Pyrochlores. XI. High-pressure studies of the antimonates A₂Sb₂O₇ (A = Ca, Sr, Cd) and preparation of the weberite Sr₂Bi₂O₇

Abstract

. 58,2221 (1980). Reactive hot-pressing experiments at 60-70 kbar and 700-900°C indicate that the relative stability of the pyrochlore and weberite phases of the A2Sb207 (A = Ca, Sr, Cd) antimonates depends primarily on the effective size of A2+ and the covalency of the A-0 bond working in conjunction. The analogous bismuthate(V) Sr,Bi207, obtained by hot pressing of SrBiO,, with an excess of KCIO,, has been shown to have the weberite-type structure.OSVALD KNOP, GERARD DEMAZEAU et PAUL HAGENMULLER. Can. J. Chem. 58,2221Chem. 58, (1980. L'ktude sous hautes pressions (60-70 kbar) des oxydes A2Sb207 (A = Ca, Sr, Cd), dans un domaine de temperature compris entre 700 et 9 W C , a montre que la stabiliti relative des phases pyrochlore et weberite dependait de la taille de I'ion A2+ ainsi que de la covalence de la liaison A-0. L'oxyde de bismuth(V), Sr2Bi207, obtenu par action sous haute pression de KCIO, sur SrBiO,,, presente la structure weberite.

Phase Chemistry in the Ca‐Mn‐Sb‐O System at 1160–1250 °C

“…According to the literature, [19] [20] Mn 2 Sb 2 O 7 has a trigonal 3T-weberite structure above 900°C [20] (its composition corresponds to the formula Mn 2 Sb 2 O 6.75 [21] ) and a cubic pyrochlore structure. [22] 6 were produced under hydrothermal conditions. [25] The naturally occurring mineral manganostibite [26] of the composition (Mn 2+ ,Mn 4+ )Mn 4 Sb 2 O 15 has an orthorhombic braunite or hausmannite structure.…”

“…At present, the compounds CaSb 2 O 6 [5,27] Ca 2 Sb 2 O 7 , [4][5][6] and Ca 6 Sb 3+ Sb 5+ O 10 [5] were registered in the Ca-Sb-O system during synthesis in air. The formula of the latter compound is written in the literature [28] as Ca 6 Sb 5+ 2 O 11 .…”

“…The crystal structure of this compound has not been established. Ca 2 Sb 2 O 7 synthesized at atmospheric pressure has an orthorhombic 2O-weberite structure [29] and at a pressure of 6 GPa [6] [5] were obtained at low temperatures without oxidizers or during mechanochemical treatment. [30] With increasing temperature, they decompose into binary oxides based on Sb 5+ .…”

“…In the ternary oxides forming this system, manganese exists in the form of Mn 4+ , Mn 3+ , and (or) Mn 2+ cations, and antimony exists mainly as Sb 5+ . [1][2][3][4][5][6] Such various oxidation states of manganese characteristic for a wide interval of temperatures, apparently, are defined by the chemical nature of oxides of manganese (II), (III) and (IV), calcium, and antimony as components of chemical reactions.…”

“…The Ca 2-x Mn x Sb 2 O 7 (0 Յ x Յ 0.23) with a 2O-weberite structure, as well as solid solutions Ca 2 Mn 1+x Sb 1-x O 6 with monoclinic (0 Յ x Յ 0.67) and orthorhombic (0.75 Յ x Յ 1) perovskite structures. The existence of a number of double and ternary oxides and solid solutions on the basis of Sb 5+ and Mn 2+ , Mn 3+ , Mn 4+ and with mixed manganese valence is confirmed.…”