Acidities of 3d and 4f element sulfides are the key factor governing the type of phase diagram in MS-Ln2S3 (M = Mn,Fe; Ln = La-Lu) systems

An equation to calculate the acidity of simple sulfides and a relative acidity scale are proposed. A correlation between the type of phase diagram and the acid-base properties of constituent simple sulfides of the system is postulated. The similarity of the acid-base properties of simple sulfides provides that they have a eutectic-type diagram. Complex sulfides are formed when the difference between the relative acidities of simple sulfides exceeds some threshold value. The complex compounds formed in MnS-Ln₂S₃ (Ln = La, Ce) systems, namely Mn₂La₆S₁₁ and MnCe₂S₄, are classified with thiomanganates. The compounds formed in FeS-Ln₂S₃ (Ln = La, Ce, Pr) systems, namely FeLn₂S₄ (Ln = La, Ce) and FeLn₄S₇ (Ln = La-Pr), are classified with thioferrates. The strengthening of acidic properties in the Ln₂S₃ (Ln = La-Lu) series results in the formation of thiolanthanates: MnLn₂S₄ (Ln = Dy-Lu), MnLn₄S₇ (Ln = Tb-Tm), FeLu₂S₄, FeLn₄S₇ (Ln = Ho-Yb), and Fe₄Ln₂S₇ (Ln = Tm, Yb, Lu).