Reinvestigation of the crystal structure of pentastrontium tetraniobate, Sr5Nb4O15

Abstract

DiscussionThe crystal structure of Sr5Nb40i5, achieved by Rietveld analysis of X-ray powder diffraction pattern using the structurafparameters of BasTa^Ois (trigonal symmetry, space group P3m\), results of a monoclinic distortion of the hexagonal cell [11. However the refinement is quite poor: no R value, no thermal factors and no justification of choosing the monoclinic system were given. Moreover many atomic distances (mainly Nb-О and О-О) are anomalous with respect to the well-known crystal chemistry of niobates. The reinvestigation was done from single crystal X-ray diffraction data and preliminary electron diffraction analysis of the reciprocal lattice. It is definitively a member η = 5 of the homologous series of complex oxides with cation-deficient perovskite-related structure with trigonal symmetry formulated as A n B n -iC>3n. The РЗс 1 space group chosen instead that of Ba5Ta40i5 [2,3] corresponds to a doubling of the oparameter due to a global reverse rotation of ТЮб octahedra (-15°) around the с axis (see figure). As observed in LasTi^is [4| and BaLa4Ti40i5 [5], this cooperative rotation of all the octahedra is the simplest way to provide Sr 2+ cations with a [6+3] coordination without any distortion of these octahedra. Due to the ionic radius of Sr 2+ , such a coordination is better suited than the [ 12| coordination. Abstract

Structure and microwave dielectric property relations in (Ba1−xSrx)5Nb4O15 system

“…The symmetry of Sr 5 Nb 4 O 15 showed a P3c1 space group with better reliability factors and goodness of fit indicator as shown in Table 1. The refinement of Sr 5 Nb 4 O 15 was consistent with the report of Teneze et al 20 and it was confirm to have hexagonal symmetry. The c-parameter is doubled due to an anti-tilting of octahedra (∼15 • ) around the c-axis.…”

Phase transformations in formation of five-layer perovskites Ca5−x SrxTiNb4O17 (x=0−5) from coprecipitated hydroxocarbonate systems

“…However, both the type of crystalline intermediates and the method for the formation of layered perovskite-like structures of this compound from them proved to be substantially different from previously known data for five-layer A 5 B 5 O 17 compounds. The primary carbonate-free crystalline product upon heat treatment of Sr,Ti,Nb-containing batches of coprecipitated hydroxocarbonates is formed at 1170 K and is a mixture of a phase based on the compound Sr 5 Nb 4 O 15 , belonging to a family of compounds different from the A n B n O 3n+2 family with a layered perovskite-like structure of general composition A n B n-1 O 3n [11], and a slight amount of a phase based on the rutile modification of TiO 2 .…”

“…The noted difference is obviously due to the phase composition of the Sr,Ti,Nb-containing intermediate crystalline products, in particular the considerable difficulties encountered in rearrangement of the structure on going from the cation-deficient layered perovskite-like structure of Sr 5 Nb 4 O 15 (the family of compounds of the A n B n-1 O 3n type [11]) to the differently constructed layered perovskite-like structure of Sr 5 TiNb 4 O 17 (the family of compounds of the A n B n O 3n+2 type). The above hypothesis is supported by the fact that under "shock" heat treatment conditions (calcination of a dehydrated X-ray amorphous batch of coprecipitated hydroxocarbonates immediately at T > 1570 K), practically pure Sr 5 TiNb 4 O 17 is obtained even at 1670 K (τ = 2 h).…”