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

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 P2(1)/C (14) and a tetragonal Pb3Cr2(PO4)(4) of space group P4(1)2(1)2 (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 = 6K, and then to a weakly ferromagnetic phase below the Curie temperature T-C = 4 K, with a magnetic hysteresis loop displayed at 2 K.Natural Science and Engineering Research Council of Canada (NSERC) ; National Natural Science Foundation of China [20725310, 20673085]; National Basic Research Program of China [2007CB815303