Gordon S. Smith

The crystal and molecular structure of beryllium hydride

Abstract The crystal and molecular structure of BeH 2 has been determined from high-resolution powder diffraction data obtained at a synchrotron radiation source. Computer indexing methods gave the unit cell as body-centered orthorhombic with a = 9.082(4), b = 4.160(2), c = 7.707(3) A V = 291.2(2) A 3 , with systematic absences corresponding to space groups, Ibam or Iba2. Essentially, single crystal methods were used for the structure determination: Patterson synthesis to locate the Be atoms and a “heavy-atom” electron-density synthesis to confirm the location of the H atoms. The crystal structure is based on a network of corner-sharing BeH 4 tetrahedra rather than flat infinite chains containing hydrogen bridges previously assumed. The Be-H bond distances are 1.38(2) A around Be(1) and 1.41(2) A around Be(2). The H-Be-H tetrahedral bond angles range from 107° to 113° and the Be-H-Be bond angle is approximately 128°. The space group in Ibam, and there are 12 BeH 2 molecules in the unit cell. The theoretical density is 0.755 g/cm 3 .

Reexamination of the crystal structure of a high‐pressure phase in praseodymium metal

H. K. Mao, R. M. Hazen, P. M. Bell, and J. Wittig [J. Appl. Phys. 52, 4572 (1981)] reported that praseodymium metal undergoes a crystallographic phase transformation from a distorted fcc structure to a distorted hcp structure at a pressure of ∼21 GPa. This phase change, further, is accompanied by a large volume decrease ∼19%. We have reexamined their powder diffraction data for the distorted hcp structure and find the data can be satisfactorily interpreted in terms of the well‐known orthorhombic α‐uranium type of structure. The volume decrease at the phase change is now estimated to be only ∼9.8%.

High-pressure phase transformation studies in gadolinium to 106 GPa

Abstract We investigated the high-pressure structural transformations in gadolinium at room temperature. Up to a pressure of 106 GPa, gadolinium metal undergoes four structural transformations: h.c.p.→ Sm-type → d.h.c.p. → f.c.c. → t.h.c.p. The corresponding transformation pressures are: 1.5 ± 0.2 GPa, 6.5 ± 0.5 GPa; onset at 24.0 GPa and completion at 29.0 GPa; and between 44.0 and 55.0 GPa, respectively. We found a much larger stability range for f.c.c.-gadolinium phase than that reported by others.

On the possibility of Pr III having a thcp structure

Abstract Crystallographic calculations have been carried out for a triple hexagonal close-packed (thcp) structure using data for Pr III, a high-pressure form shown by praseodymium metal. McMahan and Young proposed thcp as an additional high-pressure phase in the rare earth metals structural sequence. As applied to Pr III data at 14.4 GPa pressure, this structure type accounts for most but not all of the diffraction lines. Thus, we are unable to establish conclusively that Pr III has a thcp structure. However, it is shown that the thcp structure fits the Pr III data as reasonably as other structural forms proposed by other authors.

Static EOS of uranium to 100 GPa pressure

Abstract The crystal structure and compressibility of uranium has been determined by energy dispersive X-ray measurements in a diamond-cell apparatus up to pressures of 100 GPa. The alpha phase of uranium remains stable up to the highest pressures as suggested by earlier shock-Hugoniot data. An equation-of-state for alpha-uranium derived from both types of data implies that this phase also remains stable up to 2500 K at Hugoniot pressures of 100 GPa.

Diamond-anvil cell high pressure X-ray studies on thorium to 100 GPa

Abstract High pressure crystal structural changes and volume compression for thorium were investigated to 100 GPa in a diamond-anvil cell apparatus using the energy-dispersive X-ray diffraction technique. No structural change was observed in this pressure range. However, a slope change in the P-V curve was observed between 20 and 30 GPa. We surmise that this break in the slope may be due to an electronic transformation.

A new ultra-high pressure phase in samarium

Abstract Structural changes in samarium under pressure were studied in a diamond-anvil cell (DAC) to 189 GPa. A number of phase transformations were observed between room pressure and 75GPa. At about 91 GPa samarium transforms to a body-centered tetragonal structure, and the unit cell parameters at 189 GPa are a=2.402(4) A and c=4.231(17) A , V=24.40(12) A 3 and Z=2. Identification of a body-centered tetragonal phase in Sm at about 90 GPa, which is similar to that reported in Ce, suggests that this phase could appear in other rare-earth elements too.

Static high pressure studies on Nd and Sc

Abstract We have investigated the crystal structural transformations in neodymium and scandium up to 4.0 GPa pressure and at room temperature, in a diamond-anvil high pressure apparatus. Nd has a double hexagonal-close packed (dhcp) structure at ambient pressure and temperature. Then it transforms to a face-centered cubic (fcc) structure at 3.8 GPa, which further transforms to a triple hexagonal-close packed structure (thcp) at about 18.0 GPa. In scandium we observed only one transformation from the hexagonal-close packed (hcp) structure at room temperature to a tetragonal structure. This transformation occurs between 19.0 and 23.2 GPa pressure.

High pressure diamond-anvil studies on neodymium to 40.0 GPa

Abstract We have investigated the structural transformations in neodymium as a function of pressure up to 40.0 GPa. Neodymium has a double hexagonal close packed (A3′ type) structure at 1 atm that transforms to a face centered cubic structure at about 3.8 GPa, which further transforms to another structural form at about 18.0 GPa. The latter appears to have a triple hexagonal close packed structure, similar to that which we have for the first time reported in another rare earth element, praseodymium.

X-ray diffraction studies of irradiated lithium hydride: oriented lithium precipitates

Abstract Single-crystal X-ray diffraction studies of gamma-irradiated LiH show the clear presence of metallic lithium precipitates. The Iithium is in the normal bcc form and is crystallographically oriented with respect to the matrix: {110} Li ∥{111} LiH and 〈100〉 Li ∥〈110〉 LiH . The absence of diffraction broadening is evidence that th particles are relatively large ( > 50 nm). X-ray diffraction measurements of the Li content of a heavily irradiated LiH crystal indicate that a substantial fraction of the “free” Li is present in the oriented precipitates.