Structures of the nickel gallosilicate spinelloids

Abstract

The structures of four spinelloid phases (spinel plus phases V, I and II), forming at 1 atm pressure in the NiGa204-Ni2SiO4 system, have been determined from X-ray data collected from single crystals grown from SiO2-rich melts in the temperature range 1798-1923 K. The results show that, under conditions of constant pressure, a strong correlation exists between the spinelloid structures, their atomic distributions and their chemical compositions. In particular, the Si contents of the tetrahedral sites are found to be strongly correlated with the amount of tetrahedral corner sharing: 6-12, 24-35 and 65% Si in the T(1), T(2) and T(3) sites with zero, one and two shared corners respectively. Consequently, the spinelloid structures accommodate increasing amounts of NizSiO4 in the order: spinel (6mo1%), phase V (27mo1%), phase I (33mo1%) and phase II (38 mol%). The crystal chemistry of the nickel gallosilicate spinelloids is discussed in comparison with that of their high-pressure aluminosilicate counterparts.

Combinatorial modular design of the structures of spinel-type phases

“…7) [79] and the aforementioned β phase, respectively. Similar spinelloid phases were also experimentally revealed in other systems [80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96] [87][88][89][90][91][92][93][94][95][96].…”

“…The effect of the isomorphous replacement of tetrahedrally coordinated silicon ions (r ion = 0.040 nm [3]) by larger germanium ions (r ion = 0.054 nm [3]) on the structures of germanosilicate phases is identical to the effect of an increase in the pressure [1]. Possibly, that is why the spinelloid phases in the gallosilicate system are stabilized already at atmospheric pressure, because the averaged concentration of Ga 3+ ions (r ion = 0.061 nm [3]) in the tetrahedral positions of the spinelloid structures is sufficiently high (even for phase II, this concentration is equal to 62 at %) [86]. The revealed crystal chemical role of the composition and distribution of cations over the tetrahedral and octahedral positions in the stabilization of the spinelloid structures in the gallosilicate system was used in the study of spinel-type phases in other systems [68-76, 78, 87, 88, 91, 92].…”

“…Hammond and Barbier [86] thoroughly considered the crystal chemistry of nickel-containing gallosilicate spinelloid phases in comparison with the structure of the spinel similar in composition to NiGa 2 O 4 ( Table 2). It was revealed that the main characteristics of these structures are virtually identical to those for the phases in the NiAl 2 O 4 -Ni 2 SiO 4 system (Fig.…”

“…In [86], this allowed the authors to perform correctly a comparative analysis of the characteristics of spinel and spinelloids in order to reveal correlations between the chemical compositions, the structure types of the phases, and the parameters of structural disordering of cations over octahedral and tetrahedral positions. Belov [4] discussed the crystal chemical significance of Ge 4+ cations in the stabilization of germanate structures at lower pressures as compared to pressures characteristic of the structures of the corresponding silicates.…”

“…Among the modular structures constructed from modules 2 , 3 , and 4 (Table 4), four structures are known as spinel-type structures and occur in the NiAl 2 O 4 -Ni 2 SiO 4 [42,[68][69][70][71][72] and NiGa 2 O 4 -Ni 2 SiO 4 [86] systems. These are the structures of phases I (space group Pmma ), II (space group Imma ), III (space group Imma ), and V (space group Pmma ).…”