Novel reentrant high pressure phase transtion in lanthanum

Abstract

High presure x-ray diffraction experiments with synchrotron radiation in an extended pressure range up to 67 GPa reveal a novel phase transition of La: At about 60 GPa the low pressure fee structure of La is recovered from the distorted fee (^-fcc) phase. The reappearance of the high symmetry cubic structure at higher pressure implies that the ^-fcc phase, which also appears in other lanthanides, is not a precursor of the low symmetry structures caused by the so-called /delocalization. A possible mechanism for the reentrant behavior is considered and the phase diagram of La is discussed. PACS numbers: 62.50.-l-p, 64.70.Kb Under pressure the regular trivalent lanthanide elements show a well-known series of close packed structures [1]hep-> Sm-type-• dhcp-> fee .The same sequence is shown by these elements at ambient conditions with decreasing atomic number. According to band structure calculations [2][3][4], the structural stability of these lanthanide metals is controlled by the d occupation number which decreases with increasing atomic number and increases under pressure due to s~-*d transfer.Systematic high pressure x-ray diffraction studies in wider pressure ranges [5][6][7][8] have revealed one more common structure for the regular lanthanides beyond the fee phase for the elements La, Pr, Nd, Pm, Sm, Gd, Tb, and Dy. This additional structure has been designated as "distorted fee" (d-fcc) because there are additional superstructure reflections in the fee diffraction pattern. Though there are many proposals for the structure of the d-fcc phase [3,[9][10][11], final agreement has not been obtained. A separate paper is devoted to this question [12].At still higher pressures the early lanthanides following Ce become 'irregular" and form structures of low symmetry [7]. The occurrence of these phases of low symmetry is attributed to the delocalization of 4/electrons [13]. However, La has no localized / electrons at ambient conditions. If La remains an sd band transition metal up to ultrahigh pressure [14], such structures should not appear in La at high pressure. This question has stimulated the present study.Energy dispersive x-ray diffraction with synchrotron radiation at HASYLAB (DESY) was used to study the structural behavior of La under pressure up to 67 GPa. The experimental setup with diamond anvil cell and ruby manometer has been described previously [15]. The nonlinear pressure scale [16] was used for the ruby manometer and mineral oil was used as the pressure transmitting medium. The chemical analysis of the La material provided by Gschneidner is given elsewhere [7].The two phase transitions of La at 2.5 GPa from dhcp to fee and at 7 GPa to d-fcc are well known [6]. It was noticed also previously [6] that the intensity of the superstructure reflections of the d-fcc phase increase continuously with increasing pressure. At 26 GPa the superstructure reflections (101), (105), and (113) are well developed (Fig. 1). However, the present study shows that the intensity of these superstructure reflections decreases ...

First‐principles study of the pressure‐induced α → ω transition in titanium

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New Helical Chain Structure for Scandium at 240 GPa

“…The rare-earth metals (including Y, La, Pr, Nd, and so on) exhibit successive phase transitions under high pressure due to s-d transitions. The structures follow to a systematic sequence, from hcp-Sm type-dhcp-fcc [3][4][5][6][7]. By analogy with the properties of Y, metallic Sc, which has the hcp structure under ambient conditions, is also believed to follow this systematic series.…”

“…The Sc atoms of Sc-V at 242 GPa have two nearest-neighbor atoms, with an intrachain distance of 2:0522 A. The chain is formed along the c axis, with a bond angle of 149.36 (6) . The nearest-neighbor distance between chains is 2:1552 A, which is 5% longer than in the intrachain distance.…”

Phase Diagram of Samarium up to 27GPa and 800K.

“…The boundary between these two domains exhibits an hysteresis of 2 GPa. In the first domain of pressure, intensities of superlattice reflections pass through a maximum at Pimax =19.5 GPa, as already observed for Lanthanum (for which Prmax = 26 GPa) [16]. In the second one (P > 21 GPa), supplementary superlattice reflections indexable within the rhombohedral structure appear, mainly for low d-spacings.…”