Potassium-zinc monophosphate: an original polymorphic tridymite derivate

The previously reported low-temperature (LT) α-KZnPO4 crystallizes in the hexagonal space group P63. It undergoes two phase transitions at 691 and 783°C. The medium-temperature (MT) β-KZnPO4 structure is orthorhombic (space group Pna21, a = 8.7305(7), b = 9.1651(5) Å, c = 5.3539(3) Å, Z = 4. The high-temperature (HT) γ-KZnPO4 structure is orthorhombic (space group Pnma, a = 8.7695(5), b = 5.3490(3), c = 9.1817(5) Å, Z = 4). All of these structures belong to the stuffed tridymite family with a tetrahedra skeleton. The α-β transition is a new reconstructive phenomenon that proceeds by tetrahedra reversal; the β-γ transition is displacive with ferroelectric character. The effects of structural changes on relative permittivity and ionic conductivity are reported.