diff --git a/src/pymatgen/core/ion.py b/src/pymatgen/core/ion.py
index 01524ed7d06..16c479dcf29 100644
--- a/src/pymatgen/core/ion.py
+++ b/src/pymatgen/core/ion.py
@@ -231,7 +231,7 @@ def get_reduced_formula_and_factor(
         elif formula == "N[-0.33333333]":
             formula = "N3[-1]"
         elif formula == "P[-0.33333333]":
-            formula = "P[-1]"
+            formula = "P3[-1]"
         # formate # codespell:ignore
         elif formula == "HCOO[-1]":
             formula = "HCO2[-1]"