Structural chemistry of A ₂ MX ₄ compounds (X = O, F) with isolated tetrahedral anions: search for the densest structure types

Abstract

The packing density of various structures is important not only for understanding and the prediction of high-pressure phase transitions, but also because of its reported correlation with thermodynamic stability. Plotting the cube root of formula volume against the cation radii (R) for nine morphotropic series with isolated tetrahedral anions, A(2)MO(4) (M = Si, Ge, S, Se, Cr, Mn, Mo, W) and A(2)BeF(4), permits the comparison of packing densities for 13 structure types (about 80 individual compounds and several solid solutions) stable at (or near) ambient temperature. The spinel type is the densest. The next densest types are those of K(2)MoO(4), Tl(2)CrO(4), β-Ca(2)SiO(4), β-K(2)SO(4), Ag(2)CrO(4) and Sr(2)GeO(4). In three series (M = Ge, Mo, W) the densest type comes with somewhat intermediate values of R, and not the largest, in contrast to the classical homology rule. Another contradiction with traditional views is that some of the densest phases have abnormally low overall binding energies. The correlation between packing density and coordination number (CN) is better when CN of A counts entire MX(4) groups rather than individual X atoms; many, but not all, A(2)MX(4) structures have binary A(2)M analogues (of course, A and M are not necessarily the same in these structure types). The most frequent arrangement of A around M is of the Ni(2)In type: a (distorted) pentacapped trigonal prism.

Crystal structures of Zn2SiO4 III and IV synthesized at 6.5–8 GPa and 1,273 K

“…The thenardite structure is adopted by Cd 2 SiO 4 at ambient pressure, which contains Si in tetrahedral coordination and Cd in octahedral coordination (Dent Glasser and Glasser 1964). It has previously been suggested as another candidate for high-pressure Zn 2 SiO 4 polymorphs on the basis of packing density arguments (Nalbandyan and Novikova 2012). Further in situ high pressure and high temperature measurements and first-principles calculations would be helpful in resolving these puzzles.…”

Unique thermodynamic relationships for Δf H o and Δf G o for crystalline inorganic salts. I. Predicting the possible existence and synthesis of Na2SO2 and Na2SeO2

“…Emerging in recent studies (see for example, Nalbandyan & Novikova, 2012), is the usefulness of combining simple thermodynamic ideas (obtained by using volume-based thermodynamics, VBT) with structural science concepts in order to gain new insights into the structural characteristics of condensed phases.…”

Pressure–induced phase transitions of Zn₂SiO₄ III and IV studied using in–situ Raman spectroscopy

“…Although identification of crystal structures solely from Raman data is difficult, crystal structures of new phases might be predicted from proposed structure systematics for A 2 BX 4 compounds. Nalbandyan and Novikova (2012) used the packing density of A 2 BX 4 (X = O, F) compounds to understand and predict high-pressure phase transitions. Based on their study, the high-packing density structures (= high-pressure phases) after phase I (phenacite structure) will be olivine (normal olivine structure, not tetrahedral olivine), thenardite (Na 2 SO 4 ), Na 2 CrO 4 , Ag 2 CrO 4 , Sr 2 GeO 4 , β-K 2 SO 4 , larnite (Ca 2 SiO 4 ), K 2 MoO 4 , Tl 2 CrO 4 , and finally the spinel phase in increasing packing density order.…”