Preparation and HREM characterization of a protonated form of a layered perovskite tantalate from an aurivillius phase Bi₂SrTa₂O₉ via acid treatment

An Aurivillius phase, Bi₂SrTa₂O₉, which consists of perovskite-like slabs and bismuth oxide sheets, was treated with 3 M hydrochloric acid for 72 h, and the resultant product was characterized. Scanning electron microscopy investigation indicated that no morphological change occurred during the acid treatment. X-ray diffraction (XRD) analysis revealed that the product exhibited tetragonal symmetry with a = 0.39l ± 0.004 nm and c = 0.98 ± 0.01 nm, and the a parameter is consistent with a typical value for cubic perovskite oxides. High-resolution electron microscopy (HREM) observations along both [001] and [010] showed that the structure of the perovskite-like slabs in Bi₂SrTa₂O₉ was retained after the acid treatment. The compositional analyses revealed the loss of a large portion of bismuth and a part of strontium (present in the bismuth oxide sheets due to B ↔ Sr disorder) and the introduction of protons. These observations indicate that the bismuth oxide sheets in Bi₂SrTa₂O₉ were selectively leached and that protons were introduced into the interlayer space to form a protonated layered perovskite, H_1.8[Sr_0.8Bi_0.2Ta₂O₇]. Though diffraction techniques (XRD and electron diffraction) demonstrated that an average structure of H_1.8[Sr_0.8Bi_0.2Ta₂O₇] consisted of perovskite-like slabs stacked without displacement, HREM observation along [010] demonstrated that both a simple stacking sequence without displacement (P-type) and a stacking sequence with a relative displacement by (a + b)/2 (I-type) were present in H_1.8[Sr_0.8Bi_0.2Ta₂O₇].