The claim is disputed: "The pentaiodide does not exist (except perhaps as PI3·I2, but certainly not as [PI4]+I−...)". Phosphorus pentaiodide was reported to be a brown-black crystalline solid melting at 41 °C produced by the reaction of lithium iodide and phosphorus pentachloride in methyl iodide, however, this claim is disputed and probably generated a mixture of phosphorus triiodide and iodine. (wikipedia.org)
Phosphorus and iodine don't seem to form a pentaiodide. (cineriser.com)
Form1
Although phosphorus pentaiodide has been claimed to exist in the form of [PI4]+I− (tetraiodophosphonium iodide), experimental and theoretical data refutes this claim. (wikipedia.org)