M. P. Pasternak

Hysteresis in the high pressure transformation of bcc‐ to hcp‐iron

The α‐Fe/e‐Fe phase transformation has been studied by the Mossbauer effect of 57Fe in a diamond anvil cell at hydrostatic pressures up to 24 GPa. The α/e composition is easily derived as a function of intermediate pressure. For rising pressures the transition onset is about 9 GPa and half of the α‐Fe is converted to e‐Fe by 15 GPa; for lowering pressures half of the e‐Fe is converted to α‐Fe by 9 GPa. The transition pressures, widths, and hysteresis widths are compared to those obtained by other techniques.

Sub-megabar Mössbauer studies using diamond anvil cells

The development and the use of the diamond anvil cell for Mössbauer Spectroscopy (MS) to pressures nearing 100 GPa are discussed. Three types of cells and their typical performance are given. Pressure calibration, hydrostatic media, gasketing, collimation, γ-ray absorption, and sample size for MS are reviewed. New MS results showing hysteresis in the room temperature high pressure transition in iron and showing the rapid rise ofTN in NiI2 with pressure are presented.

Mössbauer effect of iodine at pressures to 30 GPa

We report the first high pressure129I Mössbauer measurements with elemental iodine at pressures to 30 GPa. A 20 mg/cm2129I2 absorber was mounted in a diamond anvil cell with an effective diameter of 0.21 mm. The source used was Mg3129mTeO6. Experiments were performed mainly at 4 K and the pressure was monitored by the ruby fluorescence method. With increasing pressure we observe a gradual decrease in ¦e2qQ¦ and an increase in IS and η values of the low pressure, molecular phase; at ∼16 GPa a new phase (HP1) is detected characterized by a change in sign of e2qQ and a smaller value of ¦e2qQ¦, and a substantial increase in η. At ∼24 GPa a new phase (HP2) is formed that is characterized by a smaller value of η. In general the population of the molecular phase decreases from 1.0 near 15 GPa to a value of 0.4 at 30 GPa. The fraction of the high pressure phase (HP1 + HP2) increases at the expense of the molecular phase and that of the HP2 at the expense of the HP1 phase. These observations are discussed in relation to the onset of a metallic phase near 16 GPa and recent x-ray diffraction studies.

119Sn Mössbauer studies in antiferromagnetic La2CuO4

Abstract Mossbauer Spectroscopy (MS) studies of 119Sn were carried out in antiferromagnetic La2(Cu.99Sn.01)O4. The 1 at% 119Sn4+ impurity in the form of SnO2 was first diffused in CuO, and its substitution for Cu2+ was confirmed. Spectra recorded at T> 120 K show a single site quadrupole splitting. At T

Quenching of magnetic fluctuations around 119Sn impurities in superconducting GdBa2(Cu0.99Sn0.01)3O7

Abstract Mossbauer Spectroscopy studies with 119 Sn substituted for Cu in GdBa 2 (Cu 0.99 Sn 0.01 ) 3 O 7 ( T c = 90 K ) reveal the occurrence of a transferred hyperfine interaction at temperatures below 60 K. The effective magnetic field is 8.3 KOe at 57 K and decreases gradually with decreasing temperature to 6 KOeat 16 K. The onset of the magnetic interaction at the non-paramagnetic 119 Sn probe is explained in terms of local depletion of holes due to the Sn 4+ doping, consequently quenching the magnetic fluctuations around the impurity. These experimental findings provide strong evidence for spin-spin correlations originating from the Cu 2+ moments and present in the superconducting state.

Mössbauer Spectroscopy of Mg0.9Fe0.1 SiO3 perovskite

Ambient pressure Mossbauer spectra of Mg0.957Fe0.1SiO3 perovskite synthesized at pressure-temperature conditions of ∼50 GPa and 1700 K show that the iron is entirely high-spin Fe2+ and appears to be primarily located in the octahedral site within the crystal structure. We observe broad Mossbauer lines, suggesting a distribution of electric-field gradients caused by disorder associated with the Fe ions. Also, the perovskite exhibits magnetic ordering at temperatures < 5 K, implying that there is a magnetic contribution to the absolute (“third-law”) entropy of this phase.

The isomer shift of elemental iodine under high pressure

Abstract The Mossbauer isomer shift (IS) in elemental 129I was measured as a function of pressure to 30 GPa unsing diamond anvil cells. The IS increases gradually with pressure reaching a value of ΔIS = 0.56(3) mm sec−1 at P > 16 GPa. No discontinuity was observed at P = 21 GPa where a crystallographic phase transition has been reported. Using the experimental V(P) relation, the IS dependence on −lnV was found to be linear to the highest pressure. The dϱ(0) dlnV value for elemental I2 was found to be -6.6(2) a−30. The volume dependence of the p-holes is deduced resulting in a 5s25p4.62 configuration at the highest pressure. The systematics of dϱ(0) dlnV for the 5sp elements from βSn to Xe are discussed.

High pressure129I Mössbauer studies of GeI4 molecular crystals

The Mössbauer effect in129I in conjunction with Diamond-Anvil-Cell high pressure techniques was applied to investigate the high pressure phase(s) of the molecular crystal GeI4. The129IQuadrupole Interaction was the main probe for characterizing theintermolecular structural transformation with pressure. With increasing pressure, at about 15 GPa, the onset of a partial molecular-association phase (HP1) is first observed. In HP1 two out of the four iodines strongly overlap to form linear chains of GeI4. The HP1 phase coexists with the low pressure (LP) molecular phase, but its population increases with increasing pressure. AtP=20 GPa a second high pressure phase (HP2) is identified where all four iodines strongly overlap to form a three dimensional, fully molecular-associated structure. With increasing pressure and atP>20 GPa, HP2 is the only phase up toP=34 GPa, the highest pressure used. A significant hysteresis of the relative abundances with pressure is observed. The isomer shift of the HP2 and HP1 structures is considerably larger than that of the LP one.

High-pressure Mössbauer spectroscopy in diamond anvil cells

Diamond anvil cells provide a means to obtain near-hydrostatic pressures from the kilobar to the megabar regime. Mössbauer spectroscopy (MS) nicely complements the optical and X-ray measurements usually made. After a brief summary of the techniques applicable to MS, we present several examples of high-pressure MS, including hysteresis in the α-ε transition in Fe, metallization in molecular crystals and the insulator-metal Mott transition in NiI2 and CoI2.

The Mössbauer effect of SnI4 at high pressures

Tetravalent tin iodide is a molecular crystal composed of SnI4 tetrahedra loosely packed into a cubic configuration. Under pressure SnI4 becomes metallic at about 15 GPa. We report a Mossbauer effect study of119Sn and129I in SnI4 to pressures of 26 GPa. The spectra exhibit dramatic changes with pressure starting at about 10 GPa and show large pressure hysteresis effects upon reducing the pressure from 26 GPa. In the intermediate region tin exists in both Sn4+ and Sn2+ states, and iodine exists in two nonequivalent sites characterized by a different symmetry and different sign and magnitude of the electric field gradient.