the above-mentioned result is in sharp contrast to n f = 1 inferred from L III absorption edge in XAS spectra [12]. The main reason for this discrepancy between XAS and XPS measurements is that, as a matter of fact, XAS spectra including d final states could not directly extract the occupancy number of Ce 4f electrons [12].…”

“…The Kondo screening mechanism and band picture could account for the localized and itinerant magnetic properties, respectively [51]. Previous experimental observation and first principles calculation also establish that the valence (or the electron occupancy number) change is not directly from an integer to another integer, but rather to an intermediate valence, upon increasing of pressure or temperature [105, 115]. Besides the temperature, the external pressure and the structural parameter of bulk CeN also affect the valence state of Ce ions in CeN.…”

Anisotropic strain in SmSe and SmTe: Implications for electronic transport

“…The piezoresistive effect in these materials shows several orders of magnitude change in resistance with a constant piezoresistive gauge (the logarithmic derivative of resistance with respect to pressure). Previous theoretical calculations using density functional theory (DFT) [16][17][18][19][20] focused on the different phases of these materials. These studies (both experimental and theoretical) have been limited to the analysis of effects of hydrostatic (i.e.…”