New high-pressure phase transition in zirconium metal

Abstract

Phase transitions in the group-IV transition metal zirconium were studied by the energy-dispersive xray-diffraction technique with a synchrotron source to a pressure of 32 GPa. A first-order phase transition between an co phase and a bcc phase (isostructural with group-V transition elements) was observed during compression and decompression in a pressure range of 30 ± 2 GPa, which is in qualitative agreement with the recent first-principles theoretical predictions. The observation of the bcc phase in Zr (a group-IV element) under pressure at room temperature signifies a pressure-induced electronic configuration similar to that of a group-V element. PACS numbers: 64.70.Kb, 61.10.-i, 62.50.-hp, 64.30.+tCrystal structures of elemental metals tend to have certain sequences when viewed as functions of atomic number or hydrostatic pressure. The most prominent example of this phenomenon is the d transition metals, where all three transition series, excluding the four magnetic 3d metals, show the canonical hep-" bcc-• hep -fee sequence as their atomic numbers increase, or as their d bands become progressively filled. 1 " 4 It is believed that the d electrons play a critical role in determining the crystal structures of these transition metals. Similar transition-metal structure sequences are expected to occur in individual transition metals with increasing pressure since compression leads to an increase in delectron population by transfer of electrons from the s band. 1,2 This pressure-induced phase-transformation sequence has received extensive experimental as well as theoretical attention. Calculations of the crystal structures using one-electron theory, based on electron transfer from the s to the d band under compression, are given in reviews by McMahan 3 and Skriver. 4 The phenomenon of electron transfer from the s to the d band under pressure is well known for transition metals. 5 Physically, this phenomenon can be attributed to the greater fraction of the atomic volume occupied by the sp core in transition metals. The bottom of the s band (B s ) rises faster than that of the d band (Bj) because the s-band electrons have a greater increase in kinetic energy than the d electrons since they are repelled by the orthogonality effect as they are more confined to the core region under compression. This larger increase in B s reduces the spacing between the s and d bands, and causes the increase in occupancy of the d band. It is this increase in ^/-band occupancy with pressure of the transition metals with a partially filled d band like Ti, Zr, and Hf that makes the pressure effects equivalent to alloying with d-rich metals. Some model calculations 6,7 show that the pressure-induced electron transfer from the s band to the d band is greater for the first several columns of transition element series. Therefore, the transition metals in the first several columns are expected to show phase transitions at modest pressures, while ultrahigh pressures greater than 200 GPa will likely be required for the phase transitions of the ...

Titanium Alloys at Extreme Pressure Conditions

“…The reported intermediate orthorhombic  phase (transforming between ~ 116 GPa -128 GPa) possessed a distorted hcp crystal structure (space group Cmcm), and the orthorhombic  phase (transforming at ~ 145 GPa) was of a distorted bcc type structure (space group Cmcm) (Akahama et al, 2001). These observations for Ti are at variance with the other Titanium Group transition metals Zr and Hf, which have been observed to follow the     pathway at RT (Jayaraman et al, 1963;Xia et al, 1990a;Xia et al, 1990b). However, Ahuja et al have observed the coexistence of a bcc-like structure (referred to by the authors as ') with the ω phase, during the compression of Ti at RT from 40 GPa to 80 GPa (Ahuja et al, 2004).…”

“…The observation of the   ω  transformation pathway at RT for Ti-6Al-4V suggests that the slope of the -phase boundary (see boundary in figure 2) is negative at high pressures, or that there are two separate areas of phase separated by the  phase (see for example, Xia, 1990a).…”

“…For both neon and mineral oil PTMs, the transformation is completed between 115 GPa and 125 GPa. The phase is formed by the splitting of the alternating (001) plane along the c axis of the  phase into two (111) planes of the phase (Xia et al, 1990a). All the phase peaks are therefore contained in the  diffraction pattern (that is, the peaks are coincident).…”

Amorphouslike Diffraction Pattern in Solid Metallic Titanium

“…All of these facts indicate that pressure is a key issue for the formation of an amorphous phase from a pure metal. Moreover, we notice that the amorphous phase of Zr is produced in a pressure-temperature range close to the triple point at about 5.5 GPa and 700 C in the phase diagram of Zr [19][20][21]. The amorphization of elemental Zr could be explained in the following way.…”

Experimental and theoretical study of Ti-6Al-4V to 220 GPa

“…On static volume compression, Ti does not follow the α → ω → β transformation sequence predicted for, and observed in, both Zr and Hf [9][10][11][12] . The transition to the ω-phase occurs at 3-9 GPa on pressure increase [13][14][15][16][17][18] , and the ω-phase can be retained as a metastable phase at ambient pressure; the reverse transition back to the α phase in Ti occurs only on heating for extended periods at 380 K 13 .…”