A PROPOSED STRUCTURE FOR ANNEALED ANODIC OXIDE FILMS OF TANTALUM

Abstract

Anodic oxide films (~1500 Å thick) formed on tantalum in saturated H 3 BO 3 , saturated Na 2 SO 4 , 86% H 3 PO 4 , 1% H 2 SO 4 , and 100% H 2 SO 4 have been examined by a simple but sensitive glancing angle X-ray technique. They are amorphous as formed but can be crystallized on annealing at various temperatures between 500 and 1000 °C. The times and temperatures required for crystallization depend on the electrolyte used. The crystallization process is competitive with the dissolution of oxygen in the substrate metal. The observed powder diffraction patterns show considerable variation from sample to sample. Similar variations have been reported in the literature for different samples of Ta 2 O 5 . A structural model is proposed for β-Ta 2 O 5 based on the isostructural relationship with U 2 O 5 , whose structure is related to that of α-UO 3 . This proposed structure consists of linear chains of Ta—O—Ta—O— parallel to the c axis with varying disorder in the ab plane. This disorder accounts for the observed variations in the patterns. The proposed structure is a defect one and the ordering of these defects may account for some of the reports of irreversible transformations observed on heating Ta 2 O 5 .