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Thus the inferred isotropic energy loss in g-rays is E G ϳ3 ϫ 10 53 erg: This is remarkably coincident with the gravitational binding energy released during the merger of two neutron stars 24 .Even though E G is much greater than E A , the afterglow measurements pose a clear problem to GRB models. At the time the ␥-rays are emitted, the shocked gas is moving with very large Lorentz factor (Γ). The initial Γ is assumed to be 10 2 -10 3 and expected to evolve as t −3/8 . Due to relativistic beaming, only a small portion, solid angle ϳΓ −2 , of the fireball is visible to us. Hence we remain ignorant of the true shape of the fireball. The estimate of E G can be reduced by assuming that the emitting surface is a narrow jet directed towards the observer. However, Γ is smaller during the optical afterglow emission than during the ␥-ray emission. Relativistic beaming is less of a limitation, and the full extent of the fireball is gradually revealed to the observer. The monotonic power-law decay at optical wavelengths seen in the OTs of previous transients 13-15 is not consistent with the narrow jet-like emitting surfaces. We recognize that more complicated models (in which energy is injected non-spherically) could account for such monotonic decays whilst having nonspherical geometry. However, the main issue that we discuss here is the inferred energetics. It is by no means clear that the energy requirements inferred from afterglow observations (when the Lorentz factor is small) is any lower than the simplest case considered here.According to the models discussed here, the afterglow emission is supposed to arise from non-radiative shocks; that is, the efficiency of the shock in converting E 0 (the total energy in the GRB event) to afterglow radiation is low. Thus, E 0 is significantly larger than the E A reported above. This finding is in direct contrast to the assumptions made by most practitioners in this field, namely E 0 ¼ 10 51 erg. As an aside we note that equation (1) predicts that the break frequency should evolve with time as t −3/2 . Measurement of the break frequency at successive epochs for future bursts offers the possibility of directly confirming or refuting the non-radiative shock assumption.The evidence presented here favours GRB models which produce energy vastly in excess of 10 51 erg; see for example, refs 24, 25, 29. The afterglow emission is similar in nature to the emission from supernovae but is more energetic by two orders of magnitude. Following the naming sequence, nova and supernova, it is only appropriate to refer to GRB afterglow as hypernova 29 .Ⅺ

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 ...

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Energy-dispersive x-ray diffraction studies were carried out on the III-V compound aluminum nitride to 65 GPa using a synchrotron x-ray source. A pressure-induced first-order phase transition from a wurtzite structure to a rocksalt structure was observed. The first peaks of the high-pressure phase appeared at 14 GPa. On further loading of the diamond anvil cell, the wurtzite peaks disappeared by 20 GPa. After unloading the cell to atmospheric pressure, the rocksalt structure persisted. The equilibrium transformation pressure lies on the interval 0–14 GPa.

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