Thermochromism, the Alexandrite Effect, and Dynamic Jahn–Teller Distortions in Ho₂Cu(TeO₃)₂(SO₄)₂

A 3d–4f heterobimetallic material with mixed anions, Ho₂Cu­(TeO₃)₂(SO₄)₂, has been prepared under hydrothermal conditions. Ho₂Cu­(TeO₃)₂(SO₄)₂ exhibits both thermochromism and the Alexandrite effect. Variable temperature single crystal X-ray diffraction and UV–vis–NIR spectroscopy reveal that changes in the Cu^II coordination geometry result in negative thermal expansion of axial Cu–O bonds that plays a role in the thermochromic transition of Ho₂Cu­(TeO₃)₂(SO₄)₂. Magnetic studies reveal an effective magnetic moment of 14.97 μB. which has a good agreement with the calculated value of 15.09 μB.