Electric field gradients in MgB2_2 synthesized at high pressure: 111^111Cd TDPAC study and ab initio calculation

We report the high-pressure synthesis of novel superconductor MgB$_2$ and some related compounds. The superconducting transition temperature of our samples of MgB$_2$ is equal to 36.6 K. The MgB$_2$ lattice parameters determined via X-ray diffraction are in excellent agreement with results of our ab initio calculations. The time-differential perturbed angular correlation (TDPAC) experiments demonstrate a small increase in quadrupole frequency of $^111$Cd probe with decreasing temperature from 293 to 4.2 K. The electric field gradient (EFG) at the B site calculated from first principles is in fair agreement with EFG obtained from $^11$B NMR spectra of MgB$_2$ reported in the literature. It is also very close to EFG found in our $^111$Cd TDPAC measurements, which suggests that the $^111$Cd probe substitutes for boron in the MgB$_2$ lattice.Comment: 10 pages, 3 figure

On the structure and dehydration of hydrous zirconia and hafnia xerogels

Hydrous zirconia and hafnia xerogels of compositions ZrO2 · 2.5H2O and HfO2 · 2.3H2O are dehydrated in two steps upon heating. First, molecular water is mostly removed from the structures to form phases of compositions ZrO2 · H2O and HfO2 · 0.5H2O. Second, polycondensation of OH groups occurs. Both processes are easier for ZrO2 · 2.5H 2O. Apart from these steps, the interaction of water molecules with zirconium-oxygen bridges was found to occur during dehydration of the zirconium compound. The composition of HfO2 · 0.5H2O should actually read as Hf4O7(H2O)(OH)2. © Pleiades Publishing, Ltd., 2010

Structure and dehydration of hydrous tin dioxide xerogel

Hydrous tin dioxide xerogel with the composition SnO2 • 1.75H 2O is built of tin-oxygen-hydroxide fragments. Water molecules (no more than 1 mol) in the grain structure are kept by hydrogen bonds. Xerogel is dehydrated in the range 50-890°C in two stages. Below 123°C, molecular water is removed and the polycondensation of ≡Sn-O(H)-Sn≡ bridge groups occurs. There also takes place the transition of some water molecules from the molecular to hydroxide form as follows: ≡Sn-O-Sn≡ + H 2O → 2≡Sn-O-H. All processes occur within individual grains. Above 123°C, water removal is due to the polycondensation of tin-oxygen groups. As a result, grains are coarsen. After 200°C, their structure is determined as cassiterite coated by tin oxyhydrate. © 2007 Pleiades Publishing, Inc

On the structure and dehydration of hydrous zirconia and hafnia xerogels

Hydrous zirconia and hafnia xerogels of compositions ZrO2 · 2.5H2O and HfO2 · 2.3H2O are dehydrated in two steps upon heating. First, molecular water is mostly removed from the structures to form phases of compositions ZrO2 · H2O and HfO2 · 0.5H2O. Second, polycondensation of OH groups occurs. Both processes are easier for ZrO2 · 2.5H 2O. Apart from these steps, the interaction of water molecules with zirconium-oxygen bridges was found to occur during dehydration of the zirconium compound. The composition of HfO2 · 0.5H2O should actually read as Hf4O7(H2O)(OH)2. © Pleiades Publishing, Ltd., 2010