Preparation and characterization of NH 4 Bi 3 F 10
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A new NH4Bi3Fio compound was prepared, which results from a reaction between Bi 2 O 3 and NH4F in an argon stationary atmosphere. It has a cubic fiuorite-related structure isotypic with KBi 3 F 10 with space group Fm3m (a = II.91A A). Further heating leads to decomposition to BiF 3 . IR spectra at different temperatures show the disappearance of NH4 group characteristic vibrations. Indexed x-ray diffraction pattern and IR spectra at different decomposition temperatures are given.
I. INTRODUCTION Inorganic fluorides are an interesting group of inorganic materials due to their ionic conductivity. Such types of materials are used in the construction of solid state batteries, electrodes, etc. After the discovery of high ionic conductors, some BiF 3 systems have been thoroughly studied, e.g., BiF 3 -BaF 2 , BiF 3 -PbF 2 , BiF 3 -KF, and BiF 3 -RbF. 1 " 6 New compounds KBi 3 F 10 and RbBi3Fio are observed by Matar et al.1 in BiF 3 -KF, RbF systems, respectively. That type of structure was first described by Vedrine et al.8 in MLn 3 F 10 (M: K, Rb; Ln: rare earth metal). The principal methods for fluoride preparation are based on the use of anhydrous hydrogen fluoride, fluorine, hydrofluoric acid, and halogen fluorides. The fluorination of Bi 2 O 3 has been described by many authors. Aurivillius9 prepared BiF 3 by the reaction of Bi 2 O 3 and hydrofluoric acid and determined the structure. Zachariasen10 finds that BiF 3 is isostructural with YF 3 (orthorhombic symmetry). He also observes the appearance of bismuth oxyfluorides Bi 2 OF 4 , Bi 4 O 3 F 6 , BiOF, and Bi 4 O 5 F 2 , depending on reaction conditions. Formation of NH 4 BiF 4 is described by Ryss11 and in the patent literature.12"14 The most simple and useful method is fluorination by NH 4 F. When NH 4 F is heated, it decomposes according to the following equation15: 2NH 4 F (crystalline) —• N H 4 H F 2 (crystalline)
In this work we present the results of Bi 2 O 3 fluorination by NH 4 F under argon at 300 °C. A compound with composition NH 4 Bi 3 F 10 was found, which appears to be isotypic with KBi 3 F 10 and decomposes at a higher temperature to BiF 3 , NH 3 , and HF. II. PREPARATION A mixture of Bi 2 O 3 and NH 4 F in the weight ratio 1:1 is mixed, reground, and put into a silver crucible. The crucible is put into an Inconel tube and placed in a horizontal furnace. After replacing air by argon the tube is heated at 300 °C for 1 h. Then, the tube is cooled at room temperature. The product is submitted to quantitative chemical analysis for NH4 and Bi determination. Fluorine is determined by the method described by Chariot.16 A fluorine selective electrode has been used for determination of the end of titration. III. PHYSICAL MEASUREMENTS X-ray powder diffraction measurements have been carried out using a vertical diffractometer Siemens KR500 with Co anode and Fe filter. NaCl is used as an internal standard. Infrared spectroscopy is used for the determination of NH 4 group vibration frequencies (FT-IR spectrophotometer Nicolet, model 5DXC). The method of KBr pre
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