| Two new high-pressure apparatus are developed and used to explore the phase diagrams of rare earth metals gadolinium, thulium, and lutetium. The first device incorporates natural diamond anvils that have been surface modified with a homoepitaxially-grown chemical-vapor-deposited layer of conducting boron-doped diamond. This layer is used as a direct electrical heating element to heat the diamond anvils independently of the rest of the diamond anvil cell. This allows for comparatively rapid, but controlled, changes in temperature at high pressure, primarily while cooling of a sample without a large energy or time expenditure. The second device incorporates a homoepitaxially-grown micro-anvil on the culet of natural diamond anvil that acts as a second-stage diamond anvil, allowing for higher pressures than could otherwise be reached, but also creating a larger-than-normal pressure gradient in the cell.;The diamonds are then used in conjunction with rapid x-ray diffraction techniques to study the crystalline phases of the three heavy rare-earth elements gadolinium, thulium, and lutetium. The high-temperature behavior of gadolinium is studied in the pressure range from 0 to 10 GPa and from 300 K to 600 K using boron-doped diamond heaters, while thulium and lutetium are investigated at room-temperature using the two-stage anvils. Equations of state of the hexagonal close-packed structures of these materials are found, and the volume changes of the observed phase transitions measured. Further, estimates of the slope of the phase boundaries are presented, and in conjunction with the measured volume changes, the latent heats of these transitions are calculated. These measurements on gadolinium are then compared and contrasted with isothermal measurements performed in a classical high-pressure heating apparatus. A phase diagram is presented summarizing the material behavior over the investigated region of pressure and temperature. |
