Synthesis and structures of sodium containing K_2-xNa_xMg₂(SO₄)₃ langbeinite phases

In this work we report a detailed structural study of the K_2-xNa_xMg₂(SO₄)₃ series in order to evaluate the effect of Na incorporation on the structure. The results show that the cubic langbeinite structure is observed at room temperature for high Na levels;K_2-xNa_xMg₂(SO₄)₃ (0 ≤ x ≤ 1.8). Increasing the Na content further, 1.8 < x ≤ 1.9, leads to significant structural distortions, with the observation of an enlarged orthorhombic cell. These latter systems resemble a highly distorted variant of the langbeinite structure, and this relationship is also illustrated by the fact that they transform to the cubic langbeinite structure at relatively low temperature (≈200 °C). In conjunction with our prior studies on Na₂Mg₂(SO₄)₃ (complex monoclinic cell, which also transforms to langbeinite at elevated temperature), this work highlights the flexibility of the langbeinite structure to accommodate high levels of Na ions, thus offering another potential avenue to manipulate the properties of materials with this structure-type.