Polymorphic structures and properties of lead chromium phosphate Pb3Cr2(PO4)4

Abstract

Single crystals of lead chromium phosphate Pb₃Cr₂(PO₄)₄ were grown by a flux technique and characterized by X-ray powder and single crystal diffraction, differential thermal analysis, polarized light microscopy and magnetic measurements. Two polymorphic structures have been identified: a monoclinic Pb₃Cr₂(PO₄)₄ of space group P2₁/c (14) and a tetragonal Pb₃Cr₂(PO₄)₄ of space group P4₁2₁2 (94), and their phase correlation is described. Both structures consist of a three-dimensional framework which is composed of PO₄ tetrahedra and CrO₆ octahedra connected via vertices. Lead ions are situated at the cavities of the framework, but the positions of lead ions in the monoclinic Pb₃Cr₂(PO₄)₄ are different from those in the tetragonal Pb₃Cr₂(PO₄)₄. The Pb₃Cr₂(PO₄)₄ crystals are found to exhibit interesting physical properties. The ferroelastic domain structures reveal characteristic patterns of the tetragonal and the monoclinic symmetry of the crystals. The birefringence of the tetragonal crystals decreases with increasing temperature. The monoclinic crystal is found to undergo a series of magnetic phase transitions upon cooling, from a paramagnetic phase (from room temperature down), to an antiferromagnetic phase below T_N = 6 K, and then to a weakly ferromagnetic phase below the Curie temperature T_C = 4 K, with a magnetic hysteresis loop displayed at 2 K.