Phase transformations during HLnTiO4 (Ln=La, Nd) thermolysis and photocatalytic activity of obtained compounds

•We studied dehydration and further thermolysis of HLnTiO4 (Ln=La, Nd) compounds.•XRD, STA and solid state IR studies were carried out.•A new series of metastable Ln2Ti2O7·yH2O compounds was obtained.•We examined the photocatalytic activity of all obtained compounds. The hydrogen evolution rate increased in the course of the structure changes during thermolysis.

Layered HLnTiO4 (Ln=La, Nd) compounds belonging to Ruddlesden–Popper phases were found to form partially hydrated compounds Ln2Ti2O7·xH2O during thermal dehydration as well as defect oxides Ln2□Ti2O7 as final products. Further heating of metastable defect Ln2□Ti2O7 substances leads to the formation of pyrochlore-type oxides Ln2Ti2O7 (p), with subsequent transformation under higher temperatures to stable layered 110-type perovskites Ln2Ti2O7. The occurring structure transformations lead to an increase of photocatalytic activity in the order of HLnTiO4

Graphical abstractLayered HLnTiO4 (Ln=La, Nd) compounds form partially hydrated Ln2Ti2O7·xH2O compounds during thermal dehydration, further heating results to the formation to defect oxides Ln2□Ti2O7, pyrochlor-type oxides Ln2Ti2O7 (p), with subsequent transformation to layered 110-type perovskites Ln2Ti2O7. Structure transformations lead to an increase of photocatalytic activity in the order of HLnTiO4