On the existence of a high-temperature polymorph of Na 2 Ca 6 Si 4 O 15 —implications for the phase equilibria in the system Na 2 O-CaO-SiO 2

摘要: Singe crystals of a new high-temperature polymorph Na2Ca6Si4O15 have been obtained from solid state reactions performed at 1300 °C. The basic crystallographic data this so-called β-phase ambient conditions are as follows: space group P1c1, a = 9.0112(5) A, b = 7.3171(5) c = 10.9723(6) β = 107.720(14)°, V = 689.14(7) A3, Z = 2. showed twinning by reticular merohedry (mimicking an orthorhombic C-centred unit cell) which was accounted for during processing and structure solution. Structure determination accomplished direct methods. Least-squares refinements resulted in residual R(|F|) = 0.043 5811 observed reflections with I > 2σ(I). From structural point view β-Na2Ca6Si4O15 can be attributed to the mixed-anion silicates containing [Si2O7]-dimers well isolated [SiO4]-tetrahedra ratio 1:2, i.e. more precisely formula written Na2Ca6[SiO4]2[Si2O7]. tetrahedral groups arranged layers parallel (100). Sodium calcium cations located between silicate anions charge compensation coordinated six eight nearest oxygen ligands. Alternatively, described mixed tetrahedral-octahedral framework based on krohnkite-type [Ca(SiO4)2O2]-chains CaO6-octahedra corner-linked bridging SiO4-tetrahedra. infinite chains running [001] concentrated (010). Adjacent shifted relative each other amount +δ or −δ along a*. Consequently, …ABABAB… stacking sequence is created. A detailed comparison related structures such α-Na2Ca6Si4O15 A2B6Si4O15 representatives including topological theoretical aspects presented. There strong indications that monoclinic Na2Ca3Si2O8 mentioned earlier studies actually misinterpreted β-Na2Ca6Si4O15. In addition analysis previously unknown compound our results will also help improve interpretation phase relationships compounds ternary system Na2O-CaO-SiO2 interest several applications field applied mineralogy materials science.