Synchrotron-radiation study of phase transitions in phosphorus at high pressures and temperatures

Abstract

Results are reported of an investigation using synchrotron radiation into the effects of temperatures up to 1173 K on pressure-induced phase transitions in phosphorus. A cubic-type multi-anvil press was employed and a diffraction pattern in an energydispersive mode was taken for a period of time, typically 200 s, without suffering from a deterioration of the sample material. The pressure of the first transition, orthorhombic-rhombohedral (As-type), decreases with increasing temperature at a rate of 2"3 MPa K -1 and the As-type structure is stable at a pressure as low as 2-6 GPa at a temperature of 1073 K. The volume discontinuity at the transition, A V, is 10% at room temperature and remains almost unchanged with increasing temperature. The axial ratio c/a, when the rhombohedral structure is referred to the hexagonal system, changes mostly with pressure but only slightly with temperature, approaching x/6=2-45 on going to the second transition, rhombohedral-simple cubic. The pressure of this transition, in contrast to the first one, is independent of temperature but AV at this transition, 3.7%, continuously decreases with increasing temperature.

Interlayer Bond Formation in Black Phosphorus at High Pressure

“…Nevertheless,incontrast to the experimental observation reporting this transition to occur sharply in pressure, [19,20] am ore complex transition dynamics emerges from our data.…”

“…Within this picture,a st he hexagonal cell approaches the sc one,t he c/a axial ratio reaches the p 6 % 2.45 theoretical limit. [20] The data clearly show ad ecrease of the c/a ratio up to 10.5 GPa, where ad iscontinuity suddenly marks the fulfillment of the c/a % 2.45 condition, approximately corresponding to as cl attice (Figure 4). Above 10.5 GPa the second step of the transition, involving the displacement of the atoms,d ominates the transition mechanism.…”

“…Ther esults,r eported in Figure 3, clearly show ad iscontinuity at 10.5 GPa, where the A7 to sc transition is expected to take place according to literature.N evertheless,w hile at this pressure a rapidly approaches the limit value of 60.0 degree, u, except for as mall discontinuity,i ss till far from the 0.250 value expected in the sc phase,w hich is indeed not yet [20,29,30] The dashed line at 10.5 GPa marks the A7 to sc phase transition accordingto current literature. Our room T data indicate that the sc phase is not achieved up to 27.6 GPa and that an intermediate pseudo simple-cubic(p-sc) structure exists between the A7 and the sc structures (see text).…”

“…[17] By increasing pressure at room temperature bP undergoes ac haracteristic phase transition at~5GPa to al ayered semimetallic rhombohedral phase (A7, R3 m, Z = 2), which is reported to transform to ametallic simple-cubic phase (sc, Pm3 m, Z = 1) at~11 GPa. [18][19][20] Thes cs tructure,r arely observed in nature (a-Poa nd high-pressure Ca-III and As-II), [21] is stable up to 103 GPa, where it transforms to an incommensurate modulated structure (P-IV,I M, Cmmm(00g)s00). [22,23] At 137 GPa P-IV converts to asimple-hexagonal phase (P-V,sh, P6/mmm, Z = 1) observed up to 282 GPa, [24,25] which is also ar arely adopted structure.…”

Metallic high-pressure modifications of main group elements

“…Black phosphorus with three covalent bonds per atom within the puckered layers plus two additional interlayer contacts is reported to become metallic at 1.7 GPa without undergoing a change of crystal structure [67]. At 4.2 GPa, a semiconducting phase with an arsenictype structure (3 þ 3 coordination) is formed which transforms to a primitive cubic arrangement (CN 6) with metallic properties at 10.2 GPa [68]. At 103 GPa, this modification transforms into an interjacent allotrope of as-yet unknown organisation which is followed at P > 137 GPa [69] by a hexagonal primitive pattern (CN 8) finally undergoing a change towards bcc (CN 8) at 262 GPa [70].…”