Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates

Abstract

Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO2–La2O3 (–Gd2O3) systems. All melts crystallized upon quenching in oxygen. Hf2La2O7 pyrochlore and Gd0.5Hf0.5O1.75 fluorite phases were identified. Gd0.5Hf0.5O1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO2‐ La2O3 melts contained 31.6–34.2 mol% La2O3. The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na2O·4MoO3 melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf2La2O7 and −48.8±4.7 kJ/mol for Hf2Gd2O7. The enthalpy of the pyrochlore–fluorite phase transition in Hf2Gd2O7 is 23.6±3.1 kJ/mol.

Energetics of rare-earth-doped hafnia

“…Annealing this phase at 1450°C for 50 h resulted in the formation of the ordered pyrochlore phase. 26 Therefore, the calculated ⌬H f,ox (Hf 0.5 La 0.5 O 1.75 , amorphous) from ⌬H f,ox (fluorite) − ⌬H cryst represents only an approximation.…”

“…7,20,26 The enthalpy of formation of the amorphous phase was calculated by combining the enthalpy of crystallization with the enthalpy of formation of the fluorite phase:…”

Formation of nanocrystalline structures in the Ln2O3-MO2 systems (Ln = Gd, Dy; M = Zr, Hf)

“…Фазовые диаграммы систем «Ln 2 которые при комнатной температуре могут иметь как кристаллическую структуру пирохлора, так и флюорита [14].…”

The electronic structure change with Gd doping of HfO2 on silicon

“…5 It has been demonstrated that Y 2 O 3 doping stabilizes HfO 2 in the cubic phase. [5][6][7][8] Doping of HfO 2 by Er, [7][8][9] Gd, 8,10,11 and many other rare earth elements 8 also induces a monoclinic to cubic phase transition. The gadolinium doping of a number of wide band gap semiconductors [12][13][14] suggested that Gd doping of HfO 2 may lead to a dilute magnetic semiconductor.…”

“…[8][9][10] The presence of a small amount of the Gd 2 Hf 2 O 7 pyrochlore phase cannot, a priori, be excluded, because it differs from fluorite only in the presence of ordering within the cation sublattice. 8,11 To determine the placement of the Fermi level, temperature-dependent angle-resolved photoemission experiments were performed using the 3 m toroidal grating monochromator beam line in an UHV chamber previously described. 18,19 The Fermi level ͑E F ͒ was established from a a͒ Electronic mail: pdowben@unl.edu…”